Inliners International Forums General Discussion Drive Train Rear axle, drive shaft mystery

Okay, I am, once again baffled and need some help with a very obvious vibration after I put the rear axle back in.
From the beginning... I had a Dana 35with a 3:54 gear in it. I had it changed to a 4:56 at a local axle shop to better utilize the capabilities of the overdrive, available power and tire size.

Before, no vibration while running on jack stands. After, serious vibration which shakes the the axle at the pinion end up and down about 1/4 to 3/8 inch in an arc, so to speak at a very low speed. (say 10 mph or so on the speedometer)

U joints are (were) good before this. I have removed the drive shaft (again)and double checked the angles at the yokes at the pinion (91 degrees) and at the transmission (89 degrees) the driveshaft is at 5 degrees at ride height. The drive shaft is in phase (both yokes pointed the same direction and checked with an inclinometer on the bench)

While under the truck I can spin the driveshaft by hand and there is no unusual movement, no slack anywhere, no up and down of the pinion yoke. The drive shaft has a slip joint in it, but the length is the same as it was before and it hasn't been a problem prior to this.

No noise from the axle when running. Ran it without the wheels and tires and no noticeable vibration. Checked the U bolts and they are torqued to spec. Put the front tires on the axle (smaller size and lighter and they vibrate less than the rears tires did but more than running with just the brake drums.

Tires? about 20 miles total since new and mounted/ balanced.

The torque at the pinion yoke is in spec and I'm inclined to think that the axle is assembled okay, but short of taking it out again I am going on faith at this point.

The heavier tires seem to magnify what ever is happening but I'm not ready to jump to the conclusion that they are the root cause.

When it's in gear and the tires are turning...driver side turns at constant speed with engine speed. Passenger side moved in a sort of coasting manner and in a rather jerky, start -coast-stop-start type of motion but at a much slower pace (obviously, it's not the driven wheel)

So there you have it...That's as much information as I can muster...
Any ideas?

Thanks:
Paul

Problem with the passenger side brakes, perhaps?

Thanks for the suggestion. I'm out of town right now but I'll check into that when I get back. I doubt that's the case but I'm up for anything at the moment.

Thanks for the input
Paul

Hi Xerxes . . .

The oscillation on the pinion end of pumpkin suggests a slight axle wrap twisting the pinion end up against the leafs and then rebounding off the springs. Stroker may be on to something in terms of drag from the brakes causing the axle to start to wrap in the first place.

I haven't got my head around why the larger tires would magnify this effect. But I am also puzzling about the impact of running the drive line while up on jack stands without any weight on the wheels.

You mention that it only happens at low rpm. So it smooths out as RPM increases?

regards,
stock49

Stock49:
No, I meant it starts at low rpm..it gets worse the faster it turns. I would expect it to start at a much higher speed if it were due to incorrect pinion angle. I also checked to see it the pinion yoke was bent, and it seems to be straight. Also, if I rotate the driveshaft by hand there is no indication that anything it out of alignment. The brake possibility seems remote to me, although I'm going to double check them...but I can easily rotate each wheel by hand without any noticeable, or unusual resistance.

At any rate, I'm back in town now. Tires go to the shop to get balanced first thing tomorrow and then it goes for a ride around the neighborhood. Win or lose it goes on the road tomorrow. I hope I don't break anything expensive.

Thanks:
Paul

Bent rim? Also, had a van that did something similar, the U-joint was binding.

Thanks all for the suggestions. I had the wheels/tires balanced. They were new tires with about 20 miles or so since I bought them and had them balanced... Oddly they were both out of balance. One by 3/4 ounce and the second by more than that... I'm thinking that the folks at store #1 didn't do such a good job... Anyway, I put them on, and gave it a test drive..No vibration, noise, shaking... Test speed was up to 30mph or so...Did a couple of engine idle coasts and nothing.

For now, I'm calling it good. Now to get the engine up to snuff..

Thanks again
Paul