Chevrolet C60s - Front Wheel "shimmy"

[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.

[motto (RIP)]

I'm still thinking wheels too Dave. Dynamic balancing would give a definitive answer on that issue.
A simple test would be to drop the rear driveshaft and jack the front wheels one at a time to spin them up using the engine. What happens could be quite a surprise.
Just keep in mind that with one on the ground the one spinning will be rotating at twice the speed showing on the speedo.
Chock the vehicle well if carrying out this test.

David

[cordenj]

For reference photos of the 6-degree wedges.

Had the offer of two wheel and tyres this morning (thanks Neil) that are old but know to be ok on a guntractor. So collected them and fitted to the Chev. Just back from short test drive and they make ALL the difference. While there is still shimmy on hitting a pot-hole/drain cover at 20+ mph it is controllable and nowhere near as violent of with the other bar-grips.
Photo shows problem "round-shouldered" tyre about to be replaced with the very old "square shouldered" type.

As the large wedges appeared to make matters worse yesterday, I'll fit the 3-degree one in place again as the front springs have still "sunk" and the caster angle will be affected.

[Dave Mills]

Agree with Andy in relation to CASTER ANGLE. Reading through some of my old Automotive Apprentice training manuals from the mid 70's, I have found the following - I quote - "Excessive caster can make a wheel shimmy at low speeds; after striking an obstruction on an otherwise smooth road, the excessive caster will cause an over-correction of the deflected wheel, causing this to shimmy from side to side. Insufficient caster can cause the vehicle to wander, as the wheels will not trail sufficiently to give forward stability. This may also contribute toward high speed shimmy".

Have been giving a lot of thought to your problem and have added in my little bits along the way but if you look at it this way, you go to the super market and grab a trolley with the caster wheels shimming and it is almost uncontrollable this is due to incorrect caster, may be the same as your truck.

Dave.

[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

[Bob Carriere]

My C15a with the 10:50 x 16 chevron tires gave me a shimmy at around 40 + mph....

I inquired about balancing the tires at a Farm tire shop.... no can do on these rims.......BUT they suggested playing with the tire pressure and moving the tires around the four corners.

Moved the front to rear and decreased the pressure from 50 pounds to 30/32 pounds by five pounds increments..... at the lower pressure the shimmy went away...... and the ride got smoother....... not sure how accurate the speedo head is but will flat out at 45 mph screaming like a banshee........ much more pleasant at 35 mph.

Bob C

[Andy Beevers]

This topic is a good one,
I was involved with John right at the beginning.
I have now got my truck back on the road, and yes I too have a shimmy!
and wheel balance issue.
just over 30 mph wheel wobble starts, compounded by shimmy.

Working on the same theory as the rest. part by part.
Steering box, done
Azle, pivot, and wheel bearing, done
New tyres
Drag link and track rod done.
Tested shock absorbers, damping good.

Stuff that is not seeming quite right.
The tyres, big difference the higher the pressure, they don't seem as perfect round as I was expecting, haven't had them balanced yet (two faults here)
Slight play in shock absorber ball joints
Lateral movement on leaf spring rear shackle lower mount.

I have removed the springs, the bushes and pins are worn and need replacing, now my question, the original bushes (C15A, 1944) are bronze,
the ones from LWG are steel, which is correct?