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MG MGB Technical - Body shake at 45-50Mph

1976 MGB - 80K. The car is very solid and stable up to 45Mph, then the body of the car begins to shake, almost in a side-to-side manner. The steering wheel stays solid, but the car is shaking around it. I've rotated and balanced the tires, replaced the front shocks and A-arm bushings but haven't noticed any change in behaviour. Above 55Mph, the car does settle down a bit, but it doesn't feel terribly stable. Thanks for your help.
Martin

It's either the prop shaft or a back wheel that whilst balanced might be horribly buckled.
Iain MacKintosh

I had the same problem and it was the drive shaft on my car. My problem started after a quick acceleration that bent the drive shaft. I had a shake that I felt more thru the steering wheel, but you are describing the same sensation. I went thru everything on the front end and wheel balancing. Let us know if this turns out to be the problem!
Todd Alkire

Can the driveshaft be balanced with weights - or is a new driveshaft in order?
Martin

Once again, could be worn U-joints --- cheap and easy to replace.
Dan Robinson

Martin,

Yes, the driveshaft can be balanced. You should be able to find a shop that can do it without too much trouble. You might as well install new u-joints while you are at it. I had a spare driveshaft, so I just swapped mine out.

One more possibly is that the bolts holding the driveshaft in place are loose or missing. That was the very first thing I had to fix on my car!

Todd
Todd Alkire

Martin,

I once experienced the almost identical problem except the shaking and rattling started at 50 mph and smoothed out again at 60 mph. I tried everything that I could think of and had my machanic/friend go thru everything from bumper to bumper and it finally turned out to be the tyres were out of round. Could not detect this problem visually, but a new set of tyres did solve the problem. Something to check.

Cheers - Dennis - Sacramento
Dennis Rainey

I had a similar problem when I first got my B 4 years ago. It is only by process of illimination that you may resolve it. I thought it was tyres and balancing. I had the tyres checked by a local "specialist" he reported that he couldn't get them accuartly balanced and that one wheel was buckled. I changed this for another Rostyle, still really no better. I then found a garage via the club magazine that specialised in balancing rostyle wheels and sure enough he did the job far better than the other company.. Something about having the right centralising boss and equipment. I also changed some old tyres which may have been distorted. I also changed my steering column as the bush at the top was worn and the steering wheel vibrated at this point. I have now just changed all shock absorbers and this has made a differance as well. Whilst doing normal checks on the car silly things come to light. I found my pulley on the main crankshaft had been chipped from presumably a front end knock at some time. I replaced this and found that it got rid of a vibration through the clutch when it was depressed to change gear. So I now have a vibration free ride through to about 70 mph, a little above this but I rarely push beyond 70 so it doesn't bother me. So my tip is persever as far as you can. Good luck
John
John Gearing

Lot of good info here but just a couple of further points. Check the UJs or preferably replace them. Then ensure that the shaft is assembled so that both joints lie in the same plain. Do this by pulling the splined joint and refitting to achieve alignment. Unless the shaft has been damaged I don't believe it will need further balancing but could be proved wrong.
Re tyres being out of round this was something which plagued us a few years ago especially with remould tyres but it seems to have largely gone although don't discount it. Finally the truth of the rostyle I feel is a real issue. For two cars I have about 20 rostlyes and I'm still trying to get the best running sets. I always take old weights off and then get a rim spun on the balancer before I sandblast otherwise there is no point in going further. The rim should basically show no imbalance whatsoever. If you can do this and provided that all tyres are new (i.e. not remould) then the finished wheel should balance out with a max of about 40 -50 grammes. If you can test the wheels one by one on a smooth running car then you can be confident that wheels & tyres are OK.

There is no simple answer to this. Some of these wheels have been around for 35 years and have been hard used, hit many kerbs etc. A visual check doesn't always work either. I have a wheel in service which looks as if it has distortion on it yet runs perfectly.

A well set up car should run up to high speed with no shake.
Iain MacKintosh

I had the same thing. Several times. There was no one sure fix in particular and required a lot of detective work, and so I can simpathize with your
plight. My own fixes involved:

Out-of-round tires.

Loose lower A-arm mounting bolts.

Bad shocks.

Worn front wheel bearings.

You might also want to check the steering arm bolts with a torque wrench, as well as the steering
knuckles.
Daniel Wong

The final piece in the puzzle for me was front wheel bearing that looked fine , but would not hold its shimmed clearence. At the moment the car is completley vibration free, but you only need to hit one pothole on our disgraceful roads .
S Best

A lot of people often comment that they don't make cars like they used to. My comment would be thank God for that, but sometimes we all try to achieve the impossible and get these cars to run like their modern day counterparts. We are unfortunately never going to get there but it should be possible to get a vibration (not noise)free ride up to at least 80mph.

I'm interested in Steve Best's comment as to why the bearing clearance does not hold after presumably the hub nut is torqued against the spacer. It almost hints that something strange is going on within one of the bearings or that the outer races weren't seated tightly by torquing the hub nut with the spacer removed.
Iain MacKintosh

Thanks for the help.

I did the U-Joints this weekend, the front one was completely shot - no rollers left under one of the caps and the back one had some movement in it as well. This actually fixed a different vibration, one that I hadn't paid much attention to due to the more severe body shaking. It was more of a rumbling that I would start to feel at higher speed.

I'm thinking I will balance the spare (same tire type) and swap it around to see if I can find the culprit. The previous owner bought these tires (Bridgestone Potenzas) about 5K miles ago - as best as I can tell - and they still have good tread, so I'd hate to replace the set. But, tires are pretty cheap with the 14" rim size.

With each step the car keeps getting more and more fun to drive, thanks again for all the help.
Martin Milkovits

Since you mentioned balancing your wheels:

I had to go through three shops before I could find one who could mount my Rostyles on the balancer correctly. Before you remove the wheels from the car, raise the car and give each wheel a spin. See how much deflection there is on each (horizontal and vertical). My personal rule of thumb says less than 1/16" is acceptable, although you really should have zero.

Then when you take your wheels to a shop and they tell you the rims are warped, you have seen with your own eyes that's not the case.
Matt Kulka

Sometime a rim can appear true and still not balance out whilst at other times a rim can have minimal but visible distortion present and balance perfectly.

The balancer is the only way to check the rim.
Iain MacKintosh

That bearing was one of the strangest things. I washed it in petrol , and inspected it , fine . Set it up and ran the car for 20 miles it was loose , OK , my mistake perhaps some crud behind it etc , did it again with the same result . Changed to new bearing , no problem since. I still have the old one somewhere it still loooks perfect , so I must consider it unfinished business .
This was done the correct way with the spacer and shims etc . The other wheel ran fine , and still does.
S Best

Yes but did you seat the bearing firmly in the first place by trying to seat the bearing by tightening the hub nut BEFORE fitting the spacer and shims. Looks like it moved further when put in to service
Iain MacKintosh

Hi Ian , I have done this job before with no probs. The manual does say tighten the nut without the shims first , it says this is to seat the outer races , but I guess it pulls everything up .If I do it again I may buy a dial indicator , and save spending hours fiddling with the shims.
S Best

Martin, congrats on the propshaft overhaul. Once the tires are balanced, and trust me - I worked for Bridgestone for a number of years and their tires never have tread separations - that will be out of the equation.
Has any major bodywork ever been performed on your car such as a front end collision repair or new rocker assemblies? Bad body alignment could cause severe driving problems as well.
I'd look first at the front suspension though. make sure all the control arm bushings are good and that the kingpins aren't loose.
Do you feel the shimmy more in the steering wheel or your seat? Does it make the pasenger seatback shake before you feel it in the steering wheel? try to determine if it's coming from the front or the back of the car for starters, then jack up each wheel and check for play - anywhere.
I've also seen severe driveline vibrations come from warped brake drums and rotors. If your brakes are warped, they will kick the pads or shoes every time they make contact and the shaking becomes more severe the more you drive it.
Jeff Schlemmer

WARNING*** Tightening the wheel bearings with the nut on the stub axle to "seat the races" will destroy the bearings. They should be driven in properly before assembly. Fully seated races will "ring" when struck, whereas unseated or "cock" races will have more of a dull clang sound.
The tightening to be done prior to checking end-float is to carefully tighten while rotating the wheel until the wheel drags. This squeezes out excess grease from between the rollers and seats so your measurement is accurate.

As far as the shimmy is concerned, I suspect either loose bearings or a bent wheel. Seeing if the shimmy reduces when cornering will indicate a loose bearing on the outside of the turn... as will disassembling for a (sounds like much needed) re-pack! A decent shop is the only way to find a bent rim.

Good luck,

Mike!
mike!

Mike,

Yes it will destroy the bearings if you go mad with a breaker bar but we are not going there. Perhaps not even as tight as the specified bearing nut torque whilst rotating the hub all the time. The point is that if the bearings are not fully home for some reason they will than tend to move when the vehicle is in service but they must be bottomed properly before fitting the shims otherwise all the work is pointless.

Steve is well up to this and it seems he has a rogue bearing.
Iain MacKintosh

For driving the new race , I used the old bearing and a wooden block , it was quite satisfying , Mike is spot on that you can hear it go home . I took a LOT of care to keep it square , i can imagine you could get it stuck .As i said before the manual tells you to tighten the bearing nut without the shims fitted , to seat the outer race . Then put too many shims in , measure the end float , take the assembly apart , do the sums put the shims you need in re-assemble check , and hopeful drive off for another 100K moiles .
S Best

Had a similar problem with my 68 B. Drove me nuts trying to find the problem. I did all the usual including having all 4 wheels dynamically balanced on the car. (a method I strongly recommend). Replacing most of the front suspension and having the drive shaft balanced. Even replaced engine and transmission mounts.

After many attempts, I finally contacted the PO and found that he had a u-joint fail and had it replaced. What we finally found was the problem was the rear bearing in the transmission was bad. Interestingly, there was not any oil leak. Whether the bearing going affected the u-joint or the u-joint failing caused the rear transmission bearing to go bad, I don't know, but it took care of the mid/high speed shakes.
Bruce Cunha

That is really intersting Bruce , if I was investigating this one check would be to get hold of the prop shaft and see if I could move it , mainly as quick check on the Hardy Spicers, did the Gearbox bearing show up when , and if , you did this ?
S Best

There was some play in the prop shaft, but at the time I was assessing the problem, I was not clear if it was abnormal. After having tghe drive shaft checked for balance and replacing the U-joints, the next step was to pull the short shaft on the transmission to check it for balance and check the splines for wear (it was fine. It was at this point that the rear seal and bearing were replaced and the problem went away. Bearing did show more slop than the new one when I inspected it once it was out.
Bruce Cunha

This thread was discussed between 11/06/2004 and 17/06/2004

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