Discussion:
Old bike suspension question
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m***@simple.gopher
2014-09-09 17:00:38 UTC
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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?
appler
2014-09-10 20:18:24 UTC
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Post by m***@simple.gopher
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.
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?
Post by m***@simple.gopher
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?
Not intentional, and almost assuredly not a manufacturing error.

I recently serviced the swingarm
Post by m***@simple.gopher
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 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.
Post by m***@simple.gopher
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.
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.
Post by m***@simple.gopher
Re-welding one of the bosses -
NO!
Post by m***@simple.gopher
probably the one on the swingarm - might also work but alignment might be
hard to achieve, no?
Find a swingarm on ebay if you have to. Fix the problem at the source,
don't compensate for it somewhere else.
m***@simple.gopher
2014-09-12 02:16:45 UTC
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Post by appler
Post by m***@simple.gopher
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.
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?
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.
Post by appler
Post by m***@simple.gopher
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?
Not intentional, and almost assuredly not a manufacturing error.
I recently serviced the swingarm
Post by m***@simple.gopher
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 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.
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.
Post by appler
Post by m***@simple.gopher
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.
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 center-stand, and measure from upper shock
mounts to the ground.
Post by m***@simple.gopher
Re-welding one of the bosses -
NO!
Post by m***@simple.gopher
probably the one on the swingarm - might also work but alignment might be
hard to achieve, no?
Find a swingarm on ebay if you have to. Fix the problem at the source,
don't compensate for it somewhere else.
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.
appler
2014-09-12 05:03:02 UTC
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Post by m***@simple.gopher
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.
Do the shocks have spring preload adjustments? They might be set
differently on each side. Or maybe one spring has sagged.

But...
"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.
m***@simple.gopher
2014-09-14 17:04:02 UTC
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Post by appler
Post by m***@simple.gopher
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.
Do the shocks have spring preload adjustments? They might be set
differently on each side. Or maybe one spring has sagged.
But...
"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.
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.
Rob Kleinschmidt
2014-09-17 00:44:15 UTC
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Post by m***@simple.gopher
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.
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?
Post by m***@simple.gopher
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.
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.
m***@simple.gopher
2014-09-17 16:08:00 UTC
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Post by Rob Kleinschmidt
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?
Ah, usenet...

Please read the whole thread; hopefully it'll make more sense to you.
Rob Kleinschmidt
2014-09-18 16:35:45 UTC
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Post by m***@simple.gopher
Post by Rob Kleinschmidt
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?
Ah, usenet...
Please read the whole thread; hopefully it'll make more sense to you.
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.
m***@simple.gopher
2014-10-03 05:18:00 UTC
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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.
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