'91 K75S hop and wobble

[kurtk75s]

Sorry misunderstood what you were asking.  I don’t know, I didn’t remove the bell housing from the engine, only the tranny from the bell housing.  So I have no idea.

[daveson]

Anyways I spose that would result in a vibration rather than a hop so unlikely. I thought maybe you removed the engine from the frame. If you look you should see the spacer between the frame and the housing.

Did you change to a k1200 rim in between, or something, or it's still original?

[frankenduck]

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.

Try Rocky Mountain ATV. (Where I get my Bridgestones.) They usually have the best prices. And the T31s are good tires too if those are still available.  Brdigestone usually has $50 rebates on a pair in the spring and early summer.

[kurtk75s]

Partial success:

I backed off the steering head bearing preload to 'just under too tight'.  I wish that I had only done that - but I also took off the front wheel and check for run out, out of round and fork alignment using the 'axle test'.  Everything looks great:  nothing is out of round, if there is any lateral run out it is probably from my measuring setup (maybe 0.5mm max?).  No dings.  No runout on the brake disks, etc.  The axle slides into the left fork tube easier than it ever has since I've owned the bike!  Just very light finger pressure.  I didn't change anything, though.  Just retorqued the bolts on reassembly.

The good news:  I no longer feel the 45-50mph wobble.  I believe it was the steering adjustment that 'cured' it:  Frankenduck called it!  The odd thing is that the wobble developed while the bike was sitting.  Then I replaced the steering bearings and still had the issue.  It wasn't until I went too tight and then backed off that anything changed. 

Bad news:  the 'hop' remains.  It is at its worst at about 35mph.  It doesn't matter what gear the bike is in, I had the same symptom in 2nd, 3rd, and 4th at the same speed.  I'm calling it a hop but it feels kind of like going over highway expansion joints that are really close together so it feels rhythmic.  It isn't a sharp jolt, just and up and down.  Applying brake pressure doesn't change it.  Worst when off the throttle or trying to maintain speed. It is most noticeable in the handlebar and front fairing - but I am not ruling out the rear end.   I don't feel the rhythm at higher or lower speeds.  Also, it happens on perfectly flat new asphalt!  Bumps aren't needed.
 At 50mph+ the bike is now feeling like it used to.

Since it is speed related, that narrows it down to drive shaft, final drive, wheels and tires, front wheel bearings, brakes.  Did I miss anything?

It appears that I can only deal with one problem at a time.  Compound issues are too much!

[frankenduck]

Do the forks feel stiff?

The 280 ml you put in is right quantity for the S forks.  Did you measure how much came out when you drained them? (I do this sometimes on recently purchased Ks.)

If the front end is "hopping" when slowing or maintaining but not accelerating then it's possible that you have too much oil in the forks.  Some aftermarket springs are thicker than stock springs so they take up a little more volume in the forks.

Another thing to try would be to drain 10-20 ml out of each fork to see if that helps.

I had a K100RS4V once that "pogo'd" on the freeway.  Draining a little oil out of each fork solved that.

[kurtk75s]

I did not measure what came out but I know that I put 280 in the last number of times I changed it.

The forks don’t feel overly stiff but I am thinking that it is suspension related.  I just got back from an hour ride where I really tried to concentrate on the source.  It really feels like it is coming from the front end.   I can see the front wheel moving when it is happening.

I will drain and refill with less oil and see what changes.

Thanks for the suggestion.

[kurtk75s]

I took out 20ml from each side.  The hop still persists but nothing felt worse.  I’ll take it on a longer ride and see if ai feel any difference.

Maybe this is like the steering bearings:  I have to keep taking oil out until SOMETHING happens.

[The Mighty Gryphon]

Hopping in the front end on smooth pavement(not sure where you can find that in New York State) tells me there is something wrong with the tire and wheel, most likely a balance problem.  It's also possible(but unlikely) that the tire could have a weak spot in the sidewall that is collapsing a little as the wheel turns and has a resonance around the speed where it feels the worst.

I would double check the wheel balance of the front wheel.  Is it possible that one or more weights have fallen off?  I do my own tires and some of the weights I have don't seem to stick that well to the wheel and I know I have lost several that I've had to replace.

If the balance is perfect as you say, try inflating the tire to 50-55 psi and see if that helps.  That might confirm a weak sidewall.  Just be very careful cornering because an over inflated, hard front tire will easily wash out if pushed even a little in turns.  Don't leave that much air in the tire beyond the time it takes do do a short test ride.  Overinflated tires are dangerous.  I found that out the hard way a long time ago.  Take the pressure back to 38 psi, that's where the Shinko's like to run.

I had a 1994 K75S, and ran Shinko 712's on it successfully.  Those tires work very well on the K75 front end.  The bike was a real pleasure to ride with those tires. 

[kurtk75s]

I would be completely on board with your assessment except for one thing:  my hop smooths out with higher speed.  I would not expect that with a grossly out of balance wheel.  When it is hopping, it surely feels like it is grossly out of round.  Maybe the roads aren’t smooth but they sure feel that way using any of our other vehicles.

Pressure is an easy test and I’ll give it a shot.  But I’m not taking the wheel off anymore.  I balanced the old tire twice, the new one once and checked it once again.  If anything is out of balance then my technique of doing it and checking it is inadequate so the old saying about doing the same thing and expecting different results…

I’m also coming to see if adding 100lbs of ballast changes anything.

[frankenduck]

55 psi? 

I wouldn't go past the max pressure on the side of the tire.

[kurtk75s]

Put 50 psi in the front wheel.  Felt about the same.

Went down 38 ft, 44 rear and then took my wife (100 lbs) out for a spin.  She is a rider, also.  The hop was more noticeable with her on the back.  She commented that she felt the hop was directly under her.  She said she could feel it at all speeds, even slow ones.  I tried sitting on the rear seat on my ride earlier but I didn't notice a difference.

I put the bike on the center stand and ran it up through the gears.  There is a little vibration at slow speed in 1st gear but nothing noticeable at 40mph in 4th (where it was very strong with her on the back).  We also both noticed a bit of rear brake rub in one part of the rotation.  Either a slightly warped rotor or a misalignment in the final drive?

There are only 3 different parts between when the bike ran OK in 2017 and when it exhibited this hopping last month:  1) front brake rotors and pads, 2) new drive shaft, 3) used final drive.   And, of course, the reassembly to add all of those things.

Maybe my rear tire 'went bad' while sitting around and then I got a new tire that exhibited the exact same symptoms?  I don't bet on those types of coincidences. 

Is it possible for a final drive to be this out of whack?  If so, how would I diagnose?

I wish I had my old drive to test with.

[Laitch]

Keep adjusting the rebound damping on the Fox shock. If you can't eliminate it by doing that, get a different shock.

[kurtk75s]

The shock has about 38 'clicks' of rebound adjustment.  I had it set at about 26 out (12 from max).   Compression adjuster is set at 3 out of 8 (the suggested starting point by FOX). 

I will certainly play around with the rebound adjuster per your suggestion. 

I will admit that the initial settings, including sag/preload were made with the help of a good friend who is a mechanical engineer, road racer and race bike builder.  At the time he was the national champion in AHRMA twins on a vintage Moto Guzzi.   I really just did whatever he said, he made the measurements and wrote everything down.  I thought I understood at the time but it was a LONG time ago.  The bike handled absolutely beautiful when he was finished with the setup.  I never adjusted anything after that.  There.  I'm not afraid to make a 'true confession'.
   

[kurtk75s]

I set the rebound adjuster at both max and min and it made no difference in the hop.  I also adjusted the compression to max.  I can't feel ANY difference in the way the bike feels or handles with ANY of the changes I made.  Either FOX shocks really aren't adjustable or mine has lost its oil/gas charge.

It doesn't handle poorly, however.  It doesn't wallow through turns or act harshly over bumps.  It feels like it always has except for the hop. 

[Laitch]

Given your decades of riding/maintenance experience and the depth of knowledge of your accomplished cohort of friends brought to bear on this machine, this simply might be a case of sensitive nether regions like those of the princess described in HC Andersen's The Princess and the Pea. Nothing can be done; it's just special circumstance. 

Still, the shock has a couple of decades and tens of thousands of miles on it that, despite the moderate load imposed on it by you and your wife, can create sufficient wear to make the whole more sensitive to inevitable imperfections of tire composition and balance. Can anything be done to a Fox Twin-Clicker to restore it to its glory days? Here's some interesting reading.

I still haven't ruled out an ancestral curse at play here. Yours wouldn't be the first. Spirits can hold a grudge. If everything else fails, Ancestry.com could hold a clue.

[kurtk75s]

The hop definitely isn't subtle or me being overly sensitive.  It is bad and really feels like I have an egg shaped tire.  My wife noticed it within a quarter mile. 

Just to be clear, my wife rarely rides on the back - she has her own bike (with more power than the K75).   If we do go 2 up it is never on the K75.

I went looking to see if I had squirreled away the old shock when I replaced it 20 years ago.  I'm pretty sure I got rid of it. 

I did send an email to Ted Porter last fall asking if he rebuilds Fox shocks (someone in his shop had experience working with Fox).  He responded that he did not.

I'll ask my friend if he has a shock suitable for a K75S that I can try out.  He has a K75T but he replaced the front end, fairing, and entire rear end from a K100RS.  The expectation was to turbocharge it but that is still a project waiting in the wings.   

Thanks for pushing me down the rear suspension path.  It would have taken me a lot longer to start looking in that direction.

[kurtk75s]

I was contemplating this question along with why the diagnosis wasn't more obvious to me.

At least I have an answer to the 'how':  when my bike was on the lift for a couple of years before I did the refurb, I now remember cleaning up a puddle of oil under the final drive and thinking 'I'll have to figure out which seal is leaking'.  And eventually forgot all about it.  When I sent my drive out to have the pinion shaft replaced, I drained the oil - and a lot of it.  I didn't even give a second thought to the fact that I should have expected LESS oil to come out if the drive was leaking.  I obviously didn't sniff the oil in the original leak - just threw a shop rag on it and cleaned it up.  I was 100% convinced my FD was leaking.  Never even thought about the shock.  I probably wouldn't have remembered that shocks HAD oil if you asked me where all the fluids were in the bike!

Even yesterday I cleaned a couple of drips of oil from the FD drain plug and the bottom half of the FD had collected a lot of dirt held in place by oil.  I still didn't attribute that to the shock. 

So the tell tale signs, the tea leaves, were there for the reading.  At the time I was illiterate.  I feel like I can now 'see Spot run'  - but that is about it.

I leave this admission to entertain the experienced and provide food for thought for the inexperienced.  I sure would have liked to read this post in early July.

[Past-my-Prime]

So possibly I'm obtuse (I've been called worse) but tell me: did you change the rear shock and the problem went away?

[kurtk75s]

Things don't happen that quickly, I'm afraid - I just made the 'discovery' that my shock is very broken yesterday. 

I've been in contact with a fellow that rebuilds Fox Twin Clickers and I just got it in the box ready to send tomorrow.  I just logged onto my computer to print out a mailing label.   Unfortunately, with shipping and his turn around time, I probably won't see it again for another couple of weeks. 

With what I know now, I would have had the shock rebuilt first before screwing around with any of the other stuff I did.  I am glad that I took the forks apart, though.  There was a lot of grunge at the bottom of the sliders.  I forgot to mention that when I initially went to drain the left fork leg, nothing came out!  The drain hole was full of 30 years of sediment.  That was the indicator that maybe I should look inside.  I did change the oil a few times in the bikes life but it hadn't been done since 2010.  Even with that neglect, the fork internals were fairly clean.  I changed the seals because it only took a couple more minutes and the ones I took out were original.  They were working fine but that is still 30 year old rubber.

I wish I hadn't changed the tires.  The ones that were on there were only 1/2 worn - it is unlikely that I will put the worn ones back on.  I guess I'll keep them for spares if one of the new ones gets damaged.  But now I have 10000 miles before I can take advantage of Frankenduck's radial suggestion. 

I will come back to this thread and update it with my findings regarding the rebuilt shock. 

[kurtk75s]

I just got the shock back from being rebuilt and installed it.   Unfortunately, it did not fix the hop.  The rear end of the bike definitely feels 'tighter'.   The intensity of the hop does seem to be reduced.  But only slightly.

At higher speeds, everything smooths out.  30 to 45 mph is where it is worst.  Severity depends on the road.  At its worst, it feels slightly like riding on a washboard gravel road.

I don't notice it under acceleration.  Worst at constant throttle and decelerating. 

Thanks for the help and ideas.  I'm disappointed that my project ended up here.  You win some and you lose some.

[Laitch]

Thanks for the update, kurt. 

[daveson]

Could the fluidbloc damper be worth considering again?

[kurtk75s]

Since the problem now is ‘up and down’ vs ‘back and forth’, I really doubt it is the fluid block.  My next moves are:

1.  Remount front tire rotated 180 degrees
2.  Heavier fork oil
3.  New fork springs

If that doesn’t work, I’ll probably go Race Tech or something.  Or maybe try a different tire.

But first I’m going to the Green Mountain rally.  On an R12RT.  😁

[Chaos]

I hesitate to mention it because I've never used them and doubted their effectiveness, but it might be worth trying dyna beads as a last resort.  Plenty of people swear by them, who knows?

[Past-my-Prime]

Since the problem now is ‘up and down’ vs ‘back and forth’, I really doubt it is the fluid block.  My next moves are:

1.  Remount front tire rotated 180 degrees
2.  Heavier fork oil
3.  New fork springs

If that doesn’t work, I’ll probably go Race Tech or something.  Or maybe try a different tire.

But first I’m going to the Green Mountain rally.  On an R12RT.  😁

Will you do each one, then rebuild, take it for a ride, and see if it fixed the problem? Or will you do all three things, put it together, and see? If you choose the former, it will be more educational for us, but with more assembly / disassembly work for you!.

Actually, fork oil change is pretty easy. I wonder if the volume is correct?