Hello,

I am using the front and rear suspension from a C4 in a project car that I built a couple years ago. I think I may have setup my rear suspension incorrectly when I built the car, so I was wondering if someone could help me out with something…

Looking down from the top of the rear diff/axle shafts, should the halfshafts be directly in line with the differential stubs, or should they be at a slight angle? In other words, should the wheel centerline / differential / opposite wheel centerline be a straight line, or should it form a slight “V”? I’ve looked online and can’t seem to find this info anywhere.

Appreciate your help.

Thanks.

 

At an angle. Covered elsewhere in this forum a few months ago, so directly relevant to your question.

https://www.corvetteforum.com/forums...uspension.html

 

That’s exactly what I was looking for. Never seen that diagram before. So yes, they do form a backwards “V”. I thought it should, but I believe I have a little too much “V” in my setup. Reason is, I have a minor vibration at highway speed, and I’m thinking it might be related to my u-joints having a little too much rearward angle. Does anyone know what the distance between differential centerline and wheel centerline should be?

And just to get ahead of the curve…ALL the u-joints in the entire driveline are new, the rear wheel bearings are new, and the Dana 44 has been rebuilt with all new bearings. Tires/wheels are balanced.

Thanks again.

 

Check driveshaft working angles.

 

If u-joints are aligned correctly on the shaft then equal angles on both sides will cancel out each other. So massive camber or toe could give vibration but moving the wheel forward / reverse or up / down should not affect vibration.

Same goes for the driveshaft. Yoke on translission and yoke on rear end should be parallell so both ends have the same angle. U-joints on driveshaft must be rotated to the same orientation.

 

I kind of doubt it's the V angle causing your vibration: think about the serious angles the halfshaft U-joints have to negotiate when the car goes over bumps, or leans in a corner, squats on acceleration, or lifts during hard braking. Also, when the suspension moves, the knuckle and wheel move in an arc dictated by the short trailing arms and so that V angle is always changing (generally getting shorter compared to their resting location).

The way this V angle would be off is if you didn't locate the trailing arm mounts on the frame at the correct location. If you want to verify the V angle, measure the distance between a line drawn through the two upper or lower trailing arm pivots (frame side) to a point on the diff snout. Compare that measurement to a stock Corvette, and you'll know how close you are to what GM engineered. If it's actually off by a significant amount, it would probably be easier to buy/build tubular links with threaded rod ends (e.g. Banski) so you could tailor and adjust the trailing arm lengths correctly.

Honestly, my guess is that you either have a tire that still isn't balanced correctly (not all shops get this correct) or one tire or wheel is perhaps out of round or has some other defect that allows it to cause a vibration even if it's balancing out. It might be helpful to take a set of rear C4 wheels and tires that are known to not vibrate and swap them on and see if that solves it. Then you'd know.

 

Since this is an 'other than C4 build' you start over and confirm all of your 'drive-line' alignment. How is the differential pinion mounted? How do you know you have no 'balance' issues? 6 new u-joints don't do any confirmation other than they're thought NOT to be an issue.

I agree with MM and I doubt what you call 'V' is at fault but you claim you have likely 'too much' - reason enough to start over and confirm what your actual issue might be.

BALANCE? More likely particularly since you mention 'minor'

 

Driveshaft working angles are correct. I’ve checked it multiple times, and it’s well within recommended angles, and u-joints are aligned correctly on the driveshaft.

I’ve checked tires/wheels for roundness, and they are good.

Honestly, it sounds and feels like a bad wheel bearing, but both rears are new. It has a minor, low pitched whoomp - whoomp sound at about 60 mph. I’m having my driveshaft shortened due to a new overdrive trans, so I’ll make sure it’s balanced as well. May be something as simple as the driveshaft itself is slightly out of balance, or a bad wheel bearing.

Thanks for the suggestions.

 

That sounds to me like tire/wheel balance more than anything else, as Matthew and WV have said. I would go to a shop that does Road Force balancing where the tire is under load and then you can see if you have any issues with the tire plies or construction. Just static and dynamic balance without road force doesn't replicate what the tire does on the actual road accurately. You'd be surprised how much a tire that's "in balance" can show out of balance forces (16-18lbs. or more) on a road force machine.

 

This is the 'balance' I was questioning! You could take some clamps and see if you can tweak the condition before you do the trans change. If the drive shaft wasn't balanced initially that was foolish. Are the 2 shafts to the wheels modified in any way or not? With clamps you can make it worse for sure and that would indicate that's likely your issue.

 

Halfshafts are not modified in any way

 

Is there a weight on the drive shaft? Is the driveshaft modified now or not? Use clamps and see if you can 'help OR make worse' the issue!

 

If you can get access to a "reed tach" or electronic vibration analyzer (EVA), using Htz reading and chart should isolate where to look. GM dealer should have a EVA.

 

Am i the only one that wants to see pics?

 

There are many 'preliminary' checks that need resolved before someone should consider use of EVA. Many 'drive-line' shops that do serious 'drive-line' also use and are familiar with EVA, large fleets also. An interesting read maybe and dependent upon location there's possibilities that are available.

https://vibratesoftware.com

 

That's wheel/tire. Drive shaft is going to be a MUCH higher frequency, somewhere around 25hz. or so, depending on rear axle ratio. But an out of balance drive shaft will "thrum" like a low bass note. Wheel/tire will give you that "whoomp - whoomp" sound that I see you describing.

I'd love to see pics too.

 

It does 'seem' likely but it would also seem that assuming the OP's knowledge that would have been considered, OP avoided any mention of tire/wheel except 'roundness'! Cheap tires?

Road Force as suggested earlier cost $$$$. If it doesn't correct or confirms a tire issue it get's more expensive. Clamps on the drive shaft to make 'worse' or 'better' is 'CHEAP'!

 

I agree....I've done the clamps on the DS and it works...just doesn't sound like the OP's issue. But maybe. And it is cheap/easy.

 

[QUOTE=WVZR-1;1604569046]There are many 'preliminary' checks that need resolved before someone should consider use of EVA. Many 'drive-line' shops that do serious 'drive-line' also use and are familiar with EVA, large fleets also. An interesting read maybe and dependent upon location there's possibilities that are available.


true regarding preliminary checks, but when someone claims to have done all of them. You have to try a different approach. Vibration correction can get tedious at times, especially without the right equipment. There are alot of unknowns with this thread.

 

But the OP hasn't actually 'confirmed' doing any!

I imagine the OP has actually previously seen the 'images' in the link provided very early in the thread or he wouldn't have been able to claim his was 'exaggerated/extreme'. You're certainly correct - there's much missing!