Instability

[georgesgiralt]

Hello !
Last week, I was running my '82 R65 and got a big bug on my screen.
I left the handlebar live it's life and opened the helmet (using both hands).
As the "cruise control" screw was not free, the bike continued to ran at the same speed and course. (around 50 ~60 km/h)
I was very surprised to see the handlebar oscillate more and more and I had to stop it before it slap the tank.
As I had a very worn out rear tire, I changed it, checked both tire pressure and did the test again. Same reaction.
Front tire is a Lasertec at 2.1 bar pressure (as per Metzeler specs) Rear is a ME77 at 2.3 bar.
The steering bearing had been greased a couple of months ago and the play adjusted. they are fine. Rear shocks are Fournales OPS at 9.0 bar pressure.
So what should I check ?

[Barry]

I don't think your experience is that unusual.  Even one hand on the bars would have been enough to damp any tendency to oscillations. Assuming the wheel and swinging arm bearings are correctly adjusted what that's telling you is there is a need for some damping in the steering system.

How much pre-load do you have on the steering bearings, just eliminating all play is not enough. There needs to be some pre-load usually judged by the rate at which the bars fall from side to side but not so much that it produces a low speed weave through over correction inputs to the bars. The safest setting is as much pre-load as you can apply without inducing a low speed weave.

Maybe there has been some bedding in since you greased the bearings and they need tightening a touch.

[georgesgiralt]

Thanks Barry,
As I've changed the clutch recently and a couple month later opened the gearbox to replace the selection spring, I'm quite confident that the swing arm bearings are properly greased and adjusted...
I feel the steering head to be too loose but a mechanic I know says it's good.
So I will try to tighten it a bit and see what change it make...
BTW, how do you tighten the curled big nut ? By hand it seems I can't go farther than it is.

[Matt Chapter]

I have a head shake on my R65, but it's more common at about 45 to 50 mph, so almost twice as fast.

Makes it quite interesting to reattach the velcro on my cuff while at speed.

I've mentioned it to my mechanic (who does race setup on weekends and really knows his stuff) and my head bearing is fine.  In my case, it could be exacerbated by the fairing.  My rear shock could stand to be replaced, so that could be a factor too.

[montmil]

BTW, how do you tighten the curled big nut ? By hand it seems I can't go farther than it is.

I tape some cloth pads on a pair of slip-joint plumbers pliers and get after it. No marks. No errors.

Properly adjusted for preload, the forks should fall gently to full lock. Not hard enough to make a big "clunk". So sayeth the guru on the mountain top.

[Barry]

I feel the steering head to be too loose but a mechanic I know says it's good.

Unless your mechanic rides an airhead I wouldn't listen to him.

This is a safety issue at least as important as wheel bearing pre-load and look at the fuss we make about that. BMW prescribes the same approach to head bearing pre-load using some special measurement device that nobody has so that's why we use the "bars falling gently to the stops" method.  

There should absolutely be some bearing pre-load to provide damping. When you have it right you should be able to feel an increase in resistance to turning the bars over and above  just eliminating all bearing play.

Bear in mind the pre-load changes when you tighten the top screw so some experimentation is usually needed.

The adjusting ring is a daft bit of design as you have almost no hope of tightening it adequately by hand. It should have had the slots for an adjusting tool just like other airheads.

[steve hawkins]

Please bear in mind that this is a short wheel base airhead.  When the first R75 was released, it also had a short wheel base and suffered from complaints of a slight instability - the result was a lengthened swing arm and the longer, larger capacity airheads.  

But the main difference between the R65 and its larger rivals is again the swing arm length.  The fork rake and trail is the same or very, very similar.  But it has the shorter swing arm, which is why the rear shocks are different length to those fitted to R50/60/75/80/90/100.

Check your bike over thoroughly and make sure that everything is working as it should be, then rejoice in the fact that you have nimble bike, rather than a long, lazy tourer.  The bike was originally marketed as a mid size bike with mild sporting pretentions.

When I rebuilt mine all those years ago, as a café racer, I also found a little wayward/playful wag of the handlebars at certain speeds, especially when I was having fun.  I thought about a steering damper, but as the 'wag' never manifested itself as anything more sinister, as long as I was holding the handlebars,  I never bothered.  I am not sure it even does it now.  But that might be because I running a 19 inch front wheel rather than an 18 inch one

If a little head shake still bothers you, buy the BMW steering damper.  Otherwise put it down to a little exuberance.

Cheers

Rev. Light

[trips]

I get a bit sometimes in mine with the side car on now, but only in specific cases, like accelerating briskly while turning left onto the road as it crosses 40 mph or so, and hits just the right bumps. Going straight down the road, I can let go of the handlebars for a bit, it goes surprisingly straight, considering the side car, and no headshake. My 2010 Rocket Touring, on the other hand, has always had headshake since new, and front tire premature scalloping, my dealer helpfully said "that's just the way they are".  [smiley=rolleyes.gif]

[nhmaf]

I also suggest a check for the proper steering head bearing preload, and, if that doesn't bear fruit, try first adding a bit more air to the front tire - You are only at about 30.4 PSI, I generally run mine at 34-36 PSI, and I KNOW that if it drops down to 25 PSI I will get a brief weave/oscillation as I approach 35-40 MPH.. I do have different tires (Bridgestone BT45) and I have progressive (stiffer) front fork springs and rear shock springs.   you might also try lowering the pressure slightly in your rear shock absorbers - sometimes these wobbles are not due to loose bearings but from mismatched suspension compliance.

[Barry]

For anyone interested I had a look at the BMW method of setting the head bearing pre-load. This is one example where the BMW manual is not useful as it gives brief description of the method then fails to provide the actual torque value needed to carry out the test.

As far as I can make out the method is to apply a temporary stop that restricts the steering lock to 20 deg which is approx. half travel. Then a torque wrench is used on the cap screw to measure the force required to return the forks to the centre position.  Snowbum gives the setting as 2.6 ft/lbs . I don’t have a torque wrench that would be reliable at that setting so unfortunately I can’t try it.

[georgesgiralt]

2.6 ft/lbs ....
Hu ho...
As I'm far from confident enough on Imperial units, can someone more knowledgeable translate this in mN or mkg ?
Mani thanks in advance !

[Barry]

3.5 Nm

[georgesgiralt]

It's what I thought.
I've a small torque wrench. Have to look at the minimal setting, but I doubt it can go so low a figure... It's a torque screwdriver's value !

[Tony Smith]

 Snowbum gives the setting as 2.6 ft/lbs . I don’t have a torque wrench that would be reliable at that setting so unfortunately I can’t try it.

Wrap a wire tightly around your favorite 1/2" cheater bar and bend a hook onto it such that the centre of the hook is 1 foot from the centre of the allen key socket. Tie a piece of string to the hook and the other end of the string to a reasonably accurate spring weight scale - the type you buy at fishing shops for weighing fish. 2.6 lbs on the scale at 1 foot along the cheater bar is 2.5ft/lbs of torque at the steering head..

Having said that i have an opinion that head shake is usually due to a tyre mismatch, a frame alignment problem and sometimes a suspension problem. Forgive me if I am wrong, but isn't the Metzler lazertech a radial and the me77 a crossply? If so, the differing reactions of the tyres could set up a shake right there.

I once tried a set of flash dunlops on the RS and it developed a terrifying tendency to tank slap at 60kph that disappeared as soon as I changed the tyres. There was nothing wrong with them, they ran just fine on something else I owned, the BMW just didn't like them at all.

[georgesgiralt]

So my smallest wrench is from 8Nm up to 54 Nm.
Not fit for the job.
I've to ask a co-worker which is a fisherman if I can borrow it's fish weighting tool  8-)
As per the Lasertech, I do not know what structure it is. All I know is that the seller had only one suitable tire that day and it was this one, V speed rated and very expensive. As the ME22 was cut open by a piece of steel found on the road, I had no other alternative...
When the new ME77 will be worn out, the lasertech will be too. And at that time I'll fit a couple of Michelin Pilot Activ tires. A couple of friends o f mine use them on an old Guzzy and on a 1000 RS with great pleasure.