I disagree. If that were true adding bigger brakes wouldn't cause the car to stop better (no need for all those upgrade brands).

What locks up when the brakes are stomped is the pad/rotor, thus causing the tires to drag.

The kinetic engergy of the car is turned into heat energy at the caliper. Bigger pads/calipers = better handling of the deceleration.

 

I think it gets much closer to 50/50 when you add the driver. That's 100# more to the rear, and 50# up front, give or take.

Thanks for the brake pressure CFI, those are a big help!

 

But it is true. Bigger brakes DO NOT stop the car quicker. In fact, most of these front brake 'upgrades' actually increase the stopping distance, because they make the front bias even worse. They might show gains simply from the new pads and fresh fluid, but it isn't because they spent $500 on C5 brakes.

Bigger brakes should stop the car as quick, and repeat it more before fading.

Stock brakes in proper working order can lock up the rotor (or activate ABS) at any speed. Car makers sacrifice a little rear bias (increasing the stopping distance) to ensure the rears don't lock up and result in loss of control.

 

AFAIK, the brakes between 84 and 87 were exactly the same.

Interesting that they increased the rear bias at some point (still not enough).

There's about 11 ft more braking potential they left out.

 

Hey CentralCoaster,
Did you receive my MPEG?
Does what they say make sense? I am having a difficult time understanding how less than 10% difference in speed at point "A" can result in over 600% difference at point "B".
Fred

 

Many other factors control how much braking force is available, not the size of the calipers/pads. Big rotors and pads help disipate the heat better, which *may* contribute to better stopping in the long run, but the stopping power itself is controlled by the friction of the pads and force exerted by the pistons in the caliper. The force exterted by the caliper piston is a function of the diameter of the piston and the line pressure.

For example, when tuning race brakes, you choose a pad/rotor/piston size combination so the pads are heated to their proper working temperature. Too hot or cool and they have no friction. Some Teams use IR temperature sensors on the rotors or heat sensitive paint to determine what the operating temperature is. You use this data to choose a rotor size and pad combination to give optimum stopping power. Race brakes (short track oval) are designed to run extremely hot, cherry red rotors in some cases.

The race brakes on the Late model short track car I work on have smaller rotors and pads than my Vette. When cool you would swear you had no brakes, but when hot (edge of cherry red) they work many times better than any street car I've ever been in. Obvisouly, this is no good for the street.

 

How did the thread starter get the numbers he got?

I got closer to the front tires together holding something like 87% of the car during 1 g braking. That is something that i sorta expect.

 

It's a simple dynamics problem, accelerate a body at a certain CG height, then sum the moments about the contact point of the front wheel, and sum the forces. I assumed acceleration only in the vertical horizontal. This gives me the load at each tire for any amount of deceleration.

The amount of force applied to each wheel by the brakes is a factor of the brake system and bias in line pressures. I used this to determine when impending lockup occurs at the front wheel. The rears still have more potential not being used at this point. This is what decreases the maximum braking ability.

If both the fronts and rears locked up at the same time, you would have perfect bias, and could in theory stop with the same G-force as the tire's friction coefficient.

I can use this, based on the rotor diameters, piston sizes, tire diameters, friction of pads, tires, pedal force, to determine what line pressures are desireable.

GM is only running the rear brake at 75% of its potential.

My next step is to predict bias spring rates, and graph the brake balance for intermediate braking. A properly designed system will not only maximize the use of front and rear at full braking, but will also use a good balance of each during normal braking to balance pad wear. I still need to learn more about the combination valve to figure the rest out.

 

Kevin,

Refer to that book I loaned you, Corvette from the Inside. I know that Dave touched on many of these numbers regarding the C4 chassis.

If my memory serves, about 80% of any cars braking potential comes from the front brakes... EXCEPT! Porsche 911s. The rear engine gives the car tremendous braking potential because the weight transfer isn't nearly as evil as it is on a front engined car.

 

Less so for lower CG values, and for *** heavy cars, like you said.

On the 84 vette, the fronts do 76%
On the 87 Vette, the fronts do 73%...based on the pressures listed.

But that doesn't mean it's the best ratio, it should be 65%, if i'm right. It's the whole basis for the doug rippie spring. They don't go that far, obviously you don't want rear lockup on a low tank.

 

Are you talking average G force during breaking or instantaneous? I would imagine the ABS screws with the instantaneous numbers as it is almost impossible to maintain constant break force.

I am not a NASA engineer or even close. I am just a curious geek.