Subaru Enthusiasts Car Club of the Sierras - View Single Post

Dean · 2004-02-21, 01:21 AM

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

2004-02-21, 01:21 AM	#131
Dean Señor Cheap Bastarde Real Name: Dean Join Date: May 2003 Location: $99 Tire Store Posts: 9,294 Car: $.04 STI Class: Fast,Cheap & Reliable=STI Deal, did somebody say Deal? Oh, Dean, yeah that's me.	************************************************ ********************************* 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 __________________ I am a Commodore PET --- Now get off my lawn you kids...