Noticing vibration 25-35mph, what could cause this?

[Ancient4X4]

I first noticed this a couple of days ago.
The truck is a 1977 Chevy Bonanza. Original transmission, orig. mileage just under 101K. I'm getting a vibration while accelerating slowly. It comes on at about 25mph and stops about when I hit 35mph. Doesn't seem to have anything to do with gear changes. What could this be?

I actually hope it is something to do with the transmission funny as that sounds because it leaks and I was planning to take it in to have it rebuilt since they have to tear it apart anyways to fix the leak.

Any suggestions?
Thanks

 

[apache]

Id look at the U joints first,sounds like ones seen better days.

 

[British Slave]

Does it come back at 50 - 70 mph?

 

[K5dreamer]

is it 4x4? if so, put it in four wheel drive and see if the vibration lessens, if so, one of your rear u-joints is getting ready to vacate the building.

 

[howdiy]

My tires are out of round on my other truck and it does that

 

[4xcrazy]

i'm still runnin the winterized air in my tires, does this now and then, need to get around to inflating with summer air, not as stale.

 

[Ancient4X4]

Does it come back at 50 - 70 mph?

Yes! I was just going to post that it comes back at about 60 mph, I noticed that today as well. Well speedo shows 60, but it's actually closer to 65mph in reality, speedo is a bit off.

This is all in third gear, I haven't tried stomping on it yet to see if I could tell if there's any difference.

Guys, thanks for the advice, I'm going to take a serious look at the U-joints because I think that may be what it is.

It is a 4wd.

To be honest I think the rear U-joint has been going out since about 1985 according to my dad, it may finally be ready to let loose.

 

[Ancient4X4]

My tires are out of round on my other truck and it does that

I just about ran the treads off the last pair (could see metal hehe) and got a new set of Uniroyal Liberators about a year or two ago, or about 2,000-3,000mi ago, so I doubt that's the problem, but thanks for the suggestion, I will double check the tires just to make sure.

 

[Ancient4X4]

is it 4x4? if so, put it in four wheel drive and see if the vibration lessens, if so, one of your rear u-joints is getting ready to vacate the building.

It is the NP203 (full time unit), so will this make a difference? I will give it a try, please let me know what you think.

Thanks.

 

[K5dreamer]

ok, full time AWD is different than 4x4, so my advice is null. For a part time 4x4, if you have the vibration in 2wd, and you put it in 4wd, and the vibration lessens, its because putting it in 4wd takes load off the rear driveshaft and lessens the forces that create the vibration.

in my case i was already screwed by the time i did it. It wound up blowing out and the driveshaft came loose from the t-case and whacked the bottom of the truck a few times until i stopped. truck looked like an excited donky sitting on the side of the road. I could have just driven it home on the front driveshaft, but had it towed to the dealership to have the U-joints fixed.

Although.... you could remove the driveshaft from the rear and drive on the front shaft, and see if the vibration goes away. That would basically tell you if the rear U-joints are the problem.

 

[British Slave]

Yes! I was just going to post that it comes back at about 60 mph, I noticed that today as well. Well speedo shows 60, but it's actually closer to 65mph in reality, speedo is a bit off.

This is all in third gear, I haven't tried stomping on it yet to see if I could tell if there's any difference.

Guys, thanks for the advice, I'm going to take a serious look at the U-joints because I think that may be what it is.

It is a 4wd.

To be honest I think the rear U-joint has been going out since about 1985 according to my dad, it may finally be ready to let loose.

tire balance. A u joint will vibrate at all speeds, a tire out of balance will vibrate at it's critical rpm(speed), pass thru it's critical rpm(speed) and then return at 2X's that rpm(speed). And if your truck will do it, the vibration will return at 4X's it's critical rpm(speed) and continue to double till it fails.

 

[Ancient4X4]

tire balance. A u joint will vibrate at all speeds, a tire out of balance will vibrate at it's critical rpm(speed), pass thru it's critical rpm(speed) and then return at 2X's that rpm(speed). And if your truck will do it, the vibration will return at 4X's it's critical rpm(speed) and continue to double till it fails.

Hmm. Is it possible that tires could need re-balancing after only 2-3k miles of mostly in town driving?

I am in need of an alignment shop, as my steering is very sloppy.

I will check into this some more, maybe the rear u-joint is not bad after all. Thank you very much for the advice!

 

[smalltruckbigcid]

If you broke a belt in a tire it will do this too.