Let's talk about brakes!

[JoelK]

MORE OT:

You can change the session length in the forum admin, General Admin section, then "Configuration." I think the default setting is 3600 seconds. Doubling or tripling it would probably do the trick.

[Kevin M]

Originally Posted by GarySheehan

Wow, I wish I was involved in this thread a little earlier. I'm going to try and address several points on this page of the thread. I fear it's going to be a little lengthy...

BAN SUVS,

Reducing front brake torque is an excellent thing, not a bad thing for the WRX. The WRX comes with too much front brake bias, as do all production cars. Moving some of it rearward helps improve front/rear brake bias, which will reduce stopping distances. With the stock front calipers and rotors, I was able to consistently lock the front brakes with Hoosier R3S03 race tires as well as with front downforce via a splitter. The WRX does not need more front brake torque! There is no such thing as adding more stopping power, unless the original system cannot lock the brakes. What you can do is increase brake torque with less pedal effort. However, when considering a brake system, you do not want a system that generates very high brake torque with very low pedal effort. Systems like that are very hard to modulate at the limit. A well designed system will take some effort to lock the brakes. As long as the effort is not fatiguing, the higher effort provides the driver a wider modulation range, making it easier to use the brakes to keep the tires on the edge of adhesion.

You also mentioned that you have yet to lock the brakes at the track. One of two things is happening. You are using ABS, which prevents the wheels from locking, or you are not braking hard enough. Unless of course you have faded the brake pads and/or fluid, which would make it impossible to lock the wheels after a few laps. But even with R compound tires, you should have no issues with locking the front wheels on the track while the brake pads and fluid are still within their operating temperature range.


Let me know if you have any questions.

Gary
Sheehan Motor Racing
www.teamSMR.com

Wow Gary, your expertise is most welcome here. Thanks for clearing some things up! Unfortunately, I think you've just eliminated StopTechs as a possibility for use on my car. Let me explain...

I have a 2001 Impreza RS. I've never faded or locked up my brakes on the track Using FHI 4 pot fronts, OE pads, rotors, and lines with Ate SuperBlue fluid. This is probably because I jsut haven't had to use my brakes that much yet- all of my track experience is at Thunder Hill, where I only have one hard braking point, and that's on the back stretch. I only get up to about 90-95 and brake to 35 or so, and my system is easily able to handle that. On the front straight I only drop from <100 to about 80-85. That track is simply faster than my car. On to why I can't use StopTechs now-

I am doing a full STi 6 speed swap, including the R180 rear end and hubs. Those hubs require the use of either STi 2 pot rears or STi Brembos. I've decided against the Brembos due to cost and limitations on wheels. That leaves me with the old STi 2 pots. Since I have STi 4 pots up front, this seems a natural combination to me (I don't know how closely matched my existing MC and bias valve are to this setup, but it can't be worse than any non-Subaru setup I suppose). I know the pedal will be somewhat softer, but that's the breaks... so to speak. Anyways, should my 4 pot setup prove incapable of handling the heat from increased speeds at the track (forgot to mention- I'm also doing a compete motor build. Basically an STi motor with a VF22) I was planning on getting StopTechs. I love the company's attitudes towards the enthusiast community, their openness with their testing and with providing concrete proof of their claims, and heck, I even know someone who works there now. But alas, adding them would destroy my brake system. I would end up with far too MUCH rear bias, and I am nowhere near skilled enough to drive a car that behaves that way. The only way I could run StopTechs at this point would be to have custom piston sizes made.

[GarySheehan]

BAN SUVS,

Well, you're going to love Stoptech even more. I just got off the phone with Bob. They will customize the piston sizes of the front calipers to work perfectly with whatever rear calipers you are going to use. All you have to do is tell them the rear rotor diameter and the rear piston sizes and they will do the rest. Is that service or what?!

They are waiting for your call

Gary
Sheehan Motor Racing
www.teamSMR.com

[Kevin M]

Originally Posted by GarySheehan

BAN SUVS,

Well, you're going to love Stoptech even more. I just got off the phone with Bob. They will customize the piston sizes of the front calipers to work perfectly with whatever rear calipers you are going to use. All you have to do is tell them the rear rotor diameter and the rear piston sizes and they will do the rest. Is that service or what?!

They are waiting for your call

Gary
Sheehan Motor Racing
www.teamSMR.com

Excellent! Now, if I hadn't already spent upwards of $20k on this car in the past few months, he wouldn't have to wait so long. I'm going to see how far 22b takeoffs can get me with good pads and fluid first.

[Kevin M]

Oh, and this is probably better off in another thread, but you mentioned the R3S03s... have you tried the R3S04s yet? And how long does a set last you typically?

[Dean]

************************************************** *************************************
Edited slightly 2/22 9:00am pacific to fix some typos and unclear language.
************************************************** *************************************

OK, last time I wrote this response, I went down your list, I think I will do it different this time..

I too wish you had been involved in this discussion earlier. We actually agree on most of the things we are discussing, so I will ignore most of those and stick to the questions you have raised in my mind, and the areas we disagree to some extent.

I am trying not to be defensive, or offensive, but hope we can all learn something from this discussion.

First, a couple easy questions before we get into the deep stuff...

I am going to concede the bigger disks are better for all the reasons you listed. I think I have been biased in favor of width since my wider/flat/directional vaned but same diameter kit on my Stealth was so much superior as compared to the not as wide/cross drilled/directional vaned but larger diameter kit that I put on my A4. I realize they are completely different systems, and it is likely that one is just a better fit for the vehicle than the other, but it has warped my impressions.

My question to you on that would be all other things held constant; would you prefer 10% more diameter, or width? OK, maybe this isn't easy...

You describe the Stoptech front only upgrade system for the WRX as shifting 10% bias to the rear. With a bigger diameter rotor, and I assume a higher coefficient of friction pad than stock supplied with the kit, the only way I can see this could be done is with significantly less piston area than stock. Do you have the stock vs. Stoptech piston area and piston circle diameter? I just don't recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock.

Now on to the theory stuff.

I don't know your background, but for the most part, I come from a geeky science/physics/engineering background. Often in that world, they take things to extremes such as my Infinite CF pin and hole example. I realize it isn't how brakes normally work, but it serves a purpose.

You said "When all four tires lock simultaneously after gradually increasing brake pressure without any steep transients, you can say that the brake system is perfectly balanced."

My example was to address precisely the transients you mention.

Let me start by describing the picture as I see it. We have two friction interfaces working against each other through the semi flexible lever that is the wheel and tire. The brake interface has two states, dynamic friction as the wheel continues to spin, and static when it stops/locks. The tire/ground interface is in a very weird world called rolling friction that is a combination of static and dynamic friction that transitions to mostly pure dynamic friction when the tire stops rotation while it is still in motion.

Both of these friction interfaces have a torque curve they go from coasting to lock up. Where those two torque curves intersect is the instantaneous point where the brake locks or the tire stops rotation.

I don't think either of these curves is linear as you describe, but I may not be reading this sentence correctly. "The state of a rolling tire versus a locked tire is dependant on the coefficient of friction between the road surface and the tire versus the braking force applied to the rotor based on pedal pressure. It is a linear progression that rolls off as the tire adhesion is exceeded." Otherwise, I think we agree on this.

My thought right or not is that whichever curve is the steepest, or more likely has the highest rate of change in slope at that point is the interface that could stand to be improved the most. A bad brake system will have a steep curve at that point, a bad tire will have a steep curve at that point.

You also state that "A brake system will apply more and more force to the pads, creating greater and greater friction, slowing the wheel and tire combination more and more." again, I agree with this, but then you say "There is never a case where the friction coefficient between the brake pad and the rotor will instantaneously jump to a level to suddenly lock the brakes. Brake systems do not work that way. There is never an instantaneous on-off. There is always a ramp." While I agree with the first sentence, there must be a instantaneous change from dynamic to static friction, or else the brake would not be able to ever actually stop the car.

My pin/hole example is the ultimate example of a bad brake system as it is nothing but a single infinite transient. It instantaneously applies an infinite brake torque through an infinite coefficient of friction as the pin stops the rotor. At that instant, the torque curve is a straight vertical line from 0 to infinity as the "brake" is applied. Some non zero time later through the flexible lever that is the tire, the tires torque curve follows curving initially as the tire gives way until it goes into a skid as the sidewal etc. runs out of elasticity.

I realize it is extreme, and not remotely real world, but IMHO it shows how a brake system that has transients, or is "grabby" for whatever reason can force a tire lockup earlier than necessary.

This is where I got into the ABS making up for a bad brake system and where Scott came up with the idea of the ultimate ABS system that would run my pin/hole brake system in which the tire would end up absorbing close to 100% of the energy of stopping. Again, a ridiculous example, but educational none the less as we see that the energy must still go somewhere and how an excelent ABS system can improve a less than optimal brake system.

I think we both agree that a huge amount of money has gone into pad formulation to minimize spikes, enhance release, provide smooth coefficient of friction curves, etc. But you can still by something close to a cow turd at your local Kragen and put it into your stock caliper with a single small pot and small pads that closely resembles a C clamp. Even though you can probably lock up a tire and/or get into ABS with this turd and C clamp brake system, I think we would both agree a better system can stop your car faster.

I agree a good ABS system can stop a car faster than most humans, but it remains to be seen if the WRX system falls into the category of "sports cars and performance cars" you describe. I was also referring to overall vehicle balance at the time when I said getting into ABS was not necessarily fast. IMHO, standing the car on it's nose every time you hit the brakes is not always the best way to get around the track.

I do not have first hand experience with this, and weld was probably a bad word, but I have heard tell of where probably the binding agent in the compound in the pad became fused to the rotor and required significant force to be dislodged.

This of course is my segway into the rotor and pad discussion.

It is really hard not to get defensive about some of this, so bear with me...

To the best of my knowledge, everything is compressible to some extent, rotors, pads or diamonds. Substances do not have to be molten to do so. Unless I am mistaken, the modulus of compression for steel is 1/160x10^9 N/m^2. Yes, this is really small, but it is not zero. Brake pads also have a non zero modulus of compression which undoubtedly varies based on compound. All of these materials are also subject to thermal expansion, but that is a whole other topic.

This compression, along with the deformation due to flexing between the vanes is probably the reason for the Jutter you describe, and some of the transients you see in some brakes.

Every friction example I can think of has the potential for more heat at the leading edge then on the remaining friction surface. That is usually due to some material compression and/or sloughing off of one or both of the materials as the leading edge molecules crash into the oncoming molecules of the surface it is in contact with.

There are a ton of things that go into the design of a caliper and the rest of a brake system. I would guess that reducing leading edge pressure and therefore temperatures are contributors to this design decision in addition to the debris issues you describe.

I think we are splitting hairs trying to differentiate between a softer and lighter pedal. Perhaps I was using a specific braking term incorrectly, but we appear to have meant the same thing, pedal pressure, not the distance the pedal travels.

And when I basically said bigger everything improves braking performance, I was referring to a single brake, not the entire car where bias and other issues come into play.
To be honest, I had not considered the possible feel issue with increase in piston area. On reading another article, it appears drivers are better at modulating to a point with a firmer pressure (non light) pedal. Would you agree with this?

Hey, at least I got that the tire stops the car right...

Sorry if any of this doesn't make sense. It is late, and I probably haven't proofread it as well as I should...

[edit] If it still doesn't make sense, I can no longer blame it on being tired

[AtomicLabMonkey]

Originally Posted by GarySheehan

The state of a rolling tire versus a locked tire is dependant on the coefficient of friction between the road surface and the tire versus the braking force applied to the rotor based on pedal pressure. It is a linear progression that rolls off as the tire adhesion is exceeded. As a matter of fact, maximum braking occurs when the tire is rotating just slightly slower than road speed.

It's also dependant on the instantaneous normal (downward) force on the tire, from weight and any aero download, since the normal force acting with the coefficient of friction is what enables the longitudinal braking force at the tire/road to be developed. Fbraking = CF * Fnormal. A tire with the same CF with the road obviously won't be able to sustain the same braking force over the crest of a sharp rise at 100mph as it will on a level road, since there's less normal force on the tire.

[GarySheehan]

Originally Posted by Dean

I am going to concede the bigger disks are better for all the reasons you listed. I think I have been biased in favor of width since my wider/flat/directional vaned but same diameter kit on my Stealth was so much superior as compared to the not as wide/cross drilled/directional vaned but larger diameter kit that I put on my A4. I realize they are completely different systems, and it is likely that one is just a better fit for the vehicle than the other, but it has warped my impressions.

My question to you on that would be all other things held constant; would you prefer 10% more diameter, or width? OK, maybe this isn't easy...

You describe the Stoptech front only upgrade system for the WRX as shifting 10% bias to the rear. With a bigger diameter rotor, and I assume a higher coefficient of friction pad than stock supplied with the kit, the only way I can see this could be done is with significantly less piston area than stock. Do you have the stock vs. Stoptech piston area and piston circle diameter? I just don't recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock.

Dean, I am glad you are willing to concede. Acknowledgment of denial is the first step in recovery!

To answer your first question, I would always take 10% diameter over width if I could fit it under my wheel. While a wider rotor will provide additional cooling, a larger diameter rotor generates less heat to begin with. Less heat generated is less heat to shed.

You said you don’t recall ever seeing an aftermarket brake kit company selling a kit with less piston area than stock. That’s because you’ve been looking at kits that are not sized properly to the application, like the kit that went on your A4. If it had larger front pistons and larger diameter front rotors, it moved even more braking bias to the front of an already front biased brake system. The end result would be a system that locks the front brakes WAY too early as well as have a longer brake pedal and lighter pedal pressure. Nasty.

The stock Subaru WRX calipers have pistons that are 42.5mm in diameter per my calipers. The Stoptech ST-40 calipers have a leading edge piston of 36mm and a trailing edge piston of 40mm. Here is an aftermarket brake kit company selling a kit that is properly designed for the application with larger front rotors and smaller front pistons.

Originally Posted by Dean

Now on to the theory stuff.

I don't know your background, but for the most part, I come from a geeky science/physics/engineering background. Often in that world, they take things to extremes such as my Infinite CF pin and hole example. I realize it isn't how brakes normally work, but it serves a purpose.

My college degree is in electrical engineering. I have had plenty of physics courses thrown in as well. I will not discuss theory with you on how brakes work. I will tell you how brakes work. There will not be any need to discuss an infinite CF pin and hole example. It does not apply. If you have questions on my explanation, feel free to ask. But don’t bring theoretical extremes into the discussion.

Originally Posted by Dean

You said "When all four tires lock simultaneously after gradually increasing brake pressure without any steep transients, you can say that the brake system is perfectly balanced."

My example was to address precisely the transients you mention.

STOP. There are no transients in the coefficient of friction between the brake pads and the rotors other than the direct input of line pressure from the driver. The transient between dynamic friction to static friction between the brake pads and rotor is IRRELEVENT because the tire has been “locked up” for a considerable and measurable amount of time before the wheel actually stops rotating. I will explain.

The tire stops the car. A tire has a limited amount of adhesion while rolling. As you add brake pressure and use the tire to slow the car, you approach the limit of the tires adhesion. As you continue to add brake pressure, the tire will actually start to rotate slower than road speed. Just as this starts to happen, maximum tire adhesion has been reached. As you further add more brake pressure, the tire continues to slow in relation to road speed. As the delta between actual road speed and the rotational speed of the tire increases, the tire loses adhesion as it transitions to sliding friction. It will get to a point where the speed difference between the road and the tire becomes so great that the friction between the tire and the road is the same as if the wheel were actually stopped, even though the tire is still rolling. For all intents and purposes, the tire is locked because minimum friction is being generated between the road and the tire.

Throughout this entire process, the coefficient of friction between the pads and the rotors HAS NOT CHANGED. There is no instantaneous transient of dynamic friction to static friction between the pads and the rotors. It is a constant dynamic friction between the pads and rotor based on increasing brake pressure and decreasing tire/road friction. When a wheel has finally locked (i.e. – stopped rotating) it is very late in the game and traction between the road and the tire has been long gone for quite some time. The difference in the coefficients of friction between a wheel rotating at 5mph in dynamic friction versus a wheel that has stopped rotating in static friction is absolutely negligible when the road speed of the vehicle is 100mph. The part that you have to understand and accept is that the driver has control of the rotational speed of the tire from full roadspeed to full lock, regardless of the speed of the vehicle. The reason it is practically impossible to control that is because the friction between the tire and road rolls off VERY fast as the tire begins to rotate slower than road speed. It has NOTHING to do with the transition of the friction characteristics of dynamic friction vs. static friction of the pads to rotors because a static state has not been reached.

In summary, the friction curve between the tire and the road will drop dramatically as the tire slows below road speed and becomes flat from a given rotational speed all the way to a fully locked wheel sliding along the pavement. The friction curve between the pads and the rotor stays very consistent throughout the speed range of the rotating tire. There isn’t a transition in the coefficient of friction between the pads and the rotor that would CAUSE a tire to lock, other than driver input.

Originally Posted by Dean

My pin/hole example is the ultimate example of a bad brake system as it is nothing but a single infinite transient. It instantaneously applies an infinite brake torque through an infinite coefficient of friction as the pin stops the rotor. At that instant, the torque curve is a straight vertical line from 0 to infinity as the "brake" is applied. Some non zero time later through the flexible lever that is the tire, the tires torque curve follows curving initially as the tire gives way until it goes into a skid as the sidewal etc. runs out of elasticity.

No, the worst braking system in the world works the same way because the tire/road interface rules all. Your infinite transient does not apply. The tire will always reach it’s minimum traction long before the tire actually stops rotating. Continuing to add brake pressure will speed up when the wheel actually locks.

Originally Posted by Dean

I realize it is extreme, and not remotely real world, but IMHO it shows how a brake system that has transients, or is "grabby" for whatever reason can force a tire lockup earlier than necessary.

It’s not extreme. It doesn’t apply. You are not understanding the friction curve between the tire and the road. In addition, we are not talking about stomping on the brake pedal as hard as humanly possible as an effective braking method. Neither of these scenarios are applicable to describing brake systems.

Originally Posted by Dean

This is where I got into the ABS making up for a bad brake system and where Scott came up with the idea of the ultimate ABS system that would run my pin/hole brake system in which the tire would end up absorbing close to 100% of the energy of stopping. Again, a ridiculous example, but educational none the less as we see that the energy must still go somewhere and how an excelent ABS system can improve a less than optimal brake system.

NO! The ultimate ABS system would keep the tire rotating just slower than roadspeed. The on-off nature of the ABS system would cycle the brakes between road speed and a tiny bit too slow for maximum traction. The average would be a tire rotating just below roadspeed in the maximum adhesion window of the tire.

Your example using the pinhole brake system would alternate between near maximum adhesion with the tire at road speed to absolute minimum adhesion with a locked wheel. That average adhesion would be DRASTICALLY lower than the average ABS system in production today. Today’s ABS systems do not sense lock-up. They sense a speed delta in the tire to itself and to the other tires, but the tire is always rotating.

Are you seeing why the pin and hole analogy doesn't work?

Originally Posted by Dean

I think we both agree that a huge amount of money has gone into pad formulation to minimize spikes, enhance release, provide smooth coefficient of friction curves, etc. But you can still by something close to a cow turd at your local Kragen and put it into your stock caliper with a single small pot and small pads that closely resembles a C clamp. Even though you can probably lock up a tire and/or get into ABS with this turd and C clamp brake system, I think we would both agree a better system can stop your car faster.

Yes, but a cow turd will still stop your car with the same principals. Brakes do not exhibit a dramatic increase in friction between the pads and rotors as rotor speed decreases. Your car comes to a smooth stop with the application of constant brake pedal pressure. It does not have a sudden jerk of deceleration as it approaches zero speed. Do not confuse the rebound of the stored energy in the springs once the car reaches a stop as a sign of increased braking force.

Originally Posted by Dean

I agree a good ABS system can stop a car faster than most humans, but it remains to be seen if the WRX system falls into the category of "sports cars and performance cars" you describe. I was also referring to overall vehicle balance at the time when I said getting into ABS was not necessarily fast. IMHO, standing the car on it's nose every time you hit the brakes is not always the best way to get around the track.

The WRX does not fall into this category. It’s ABS system is not the best.

Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking). ABS will actually compensate for an unbalanced system by allowing all four wheels to do the maximum amount of work possible.

Originally Posted by Dean

I do not have first hand experience with this, and weld was probably a bad word, but I have heard tell of where probably the binding agent in the compound in the pad became fused to the rotor and required significant force to be dislodged.

You can transfer pad material from the pad to the rotor. It’s called pad deposition and it occurs when you apply a very hot pad to a very hot rotor when the rotor is stopped. But this is a result of a stopped wheel, it does not cause a locked wheel by welding itself to the rotor when the wheel is rotating.

Originally Posted by Dean

It is really hard not to get defensive about some of this, so bear with me...

To the best of my knowledge, everything is compressible to some extent, rotors, pads or diamonds. Substances do not have to be molten to do so. Unless I am mistaken, the modulus of compression for steel is 1/160x10^9 N/m^2. Yes, this is really small, but it is not zero. Brake pads also have a non zero modulus of compression which undoubtedly varies based on compound. All of these materials are also subject to thermal expansion, but that is a whole other topic.

This compression, along with the deformation due to flexing between the vanes is probably the reason for the Jutter you describe, and some of the transients you see in some brakes.

Every friction example I can think of has the potential for more heat at the leading edge then on the remaining friction surface. That is usually due to some material compression and/or sloughing off of one or both of the materials as the leading edge molecules crash into the oncoming molecules of the surface it is in contact with.

There are a ton of things that go into the design of a caliper and the rest of a brake system. I would guess that reducing leading edge pressure and therefore temperatures are contributors to this design decision in addition to the debris issues you describe.

Okay, so don't get defensive of your theory. Just accept the facts I am about to give you. I have explained that differential piston sizes are there to compensate for the debris field that forms along the pad surface. Additional force on the trailing piston is required to force the pad through the debris field and make the entire surface of the pad usable. This is not a theory of mine. It is what I have learned directly from brake manufacturers, brake designers and brake engineers. If you don't agree with what I have stated above, stop theorizing and start talking to the people that design and build brakes.

Originally Posted by Dean

I think we are splitting hairs trying to differentiate between a softer and lighter pedal. Perhaps I was using a specific braking term incorrectly, but we appear to have meant the same thing, pedal pressure, not the distance the pedal travels.

I was not splitting hairs. There is terminology in the braking world that are used to describe the characteristics of a brake system. A soft pedal and a light pedal are two different things. Regardless of what you implied, it’s important to clarify this for the other readers of the thread, because what you described was not what you were experiencing.

Originally Posted by Dean

And when I basically said bigger everything improves braking performance, I was referring to a single brake, not the entire car where bias and other issues come into play.

But you can’t speak about a car’s four brake system in those kind of generalities. They don’t work that way.

Originally Posted by Dean

To be honest, I had not considered the possible feel issue with increase in piston area. On reading another article, it appears drivers are better at modulating to a point with a firmer pressure (non light) pedal. Would you agree with this?

The driver’s ability to use the brake system effectively is critical to the operation of the brakes. This thread is discussing improving brake performance and interface to the driver is very important. Yes, a pedal that requires higher effort is easier to modulate, as long as it doesn’t require so much pressure as to be fatiquing. Think about it, with your hand on a scale, is it easier to control a range from zero to an ounce or from zero to 10 pounds? It’s much easier to select any point between zero and 10 pound than between zero and one ounce. However, if you changed it to control between zero and 100 pounds with your hand, it isn’t feasible.

I’m ready to answer questions you may have on any of the above.

Gary
Sheehan Motor Racing
www.teamSMR.com

[sperry]

Well put Gary, I think you've help me understand braking systems a bunch more. I think the failure in our earlier discussion was a result of a brake-centric train of thought over a tire-centric train of thought. Basically we were tracing the effects of the brakes on the tires, rather than the effects of the tires on the brakes. Couple that with an incomplete understanding of a tire's dynamic to static friction transition, and I think you can see where the pin-in-hole brakes came from.

Taking a step back regarding the sizing of pistons in the front brakes to match an existing rear caliper, how is that different from using a brake proportioning valve? I'm not quite sure how a valve would work on a WRX due to the RF-LR/LF-RR dual circuit nature of the WRX's brake system (two valves?), but I infer that they're quite popular for tuning the brakes on solid-rear axle cars that use a front/rear circuit. Is there some drawback to using a valve? It seems the ability to adjust brake bias would be very useful! Plus, you could potentially lower your initial brake purchase costs by not having to have custom pistons up front, just get the less expensive, mass produced calipers, and adjust the bias w/ a valve.

[Dean]

I appreciate your further description of brake operation, especially the tire dynamics.

I'm sorry you don't want to discuss theory, torque curves etc., but that is your choice.

Unless I am mistaken , the single 45mm stock piston has less surface area than 2x36mm and 2 x40mm Stoptech pistons. Unless my math is wrong, that is 222mm^2 vs. 2 x 177.6 + 2 x 197.4 = 750mm^2. You didn't provide piston ring diameter, but it is undoubtedly larger for the larger rotors which only going to make it worse. How does this shift bias 10% to the rear?

You may want to discuss leading edge temperatures and pressures a little more with your “brake manufacturers, brake designers and brake engineers” to better understand the "facts" behind leading piston size.

I quote from http://www.stoptech.com/whitepapers/glossary/t.htm

“Uneven wear of brake pads caused by geometry, by the difference in temperature between leading and trailing edges and/or by lack of stiffness in the caliper.”

In another article, they mention debris etc., but it only effects the system to “some extent” . Leading edge temperature is the primary issue.
From: http://www.stoptech.com/whitepapers/...ons_122701.htm

“The trailing area (portion) of the pad, to some extent "floats" on the entrapped gasses and particulate matter generated from the leading portion of the pad. The leading portion of the pad will always be hotter than the trailing portion and so will correspondingly, wear faster - resulting in a pad that is tapered when viewed from the edge. This phenomenon is termed "longitudinal taper".

The differential in heat generated across the pad surface, leading to trailing, is characteristic regardless of caliper and pad design. This is why all racing calipers and most high performance street calipers have differential piston bores. Most high performance pads also feature a tapered leading edge”

A good analogy I found in another technical paper on brakes might help explain it: http://www.dietersmotorsports.com/tech/2000/1-00.html

“The relationship between the pistons, brake pads, and rotors is not as simple as it seems. The caliper must load the brake on the trailing edge of the pad. This is done so the pad bites into the rotor evenly for more stopping power. Think of it as if you were moving a 100-pound bag of sand across a dirt lot. If you pull it behind you, the front of the bag will be up and skim across the top of the sand (greatest load to the rear), leaving an even trail behind you. If you get behind the bag and try to push it (greatest load to the front), you will cause the front of the bag to dig into the dirt and create a hole. In the case of the brakes, the leading edge of the pad would then see greatly increased uneven wear.”

Enough of that.

I can't believe you said this:

”Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking).”

Perhaps you consider every corner entry where you want to set the front end of the car, or need to scrub a small amount of speed trail braking.

Can you honestly tell me that every time you press the brake pedal, you are trying to reach maximum braking potential, ABS or not for however short of a time?

[GarySheehan]

Dean,

The WRX has two 42.5mm pistons in the caliper. The Stoptech caliper uses one 36mm piston and one 40mm piston. You do not factor in all four pistons in the area, only two. This is because the two piston floating stock caliper applies the same force to both sides of the rotor (i.e.-a 2 piston floating caliper equals a 4 piston solid mount caliper with equal piston sizes).

All of your references to the differential in temperature across the pad is because the trailing edge of the pad is impeded by the debris and outgassing. More force is required at the trailing edge to force it through the debris.

In discussing the fast way around the track, you are correct, there are instances where you do not brake at the maximum because of balance of the car, etc. But our entire discussion to this point has been discussing maximum braking ability of a system. So, in all of the major braking zones where the intent is to scrub off a significant amount of speed, I use the brakes to the maximum of the tire's ability. Standing on it's nose. As I transfer straight line braking for turning, I am still braking at the limit of the tire just prior to lock-up.

Very short braking areas where there is only a little bit of speed to scrub, you don't hit the brakes to the point to upset the car. But you wouldn't be utilizing ABS to have the car stand on it's nose as you described either.

Take Sears Point. There are six "stand the car on it's nose" braking zones and only two places where you do not brake at absolute maximum, because you don't need to shed that much speed.

You didn't mention anything about the majority of the discussion revolving around how the tire traction breaks away quite a long time before wheel lock-up. That was actually the most important topic in this thread. Did you understand what I was trying to convey?

Gary
Sheehan Motor Racing
www.teamSMR.com

[AtomicLabMonkey]

Originally Posted by Dean

I can't believe you said this:

”Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking).”

Perhaps you consider every corner entry where you want to set the front end of the car, or need to scrub a small amount of speed trail braking.

Can you honestly tell me that every time you press the brake pedal, you are trying to reach maximum braking potential, ABS or not for however short of a time?

I'm not going to speak for Gary, (since he is more than capable of doing that for himself ) but assuming that the racetrack is made up of discreet straights and corners, and the racecar is of sufficient power to weight ratio (therefore having a higher speed at the end of every straight than the car is capable of cornering at), the car should absolutely be braked at maximum capacity leading up to every single corner. This is fairly obvious. Now, when you look at some real world racetrack examples that have many combination corners, and get out there in a production car that doesn't have typical racecar-like acceleration & speed, then it's a toss-up as to what will be required on different parts of the track. If you come out of a slow hairpin and then lead into a large-radius sweeper just a second or two later, the car might just not be going fast enough entering the sweeper to require more than a gentle tap on the brakes to get it to turn in sufficiently. I think this is what you were getting after, Dean?

[GarySheehan]

Scott,

I forgot to answer your question. The prop valve is a crutch for a system that isn't sized properly. Manufacturers don't want to design brand new components for every vehicle. It's cost prohibitive. A prop valve reduces line pressure to the output line of the valve. It's typically done to reduce the amount of rear brake.

The WRX has two (in the same package). They allow line pressure to build equally until a given pressure is reached. Once that pressure is reached, the output pressure "knees" to a less steep ramp as input pressure is increased. Essentially it will allow the rear brakes to do more work at light brake application, but ensures that the fronts do more work as brake pressure is increased, keeping the car safe.

Gary
Sheehan Motor Racing
www.teamSMR.com

[sperry]

Originally Posted by Dean

Unless I am mistaken , the single 45mm stock piston has less surface area than 2x36mm and 2 x40mm Stoptech pistons. Unless my math is wrong, that is 222mm^2 vs. 2 x 177.6 + 2 x 197.4 = 750mm^2.

Stock WRX calipers are 2 pots both on one side of the caliper.

Originally Posted by Dean

”Standing a car on it’s nose every time you hit the brakes IS the best, fastest way to get around the track. Regardless of being at maximum braking with ABS or maximum braking on your own, you should always be braking at your maximum in the braking zones (except while trailbraking).”

Perhaps you consider every corner entry where you want to set the front end of the car, or need to scrub a small amount of speed trail braking.

Can you honestly tell me that every time you press the brake pedal, you are trying to reach maximum braking potential, ABS or not for however short of a time?

I think Gary's point was "If you need to slow down, do it as fast as possible." Which makes sense to me... the more time you spend at top speed the faster your laps will be. Don't waste time slowing down at less than the maximum braking ability of the car.

I don't think he was talking about using the brakes to transfer weight to help the car turn, and he explicity said he wasn't talking about trail braking. Just when you need to slow down, which should be in a straight line braking area for the most part.

Edit: I dunno why this ended up so far down the thread.. it was posted right after Dean's calculations!

[Dean]

Originally Posted by GarySheehan

The WRX has two 42.5mm pistons in the caliper. The Stoptech caliper uses one 36mm piston and one 40mm piston. You do not factor in all four pistons in the area, only two. This is because the two piston floating stock caliper applies the same force to both sides of the rotor (i.e.-a 2 piston floating caliper equals a 4 piston solid mount caliper with equal piston sizes).

Sorry, my math was in error. I typed 45, not 42.5 into my calculator. 42.5 results in 209.7 x 2 = 419.4 and the Stoptechs comes down to 375. This is 89% the surface area. I don't have the piston circles, but the Stoptechs appear to be 328mm rotors, and stock are approximately 290mm. This is 13% more rotor diameter. So we have 89% the piston area and 113% the diameter. I am still having a hard time understanding how this can shift 10% bias to the rear. Any thoughts?

Originally Posted by GarySheehan

All of your references to the differential in temperature across the pad is because the trailing edge of the pad is impeded by the debris and outgassing. More force is required at the trailing edge to force it through the debris.

I'm sorry, I disagree, and Stoptechs own documentation and physics appear to disagree as well. As I understand it, debris and outgassing are mostly a thing of the past with modern pad formulation and manufacturing processes... You clearly have access to people and resources I don't. Given these documents, would it be possible for you to discuss this with the enginerers at Stoptech?

Originally Posted by GarySheehan

In discussing the fast way around the track, you are correct, there are instances where you do not brake at the maximum because of balance of the car, etc. But our entire discussion to this point has been discussing maximum braking ability of a system.

I believe this discussion started as a discussion about brake options to increase feel and minimize fade. And It's good to see that you agree that balance etc. is an issue when not in a maximum braking zone as was my original statement.

Originally Posted by GarySheehan

You didn't mention anything about the majority of the discussion revolving around how the tire traction breaks away quite a long time before wheel lock-up. That was actually the most important topic in this thread. Did you understand what I was trying to convey?

I said "I appreciate your further description of brake operation, especially the tire dynamics", and I do. Since you don't wish to discuss the intersection of torque curves, transients, etc. I'm not sure there is anything left to discuss on that front.

[GarySheehan]

Dean,

Your replies are sounding more and more defensive. This is not an attack on you. Please try to stay objective.

Originally Posted by Dean

I'm sorry, I disagree, and Stoptechs own documentation and physics appear to disagree as well. As I understand it, debris and outgassing are mostly a thing of the past with modern pad formulation and manufacturing processes... You clearly have access to people and resources I don't. Given these documents, would it be possible for you to discuss this with the enginerers at Stoptech?

I will go back and read the whitepapers you posted regarding offset piston sizes as well as follow up with the Stoptech folks. It's possible they didn't feel the need to go into all of the engineering details with a "dumb race car driver." (that's a self description, by the way)

Originally Posted by Dean

I believe this discussion started as a discussion about brake options to increase feel and minimize fade. And It's good to see that you agree that balance etc. is an issue when not in a maximum braking zone as was my original statement.

Just to be clear, I meant chassis balance, not brake balance. Brake balance is insignificant when not braking at the maximum.

Originally Posted by Dean

I said "I appreciate your further description of brake operation, especially the tire dynamics", and I do. Since you don't wish to discuss the intersection of torque curves, transients, etc. I'm not sure there is anything left to discuss on that front.

I don't know how to interpret that. It may mean that you still believe your pin/hole theory is applicable. Or not. It's not clear to me.

Originally Posted by Dean

Sorry, my math was in error. I typed 45, not 42.5 into my calculator. 42.5 results in 209.7 x 2 = 419.4 and the Stoptechs comes down to 375. This is 89% the surface area. I don't have the piston circles, but the Stoptechs appear to be 328mm rotors, and stock are approximately 290mm. This is 13% more rotor diameter. So we have 89% the piston area and 113% the diameter. I am still having a hard time understanding how this can shift 10% bias to the rear. Any thoughts?

I tried for quite some time to figure out what you were doing with your math. I'm still not sure I know what you were trying to calculate.

The pressure exerted on the pad is the brake line pressure multiplied by the surface area of the pistons. Lets assume that brake line pressure and the cf between the pad and rotors are constant for both systems. The area of a circle is pi*r^2. For the stock pistons, that's 3.1415 * 21.25mm^2 = 1,418mm^2. Since there's two pistons, the total piston area is 2,837mm^2. The Stoptech is 3.1415 * (18mm^2 + 20mm^2) = 2,274.5mm^2.

Given the same cf brake pads and the same brake line pressure, that gives the stock system a brake torque constant of 290mm / 2 * 2,837mm^2 = 411,365mm^3. The Stoptech system has a brake torque constant of 328mm / 2 * 2,274.5mm^2 = 373,018mm^3. The difference is (411,365mm^3 - 373,018mm^3) / 373,018mm^3 = 10.28%. The Stoptech system is 10.28% less than the stock system. Unless I screwed up my math somewhere.

Regardless of how this thread was started, I entered into it because I saw several issues with the way you were discussing brake systems (i.e.-wider rotors better than larger diameter rotors, NASCAR as an example to back that up, poor brake design causing lock-up prior to the tires adhesion limits being exceeded, braking and ABS described with a pin and hole analogy, etc.) My only purpose was to help you and the readers of this thread understand how brakes work.

When I read your last post, it seemed you were more interested in showing where I may be inaccurate regarding sidebars rather than digging further into how brakes work. Perhaps I used the wrong word when I said I don't want to talk about theory. What I really meant was I don't want use your theories of extreme analogies to attempt to describe a braking system. I am interested in further discussion of torque curves, etc.

Gary
Sheehan Motor Racing
www.teamSMR.com

[ZER026D]

All I can add to this is that I have made the choice to go with Alcon brakes. The SWRT uses them and there is no way in hell they will use anything second best on that car. They have some of the top guys working on that car and more money then god so that is hands down where my money is going.

[GarySheehan]

Zero26D,

Out of curiousity, how much are you spending on the exact same Alcons that the SWRT is using and what will you be using them for?

It's also important to point out that a rally brake system does not require the same thermal capacity as a roadrace brake system.

Gary
Sheehan Motor Racing
www.teamSMR.com

[sperry]

Originally Posted by ZER026D

All I can add to this is that I have made the choice to go with Alcon brakes. The SWRT uses them and there is no way in hell they will use anything second best on that car. They have some of the top guys working on that car and more money then god so that is hands down where my money is going.

Just because SWRT uses a certain setup, that doesn't make it the best for your needs. We've already had the NASCAR example. NASCAR teams have tons of money, but their brakes are limited by series rules and the fact that most ovals don't require heavy braking. Similarily, WRC competition and rules may not allow for a brake system that will be useful on the street. I don't know the details on the Alcon's used by SWRT, but I would certainly find out before deeming them the best brakes. Not to mention the likelihood that they'll cost way too much.

[Kevin M]

Originally Posted by sperry

Just because SWRT uses a certain setup, that doesn't make it the best for your needs. We've already had the NASCAR example. NASCAR teams have tons of money, but their brakes are limited by series rules and the fact that most ovals don't require heavy braking. Similarily, WRC competition and rules may not allow for a brake system that will be useful on the street. I don't know the details on the Alcon's used by SWRT, but I would certainly find out before deeming them the best brakes. Not to mention the likelihood that they'll cost way too much.

Hopefully he got the Tarmac version. Gravel/snow brakes have to fit under 15" wheels. :-p They're little better performance-wise than 4 pots, although I'm sure the build quality is super. Tarmac brakes use something like a 14" disc, vented, slotted, all the bells and whistles. Do WRC cars allow water-cooled brakes? I had a good time once going through the AP caliper catalog... nifty stuff in there. Loved the line about F1/Indy car calipers... "please call for information on ordering." Ha

[Dean]

Originally Posted by GarySheehan

I will go back and read the whitepapers you posted regarding offset piston sizes as well as follow up with the Stoptech folks. It's possible they didn't feel the need to go into all of the engineering details with a "dumb race car driver." (that's a self description, by the way)

Thanks. I really do appreciate all your real world experience with this as well as the technical insight and I would like to understand the theory as well. Please let us know what you find out.

Originally Posted by GarySheehan

Just to be clear, I meant chassis balance, not brake balance. Brake balance is insignificant when not braking at the maximum.

Agreed…

Originally Posted by GarySheehan

It may mean that you still believe your pin/hole theory is applicable. Or not. It's not clear to me.

I’m not certain my example is entirely without merit, but do agree it is not how brakes work under normal conditions. I do think your description of the tire friction and release was very helpful to my understanding. It would be interesting to understand what the torque curves for both these friction zones is just to see what the look like on top of each other.

Originally Posted by GarySheehan

For the stock pistons, that's 3.1415 * 21.25mm^2 = 1,418mm^2. Since there's two pistons, the total piston area is 2,837mm^2. The Stoptech is 3.1415 * (18mm^2 + 20mm^2) = 2,274.5mm^2.

Given the same cf brake pads and the same brake line pressure, that gives the stock system a brake torque constant of 290mm / 2 * 2,837mm^2 = 411,365mm^3. The Stoptech system has a brake torque constant of 328mm / 2 * 2,274.5mm^2 = 373,018mm^3. The difference is (411,365mm^3 - 373,018mm^3) / 373,018mm^3 = 10.28%. The Stoptech system is 10.28% less than the stock system. Unless I screwed up my math somewhere.

Thanks, your math looks 100% correct, I appreciate you going through the exercise. My math was entirely in error because the idiot behind the keyboard entered the wrong formula into the very first Excel cell and then replicated it for the remaining cells… pi^2r. Oops.

I apologize for becoming defensive. My bad math combined with my bad interpretation of some of your comments led me astray. I personally have a much better understanding now of how all this works together now. I am even more convinced that I want a better ABS system, but oh well, just keep learning to drive better I guess. Do you know if the STI system is any better?

P.S. I hope we can get your insights into swaybars and other suspension goodies next. But that is for another thread.

[GarySheehan]

Dean,

I have no experience with the STi ABS system. Or the WRX ABS system for that matter. We removed the ABS system completely at the start of the season.

I have heard lots of people complain about Subaru ABS, though.

Gary
Sheehan Motor Racing
www.teamSMR.com

[sperry]

Originally Posted by GarySheehan

Dean,

I have no experience with the STi ABS system. Or the WRX ABS system for that matter. We removed the ABS system completely at the start of the season.

I have heard lots of people complain about Subaru ABS, though.

Gary
Sheehan Motor Racing
www.teamSMR.com

It's especially bad on snow-covered ice or loose surfaces where locking the tires would actually help. I tried stopping once from 5mph with 15 feet of room before the stop sign and ended up sliding out into the street, and I had my snow tires on! :shock: You'd think the ABS wouldn't ever just disable the brakes, especially at such a low speed.

I miss my Thunderbird's ABS... it cycled much quicker than the Subaru system, and seemed be much less finickey about triggering when you don't need it. Part of that may be due to the T'Bird's 4000+ lb weight... but overall it just felt and worked much better.

If you'd like to ditch the ABS Dean, you can simply pull the ABS fuse from the inside fuse box. In fact, lots of people have installed a switch in there to allow easy on/off of the ABS. 'Course, I'm not sure about all the side-effects... like switching it on/off while driving, or what your insurance company will say if you get into a wreck w/ the ABS disabled.

[AtomicLabMonkey]

Originally Posted by sperry

I miss my Thunderbird's ABS... it cycled much quicker than the Subaru system, and seemed be much less finickey about triggering when you don't need it. Part of that may be due to the T'Bird's 4000+ lb weight... but overall it just felt and worked much better.

It must have been better than what they put in the Mustangs... its ABS is always disconcerting, the pedal gets harder and it "feels" like it's grinding bare metal on metal somewhere.

[sperry]

Originally Posted by AtomicLabMonkey

It must have been better than what they put in the Mustangs... its ABS is always disconcerting, the pedal gets harder and it "feels" like it's grinding bare metal on metal somewhere.

I've been told that the '96+ Mustangs got the good ABS. I'll bet that's what my '95 TBird had, since my '95 also had the 4.6L motor that went into the '96 Mustang, while the Mustang lingered on the 5.0.