Chevrolet C60s - Front Wheel "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?

[Andy Beevers]

Just to clarify a few details.
The vehicle was designed without the shimmy
therfore should be able to cure the shimmy

The shimmy is when the wheels start to turn left and right by a few degrees on their own, violently.
A steering damper will stop or nulify the problem ( can be fitted) but is not fixing the problem.

As has been said, there must be play somewhere or insufficient resistance to create the scenario for this to happen.
The wheel balance issue brings on the shimmy automatically, so will look into that once the shimmy is sorted, otherwise you need to be at a speed of 30 mph+ and hit a pothole or even a cats eye to instigate the shimmy.

[Harry Moon]

With my C15a and the 10.50 16 chevron tires i had a little bit of shimmy between 30-35 MPH, faster than that it went away. I did as i previously noted reduced running pressure from 50 to 38 and the ride improved but the shimmy was only slightly improved. As a side note i used a lazer temperature gauge to monitor tire pressure and temperature so now when i drive fully loaded I put the rear tires to 43 and then they keep the same temperature. early this year i rotated the wheels and tires front to back and the onset of the wobble started much earlier about 28MPH and was worse and lasted until about 42MPH.
I took it into a truck shop and had the wheels balanced on the heavy truck machine, not the automotive one. Added as much as 9 ounces to one of the now front wheels. the rear ones which had been on the front only needed an ounce or two. Took it out on the freeway for test drive and it was better than ever. Never picked up a shimmy at all.