'91 K75S hop and wobble

[kurtk75s]

My K75S was put back on the road in July (I have a refurb thread in the Classic Projects section).

Everything is running great: except I have a slight 'hop' at slow(ish) speeds (30mph in villages) and a bar wobble in the 45-50 mph range.  I can feel the bar wobble and if I take my hands off it is really noticeable.  I did not have this issue when I started in on the project back in 2017.  I was pretty in-tune with the bike:  the last ride I took before the refurb was a 4500 mile cross country trip.

Here is what I've done so far:

1) Forks: Disassembled, cleaned, new seals and dust caps, new 7.5w fork oil.
2) Steering:  new steering head bearings, inspected fluid blok - still had grease and appeared to be functioning as designed.
3) Reassembled front end paying extreme attention to getting the 'axle' test right.  Steering head bearings adjusted.  I will need to check that in a few miles since I just put in the new bearings.

The above made no difference to my symptoms.

4) While I had the front end off, I checked the roundness of the tire, the wheel for any dings or bends and overall balance.  Everything looked great but the tire did have 4900 miles.
5) Replaced the front tire and rebalance.  Again checked to make sure that the axle slides in with almost no effort.

Again, no change to the symptom.

6) The last thing I did was to pull the rear wheel, check for roundness and for any dents (looked fine).  Also rebalanced it.  Ended up taking off the 14 grams that I put on 400 miles ago. 

Again, no change.

So now I am here because I'm stumped.   

Other details:  the bike has 75000 miles.  Front suspension is the stock 'sport' forks (s stamp) - I put in 280ml of fork oil per leg.  Rear shock is a Fox Twin Clicker that has about 45000 miles (20 years old).   The final drive and drive shaft were replaced during the refurb.

Any ideas?  I'm at the point where I don't even know where to go from here.  I am really surprised that none of the stuff that I've done so far has made ANY change.

Thanks.

[Laitch]

5) Replaced the front tire and rebalance.

What model is the tire, what is its size, how was it balanced, what is its tire pressure spec, what pressure do you keep in it and how much do you weigh? Is the tire valve rubber or metal; angled or straight?    This condition started in July 2021?

[kurtk75s]

Shinko 712.  100/90.  I weigh 185.  I have tried pressures from 28 to 40 psi in the front.  Tire pressure does not appear to change the severity of the symptom.  Valves are the straight rubber kind.  I did not change the valve when changing to the new tire.  The new tire is the same Shinko because I couldn’t find Michelins anywhere.  The original shinko did not exhibit the hop and wobble for the first 4500 miles of its life.  My assumption was that, if it was the tire, it was because of wear or deformation, not the intrinsic nature of the tire. Yes, this was a ‘new’ thing that I noticed when the bike went back on the road in July.

[kurtk75s]

Also, the front end was unmolested in the refurb except for new EBC brake rotors.  I did verify that the wheel spins freely with only very light brake pad rubbing and it is uniform throughout rotation - no binding at all.  All of the fork work was done after I noticed the wobble.

Wheel bearings were replaced about 30000 miles ago.  I don’t notice any binding, grittiness, or play in them and they rotate nicely on the axle.

[Laitch]

How was it balanced—static with weights, dynamic with weight or internally with beads?

[kurtk75s]

Static with weights.  The old shinko took 14 grams.  The new one took 56 grams.

[Laitch]

The last tire went 4500 miles then the moto developed hop-and-wobble during its next 400 miles on that tire; the moto hops and wobbles from the get-go despite a new tire. Hop-and-wobble preceded the front suspension service including steering bearing replacement and a rear drive/driveshaft replacement; it persists afterwards. Tire pressure makes no difference. What's left to consider? Those would be conditions that might have preceded all the service—rim defects, rear tire defects, rear shock absorber defects. Each could be setting up possible migrating vibration. Additionally, loose frame-to-engine bolts or rear wheel bolts could be in the mix. Even loose rotating parts like the clutch housing might be considered.

[kurtk75s]

The only correction that I will make to your analysis is that the final drive and driveshaft were replaced before the wobble was noticed. 

I did loosen and then re torque the frame to engine bolts.  I have also retorqued the rear wheel mounting bolts to 77 ft-lbs.  I do that every time I have the rear wheel off.  All bolts I have touched have been torqued using a clicker type torque wrench (I have a few) using the Clymer manual values. 

If a loose clutch housing can cause a consistent handlebar wobble at 45 ish mph then I may as well hang it up.  I just can’t make that connection in my brain.

I have a new rear tire that I will replace, since I have it.  I’ll check the rear wheel while I have the tire off.  I’m not holding my breath, though. 

I doubt that defects in the wheels or the shock occurred while the bike was sitting.  I would guess it is something that I touched - could the used final drive or the new drive shaft cause a wobble?  I don’t have any experience with that nor does it seem like a logical culprit but I seem to be beyond logic at this point.

[Laitch]

I doubt that defects in the wheels or the shock occurred while the bike was sitting. If a loose clutch housing can cause a consistent handlebar wobble at 45 ish mph then I may as well hang it up.

Defects in the wheels or shocks could have developed in the 4500 miles you rode it then manifested themselves in the hop-and-wobble you described that occurred in the last 400 miles before you put it up for service, if I'm reading your tale of woe accurately. A failing bearing in the crown wheel of the rear drive can cause wobble.

As far as hanging it up goes, if an open-minded approach to investigation is beyond reach, the remaining choices would seem to be hanging it up, selling it or adapting to the wobble and enjoying the ride.    I haven't ruled out exorcism, either.

[frankenduck]

Which way does the rotational arrow point on the front tire? Hopefully the correct direction.

Jack up the front end and confirm that the steering bearings aren't lose.

Next time buy radial tires.

[kurtk75s]

Laitch - I think you may be misunderstanding something.  I rode the bike for 4500 miles with the shinkos and everything was fine.  Then the bike sat for a couple years.  Then I refurbed it over the last year.  Then I rode it for 400 miles since the refurb.  It has been hopping and wobbling for the FULL 400 miles since being on the road.  This DID NOT develop during the last 400 miles of riding. 

You are spot on with the possible options that you laid out.  The one that I will absolutely not entertain is 'live with it'.

[kurtk75s]

Frankenduck,

Thanks for your response.  in your experience, my problem HAS to be with the tires or the steering head bearings?

What radials are available for the K75S?  I have only used bias ply in my 72000 miles, 26 years, of owning this bike and have not had this issue before.

[kurtk75s]

I have been thinking a lot about my problem with the K75.   

This series of videos pointed me in the direction of 'stiction' as a possible cause:



I definitely had a slight misalignment of my forks when this issue popped up - BUT:  that alignment issue had been there for ever.  I don't think I've ever removed the forks from the bike prior to last week.  When I did reassemble everything, I made sure that the fork internals were spotless and then torqued meticulously from the top triple tree, then the axle bolt, and left side clamps, then adjusted the steering head, then bouncing the forks up and down, then torquing the fork brace, make sure the axle slides easily, check steering head again, then bounce, then lower triple tree clamps, bounce and, finally right side axle clamps.  Then I applied the brakes and tightened the brake caliper attachment bolts.  I am positive that I now have good alignment of the forks and that the steering head is adjusted properly. 

And all of the above made no difference.   The only wear item I haven't replaced are the fork springs but I don't notice any change in how they perform - it isn't like they got real soft or broke.

So I don't believe my issue has to do with stiction, fork performance or steering head issues.

I also don't think my problem is with my rear shock.  Again, it moves freely when I bounce on the seat and it 'feels' just like it did before the issue.

It may just be my intense focus on the issue while riding, but it may have been a little WORSE after replacing the front tire.  Definitely no better.   

Hmmm, if it did get a little worse, maybe it is the more 'defined' tread interacting with whatever is causing the problem. 

I will focus on the back end now.  Replace the tire, check the wheel, disassemble and re assemble the drive shaft/final drive/rear brake/shock.  I'll run the bike in gear without the wheel to see if there is any noticeable defect with the final drive.

BTW, what's a rotation arrow?

Just kidding. 

[Laitch]

This series of videos pointed me in the direction of 'stiction' as a possible cause . . .

Frank is definitely the Mr. Rogers of BMW motorrad wrenchers. 

[frankenduck]

Frankenduck,

Thanks for your response.  in your experience, my problem HAS to be with the tires or the steering head bearings?

What radials are available for the K75S?  I have only used bias ply in my 72000 miles, 26 years, of owning this bike and have not had this issue before.

Radial tire info here: http://bit.ly/new2kbikes  (I don't mean to imply that radials will solve this issue. You shouldn't have wobble issues with bias ply tires either. I just mention that because K75s and K100s ride tons better on radials.)

IN MY EXPERIENCE, I've only had one tire that wobbled similar to yours (when the bike got up to about 30 MPH.) That one turned out to be tire treads that had worn poorly because the issue went away with installation of a new tire. I've also had low speed wobbles that were caused by loose/worn steering head bearings.

I also had very low speed wobble issues putting a K1100 front end on a K75. Since the steering stem of the K1100 forks has a narrower diameter than that of K75 forks the fluid-bloc steering damper inside of the K75 steering head is taken out of the picture. With bias ply tires the front end would wobble a bit at stop-n-go traffic speeds. When I swapped in radial tires that wobble went away.

Not sure if this will help but here's how I install forks:

a) Insert them into fork trees at the same height and torque fork tree pinch bolts.

b) Install the fork brace but leave the four bolts for the fork brace a tad loose.

c) Install the front wheel (and brake calipers), leaving the axle pinch bolts and axle end bolt loose. (Like the rider manual says.)

d) Get on the center-standed bike and jump the front end up and down a few times to get the alignment of things "settled." (With the front brake applied so that you don't accidentally rock the bike off of the center stand.)

e) Torque the axle end bolt, then pinch bolts.

f) THEN torque the bolts for the fork brace.

My theory in doing things in this order is that if you tighten the fork brace bolts too early then you can introduce pre-stresses in the front end. By tightening those last you shouldn't have that issue.

One thing I would try in your current situation is to loosen the fork brace bolts, axle pinch bolts and axle end bolt.  Get on the bike and jump the front end up and down a few times. Then retorque the bolts in the following order:

1 - Axle end bolt.
2 - Axle pinch bolts.
3 - Fork brace bolts.

Then go for a ride to see if that helps get rid of the wobble.

One other thing: Instead of balancing tires you might want to consider using Ride-On.

[The Mighty Gryphon]

What the Frankenduck said!!!  fork alignment is important and worth double checking.

[kurtk75s]

I'll give the front end one more shot.  If it does end up being that, it is crazy that I never gave fork alignment a thought in 74500 miles.  All of a sudden a problem shows up where the only thing I did on the front end is replace the brake disks and pads.  After that, I've tried to gnats ass the fork alignment multiple times and I can't get the issue to change at all. 

This morning I took off the final drive and assorted bits.  I reassembled and retorqued.  Changed the rear tire and balanced (only needed 14 grams).  Checked runout on brake disk to final drive, rim without tire mounted, and with tire mounted.  If there is an issue, I will need a dial indicator to see/measure.  I don't have one so I was simply using a magnetic pickup tool as a pointer to give me a reference as I rotated in my balancing stand.  I just don't see anything wrong.

As I expected, none of this did anything toward solving my problem.

One thing that I noticed:  with the bike in gear I can rotate the rear wheel a little bit.  Using the rear brake caliper as a reference, I measured 15mm of rotational 'play' at the ABS sensor ring.  Is that normal? 

[kurtk75s]

BTW, the only thing I did differently than Frankenduck's suggested method is that I torqued the fork brace second to last and then the axle clamp on the right side absolutely last.   I did leave everything loose and compressed the front end many times before beginning the torque procedure. 

It really feels like my front wheel is out of round but I don't see any indication of it.

[frankenduck]

One thing that I noticed:  with the bike in gear I can rotate the rear wheel a little bit.  Using the rear brake caliper as a reference, I measured 15mm of rotational 'play' at the ABS sensor ring.  Is that normal?

Yes, it is normal to have a little play in the drive train.

[kurtk75s]

I made a typo in the title:  my bike is a '91!   I went to the main Workshop page and thought - 'hey, someone with a '92 is having the same problem as me'!

I tried to take the bike out for a short ride and it feels like I'm riding a pogo stick between 25 and 35 mph.   It really feels like the new rubber made the issue worse.  So I turned around and went back to the garage and tightened my steering head bearings until they were definitely too tight.  Hop and wobble feels exactly the same except now making U turns is a lot more exciting. 

I know that I have a major problem with some component on my bike I just don't have enough experience to figure out what it is.  The symptoms are so bad that I expected to see SOMETHING that looked out of whack.   

When installing the new rubber (this isn't my first time, I've been installing my own tires on many bikes for over 30 years - I've been using the Metzler static balancer since the mid '90s),  the tires went on easily, they balanced fine and I even spun them as fast as I could to see if I could notice any 'washing machine' action.  They looked as good as any tire I've mounted in the past.  They didn't need excessive weights (although I should have payed more attention to where I put the light spot on the front wheel). 

The wrench to ride ratio for this bike is exceeding my tolerance level and I'm pretty frustrated.

Thanks for the thoughts on the topic.

[kurtk75s]

I just checked out the prices on the Bridgestone T32 at RevZilla.  Ouch.  Over $400 for the set.  I thought the Michelin Pilot tires were expensive (I just bought 2 sets of 'sport bike' size tires (120 ft, 180rear) for $689 shipped to my door).

If I knew that would fix my problem I would pay it, though.

[daveson]

Consider this after exhausting all the other options cause this is another wild shot.

You've had the wobble since it sat for a few years. I've had plenty of cars that sat for years until I got them. I'd drive home, hopping along cause the tyres got a flat spot where they sat. They might come good after an hour, maybe more. Dunno if a bikes heavy enough to do this, but if it sat on the side stand for years it could be a flat spot on the rear tyre. Maybe it would take longer to fix itself on a bike, there's not much weight, but then the contact patch is smaller too. If driven too long the high point with have thinner tread.

When you tighten the frame to engine bolts they should be done in a specific sequence for a k75. I think the sequence in the Clymer manual is not for a k75, but you'll find it in the BMW manual on this site page 46 - 67.

Hopefully the problem disappears when you change the back tyre.

[kurtk75s]

Thanks for the reply. 

Both tires have been changed but the issue remains.  Maybe even worse.

I had the same thought as you.  The front tire was in a clamp on my lift over the winter.  I was sure that was the problem.  But the problem remains after changing the front tire.  The rear wasn’t on the bike until this spring and even then it didn’t have any weight on it.  No change after changing out the rear, either.

I have an old Miata that sits on it tires over winter.  You can feel he flat spot until the tires warm up.  The problem with the K feels  a little different.  It becomes apparent at 20ish mph.  Then it feels like a hop.  At 45 the bars wobble back and forth.

[daveson]

Do you have a spacing washer between the engine and the bell housing? I think you should have.

[kurtk75s]

Umm.  No.  I think.