RKA

I'm not sure I understand how the stock caliper is designed vs. something like stoptechs, and why the design of the stock caliper is inferior.

Thanks!

ryans4

The OEM S4 front (also found on the A6 2.7t, A6 4.2, and S6) is a floating caliper with two 43mm pistons.

There are two pistons on the inside of the caliper, and a floating bridge with a cast piece on the outside of the caliper. As you apply brake pressure, the caliper contracts and pulls the outside cast piece against the rotor. This means that the front caliper has a lot of flex to it.

As you apply brake pressure, the entire front caliper (and OEM brake lines) flex before the system actually begins applying force to the rotor, thus absorbing energy. This flex results in a very "grabby" pedal feel. As you apply light brake pressure, all of that effort is used up by the caliper to flex before you get any brake force.

When you moved to an opposed piston caliper, you have two sets of pistons on either side of the rotor (assuming a 4 piston caliper). As you apply pressure, there is much less flex since the opposed piston design is more efficient than the floating design. This means that brake modulation under light brake effort is capable.

The end result in an opposed piston caliper is a more linear brake feel. As you apply pressure on the brake pedal, you get a very responsive "feel" from the brakes. The easiest way to realize this is under light braking in an opposed piston caliper vs. the OEM floating caliper.

So to get a brake system that stops your car, OEM is as good as most of the kits out there. To stop in a shorter distance once (or with a lot of time in between stopping), get better tires. If you want to improve brake efficiency, get better calipers and rotors. If you want to improve brake feel (in terms of pedal effort), go with an opposed piston caliper. If you want to go to the track and abuse your brakes and have them feel great on the drive home from the track as well, go with a floating front rotor, with a larger air gap and directional venting, gas slotted, add an efficient hat that is capable of assisting in heat evacuation of the rotor, and get an opposed piston caliper and run dedicated track pads with a good coeficient of friction for the heat range you expect to see while on track.

Everything in a street car is a compromise, decide what is most important, and that usually dictates which route is the most cost effective.

RKA

I think I have a handle on the design differences you pointed out between the stock caliper and the opposed piston caliper. Where I lose you is when you state that the caliper flex in the stock calipers causes the brakes to be grabby.

Upon initial modulation of the brake pedal, if what you state is correct, the braking force exherted is absorbed by the caliper in the way of flex, so I would expect that absorbed braking force to result in spongy brakes initially. Once the caliper body has absorbed as much energy as it can, then I would expect the calipers to transmit those braking forces to the rotor, at which point, I might expect the brakes to feel grabby. So, in all, if what you're stating is correct, then the brakes in the S4 would have some non-linear characteristics, feeling soft at first, and then grabby.

In my experience, the S4 brakes are crabby right from the onset, which would seem to me to be a function of the pads used. They also feel very linear (on the street) to me. I've never tracked the car, so I don't have any experience with how they will behave under extreme heat.

The rest of your post is pretty intuitive. What I'm trying to understand is why the stock calipers won't work as well as aftermarket equipment in a track environment. Thanks for your insight.

ryans4

...the grabby brake feel is not immediate as you apply brake pressure.

Basically, you will start to apply brake pressure, and not much happens until a point where the brakes grab fairly abrubtly. This is what I mean by grabby.

Not that as soon as you apply pressure the brakes grab. I should have been a little more descriptive about that.

The OEM calipers will work fine on the track, the major drawbacks are: Not much light brake modulation, limited choice of track pads, expensive pads (that design).

Where the OEM system fails on track is the inability of the OEM rotor to efficiently evacuate heat. In addition, on a solid front rotor, you have no room for expansion like you do in a floating front rotor setup.

Lastly, you stand to lose a bit of unsprung weight when upgrading your brakes, and that is definitely worth something.

Singletrack

Thanks for all the info Ryan, I reserve the right to bother you with questions next spring : )

RKA

Okay! Thanks for all the info! :-)