weird vibration at speed 60 plus

[75chev4x4]

I have a 75 k20 with a 350/400 and have noticed a weird vibration when on the freeway. It is only at about 3k rpm so I'm guessing around 60 mph speedo doesn't work I get a vibration in the drivetrain it feels like. After 3k and up the vibration gets faster. It will vibrate for 1 second then stop for a half second then back again. It kind sounds like a train going by and as you go faster it does too. No vibration at all before 3k then it just starts. I have brought it up to 3k plus in park with no vibration so its not in the motor. Any suggestions?

[crazeetxn]

Check your tires. I would get a vibration going down the road that would shake a filling loose. Went to get the tires balanced and one of them was actually showing the belt.

Didn't really notice it till it got up on the rack. Once the tires were replaced, no vibrations.

[75chev4x4]

Yea was thinking that but I just had these ones balanced and they look to be in good shape.

[Keith Seymore]

When it comes and goes like that it's called a "boom/beat" - and it means there are TWO vibrations going on at the same time. They are slightly out of phase so when both amplitudes build then you hear/feel it and then when they go out of phase they cancel out and you no longer hear it.

So - you need to look for two vibration going on - like driveline + tires - and if you make one of those go away that should take care of it.

K

[Keith Seymore]

What's your rear axle ratio, and what size tires?

Is it something you hear, or something you feel?

If it is something you feel, where do you feel it (in the steering wheel, or in the seat)?

K

[75chev4x4]

4.10 gears and 265/75/16 tires. You can feel it in the whole cab floor boards not so much in the steering wheel.

[geezer#99]

U-joints! Or bent driveshaft! Or out of faze driveshaft! Or wrong pinion angle!

[75chev4x4]

Changed all the u joints. So would I need to have the driveshaft rebalanced? What does that cost usually?

[geezer#99]

Is your truck lifted from stock?
2 piece driveshaft or one piece?

[75chev4x4]

Lifted 2in from stock. Drive Shaft goes from the transfer case to the rear diff but its two pieces that slide together.

[75chev4x4]

Guess that's a one piece than

[geezer#99]

Did you change the pinion angle when it was lifted?
Sometimes you need to install some tapered shims between the springs and the spring saddles on the rearend.

[75chev4x4]

I just put the springs on to lift it didn't think two inches would effect it that much but I could be wrong.

[geezer#99]

It can if it was marginal in the first place. Find an angle finder like this link. Home Depot has them or some hardware stores.
http://sadoun.com/Sat/Products/Acces...nson-Level.htm

Here's a link to driveshaft phasing, pinion angles etc.
http://www.pirate4x4.com/tech/billav...ft/index2.html

A quick test to see if it's driveshaft related or pinion angle is to unbolt the ujoint at the rearend and rotate the shaft 180 degrees and bolt it back on. If it gets better then its driveshaft phasing/balance.
My truck had the same problem. New ujoints, checking the pinion angle, balancing the tire didn't help. I rotated the DShaft 180 and it cured it.

[75chev4x4]

Ok I will do that. Thank you for your help. I will post back results

[75chev4x4]

Hey guys I flipped the driveshaft and the vibration was less than before but still there. My question is if it is a driveshaft angle causing this from my lift how do I fix it?

[MadManAndrew]

Originally Posted by geezer#99

Sometimes you need to install some tapered shims between the springs and the spring saddles on the rearend.

Where to get them? I don't know.
Posted via Mobile Device

[75chev4x4]

Thanks mad man didn't even see that haha. I thought I saw on here where to get them.

[geezer#99]

http://www.summitracing.com/parts/sum-770025

[75chev4x4]

Ok so I got an angle finder and measured at my u joint at tcase, the driveshaft tube, and the rear ujoint. I'm not sure how to interperet these results maybe you guys can help. At tcase 7.5 degrees. Drive shaft 7 degrees, and rear pinion ujoint 7.5 degrees. These are all sloping down angles. Trying to find my vibration in the driveshaft. Thanks!

[geezer#99]

If you are indeed 7.5* down on the pinion then no wonder it vibrates.
Here's a link with some good info.
http://speedtalk.com/forum/viewtopic.php?f=19&t=28785

[75chev4x4]

Thanks for that geezer. I'm gonna have to read a few times though little over my head haha.

[75chev4x4]

Wow this stuff is confusing. So what pinion angle should I be looking for to get rid of this vibration?

[Keith Seymore]

Originally Posted by 75chev4x4

Wow this stuff is confusing. So what pinion angle should I be looking for to get rid of this vibration?

Less than three degrees (U joint working angle) and preferably so that the rear joint cancels with the front joint (equal and opposite).

K

[geezer#99]

You want the centre line of your motor and transmission the same as the centre line of your pinion.
If your tranny is pointed down 7.5* then your pinion should be pointed up 7.5*.
Most motor tranny set ups are only 3-4* down.
You got any pics of your setup?