You're running BMW wheels, yes? The wheels should be mounted "hubcentric" (indexed on the hub), and given the difference in bolt pattern between GM and BMW...

Know anyone with a set of GM wheels you can borrow?

I have hub and wheel centric adapters from Motorsport-Tech/BORA to correct the bolt circle and hub diameter differences. I've also removed the adapters and bolted the wheels directly to the car since the bolt pattern difference is very minor. There was no change in the vibration even though I could only center the wheel using the lugs.

Yes, factory driveshaft was shortened back in 09 when I did the LT1/T56 install. I had a good, local company do the work. I didn't have the vibration issue until 2014. Shaft length was perfect until I put the new suspension on the car. That raised the rear a little which caused the length needed to change. Problem is, I did more than just the suspension at one time. I also can't remember at this point if I had this vibration before I changed to the 3.90 gears which would change shaft speed. I also had other vibrations like tires being old and out of balance and a u-joint not perfectly secured that I found during my searching for a vibration solution. It seemed like I'd fix one thing only to realize it was only part of the issue. This current issue just won't go away.

I appreciate the insight. I was considering them or Denny's. I was also only considering steel since I'm no race car. I've had my drive shaft balance checked by multiple shops and it came back perfect each time. U-joints are good too which is why I say the yoke stick out is the only thing not perfect.


I'm going to measure soon but I think I'm close to 54 inches. Maybe it's 45. I can't remember. It's the factory shaft with 3R u joints whatever it is. Plenty strong enough for the power and I don't run road courses so every thing I've read is it's fine to use. Maybe they disagree and can recommend a solution. I hate this vibration crap too. Love the power curve and drive characteristics but dear lord is this vibration ruinous to the enjoyment of the car.

Thank you all. I appreciate the responses and thoughts.

Get the 3 1/2" aluminum driveshaft, vibration reducer mod, and it will handle anything that you need. With the aluminum the critical speed was more than enough for what i was doing. Again 1 of 3 companys that do high speed balancing. The guy below is who i delt with. My buddy also had the same issue in his 69. Same issues as you and mine. Called the driveshaft shop and had one within 2 weeks as well and fixed his issue. My total length was 48 3/4". If you have a email i can send you a email with all that i delt with, measured etc

Lucas Bartholomew

The Driveshaft Shop
4530 Southmark Dr.
Salisbury, NC 28147
800-564-2244

Thanks Randy. I'll give Lucas a call.

I measured 48 9/16" so 48.5" I'm sure is what they'll cut it to. I confirmed again that I'm about 3/4" short now.

Thanks everyone

Trey, I sure hope that a new shaft fixes the car. I have a relative in the driveshaft business, and maybe I can save you a buck or two if your interested? Send me a pm...

Me too, Scott. I appreciate the offer. I've already started working with Drive Shaft Shop. We'll see what happens.

Thanks

I believe I've goofed on my working angles and I'm way over the 3* limit. I had always taken my measurements with the car sitting at ride height. Well, tonight I took them with only the rear off the ground and it became obvious to me that I think I screwed that math up. I'll see what Lucas at Drive Shaft Shop says in the morning.

Thanks

Easy enough to do ----

Motor should be about 2 to 3* DOWN at the tranny pan --- REAR END should be about 2* UP --- and if the slip yoke is correct (they make all kinds of lengths) -- you should be close.

The other thing I've seen --- a too small diameter drive shaft for the overdrive tranny / rear end gear combo. The shaft has a working RPM and when exceeded can go into harmonic death wobble. LOL

Glad you're on it.

Those values are actually pretty close to what I have for pinion angles now. The issue is my rear end if higher than the rear of the transmission so the drive shaft is down in front. The solution is to raise the trans more. I'm not sure I have the room due to my headers though.

I didn't want to get into it just yet as I'm waiting to see what the Drive Shaft Shop has to say, but I'm getting really concerned with some things about my build compared to what others have had to do with their similar builds. This whole car is a learning experience for me. I'm worried I may have missed or done some critical stuff wrong after all. We'll see what happens. I hope I'm just over reacting.

Thanks