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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: kurtk75s on August 19, 2021, 08:27:15 PM
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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.
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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? :laughing4-giggles: This condition started in July 2021?
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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.
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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.
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How was it balanced—static with weights, dynamic with weight or internally with beads?
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Static with weights. The old shinko took 14 grams. The new one took 56 grams.
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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.
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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.
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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. :laughing4-giggles: I haven't ruled out exorcism, either.
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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.
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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'.
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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.
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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:
https://www.youtube.com/watch?v=tVWHfyG4jB4
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.
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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. icon_cheers
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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.
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What the Frankenduck said!!! fork alignment is important and worth double checking.
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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Do you have a spacing washer between the engine and the bell housing? I think you should have.
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Umm. No. I think.
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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.
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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?
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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.
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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!
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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.
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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.
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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.
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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.
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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.
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55 psi? :johnny
I wouldn't go past the max pressure on the side of the tire.
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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.
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Keep adjusting the rebound damping on the Fox shock. If you can't eliminate it by doing that, get a different shock.
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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'.
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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.
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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. :laughing4-giggles:
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 (http://mahonkin.com/~milktree/motorcycle/fox-rebuild/Fox-twin-clicker-rebuild.pdf).
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. :popcorm
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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.
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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.
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So possibly I'm obtuse (I've been called worse) but tell me: did you change the rear shock and the problem went away?
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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.
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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.
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Thanks for the update, kurt. 112350
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Could the fluidbloc damper be worth considering again?
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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. 😁
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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?
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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?
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Good suggestion. It might come to that.
The first time I changed tires on my RT, after breaking the bead I went to remove the tire and f-Ing dyna beads scattered all over my garage floor. I was not happy. So there is source of my prejudice against dyna beads. I’ve never had a balance problem in the last 33 years so I just thought they were a solution looking for a problem.
I’ve convinced myself that it is a problem with either: wheels, tire, front suspension. The rear shock is still a possibility but will be a last resort.
Dyna beads are relatively cheap to try, though.
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PMP, I will do each independently. The only thing I will double up on is: when I have the wheel off, I’m going to rotate the one of the brake disks 180 degrees as well. Changing the oil will only take a few minutes. Fork springs only slightly longer. I’m getting pretty quick at disassembling and reassembling both the front and rear end of the bike thanks to this issue. I think my last tire change and balance took less than an hour for both wheels!
Regarding fluid level. I may experiment with that AFTER the springs if I still have issues.
As a side note: I’ve started my winter project which is a frame up on an R90/6. I’m going much farther with that project because all of the seals need to be replaced. It only has 15k miles but sat in a barn neglected for 30 years. I am approaching it as an educational exercise. The K75 was too nice to really dig into. I can’t make the airhead worse than it is 😁.
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I read with interest this trail. Since your wife commented about the hop being under the seat, could it be that you have a stiff U-joint? That would be really bad for the splines too.
Alex
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Have you checked the swingarm and bearings for play or stiffness? Just tossing stuff out there.
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Probably all's I can toss in.
Don't remember if it's at the same speed in another gear. In a lower gear if it's still happening at the same speed, could point to suspension. If it's revs related, could point to the drivetrain.
Could a dynamic balance do the trick.
Maybe get to the trouble speed on the centre stand, might help to eliminate some stuff, dunno if that's been mentioned yet.
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Thanks for the comments.
1. If the U joint is bad I’ll be pissed because this is a brand new $$$ driveshaft. I think my wife may have noticed the bounce because the bad rear shock was amplifying it from her perspective. When I run the bike on the center stand, it appears to be smooth so that is why I think it is tire, wheel, or suspension.
2. At the Green Mountain rally, a friend also suggested swing arm bearings as something to look at if all else fails. I was going to ask this group if anyone has experience with swingarm bearings going bad - what are the symptoms? My friend is a very experienced professional mechanic, Connecticut airhead air Marshall, and has restored a number of beautiful bikes. He said that he had a similar hop on an R100 and he never figured out what it was. It eventually went away. That didn’t give me a warm fuzzy feeling.
3. The symptom is the same no matter what gear the bike is in or if the clutch is pulled in.
This is a crazy situation where I have multiple issues compounding each other. I always assume that if something is wrong after I work on it, it is related to something that I touched even if it is seemingly unrelated. So far this hasn’t been the case with this problem. The shock and steering bearing looseness were things I hadn’t messed with.
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You might get an opportunity to swap the final drive. Or other bits. I know trial and error isn't the proper way but that's what I sometimes do.