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MG Car Repair/MGB vibration back end

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QUESTION: hi Howard - my MGB 1974 with 80k miles.
I was experiencing a vibration -  ( not in the steering ) at high speed, so assumed it was from the rear end. I checked the u-joints and at the diff was just a tiny bit sloppy, so I changed out both. This turned out to be a nice easy job.  Less vibration now, but still some there at 55 -60 mph. I noticed that there is a bit of play in the driveshaft to differential. ie; when I twist it either way there is a gap of 2-3' ( as in minutes of a clock face )before I can feel it engage the gears. Is this normal? otherwise what are other things I can check. Could a bad wheel bearing cause this?
many thanks
P

ANSWER: Hi Paul,

When you have a "Speed" oriented vibration it is a balance problem. Meaning it comes in at a certain speed and has no change under load or road condition. Even a good "U" joint can vibrate if the cups were not installed correctly. This causes the driveshaft to not be centered and thus out of balance. This can be roughly checked by holding a marker or piece of chalk close to the driveshaft itself while it is rotated to see if it is not spinning true. this should be done at the front as well as the rear.

This produces a fine high high speed vibration at moderate speeds. An out of balance rear wheel gives a rougher vibration that is most noted in the seat and steering wheel in an Up/down vibration not a rotational vibration like a front wheel out of balance.

The rotational free play is normal in the pinion shaft in the differential. As long as there is NO up / down or in /out play in the flange. The gear on the pinion shaft is a hypoid gear so it moves a lot to engage the ring gear. Rotational movement of the ring gear is very little. But that can only be checked with the differential open.

I have had to balance many driveshafts but only long ones. The MGB drive shaft is short compared to other cars and rarely needs balancing. It is easy to do though even with no equipment.

Put the car on a lift and have someone run it up in gear to the speed where the vibration is felt. Most of the time if it is a driveshaft causing the problem you can see it vibrating. At that time you just hold a piece of chalk up to the shaft and lightly touch it with the chalk. Then stop the car and install two standard hose clamps on the driveshaft with the screws exactly opposite the chalk mark and run it again. And mark it again. If the mark is in the same place you need another hose clamp. If the new mark is now in line with the clamp screws, Spread the location of the screws apart about 30 degrees each on either side of the new line. Always think of the chalk mark as the heavy HALF of the shaft. Splitting the clamp screws lightens that half and moving them closer together increases the weight. If your shaft requires more then two clamps then you have a "U" joint problem. The front of the shaft and the back should be checked that way. I balanced many driveshafts in my years and only had to do a few MGBs. almost all long driveshafts need it done. An out of balance driveshaft will shorten the life of differential pinion bearings and rear transmission bearings not to mention "U" joints.

Let me know what you find,
Howard    

---------- FOLLOW-UP ----------

QUESTION: THANKS HOWARD for super prompt reply.  I will see if my mechanic can provide the lift for me... meanwhile there is another question - when I reassembled the splined end of the driveshaft into the front yoke I may have installed it 180 degrees out of phase compared to the original position ( my chalk marks were smeared so cannot be sure, but I would have a 50 \50 chance right?  ) Should I re-install and test drive?

Answer
Hi Paul,

If you are talking about taking the driveshaft apart at the slip spline. It has been many years since I took one apart but I thought there was just one wide spline and it could not be assembled wrong. If it can go together 180 degrees try rotating it but it should not matter because the spline was set with the large tooth so that you could not get the Spicer type "U" joints aligned wrong since Spicer type "U" joints when not perfectly straight cause the driven side to change speed as it rotates. Thus the design of CV joints.

First you need to confirm where the vibration is coming from, the driveshaft or the tires. My system served me well for many years in many different dealerships on many different brands of cars.

Drive the car on different surfaces and see if the vibration comes in at a specific speed and then speed up to see if it fades out at a higher speed and returns when slowing to that speed. When the vibration is in at it's worst does acceleration or deceleration eliminate the vibration or enhance it? Does making mild "S" turns at the vibration speed change it?

A 100% chance that it is a balance problem if it is speed oriented vibration on any road surface straight or in a turn. Next is it wheel speed frequency or driveshaft frequency? Keep in mind that driveshaft vibrations are from 3 to 4 times as fast in frequency then wheel speed vibrations. Then put it on a lift that supports the body not the axle and have someone run it at the vibration while you watch with a good light at the front and rear of the driveshaft and each rear tire. You can always see a vibration.

Then you can go after the cause of the vibration. First ID where it is. Then go after what it is. Let me know,

Howard  

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Howard M. Fitzcharles III

Expertise

MG from 1956 (USA versions only) up and Engine theory.

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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Associate member SAE EAA member

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Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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