Rear-End Knocking

[shawec]

I have a 1990 ZR-1 with about 65,000 miles and I recently noticed knocking coming from the rear end when the car's momentum is shifted from side to side. The sound is not particularly loud outside the car, but is audible inside. The knock occurs when cornering or going over un-even bumps which cause the car to rock from side-to-side. I checked the obvious such as loose exhaust, wheels or spare tire and found nothing that simple. It's difficult to tell if the sound comes from the rear axle or suspension areas, as I can't reproduce the sound while the car is stopped. Does anyone have any theories and/or suggestions???

Thanks in advance!

Eric S.
1990 Bright Red ZR-1 #780

[Bill]

A friend of mine just had a similar sounding problem on a C3. He found that the rubber bushings that mount the rear spring were worn out. This allowed the rear spring to move about ¼ inch. Before repair the steering wheel centered about 11 o’clock after repair it returned to 12 and all noise disappeared. I’m not sure if this applies to our cars or not but it’s worth looking at.

[tomtom72]

Just a thought from left field...if the bushings on the "bat wing" &/or the control arms are okay...big if, could one of the shocks have come loose somehow???

Another thought: how about checking the U-joints and stub axels for play. I know they were a source of minor trouble on the C3's. Another thought, have you been keeping up with the rear gear fluid changes?? When the fluid looses it's lubricity the clutches have a tendency to "hang up" when going around corners first and if you leave the fluid long enough they will hang up even in a straight line???

I know my C3 was a bit of a PIA when it came to the rear fluid change intervals...finally I gave up & started changing it every 10K miles. GM fluid + GM additive every yr solved my rear noise problems.

Good luck & Gingle Bells!
Tom

[ZR-1 Mack]

Tom,

Do you still have the C3?

[tomtom72]

Hi Mack, No I sold it when I got married!
It was my first vette, 72 LT-1 A/C T-top w/ 69 style side pipes & 70 LT-1 heads, F-41, M-22, 3.55 rear, no mumbers match. It was a used road race car that I had to put back together. It was rude! I like my ZR-1 better because it is all that and it can be civilized at the same time. Nostaglically speaking I would like to have it back though, the side pipes were way rude on downshifts! But you couldn't get caught in traffic with the A/C on for very long! You couldn't hear the radio much & if you drove it a while you had this droning in your ears. But I was 26 when I bought it & well ya know...more $ than brains!

Gingle bells!
Tom