Contour Enthusiasts Group Archives Forums Modifications Brakes Performance Brake Upgrade

No, I know what you were trying to say, and it's a very commonly-held belief, I'm just saying it's not that simple and I'm really wishing I had the engineering background to explain myself better.

But I'll try again. Just because a tire locks due to the braking force being applied to it does not mean the tire was incapable of accepting a higher braking force prior to the point of locking if the brakes were indeed capable of generating that force WITHOUT locking. I know and understand that this is counterintuitive to those who believe that tire traction is what it is and no more braking can be put through it if it's locking anyway, but such is not universally the case.

Matter of fact, think of it this way: It's not the brakes putting forces through the tires, it's the tires putting force through the brakes. Nearly any braking system is capable flashing over to complete lockup and overwhelming the tire but a better braking system allows a higher level of torque to be managed before going to complete lockup.

Put simply, a braking system can force a lockup BEFORE it's taken advantage of all the tire had to offer, and this is where a larger, higher-torque rotor and good pad come into play.

I'm following, (I think) that old rule about tires don't like to be suprised.

So if you stab the brake pedal, you can easily lock the tires, but you modulate your application of the brakes and get more braking force, and avoid lockup.

I think I'm following you.

TB
This really seems like Deja Vu all of the sudden.

Most of what you say addresses braking feel (modulation, initial bite, fade resistance) and has nothing to do with stopping distance.

Your statement about drum brakes is just wrong.

A sliding tire is a sliding tire. The advantage of discs over drums is cooling/fade resistance. All other things held constant, the sliding tire with drum brakes will slide just as far as the sliding tire with 13" discs and 4-piston calipers.

The problem here is that the word "performance" encompasses many factors besides just stopping distance, and yes, it's a lot more complex than just how good your tires are. But, at the upper reaches of the performance stratosphere, there are a lot of things that come down to driver preference.

That's part of it (which also starts to deal with weight transfer)...but the important thing I'm trying to convey is that tire lockup isn't *all* about the tire.

Try this: A brake's torque generation is not necessarily linear, and can go from a value far beneath the point of lockup to a value beyond lockup in an instant, while leaving a lot of potential torque (torque that the tire COULD handle) on the table. A better, higher-torque brake system can take advantage of the tire's capability before "spiking" to lockup.

I'll see if I can get some people more versed in this stuff to better illustrate the point.

Originally posted by RogerB:

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Your statement about drum brakes is just wrong.

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What's wrong about it? Do you think 4-wheel drum brakes couldn't lock, or do you think that tires and vehicle weight being equal a drum brake will stop just as fast as a disc? Because if you think either of those things then *you* are wrong.

In reply to:

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A sliding tire is a sliding tire.

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Yes. The point you're missing is that there's more to lockup than "that's all the tire had to give". Give me the courtesy of reading what I've written.

In reply to:

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The advantage of discs over drums is cooling/fade resistance.

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...which goes directly to stopping distance...

In reply to:

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All other things held constant, the sliding tire with drum brakes will slide just as far as the sliding tire with 13" discs and 4-piston calipers.

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Do you really think I'm that stupid? I'm talking about BEFORE LOCKUP, again, please take a moment and read the above replies.

I just spoke to an engineer I know at Baer who confirmed what I'm trying to say, and pointed me to their website for a little bit of an explanation. Unfortunately it doesn't go into the engineering detail I was looking for and he didn't sound like he had the time to give me his treatise personally , but anyway, it says: Although virtually any brake system can be spiked into locking a rotor or activating the ABS, the true measure of a performance brake system is how close it allows you to come to the point just shy of locking. This is affected by a number of elements. Some of the most important are caliper stability and leverage affect on the rotor, which increases with rotor diameter. In simple terms, the more stable a caliper and the more evenly it can apply the pads to the rotor surface, the faster your car will stop.

These guys aren't guessing at this...

OK, I read your whole post, but my reply was perhaps not clear.

Grant me the same courtesy.

Whether a driver is able to effectively threshold brake is the result of many "performance" factors, such as those noted in the Baer quote. I think their statement is a bit misleading, though, and what they should say is, "...the better a driver will be able to stop."

Now, as to your other point, what you are saying is that, in effect, the way you approach the tire's saturation point actually effects the total traction available at that moment.

I have read many things on the subject of tire traction, and that's the first time I've heard that one. That doesn't mean it's wrong, though, but it is interesting. Wish that you or I were better equipped to explain it. For now I'll have to concede and keep an open mind.

Later

We call it 'threashold braking' and few master it with stock brakes because it not only generates a lot of heat but the modulation of stock parts is marginal at best.

I agree, cars are capable of better braking in this enviroment when supplied with the proper parts to do so. You can 'jam on' the brakes on a race car and lock up the stickiest of tires. That only proves that under extreme pressures the torque generated by the brakes can exceed that provided by the tire.

But when a proper techinque is combined with the proper parts the torque of the tire can be fooled as I call it, and the mass of the car can be absorbed by the brakes while keeping the tire at its maximum adhesion.

And the queistion of pads, it's about Cf. Thus more heat too.

Actually, I'm beginning to understand MFE's point.

Consider that when you stretch silly putty slowly, it will keep going until it's a thin, hairlike string. But, if you stretch it fast enough, it breaks. The behavior of this elastic material varies by rate. I can see where tires, also elastic, would behave similarly.

As I said, "performance" encompasses many factors, but "stopping distance" is fairly specific. Yes, the right tools will allow the driver to use more of the available stopping power more effectively, which could lead to shorter stopping distance for that driver. More importantly, depending on the intended use, it could mean shorter lap times, etc.

But my original point stands, that there are many ways to improve overall braking performance, before we spec larger rotors, which in my opinion have the worst tradeoff cost, which is higher unsprung weight.

You're gettin' it, but quit thinking about the TIRES goddammit LOL...think of the grip between the rotor and the pads not being linear, and that at some point far short of what the tires really have to offer, the grip at the rotor/pad suddenly goes to infinite and THEN the tire locks. Bigger rotor/better pad, more grip before snapping to infinite.