Measuring Rear Pinion - help with method and numbers

[ajftj]

I'm trying to track down driving vibration and read lots about rear pinion angle.
Jeep has 2" lift and 20mm spacers under transfer case plate.
31x10.5x15 rubber.

Can you confirm if what I'm measuring makes sense, and if the numbers are good?

Pictures show the technique, degrees measured on the gauge are 8.5 for both front and rear measurements. The garage is 1 degree tilt, so I guess the diff and case are at 7.5.

If my measurement method is OK, does both ends being equal mean the whole thing is running true?
Is 7.5 a good number in this case?

Also... I've read you can pull the rear shaft out and drive in 4x4 on the front to help diagnose. When you do this do you loose all your transfer case oil? Do you tape over the end to avoid a trail?

Thanks guys.

[xjdavid]

when do you get vibrations. What is the angle of the driveshaft. How many degrees does the pinion point up under power while in the garage.

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[ajftj]

that was quick!
I need to get a bit scientific when driving to answer the"when" question properly.
I'm still celebrating having fixed my DW so have just moved on to this problem and not been too focused. Next drive ill try a few things. That's partly why I was wondering about removing the rear driveshaft.

All rear control arm bushes are new and standard rubber.
Not able to measure the angle under load on my own at the moment, seems a little dangerous

It's generally more noticeable as vehicle speed is increased and no dependency on engine speed.

Driveshaft itself measures 7.5 degrees with gauge stuck in tube.

[xjdavid]

so you only have 1 degree on the u-joints. need to know if it vibrates under load or when acellerating

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[ajftj]

OK road tested with my notepad in hand..

Driving uphill under light load vibration becomes noticeable from 40 and obvious 45 to 60 increasing in frequency with road speed, thereafter intensity of vibration is fairly constant and very obvious.

Vibration is most obvious when driveline is all connected, clutch is up, if you press the clutch it's 80% gone. i.e. on a slight slope pausing between accelerations between gears.

Coasting downhill in gear it is still evident but much reduced in intensity. Pressing the clutch now, pretty much 95% gone. If you force engine braking - say shift from 70 in 3rd to 2nd the intensity increases quite a bit. But still not as bad as when under load accelerating. (There is some minor oscillation in the steering, like nibble on the overrun going downhill, but I suspect this is another issue. Also not currently running a steering damper)

If you accelerate briskly from stationary you are more aware of the frequency than the intensity - if that makes any sense. So on the flip side if you accelerate gently from rest on a flat road it's really not very evident until higher road speed. Starts to be obvious around 55.

If you find a long hill and accelerate from level into the hill, say 2nd then 3rd, while you are briskly accelerating it's very obvious. If you continue to hold steady acceleration in 3rd in the long hill the severity of the oscillation settles as you hold torque. Its still obvious but not as aggressive - more like a shimmy than a bad wheel balance.

(you know as I write this I realise that I have not personally balanced the rears, I have a receipt from when they were balanced but I didn't check myself - oops)

There is a little dependency on engine speed. It's more unpleasant in 2nd than 3rd at the same road speed, but this is accelerating moderately from 40 to 60 in 2nd, when you shift to 3rd quickly the engine speed drops, the road speed continues to increase but the oscillation is more tolerable.

Hope this helps to point me in the right direction..

[Classic Boy]

There is a very cool commercial app that uses you smartphones built in accelerometer to diagnose drive line vibration.
See u-tube, pay for and install the app, fill in the details of your car and put you phone on the dash or console and go for a drive it will give feedback on where the source is likely to be.

I once saw a similar unusual vibration in a manual trans falcon many years ago and it was a collapsed spigot bush in the rear of the crankshaft, its failure allowed the gearbox input shaft to be not centralally located in the back of the crank therefore the centre plate of the clutch was out of alignment with the flywheel. Not saying it is your problem but it was a weird driveline vib that was hard to find, that came and went with the clutch pedal use.

[ajftj]

Interesting about the app, will hold that thought thanks.

A little worrying about the spigot bearing too, trouble is with something of this age and mileage there's lots of little things that stack up and create multiple problems.

Im trying to eliminate the basics first, I want to watch the rears get balanced as I did with the front to eliminate anything there / wheel irregularity etc. then ill be back!