Chevrolet C60s - Front Wheel "shimmy"

[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

[Lynn Eades]

Even with all that done (wheels and tyres supposedly"perfect") you can still get a shimmy if the hubs are not set up correctly, because any minor play is magnified.
One needs to first accept that any balancing system is limited by its degree of accuracy (in most circumstances it is enough)
If your front axle joints are worn, not centered, you have worn balls in your steering joints or any other un acceptable clearance, then it can quickly magnify into a shimmy.
Just imagine the simplest situation. Imagine a tiny bit of play in a ball joint at the end of your steering arms. When your travelling down the road, the wheels are as per wheel alignment specs. Now just touch the brakes.
The result is that the two front wheels are now trying to retard the forward movement of the veh. The result is that the two front hubs have pivoted back (loading the joints)to a toe out situation. Then when you release the brakes they "flop" back to an "unloaded" position. This is the first part of a shimmy, which can be aggravated by so many things.
Sorting the tyres and wheels often will "fix" the problem, but the "inherent" (tendency to shimmy) problem begins elsewhere.

[Andy Beevers]

Lynn Eades,
I agree completely.

I have used a smaller version of the on car balancer, you actually sat on it and pushed it into the wheel to get wheel to spin, then a small sensor triggered the strobe and likewise the datum mark point, however, wheel balance on the majority is in the vertical plane, the shimmy is in the horizontal plane.
Therfore I see that there are two faults, one can compound the other.

If I can isolate the source of the play, I should then just have a wheel balance problem, which I will recognise.

Does anyone know if the spring eye bushes should be bronze or steel in the C15A 1944 model, this has the later steering box and 2 inch leaf springs.
It has bronze bushes in at the moment, both pins and bushes are worn out.
I have been supplied steel bushes which I would need to cut down and ream.

With the rolled steel bush there is a joint, this means I cannot use a straight flute reamer, helical ones are not made anymore for that size, I can have one made.

My thought is to fit bronze bushes back in, ream to the size of the new pins, and shim up to take out the lateral movement.

Once this is done, road test and see what shimmy I have, then balance wheels.

I wish to thank everyone for the comments, all are valued, appreciated and helpfull.

I will post continual, but slow, progress untill the solution is found.

Andy