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60mph+ vibration... DESPERATE!!

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Old 06-09-2009, 02:34 PM   #1
desdain
 
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Default 60mph+ vibration... DESPERATE!!

Okay, here is my setup and problem. I have a 1966 Mustang fastback with a 347 stroker, C4 transmission, stock driveshaft, 8 inch rear axle with posi, original leafsprings with slide-o-link traction bars, and Wilwood disc brakes.

When driving at speeds of 60mph or higher, there is a severe cyclical reverberating hum that rattles and vibrates the entire car. It's cycles in and out about once a second, faster at higher speeds. I have put the car in neutral while rolling downhill and the hum is still there, but it seems to become more constant rather than cyclical then. This leads me to believe it's a problem with the drivesystem.

Here is what I have done so far:

- Set the angle between the engine and rear axle by adding wedges between the axle and leafsprings (they were off by 4 degrees, but now they are exactly parallel)
- Replaced the rear bushing in the transmission (the slip yolk had some play in it, but now there is very little play)
- Straightened and milled the axle shafts (when the car was on stands and wheels rolling, there was severe wobbling in the wheels. This wobble is now gone).
- Reduced driveshaft wobble (there was a 1.5mm wobble in the driveshaft caused by bad rear u-joint mounting holes, but this has now been reduced to 50 microns by positioning the u-joint correctly with some tack welds. Yeah, I know this is a no no, but it was our only option. The driveshaft itself is quite straight)

Things I have not done yet:
- Balanced the rear wheels
- Replaced the transmission mounting bracket
- Tried a different driveshaft
- Removed the traction bars
- Two sticks of dynamite up the tailpipes ;-)

I'm getting really frustrated by this as we keep fixing all these different issues, but nothing seems to give any results. Now I'm hoping that someone here can point me in a direction that might give me some results. I have searched the forum for similar posts, but none of the ones I have found have helped so far.

Thanks a lot!
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Old 06-09-2009, 03:14 PM   #2
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Run your hand along the side of the driveshaft. Remember, think of it as a straw. Any dents in it will cause an out of balance condition. Had a friend the hit a box in the road and it dented the shaft just enough to get a vib around 50 mph. Good luck.
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Old 06-09-2009, 03:54 PM   #3
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Spin balance the wheels. You may have an out of round tire or two.
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Old 06-09-2009, 06:13 PM   #4
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Quote:
Originally Posted by desdain View Post
- Straightened and milled the axle shafts (when the car was on stands and wheels rolling, there was severe wobbling in the wheels. This wobble is now gone).
Did you jump the car off a ramp trying to play Dukes of Hazzard?

If it was dropped hard enough to bend the axles, the bearings might have dimples in the races. that could cause your symptoms. the axle housing itself might even be bent.
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Old 06-09-2009, 08:19 PM   #5
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You need to get the driveshaft balanced.
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Old 06-09-2009, 08:21 PM   #6
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u joints?
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Old 06-10-2009, 12:15 AM   #7
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Well, I did hit a really big hole in the road in a construction zone last year. They had cut a 2 ft section out of the road and no markings. Didn't see it until I was in it basically and it really hit my rear end hard. I'm suspecting that's what messed up my axle shafts, but I can't be sure.
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Old 06-10-2009, 02:56 AM   #8
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u joints?
Front u-joint is new and rear u-joint checked to be okay. This is the one we tack welded to be able to center the shaft.
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Old 06-10-2009, 07:18 AM   #9
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maybe your axles are shot after that pot hole hit
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Old 06-10-2009, 07:39 AM   #10
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The first most common thing that causes vibration at 60-70mph is tire balancing. Have your tires road balanced if possible or at least spin balanced. After they are known to be good, think driveshaft balance, replace axle bearings, check for proper driveshaft slip on the tranny yoke, check for proper tranny yoke fitment (too much play will cause vibration...ask Gun Jam), check engine and tranny mounts, and check exhaust mounts (they too can cause harmonic vibrations).
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Old 06-10-2009, 02:42 PM   #11
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Thanks for your comments. I'll post the results as they come in...
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Old 06-11-2009, 11:13 AM   #12
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Okay, I got the wheels balanced and it seemed like the guy had to add a lot of weights to them. Anyway, the balancer showed perfect and I took the car out for a test drive. Long story short, no fix. If anything it seemed like it started vibrating a little earlier (at 55mph).

Next I'm gonna put it up on jack stands run it up to 60mph on the speedo and then start removing stuff until it stops vibrating (first wheels, then discs, then drive shaft, etc.). With my luck this is something that only happens when it under load :-(

Anyway, keep the suggestions coming.

Last edited by desdain; 06-11-2009 at 11:17 AM.
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Old 06-11-2009, 11:28 AM   #13
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I had a bent axle once...

Its easy to check for take the wheel and drum off and placed a fixed indicator of some type so the point just touches the flange. A center punch C clamped to a solid free standing object works well.

Rotate the axle and watch the point if the axle flange runs into or away from the point you have a bent axle...I could clearly see the runout on the axle using this method...it was .009" out of round and was very noticeable. I bet this could be your issue.

-Gun
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Old 06-11-2009, 11:35 AM   #14
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My father had an old truck (1976 F150) that had a nasty vibration to it from 45-65mph, it was a failing Ujoint. It might be worth it to look at them.
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Old 06-11-2009, 01:15 PM   #15
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Quote:
Originally Posted by Gun Jam View Post
I had a bent axle once...

Its easy to check for take the wheel and drum off and placed a fixed indicator of some type so the point just touches the flange. A center punch C clamped to a solid free standing object works well.

Rotate the axle and watch the point if the axle flange runs into or away from the point you have a bent axle...I could clearly see the runout on the axle using this method...it was .009" out of round and was very noticeable. I bet this could be your issue.

-Gun
Quite possible, they were pretty bad, but we were able to fix them to the point of ~no wobble. It's down to 0.05mm now, but I'll probably have to replace them during this coming winter, I just want to get is "pretty good" for this summer.
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Old 06-11-2009, 01:18 PM   #16
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Quote:
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My father had an old truck (1976 F150) that had a nasty vibration to it from 45-65mph, it was a failing Ujoint. It might be worth it to look at them.
Yeah, this could also be an issue. The front u-joint is new, but the rear is the old one. It looked okay though, and now that it's been tacked into place, it's hard to replace. I think I need to borrow a good driveshaft from someone and try it before buying a new one.
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Old 06-11-2009, 01:59 PM   #17
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wait....what?

You were able to fix them? as in the axles were bent and you made them straight?

how many mm did they run out before you fixed them?

How did you fix them?

-Gun

P.s a 1/2 mm is .0196" runout it was pretty bad at .009" (inch) in my case and .002" is maximum allowable

Reality check!!! you are 194 thousandths above maximum allowable runout
dont mess around replace both axles and bearings.
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Old 06-11-2009, 02:04 PM   #18
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on my 6 banger one of them was out of whack. Extremely hard to find replacements. threw the sucker on a lathe and made them a bit better. Obviously difficult if you dont have the machines or tools
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Old 06-11-2009, 02:12 PM   #19
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just to make sure you fully appreciate the axle situation let me state it this way...



you are 9700% above tolerable upper runout limit

I hope I have reached you

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Old 06-11-2009, 02:35 PM   #20
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wait....what?

You were able to fix them? as in the axles were bent and you made them straight?

how many mm did they run out before you fixed them?

How did you fix them?

-Gun

P.s a 1/2 mm is .0196" runout it was pretty bad at .009" (inch) in my case and .002" is maximum allowable

Reality check!!! you are 194 thousandths above maximum allowable runout
dont mess around replace both axles and bearings.
We quickly heated up the "concave" side of shaft and it pulled it self more or less straight, but not enough heat to compromise the integrity of the steel. Then we also milled the flanges straight.

Btw. it's 0.05mm not 1/2mm, so that's 0.00196'', which is less than the maximum allowable 0.002'' you mentioned.

Thanks for the continous feedback. It's really appreciated.

Last edited by desdain; 06-11-2009 at 02:41 PM.
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Old 06-11-2009, 02:35 PM
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03, 1967, 50mph, 60, axle, c4, driveshaft, ford, mustang, shaft, t5, trans, transmission, vibration, wc, wobble

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