Larger calipers will not make the car stop faster. It will be harder to stop. You will have to move a larger volume of brake fluid which means you'll have to push the brake pedal further to the floor. The best you can hope for is brakes that are strong enough to lock up the wheels. What more can they possibly do? The rest of the equation is up to the traction rating of the tires and the road surface.
You already have the best stopping power with the ABS. I have a '93 Dynasty with just over 4000 miles, with the Bendix-10 system. This car stops so hard it just about tears the seat belts off the hinges!
Part of the problem is your perception of stopping distances because you can't lock up the wheels. The first thing to understand is a skidding tire has no traction, therefore very little stopping power. Maximum stopping force is achieved just before wheel lockup. Your system operates by four sensors watching individual wheel speeds all the time. When the brake pedal is pressed, the computer prepares to modify hydraulic pressure to any individual wheel. First it finds the one wheel with the fastest speed, ... And disregards it. Of the three remaining wheels, if one is turning a certain percentage slower than the other two, imminent lockup is suspected and the computer operates valves in the hydraulic control unit. First it blocks any additional fluid flow to that wheel even if you push harder on the pedal. If lockup continues, a different valve opens to bleed pressure from that wheel. Once the wheel gets back up the same speed as the others, a third valve opens to apply additional fluid from the high pressure accumulator. This block / bleed / apply sequence can occur up to 30 times per second. 15 times per second is more typical.
Because no tire ever is allowed to skid and lose traction, all four are stopping to their maximum potential which is why these brakes are so effective. Imagine if you were trying to stop as quickly as possible to avoid a crash. You push really hard on the pedal until one wheel skids. To maintain traction, you let off the pedal a little. Three tires were not stopping to their maximum potential, ... But you let off the pedal to prevent the one tire from losing traction. Now those three wheels are doing even less stopping.
A similar Bendix-9 system is used on the Chevy Caprice Classic, commonly used for police cars. Those cars take a very long time to stop. The only difference from the Bendix-10 is they modulate fluid pressure to both rear wheels together so the system is a little less expensive. Other design concessions lead to the longer stopping times. A friend who's a county deputy says the cars stop faster without ABS. It's important to remember that even though most Chryslers have terrifyingly short stopping times, the ONLY purpose of ABS is to maintain steering control. Dramatically reduced stopping distances is just a dandy bonus for us Chrysler owners.
Your car uses a Teves system which works on the same principles as my Bendix system. You also have rear disc brakes. Even though the rear brakes only contribute around 20 percent to the stopping power on front wheel drive cars, disc brakes eliminate the lag time involved in moving shoes out to the rotating drum on drum brakes. It's also easier to move the light weight pads back and forth during ABS operation.
Some other things to keep in mind are the service procedures when doing a brake job are exactly the same as on a car without ABS, and power assist comes from from pressurized brake fluid rather than engine vacuum. With conventional power brakes, if the engine stalls, there is enough vacuum stored in the booster for two to three power assisted stops; enough to get you to the side of the road safely. With your ABS system, every two or three pedal applications will reduce stored pressure enough that the pump will run to build stored pressure back up to around 2200 psi. In the event the engine stalls, you will always have power assist until the battery goes dead, which could be hours!
One last point of interest; Chrysler was the first company to use anti-lock brakes on one of their cars, ... In 1969. It wasn't very effective because it only modulated hydraulic pressure the equivalent of pumping the pedal three times per second, about the same as a driver can do manually. Still, it was the first attempt at increasing safety.
To get back to your question, smaller caliper pistons will make the brakes respond faster but will require much more pedal effort. There are many problems with this approach. First, the engineers spent a lot of research and development time matching the front brakes to the rear ones to produce a balanced system. Smaller pistons on front will reduce pressure on the pads resulting in the rear wheels locking up very easily and the rear pads wearing out too quickly. Different size calipers will only be found on vehicles that are available with a variety of drive trains of substantially different weight. You will likely need to replace the mounting knuckles or spindles, use different wheels, and in some cases, different hub / wheel bearing assemblies. Next, you'll need to replace the combination valve assembly although most cars equipped with ABS don't use this valve. Part of its job is to limit fluid pressure to the rear brakes to prevent rear wheel lockup under hard braking. Changing one of the design criteria affects all the other variables.
Finally, you can't overlook the legal issues involved when modifying safety systems. Lawyers are involved in every step of the design phase of new cars. In the event of a crash, even one that is not your fault, good lawyers will pick your car apart and use any modifications, worn out parts, (especially under-size rotors), and obvious lack of maintenance to convince a jury there were things you could have done to avoid the crash if you hadn't eliminated or modified what the manufacturer designed.
Caradiodoc
Tuesday, November 10th, 2009 AT 1:43 AM
