Chevrolet C60s - Front Wheel "shimmy"

[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

[cordenj]

Fitted the larger 6 degree wedges last night and lowered the tyre pressures on front to 36 psi from 43.

At same time checked shock absorbers when the connecting arm was detached from axle, and both appear to be working correctly.

Just been for a short test drive and the shimmy is WORSE. Any speed above 20 mph it will start without warning and not needing a pothole, so that rather stuffs it for W&P!

Could it really be the tyres? I didn't get shimmy with the "square shouldered" tyres that were on it but changed them as they looked cracked. Tragedy is that I got rid of them as thought they were scrap!! The replacement ex-Bedford RL 11x20 bar-grips are far more "round shouldered".

So, any ideas welcome now as I really thought the logic of sagged springs leading to reduced caster angle made a lot of sense.

All I can think of is waiting for the new tyres we are getting at W&P and fitting them on the front.

[Dave Mills]

Have you had the wheels dynamically balanced? They would have to be massively out of balance to cause this type of trouble. Just a thought. Also, if your tyres have weakened side walls this can also create shimmy.

[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.

[motto (RIP)]

I understand John that you have had enough of this vexing problem and would like to hear no more on the subject but your point 6 'New tyres made a MAJOR improvement' begs the question. What is it about new tyres that made the difference? The improvement was not only with new tyres but was also experienced with a previous wheel change.
I will quite understand if you don't wish to respond to this post and the last thing I want is to engender any ill feeling but unless I've missed something I believe that you could have possibly achieved the same result with a dynamic (on vehicle) balance. Some vehicles are more tolerant of imbalance than others but on a 20" wheel my experience tells me that balance can be important. Imbalance sometimes having a similar effect to a buckled wheel even at comparatively low speeds. My suggestion that you drop the rear drive shaft and spin up the front wheels one at a time would have shown any imperfection immediately. The violence when this is done can be quite startling and can commence at surprisingly low speeds. Tyre out of round can do this even if balanced.
If balance is irrelevant then about the only explanation I can think of would be a variation in flex or stiffness at different points in the tyre. Very difficult to determine.
I have no wish to start an argument or upset anyone but would sure like to pinpoint the reason for the improvement elusive and all as it may be.

Yours sincerely

David

[Phil Waterman]

Hi John

I've revised this thread, you started, several times, in search of some little tidbit of information that would make shimmy on my Pat 12 C60L go away. Well this summer the C60L nick name "Canada" has had a profound shimmy starting at 35MPH continuing to get worse till 40MPH then suddenly it basically goes away.

Like you I had gone through it by the book and adjusted everything to essentially "0" play. I've even shot GoPro Video under the truck while driving through the shimmy range trying to spot something(I'll try and post that soon).

No good, still shimmied was about to start swapping the 11:00x20 wheels and tires from the truck with the shimmy to the C60S with no shimmy. But this after noon thought what else could it be? Checked my maintenance log and spotted a difference tire pressure. The C60L with just the cargo box is pretty light so I was running the tires a 38PSI to cushion the ride a little and get better traction off road, the other truck with the radio box is considerably heavier doesn't go off roading so run it at 60PSI.

Well this afternoon aired the C60L up to 60PSI all around and shimmy is essentially gone. Truck rolls right up to 50 MPH on the speedometer. Glad I found the solution the shimmy was beating the daylights out of shoulders probably wasn't help the truck much either.

Cheers Phil