Chevrolet C60s - Front Wheel "shimmy"

[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

[Lynn Eades]

Andy, You could shelve your bushes and have a machinist turn up some up. I would go with bronze. Maybe you can buy some off the shelf with the right O.D. and I.D. If not the correct I.D. you could put a drill through in a lathe and then ream. Steel to steel probably requires more diligence with your greasing.
Have you dropped the tie rod off (with the front jacked up) and checked the pre-load at the knuckles? Is there any resistance to them moving?
To minimise the risk of shimmy, everything has to be in spec.
Good luck as you move forward with this. (As it's a Chev, I suppose I should have said "ahead")

[Andy Beevers]

Hi Lynn,
Yes the swivel hubs have been completely stripped down, had the shafts out, replaced some wheel bearings as required. the preload on the hubs has been set to the requested specification, interesting if you go over the specification the hub starts to distort.

I have located bronze bushes and will order them Monday and an adjustable reamer, the manual gives a clearance of 2-8 thou.

I am thinking the bushes I have been supplied are not correct, steel on steel is not my first choice, = rust!

the sizes of bronze bush are on the shelf, then a gentle ream, bit of grease, perfick.

Andy

[Hanno Spoelstra]

Originally Posted by cordenj

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:
*snip*
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)

Originally Posted by Richard Farrant

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.

Just adding my 2 cents, worth... The big, heavy tyres used on CMPs certainly play a role in making any play in the steering and suspension apparent.

I had flat spots on my tyres from standing still. It was so bad it would not go away by driving a long distance to heat them up.

So I had the tyres trued up to make them perfectly round again. After this: no more shimmy.

HTH,
Hanno

[cordenj]

Hi Andy,

I fitted NOS bronze bushes and pins when replaced the front springs in the summer. One of the "Y-shaped" shackles had very worn bushes.
I have a reamer and rigged up a threaded special tool to fit the bushes, welcome to borrow both.


The truck runs well at a comfortable 25mph, but if I try to go faster and then hit any blemish in the road it can still sometimes wobble.

Interestingly when we took it to Dieppe in August, it ran without any shimmy any any speed I could manage (slightly, but not a lot more than 25mph) on the perfectly maintained French roads. It might be an issue with the cart-tracks called roads in UK that sets it off, but I'd have thought these vehicles should be able to cope with rough roads.

I have also jacked up and run the front axle. There is definitely a noticeable variation in the diameter tyre as the wheel rotates. Not sure balancing would help that would it? 25mph seems to be the solution for me at moment, but will interested to hear how you get on with your lighter "sports" model.

[Andy Beevers]

Hi Hanno,
new tyres all around, yes they go out of shape when parked, in fact they are terrible!
I inflated the tyres to around 60psi, left the truck on axle stands for a while, was doing other work underneath, then spun the wheels by hand with an axle stand positioned as a datum reference, the rims spin true.
The tyres, holy moly. reseated still not nice.
My intention is to get to a truck garage and do a little tyre phasing and balancing. however at the moment the wheel balance issue is making it easy to get the shimmy started, once I have the shimmy reduced to an acceptable level the tyres will get attention and may indeed need to be taken back to from whence they came, if it proves to be out of round tyres due to moulding issues, they are of no use to me, I want round ones!

John,
many thanks, I will see what turns up when, out of interest, my pins are 7/8 inch, got the last 4 from Stefan did you manage to get them from elsewhere?
Was going to try a few leafspring manufacturers Monday and see what they have in the way of bushes, the ones I had found are a little short, would prefer a single bush.