Thanks Bob/Richard,

I had suspected the replacement secondhand bar grips from the start, as I didn't get shimmy on the previous tyres that had a much "squarer" shoulder (but only did few miles on them before changing.

I can try to borrow a pair of wheels/tyres, but also have a pair of new modern 11x20s bar grips being delivered to W&P, so perhaps will fit them to see what happens.

The springs are definitely past their best and when the weight is on them, sit flat. Am trying to find a leaf spring refurbisher in south-east.

Will keep you posted on the 6-degree wedge test!

OK John.
Hope the tyres correct the problem. See you at the show.

regards, Richard

Speedless wobbles
 

I get the same shimmy on my F15. It usually happens immediately after bounding out of a moderate pothole, of which we have MANY around my area. Have not had a chance to crack open the steering box yet, just learnt to tolerate the problem for now. I find the shimmy worsens quickly if I don't make a correction immediately. If a little steering force is applied L or R, then the wobble can be stopped in it's tracks. The first time it happened, I nearly pooped myself, because it was VERY violent. I hadn't learnt how to stop it at that stage, and because it was an isolated country road, I went for the brakes! It sure stopped the wobbles, because the truck literally came to a screeching halt. The silver lining was that I discovered the braking system is extremely good.

Curiously, I have not had this problem at a reasonable speed! In fact, once I go over 40-50km/h I can relax. Perhaps centrifugal force stops the problem occuring at higher speeds?!?! I do know the springs are good, so they shouldn't be an issue. Can't speak for the steering linkages, except they have no play which can be induced, or which is observable with movements of steering wheel back and forth. In fact even minor movements result in that movement being visible right through to the wheels. Can't feel any play in steering box at all, so the almost continual but very minor corrections needed to keep on course has me intrigued. For now, I am trying to resist the urge to disassemble the steering box, because I know it will become a full rebuild that I can't really spare the cash for just yet. Besides, I wouldn't be able to enjoy the challenge of driving a CMP. :D

The tyres I have on, are almost new. They are, however, Indian imports. If left sitting for some time (>1-2 days), they develop a flat spot that takes a couple of km driving to disappear. Once tyres are warmed up, they are fine. I think they have a bit of road noise, but can not really tell. The diesel engine is too noisy for me to be certain.

In near future I will be swapping the current tyres for a set of new NDBT ones. I will be curious to see if that change makes much, if any, difference to the wobbles. I will get the new ensemble professionally balanced before they are put into service. The current tyres/wheels had been balanced too, so I assume that isn't contributing (how's that for optomism!).

I am following this thread very closely. While certainly no comfort to know others are suffering same scenario, the suggestions are very, very, valuable to me, and if the tyre change doesn't fix the issue, the suggestions will assist me to know where to look next.

Thanks, to all contributors here :cheers:

Try wheel change
 

One thing I don't see specifically addressed is wheel straightness.

It's fairly easy to bend one of these rims, and you may have had the front wheels balanced but I'll bet they weren't turning very fast while they did it.

If it was OK before the replacement tyres, and isn't now, it may just be that one of the wheels is slightly bent, was on the rear, and you have put it on the front.

Spring hanger pins and bushings...
 

Hi John

As suggested by Richard, when you do the springs have a close look at the spring pins and bushing they can really wear off to a loose oval and I beleive Dirk still has them in stock...... that may be the last weak/loose link.

My worst shimmy experience was witrh a DOdge M37 thast had worn out tapered holes where the tapered tie rod fitted into...... had to replace the whole lower unit of the eggcup. The front suspension got worn that way after one Winter of driving with chains on the front axle. It shook so bad we had to come to a stop whenever we drove across a level rail road track crossing.

Good luck.

Thanks Gordon but wheels were not swopped front to back, and since this started have also moved them around to check it isnt a wheel issue

It was worth a try
 

I got rid of this problem once on a C15A by dropping the tyre pressure to the minimum - can't remember what it was though.

Funny you should say that, as am also going to reduce from 43 to 36 psi

Further...
 

Hi all, whilst out with John in the shimmy truck, I leaned out and watched the wheels for rotation at +/- 25mph, the ones on my side were spinning true.
It is a good point about the shocks, they will be tested, however the oscillation is more lateral than vertical.
It may be worth tightening the steering box up a bit more, it is pretty much correct at the moment.
The ball joints on the drag link are round and good, pre-load is set correct.
The suspension bushes were 'lever' tested and no excessive movement was found. With wheels off the ground the steering is smooth to turn from lock to lock, no tight spots.

On a straight flat road service no problems occur, this should remove the potential of a wheel being the cause, the wheel bearings have been cleaned inspected and re-packed.

My thoughts from all the information that has been gathered are leading me to think that the weight from all the recovery gear on the back after all these years has caused the suspension to settle, this has shifted the wheel alignment, castor, either from the front springs or both the front and rear springs.

The suggestion of setting the toe in a little more than specification has merits this would cause a bit more drag on the front wheels and create a dampening effect, however may cause a little tram lining and constant correction from the driver, better than a shimmy.

Andy

Large Wedges = no improvement, possibly worse
 

Fitted the larger 6 degree wedges last night and lowered the tyre pressures on front to 36 psi from 43.

At same time checked shock absorbers when the connecting arm was detached from axle, and both appear to be working correctly.

Just been for a short test drive and the shimmy is WORSE. :bang: Any speed above 20 mph it will start without warning and not needing a pothole, so that rather stuffs it for W&P!

Could it really be the tyres? I didn't get shimmy with the "square shouldered" tyres that were on it but changed them as they looked cracked. Tragedy is that I got rid of them as thought they were scrap!! The replacement ex-Bedford RL 11x20 bar-grips are far more "round shouldered".

So, any ideas welcome now as I really thought the logic of sagged springs leading to reduced caster angle made a lot of sense.

All I can think of is waiting for the new tyres we are getting at W&P and fitting them on the front.

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.

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

Tyres seem to be the "final straw" = violent shimmy
 

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.

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.

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 :rolleyes

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

I wonder what.......
 

........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

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)

Sorry to hear about your problems John.

I agree with Richard, replacing or re-arcing the springs, combined with non-worn bushings would be the best basis for the rest of the improvements.

What are you using to measure the caster?
 

Hi

Like everyone else this is a puzzler, would be interesting to measure the caster of a CMP that dose not shimmy so what are you using to measure caster and camber?

My C60S is the best driver of my three with no hint of shimmy. Just had it out this morning running along on fresh asphalt roads at 50-60 MPH it is just as smooth as you could want. I don't have a front end gauge unit to measure what the various angles. Suggestions on what type equipment you are using and I'll check around.

My C60L and HUP will shimmy but it is well above their comfortable driving speed neither of these trucks is really happy above 45 MPH.

As other people have said the geometry of steering is complicated, may not even be the front springs at fault but flattened out rear springs changing the angle at the front end.

Cheers Phil

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.

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.

Agreed Richard,
Aldershot isn't too far away. We thought there may also be somewhere near Edenbridge.
I do think new/re-set springs and pins is the ultimate answer....was trying to get a temporary solution for W&P this year though.

Hi Phil,
I've measured camber with a spirit level and steel rule.
I have no way to measure castor, so have been experimenting. The previous owner had fitted what I am guessing are 1-degree wedges, so I've tried 3 and 6 degree. A previous reply on this thread referred to 9-degree set, but my 6-degree seemed to make matters worse.

I must say, I've been concentrating on the front springs, that look flat with weight on them, no real visible curve. The rear springs look to still have a good amount of curve.

John, do you have a smart phone?
I have a Samsung S5. I have downloaded a clinometer app.
Make sure your veh. is on level ground and then lay the edge of the phone against your spirit level.
Does that help?

Thanks Lynn,

Good idea. I dont have a smartphone but my children do

More progress
 

Some more progress tonight

Picked up two wheels and "round shouldered" 11x20s from James's LAAT. Identical tyres to my troublesome set, thought it was worth a try to see how they went but was expecting bad shimmy.
Fitted these to the Chev.
Replaced 6-degree wedges with 3-degree versions


Test drive on James's tyres and they performed the same as Neil's . i.e. ran well at reasonable speeds, some minor shimmy when hit a pothole but controllable. :thup:
Removed these wheels tonight (as not fit for a long run) and Chev on blocks ready for new tyres in a couple of weeks.

So it means that my first replacement ex-Bedford RL tyres were probably a major part of the issue.....don't know why as look to be a better set than the ones I've trialled tonight.
But truck is also now running 3-degree wedges, new Pivot bearings on one side and numerous other tweaks and adjustments.

As a previous poster suggested....shimmy is probably due to a range of minor issues that cumulatively cause the violent reaction.

Am also going to tweak up the steering box as per the manual. Seems to me I need a 2x2/3" spanner....any recommended work arounds? I see previous owners have used a hammer and chisel, but don't want to follow suit.

Plan to investigate new springs later in Summer/Autumn

Thanks everyone for all the ideas, support and comments

In my experience big spanners are too expensive for one off jobs. The easiest way to manage the nut and screw on the Chev steering box is to weld two bits of flat to a bar to make a spanner and drill a hole between the flats for screw driver access to hold the bolt steady.

Steering box spanner
 

Thanks Robert,

I had come to same conclusion. Then found that a Fan Clutch Hub Spanner, for the MERCEDES AT174 is 65mm and available as a special tool from Ebay for £5. I doubt the nut is massively tight on the chev, so will try one of these.

Now that I read this I do have to say that I used to complain to myself about a shimmy in the 35-40 mph area when I ran at 50 psi. Dropped the psi to 35-rear (45 loaded) and 35 front. Shimmy sometimes comes on but I hardly run across it and my last trip was 2 hours strait driving at 50mph and I never noticed any strong shimmy.