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Need help fixing vibration

Discussion in 'The Garage' started by KFunk740, Sep 2, 2015.

  1. KFunk740

    KFunk740 XS400 Member

    The bike is a mostly stock '81 XS400-SH with 7500 miles (had it since 1900 miles around 6 years ago. It's fairly beat-up, but been a solid bike for getting around for several years now. It runs just fine. I use it frequently now because getting parking on campus in the daytime is impossible. I'm running a one tooth bigger front sprocket, and XS650 shocks for more comfort on my commute at 60+ mph.

    I recently put on a new set of Duro tires (same size and everything that I previously ran for 5000+ miles and corded), and went ahead and tightened the chain to spec at the same time and made sure it was running straight (never paid much attention to that before). It ran perfectly well for around 15 miles, then I felt a vibration that made me uneasy. I pulled over and checked axle nuts and anything I had recently reassembled, and all looked OK. I suspected maybe I lost one of the new wheel weights, so I took the wheels to a different shop, and they said they were perfectly balanced.
    Neck bearings seem OK, wheel bearings feel OK, rear swingarm bushing is greased. Chain had been rather dirty and stiff, so I took it off and cleaned it thoroughly and lubed it up. I changed rear brake assembly too (I boogered up threads on a lever arm before, and just got a different assembly and shoes on ebay).
    Still, either I'm just paranoid, or it feels shaky. I can't track it to a certain RPM or speed, but it does gradually increase until around 55mph, and at 65mph it starts to get less noticeable.
    I can't pinpoint it to front or rear. I can feel it slightly through my handlebars, seat, and footpegs.

    Any ideas what else to check? It almost feels like its something unbalanced in the engine itself, but it doesn't seem to be at just one RPM.
    I'm wondering if some imperfections on the surface of the tires just need to be worn off, but my last set never had this problem, and its weird it just started 15 miles after putting them on.
    I've thought about loosening up the chain a bunch to see if it helps, but that doesn't seem proper, and the cleaning/lubing it didn't seem to affect it.

    Thanks for any help! :bike:

    Attached Files:

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  2. Wolfe_11B

    Wolfe_11B XS400 Guru Top Contributor

    Tire slightly out of round maybe? It can balance perfect if so, but shake bad on the road. I used to work for a tire shop and Goodyear's and cheapo tires were notorious for this.
  3. KFunk740

    KFunk740 XS400 Member

    Any ideas on how to check that? Maybe spinning it and measuring the run-out somehow?
    If it is out of round, can it smooth out over time?

    I don't really want to spend money on another set of 2 tires right now unnecessarily. I'm still doubtful its them, as they did fine for 15 miles or so, and then suddenly got shaky.
    It'd also be nice to pinpoint which one it is (if that), and replace only that one. But yeah, can't tell if shake is coming from front or rear.

    I tried loosening the chain up some, and it may have made it a little smoother ride on my way to work this morning, but its hard to tell.
  4. xschris

    xschris XS400 Guru Top Contributor

    Make sure the tire are inflated to the proper pressure that is on the neck of the bike not what the tire says. Are the back brakes rubbing? Make sure that are not only when you press the pedal. Also if the chain it rusty at all or was stiff at any point it is time for a new one. New sprockets would also be a good thing to do at this time.
  5. KFunk740

    KFunk740 XS400 Member

    front sprocket almost brand new, rear sprocket has no real wear at all. tires are at correct pressure according to manual (28 front, 29 rear i think). rear brakes were nowhere near rubbing at all (barely engaging at all until i tightened them this morning). no rust on chain and smooth and very flexible, just had crud on it before.
  6. Wolfe_11B

    Wolfe_11B XS400 Guru Top Contributor

    If its badly out of round you may be able to spin it by hand and see, but most of the time it's an issue with the belts internally and not always visible.
  7. PEPXS400

    PEPXS400 XS400 Enthusiast

    I had a vibration in the front when I first bought mine, changed the wheel bearings and brake shoes. No more vibration. When I removed the wheel bearings. they seemed ok too.
  8. KFunk740

    KFunk740 XS400 Member

    OK, fixed. Also, I've learned that I'm stupid.

    The front brake (disc) was binding. I hadn't thought to just try spinning the front wheel with it off the ground before, and when I did I noticed that there was resistance. I made sure the caliper wasn't stuck, and also found that one of the pads may not have been in quite perfect position (stuck on the little tab in the center). I'm not sure how I would've messed it up two different times I took the wheel off, but I probably did.

    Of course, I already purchased a new set of front bearings (haven't installed yet), and a new front tire (also haven't installed). I'll save the bearings for another time.

    Anybody want to buy a new front tire for cheap (like $30)? I'll put it in the classifieds when I get a chance. I figure it's not worth saving, since I'll probably exceed good shelf life before I need it.
  9. Wolfe_11B

    Wolfe_11B XS400 Guru Top Contributor

    Hold onto it, you never know when it will be needed!

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