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Odd shimmy at 40mph when on the brakes... help.

BRE Fabrications LTD

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Hey Guys,

In the last 3-4 months my 80 has developed a shimmy of sorts. It happens under very specific conditions and is easily repeatable.

My observations are:
1. When I'm on the brakes, slowing from any speed over 40mph, when I get down to 40mph a shimmy developes in the steering wheel. At around 35mph it goes away.
2. I can be doing 70mph on the freeway and then take an off ramp and while slowing down all is good but as soon as I get to 40mph the shimmy kicks in. At around 35mph it dissipates.
3. The severity depends on how fast I'm slowing down, the faster I am slowing down the more severe.
4. Under acceleration no shimmy occurs at any speed. If I am on the brakes at 40mph and the shimmy kicks in, if I let off the brakes the shimmy dissipates.

I have checked the following:
Wheel bearings for play (Wheel jacked up, hands at 12 & 6, no play whatsoever, I checked with hands at 3 & 9 as well (TRE's?), no play either).
Panhard bushes were replaced today, so they are good, old ones were a bit knackered.
All radius arm bushes were replaced in the last 6-9months (can't remember exactly when I did it last year, but I did).
I had my own set of castor correction plates made last year out of 4mm plate, they were a bit crap quality wise so I suspected these. Today I replaced them with a new set I had laser cut that are good quality wise. But still the shimmy is persisting.

I am at a loss with regards to what else to check. Any suggestions?
 
Wheels out of balance, warped disc, tyre out of true. try back wheels on front
 
Mine does it because my left front tyre weighs a little less than the right , rubbed while on the trailer altering the tread pattern a bit .
 
Check the backside of the brake disks. Guessing a build up of rust at the inner or outer edge of the brake disk. You might find that if you brake really hard, the shimmy is less noticable as well.
 
Thanks guys, I'll give those items mentioned a check.

Wheels were balanced about 6 months ago also and the weights are still there.

With regards to a possibly warped disc, now that it's been mentioned I do feel a very slight pulse in the pedal when slowing down at slow speeds, less than 20mph, but it really is very slight.

How far out of true does a disc need to be to be warped?
I can check it with a dial indicator this coming weekend.


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It is the thickness of the disc that needs to be measured. It should be of constant thickness all the way round. Secondly check with a dial gauge. As uHu says the inner surface can get quite bad without you seeing it so condition is important.
 
Worth checking the operation of all 8 pistons too Ryan. I have had this before and the cause of the warped disc was heavy braking with opposite pistons not working on one caliper. I braked hard (very hard) and the diagonal force across the disc put it out of true. I know exactly what you mean about the little shimmy. Had the very same thing myself.
 
Top and bottom swivels worn or the pre-load needs adjusting?

EDIT: Now noting the 'pulsing through the brake pedal', this can only be caused by out of true discs or bearings (which you mention) allowing pad knock off which is not the issue here. As per Andycook, better to use a DG when fitting and of course run it on your old disc to look for run out max (IMO) .004"

regards

Dave
 
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I'd take Chris' lead and have the caliper Pistons checked and for the price of them, I'd change the discs.
 
I'd take Chris' lead and have the calliper Pistons checked and for the price of them, I'd change the discs.

Agreed. Sticking pistons will soon overheat your new discs and cause more warping. The person fitting them would know if the piston dust covers were damaged or the pistons sticking but, if you can 'feel' pulsing through the brake pedal, then there is only two ways AFAIK that can cause this, wheel bearing movement allowing the disc to apply pressure to the pads and hence back to the pedal via the master cylinder, or the disc/s is out of true and the results are the same. You have ruled out the wheel bearings this leaves discs, but above, what caused it to happen? Sticking pistons, sticking pads, discs worn thin?

regards

Dave
 
Could alignment be the issue?

Thanks for the further input guys. Just a few items to put out there.

1. Calipers were rebuilt approx about a year or so ago by myself, possibly more. My memory is crap.
2. Front and Rear Discs were replaced in 2012 with cryod Nitrac Drilled and slotted discs again by me.
3. Brake pads were replaced at the same time I rebuilt the calipers, when I had the wheels off this last weekend the wear on the pads looked uniform.
4. Knuckles and Axle had a rebuild around 2014 also with new bearings all round.

I'm trying to get my head around the idea it could be the discs, surely if they were warped then I'd feel the pulsing at all speeds? When I say I can feel a very slight pulse at 20mph or less, I mean very slight. and it's a slow pulse. At higher speeds other than the 40-35mph zone I don't feel anything. When the Shimmy kicks in it can sometimes be quite "violent" (for lack of a better word) but I havent noticed any pulsing in the brake pedal when this is happening.

Having given this even more thought and straining my appalling memory, I do recall on my last trip to Wales on the end section of the Strata Florida lane I caught either the drag link or tie rod on a rock which yanked the steering wheel quite violently. After that the steering wheel was a little off centre, I have since adjusted the tie rod to re centre it. Nothing appears obviously bent but could an alignment issue cause this? I'm thinking if I have slightly bent the tie rod could it be possible that the drag link was also bent and that the alignment is out of whack? I didn't have this issue before that Wales trip...
 
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@Ryan Thomson

Without wanting to sound like an arse, you are making it very difficult to diagnose!

Post 1, started with 'a shimmy of sorts', Post 2, you inform of a brake pedal pulsing, Post 3, we get 'can be quite violent'?

So to clarify, if you have a slightly out of true disc this WILL cause pulsing at the pedal, if you go slow then the pulse is slow, if you go fast the pulse can disappear, think of it as a light on the end of a piece of string, if you spin it fast enough it will appear to be forming a circle, with a buckled disc going fast, the pulse will come and go so fast you might not notice it. Wheels can go out of balance within two seconds if the weights are on the rim and have not fitted tight, or the stick on versions can also disappear, but being inside the rim they do not get noticed. They can of course cause a shimmy, but not brake pedal pulsing. Now to the violent shimmy, if the top and bottom swivels have play, then they often need something to 'set them off', that is they can cause an oscillation that will 'bounce' from wheel to wheel and can be felt through the car and through the steering. This 'death wobble' as it is often named can be set of by a buckled disc, disc out of true, loose wheel bearings, other worn components in the suspension steering, hitting the broken white lines as you exit a motorway onto the slip road is a classic cause, or a bump in the road when on a bend i.e. you have loaded the steering but, barring the wheel bearings, none of this will cause a pulsing brake pedal. Something else to consider, your discs could be true when cold but, as they heat up (sticking piston for example) they start to run out, rare but it does happen.

regards

Dave
 
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@Ryan Thomson

Without wanting to sound like an arse, you are making it very difficult to diagnose!

Post 1, started with 'a shimmy of sorts', Post 2, you inform of a brake pedal pulsing, Post 3, we get 'can be quite violent'?

So to clarify, if you have a slightly out of true disc this WILL cause pulsing at the pedal, if you go slow then the pulse is slow, if you go fast the pulse can disappear, think of it as a light on the end of a piece of string, if you spin it fast enough it will appear to be forming a circle, with a buckled disc going fast, the pulse will come and go so fast you might not notice it. Wheels can go out of balance within two seconds if the weights are on the rim and have not fitted tight, or the stick on versions can also disappear, but being inside the rim they do not get noticed. They can of course cause a shimmy, but not brake pedal pulsing. Now to the violent shimmy, if the top and bottom swivels have play, then they often need something to 'set them off', that is they can cause an oscillation that will 'bounce' from wheel to wheel and can be felt through the car and through the steering. This 'death wobble' as it is often named can be set of by a buckled disc, disc out of true, loose wheel bearings, other worn components in the suspension steering, hitting the broken white lines as you exit a motorway onto the slip road is a classic cause, or a bump in the road when on a bend i.e. you have loaded the steering but, barring the wheel bearings, none of this will cause a pulsing brake pedal. Something else to consider, your discs could be true when cold but, as they heat up (sticking piston for example) they start to run out, rare but it does happen.

regards

Dave

@ Dave, that is without doubt the most comprehensive post in this thread and I am being completely serious when saying it's very much appreciated. With regards to the issue, imho It is a shimmy, I probably wouldn't go as far as death wobble but it does seem a touch subjective.

I'm at a loss with this issue I am having. I tend to research things to death, and in this instance have researched the web on causes. I am coming up with a blank hence the post on here. I am well aware of what I don't know and that's why I'm asking here as well as posting up as much info as I can.

I'll be checking the alignment as well as discs this coming weekend with a dial gauge and micrometer to establish whether or not they are our of true or if the thickness varies. Fingers crossed, with the collective help of the forum I'll get to the bottom of this. Again all input, suggestions and comments are much appreciated. I'm no pro at this so I need all the help I can get.

Cheers
Ryan
 
I sure know "death wobble"! And violent with that, seriously scary.:wtf: I have 5" lift and equivalent of 34's... I previously had not enough castor correction. . The drop boxes have taken the violent part away from it thankfully but I have grown to believe that there definitely something that WILL get it to mildly wobble. Big tyres do have massive leverage on the steering. But the camber in the road and slowing down has direct influence on the intensity of the wobble which is only on the passenger side.

The other day I clicked on your thread of your 12"fox slinky setup Ryan with spacers in the front too. You seam to have a tall lift and only correcting on the axle.. From my experience but dont know the exact quantity of lift? Have you sufficient castor correction?

In my experience of 80 series front axles. ..... I dont like them (sorry to offend ) 8" differential. .2 bolts in the top knuckle. .. both steering rods on the lower 4 bolts (passenger side )wtf. If I keep wheeling and running my 81 I aim to fit the far superior 60 series front axle 9.5" diff and a much stronger steering/knuckle setup.

Thats all!:think:
 
Ryan, I really don't think that you can take anything as read here. Rebuilt calipers ... New discs ... It has to be one of the things on the lists mentioned. There aren't any other components. You just need to go through each one again. Reason I say this is that I put new cals on the back and fully rebuilt the front. In the Summer. Couple of weeks back, found pistons so jammed I had to jack them out. If all we did was drive to work and back, I'd be surprised. But trips to Wales, Morocco, winter salt etc just changes the landscape so much. Add to that your 37's which need more braking effort and there could be all sorts out of kilter. I have, as I said, had a shimmy just like yours so I don't doubt it. I have also had a killer death wobble and I really mean killer. And I have had the pedal pulsing. If this was Frank's showroom standard truck I'd be at a loss, but yours has had a fair bit of work and stuff doesn't always even out the way we'd like. Things change. I tried all sorts on mine and succeeded in just moving issues around. Then one day found that the front, lower, PH rod captive nut had come adrift inside the bracket. Purely by chance. It had even been though a decent MOT like that. Wiggling and jiggling doesn't always show up these issues. 3 tonnes of metal braking from 70 is sometimes the only way there's enough force involved. I'd get the truck on 4 stands and work though each component. It may only be a little bit of one thing and a bit of the other. But I'd suggest disc check first. Then all big joints and bushes. Get a pry bar in there with no weight on the wheels and give it some right proper wiggle.

Remember that with wheel balance you often on get wobble at a particular speed range, so hardly odd that you'd get it slowing down for a short period too.
 
Just a thought with regards to the steering impact that put your steering off centre. You say you altered the tie rod but nothing appeared bent, is it possible you have a twisted sector shaft as it leaves the steering box? If you have then whilst that would not cause the shimmy, it could leave your steering with a weak point. Also check the welds around the panhard rod mount, and the metal for cracking around the steering box. Again this will not cause pulsing on the brake pedal, but may be worth checking from a safety point of view?

regards

Dave
 
Well it looks like I have plenty of homework this weekend. Thanks for the input guys. I'll report back on Sunday.


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