Chevrolet C60s - Front Wheel "shimmy"

[Dave Mills]

Have you had the wheels dynamically balanced? They would have to be massively out of balance to cause this type of trouble. Just a thought. Also, if your tyres have weakened side walls this can also create shimmy.

[motto (RIP)]

I'm still thinking wheels too Dave. Dynamic balancing would give a definitive answer on that issue.
A simple test would be to drop the rear driveshaft and jack the front wheels one at a time to spin them up using the engine. What happens could be quite a surprise.
Just keep in mind that with one on the ground the one spinning will be rotating at twice the speed showing on the speedo.
Chock the vehicle well if carrying out this test.

David

[cordenj]

For reference photos of the 6-degree wedges.

Had the offer of two wheel and tyres this morning (thanks Neil) that are old but know to be ok on a guntractor. So collected them and fitted to the Chev. Just back from short test drive and they make ALL the difference. While there is still shimmy on hitting a pot-hole/drain cover at 20+ mph it is controllable and nowhere near as violent of with the other bar-grips.
Photo shows problem "round-shouldered" tyre about to be replaced with the very old "square shouldered" type.

As the large wedges appeared to make matters worse yesterday, I'll fit the 3-degree one in place again as the front springs have still "sunk" and the caster angle will be affected.

[Dave Mills]

Agree with Andy in relation to CASTER ANGLE. Reading through some of my old Automotive Apprentice training manuals from the mid 70's, I have found the following - I quote - "Excessive caster can make a wheel shimmy at low speeds; after striking an obstruction on an otherwise smooth road, the excessive caster will cause an over-correction of the deflected wheel, causing this to shimmy from side to side. Insufficient caster can cause the vehicle to wander, as the wheels will not trail sufficiently to give forward stability. This may also contribute toward high speed shimmy".

Have been giving a lot of thought to your problem and have added in my little bits along the way but if you look at it this way, you go to the super market and grab a trolley with the caster wheels shimming and it is almost uncontrollable this is due to incorrect caster, may be the same as your truck.

Dave.

[cordenj]

Hi Dave,

I suspect that this problem is combination of reduced caster angle (overcorrected by the 6-degree wedges so have put the 3-degree wedges in) and the tyre design.

Yesterday's test with wider tyre design gave some real progress.

I've been out to borrow another pair of wheels this morning ....what it is to have friends with CMPs (thanks James).
These wheels have the same pattern "round shouldered" design that originally caused the shimmy, so the test will be to see if they do the same.

All I can say is that I'm getting of of practice in changing 20" wheels

[Richard Farrant]

John,
In MB-C1 manual under the heading FRONT WHEEL SHIMMY it says the following;
Broken or inoperative shock absorbers increase front wheel shimmy and tramp which is annoying and dangerous.

You might need to disconnect them to operate by hand to check their operation, assuming they are topped up of course.

regards, Richard

[Bob Carriere]

........would happen if the 3 degree wedge was installed backwards to decrease camber????? and maybe compensate for the sagging springs..........

Carry on Sherlock ......

Bob C

[Richard Farrant]

Much better to reset the springs than go for all these compromises, the settings for springs are listed in the manual.

Might be worth talking to these people at Aldershot;
http://www.frmspares.co.uk/springs.html

(no connection and never used them, but being reasonably near they could be worth contacting)

[cordenj]

Originally Posted by Bob Carriere

........would happen if the 3 degree wedge was installed backwards to decrease camber????? and maybe compensate for the sagging springs..........

Carry on Sherlock ......

Bob C

Bob,

I dont know.
The wedges were made and fitted on entirely logical advice from Rick Cove and other replies to this thread. Sagging springs would reduce or remove the Castor angle, so fitting wedges from rear of springs re-instantes it.
My experiences over this weekend suggest that 6-degrees is too much, the reason being described by Dave Mills in an earlier post today. Hence I've left the 3-degree wedges in.

[cordenj]

Originally Posted by Richard Farrant

John,
In MB-C1 manual under the heading FRONT WHEEL SHIMMY it says the following;
Broken or inoperative shock absorbers increase front wheel shimmy and tramp which is annoying and dangerous.

You might need to disconnect them to operate by hand to check their operation, assuming they are topped up of course.

regards, Richard

Evening Richard,
I did check the operation of the shock absorbers when I dropped the axle for wedge fitting. Did this by disconnecting the drop arm and manually tested that their was resistance to upward and downward force.
Maybe not very scientific but there was what seemed like an equal resisitance.