question Bedford MJ Cab shake

[robin craig]

Our Bedford CALM (crane lorry mounted) has developed a shake between 40 and 50 mph.

 

We have had a go at swapping wheels and tyres around which has calmed things down a bit. One was out of round it would appear.

 

We also have dropped and replaced the oil in the front diff and found a copper residue in the oil.

 

Th shake induced is an up and down rythm which still leads me to look back at tyres.

 

Does anyone have any thoughts and comments?

 

Regards

 

Robin

[Richard Farrant]

Our Bedford CALM (crane lorry mounted) has developed a shake between 40 and 50 mph.

 

We have had a go at swapping wheels and tyres around which has calmed things down a bit. One was out of round it would appear.

 

We also have dropped and replaced the oil in the front diff and found a copper residue in the oil.

 

Th shake induced is an up and down rythm which still leads me to look back at tyres.

 

Does anyone have any thoughts and comments?

 

Regards

 

Robin

 

 

They used to suffer from wheel imbalance with the old bartead tyres. The workshop i was at, had a rig to balance the front wheels on the vehicle, a dummy hub drive flange was used, so driveline did not turn. In later years the bartreads are no longer fitted, so I do not have experience of how they new tyres perform.

[robin craig]

Richard,

 

good point, yes we are on the newer tyres not the bar treads.

 

I am still thinking that the tyre being out of balance is capable of imparting such a shake, just in a general principle.

 

My clue is that the steering wheel shakes a little in time with the cab shake.

 

Any idea on the copper material in the diff oil?

 

R

[paulbrook]

Mmm tricky

 

Everything has to be just so and even then they still do it - 50mph is bang on the resonant frequency.

 

Sometimes it would be easier stopping the bears from going in the woods...

 

Try increasing the tyre pressures by 10 psi (and then try reducing them by 10...)

[robin craig]

I'm interested to hear about adjusting pressure, what is the max you would run?

 

What is the logic behind adjusting up and down, is it to change the point of balance of the tyre?

 

Has standing the vehicle for considerable time done this?

 

What can be done to prevent it in the future?

 

No comments about copper residue in the diff?

 

R

[N.O.S.]

Have you noticed that sometimes the oscillation is up and down, as if on both wheels, then gradually progresses to a rocking (side to side) motion?

 

And does it wear off when tyres warm up?

[Degsy]

Standing the vehicle for a long period does temporarily 'flat spot' tyres but it normally wears off when the tyres warm up.

Not familiar with MJ diffs but do they contain bronze and if so are you using the correct oil as EP oil will react with bronze.

No doubt Richard will know the definitive answer on the diffs.

[Richard Farrant]

Not familiar with MJ diffs but do they contain bronze and if so are you using the correct oil as EP oil will react with bronze. No doubt Richard will know the definitive answer on the diffs.

 

I can only think of the the thrust washers within the diff assembly, which I think are steel with a bronze thrust face. It is probably from these, and just a process of normal bedding in, if the trace in the oil is very fine. If the oil is the same as was specified by the British Army, then it is compatable with yellow metals.

[robin craig]

You make a good point Richard about the quality of the diff oil.

 

That truck gets serviced off site.

 

I'm sure they are using a commercial grade oil of the same viscosity but as you point out it may not be compatible with what you call "yellow metal".

 

R

[robin craig]

NOS

 

Not quite what you are describing. It is very obvious between 40 and 50 mph and the cab physically lifts.

 

R

[N.O.S.]

Tyre 'flat' bounce is noticeable at very slow speeds, so that can be ruled out.

 

Checked the play on top/bottom swivel housing pins?

 

You've swapped front and back wheels around?

 

Build-up of mud on inside of rim where you don't notice it?

[Burian]

I have an MJ with a radio repair body, initially with canvas. TP were 87 rear 54 front. After fitting body things were ok for about 10 months and then violent shaking. I tried taking the pressures up and down but no changes, the tyres seemed to have a hollow on the outer edge, front wheels.

Reading up on my toyota 4x4 I noted a reference to spooning which is the effect I had noticed on the Bedford. I discussed the issue with ATS and the guy suggested upping the pressure or short term, swop tyres with rears. I upped the pressure and it took 4/5 months for the shaking to reduce and the hollowing to visibly reduce. I only use the vehicle for shows so depends on your usage. You can feel the hollow if you run your finger over the tyre, which are the modern tyres.

Just thought as you have the heavy crane up front it may be part of your problem.

[Big ray]

Vibration at a particular speed is usually indicitive of severe wear on the centre bearings on the prop shaft, check for excessive movement in those bearings. The propshaft(s) will vibrate due to centrifugal forces at a particular speed.

You could of course have loose bolts in the flanges of the u/js, again the result would be violent shaking of the vehicle.

[Big ray]

You might also want to check that the dampers are working OK, but it still sounds like centre bearings.

[Richard Farrant]

Vibration at a particular speed is usually indicitive of severe wear on the centre bearings on the prop shaft, check for excessive movement in those bearings. The propshaft(s) will vibrate due to centrifugal forces at a particular speed.

 

 

Ray,

There are no centre bearings on a MJ propshaft, as it only runs from the transfer box.

 

This was an age old problem on MK and MJ, at a certain speed, as I have explained earlier. REME workshops had a rig to balance front wheels on the vehicle, this was in the days of them being fitted with Goodyear cross ply bar grips, seems like with new types of tyres, the shake is still there.

[Big ray]

Ray,

There are no centre bearings on a MJ propshaft, as it only runs from the transfer box.

 

This was an age old problem on MK and MJ, at a certain speed, as I have explained earlier. REME workshops had a rig to balance front wheels on the vehicle, this was in the days of them being fitted with Goodyear cross ply bar grips, seems like with new types of tyres, the shake is still there.

 

Sorry, I stand corrected.

[robin craig]

Our fleet as you can tell does not get a lot of use.

 

The Bedford CALM has not been on the road proper since our previous investigations.

 

Since that time I have been fortunate enough to come into the possession of some quite lengthy manuals in PDF format plus we have a parts book in good old paper format.

 

Looking through the manuals there is a procedure that involves removing the end caps in the centre of the hubs on the outside. The one lone bolt captures the end of the tracta joint shaft. There are some quite definitive measurements to take and we did this tonight.

 

Bingo, out of tolerance on the specs. The diagnosed failure in the manual is the thrust bushing is crushed / worn. I have some pictures of left and right, in and out of adjustment, will load them tomorrow.

 

Our plan is to go check our parts truck and if within tolerance then we will pull the end of the axle off with the hub as a complete unit and transplant it. It avoids the need to drop the whole axle. Much less work.

 

So I guess next Wednesday night we will measure the parts truck and report back.

 

I finally feel like we are getting somewhere, as usual, with the proper manuals.

 

Thanks to those who provided them

 

R

Edited January 17, 2014 by robin craig

[robin craig]

I did not have the info at the time, so here it is.

 

This procedure is laid out in the following:-

 

AESP 2320 H 100 523 DEC 83 reprinted Aug 93 with amdts 1 to 3 included.

 

It is all in chapter 6 para 24 + 25 to be found on page 4 of the chapter.

 

If anyone does not have this please feel to contact me.

 

R