wobbly chainrings

In article <[email hidden]>,

Dan Lenski said:

The triple chainrings on my road bike wobble side to side about 1-2 mm.
They all wobble in the same places, so I think that the problem is not
bent rings. This makes me have to trim the front derailer more than I
would otherwise.

I tightened up the crank bolts as hard as I could, and this improved
the wobble only very slightly, but not a lot. I am thinking the
interface between the crank and BB spindle is worn out.

Is it worth replacing the BB, or do I need a whole new crankset? I am
thinking the aluminum crankset would wear out faster than the steel
spindle. This is a 1990 sakae crankset and suntour adjustable bottom
bracket, for what it's worth.

Thanks,
Dan

As you surmised, it's unlikely that the spindle is bent, but it's easy
to check that on a flat surface. The same surface can show that the
rings are planar, so that leaves the crank/spider as the primary suspect.

Do you have another crank lying around to check?

--
Ted Bennett

Dan Lenski said:

The triple chainrings on my road bike wobble side to side about 1-2 mm.
They all wobble in the same places, so I think that the problem is not
bent rings. This makes me have to trim the front derailer more than I
would otherwise.

I tightened up the crank bolts as hard as I could, and this improved
the wobble only very slightly, but not a lot.

What tool did you use to tighten the crank bolts? If an allen key you
would have a hard time getting the bolts properly torqued.

I ALWAYS use a torque wrench for this job, and I'm continually amazed
at the number of cranks on the wrench between "tight" and torqued
properly.

Of course I'm assuming you have a square taper BB. If it's splined
it's hard to get the cranks improperly aligned on the spindle.

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3

On 26 May 2005 09:34:03 -0700, "Dan Lenski" <[email hidden]> wrote:

<The triple chainrings on my road bike wobble side to side about 1-2 mm.
< They all wobble in the same places, so I think that the problem is not
<bent rings. This makes me have to trim the front derailer more than I
<would otherwise.
<
<I tightened up the crank bolts as hard as I could, and this improved
<the wobble only very slightly, but not a lot. I am thinking the
<interface between the crank and BB spindle is worn out.
<
<Is it worth replacing the BB, or do I need a whole new crankset? I am
<thinking the aluminum crankset would wear out faster than the steel
<spindle. This is a 1990 sakae crankset and suntour adjustable bottom
<bracket, for what it's worth.

You answered your own question .... "adjustable bottom bracket". You have
bearings in there, probably in a cage, maybe the cage is broken. Replacement
bearings are inexpensive.

There really shouldn't be any play in there, particularly if there's any
scraping.

Dan Lenski said:

The triple chainrings on my road bike wobble side to side about 1-2 mm.
They all wobble in the same places, so I think that the problem is not
bent rings. This makes me have to trim the front derailer more than I
would otherwise.

I tightened up the crank bolts as hard as I could,

Probably overtightened, then. This is not a "twist until the wrench
breaks" fastener. "Tight enough" may not be as tight as you think it
would have been.

Quoted message said:

and this improved
the wobble only very slightly, but not a lot.

Was the right crank loose at all? If so, you need pursue this no
farther; the crank will need to be swapped out if you're serious about
getting rid of the wobble.

Quoted message said:

I am thinking the
interface between the crank and BB spindle is worn out.

You're probably right about that, though a bent right crank is also a
possibility.

Quoted message said:

Is it worth replacing the BB

This is unlikely to solve the problem in my opinion.

Quoted message said:

or do I need a whole new crankset?

Probably.

Quoted message said:

I am
thinking the aluminum crankset would wear out faster than the steel
spindle. This is a 1990 sakae crankset

Are the sprockets bolted on, or ar they riveted together? If they're
riveted, and if the sprocket group attaches to the crank at the center
of the assembly rather than out on the arms of the spider, you may
just have a bent sprocket cluster. This can sometimes be
straightened, but such crank sets are not exactly high-tech gear; if
that's what you've got, I'd consider swapping up to a better crank set
anyway.

Quoted message said:

and suntour adjustable bottom
bracket, for what it's worth.

Okay, before you do anything else, see if there's lateral play in the
BB shaft. If there is, I'd recommend pulling the BB apart, cleaning
it, inspecting it for wear and corrosion, and then (assuming it passes
inspection) reassembling and properly adjusting it. If this removes
the wobble, smile and ride on. If it doesn't, you're only out a
little time and grease. If there's no play in the BB now, your wobble
is almost certain to be in the crank. If you want to verify that,
mark the "high spot" of the wobble, pull the right crank off,
reinstall it 90 degrees off from where it was, and recheck the
location of the high spot. If it's still in the same place, the
problem's in the crank. It it moved 90 degrees, then the BB is
probably bad...but the crank still may not be good.

--
Typoes are a feature, not a bug.
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