This thread from last September left an impression on me. Like JB, I've had a rear-end imbalance, mine at speeds above about 50 mph, sometimes there, sometimes not, feeling generally harmonic. Wheel balancing did not cure the problem. With my S2 still in its seasonal layup, I decided to have its drums dynamically balanced. I know these are the original drums for the car because I'm the only DPO.
I took the drums to an engineering shop and got a phone call a few days later. They'd balanced one but the other, they said, was oval and they didn't want to do it.
I took back the oval drum and remounted it to the hub. Sure enough, once I knew what to look for I could see it ran out-of-round. I can't measure runout precisely but it totals about 1.6 mm. I ordered a new drum from RD.
The one old drum was balanced by adding weight in two spots. The new drum was balanced by lightly grinding two spots.
I measured the distance from the lip of each bolt hole to the edge of the centre hole on the oval wheel. My four readings were (mm): 9.87, 10.07, 10.15, 9.98. Hmm. I similarly measured the new drum: 10.02, 10.02, 9.98, 10.07, which probably says something about my technique but indicates where the problem might have come from. Once the car is back on the road I'll report whether this work had any real effect.
