As for it not doing a single thing for braking performance; could you explain that for me? If that is the case then why do SN95 cars upgrade to the 99 pbrs? Why do Fox bodies upgrade to SVO calipers? Maybe because of the increased surface area of the pads, increased clamping force, or even heat dispersion. Maybe I'm just dumb (sarcasm).
This seems really uncharacteristic of you MFE. What gives?
MFW certainly knows his stuff, but since he hasn't replied yet, I will...
Sit back and grab a cool one. this is going to be a long one...
The purpose of the brakes is to convert momentum into heat. Not surprisingly, brakes do that exceedingly well. Even the punniest brake system can decelerate your car MUCH quicker than the engine can accelerate it. In that sense, the brakes are the single most powerful system on the car.
But, small brakes are likely to decelerate the car from triple-digit speeds once, or maybe twice. Good systems might
do it three times in a row -- maybe.
Why? HEAT. Brakes generate an enormous amount of heat and it has to go somewhere. Some is radiated (you can aceually see it on some race cars, particularly at dusk/night.) But the majority of heat is absorbed by the brake components (pads, rotor, and caliper, in that order, roughly.) If those components get too hot, the brakes can fail:
pad - pad fade happens when the pad's temperature gets too hot for the friction material to operate effectively. This is why street pads don't work well on race tracks.
rotor - rotor failure -- cracking usually.
caliper -- boiled brake fluid.
So, since even punny brakes, when cool enough, can stop the car very very well, what most folks do when they upgrade their brakes is increase their brake's heat capacity. That's so they can slow the car down time and time again without a loss in brake performance.
How? They upgrade the brake pads first, since on a street car these are the first things to fail. Next, usually is the brake fluid, which is a cheap upgrade -- About $100 to flush out the old an replace it with high-temperature DOT 5.1 fluid (NOT DOT5!!!)
So, now the pads will live, and the fluid won't boil away. The rotor is the next weak link. The hotter the rotor gets, the more thermal stress it sees and if it sees enough, it will
crack. The trick is to keep the temps down. You can do this somewhat by venting fresh air to the center of the rotor, where the cooling vanes inside the rotor will pick it up. This is a band-aid at best, but an effective one if your brakes are getting just a little too hot.
To get big improvements, we have to increase the mass of the rotor. We can make a solid rotor (i.e. no internal cooling vanes,) but it would have a very difficult time cooling itself. We could make it thicker, which is effective, but at some point both packaging and caliper stiffness would start being a problem -- the upper limit seems to be 1.25" for most racing brake systems.
The big gain is in increasing rotor diameter. This not only adds mass, but also increases surface area meaning the rotor will cool itself faster.
So, the BIG THREE things folks do to increase the brake systems heat capacity are:
- better pads
- better fluid
- bigger rotors
So why not better calipers? Because there's very little you can do to increase the systems heat capacity by changing caliper designs. (And OEM designs are pretty darned good these days -- that cast-iron lump does a darned good job, even if it looks like crap.)
But-but-but... Why do race cars have these sexy Alcon and Brembo calipers?
Once you have a brake system that can handle all the heat you'll throw at it, there are other aspects that that you can tweak to make components last longer, or make the brake pedal feel a little better.
Many times, sliding caliper systems (i.e. most OEM calipers,) don't slide too well if they get very, very hot, or if they get old. If they stick, only the brake pad on the piston's side will end up doing any work, meaning that pad will have a short, violent life, while the other pad will be virtually unused. Many brake systems solve this by bolting the caliper solidly and having pistons on BOTH sides of the rotor. It take up a lot of space, but it is effective. Brembo uses this approach, as do almost all racing calipers.
Many OEM calipers use a single piston, and sometimes a small one to boot (which lets you use a smaller, lighter -- and cheaper -- caliper.) However, under extreme situations, a small piston can actually cause the brake pad to bend and flex. The brake pad ends up wearing in the center, where the piston is, and wearing less on the pad's perimeter, where the piston's pressure is less due to brake pad flex.
Ford had this problem with their Crown Victorias that were built for fleet use (usually taxicabs and police cars.) Fleet maintenance costs were high because they were having to replace brake pads that still had plenty of life left in them because the pads were wearing unevenly. Ford not only solved this by increasing the piston size on the caliper to 73mm, but they also specified a new pad outline that had a thicker steel backing plate.
This proved to be so successful that the same setup was used on the Lincoln LSC and the Mustang SVO. One was a heavy car that could benefit form a beefier brake system. The other was a sports car that was likely to be driven hard.
Note that none of the reasons for developing the 73mm caliper had anything to do with fighting brake fade.
Very high-end callipers, and racing calipers spread the piston pressure out more effectively by using multiple pistons, sometimes as many as four per side, giving you an 8-piston caliper. Designers can also fine-tune the force behind the brake pad by varying the diameter of the pistons themselves to perfect the brake pad wear characteristics. The 99+ SN95 caliper is a dual-piston design which supports the brake pads better than the previous single-piston design did.
So we've seen that caliper design affects how the pads wear. What else?
As the pistons press inwards on the brake pads, they also push outwards on the caliper body (well, the hydraulic pressure in the caliper actually does that.) If the caliper isn't stiff enough, it can actually flex. This results in a slightly longer pedal for the driver sometimes. On street cars, this usually isn't much of an issue because the hydraulic pressures are much lower, and because iron OEM calipers are incredibly stiff (and heavy.) More OEMs are going to aluminum calipers, which can sometimes be a probem. I've seen the PBR Cobra calipers get BENT from hard usage. A combination of high temperaturs and high pressures will actually spread the caliper apart -- permnently. At that point the caliper is junk.
Both OEM and racing caliper designers are constantly looking for a way to make their calipers stiffer without adding more material to the caliper, which makes it heavier and can make it larger. The Holy Grail of caliper design is high stiffness with low size and weight. Check out this radical design from Alcon:
So, in a nutshell, better calipers give you better pad wear (important when your brake pads cost $300 per axle set ad have to last for an eitire race!) and peter (more consistant,) pedal feel which is critcal for consistant braking during a race.
But none of this cool technology will actually help you stop much shorter -- but I'll save that for another post...