Driveline angles: Is 0.5° really killing me?

[SkinnyG]

I have a mild vibration between 59 and 61 mph. It comes and goes. It is not tire or driveshaft balance. It is not consistent.

My drivetrain is 1.7° pointing down and my pinion is 1.2° up. My driveshaft is 0.7°.

I built it with a small discrepancy to account for bushing compliance.

If I air the truck up or down it does not change the vibration.

Is the 0.5° really causing all my problems?

[hewittca]

I'm interested to see what feedback you get on this. I am doubtful that a half degree difference in your driveshaft angle would cause a vibration. The angle probably changes by more than a half degree just going through normal suspension travels. I also have a similar problem with my truck. I have done countless tests changing wheels, tires, driveshaft angles, pinion angle, in gear, coasting, etc. Throughout all those tests I have had the same vibration that is sometimes there, sometimes not. I describe it as more of a wobble. Feels like it's in the rear but sometimes migrates to under my seat. The only thing that even changed the feel of the vibration was putting wider wheels on the rear of my truck. With those the vibration was more intense. My only conclusion so far is that I am just hitting a resonant frequency of my truck that I have introduced due to all the heavy modifications I have done to it. Currently, my only solution is to either drive above or below that mph window! If I get motivated enough, I will probably try swapping to poly bushings and maybe try to find stiffer springs or shocks.

[Keith Seymore]

Originally Posted by SkinnyG

Is the 0.5° really causing all my problems?

No.

[Keith Seymore]

Originally Posted by SkinnyG

I have a mild vibration between 59 and 61 mph. It comes and goes. It is not tire or driveshaft balance. It is not consistent.

How do you know it is not tire or driveshaft balance?

K

[SkinnyG]

I've balanced the tires, I've moved the tires on the rim, I've watched the tires on the balancer and on the vehicle at speed on the lift.

I'm also foolishly assuming the driveshaft I had made and balanced was actually balanced properly; I have no way of checking, other than I can physically see that it HAS been balanced.

The trans is rebuilt, with all new bushings. The axle bearings are new. I changed the poly trans mount to rubber.

The engine mounts are poly 4-link bushes, and pretty rigid.

The vibration feels a bit like a "wagging" from the back of the truck.

The fact that it's inconsistently cyclic makes me think it's driveline.

I've chased a similar "wagging" vibration for YEARS in my squarebody and never fully got it resolved.

[Short 3/4]

I’m going to be following this one closely, as I’m experiencing EXACTLY the same thing. Have done everything same as you, plus much much more. Have had two custom driveshafts made (very long story), plus everything else possible either replaced or rebuilt. Ironically it’s not my old abused Chevy having the problem, it’s smooth as glass. It’s a Toyota truck (oops, can I say the “T” word here without being thrown out for obscenity?) Being a 4wd I was able to take out the rear driveshaft and run it in front wheel drive. When the problem went away I first assumed driveshaft, but now I’m convinced it’s being transmitted rather than caused by the driveshaft.
Unfortunately I’m out of ideas, so hopefully I’ll once again learn something useful on this forum.
Good luck on your troubleshooting,
Tom

[Braunschweiger]

I've tried everything, with vibrations and noises.
I've now re-mounted and adjusted the differential, new axle shafts and new bearings,
it got better but I still had a slight resonance, now I've put the camper top on and the vibrations were back,
now I've checked the angles of the gearbox, drive shaft and differential and that was a disaster,
the Pontiac axle was welded in with a 2° negative,
I've now fitted 4°+ axle wedges, test drove today and I have a big grin on my face, it now drives very smoothly, up to 80 mph.
I'm very satisfied.

[Braunschweiger]

Double

[pdxhall]

I'm just throwing this out there. have you checked out the centering of the rear end? Are the rims exactly the same distance from the wheel wells?

[SkinnyG]

Keith, you might still be that genius!

Today, as I was tail-wagging my way to work, I thought to myself "what if it is actually the fronts causing all this..."

I need a few more commutes to confirm, but I rotated one front tire on the rim and balanced both, and the vibration appears gone.

Could my butt-dyno have been lying to me all along???

[SkinnyG]

Today's commute was very nice at 60mph.

There are other vibrations, vibrations that say "too much polyurethane," but not the hippy-shake-shake that had before re-balancing the fronts. Interesting.

We'll see tomorrow's commute.

[Keith Seymore]

Originally Posted by SkinnyG

Keith, you might still be that genius!

Today, as I was tail-wagging my way to work, I thought to myself "what if it is actually the fronts causing all this..."

I need a few more commutes to confirm, but I rotated one front tire on the rim and balanced both, and the vibration appears gone.

Could my butt-dyno have been lying to me all along???

Still?!

(lol)

All I know is - when you've tried everything and nothing worked it is time to go back and challenge some of those original assumptions.

K