Limited Slip Differential--Theoretically Speaking

[Diogenes]

Is it possible for a clutch type LSD to create a vibration during normal, straight-line driving? I have begun to suspect my 9in. aftermarket third member (about 2 years old) as being the possible source of a vibration that I have been tracking down, one that I thought I had solved.

I was having a vibration in the chassis (not steering wheel) at 55-60 MPH, though it sometimes varied a bit. It was not tire or wheel related. I installed new u-joints, didn't solve the issue. I spun the driveshaft 180 degrees on the differential yoke, which seemed to improve slightly, but not a solution. I then installed a spare driveshaft, which seemed to solve the problem, making me suspect my original driveshaft needs balanced. However, yesterday during a drive, it seemed to momentarily return, and under some strange circumstances.

I turned left from a stop sign under normal driving throttle, whereupon initial throttle application, a little "chirp" from the rear left tire occurred, as if it had been sitting on a rock, which was spit out and caused the tire to "chirp". I state "as if", because I don't think the left rear tire was on anything but clean pavement (though I can't be 100% certain).

I drove to the next stop sign and turned right under normal throttle, which is when my vibration returned at 50-60 MPH. I instantly thought of the differential, and was able to perform a couple somewhat aggressive left and right turns from a stop, and the vibration hasn't returned as of yet.

Motor and trans mounts are good. Inspected driveline looking for issues, nothing seems out of the ordinary.

I'm wondering if it needs another bottle of friction modifier and another "break-in" period in an abandonned parking lot.

Any thoughts and advice?

[galaxiex]

What vehicle?

Going by old guy memory here, it's been a minute or 2 since I had to fix one of these....
Some F150's had a problem with the factory LSD chattering and vibration in straight line driving.
There was a service package kit with new clutches etc, and special oil additive to fix the issue.

So ya, clutch type LSD can cause vibration... sometimes...

Also uneven tire wear with one side even slightly different rolling diameter can cause issues.

We have a tire diameter gauge and sometimes see a significant difference in tires L to R and Front to Back.
Can cause problems, esp on 4X4's.

[Diogenes]

What vehicle?

It is in my 66 Galaxie.

Tires look evenly worn.

[cjshaker]

A friend had a swap meet bought LSD that he put in his Cleveland powered '72 Mach 1. It was new when he bought and installed it. He had some similar issues with the unit, and after a couple of weeks, the differential grenaded. It didn't vibrate, as I remember, but kept chirping the tires on turns. Turns out it was shimmed too tight, and eventually it couldn't handle the stresses from the turns. If the rear tires were off a little on OD, I could see it causing the straight line issues that you're having.

Just relaying the experience because it seems to have some similarities to what you're describing. Luckily, he contacted the guy who rebuilt it and he replaced it free of charge.

[galaxiex]

What vehicle?

It is in my 66 Galaxie.

Tires look evenly worn.

Looks can be deceiving.
Only way to know for sure is to put a gauge on the tires.
Most good tire and alignment shops should have a tire diameter gauge.
They have to deal with this stuff all the time.

Some LSD diffs don't like even a small mis-match in diameters side to side.
As little as 1/8 to 3/16 difference can cause issues sometimes.

Usually don't see problems until you get to 1/4" or more difference, but some are very sensitive.
You cannot eyeball even a 1/4" difference. You will never "see" it.
Hence the tire gauge.

[Diogenes]

It was odd when I heard the chirp, as it occurred before the car was actually turning. It happened immediately upon engaging the throttle and letting up on the clutch pedal, before I entered the lane to make my turn. I've never heard or felt anything odd when turning. I'm definitely going to take it to an empty lot and do some Turning Circles (a little Judas Priest reference), but I'll make sure to be close to the house.

Referring to the tire gauge--yes, I'm sure you're right, I couldn't spot that small of a variation. I'll have to check into that.

[Joe-JDC]

Tire pressure equal left to right in rear tires?  U bolts tight, shocks tight?  Joe-JDC

[Diogenes]

Tire pressure equal left to right in rear tires?  U bolts tight, shocks tight?  Joe-JDC

Yes, tire pressures are equal, shocks are tight, though I do need to double check the rear control arms (this generation Galaxie has coil springs), though I'm fairly confident they are good.

[pbf777]

Referring to the tire gauge...................... I'll have to check into that.

      Rather than attempting to establish the diameter of the tires, what you really want to know is the circumference, and the simplest measurement tool for this is a dress-makers measuring tape!   

      Another process would be to:  park the vehicle on flat and level pavement, make a chalk at six o'clock on the side wall of the tire and a corresponding mark on the pavement; now push the vehicle forward one revolution (actually additional revolutions and dividing by the sum of, makes for greater accuracy) until the side wall chalk mark is at six o'clock again, make another corresponding mark on the pavement, now measure the distance between the two marks and you have the travel distance ("roll-out") of each revolution of the tire, measure both and compare.   

      Scott.

[Diogenes]

Referring to the tire gauge...................... I'll have to check into that.

      Rather than attempting to establish the diameter of the tires, what you really want to know is the circumference, and the simplest measurement tool for this is a dress-makers measuring tape!   

      Another process would be to:  park the vehicle on flat and level pavement, make a chalk at six o'clock on the side wall of the tire and a corresponding mark on the pavement; now push the vehicle forward one revolution (actually additional revolutions and dividing but the sum of, makes for greater accuracy) until the side wall chalk mark is at six o'clock again, make another corresponding mark on the pavement, now measure the distance between the two marks and you have the travel distance ("roll-out") of each revolution of the tire, measure both and compare.   

      Scott.

I love it! A nice and simple process to identify a potential problem--the kind of simple, common sense approach to problem-solving that we often overlook or don't even think to use these days; such is the state of things with our over reliance on technology. Thanks, a great suggestion!

Rear suspension checks out, so things seem to point to the differential, but that is yet to be determined definitively.