Chevrolet C60s - Front Wheel "shimmy"

[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.

[Richard Farrant]

When working in REME workshops years ago, we had problems like this with the Bedford MK 4x4 trucks. We had an electric motor powered roller and a dummy flange for the front wheel so the wheel did not rotate the drive shafts. The wheel was jacked up and roller run up to speed, you soon knew when the critical wheel speed was achieved, by the shaking. There was an optical instrument or strobe, details escape me at present and by using this we could then add weights to the rim at the precise point. It was incredible how much weight was needed sometimes and the thought of one of them coming off the rim on the road was horrifying. I believe the actual reason is tyre circumference not concentric with the wheel rim in a lot of cases, which will upset the balance, also these vehicles stand around a lot and it takes time for them to warm up and the casing to reshape itself. Bar tread type tyres can develop strange wear patterns on the front wheels, none of this helps.

regards, Richard

[Phil Waterman]

Hi Richard

Your memory is dead on, there were commercial units that had a wheel spinning motor and a very simple trembler switch which triggered a strobe. The switch had a magnetic base that you could stick on axle top (preferred) Use was fairly simple you chalked numbers 12, 3, 6, and 9 (clock positions) on the tire then spun the wheel up to speed. The light would flash when the imbalance was in the vertical plane. You would note which number was top and place the weight accordingly. Now my memory is failing whether you put the weight opposite or used two smaller weights 120 degrees from the heavy point. Remember that it took a lot of playing around to get to understand how to place weights, tricks like inside and outside of the rim. End results were pretty good, my dad had one that he got from a garage when they went over to a off the vehicle spin balancer.

Remember my dad having a set of tires very carefully balanced on a high speed off the car balancer. Then put the wheels and tires on car and used the on vehicle balancer to find which wheel assembly was out of balance, turned out there was a drum and hub that were, to him, badly out of balance.

But that you were also balancing the brake drum and hub was one of its failing as you could not necessarily move the tires around without rebalancing.

Cheers Phil