I've had my car for 23 years now. For a good part of that, I've fought vibration...but only the hwy variety. I've had at least 4 sets of tires and 2 sets were road-balanced. Because tires wear so fast, I've never sprung for the high-dollar variety. Set 1 (original Gatorbacks), Set 2 (Nitto 500's), Set's 3 and 4 Sumi HTR-Zs. (Actually, I'm on the 5th rear set :-))

My point is there have been times where shake, shutter, and vibes have been low -- but I can't remember if/when they were non-existent? MAYBE from 2005-2007? With the last set (installed 3-4 years ago), road-force got it pretty darn smooth. But, I rarely drove over 70...the speed limit around here. Since Covid, the highways have turned into the "Old West". Cars regularly scream by at 80-90mph...even higher. As such, I get more opportunity to observe vibration(s) at 70+ mph.

I am convinced I have a drivetrain vibration. Since my motor build in 2010, I've chased after everything suspension related. New bearings, tires, half-shaft u-joints, bushings, etc... It's never been PERFECT above 70mph. I even questioned the crankshaft balancing method in SpeedTalk. Here's why....

If I'm driving along at 75mph...an oscillating vibration goes in/out. If I push the clutch in...it disappears. It's repeatable. Until recently, that ONLY happened above 70mph and in 6th. This week, I finally felt it as I slowed from 70mph in 5th! It didn't quit until 60mph. One thing I NEVER replaced was the front u-joint. (The rear was bad in 2010). OTOH, I have had the driveshaft out 2-3 times. Problem is I didn't mark it AND I can't tell for sure if/what the factory marks mean. It MIGHT be a bit worse as currently oriented?

Looking through threads, I've failed to understand the significance of orientation. Or..what the (rubber?) balancer does on the yoke? Or...if it matters since some have knocked it off. Maybe the issue is like "Whack a mole" or maybe I need to send my driveshaft to a nearby driveline shop? They specialize in trucks though -- so I'm not sure of their Corvette "expertise". They DID balance and cure my SUV DS problem 10yrs ago. So, they seem good. (Inland Truck BTW).

Can anyone fill me in on special considerations for checking balance...to pass onto this type of shop? Also, what u-joints are best -- if I need new ones (for a 400wtq/400whp application)? Or balancer "replacement"? Finally, does this sound like a DS/u-joint symptom? I don't hear any odd DS-like noises but I do have a modestly aggressive exhaust (sidepipes).

(BTW...Someone else I know suggested motor mounts too...but I don't see any "radical" tilt in the motor when I rev it. Gotta say I considered it though...especially when you see how much it needs to be tilted for r/r! On a side note: I'm toying with the idea of a bigger cam. Anyone think it will damage 12-yr-old rubber mounts to tilt enough for the cam to clear the front frame brace? They were replaced in 2010).

Thanks in advance.

 

Check the front u-joint and also the tail shaft bushing in the transmission.

 

If the vibration stops when you push the clutch in, it's either U-joints or it's not the drivetrain. The torque through the drivetrain can push a bad U-joint off the driveshaft's (or halfshaft's) axis of rotation and cause a vibration; then, when you let off the gas or push the clutch in the joint pops back onto axis and the vibration goes away. However, if the vibration doesn't also disappear when you just let off the gas but keep the clutch pushed in, then it's probably not a U-joint at all.

If the vibration only disappears with the clutch disengaged, regardless of throttle setting, it's probably something forward of the transmission input shaft that is slowing down when you push in the clutch. It should be repeatable regardless of gear or vehicle speed. If that's the case, I'd be checking the balancer as well as the flywheel for balance.

 

just in case I missed this - but what year is your car, how many miles on it, coupe or vert, auto or manual, and what are the mods.

 

Until this past week, I've never felt it except in 6th. Pushing in the clutch or letting off the gas makes it stop. I've never felt it in any other gear UNTIL last week. Finally, it did it in 5th at higher rpm (2500). Prior to that, it seemed more prevalent around 1600-1700 rpms....though I COULD say (like a imbalanced tire) it smoothed out quite a bit ... above that. Sometimes it seems worse than others. For example not as bad going up/down hill versus level. It's always seemed worse when cold.

I considered tires heating up and getting firmer, but multiple sets? Maybe the u-joint swells with heat and doesn't "wobble" as much when warm?

 

89 Coupe with camber brace across front of engine (brace targeted frame stability due to this longterm issue)

I've been hit twice: 1) FL upper a-arm mount bent and restored via frame machine 2) RR stabilizer bar bent and replaced (all wheel bearings replaced as precaution). Everything aligns.

Engine: 383 stroker, active side-pipe system, mega-ported SLP intake, 214/214 cam .0544/.544 112LSA on 108ICL. 4-2-1 headers ZF6 using Short-throw shifter with hard-ball (increases feel for vibration) 315/35 rears on A-molds (FW replaced with new one during build. ZF6 doc supplied upgraded TO bushing)

I think other mods/updates are in OP.

 

Oh, okay. If the vibration also stops when you let off the gas, then I'd definitely be looking at the U-joints. If there is no change of direction with gas on vs gas off (i.e. you don't have to correct the steering), then you should pay extra attention to the center/main driveshaft rather than the halfshafts. I'm not sure why temps would have anything to do with it, but maybe when it's colder the tires and rubber bushings are less compliant and therefore transmitting more of the vibration into the frame?

 

Yeah...my real question is how to distinguish between DA and RA vibrations. Because any outright "wobble" has only been observed in 6th (meaning higher DS RPMs), I've been leaning toward DS. The appearance in 5th (at a higher rpm) means a u-joint is getting worse OR it's RA related. I'm hoping it's an issue with the DS though I started with a discussion on my RA balance technique in Speedtalk a few years back.

My main goal is to understand balance issues beyond just u-joints IF I take the shaft to Inland Truck for balancing. As I mentioned in the OP, I want to understand the role of orientation and the rear "dampener" in the scheme of things. I don't know that a truck driveline specialty shop will know everything Corvette related -- though I think they are very good. Maybe at least one of these "issues" (damper/orientation) affects balancing technique?

I wasn't aggressive enough when learning the ins/outs of RA balancing years ago. Trying not to overlook something again.

 

Mine was toast. Check for scoring or marks on the yoke input.
The vibration for that will be speed sensitive.
Not sure why, but, nearly every mechanic was dismissive of that possibility.
Always, tires, joints, mystery....
Just a bushing.

 

Do you guys have a picture of the tail shaft bushing?

I have some vibrations under hard acceleration and would like to locate it.

Thanks

 

https://www.transmissionpartsdistrib...s-82-gmt-3-2t/