In Reply to: recirculating ball v. rack and pinion systems posted by (more) on July 26, 2000 at 20:49:05:
Recirc Ball - Steering wheel attached to rod, rod has screw gear at end, gear is attached to another gear that moves a lever left and right (its more complicated than this). There are ball bearings in between the gears and as they move in the groves, when they get to the end of the screw gear they are recirculated to the other end of the gear. The lever is attached to rods that are attached the wheels.
The biggest difference from a drivers perspective has more to do with how each is engineered/implemented than the actual theoretical differences. By that I mean, the best recirc ball system (probably the E39M5's) is much better than the worst R&P.
That aside, R&P is generally easier to get right for performance cars since it is a more direct/simple design. I've had an E36M3 (R&P) and a E39 540 (recirc ball not as good as M5's) and the steering between the two was quite different. M3 has more road feel and is much more responsive. 540 has slower steering response and much more muted. Recir ball has many more places for road feedback to be absorbed so it is hard to get the same feel as from R&P.
The reason BMW used recirc ball in the the 5 and 7 series is because with the V8 and V12, the R&P system would not fit. Note that the 528 has R&P since it fits with the I6.
I hope that helps.