Chevrolet C60s - Front Wheel "shimmy"

[cordenj]

Thanks Bob/Richard,

I had suspected the replacement secondhand bar grips from the start, as I didn't get shimmy on the previous tyres that had a much "squarer" shoulder (but only did few miles on them before changing.

I can try to borrow a pair of wheels/tyres, but also have a pair of new modern 11x20s bar grips being delivered to W&P, so perhaps will fit them to see what happens.

The springs are definitely past their best and when the weight is on them, sit flat. Am trying to find a leaf spring refurbisher in south-east.

Will keep you posted on the 6-degree wedge test!

[Richard Farrant]

Originally Posted by cordenj

Thanks Bob/Richard,

I had suspected the replacement secondhand bar grips from the start, as I didn't get shimmy on the previous tyres that had a much "squarer" shoulder (but only did few miles on them before changing.

I can try to borrow a pair of wheels/tyres, but also have a pair of new modern 11x20s bar grips being delivered to W&P, so perhaps will fit them to see what happens.

The springs are definitely past their best and when the weight is on them, sit flat. Am trying to find a leaf spring refurbisher in south-east.

Will keep you posted on the 6-degree wedge test!

OK John.
Hope the tyres correct the problem. See you at the show.

regards, Richard

[Private_collector]

I get the same shimmy on my F15. It usually happens immediately after bounding out of a moderate pothole, of which we have MANY around my area. Have not had a chance to crack open the steering box yet, just learnt to tolerate the problem for now. I find the shimmy worsens quickly if I don't make a correction immediately. If a little steering force is applied L or R, then the wobble can be stopped in it's tracks. The first time it happened, I nearly pooped myself, because it was VERY violent. I hadn't learnt how to stop it at that stage, and because it was an isolated country road, I went for the brakes! It sure stopped the wobbles, because the truck literally came to a screeching halt. The silver lining was that I discovered the braking system is extremely good.

Curiously, I have not had this problem at a reasonable speed! In fact, once I go over 40-50km/h I can relax. Perhaps centrifugal force stops the problem occuring at higher speeds?!?! I do know the springs are good, so they shouldn't be an issue. Can't speak for the steering linkages, except they have no play which can be induced, or which is observable with movements of steering wheel back and forth. In fact even minor movements result in that movement being visible right through to the wheels. Can't feel any play in steering box at all, so the almost continual but very minor corrections needed to keep on course has me intrigued. For now, I am trying to resist the urge to disassemble the steering box, because I know it will become a full rebuild that I can't really spare the cash for just yet. Besides, I wouldn't be able to enjoy the challenge of driving a CMP.

The tyres I have on, are almost new. They are, however, Indian imports. If left sitting for some time (>1-2 days), they develop a flat spot that takes a couple of km driving to disappear. Once tyres are warmed up, they are fine. I think they have a bit of road noise, but can not really tell. The diesel engine is too noisy for me to be certain.

In near future I will be swapping the current tyres for a set of new NDBT ones. I will be curious to see if that change makes much, if any, difference to the wobbles. I will get the new ensemble professionally balanced before they are put into service. The current tyres/wheels had been balanced too, so I assume that isn't contributing (how's that for optomism!).

I am following this thread very closely. While certainly no comfort to know others are suffering same scenario, the suggestions are very, very, valuable to me, and if the tyre change doesn't fix the issue, the suggestions will assist me to know where to look next.

Thanks, to all contributors here

[gordon]

One thing I don't see specifically addressed is wheel straightness.

It's fairly easy to bend one of these rims, and you may have had the front wheels balanced but I'll bet they weren't turning very fast while they did it.

If it was OK before the replacement tyres, and isn't now, it may just be that one of the wheels is slightly bent, was on the rear, and you have put it on the front.

[cordenj]

Originally Posted by gordon

One thing I don't see specifically addressed is wheel straightness.

It's fairly easy to bend one of these rims, and you may have had the front wheels balanced but I'll bet they weren't turning very fast while they did it.

If it was OK before the replacement tyres, and isn't now, it may just be that one of the wheels is slightly bent, was on the rear, and you have put it on the front.

Thanks Gordon but wheels were not swopped front to back, and since this started have also moved them around to check it isnt a wheel issue

[gordon]

I got rid of this problem once on a C15A by dropping the tyre pressure to the minimum - can't remember what it was though.

[cordenj]

Funny you should say that, as am also going to reduce from 43 to 36 psi

[Andy Beevers]

Hi all, whilst out with John in the shimmy truck, I leaned out and watched the wheels for rotation at +/- 25mph, the ones on my side were spinning true.
It is a good point about the shocks, they will be tested, however the oscillation is more lateral than vertical.
It may be worth tightening the steering box up a bit more, it is pretty much correct at the moment.
The ball joints on the drag link are round and good, pre-load is set correct.
The suspension bushes were 'lever' tested and no excessive movement was found. With wheels off the ground the steering is smooth to turn from lock to lock, no tight spots.

On a straight flat road service no problems occur, this should remove the potential of a wheel being the cause, the wheel bearings have been cleaned inspected and re-packed.

My thoughts from all the information that has been gathered are leading me to think that the weight from all the recovery gear on the back after all these years has caused the suspension to settle, this has shifted the wheel alignment, castor, either from the front springs or both the front and rear springs.

The suggestion of setting the toe in a little more than specification has merits this would cause a bit more drag on the front wheels and create a dampening effect, however may cause a little tram lining and constant correction from the driver, better than a shimmy.

Andy

[Harry Moon]

Originally Posted by gordon

I got rid of this problem once on a C15A by dropping the tyre pressure to the minimum - can't remember what it was though.

Now that I read this I do have to say that I used to complain to myself about a shimmy in the 35-40 mph area when I ran at 50 psi. Dropped the psi to 35-rear (45 loaded) and 35 front. Shimmy sometimes comes on but I hardly run across it and my last trip was 2 hours strait driving at 50mph and I never noticed any strong shimmy.

[cordenj]

An update on the "shimmy":

We collected our NoS Fidelity 11x20 US made Bar treaded tyres from Belgian dealer, Rudi, at War & Peace.

I've fitted four on the Chev, and on the advice of Alain (who drove has CMP from Switzerland to the Guernsey tour in May on them) inflated all to 55 psi.

Just back from 10 mile run and what a difference! Steering improved at low speed and "shimmy" MUCH reduced, but still not completely eliminated.

I can now live with this, and the problem only occurs when can't avoid hitting a large pot-hole and is far more easily controlled than before. On the rare pieces of smooth tarmac on the backroads of Surrey, it ran perfectly.

So what lessons from all of this that might be useful to others?

Well my interpretation based on the practical lessons of my truck of last 3 months:

1. Unacceptable Shimmy was caused by a number of issues, no one simple fix

2. Wedges are NOT the simple solution. 6 degree wedges made it worse, currently running on 3 degree ones, but only because they appear to make matters no worse (or better!)

3. Adjusting steering box to the specs in manual made a slight improvement and worth doing anyway

4. Slightly worn Pivot Bearings on one side, may have added to the problem, but worthwhile replacing them anyway

5. Correctly adjusted Pivots bearings on both sides to spec in manual, eliminated that possible cause....but no evidence that it was connected to shimmy though

6. New tyres made a MAJOR improvement.

7. Tyre pressures seems critical on my truck. New tyres printed with recommendation of 65 psi; practical user recommendation of 55 psi seems to work well. I had previously tried 35 and 43 psi on the other second hand ex-Bedford RL tyres

8. Wheel balancing: made no difference to onset of shimmy. So a waste of time in my view at speeds I am going at (30 -35mph)

Thats it for now and thanks to everyone who gave advice on this thread.

[Bob Carriere]

Hi John

As suggested by Richard, when you do the springs have a close look at the spring pins and bushing they can really wear off to a loose oval and I beleive Dirk still has them in stock...... that may be the last weak/loose link.

My worst shimmy experience was witrh a DOdge M37 thast had worn out tapered holes where the tapered tie rod fitted into...... had to replace the whole lower unit of the eggcup. The front suspension got worn that way after one Winter of driving with chains on the front axle. It shook so bad we had to come to a stop whenever we drove across a level rail road track crossing.

Good luck.