Let's talk about brakes!

[Dean]

Originally Posted by BAN SUVS

Too bad the front/rear Wilwood systems for the WRX suck. I do know I will not be upgrading my brakes further until I find something that will increase braking power, not just fade resistance.

What source do you have for this assertion? I have not thuroughly researched the Wilwoods, but they appear to be the same Superlite 6s I have on my Stealth for the fronts, and I'm not sure which 4 pots for the rear? The 6 pots are quite impressive IMHO. Is this just a dust sheild issue, or is there imperical data out there on the Wilwoods?

[Dean]

Originally Posted by BAN SUVS

I have yet to lock up my brakes at the track... and I'm still considering running r-compounds.

This is one of the common misconceptions. Getting into ABS is not necessarily fast, and in fact is often fairly slow. Being fast is about balancing forces in a smooth fashion. But that is a whole different discussion. Getting into ABS also does not mean your brakes are operating at their maximum performance, it actually means they have failed.


Warning: Dean's Soapbox mode enabled. Preaching begins in the next paragraph. You have been warned.

And actually, brake selection is lagely about modulation, or the systems ability to create high levels of friction without locking up. I know that sounds contridictory, but it isn't. This is where heat capacity, cooling and pad compound come in. I'll try and explain with a exagerated example.

When a rotor is extrmely hot, it actually deflects under the pressure of the pads. This is bad! So at the leading edge of the pad, a slightly thicker rotor is continiously being compressed. As I understand, this area of interferance is where lockup actually begins, and where if you do not release the lockup the pads can actually fuse/weld themselves to the rotor. If instead, the system as a whole, but mostly the rotor stays cooler and therefore deflects less, minimizing this interferance area, you will see an increase in braking performance. This area is also why in most any fixed caliper multi piston per side configuration, the leading piston will be smaller, allowing the leading edge pressure to be lower than the mid to trailing edge.

Thsi is why I keep emphesising that the diamter of your rotor is not very important, but the width and cooling fins are! Even thermal mass doesn't do you any good if you can't cool it.

Look at the many NASCAR series... As far as I know, they are all running under a 12" rotor.(11.75"x1.25" I believe is most common). Do any of us really think we in our weekend warrior roles can hold a candle to the demands they put on their brakes? Other series run even smaller diameter brakes. If bigger was better, don't you think some series somewhere would run them?

Braking is not about stopping the rotor, it is about stopping the car which whether you believe it or not is done by the tires! Dispating energy without locking up is how you do that. getting into ABS means your brake system has failed, not that it is running at maximum performance. I'm not saying ABS is bad, only that is not a valid measuring point.


Soapbox mode disabled.

I am far from an expert on this, and would love to hear other technical opinions on this becasue there is certianly more I can learn on the subject. I just feel obligated to speak up when I see/hear these types of automotive myths being discussed.

[sperry]

Good soapbox rant Dean!

My point regarding the whole ABS thing wasn't to say that hitting the ABS is the end-all limit of braking. My point was that at some point, considering the tires were using, "better" brakes aren't going to actually help us to stop faster.

Once your brake setup feels good, doesn't fade, and allows you to brake at the limits of your tires, there isn't any reason to upgrade anymore!

[Dean]

Originally Posted by sperry

Good soapbox rant Dean!

My point regarding the whole ABS thing wasn't to say that hitting the ABS is the end-all limit of braking. My point was that at some point, considering the tires were using, "better" brakes aren't going to actually help us to stop faster.

Once your brake setup feels good, doesn't fade, and allows you to brake at the limits of your tires, there isn't any reason to upgrade anymore!

OK, I have my instructor hat on now. How do you know you are at the limits of your tires?

In most cases, braking isn't about stopping. It is about slowing. How the brake slows the car under differnt circumstances including but not limited to speed, pedal pressure, temperature, pad depth, etc... Can you describe the ideal brake system? You only refer to feel, and fade as parameters. Can you be a little more specfic?

[sperry]

Originally Posted by Dean

OK, I have my instructor hat on now. How do you know you are at the limits of your tires?

In most cases, braking isn't about stopping. It is about slowing. How the brake slows the car under differnt circumstances including but not limited to speed, pedal pressure, temperature, pad depth, etc... Can you describe the ideal brake system? You only refer to feel, and fade as parameters. Can you be a little more specfic?

I'm not at the limits of my tires, which is why I don't need a huge brake kit yet. I just want a stiffer pedal, primarily to allow me to brake harder and still have proper pedal alignment for heel-toe.

Now, if I were able to lock all 4 tires at the same time (or the fronts just slightly before the rears) during straight line braking from high speed, then I'd say that I'm "at the braking limits of the tires". At that point, my bias is just right, so I am using all my available traction to slow the car. And if the pedal feel is such that I'm comfortable and modulation is predictable allowing me to brake just above the locking threshold, and if the brakes don't fade, then I'd say that my brakes couldn't be improved.

[AtomicLabMonkey]

Originally Posted by Dean

Look at the many NASCAR series... As far as I know, they are all running under a 12" rotor.(11.75"x1.25" I believe is most common). Do any of us really think we in our weekend warrior roles can hold a candle to the demands they put on their brakes? Other series run even smaller diameter brakes. If bigger was better, don't you think some series somewhere would run them?

Dean,

Cup teams use small diameter rotors because NASCAR mandates a maximum wheel diameter of 15"; their packaging constraints within such a small wheel are what limit them to relatively small braking systems compared to what teams in most road racing series use.

[Dean]

Originally Posted by AtomicLabMonkey

Cup teams use small diameter rotors because NASCAR mandates a maximum wheel diameter of 15"; their packaging constraints within such a small wheel are what limit them to relatively small braking systems compared to what teams in most road racing series use.

Sorry, you are right, I was thinking of NASCAR only at the time. I do think it is all the NASCAR series, not just the cup, but that isn't really important and I may well be wrong on that count as well.

Most production car based series are limited to a 17" wheel which keeps to 14" rotors, though there is often a weight penalty for using larger rotors. Many open wheel series have 13-15" wheels, so they are on 12s, and smaller. WRC uses about 12" on everything but tarmac where they use whoping 14.4"ers.

I guess my point was we don't push them anywhere as hard as these guys do, and they run for many more miles etc.

[Dean]

Originally Posted by sperry

I'm not at the limits of my tires, which is why I don't need a huge brake kit yet. I just want a stiffer pedal, primarily to allow me to brake harder and still have proper pedal alignment for heel-toe.

Now, if I were able to lock all 4 tires at the same time (or the fronts just slightly before the rears) during straight line braking from high speed, then I'd say that I'm "at the braking limits of the tires". At that point, my bias is just right, so I am using all my available traction to slow the car. And if the pedal feel is such that I'm comfortable and modulation is predictable allowing me to brake just above the locking threshold, and if the brakes don't fade, then I'd say that my brakes couldn't be improved.

Ha, you have fallen into my trap... It was kind of a trick question to get you to think about it.

If all 4 wheels lock, you are at the limits of the brakes, not the tires necesarily. This is where people get confused. If instead of locking, the brakes continued to apply even greater decelerating force without locking the pad to rotor contact, you would not actualy reach the limits of the tires until the tires started to substantially slip while still rotating. Unfortunately, this is often closely followed by brake lockup because the reduced torque from the tire/road loss of friction shifts the balance and the pad/rotor interface locks.

I'm not saying any particular brake system is not capabe of reaching the limits of a given tire prior to locking, only to change focus from the brakes to the tires and get you to think about how limitations in the brake system could be percieved as using all the tire's traction when it may not be. The equations for all of this are way out of my league.

Perhaps an example would help. Pretend you had a set of infinitely strong cross drilled rotors and the calipers have infinitely hard pins instead of pads connected to the pots. These binary brakes would be on or off, applying either zero or infinite friction instantly to the "pad"/rotor interface causing it to clock, this locking of the brake system is what I mean by a failure which prevents the tire from rolling causing the loss of traction at the tire/road interface.

This is why pad materials are so critical and why spreading force over an area instead of a point allows a brake system to apply braking force while still allowing rotation.

[Kevin M]

Originally Posted by Dean

What source do you have for this assertion? I have not thuroughly researched the Wilwoods, but they appear to be the same Superlite 6s I have on my Stealth for the fronts, and I'm not sure which 4 pots for the rear? The 6 pots are quite impressive IMHO. Is this just a dust sheild issue, or is there imperical data out there on the Wilwoods?

The Wilwoods flex like Hanz and Franz under heavy pressure. Causes poor pedal feel. I had only heard this as a rumor until recently, but then a customer came into S-Squared complaining of poor pedal feel. 3 liters of bleeding later, it still felt like crap. Finally, during the last bleed session, Nate noticed the calipers flexing about 15 degrees before full pedal pressure had been reached. These were the 4 pot calipers, btw, which should be actually stiffer than the 6 pots.

[Dean]

Originally Posted by BAN SUVS

The Wilwoods flex like Hanz and Franz under heavy pressure. Causes poor pedal feel. I had only heard this as a rumor until recently, but then a customer came into S-Squared complaining of poor pedal feel. 3 liters of bleeding later, it still felt like crap. Finally, during the last bleed session, Nate noticed the calipers flexing about 15 degrees before full pedal pressure had been reached. These were the 4 pot calipers, btw, which should be actually stiffer than the 6 pots.

15 degrees on what axis?
Which 4 pots? Wilwood makes two or three different 4 pot calipers.

Why do you feel the 6 pots would be less stiff? The 6s are a much larger caliper in every dimension with a different configuration than the 4s.

Also, I am unaware of a Wilwood 4 pot application for the WRX from Wilwood. The 6 pot Billet Superlite front kit from Wilwood is here: http://www.wilwood.com/products/kits/sl6bbhk/index.asp
The rear is a 4 pot kit based on the Billet Dynalite 4 pot caliper. It is only in their PDF file. http://www.wilwood.org/2003kitcatalog.pdf

I think someone else must have built a 4 pot kit using Wilwood parts that you saw or heard about, or they discontinued because I do not see a pert number for it in their stuff.Sounds like somebody threw together something with the wrong parts for the application.

[AtomicLabMonkey]

Originally Posted by Dean

If all 4 wheels lock, you are at the limits of the brakes, not the tires necesarily. This is where people get confused. If instead of locking, the brakes continued to apply even greater decelerating force without locking the pad to rotor contact, you would not actualy reach the limits of the tires until the tires started to substantially slip while still rotating. Unfortunately, this is often closely followed by brake lockup because the reduced torque from the tire/road loss of friction shifts the balance and the pad/rotor interface locks.

I'm not saying any particular brake system is not capabe of reaching the limits of a given tire prior to locking, only to change focus from the brakes to the tires and get you to think about how limitations in the brake system could be percieved as using all the tire's traction when it may not be. The equations for all of this are way out of my league.

Dean,

I think I have to disagree with some of what you said. If a tire locks up, by definition the caliper has applied more braking torque, and hence more force opposite the direction of tire rotation, than the tire was capable of developing for its given (instantaneous) normal force and coefficient of friction with the road surface. Take a given tire with a given downforce, section of road, and coefficient of friction of 1.0 which locks up at a braking torque of 1000lb*ft. If you strap a different tire on the car with a coefficient of friction of 2.0 on that section of road, and apply the exact same braking torque of 1000lb*ft, the wheel will now not lock up; it will now take 2000lb*ft to lock up the wheel. I think this pretty much defines being deceleration-limited by the tires, not the brakes.

I think maybe what you were trying to say was more concerning how effective a braking system (and operator) is at modulating force near the limit of what the tire can produce?

[Kevin M]

Originally Posted by Dean

15 degrees on what axis?
Which 4 pots? Wilwood makes two or three different 4 pot calipers.

Why do you feel the 6 pots would be less stiff? The 6s are a much larger caliper in every dimension with a different configuration than the 4s.

Also, I am unaware of a Wilwood 4 pot application for the WRX from Wilwood. The 6 pot Billet Superlite front kit from Wilwood is here: http://www.wilwood.com/products/kits/sl6bbhk/index.asp
The rear is a 4 pot kit based on the Billet Dynalite 4 pot caliper. It is only in their PDF file. http://www.wilwood.org/2003kitcatalog.pdf

I think someone else must have built a 4 pot kit using Wilwood parts that you saw or heard about, or they discontinued because I do not see a pert number for it in their stuff.Sounds like somebody threw together something with the wrong parts for the application.

They opened up like a clamshell. 15 degrees is just me typing before thinking- who knows if it was more or less than that, but it is quite noticeable. I said 4 pots are stiffer than 6 pots, typically, based on StopTech's White Paper on caliper build and design. And it may have been the 6 pot kit- my brain is mush lately. I believe it was the Willwood-built Perrin kit, but I don't remember for sure. Aaron saw it as well, maybe he can clarify my details.

[Dean]

Originally Posted by AtomicLabMonkey

I think I have to disagree with some of what you said. If a tire locks up, by definition the caliper has applied more braking torque, and hence more force opposite the direction of tire rotation, than the tire was capable of developing for its given (instantaneous) normal force and coefficient of friction with the road surface. Take a given tire with a given downforce, section of road, and coefficient of friction of 1.0 which locks up at a braking torque of 1000lb*ft. If you strap a different tire on the car with a coefficient of friction of 2.0 on that section of road, and apply the exact same braking torque of 1000lb*ft, the wheel will now not lock up; it will now take 2000lb*ft to lock up the wheel. I think this pretty much defines being deceleration-limited by the tires, not the brakes.

I think maybe what you were trying to say was more concerning how effective a braking system (and operator) is at modulating force near the limit of what the tire can produce?

Yes and no. I did actualy use the word modulation in the first sentance of my soapbox, but...

Take my Pin and Hole example. The brake interface transitions from dynamic to static friction before the tire transitions from rolling/static friction to dynamic of skidding. the tire itself provides the delay between the two events. In all but a "perfect" system one of the two events must happen first, and that is what I am getting at.

An incrediable amount of money goes into just designing pads with different friction charecteristics. Some transition from dynamic to static friction more easily than others at a given load and temperature.

In many ways, ABS is a primitave way to make up for other shortcomings in a brake system after the fact. With a soft enough sidewall, and a fast enough ABS system the tire may never even transition to dynamic friction even though the brakes are swithcing back and forth from static to dynamic as they lock and unlock. The tire never locks even though the brakes do repeatedly.

Do you see what I mean by the brakes failing/transitioning before the tire?

[Dean]

Oh, and I keep forgetting to mention something most people don't realize.

Bigger brakes with more piston area corresponds to a SOFTER, not firmer pedal.

And there is actually a fairly good spiel on scoobymods, though they don't talk about modulation as much as I'd like.

http://www.scoobymods.com/forums/showthread/t-1122.html

[AtomicLabMonkey]

Ok, I see what you were getting at now.

[sperry]

Alright Dean...

I'm a little confused by this "trap" I fell into. I understand what you're saying about the brake's transition from dynamic to static friction being able to lock tires that would otherwise still have rolling friction to spare. But if you read my post again:

Originally Posted by me

Now, if I were able to lock all 4 tires at the same time (or the fronts just slightly before the rears) during straight line braking from high speed, then I'd say that I'm "at the braking limits of the tires". At that point, my bias is just right, so I am using all my available traction to slow the car. And if the pedal feel is such that I'm comfortable and modulation is predictable allowing me to brake just above the locking threshold, and if the brakes don't fade, then I'd say that my brakes couldn't be improved.

I mentioned being able to modulate the brakes to stay above locking up the tires. Perhaps I wasn't explict enough, but it was my intention to describe the situation you later fleshed out: brakes that decelerate the car at the maximum dynamic friction of the tires. At that point, your brakes cannot be better... they are not the bottleneck in your braking performance.

Let's look again at your "binary brake" system. That extreme example actually demonstrates that *any* brake that can lock the rotor can be used to brake "at the limits of the tire" provided that they can be modulated fast and accurately enough. As you stated, you can theoretically lock the rotor and unlock it w/o the tire itself ever locking up as long as you unlock the rotor before the tire's sidewalls run out of flex. Using this knowledge I could actually design a computer controlled brake that would lock and unlock thousands of times a second to bring the tire to its limit, and threshold brake without ever locking up a tire. (Although you'd heat the bejesus out of the tires... in fact the brake rotor wouldn't heat up at all... those tires would have to generate all the heat that results from stopping the car!! )


The same goes with regular brakes. Lets say I've got a crappy pad that has a very quick and harsh transition from dynamic to static friction. i.e. the pad would cause the tires to lock at a deceleration rate less than the tires "maximum" rolling friction. You would argue that a "better" pad with a less harsh transition would prevent this from happening, and thus would improve the overall brake system. While I won't disagree, I want to point out that a better driver, or a super-fucking-good ABS is also a solution. If I were to be able to modulate those crappy pads, they'd also allow tire-threshold braking, just as in our example binary brakes.

What's my point? Well, I'm not sure. Given the choice of harsh transition or smooth transition pads, I'd clearly go with the smooth pads since they make driving at the limit easier, however I'm not exactly sure how to find out which pads those are... it's not like that's labeled on the box. Frankly, I just want brakes that stop well, resist fade, and don't feel like I'm stepping on a wet sponge.

[Kevin M]

Maybe we're beginning to drastically overcomplicate things. Let's get back to basics for this thread.

Scott wants advice and opinions on what to do with his brakes. Since Scott is a real person, with a real job and real bills and a real life (such as it is ) he has only so many options. These would seem to include:

subaru 4 pots
Stoptechs
STi Brembos
Wilwoods
Rotora


Of those, I personally would choose either StopTech or the 4 pots, basing the last choice on cost, ultimate functionality, and secondary effects such as wheel concerns. 4 pots require replacing one set of wheels with a set that costs roughly $250. The Stoptechs require a 3rd set of 17s for winter, let's call that $450 for rotas. Stoptechs also cost about twice as much as 4 pots. Both have excellent fade resistance. Both retain good "stopping power." Personally, I think that the performance ceiling of a StopTech kit may be a little higher, but value-wise, 4 pots are king.

[Dean]

Originally Posted by sperry

The same goes with regular brakes. Lets say I've got a crappy pad that has a very quick and harsh transition from dynamic to static friction. i.e. the pad would cause the tires to lock at a deceleration rate less than the tires "maximum" rolling friction. You would argue that a "better" pad with a less harsh transition would prevent this from happening, and thus would improve the overall brake system. While I won't disagree, I want to point out that a better driver, or a super-fucking-good ABS is also a solution. If I were to be able to modulate those crappy pads, they'd also allow tire-threshold braking, just as in our example binary brakes.

OK, now we are talking... I agree with you completely! And rereading, maybe you didn't fall into the trap exactly, but you did go where I wanted you to, the tire to road interface.

Everybody gets so caught up in the brakes they forget about the tire characteristics, and that the tire actually stops the car, not the brakes. As we go to stiffer and stiffer sidewalls to achieve better handling, we take some of the tolerance out of our brake systems. Anything we can do to minimize the spikes in the brake system aid the tire in staying hooked to the pavement at the limit. Have you ever heard tires under ABS? In many cases, they chirp about 8 times per revolution as the brake locks, and releases causing the tire to slip.

More pad area(calipers), better pad compounds(Pads), cooler temperatures(better rotors and/or ducting) and to an extent, more piston area(calipers) all contribute to better brake performance at the limit. OK, large diameter rotors does to, but I hate to admit it.

Pads are often the cheapest of the above followed by rotors and finaly calipers. This is why I sugested what I did originally, what you had already done, pads, and rotors. If you have exhausted those options for the most part and are still not happy, the only place to go is calipers. I still think if you can get a used set of JDM/CDM 4 pots cheap, that would be a great option. If they don't meet your needs, they would be easy to unload to someone else with little loss.

[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...

Dean,

The Stoptech kit is only a front upgrade system. The pistons are custom sized to the rotor AND the WRX platform to move brake bias 10% more towards the rear over stock. Stoptech hardly throws bigger parts at anything (unlike many of it's competitors). They design, engineer and extensively test every application before it becomes available for sale. Stoptech uses the same caliper (ST-40) for most of it's applications. However, they machine unique piston sizes for each of the applications. Stoptech has 14 different piston sizes they can utilize to generate optimum brake bias, pedal travel and pad bite characteristics. Not taking piston size into consideration in your analysis ignores one of the more critical factors in a braking system.

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.

Dean,

The Wilwood calipers are known to be some of the most flexible calipers in the aftermarket. See this chart here: http://www.stoptech.com/technical/ca...ctionchart.htm
Company W is Wilwood. The higher the caliper flex, the worse the pedal feel, because the effort you are putting into the pedal is being wasted by flexing the caliper. Very similar to flexible rubber brake lines. Since this is widely known, I feel confident that the flexing described in this thread above is a normal occurance in a Wilwood caliper, not a hobbled together caliper made of Wilwood components.

You also mentioned that getting into ABS is not necessarily fast, and is often fairly slow. This is entirely dependant on the ABS braking system and you will find that in the higher classes of sports cars and performance cars, the ABS systems are excellent and hard to outperform. They react extremely fast and keep the tire rotating at a speed that provides optimal adhesion.

Your soapbox conclusions are mostly inaccurate. The trailing piston is larger to counteract the debris field and slight outgassing that starts developing at the leading edge of the pad and continues along the length of the pad. More force is applied to the rear of the pad to counteract this and provide even pad wear.

The melting point of iron is 2,800F. Brake rotors will glow at 1,200F. Brake rotor iron does not become maleable until it reaches the 1,700F - 1,800F degree range, however, it will undergo a loss of tensile strength at 1,600F, but will not deform. There is a phenominon known as judder associated with straight vaned rotors, where the leading edge of the pad will set up a pressure wave between vanes and will cause slight pedal kickback, but this is cured by using a curved vane rotor, which also improves cooling efficiency.

The majority of OEMs and many aftermarket BBK vendors will use rotor iron with a tensile strength of 18,000psi. This is also known as dampened iron and is used to reduce rotor noise and decrease NVH issues. This is a softer iron. Stoptech, as well as the top racing brake rotor manufacturers use iron with a tensile strength of 40,000psi, which dramatically increases it's strength all the way up to the melting point, in which case both rotors turn into puddles.

Saying that piston size differences in the caliper is designed to eliminate distortion of the rotor is completely inaccurate.

You used NASCAR as proof that larger rotors are not required. This is an unfortunate example, since on many race tracks that NASCAR runs on, they never even use the brakes. The answer to your question is yes, many weekend warriors will dramatically exceed the demands put on the brakes over NASCAR drivers. Other tracks in NASCAR, like Bristol, put a huge toll on the brakes. The cars use a much wider rotor on those tracks, but do not use a bigger diameter rotor because of NASCAR mandated wheel diameter and rotor diameter. Using NASCAR as a technological example is horrible because their rules have locked them into very old technology. The formula for NASCAR is not to produce the fastest, most technological racing in the world. It is designed to produce the closest, most cost effective racing possible.

That is why true road race cars, like the Daytona Prototypes, ALMS Prototypes, Indycars, Grand Am GT and GS and such use huge diameter rotors. These cars are much more technologically advanced than anything you would find in the NASCAR series and they all use big rotors. I'll get into why larger rotors are better at the end of this post.

On to the next point. 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. You mentioned the difference between locking the brakes before reaching the maximum traction of the tires. This is wrong. Your example of an infinitely hard pin example, first of all, doesn't work they way you explain, and secondly is completely irrelavent to how brakes work.

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. 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. You cannot weld a pad to a rotor.

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.

The characteristics of how quickly the coefficient of friction between the pad and the rotor increases is dependant on pad material. Some pads can come on very quickly, others have a more gentle ramp. NONE of them will lock a wheel prior overcoming the coefficient of friction between the tire and the road.

Furthermore, brake pads are not designed to transition from dynamic to static friction. They are designed to provide a stable coefficient of friction in a given temperature range and pressure range. Quite a bit of effort has gone into the "release" characteristics of a pad, but that is as you are coming off of the brakes, not adding pressure to the locking point. Given a stable braking system regardless of bias, locking of the brakes is a driver error.

You mentioned that bigger brakes with bigger pistons create a softer pedal. This is not true. Larger pistons will create a longer pedal, because you have to displace more fluid before the pads touch the rotor. Once the pads do touch the rotor, the pedal movement is based solely on the flexibility of the system. A very stiff system will not have a softer pedal with larger pistions. Finally, with a larger rotor and larger pistons, it will require less pedal effort to generate the previous brake torque. This is not a softer pedal, it is lighter pedal effort.

In your closing comments, you said that more pad area, better pad compounds, more piston area and hating to admit it, larger rotors contribute to better brake performance. As written, this is inaccurate. What you are saying is that if you increase the size of everything, your braking performance will improve. Absolutely not! What you've done is drastically increase the brake torque, which may not be a good thing for brake bias or pedal feel. You have to take everything into consideration to design an appropriate brake system for an application.

If you increase front rotor diameter, you will need to reduce piston size to prevent too much front brake bias. If you just increase the rotor size front and rear, the original size pistons may be too big, creating a brake pedal that is far too light and difficult to modulate. All of the components that make up a brake system must be carefully analyzed when making changes, and an increase in one parameter may require a decrease in another parameter.

Finally, I want to explain why a larger diameter rotor is better, even though you don't like the idea. A larger diameter rotor requires less clamping force to generate the same brake torque as a smaller rotor, since the torque is applied further away from the center of the wheel. Less clamping force means less heat is generated per square inch over the face of the rotor. Less heat is a good thing. Also, the air speed in the vanes of a larger diameter rotor is faster than a smaller rotor, since the larger rotor is passing through more air per revolution, allowing it to draw heat away from the rotor more quickly. Smaller rotors are built wider to help flow more air, but they are also built with thicker steel, because brake temps will be higher (due to higher clamping forces required to slow the tire) and the additional mass is used to strengthen the rotors to run at the higher temperatures.

One of the major benefits to a larger rotor is the ability to modulate the brakes closer to the edge of the tire's adhesion. This is because there are more rotor inches passing through the caliper. This gives you higher "resolution" with the larger rotor. It makes the brakes easier to modulate, keeping the tire consistently closer to the limit of adhesion.

To your credit, you did mention that the tire stops the car and the tire's characteristics are very important. Somehow along the way, the information provided to you led you astray and you contradicted yourself several times as well as used analogies that do not relate to braking systems.

Please don't be offended by any of my comments. You said yourself you are not a brake expert and can stand to learn a few things. I am not a brake expert either, but 11 years of roadracing experience and working closely with brake manufacturers have provided me with a solid working understanding of brake systems.

Let me know if you have any questions.

Gary
Sheehan Motor Racing
www.teamSMR.com

[ArthurS]

WHOA. Thanks for such a informative post Gary.

[sperry]

Wow! Thanks for the info Gary.

[GarySheehan]

Also, here is some interesting information (and misinformation) regarding monobloc brake calipers...

http://www.stoptech.com/whitepapers/monobloc3.htm

Gary
Sheehan Motor Racing
www.teamSMR.com

[Dean]

Originally Posted by GarySheehan

Please don't be offended by any of my comments. You said yourself you are not a brake expert and can stand to learn a few things. I am not a brake expert either, but 11 years of roadracing experience and working closely with brake manufacturers have provided me with a solid working understanding of brake systems.

Let me know if you have any questions.

Gary
Sheehan Motor Racing
www.teamSMR.com

Not offended, a ton of questions and comments and clarrifications that I just finished typing but aparently my session timed out, so I will have to retype it over the weekend since it is gone forever!!! Stupid computers!

[sperry]

Originally Posted by Dean

Not offended, a ton of questions and comments and clarrifications that I just finished typing but aparently my session timed out, so I will have to retype it over the weekend since it is gone forever!!! Stupid computers!

<OT>

This board sucks for very long posts. If you're working on a long post, make sure to edit in a text editor, then hit reply and paste in your response. I've been burned before too.

</OT, please resume the brake discussion!>

[GarySheehan]

Oh no! Dean, sorry to hear about that. I know how frustrating that is.

As a matter of fact, I was worried about my computer crashing, so I copied to Notepad.exe and saved just in case. I'm glad I did. I wasn't aware that a session could time out on SECCS. I don't think I would have typed all that in again!

I'm looking forward to your questions this weekend.

Gary
Sheehan Motor Racing
www.teamSMR.com