Chevrolet C60s - Front Wheel "shimmy"

[Phil Waterman]

Hi

In 30 years of driving CMPs I've had this problem with all three of my trucks. In each case I've done as suggested check setting, looked for play installed heavy-duty steering damper. All good things to do any way, some improvement but no complete fix, but each time changing the tires solved the problem or moved the speed that it starts at well above normal driving speed.

My C60S is the happiest highway truck of the three, even with the radio box the 261 engine and 11:00x20 tires it is smooth at 50-60 MPH ( with 11:00 tires the truck thinks it is doing 50 ) All three of my trucks have recent manufacture new tires. My C60L has a little shimmy as you get to 45-50 MPH which I suspect is from no front shocks which is letting little wheel hop feed in to steering. I suspect that the steering geometry of the pitman arm steering connector link is not perfect that allows the up down motion to be converted to lateral motion in the steering. Will see if this goes away once the shocks are bolted in.

Given my experience would try the following with the wheels jacked up take large block position so it just doesn't touch the tire hold something on the block and slowly turn the wheels to mark a line down the center of the tire. Then look at your line is it staying centered on the tread and did then your marker try to move in and out as you turned the wheel. Assuming the tires are running true an are not egg shaped, then would try moving your tires around front to rear side to side, does the shimmy change?

Cheers Phil

[cordenj]

Originally Posted by Phil Waterman

Given my experience would try the following with the wheels jacked up take large block position so it just doesn't touch the tire hold something on the block and slowly turn the wheels to mark a line down the center of the tire. Then look at your line is it staying centered on the tread and did then your marker try to move in and out as you turned the wheel. Assuming the tires are running true an are not egg shaped, then would try moving your tires around front to rear side to side, does the shimmy change?

Thanks Phil,
I did wonder if changing the tyres was partly the cause. The ones on it when I bought the truck last autumn were very badly cracked - so I sourced some good secondhand 11x20s from a Bedford RL. But didn't get to drive it very fast with the old tyres, so I don't know if the shimmer was there.

I'd have hoped that the professional balancing of the front wheels/tyres would have highlit any defects, but I'll try your recommendation.
I might also take up a offer to try some other wheels with 12x20s fitted to see if that cures it.

[lynx42]

Check the caster. My C60S was more than terrible and I found that the front springs had sagged a bit and the caster was way out.

I machined up a pair of wedges to, from memory, 9degrees and fitted them from the rear. Shimmy gone straight away even with old tyres.

Regards Rick.

[cordenj]

Originally Posted by lynx42

Check the caster. My C60S was more than terrible and I found that the front springs had sagged a bit and the caster was way out.

I machined up a pair of wedges to, from memory, 9degrees and fitted them from the rear. Shimmy gone straight away even with old tyres.

Regards Rick.

Thanks Rick,
I'd not measured the castor as have never needed to consider that on other vehicles I've had . I see the manual says 1 1/2 - 2 inches, so will check

[Lynn Eades]

Many types of 4 wheel drives can suffer from this. To eliminate the problem requires that all aspects of the front end are up to spec.after an initial general check, I would check the king pin preload (as previously suggested) Every thing helps to either eliminate or compound the problem. Things like lateral float at spring shackles, spring set (loss of castor) a slightly grabbing brake. Tyre, wheel, brake drum balance, out of round, wheel bearing play, any aspect of a tyre (off set in mold?)that is not perfect is detrimental. (some tyres will go out of round when parked with a load on them, and will not come right until they are once again warm from driving)

If the truck has Timken taper rollers as king pin bearings (Dodge used bronze upper cones in a Timken cup (I suspect they had condensation problems rusting the upper cones) they wear in one place becoming loose in the straight ahead position. Correct preload causes a somewhat alarming distortion of the housing but they then drive well (assuming everything else is good as mentioned previous posts)
Another possibility is the state of the universal joint in the front half shafts, and also the centering of the joint on the king pin axis line (shimmed in a jeep, depending on axle joint type)

Any one of these things can help to set off this shimmy.
Hope this is food for thought.

[David Dunlop]

John.

I had the same problem with an M38. After a wheel balance failed to cure it, I started checking everything from the steering knuckles inward and up to the steering box, replacing every suspect bearing, pin and tie rod end. The problem gradually improved as each suspect part disappeared from the system, but still showed up whenever I hit a serious pothole or bad railway crossing. At that point i noticed the front spring assemblies had a noticible lack if arch to them. put new springs, shackes and pins on the front end and problem finally solved. Never came back either.

As others said, it is an area where a number of smaller problems seem to add up and have a very alarming effect. Definitely an area where it pays to take your time sorting it out.

David

[cordenj]

Originally Posted by David Dunlop

John.

I had the same problem with an M38. After a wheel balance failed to cure it, I started checking everything from the steering knuckles inward and up to the steering box, replacing every suspect bearing, pin and tie rod end. The problem gradually improved as each suspect part disappeared from the system, but still showed up whenever I hit a serious pothole or bad railway crossing. At that point i noticed the front spring assemblies had a noticible lack if arch to them. put new springs, shackes and pins on the front end and problem finally solved. Never came back either.

As others said, it is an area where a number of smaller problems seem to add up and have a very alarming effect. Definitely an area where it pays to take your time sorting it out.

David

Thanks David.
Yes I think it is a case of slowly working through the possible causes. That is why everyone's thoughts here are very useful

[motto (RIP)]

Mention has been made of wheel balancing but what type of balance was done, static or dynamic?
Static balancing has definite limitations in that the location of the 'heavy spot' is not defined axially i.e. it may be toward the inside or outside of the wheel. When weights are added to 'balance ' the wheel they can be just as easily be put on the opposite side of the wheel to the heavy spot and make the dynamic imbalance worse.
Dynamic, on vehicle balancing is a far better option if you can find someone with the equipment to do it as this removes the unknown and also takes the hub and drum into account.

David

[Lynn Eades]

Wheel balance would generally be less of a problem with big old M.Vs. than the likes of modern vehicles with smaller dia. wheels traveling at higher speeds.

In saying that, I do not disagree with any of the above.

John If you have measured correctly then something is definately wrong (1" difference in camber??) were you on level ground to start with?

Jack it up and see if you have any play in the vertical plane (grab the tyre top and bottom and try to wobble the wheel)
If there is any movement get someone else to apply the brakes and try it again
If the movement has gone, it is wheel bearing adjustment. If it is still there, it is in the king pins.
You also need to deal with the toe in as well. 3/8" will be scrubbing the hell out of your tyres with each wheel trying to go in different directions.

If you disconnect the drag link (steering arm end)and then swing the wheel asemblies through their steering arc (left and right) you may be able to pick up any roughness in the king pins.

If you do have to do king pins, keep in mind that king pin bearing preload is normally set without the knuckle seals fitted.

[rob love]

Originally Posted by Lynn Eades

If you disconnect the drag link (steering arm end)and then swing the wheel assemblies through their steering arc (left and right) you may be able to pick up any roughness in the king pins.

Not sure one can swing a pair of 20" tires with the wheels still on the ground unless you have a pair of alignment plates underneath. If the axle is jacked up and the wheels are in the air, then the problem may not be evident as you have transferred the weight away from the kingpin normally under load to the kingpin that normally does not carry a load.

[cordenj]

Originally Posted by Lynn Eades

Wheel balance would generally be less of a problem with big old M.Vs. than the likes of modern vehicles with smaller dia. wheels traveling at higher speeds.

In saying that, I do not disagree with any of the above.

John If you have measured correctly then something is definately wrong (1" difference in camber??) were you on level ground to start with?

Jack it up and see if you have any play in the vertical plane (grab the tyre top and bottom and try to wobble the wheel)
If there is any movement get someone else to apply the brakes and try it again
If the movement has gone, it is wheel bearing adjustment. If it is still there, it is in the king pins.
You also need to deal with the toe in as well. 3/8" will be scrubbing the hell out of your tyres with each wheel trying to go in different directions.

If you disconnect the drag link (steering arm end)and then swing the wheel asemblies through their steering arc (left and right) you may be able to pick up any roughness in the king pins.

If you do have to do king pins, keep in mind that king pin bearing preload is normally set without the knuckle seals fitted.

Hi Lynn,
I rechecked the camber with wheels on level ground, the figures decreased slightly on both side with the weight of the truck on them, but still a difference between left and right hand side.
With the wheels jacked up, I tried shaking wheels to see if any play. There was no perceptible movement, even with a lever under the wheel, so that implies to me that the bearings are not too loose. The wheels rotate freely too, so no reason for possible "grabbing" being the cause.

[Dave Mills]

I am certainly no expert on the CMP and have watched with interest the conversation. I am delving into the unknown here but may I suggest that you check your shock absorbers. If you have checked everything else it seems like you may need to check these, if they are faulty they can cause a shimmy once you increase speed and hit any form of undulation in the road surface.

Dave.

[cordenj]

Originally Posted by motto

Mention has been made of wheel balancing but what type of balance was done, static or dynamic?
Static balancing has definite limitations in that the location of the 'heavy spot' is not defined axially i.e. it may be toward the inside or outside of the wheel. When weights are added to 'balance ' the wheel they can be just as easily be put on the opposite side of the wheel to the heavy spot and make the dynamic imbalance worse.
Dynamic, on vehicle balancing is a far better option if you can find someone with the equipment to do it as this removes the unknown and also takes the hub and drum into account.

David

Hi David,

They were static balanced (on a large machine but off the vehicle) at a specialist truck tyre company. The wheels ended up with weights on both inner and outer faces. Before I changed the tyres, the wheels had large amounts of weight added so clearly been balanced before; so thought that might be the shimmy cause.