[jritt@essex]

Originally Posted by DocWeatherington

Awesome post and information.

Thanks!

Have you done any tests vs oem in stopping performance?

Before getting too deep into any discussion of testing, it's important that the term 'stopping performance' is first defined. The term, 'stopping power' is a similar term that is often loosely thrown around, but we need to be more specific. When many people ask about 'increased stopping power', what they actually mean is, "Will the car stop in a shorter distance than it would previously?"

When discussing overall brake performance, stopping distance is just one measure of performance. Other thing to consider are pad and disc temperature rise, length of brake pedal stroke, maintaining proper front-to-rear bias, response/activation time, mass reduction, cooling efficiency, etc. Subjective factors such as feel and modulation, controlability of ABS intervention, etc. also come into play.

The most fundamental point to make with regards to stopping distances is: Brakes don't stop your car, tires do. Brakes convert the energy of the spinning discs into heat. If you want your car to stop in a shorter distance, you need to increase the grip that the car has on the road surface, and the primary way to do that is via stickier tires.

All else held equal, a properly designed aftermarket brake system should stop a vehicle in a similar distance as the factory brakes on the first stop. Resistance to the heat generated by repeated stops is where aftermarket brake systems really come into play.

When we look at the 'stopping performance' of our system vs. an OEM system, our brake kits typically have gains/advantages in the numerous areas. Those areas are related to specific design elements of both the caliper and discs, as well component material choices. We really get down into the nitty gritty details throughout the entire process.

The fundamental issue with brakes on a track car is how effectively/efficiently can the brakes absorb and shed heat? A lot of the effectiveness hinges on the performance of the brake discs. Compared to OEM discs, more heat can be poured into our J Hook discs in a shorter period of time, without creating as much of a temperature rise. In other words, our discs are able to more efficiently absorb and shed the heat generated by repeated stops. This is accomplished by the number and shape of the internal vanes, careful consideration of the disc's dimensions such as air gap (space between disc faces) and disc face wall thickness, iron metallurgy, etc. This translates into the discs not getting as hot as an OEM disc under the same conditions (e.g. 15 stops per lap, multiplied by 10 laps).

Properly designed, directional internal cooling vanes push more cooling air through the discs, slowing their temperature rise. OEM discs usually have more primitive, non-directional cooling vanes. The vanes don't flow much air, or turbulence is created when the air hits them. A proper internal vane design also accommodate a higher flow of cooling air brought into the discs via brake ducts for example. When the discs run cooler, all brake components 'downstream' from the discs run cooler as well. If the discs don't get as hot, the pads clamping them won't get as hot, the metal pistons behind the pad backing plates won't get as hot, the fluid pushing the backside of the pistons won't get as hot, and the caliper body holding that fluid won't get as hot.

The disc hat mounting mechanism also impacts how much heat a disc can take before starts to create problems. Our mounting system allows for more float than the OEM dual-cast hat attachment method. In other words, the discs are free to get hotter and grow larger, without distortion. When the disc hat restrains the disc's growth, the disc cones, or the edges pull towards the center. A free-floating disc remains centered in the calipers better, which means they're less stressed, less prone to cracking, and more likely to provide even pad wear.

Having a high internal vane count also provides a strong foundational structure behind the disc face, creating a more even distribution of heat and less deformation. The pads tend to stick more evenly around the entire face of the discs when there aren't any hot spots or cool spots, which means less brake judder from uneven pad deposits. The J Hook slot pattern also leaves no substantial gaps on the disc face, which means it more evenly distributes heat throughout the disc face. Again, that helps prevent uneven pad deposits and judder.

The shape of the Radi-CAL calipers also allows for greater airflow and heat capacity (although stiffness with less mass is arguably their greatest benefit). All of the voids and gaps around and through the calipers provide more surface area in contact with cooling area, and more heat can escape the calipers via radiation. In other words, all those gaps provide more places for cooling air to touch the caliper and draw heat away.

Another area in which our systems have greater heat capacity is often the pads. Thicker pads provide a greater buffer and heat sink between the brake disc and the caliper pistons and brake fluid. This is especially true of our CP9668 kits with the full inch thick pads. The thicker pads can absorb, hold, and radiate more heat into the air than thinner pads. When they run cooler, they also burn slower, which means they last considerably longer.

Once you get past overall heat capacity, you get into material choices specifically chosen for heat rejection. Stainless steel pistons absorb and transfer heat much more slowly than aluminum. That means heat from the pads doesn't flow as quickly into your brake fluid, so you're less likely to experience brake fluid fade (soft pedal). Your fluid also doesn't get broken down as rapidly since it runs at lower temperatures, which means less need for bleeding. The pistons are also ventilated to allow air to flow inside them, behind the pad backing plate, again lowering temps. AP's iron disc material has been formulated across the span of over 50 years at the elite level of racing. It is designed to be crack-resistant with unique ingredients, unlike OEM disc materials which tend to focus on low cost as the primary goal.

We also see improvements in response vs. OEM brake equipment. AP's Radi-CALs are the stiffest calipers in the world, since they were produced explicitly to combat the dynamic forces acting upon a caliper during a brake event (read The AP Racing Radi-CAL Story for lots more details). That means there is less lag-time from the moment you press the brake pedal until the calipers squeeze the disc.

Tied to all the above are subjective improvements to the system such as feel and control. The stiffness of the calipers and increased responsiveness gives the driver greater control and feel for what is going on at the wheel ends. It's easier to modulate the pedal, feel when ABS is about to engage (or lock up a car without ABS), trail brake, etc. You have better control over the car, which is what the pro racers really rave over about the Radi-CAL.

Okay...I got a bit carried away on that post ...but I wanted there to be an understanding about what the objectives are with a race-spec brake system, and how we look at an OEM system and try to overcome its deficiencies.