I want to stop - faster

There has been a lot of discussion about big brakes and their benefit (or lack of benefit). Clearly big brakes deliver superior fade resistance through greater heat dissipation capacity.

But fade resistance aside, what can make you stop in shorter distance with brakes that are not fading?

Here are my rambling thoughts, I am curious what others think about this -

Maximum braking action occurs at the threshold of wheel lock up. For the sake of discussion, I define the threshold as the amount of braking force that is generated immediately before wheel lock up.

Any P car brake can lock up the wheels hence is capable of hitting the threshold and producing the maximum braking action that given vehicle is capable of producing. This argues for "big brakes don't matter" at least in theory.

But it is one thing to pass the brakes through the threshold momentarily and another to keep it there as the car comes to a stop. So I would conclude that brakes that make it easier to stay at the threshold can stop the car faster. So if these big brakes could somehow make it easier to stay at the threshold, they could in fact stop the car faster in the real world.

I should think that brakes with longer pedal travel and more "feel" should be easier to hold at the threshold. This argues for non-boosted brakes and smaller master cylinders (not very popular options).

This also says the driver skill at maintaining the threshold should be a significant factor in braking distance.

Second thought is that tire grip, and tire grip alone, will determine where the threshold is. So grippier tires will allow you to stop faster by increasing the threshold.

Third thought is lighter cars can stop faster than heavier cars, all else being equal. This is becuase tire grip will increase with vehicle weight, but at a decreasing rate (it is not a linear relationship). If you double the vehicle weight, grip and threshold will increase but less than double.

Final thought is that maintaining proper brake bias is also key. The bias is correct when both front and rear are at their respective thresholds at the same time. Any other bias is less than optimized.

So my conclusions are that to stop faster you want 1) brakes and skilled driver that easily stay at or near the threshold, 2) grippy tires, 3) light weight, 4) optimized bias

Short of throwing out a parachute, I don't think anything else matters.

So do big brakes help? I would say only if they can make it easier for the driver to maintain the threshold, and I can't think of why they would.

What have I missed?

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'86 Cab, '77 Targa, Family Truckster

Clark, have you tried attaching more family pets to the rear bumper to see if it improves braking???

There is an excellent article in Grass Roots Motorsports regarding this issue. Though I am not a brake expert, I beleive the main point was to improve tires, thereby improving braking.

I'm no expert or engineer, but I think the argument for the heat-dissipating qualities of bigger brakes kicks in when you have increased the roles of grippy tires, light weight and optimized bias to the point where the braking system is doing a lot more work, and heating up proportionately. There's an argument to be made (I don't agree with it, fully) that if you're not experiencing brake fade on a track, then you're not fully utilizing your stock brakes.

I should think that brakes with longer pedal travel and more "feel" should be easier to hold at the threshold. This argues for non-boosted brakes and smaller master cylinders (not very popular options).

This is actually exactly the setup I'm running right now (Turbo brakes, 19mm master cylinder), and my unscientific analysis is that it does feel like I have greater control over the braking (less 'stabbing' stops, unless I hit it hard). I still plan on going to the 23mm, eventually, but I'm going to try it this way for a little while longer.


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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net

Interesting.
I think that Vic Elford said it best. You mash real hard at first to get weight transfer and bite and when this has been done you ease off just enough to prevent lockup.
Caroll Smith backs it up when he talks about the incredible feel that people like Fangio have for tire deformation and ultimate grip.
It is the ability to maintain as close to the threshold for as long as possible or needed to attain max stopping or cornering. Short stroke master cylinders are certainloy not the rule on performance cars, and it is possible that over mechanical advantage may actually reduce the feel. That feel is where it is really at. Just my read on it.

Diverdan

Clarke,
You have hit most of the pertinent points. As with Jack I have big calipers / rotors / wheels / tires / light weight. I did use the 19mm mc for a year before switching to the 23mm.
First, it is the size of the caliper pistons that dictatates the size of the master cylinder bore which is required. Not all big calipers have the same piston sizes, there is actually quite a wide range available.
Second, bias is just the ratio between front and rear piston sizes, by properly choosing calipers with the correct piston sizes, bias will be correct out of the box. If caliper piston mismatches exist then bias valves or in extreme cases twin mc with a balance bar are used.
The 19mm mc will work fine for up to fairly large piston sizes for NORMAL stops. I found on my car that under extreme stopping conditions that the mc volume was insufficient to supply fluid to all(8) of the pistons at once. What would happen is that the fluid would take the path of least resistance and than suffer from a hydraulic lock(fluid could not redistributete(modulate) itself quick enough to compensate for changing needs at each end of the car). The net result was that under these extreme conditions the rear woud have a tendancy to lock. The 23m mc has totally cured this tendancy. You do want the smallest mc that works properly (the only choices are 17mm, 19mm,23mm, 25mm)
Third, the larger capacity of the bigger brakes/tires/wheels is easier to modulate(hold at or under the threshold for lockup) You are walking a wider plank so to speak.

[This message has been edited by Bill Verburg (edited 05-08-2001).]

One item that was not mentioned and is one of the most important factors in high performance braking is proper pad selection. I am speaking mainly about 84 to 89 Carreras. A good pad in your Properly maintained stock system will give all the benefits you desire, Threshold braking, fade resistance, heat degredation protection. Brakes are not like engine modifications in that altering a part is not always for the greater good of the entire system. They are designed as a complete system and should be addressed as so. At every DE I have been to, we have multiple cars of every type. One thing that is usually mentioned at these events at the classrooms is that Porsche's have great brakes! Seems to be a universal thought. I have no problem with improving on old design, I just think that the products mentioned occasionally have no backing to their claims. Has anybody seen a comparison in actual numbers of a car equiped with Boxter calipers and is original stock calipers in stopping distances for 84-89 Carreras? In theory it may seem logical, but I need actual data. These companies would like to sell us anything and everything. Get some good pads, get your brakes nice and hot and feel the threshold are of the brake engagement. Funny thing is the more you do these track things, the less you actually use your brakes.

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8 9 9 1 1, The last of the line.