Ujoint play on Corvette suspension

[John]

Dave,
I was interested in your comments on the combination of Ujoints and CV joints on the same half shaft. I get a shudder when I accelerate from about 2K to 4K - it only happens is 2nd gear and higher gears - it does not happen in 1st gear. If the Ujoint/CV Joint is a bad design, what kind of problems could I expect? If that combination is bad, wouldn't I experience a problem in all gears?

John
epper:

 

[Dave Bilyk]

Hi John,
yes, an interesting question.
On its own, the effect will be quite small, but in speed ranges where it excites torsional resonance in the drive train, it will be magnified and show up as a vibration. If that is what your symptom is, you might find that it only shows up over a small speed range, and that speed range might even be different for each gear, or it might show up in some gears but not others. The effect will also be very small if the UJ angle is small (parallel input / output) and so could be affected by how hard you accelerate because the rear squat changes the angle. I am by no means sure that this is the actual cause, it would take vibration and torque measurements to diagnose with certainty, but it is certainly possible.

What is the drive shaft angle or articulation angle of the UJ, is your transaxle output centre line higher than the shaft? I am thinking that if the vibration is caused by the single UJ, the effect would be minimised if the the transaxle output centreline is slightly higher than the wheel centreline - so that under acceleration squat the UJ angle tends to zero.
Conversely, if the transaxle output shaft centreline is lower than the wheel centreline, the effect would get worse under acceleration as the UJ angle would increase. So what does your drive shaft do, straight, slope up to the wheels, or down to the wheels?

Dave

 

[John]

Hi John,
yes, an interesting question.
On its own, the effect will be quite small, but in speed ranges where it excites torsional resonance in the drive train, it will be magnified and show up as a vibration. If that is what your symptom is, you might find that it only shows up over a small speed range, and that speed range might even be different for each gear, or it might show up in some gears but not others. The effect will also be very small if the UJ angle is small (parallel input / output) and so could be affected by how hard you accelerate because the rear squat changes the angle. I am by no means sure that this is the actual cause, it would take vibration and torque measurements to diagnose with certainty, but it is certainly possible.

What is the drive shaft angle or articulation angle of the UJ, is your transaxle output centre line higher than the shaft? I am thinking that if the vibration is caused by the single UJ, the effect would be minimised if the the transaxle output centreline is slightly higher than the wheel centreline - so that under acceleration squat the UJ angle tends to zero.
Conversely, if the transaxle output shaft centreline is lower than the wheel centreline, the effect would get worse under acceleration as the UJ angle would increase. So what does your drive shaft do, straight, slope up to the wheels, or down to the wheels?

Dave

Dave,

My transaxle centerline is lower than the wheel centerline. Also the CV joints are on the transaxle side (930 Porsche) and the u-joints are on the Corvette hub side. I doubt whether much squatting is going on under acceleration as the shudder normally occurs when I'm driving about 30-40 mph and accelerating in 2nd gear. Also, the suspension is rather stiff so there's probably not a lot of squatting and increasing the ujoint angle. The drive shaft slopes up to the wheels.

I solved the ujoint play problem by replacing it with a "genuine GM part" but the shuddering still persists!

John

John
epper:

 

[Randy V]

Dave,

My transaxle centerline is lower than the wheel centerline. Also the CV joints are on the transaxle side (930 Porsche) and the u-joints are on the Corvette hub side. I doubt whether much squatting is going on under acceleration as the shudder normally occurs when I'm driving about 30-40 mph and accelerating in 2nd gear. Also, the suspension is rather stiff so there's probably not a lot of squatting and increasing the ujoint angle. The drive shaft slopes up to the wheels.

I solved the ujoint play problem by replacing it with a "genuine GM part" but the shuddering still persists!

John

John

Presuming you have adjustable coil-over shocks like the rest of us - try this experiment -

Put a hose-clamp around the shock body up against the adjuster ring. This will allow you to change the ride height and to return precisely back to where you are today.

Lift the chassis to unload the wheels and adjust the springs tighter by 1" (this will raise the rear of the car).

Go for a ride and see if the vibration has change (increased / reduced) and if the speed has changed.

If my guess is right, the speed at which the vibration comes will be higher and the frequency will be higher as well - but reduced overall..

What to do if this is the case? Perhaps elevating the transaxle? Is the transaxle inverted?

I would probably look into how to implement outboard CV joints..

 

[John]

Presuming you have adjustable coil-over shocks like the rest of us - try this experiment -

Put a hose-clamp around the shock body up against the adjuster ring. This will allow you to change the ride height and to return precisely back to where you are today.

Lift the chassis to unload the wheels and adjust the springs tighter by 1" (this will raise the rear of the car).

Go for a ride and see if the vibration has change (increased / reduced) and if the speed has changed.

If my guess is right, the speed at which the vibration comes will be higher and the frequency will be higher as well - but reduced overall..

What to do if this is the case? Perhaps elevating the transaxle? Is the transaxle inverted?

I would probably look into how to implement outboard CV joints..

Randy,

It's funny that you would suggest this - after digesting Dave's thoughts on the harmonics introduced by the CV Joint/Ujoint combination - I thought to myself that if Dave is right, then adjusting the shock height or tension should cause the the vibration to dampen or occur at a different RPM. This is precisely the test you are suggesting!

I have Hal-Proactive shocks that have dampening settings. Before adjusting the ride height, I'm going to try changing the dampening settings to see if that affects the shuddering. If that does not affect, then I'll try the ride height change per your suggestion.

While I am not a auto engineer, I am a scientist and had one thought on Dave's assessment, as I have ujoints on the end of each drive axle, would not the drive assembly behave as a straight angle and the ujoint motion on each end cancel the effects of the U-joint/CV joint combination?

Thanks to both you and Dave for your thoughtful suggestions!

John
epper:

 

[John]

Randy,

It's funny that you would suggest this - after digesting Dave's thoughts on the harmonics introduced by the CV Joint/Ujoint combination - I thought to myself that if Dave is right, then adjusting the shock height or tension should cause the the vibration to dampen or occur at a different RPM. This is precisely the test you are suggesting!

I have Hal-Proactive shocks that have dampening settings. Before adjusting the ride height, I'm going to try changing the dampening settings to see if that affects the shuddering. If that does not affect, then I'll try the ride height change per your suggestion.

While I am not a auto engineer, I am a scientist and had one thought on Dave's assessment, as I have ujoints on the end of each drive axle, would not the drive assembly behave as a straight axle and the ujoint motion on each end cancel the effects of the U-joint/CV joint combination?

Thanks to both you and Dave for your thoughtful suggestions!

John
epper:

I CORRECTED ONE PARAGRAPH:

While I am not a auto engineer, I am a scientist and had one thought on Dave's assessment, as I have ujoints on the end of each drive axle, would not the drive assembly behave as a straight axle and the ujoint motion on each end cancel the effects of the U-joint/CV joint combination?

John

 

[Dave Bilyk]

John,

agree with Randy and your ideas. Increasing the ride height will bring the shafts more level since the transaxle is lower than the wheel centreline. If you can get the shafts level that would be good. If it changes the effect, then we may have a diagnosis.

While I am not a auto engineer, I am a scientist and had one thought on Dave's assessment, as I have ujoints on the end of each drive axle, would not the drive assembly behave as a straight axle and the ujoint motion on each end cancel the effects of the U-joint/CV joint combination?

It would if you were driving from one wheel to the other, but you are driving from a centre differential, so something has to give to accommodate the non uniform motion.

Dave