[CZ-75]

http://www.geocities.com/nosro/abs_f...%20aftermarket

What happens if I install aftermarket tires, wheels, brake linings, calipers, springs, . . . ?

It is anybody's guess. Tires and brake linings are fairly safe items to experiment with. Having said this, I will point out that I am familiar with at least one case where the installation of high performance tires (where smooth-riding all-season tires were original equipment) resulted in a reaction with the suspension that ABS had not been tuned to anticipate. In this case, the installation of high performance tires resulted in significantly longer stopping distances because the extra tire traction was causing the front suspension to literally bounce off the jounce bumpers repeatedly, causing very violent changes in tire grip that ABS was not expecting. From a strictly ABS standpoint, it is best to stick with a tire, wheel, or brake lining that is similar in characteristic to the original equipment parts, or at least similar to any of the parts that are available as an option on that vehicle.

Toying with the hydraulic system is slightly riskier. (This would include parts such as the master cylinder, calipers, and brake lines.) If you change the hydraulic characteristics of the brake system, you may reduce the efficiency that ABS operates with. For example, ABS may deteriorate from a 95% utilization of peak tire adhesion to a 90% utilization of peak adhesion. This is due to the fact that ABS is tuned to the hydraulic characteristics of the original brake system, as noted above. Another way to think of this is that ABS, like any other control system, performs some amount of prediction or anticipation to maximize performance. If you change the hydraulic characteristics of the brake system, you render the predictions or anticipations of the system less than ideal. That means ABS will need to react and adapt to the change. Reacting and adapting takes some time and this translates to lowered efficiency.

There are many variables that go into characterizing the hydraulics of a brake system and some are not always obvious. For example, if you install larger race-specifications calipers, you may assume that your new calipers will consume more fluid. This is not necessarily the case. It is true that larger pistons in your caliper will result in greater fluid consumption, but these race-specification calipers are probably also much stiffer than the stock OEM calipers. This stiffness actually reduces overall fluid consumptions. Also counter to your intuition, large race-specification calipers may have multiple small pistons than result in an overall volume displacement that is smaller than the single large piston in your original calipers.

A driving enthusiast may point out how a certain magazine tested a particular vehicle, after installing big brake calipers and rotors, that stopped 10% shorter than the stock car. The problem here is that these changes are almost always made along with a change from all-season tires to high performance tires or some other significant change. As I have noted above, engineers spend years tuning the ABS hydraulic controller to work well with the hydraulic characteristics of the brake system. It is true that your final braking performance may be superior (less brake fade, for example), but speaking strictly from an ABS perspective, it is a crap shoot.

More recently, the June 2001 issue of Car and Driver magazine featured a Roush modified Mustang, featuring race-specification brakes and suspension, and extremely low profile tires. They found the stopping distance to be surprisingly long given the quality of the parts. They attributed this disappointment to the fact that the ABS was not originally designed for such components - my assessment exactly.

In contrast, the September 2002 issue of Car and Driver magazine (they seem to appreciate brakes more than any other car magazine) featured a comparison of four aftermarket big-brake kits for the Subaru WRX. They expected the big-brake kits to show their mettle during their brake fade test, but they did not expect any stopping distance improvements with cold or merely warm brakes. After all, if the brakes can invoke ABS control, the vehicle must be tire-limited rather than brake-limited. However, with every big-brake kit, they found improvements in stopping distance even with cold brakes. There are many possible reasons why this is the case. My best assessment is that, according to my experience driving a Subaru WRX, the ABS was tuned to be very conservative in order to extract stability (resist spinning or oversteer) on ice, resulting in the trade-off of reduced asphalt performance. This is a classic compromise for ABS engineers. Installation of the big-brake kits may have fooled the Subaru ABS into a more aggressive ABS control (for example, with a shorter time between slip cycles) whereas the engineers had intended a more conservative ABS control (to improve resistance to spinning out). Put another way, I believe that the Subaru WRX with one of the big-brake kits featured in this article may have significantly reduced ability to resist spinning out on ice.

I know that StopTech makes aftermarket calipers that are custom tailored to each application by modifying the piston size to closely match total piston area of the stock vehicle. This is smart and shows due care. There may be others, but I have not researched this matter. (I own Stoptech products, but I have no relationship with them.) Some other aftermarket caliper manufacturers take a single design (with one piston size) and use it for everything from a sub-compact to a large sedan. In such cases, your results will depend on being lucky enough to have the one available piston size match your stock piston area.

Suspension modifications are another interesting area. Engineers spend considerable time tuning ABS performance while in an actual prototype vehicle. This means they are working with the entire vehicle - the entire vehicle includes the suspension. In fact, engineers will often find themselves working against the suspension. When you mash the brake pedal, the vehicle begins pitching as weight is transferred away from the rear tires and towards the front tires. This pitching can create difficulties in extracting the last 10 feet of stopping distance. ABS software is sometimes made to anticipate the pitching and to modify the manner in which brake force is modulated. Therefore, if you modify the suspension, you may be reducing the efficacy of ABS.