a8 rear brake upgrade

[Dynamite]

I have seen a couple people on here upgrading their rear brakes with the A8 rear carrier to take the b5 s4 rear caliper. Is there any reason why the A8 caliper couldn't be used or why the s4 one is better? Also do both of them have the assembly to keep the handbrake on the same caliper like stock?

[034Motorsport]

I have seen a couple people on here upgrading their rear brakes with the A8 rear carrier to take the b5 s4 rear caliper. Is there any reason why the A8 caliper couldn't be used or why the s4 one is better? Also do both of them have the assembly to keep the handbrake on the same caliper like stock?

Both the A8 and S4 rear brakes are vented 22mm wide discs, versus the unvented far thinner A4 rotors. This is the primary reason to upgrade, and requires the S4 calipers. I do not know if the A8 utilize the same e-Brake, but the S4 calipers do.

The more popular upgrade is to now use B7 S4 300mm rear rotors instead of the 278mm of the A8. They are the same 22mm wide, but a full 2" larger then the stock, and about 20mm larger then the A8.

We sell these as a kit with or without calipers, although you can source used calipers for cheaper. We only offer brand new calipers with the kit. You keep all the factory e-brake system with the S4 calipers. I can't confirm the A8 calipers.

Our kit with Brand New calipers:
http://www.034motorsport.com/chassis...mm-p-1170.html

Minus $980 from the price if you source your own calipers. We also offer the brackets themselves for $75.

PS. Stainless brake lines with a banjo ARE required do the caliper position; stock lines (steel or rubber) will not work.

[$teady$upreme]

Hey Max, if upgraded to B7 S4 300mm, can i reuse stock shield?? I currently have the B5 S4 rear.

[034Motorsport]

Hey Max, if upgraded to B7 S4 300mm, can i reuse stock shield?? I currently have the B5 S4 rear.

Yep. The reason you can utilize the stock shield is because the B7 rotors have far less offset, so they stick out out more and clear the shield. I am using the stock shields on mine.

[Dynamite]

With tue stock calipers and the 312mm a8/TT upgrade would the size of the 300mm rear rotors and the b7 s4 calipers throw off the braking bias? It seams like a lot more in the rear without going to boxter calipers up front.

[$teady$upreme]

I think you got it wrong here. You don't use the B7 S4 caliper, you will need B5 S4 caliper for the 300mm rear set up.

With tue stock calipers and the 312mm a8/TT upgrade would the size of the 300mm rear rotors and the b7 s4 calipers throw off the braking bias? It seams like a lot more in the rear without going to boxter calipers up front.

[$teady$upreme]

Very nice to know. Cause with the B5 S4 rear rotors, the rotors barely stay within the lip of the brake shield. Good to know that the B7 rotors have far less offset.

Yep. The reason you can utilize the stock shield is because the B7 rotors have far less offset, so they stick out out more and clear the shield. I am using the stock shields on mine.

[Dynamite]

ah that makes more sense thanks for the clarification. How much different in piston area is the s4 calliper to the a4 one?

Not to start another brake shit fight, but I am a little lost on how this whole bias thing works, (I have read a few threads and the stoptech articles etc)

My understanding currently (im probably wrong so correct me) is that when we take the calliper outboard as in the TT/A8 setup then we improve the heat capacity of the rotor making it less likely to fade. By doing this then we now have a bigger lever arm from the front calliper to the hub this creates more braking force on the hub (even though the calliper isnt working any harder) from the leaver arm. This means that we get more stopping power in the front by the factor 312/288, however the rear is left as stock. so we have ~8 present more braking force now on the front wheels. this is not enough to through the bias out (apparently) so never causes any issues.

If we want vented rotors in the rear then we have to change the callipers (vented rotors wont fit in the stock ones) now it seams to get tricky, we upgrade the disk to the 300mm vented ones and to the s4 calliper, if the calliper was the same as stock (a4) for as far as x pressure does y work on the rotor then if we keep the ratio of 288/245 the same when we went up to the 312 disks giving a 267mm rear disk. However it is a 300mm disk so we now have a bigger lever arm on the rear than with the (“optimum”) 267mm disk so the rear wheels see more stopping power than it would with the 267mm disk and hence the system becomes more rear bias than stock.

As the rear is now a 300mm disk then we have a calliper that needs to do LESS than y work for x pressure as we have the increased distance from the hub to the centre of where the callipers force is acting. (it is more than 267mm) so we need a calliper that does 0.89 y work on the disk for x pressure.

Now because the calliper (I assume) from the s4 does more work for y pressure then we now are doing more work comparatively to the front calliper and further away from the hub (so the wheel sees even more stopping power) than it did before making the bias even more rear favoured.

To upgrade the rear rotors to 300mm then to keep the stock bias (which I assume audi did the correct job of) then we would need a calliper that did less work than the stock one for the same pressure when we change to the A8/TT front brakes. Or a proportioning valve to let less pressure get to the rear callipers.

Is this correct? Am I making incorrect assumptions?

[Dynamite]

Now if we were to change to say boxter callipers up front then they have multiple pistons and hence bigger pads, so more friction on the rotor. With more pistons then for x pressure then we get more work done on the rotor and hence more stopping power on the front, moving the bias forward again. How do we calculate how much further forwarded the bias has moved? Is this the best way to balance out the rear bias caused by the 300mm rear setup?

[walky_talky20]

I would think that one way to approach even-ness would be to use B7 S4 brakes front and rear. The B7 chassis is probably quite similar in weight distribution as it uses the same drivetrain layout. I don't know what kind of extra weight the V8 engine is, but there aren't turbos and it's all aluminum, so I'd guess it would be in the ballpark anyway. Using the front and rears would be a sort of "matched pair". Just a thought that I had. I just bought my A8 front rotors today. My brakes completely suck right now.

[Dynamite]

Then you would need at lest 18s and I'm not keen on that plus it would be very expensive. Are you planing on upgrading your rears with the 312s up front?

[walky_talky20]

Wasn't planning on it, but when I need new rear rotors next time I may consider it. That's how I did the fronts. Current rotors are completely wiped out, so I'm putting on new A8's for the replacements.

[034Motorsport]

I put out the 300mm kit for those running a large BBK up front. I would not recommend it in conjunction with a 312mm single piston front brake kit. That would indeed throw brake balance off.

However, when you get up to a 330mm or larger front BBK, especially with 4 or more pistons, you NEED the 300mm rear rotor to help balance things back off. We have front heavy front bias cars, but they can benefit from a slight increase in rear power to help the car rotate into corners. If you don't drive aggressively, not necessary.

I run a stock 254mm vented B5 S4 rear setup on my B5 A4. It is paired with a Boxter/312mm front setup, and is perfect. I noticed a dramatic increase in fade resistance by moving to the vented rotor.

[Dynamite]

So do the rear b5 s4 carriers and calipers bolt straight up or are they like the front where you have to swap all the uprights etc?
also are the D2 A8 callipers and carriers different to the b5 s4 ones?
brake fade is really why im looking at these options.

[RS4Avant2be]

If you are looking at saving some money, you might want to consider the A6 rear upgrade. Goes from 245 to 255mm (unvented), 10mm, w/ 1.5" offset. A6 2002 3.0L or 2.7TL Q (C-5) rotors [4B0-615-425G] and A4 2002 3.0L Q (B6) rear caliper carriers [8E0-615-425G]. Just be carefull of where you get the carriers as my first set were the wrong size, ended up going directly to an Audi dealer. Uses the original calipers, just need to replace the pads. I have StopTech disks and PBR Ultimate Ceramic pads both front and rear (A8 upgrade in front) and am pleased with the results. Good balance for DD as I do not track my Avant.

[034Motorsport]

So do the rear b5 s4 carriers and calipers bolt straight up or are they like the front where you have to swap all the uprights etc?
also are the D2 A8 callipers and carriers different to the b5 s4 ones?
brake fade is really why im looking at these options.

As I mentioned originally, the uprights are the exact same. The S4 calipers and rotors bolt right up WITH the S4 caliper bracket/carrier. E-brake works and all. Been mentioned a couple times.

Venting = far better fade resistance