Chevrolet C60s - Front Wheel "shimmy"

[cordenj]

An update on the "shimmy":

We collected our NoS Fidelity 11x20 US made Bar treaded tyres from Belgian dealer, Rudi, at War & Peace.

I've fitted four on the Chev, and on the advice of Alain (who drove has CMP from Switzerland to the Guernsey tour in May on them) inflated all to 55 psi.

Just back from 10 mile run and what a difference! Steering improved at low speed and "shimmy" MUCH reduced, but still not completely eliminated.

I can now live with this, and the problem only occurs when can't avoid hitting a large pot-hole and is far more easily controlled than before. On the rare pieces of smooth tarmac on the backroads of Surrey, it ran perfectly.

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:

1. Unacceptable Shimmy was caused by a number of issues, no one simple fix

2. Wedges are NOT the simple solution. 6 degree wedges made it worse, currently running on 3 degree ones, but only because they appear to make matters no worse (or better!)

3. Adjusting steering box to the specs in manual made a slight improvement and worth doing anyway

4. Slightly worn Pivot Bearings on one side, may have added to the problem, but worthwhile replacing them anyway

5. Correctly adjusted Pivots bearings on both sides to spec in manual, eliminated that possible cause....but no evidence that it was connected to shimmy though

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)

Thats it for now and thanks to everyone who gave advice on this thread.

[motto]

I understand John that you have had enough of this vexing problem and would like to hear no more on the subject but your point 6 'New tyres made a MAJOR improvement' begs the question. What is it about new tyres that made the difference? The improvement was not only with new tyres but was also experienced with a previous wheel change.
I will quite understand if you don't wish to respond to this post and the last thing I want is to engender any ill feeling but unless I've missed something I believe that you could have possibly achieved the same result with a dynamic (on vehicle) balance. Some vehicles are more tolerant of imbalance than others but on a 20" wheel my experience tells me that balance can be important. Imbalance sometimes having a similar effect to a buckled wheel even at comparatively low speeds. My suggestion that you drop the rear drive shaft and spin up the front wheels one at a time would have shown any imperfection immediately. The violence when this is done can be quite startling and can commence at surprisingly low speeds. Tyre out of round can do this even if balanced.
If balance is irrelevant then about the only explanation I can think of would be a variation in flex or stiffness at different points in the tyre. Very difficult to determine.
I have no wish to start an argument or upset anyone but would sure like to pinpoint the reason for the improvement elusive and all as it may be.

Yours sincerely

David

[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

[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.

[cordenj]

Originally Posted by Andy Beevers

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.

Andy - yes, pins from Stefan last year. I used Jones Springs in West Midlands to reset the seconhand set of C60 front springs. Would recommend them.

[Phil Waterman]

Hi All

Been working on this for awhile takes almost as much time to upload as to shoot the video. This is my 1941 Pattern 12 C60L road test video shimmy between 35 & 40 MPH. Since then I've reduced the shimmy and raised its threshold to 45+ MPH. Comments please.



Click here for video I can't get the embed to work correctly

https://youtu.be/vTPqSBq9klA

Cheers Phil

[cordenj]

Excellent video Phil.

All seems very familiar to me!

I jacked up each axle and set up a pointer and ran each axle static. I had found one of my wheels to have a significant variation at the rim edge. changed that wheel with a spare I managed to obtain.

In the first two years of ownership, I had these split rims apart and together more times than I like to think about (good exercise I suppose). I made sure the mating faces of the two halves were clean and that they were bolting together correctly as I appreciate that a small error here will cause the rim movement we are seeing.

All 4 wheels are now reasonably straight in that there is only slightly movement at the rim on each one, certainly less than on you truck.

Tyres do have some "out of round" movement when run static, but am not sure how much of that is from sitting for a month between giving them some exercise.

Still, As I said at 25mph the truck runs well, and that is not a problem in the areas in tend to use it.

[Lynn Eades]

Phil, the wheels are forged and I think that "eccentric" extra bit, is commonplace. The military Dodge rim was factory balanced. (welded on steel weight) I assume CMP wheels were as well.
I noticed the brake drum is not running exactly true. An out of round drum can be a culprit.

[Andy Beevers]

John,
such a small world at times.
Somebody else reccomended Jones, called them this morning, in the post now, should be here tomorrow.
I have already got a bush puller and replacer so good on that thank you.
I have ordered an H10 adjustable reamer which should allow for what I need.

Phil, excellent video, however once you start on you tube difficult to stop, liked the HUP going out in the snow too.
Once those wheels start to go shimmy, it takes everything with it.
I did notice a little movement on the pitman arm draglink, (tighten screw all the way in, then back off half turn, fill with grease, sorry sucking eggs etc.) but from that view the temptation is to say the wheels start the shimmy and it just gets bigger till centrifugal force wins and pulls the shimmy back out like a gyroscope.

I will still follow the engineering principal of 1 item at a time, then I will know.
So, will get the springs done, then balance the wheels, and try to post a video like you have done, might take longer for me to do that than strip the truck!

Should be out for a roadtest this coming weekend, if all goes as promised.

[Andy Beevers]

Phil,
Some other points,
truck sounds real good, accelerates well, gear changes, wow, so smooth and quiet.

The following are random thoughts that may get me somewhere, or not.
I am tempted to try something a bit different.
What if a weight was placed, of a significant size, on the wheel and then run again. If the shimmy was worse or better, then wheel/drum imbalance is the cause. It is a theory, if it can be made worse, then the opposite should make it better.

I have a box of stick on weights, will try, 100g strips, a couple should do something.
Then using the logic of the on car balancer, using the tyre valve as a starting point, should be able to do on road balancing. Problem though, is one wheel at a time is the best way, some more thought perhaps.
Not overly keen on testing one wheel at a time on axle stands, the way it shakes could be a bit more excitement than I need.

[motto]

Originally Posted by Lynn Eades

Phil, the wheels are forged and I think that "eccentric" extra bit, is commonplace. The military Dodge rim was factory balanced. (welded on steel weight) I assume CMP wheels were as well.

The welded steel weights on the 3/4 ton Dodge wheels (I think supplied by Budd) seem to be a standard weight in a standard position to compensate for the valve stem cutout. I suspect they were not individually balanced though I have not compared wheels to confirm this. It would be an oddity if they were as I don't know of ex factory weights on Jeep or GMC/Stude wheels etc.

David

[Lang]

These balance beads work absolutely wonderfully. They take into account any unbalance caused by bearings, drum weight, wheel weight and tyre weight and even slightly out of round.

The physics behind them is very complex but they are very widely used, even on 200mph race cars. They are relatively cheap and many brands.

[Tony Smith]

Do they work with inner tubes? I understood them to used by putting a handful inside tubeless tyres when fitting to the rim. Would you have to remove the valve from the inner tube stem and feed in each bead one by one through the stem?

[Lang]

Tony

You can get small size balls to feed into a tube stem.

I saw a set of tyres (tires) fitted in USA for a Dodge WC and the kit came with the balls and a little funnel arrangement. Took some time and a little jiggling but the result was perfect.

You are right about the tubeless tyres, just throw in the bag, which quickly disintegrates to release the balls when fitting the tyre to the rim.

There is also heavy liquid to do the same job on the market which I presume would be easier for tubes. Motorcycles use this a lot.

Antique and Classic Car owners have discovered Dyna Beads®!
Classic Car and Antique Automobile tires, whether tube or tubeless, need Dyna Beads®. This long era of automobiles and the styles of tires that were used back then lend themselves perfectly to using Dyna Beads for tire balancing.

If you look at this video at around 2.25 minutes you will see a valve stem filling system.

https://www.youtube.com/watch?v=z4h0H7ycIBg

Lang

[Phil Waterman]

Hi

Simple caution about using Dyna Beads, be sure to use good metal value stem caps. Reason I say this is that years ago had a version of the beads put in the tires of my C60S tires by a heavy truck tire shop. No problem airing up the tires but later after adjusting air pressure down the the stem valves would not always seal complete. Blow a little air in problem would go away. Replaced the stem valves, same issue bleed air out valves would slow leak, quick blast of air in they sealed again. As a precaution replaced all the plastic value caps with good metal ones.

Now as to did the beads balance the tires no. These were used 11:00x20 military tires probably 50% tread cronic out of balance in the end replacement was the only fix.

Cheers

[Bob Carriere]

Tried twice getting the wheel balanced at commercial truck shops.....one look at the wheel/1050x16 none were interested citing various reason...... will not fit there machine...... we do not spin balance on the truck....... a few ounces will not mean much against the mass of the tire wheel combination.

Suggested moving the tired in different location and play with tire pressure snd that resolved 90 of the problems.......Shimmies now at very top speed of 44 or 45 MPH and since I find that speed dcsry the more comfortable 35 mph has no shimmy.

Even a guy in the States that had 900x16 (power wagon)shaved and claims they are now smooth as silk BUT tires had to be marked for corner location and same bolt pattern for each wheel

Considering beads now.......

What experience do others have had........

I have a new spare set of Michelin 900x16 to try out next SUmmer.

Cheers

[cordenj]

I had also wondered about using these beads:
http://www.dynabeads.co.uk/dynabeads_4x4.php
But as the shimmy was not such an issue after other upgrades had not pursued further.

I know they are being used on vintage Rolls Royces...so should be ok for CMPs!

[motto]

Originally Posted by Bob Carriere

I have a new spare set of Michelin 900x16 to try out next SUmmer

If the present day Michelins are as good as they used to be you will have no trouble with balance.

David