Old bike suspension question

Discussion in 'Motorbike Technical Discussion' started by mrmoto, Sep 9, 2014.

  1. mrmoto

    mrmoto Guest

    Got an old 1980s 125cc Suzuki that appears to have ~0.25" less distance
    between the left-hand rear shock mount bosses compared to the right-hand
    side. This difference appears to force the left side of the swingarm down
    farther which is noticeable when the bike is up on the center stand.
    First thought was it's flaw - misalignment in a tooling fixture during
    manufacture maybe? - but then I thought perhaps it's intentional to
    counteract the force of the drive chain? I recently serviced the swingarm
    bushings and noticed wear patterns consistent with what I'd expect from
    a torsional twisting load from uneven shocks; doesn't seem like intentional
    design to me.

    The above leads to my next question - if it's a flaw how best to correct
    the problem? Other than custom shocks I think it may be possible to bend
    the swingarm enough to even things up. Re-welding one of the bosses -
    probably the one on the swingarm - might also work but alignment might be
    hard to achieve, no?
    mrmoto, Sep 9, 2014
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  2. mrmoto

    appler Guest

    1/4" difference in spring compression between left and right sides
    shouldn't visibly tweak the swingarm. Is the 1/4" still there when only
    one (or no) shocks are fitted?
    Not intentional, and almost assuredly not a manufacturing error.

    I recently serviced the swingarm
    The swingarm axle holes in the frame may have become oval'ed out
    sometime during the last 30 years. Or maybe the bike was run over and
    the frame is bent.
    Set the swingarm on a flat surface and see if the problem is there.
    Use a bubble level and large triangle to put the bike perpendicular to
    level ground when it's on the centerstand, and measure from upper shock
    mounts to the ground.
    Find a swingarm on ebay if you have to. Fix the problem at the source,
    don't compensate for it somewhere else.
    appler, Sep 10, 2014
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  3. mrmoto

    mrmoto Guest

    Think so. I usually install the RH shock first due to the muffler
    obstructing access to the lower boss. With the RH shock installed I've
    got to wrest the LH shock onto it's mounts. That was actually how I
    noticed the issue while reinstalling the shocks after cleaning and
    greasing the swingarm bushings.

    Don't think it was run over or otherwise abused, at least there are no
    signs of an impact like dents, missing paint, etc. The swingarm bushings
    are a mix of metal sleeves and some sort of non-metallic material. The
    torquing of the swingarm axle seems to compress the non-metallic material
    so that there is very little play once assembled. Also, I think the lower
    shock mounts are fine relative to the rear wheel axle as the rear wheel
    is titled an equal amount with the LH shock installed. Without the LH
    shock it look level.

    For the reasons previous stated I no longer think the swingarm is the
    problem. Seems it's the upper LH shock mount that's too low. Sounds
    like you don't think re-welding is a good idea; it's certainly not my
    preferred option. Perhaps replacing the swingarm bushings would help.
    I also thought I could either look for a source of eccentric rubber
    bushings for the shock absorber eyelets, or modify the existing ones with
    a half-round file. 1/16" on all four shock bushings would remove the
    difference without removing a whole lot of rubber from any one.
    mrmoto, Sep 12, 2014
  4. mrmoto

    appler Guest

    Do the shocks have spring preload adjustments? They might be set
    differently on each side. Or maybe one spring has sagged.

    "the rear wheel
    is titled an equal amount with the LH shock installed. Without the LH
    shock it look level."
    says there's some flimsy structure somewhere, and you're not going to
    strengthen it by altering the shock length or mount points.
    appler, Sep 12, 2014
  5. mrmoto

    mrmoto Guest

    My guess is the non-metallic swingarm bushing material is more malleable
    than it appears - it's like really hard oil-impregnated gasket material.

    Latest work-around:
    I bought a pair of shocks off ebay that are sold as OEM replacements for
    this bike. They look nearly identical to the originals but the eye-to-eye
    dimension is ~1/4" longer and they are a bit stiffer. I put one of these
    on the RH side with the preload set to lowest level, and left an OEM shock
    on the LH side with preload set to the 3rd from lowest position which,
    using a floor scale, I determined was approximately equal to the new shock
    as installed. The rear axle and wheel look level now and the bike seems
    to ride well. It's a 125cc bike so there isn't the safety and performance
    concerns one has with a bigger bike; more akin to an around-town scooter.
    mrmoto, Sep 14, 2014
  6. Why would you do this ???? Wouldn't it make sense to just install both
    of the newly bought pair of matched shocks ? Or is that exactly what
    they'd be expecting you to do?
    Right now there may not be the safety and performance concerns that one
    has with a bigger bike, but I'm sure you'll soon figure out a workaround
    to that problem.
    Rob Kleinschmidt, Sep 17, 2014
  7. mrmoto

    mrmoto Guest

    Ah, usenet...

    Please read the whole thread; hopefully it'll make more sense to you.
    mrmoto, Sep 17, 2014
  8. Actually, no. In re-reading the thread, you seem to believe that
    you "fixed" a problem by installing a cockamamie mix of parts.

    Best of luck and hope that whatever else is really wrong, (a worn
    swingarm bushing and a possibly tweaked frame seem likely) doesn't
    bite you in the ass.
    Rob Kleinschmidt, Sep 18, 2014
  9. mrmoto

    mrmoto Guest

    Another follow-up: snagged a pair of slightly used swingarm bushings off
    ebay and got them installed the other day. Before installing I checked
    the old and new parts with calipers; the old outer bushings were still
    within spec but just barely. Even though the differences in I.D. wasn't
    much, it was apparently enough to vastly reduce the difference I was seeing
    between the left and right shock mount distances. With both new shocks
    installed the rear wheel looks straight and the bike ride better.

    Many thanks to appler for suggesting I look at the swingarm components.
    mrmoto, Oct 3, 2014
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