Handlebar oscillation

[Macman900]

Hi there, this is turning into a bit of an epic and I'm losing hair and sleep in trying to sort this. Just wanted to put this out there to get some feedback on a problem thats developed on my 1994 M900. Any helpful thoughts from folk who may have experienced similar issues gratefully appreciated.

Problem:
From pulling off up to about 35MPH there is an oscillation of the bars. Sort of a constant push-pull effect. Only a slight shimmy and not a full on wobble. After about 35MPH it stops and appears fine (probably due to the spinning/rotating effect of the wheel at speed). It appears again when returning to around the 35MPH mark and below. It's most noticeable and annoying when filtering. Another characteristic of the problem and more worrying, is the loss of feel from the front on roundabouts, corners, bends etc. To stop it dropping in more than usual (an boy is it noticeable sometimes!) I'm having to weight the opposite bar or foot peg it's that noticeable. Also when stationary and moving the bars from left to right (backing into a parking slot etc), theres definite drag on the tyre or front end as if the tyre was completely flat.

The bike is off the road until I can find out what ever is causing this effect. Below is whats known so far and whats been checked. I've had the bike for 19 years so know its handling characteristics so I know something ain't right.

Tyres:
Pressures are as per specs. Both front and rear tyres are new (600miles so far) and problem was there while old tyres were fitted. Both wheels show no obvious signs of damage, cracks, dings, dents etc. Both spindle/axles are straight and true, no damage. Cush drive rubbers in rear wheel are fine and rear wheel alignment is good.

Brakes:
Both callipers, pistons, pads etc are fine, no problems with binding or dragging.

Steering head bearings:
Only thing I've checked here is the amount of play when off the ground. It feels fine with no obvious movement or play. Though folk have told me that there could still be wear in the bearings even if play isn't felt. Not sure about that one!!

Rear shock/Swing arm:
Rear shock is a non-standard Ohlins item which sits in its mid-adjustment position and never gets played with. Both top and bottom bushes are fine and theres no obvious sign of wear to the shock and its functioning. Up and down check of the swing arm feels ok with back wheel fitted. Theres no obvious side-to-side play in the swing arm with the rear wheel removed. Tie-rod end bearing are new on the suspension hoop and the hoops linages/bearings etc feel good. The height of the tie rod end bearings on the rear hoop were set to the height of the ones that were replaced.

Front forks:
Non-adjustable type which have had new bushes, oil, seals. Spring length checked, damper rods checked. Not serviced by me. Have had contact with an American bloke online who said he had almost same problem with his M750. Turned out that a damper rod had worked loose at the bottom of the fork leg and so that leg was only operating on its spring. Not sure about that one. Not sure how the forks work if one was out of sync if thats the right word. Other than that they have yet to be pulled apart for further inspection. I'm assuming they were put together correctly by someone I do trust.

So any further help in sorting this one out would be great. I'm sort of at the end of my knowledge really. I may have missed some stuff I've checked but am happy to go over old ground as it were.

Oh yeah...I can see no obvious cracks or splits in the frame that I can see. And you can see quite a bit of frame on a Monster!

Cheers in advance...

[rollo22]

First thing I would try is getting the front re balanced.
Could have been done in a hurry

[Nickj]

Get the balance checked and look for wear on the tyre, often a gentle ripple around the tyre surface can do this.
Otherwise it's probably just a combination of wear from the head bearings down, change one factor a little and it'll go away.

[Tosh]

Hi there, have you tried getting front wheel off the ground and spinning it by hand ? check both the rim and the tyre, also look at the discs. If all looks ok at the front check the rear also.
Maybe even prise the pads back off the discs and give the wheel a good spin, might show up then.
Good luck

[Dookbob]

If you havn,t ever changed the steering head bearings during your ownership, then I would change them now. Then make sure that after you have adjusted the new bearings, the bars will flop from one lock to the other very easily with no sign of any drag whatsoever. It is very easy to over tighten the head bearings. There is very little adjustment between too loose and too tight, you need to persivere until you are happy with it. Before removing the old bearings have a feel at the steering with the front wheel off the ground, any notchiness will point to this being your problem.

[Mr Gazza]

Sorry to hear about your problem, but congratulations on your scientific analysis and comprhensive checks.

You are not leaving many chinks in your diagnosis that I can find...Very mysterious.!

Normally I would point at the rear tyre as the prime suspect. Low pressure or squaring off being the usual reason. But you have covered that and it follows through from the previous tyres. The only thing that you don't mention is having checked the bead is correctly and evenly seated all the way round...Also that the direction of rotation is correct....Unlikely I know, but possible.

Are you useing the same pressure gauge all the time? Could that be at fault?
Have you tried putting 3 or 4 more psi in the rear as an experiment? If doing that alters the symptoms in any way, it could still implicate the rear tyre.

Exhaustive as your checks have been, the only other chink I can find would be in the steering head bearings. Over tight bearings can cause the oscilation that you have , but something would have to have been deliberatly done for them to be too tight?..Did you tighten them about 700 miles ago?
Perhaps they are on the verge of getting notchy? Have you very slowly and gently tuned the bars from lock to lock with the wheel off the ground to ensure the movement is perfectly smooth all the way round? Sometimes the MOT tester will have a rotating plate in the floor so he can test the lock to lock movement with the wieght on the bearings....Maybe worth a try, the steering head bearings are notoriously difficult to feel.

Sorry I can't be much help buddy, I love a good mystery, but I hope you get to the bottom of it.

Edit...Dookbob posted while I was posting....I agree wth him.

[jerry]

put in Taper roller head bearing from all balls£40 ,, much better than standard ,,

[Saint aka ML]

How did you check the brakes are not binding? Best is to make sure they are fully backed in to the calliper. Take calliper off and push the pads all the way in, then jam in a pieces of wood there.
Then take 2nd calliper off and do the same. Then put the callipers back on the bike but do not touch the brake lever at all at any time.

Go for a quick spin in safe environment as you will only have back brake.
If all is still there take front wheel off and remove discs then go for a spin.
Both tests will rule out brakes/discs as issue. Also when brakes and discs are off walking the biek should be very, very, very easy.

The oscillation could happen due to warped disc and pad binding. The drag on tyre would point to damaged front rim bearings, binding pads or damaged tyre/rim.

[crust]

If all is as it should be, tyres, bearings wise, it sounds like the previous owner has played around with the bikes set up.

First I'd go back to standard and work from there.

Front end should be 280mm from the underside of the bottom yoke to the top of the fork leg.
Rear end - the rod ends should be wound all the way into the hoop.

That should slow the steering back to normal. From there raise the rear in small increments. To keep seat height, raise the forks in the yokes a few mm at a time.

A lot of this is personal preference, most people go too far and create an unstable wobbly bike. Stick to small changes.

[bluestoesonnose]

Get the front tyre balanced, if that doesn't cure it check the front wheel for run out (it may be bent, it's quite easy to put an alloy out of true if you catch it wrong). If it's still happening check the rear wheel (balance and check for true) if it persists then it's into the S/A bearings and the head stock bearings and fork bushes.

Just a thought, you have checked the wheel bearings haven't you?