Took some time to play with the RS4 rear suspension parts that I got from 65vetteC6 to see what the differences were when compared to my B6 S4 parts. I wanted to see what it would take to put RS4 rear brakes on my car and it was a lot more than I originally thought. It was worth it to me to get these parts just to see and hopefully somebody finds this information helpful.
Overview shot of the S4 components installed on my car:
Overview picture of the RS4 components installed on my car:
In the above pictures I have the 034 Motorsport adjustable rear upper control arms since the upper arms are the same for the S4s and RS4s.
Below I have a picture of the normal rear upper control arm.
The rear subframes are also the same between the B6/7 S4s and the RS4s. Below I have a picture of my rear subframe with solid bushings installed but normally they would be rubber bushings.
The cast weirdly shaped lower control arms that hold the coil springs are angled differently to probably allow for better clearance of the brakes and wheels. The measurements that I took are done based off using the lift platform as a stop. The RS4 rear suspension was about an inch higher than the S4 suspension.
The black lower tie rod arms are different lengths. The RS4 arm is noticeably shorter than the S4 arm.
The rear wheel bearing housings are different, the rear wheel bearings are the same, the rear wheel hubs are different between my B6 S4 and the RS4 but it is fuzzy for the later B7s off part numbers. The rear wheel bearing housings are angled differently to match the way that the cast weirdly shaped lower control arms. The rear brake caliper bracket holes are different sizes, the S4 uses 10 mm bolts as the RS4 uses 12 mm bolts. The rear wheel hubs are spaced out farther to accommodate the offset of the larger rear brake rotors on the RS4. This is why CHECKERED had to modify his hubs and housings in this thread:
RS4 wheel hub offset measurement:
The rear axles are different lengths with the RS4 rear axles being a little longer. The S4 rear axles are on the bottom and the RS4 rear axles are on top. The tape measure was used to better illustrate the different lengths.
Look at the outer CV joints. The RS4 outer CV joints are bigger and beefier. The splines are the same for the outer CV joints where they go into the wheel hubs.
With the RS4 rear suspension still installed, I put in the rear RS4 axle to see how they fit to with my S4 rear diff.
Just for shits and giggles, I left the RS4 rear suspension in and put in the rear S4 axle to see how the combination would fit at the rear diff. The rear inner CV joint is not even close to the flange of the rear diff.
The rear differentials are also different between the S4 and RS4. Here is a picture from koolade9’s build thread on Audizine. The RS4 rear diff is on the right and the rear diff on the left is probably not from a B6/7 S4 but it looks very similar to what would be correct.
That pretty much covers everything.
Now for reflection on my idea of using RS4 rear brakes on my S4. Here are some numbers on some of the rear brake options for the B6/7 S4s and RS4s:
- Stock B6/7 S4 rear brakes use 300 mm brake rotors and single piston sliding brake calipers.
- Stock B7 RS4 rear brakes use 325 mm brake rotors and single piston sliding brake calipers.
- JHM sells a B6/7 S4 rear BBK which uses 330 mm brake rotors and it reuses the stock single piston sliding brake calipers.
- Brembo sells a B7 RS4 rear BBK that uses 380 mm brake rotors and four piston fixed calipers but you lose the parking brake.
- Stoptech sells a B7 RS4 rear BBK that uses 355 mm brake rotors, four piston fixed calipers, and they include a parking brake caliper setup to retain the parking brake.
Cost wise it just doesn’t make sense for a B6/7 S4 owner to switch to a RS4 rear brake setup unless they are going with a RS4 rear BBK from Brembo or Stoptech since in addition to the BBK they would need the rear suspension and axles to make it all work. Even then it is a stretch since the cost would still be outrageous!