driveshaft angles

[Impala Matt]

I have a slight vibration in my Impala at hwy speeds. Only at certain speeds. I started with the wheels and tires but they are ok. Next I had the driveshaft looked at. It needed 2 u joints (which is odd because the unit only had 20-25,000 miles on it) After putting it back in I find the vibration is worse and now at a different speed. I will take the driveshaft back and have it re-checked but I want to know if I should be looking for other issues. My car sits about 3" lower than stock which leaves the yoke on the diff pointing somewhat down. The car has a T-56 and a custom trans crossmember.
The vibration will go away if the car is coasting of if the clutch is in. Even if its at the speed where it usually shows up. Could I have too small an angle between my low tranny and the rear end which is up into the floorpan? I know the driveshaft is not supposed to run straight but I have never measured the angle because it has not been an issue until recently.

[Pontiac Chris]

If the vibration goes away when you push the clutch in, it sounds like its an engine vibration, do you get the same vibration when you hold the engine at freeway rpm whilst the car is sitting still? I'm chasing a 75mph+ vibration myself but it does not go away when I push the clutch in on the freeway.
If you get your driveshaft re-balanced ask at what rpm they balance it to, I think 3000 rpm is common for the average shop, but I think mine is spinning around 4000 rpm with 3.73's and a 26" tire at 80 mph. You might find your driveshaft speed is above what the shop is balancing it at so it shows up fine to them.
Check out Inland Empire's driveline setup guide, has info on angles:
http://www.iedls.com/asp/admin/getFile.asp?RID=5&TID=28&FN=PDF

Chris.

[Motor City Impala]

Matt
One way I have found to eliminate the engine as a suspect is to shut the engine off while driving. Since you have a manual and not an automatic this is much easier to do. I tend to get to the speed I feel the vibration is at its worse....increse speed slightly, push clutch in, shut off engine, MAKE SURE TRANS IS IN NEUTRAL!!!!! and then ease off the clutch. If the vibration still exsists you have just eliminated the engine. If it is still there then I would look at pilot bearing and input shaft.

[If you get your driveshaft re-balanced ask at what rpm they balance it to, I think 3000 rpm is common for the average shop, but I think mine is spinning around 4000 rpm with 3.73's and a 26" tire at 80 mph. You might find your driveshaft speed is above what the shop is balancing it at so it shows up fine to them.
Check out Inland Empire's driveline setup guide, has info on angles:
http://www.iedls.com/asp/admin/getFile.asp?RID=5&TID=28&FN=PDF

Chris.[/QUOTE]

Excellent point Chris. Balance speed is very critical and as far as I am concerned it should be one of the questions asked when a driveshaft gets balanced.

Todd

[rustomatic]

I've been having similar issues, along with springs settling in and slightly changing ride height as mileage builds on new stuff. My engine angle is not stock (right), but then I realized that my crummy balancer has a wobble--this changes at various (or to various) speeds. Now, I'm too lazy to put the new one on, until my new steering rack shows up...

Check the balancer, just to be sure, because if the rubber/poly is wearing, it will change activation speeds (for the wobble), depending on heat/usage/day.