General Brake Design Theory Question

A co worker and I were discussing brakes and upgrades this AM. Long story short he purchased a larger diameter rotor kit for his SUV. This comes with bigger rotors and new caliper mounting brackets. The same calipers and pads are used. This lead us into a discussion of the following.

Is there a point where you can have too much stopping power and break the friction between the pavement and the tire too quickly relative to vehicle weight and speed? Is there an engineering formula to go by to prevent wheel lock up every time you press the pedal?

What is the point of larger rotors with stock calipers? Better heat dissipation? Less fade?

 

Yes -- you actually lose a little on the control/modulation side of things.

 

That is 100% the opposite of my experience on many different cars. just sayin

 

It's pretty hard to "over brake" any car, but the stock ABS system can definitely be affected and not perform as well with certain mods to the hardware.

As for heat, nothing really is gained by increasing rotor size but retaining stock calipers and pads. The rotor temps will drop, but the caliper and pad (and, thus brake fluid) temps will remain the same under most braking scenarios.

Only reason to go for bigger brakes is if the car is "under braked" in the stock set up. And, very few high performance cars leave the factory with inadequate brakes ---- too much design attention is paid to braking systems these days.

One recent, notable exception was the first generation of the Lotus Esprit V8 ---- the original Brembo calipers and pads were not adequate when driving the car hard, so Lotus upgraded the stock equipment to larger AP calipers and pads which were much better and even plenty sufficient for track use.

 

Would not the larger rotors cause an increase in the contact speed between the rotor and the pads for a given vehicle speed?

 

Interesting replies. So does maximum stopping power occur when the friction of the brake pads on the rotor are closest to causing the tire to loose traction and skid on the road? In a laboratory world Is there a way to modulate this to provide maximum stopping power? I.e. A lot of power at the initial pedal press then taper off as you slow? Kind of like a Variable geometric turbo? Or is constant pressure thru out the stopping process the best?

 

This happens anyway, no? I am no engineer, but I do know brakes convert kinetic energy into heat. So, as the ability to dissipate heat drops (pads, rotors etc heat up), doesn't the brake become less efficient anyway. Our experience of this once it goes past it's operational temperature is fade.

Philip

 

Maximum braking occurs when the tire is rolling 7% slower than the road surface traveling underneath it. This is the traction limited deceleration rate. And lower and you are not stopping fast enough, and higher and the tire will begin to decelerate without transmitting as much force to the road surface.

You measure the speed of the road surface (radar/laser) and then measure the speed of the wheels (sensors), and use the two signals to determine the appropriate pressure in the calipers to achieve the traction limited deceleration rate.

This is what we used to do with completely unassisted brakes.

Given the parameters of tire moment of inertia, wheel moment of inertia, maximum brake force on rotor (pad force per pressure and per temperature), and a graph of tire traction per temperature, and a sensor for tire temperature, and rotor temperature, the algorithm to connect the dots is straightforward.

What is hard is when the user changes the tire MoI, wheel MoI, tire traction (r-compound), brake pads; and still expects the ABS to save his a$$.

 

Not an engineer so I ask, "why not?" In my experience we do these changes all the time racing wheel to wheel modified streetcars. We slap on wheels and DOT-R tires and run them. We switch to big brake kits that we hope are well engineered. We threshold brake until the ABS comes on and the ABS seems to work just like on street tires and stock wheels. Doesn't the ABS wheel speed sensors tell the ABS computer what the wheels speeds are regardless of tire compound or tire and wheels used? Then the ABS algorithm does its thing and voila you stop on command.

 

"Luck". ABS is much more sophisticated than it would appear at first, with lots of elements tuned for the specific hardware it's developed for.

Because the ABS system expects the tire to behave in a certain manner, and race tires tend to prefer much higher slip levels than street tires (not to mention pulling higher decel than expected, and having varying recovery characteristics from sloppy to edgy), so that when the ABS expects the tire to "depart" and it still doesn't, it applies even faster, and when the tire does eventually go into significant slip, it often winds up being pretty deep slip. Street ABS systems also sometimes aren't programmed to handle the decel rates of race tires. I know Ford Racing sells special ABS modules for Mustang race cars.
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Changing the ratio of pedal force (actually measured as master cylinder pressure) to braking torque can also throw off many calculations in the ABS software that attempt to estimate surface type and influence pressure control.

Many racers of many types of cars over the last 20 years have problems with ABS on street-derived race cars, most commonly the dreaded "ice mode", also deep slip on R-compound tires or other undesirable pressure clipping. Some systems are more adaptable than others, I suppose.

* These are my own thoughts and do not represent official positions of my employer.

 

Mitch's answer was decent; I'd add that different types of tires have different curves of how much slip generates how much traction.

Trying to control to this in the real world becomes so tricky that (as a general rule) even ABS systems don't try to do so. Instead, they allow pressure to the wheel until they see the wheel "depart" and begin to significantly break traction, then reduce pressure to bring the wheel back up to speed and repeat. Of course, they do this independently on each wheel (unless they choose to control both rears together) and their feedback loop is faster and more sensitive than most of us, and can compensate for combined stopping and steering.

* This represents my own thoughts and does not reflect an official position of my employer.

 

The ABS algorithm depends on a rate of deceleration of the MoI of the wheel/tire/rotor combination when the tire looses traction. This deceleration depends on the forces on the brake rotor and the rotational inertia of the wheel being decelerated. Put racing slicks and lightweight forged magnesium wheels and you are now operating with 60%-65% of the MoI the ABS was programmed for.

So when ABS decides to intervene instead of modulating the wheel a little slower-a little faster-a little slower-a little faster as it goes in and out of operation;
Instead, the wheel/tire spins up to road speed- goes to a complete stop-back to road speed
This "harshness" of operation makes it a lot easier to "loose" the car than if the original wheel/tire combination was left on the car.

Many times the wheel/tire combination cancels itself out when larger wheels (widths) are accompanied by lighter weight tires. Big brake kits add to the MoI of the wheel/tire package allowing the wheels and/or tires to be lighter and stay within a window where ABS works smoothly and not harshly.

 

I don't think any of these answers get to the OP's question. First, if you install an aftermarket brake system for the model it will be an upgrade to the stock system that should work fine depending on the degree of difference to the stock system. These guys are talking about going to the extreme. You are not. The best is to upgrade all 4 corners. Just doing the fronts is probably not going to give you radically faster stopping distances. Your brakes will resist fade better so braking down long grades is upgraded. Doing all 4 will improve overall braking. However, if you are supassing other component's service ratings you get nothing. This includes the power braking system - master and boost. You may exceed the capacity of these components. You cannot just slap on the biggest rotors, calipers and wheels on a moderm ABS and traction controlled vehicle and end up with a vehicle that will bug your eyes out when you slam on the binders.

However, if you option for a prepackaged upgrade from a reputable manufacterer you will generally achieve your goal.

 

This is how I think ABS works and why I think of the ABS as not knowing whether I got sticky tires or not. The ABS defacto seems to read traction. If I got sticky tires on a loose surface it reacts. If I have sticky tires on a sticky surface it reacts. I'm not the smartest guy in the room and all that stuff that Mitch said already left my other ear.

 

Tell me about Ice Mode. What is happening. There are a group of about 10 local guys who share information all racing corvettes with big brakes and one guy with modified OEM brakes. Just about 100% of us have experienced what was claimed to be Ice mode at least once. The result is 4 wheel lock-up and no braking control even if off the brakes once lock-up occurs. This always happens on sticky tires and threshold braking into ABS. After the event the ABS light is on. Sometimes we can turn the car off and turn the car on and the ABS light goes out. Sometimes we take the wheels off disconnect and reconnect the wheel speed sensors and all if good again. It is a very rare occurrence like once in every 3 years of racing.

Can you tell me what is happening here?

 

My guess is ice mode is really slush and gravel mode where the system translates the braking environment to allow full lock up so the tires can grind down into a slippery surface instead of releasing when it detects wheel slip. Anyone experiencing a car on a sloped snow covered surface that starts sliding can appreciate this. Flooring the pedal gets you a vibrating pedal and no wheel lock. Same for a gravel coated surface. Always been an issue for ABS. Sometimes you want to turn the brain off. Later cars may allow this. I know my mid-2000 BMWs don't. I would think the ABS computer could be programmed to determine when this is the case and allow for lock up in certain situations. Not sure I trust that little box though.

 

Interestingly, we have found ways around it and ways to trigger it. Sometimes it still happens on good pavement. The number 1 cause seems to be stabbing the brake pedal while loosing concentration as to basics and concentrating instead on finding the last 1/10th of a second. Impatience! The interesting thing is ABS is supposed to save your bacon of Joe average on the street. He will us it when a kid runs in front of his car and just slam the brakes. Well that is the #1 way to enter ice mode especially on mildly uneven pavement where traction can be initially lost to one wheel like on bumpy L.A. roads. A better ABS default mode would be to lock all for wheels vs. kill pressure to the brakes still allowing steering from a safety standpoint. I would rather skid and slow than not slow at all. It seems that on corvettes we get both these ice mode versions but we don't know the reason for which one activates. For example, I was racing at bumpywillow and flat spotted 4 new race tires to the metal cords while flying off the track in an ABS failure we call ice mode. Another friend racing at Long beach hit the T1 wall when his vette lost all braking with his foot mashed but he was able to steer and put it sideways into the wall. That is a more typical ice mode most talk about.

 

Out of curiosity..

Would increasing the brake swept area on a stock rotor, be equivelent to a larger rotor with an equal size caliper..
If that was the case...why not have 2 smaller calipers..mounted fore and aft of the axle line with a swept area still greater than the stock unit..weight savings not withstanding...

I seem to recall seeing , of all cars an older Bently with calipers fore and aft on a single disc.

 

Area of the pad contact isn't much of a factor -- there is no "A" in "F= uN" (that more affects the thickness wear rate of the pad material as more area gives more volume for a given thickness), and you don't get the benefit of a slightly larger rotor being a slightly better "radiator" to conduct heat away from the rotor-to-pad contact. Also, you hit the other downsides -- extra complexity and (probably) weight.

 

It is amazing that no one has built a better mouse trap since the intro of the Disc brake.
Why are caliper and rotors not cooled by satelite radiators.
Why can't the discs consist of 3 plates the outside 2 stationary and the inside rotational and use the 2 stationary plates as calipers..
Talk about maximum swept area

Better yet..Reverse thrust on all four wheels!!!!!

 

When cost is included in "better", it's a lot harder to be "better"

 

I'm 100% with you here. The modern ABS computer is looking at each wheel. If it detects that a wheel is locked up, it cuts the pressure being applied so as to unlock said wheel. Doesn't matter what surface we're on, what compound tires we're running, the state of the pads, or the phase of the moon.

With all due respect, I'm going to need a much deeper explanation than that before buying this..... Why 7%? Why not 5%? Or 10%?

I may be wrong here, and am keeping an open mind to be corrected, but I fail to see what any MoI, tire compound or pad chemistry has to do with how my ABS behaves....

It detects a lock up, it cuts the pressure. Once unlocked, pressure is again increased. Works beautifully. In general. (We'll leave ice mode intervention and other confusion it may suffer out of the debate for now. )

Cheers,
Ian

 

Airplanes have been doing that for years
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You are correct. One of the white papers Stoptech used to have on it's website said all things equal increasing rotor diameter didn't increase braking torque very much. It's mostly done for looks.

 

Stopping distance is determined by the tire and road surface. Auto brakes are used more than airplane brakes and the rotors need exposed area to cool off.