Fixed gear chainrings -- how round is round enough?

kevinkiller said:

Hi,

Now I notice that when I'm using the 41/14 combo that the chainring is out
of round by about 1 to 1.5 mm.

Are you sure all the bolts are centered in their holes? In other words,
that the ring is exactly centered on the spider?

kevinkiller said:

Now I notice that when I'm using the 41/14 combo that the chainring is out
of round by about 1 to 1.5 mm. This is enough to cause a pretty big change
in chain tension as the crank rotates. The transition from loose to tight
tension causes the chain's vibration frequency to ramp up very quickly and
can be felt in the pedals as a kind of grinding sensation.

That's a long way out of round; assuming you've tried it in all 5
positions and done the usual trick of getting the bolts thumb tight
and turning it to find a sweet spot (the ring might float a little,
though if it's well made and the base circle is snug on the spider
shoulders, this should be negligible), then I'd send it back. Based on
my experience with screw adjusters on my horizontal dropouts (a
quarter turn on a M4 screw is a lot of difference in chain tension),
I'd want chainrings to have a radial runout of less than 0.1mm.

I'd also expect the runout tolerance on the spider shoulders of track
cranks to be about the same, but there's no need for such close
tolerance on a road crank, and this might be contributing to the
problem. You don't say which crank you're using, but I doubt if
low-end square taper cranks are very accurately made.
Kinky Cowboy*

*Batteries not included
May contain traces of nuts
Your milage may vary

kevinkiller said:

So finally to the question: Is it reasonable for chainrings to be

out of

Quoted message said:

round this much? My LBS guy seems to think so, but I can't see how
chainrings made especially for singlespeed, fixed gear, or bmx bikes

can get

Quoted message said:

away with this much slop. What do you think?

I think you should use some sort of a tool, never fingers, while
checking chain tightness for the purpose of adjusting chainrings for
roundness, so that your digits don't get sucked into the space between
chain and rotating cog/chainwheel teeth. I think you should never
forget to use a tool for this job. Or they'll start calling you
"Stumpy" or something else you'd prefer not to be known as.

More Sheldon, and this roundness adjustment drill is what they showed
me when I was brand-newbie at the track:

http://sheldonbrown.com/fixedb.html#tension

IME, works well on Campy and SugiNo.

Chainline: For winter bikes especially, I've used total [censored] old
worn-out cogs, chains, chainwheels on fixed, chainlines off by miles,
and chain run loose so it would run <g>, all without tossing a chain.
Since seeing a couple of thrown chains at the velodrome, I don't do it
that way anymore. Sheldon has good chainline info, if you haven't
checked it out. Also (John Dacey's Business Cycles, track supply
specialist/frequent contributor to r.b.t.):

<http://www.businesscycles.com/tr-refspec.htm>

BTW, I had an already kinked metric steel tape lying about that I broke
a couple of short sections off right at "10" mm lines; that made it
easy to measure to tube and cog/chain centers.
I just realized you didn't mention chainline problems, so 'scuse please
if you've already got that set. --D-y

Jay S. Hill said:

kevinkiller said:

Hi,

Now I notice that when I'm using the 41/14 combo that the chainring is
out
of round by about 1 to 1.5 mm.

Are you sure all the bolts are centered in their holes? In other words,
that the ring is exactly centered on the spider?

Jay,

Thanks for the reply. To answer your question, yes. I've tried the
Sheldon Brown loosen the crank bolts and bang the chain. That may work
for 1/4 mm or so, but I'm talking 1 to 1 1/2 mm ... there is no where
near the kind of slop in the chainring holes.

John.

jack sprat said:

Kinky Cowboy said:

kevinkiller said:

Now I notice that when I'm using the 41/14 combo that the chainring is out
of round by about 1 to 1.5 mm. This is enough to cause a pretty big change

I don't understand how rotating the crank holes through the spider arms
can make any sort of difference other than moving the tight/loose spot
relative to the crank arm itself even considering for the possibility of
out of round spider arms. I suppose if the spider arms were out of
round as well as the chainring, then I guess you could find an
orientation that would cancel out. Is that what you mean?

Yep.

Quoted message said:

Does anyone know where I can get a nice round 130 bcd 41T chainring?

TA do a 130bcd 41t in normal 3/32" format
Kinky Cowboy*

*Batteries not included
May contain traces of nuts
Your milage may vary

Quoted message said:

I think you should use some sort of a tool, never fingers, while
checking chain tightness for the purpose of adjusting chainrings for
roundness, so that your digits don't get sucked into the space between
chain and rotating cog/chainwheel teeth. I think you should never
forget to use a tool for this job. Or they'll start calling you
"Stumpy" or something else you'd prefer not to be known as.

Or you could check it by pushing from the outside of the chain, instead of
the inside. Worst that can happen then is getting sucked through the
derailer.

Jasper

Jasper Janssen said:

Or you could check it by pushing from the outside of the chain, instead of
the inside. Worst that can happen then is getting sucked through the
derailer.

No derailer, this is fixed gear. Checked on top, forward rotation,
chain taut. Fingers away. --TP

[email hidden] at [email hidden] wrote on 6/7/05 7:04 AM:

Quoted message said:

Jasper Janssen said:

Or you could check it by pushing from the outside of the chain, instead of
the inside. Worst that can happen then is getting sucked through the
derailer.

No derailer, this is fixed gear. Checked on top, forward rotation,
chain taut. Fingers away. --TP

I usually squeeze the top and bottom chain run together about midway between
the chainring and rear sprocket (from the outside). No/Little give = good.
Start with the driveside crank at 12 o'clock, turn quarter turn - squeeze -
turn quarter turn - squeeze - turn quarter turn - squeeze. That usually
ferrets out any binding or looseness.

Just make sure the ferrets don't get on the inside of the chain. Big mess.

-- Jim

--

CycloFiend

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