Fox Drive line vibration at specific speed

[93CalypsoConvert]

I'm getting some vibration at 70-76 mph. Absolutely no vibration above or below these speeds. I can hear it and feel it on my shifter. It is a very fast vibration, so its unlikely to be a tire or brake. It is still present with clutch in. The vibration is so fast, the numbers on my shift stick look blurry. U-joints are in great shape. Any ideas?

 

[General karthief]

Drive shaft balance?
Or rather imbalance

 

[Mustang5L5]

It might be driveshaft critical speed is being reached.

This is actually what the dogbone on the rear axle that everyone removes is for. Of course, if you are feeling it that badly, there's something more than what the weight would solve going on.


I would rule out other stuff first.

 

[93CalypsoConvert]

Could it be drive line angle? The entire drive train was swapped by previous owner, so who knows what may or may not be correct under there.

 

[Mustang5L5]

Yes, driveline angle could play a role here as well

 

[AeroCoupe]

With the clutch pedal depressed and the transmission in gear the vibration could be from the tires all the way through the drive train and up to and including the input shaft of the transmission in all gears except 4th. In 4th the counter shaft will not be turning.

With the clutch pedal depressed and the transmission in neutral the vibration could be from the tires all the way though the drive train and up and including to the output shaft of the transmission. The input shaft and counter shaft will not be rotating.

Hopefully I thought the two statements though correctly. Question I have is the vibration present with the clutch pedal in and the transmission in gear and/or in neutral?

Reason I asked is I chased a vibration in the T-Bird for years after I swapped a completely different drive train into the car. New motor, new Tremec TKO, new aluminum driveshaft, and new to me and completely rebuilt from brake to brake 8.8 rear end. After all was said and done it ended up being the TKO input was not shimmed correctly from TREMEC. Problem on this one was a specific speed range and was cyclical like a washing machine.

Had the u-joints replaced in the aluminum Motorsport drive shaft in the Mustang and it vibrated so I thought the shop that did it F'd it up and they were the only drive train place around. I bought a new drive shaft and the problem when away. Years later and after two moves I had the old shaft checked and all it ended up being was the trans yolk needing to be rotated 180 degrees. Vibration on this one started at 60 mph and just got worse.

Trying to isolate things to that you can trouble shoot a vibration at speed is a pain in the ass but you need to work through it methodically and one thing at a time. Check the pinion angle first as that is easy and doesn't take a lot of time or cost money unless you find a problem. If the wheels and tires have been balanced at one shop take it to another shop for verification.

 

[93CalypsoConvert]

I'm going to check my drive line angle when I have time. I believe the vibration is coming from the driveshaft, transmission, or drive angle. At 75 mph my drive shaft is spinning at around 3700 rpm, and I feel like the vibration is certainly that fast.

 

[AeroCoupe]

Steel or aluminum DS and what rear end gears? Reason I ask is due to what Mike eluded to in that it could be a critical speed issue. Typically you see Mustangs moving to an aluminum DS when the rear gears are swapped to 3.55 and lower gears. Again, I said typically as it varies from car to car depending on max rpm, tire height, trans gearing, rear end gearing, and the length of the driveshaft.

In case someone is not familiar with critical speed here is the mechanical definition:

An object is said to reach critical speed when the speed of its rotation corresponds to one of its natural frequencies. The natural frequency will vary with the type of material (steel, aluminum, carbon fiber, etc.), diameter, and length.

Modern DriveLine has a really good explanation of all of this here:

TECH TALK: A LITTLE INFORMATION ABOUT DRIVELINE VIBRATION - Modern Driveline

Common causes of driveline vibration-U-joints, slip splines, yokes, out-of-balance components.

www.moderndriveline.com

There is an equation to calculate the critical speed of your combination and then a chart that shows the critical speed of a driveshaft based on the material of construction, diameter, and length. I would look at this and make sure that driveshaft that is currently in the car does not have a critical speed that is lower than what is needed in all forward gears.

Check the pinion angle and if that is good to go then I would do these calcs and determine whether the driveshaft is good to go or not. If it is good to go then take it to a driveline shop and have them spin it up and see if it is out of balance, has bad u-joints, or a bad yolk.

 

[93CalypsoConvert]

Stock DS from 1985 GT, 3.73 rear end. Looks like I'm safe from critical speed.

 

[AeroCoupe]

Mine sucked with the stock DS and 3.73’s. Tossed a Motorsport aluminum DS in it about 20 years ago and problem gone. I never did calculate any of this as I just listened to some old fart that evidently knew what he was talking about.

 

[Mustang5L5]

The motorsport one is a larger diameter. That one would raise the rpm of its critical speed. That’s why they tend to cure vibration issues and allow for the diff weight to be removed.

I almost don’t want to post this, but here’s and idea. Here’s what I would do. And yes, this is a bit risky to do and potentially dangerous, so proceed at your own risk and don’t do it at all if you are weary. Always be ready to jump on the clutch/brake if things seem sketchy.

Put the rear end on jackstands securely, and chock the front wheels. “Drive the car” like this. See if the vibrations occur here.

Next, swap the front wheels to the rear and repeat. This will at least eliminate the wheels from the potential suspects.

A camera under the car would help here. You can position it pointing at the universals and the shaft and see if you can notice any vibrations.

Again, this will be a bit dangerous, so only proceed if comfortable. I’ve done similar testing like this in the past and with robust jackstands under the axle and chocks, the car stays solid.

 

[AeroCoupe]

Sketchy? Yes. Have most of us done this? Yes. Do not blip the throttle! Ease into the speed and slowly change gears. Best to do this outside and not in your garage…duh.

 

[manicmechanic007]

Only about 125 to have the driveline balanced at a driveline shop like Slyco
Ford warranty would not pay us to send them out unless we jumped thru the hoops
The big hose clamps on the driveline and moving them around to see if we could make it better
Then of course we could not let a under warranty car leave with hose clamps on the driveline, we would send the driveline out
The driveline angle should not have changed unless something major was done to the suspension or mounts
The transmission angle out (to the driveline) and the angle in to the rear end (from the driveline) need to be roughly the same (optimal)

 

[93CalypsoConvert]

Drive line angle seems ok. There is only a 0.3 degree difference. I highly doubt that would effect it. I reinspected u-joints and found nothing wrong. No transmission bushing or pinion slop. My rear end does have a significant amount of backlash probably about 30 degrees or so...

I took a video with the car up on stands and couldn't see anything wrong but here it is anyways:


View: https://youtu.be/nMtfWJGKkS4

This is between 70-80mph.

I suppose Ill be visiting the drive shaft balancing shop.

 

[manicmechanic007]

Good luck

 

[AeroCoupe]

Do you have a dial indicator with a magnetic base? Reason I ask is you could check the run out on the DS with it. Basically the base would magnetize to the floor board and you then set the dial indicator up so the tip touches the DS. Zero the dial then spin the DS by hand and see if it has any run out. Do this a couple inches away from each u-Joint then every 12” or so. I use 0.015” as maximum runout. Just eliminates a bent or warped DS.

Not the greatest video but it’s short and to the point on how it’s done.


View: https://youtu.be/izX4pSXTF4M

Driveline shop can also do it and should do it when checking balance.

 

[2000xp8]

Personally i would not be removing a steel driveshaft and paying to have anything done to it.
I'd be buying a ford racing aluminum.
Looks like they are pricey now, but i'd still buy it.
One of the best mods.
LMR is out of stock for a few weeks and the price is way higher than it used to be, but fox parts are way up in general, so...

I look at it this way. Might was well do new universals (they are $50 each) while the shaft is out, then you are going to pay i'd have to guess $100+ for any shop to work on a part.
So you are at $200 and even if it's fixed you didn't really gain anything.
At least if you go aluminum you get a smoother lighter drivetrain.

 

[ThinBlue502]

How much play is there in the Tailshaft bushing? I was having identical issues, did an aluminum DS and noticed the Tailshaft bushing was allowing a lot of movement. Bought one of those Tailshaft bushing tools and changed it out really quickly and that reduced the issue substantially. I think I have a thread on here about it

 

[crucifyd]

I had a similar issue on a 2003 Grand Cherokee...
crazy vibration at only around 65-70mph....
how I have no idea but it turned out the output shaft on the transfer case was bent

 

[93CalypsoConvert]

My tail housing bushing is in great shape. No movement. A local shop will inspect and balance for $60. I'll see what they have to say.