I’ve been getting my EBay 6-footer ready for action – cleaning it up,
re-packing the bearings, etc. I guess the thing has spent the last 10 or
15 years in someone’s garage.
The Uni is structurally excellent but there were a couple of little
things the seller forgot to disclose: a mashed pedal (unusable), and
mismatched crank arm lengths (127mm vs 140mm). This kind of glitch must
be pretty common with used items on EBay.
After putting it all back together, I turn the crank and the chain
clatters in a very unpleasant way. I discover that the chain is randomly
catching on the teeth of the lower sprocket (but not the upper one).
Pushing the downward-moving chain inward with my finger quiets the
clatter so it occurs to me that the chain is approaching the sprocket
from a slight angle. Measuring with the ruler I find that the upper
sprocket is 1/2 inch farther out from the center line than the lower
sprocket so the two sprockets are not in line with each other. Must be
that it was designed that way.
My cure was to tilt the wheel (so the top is nearer to one side of the
frame than the other).
Some lousy ascii art to illustrate (the asterisks represent the wheel):
| <-- tubing going up to the “bottom bracket”
|
.-----.
| * | <-- wheel much closer to frame on this side
| * |
| * |
| * |
| * |
| * |
| * |
| * |
-|–*--|- <-- axle (imagine tilted slightly,
left end higher)
With the wheel tilted, the chain runs real smooth because the plane of
the lower sprocket is now approximately consistent with the angle that
the chain approaches it.
Has anyone else had this problem? Is this a reasonable solution?
(It is a mystery why the upper sprocket doesn’t clatter, but lucky since
I can’t tilt it.)
i think you will have a hard time going straight if your wheel is even slightly crooked. maybe try some spacers on the inside of the sprocket (if it’s not welded) to move it to the right . any bike store should have them. pull the cranks out and see if there are any spacers kicking that top chainring way out there. take a good look at the frame, make sure it isn’t bent. i’d contact the seller regarding those issues. you should get a couple of bucks back. i remember looking at that giraffe and i had a couple of bids on it, i thought it was in better shape than that. good luck and let us know if that helps.
> i think you will have a hard time going straight if your wheel is even
> slightly crooked.
This could be true. Also if the wheel is tilted more than a tiny bit, it
could compromise the wheel’s strength, making it more prone to tacoing.
> maybe try some spacers on the inside of the sprocket
Spacers sound like a great option to start with. Any way to move either the
sprocket over, or if necessary the whole wheel. Having a vertical wheel
that’s a little off to the side should be less noticeable than a tilted
wheel.
Is this a homemade giraffe? I assume it is.
Another possibility is to true with wheel with a dish in it, where the rim
is off-center from the hub. But if you have to correct a half inch, that’s
probably too much for this method. Plus it also compromises wheel strength a
little (though it’s how the back wheel on all bikes with derailleurs are
built).
I think my ascii art exaggerated the degree of tilt.
Here is a picture:
(all cleaned up, new pedals, new tire)
The tilted wheel is probably about 1/4 inch closer to the frame on the
right. Doesn’t seem to me that it is off-center enough to stress the wheel
or to keep it from going straight (but I haven’t gotten up on it yet to find
out – tomorrow is the big day).
The uni is actually in pretty good shape. I don’t think anything is bent.
The sprockets have no visible wear. I ran a taught string down the tube and
didn’t see any bend from left to right.
BTW, this is the Marshall uni that I mentioned in an earlier post.
Did some remeasuring of the sprocket-to-center-line distances just now. They
are kind of hard to measure. I am beginning to think I was mistaken about
the 1/2 inch discrepancy between upper and lower. As near as I can determine
now, they are both 1 and 5/8 inch from the center line.
I may try a new chain to see if that solves the clatter problem.
–Mark
angelasdad wrote:
> i think you will have a hard time going straight if your wheel is even
> slightly crooked. maybe try some spacers on the inside of the sprocket
> (if it’s not welded) to move it to the right . any bike store should
> have them. pull the cranks out and see if there are any spacers kicking
> that top chainring way out there. take a good look at the frame, make
> sure it isn’t bent. i’d contact the seller regarding those issues. you
> should get a couple of bucks back. i remember looking at that giraffe
> and i had a couple of bids on it, i thought it was in better shape than
> that. good luck and let us know if that helps.
>
> –
> angelasdad
> ------------------------------------------------------------------------
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