BB spindle slightly too short
f.c...@gmail.com
I am building my first bike.
I bought a BB with a 110mm spindle (square taper) and a new-old-stock
Shimano FC-M510 crankset. According to the Shimano's specs, this
should give a 47.5mm chainline.
My measured chainline is, however, a few mm's shorter. If I remove the
crank bolts, I can see that the right crank almost mushrooms the
spindle (2mm left), while the left crank definitely has a few more
mm's to go.
So, the diagnosis seems to be that the right crank isn't really
perfect...
What should I do? Replace the BB with a 113mm spindle?
What if I add a tiny shim between the spindle and the crank? Will it
do the trick or just destroy the crank?
Thanks,
Federico
Sheldon Brown
I've actually had some success shimming this way, to salvage toasted
cranks. I made a shim from an aluminum beverage can, just a right-
angled piece to cover two adjacent faces of the square. I used lots
of grease and tightened the bejasus out of the fixing bolt.
(This is not something I'd do for a paying customer, but I did it as a
"charity case" for a local homeless guy, and it seemed to work pretty
well.)
Another option is to put a spacer under the right-side bottom bracket
mounting ring.
This is the traditional item for that:
http://harriscyclery.net/itemdetails.cfm?ID=836
Sheldon "Where There's A Will..." Brown
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Ozark Bicycle
> Hello all,
> I am building my first bike.
> I bought a BB with a 110mm spindle (square taper) and a new-old-stock
> Shimano FC-M510 crankset. According to the Shimano's specs, this
> should give a 47.5mm chainline.
> My measured chainline is, however, a few mm's shorter. If I remove the
> crank bolts, I can see that the right crank almost mushrooms the
> spindle (2mm left), while the left crank definitely has a few more
> mm's to go.
> So, the diagnosis seems to be that the right crank isn't really
> perfect...
How did you arrive at that diagnosis, especially if the crankset
*really is* NOS? Are you certain the BB spindle is symmetrical (L/R)?
> What should I do? Replace the BB with a 113mm spindle?
> What if I add a tiny shim between the spindle and the crank? Will it
> do the trick or just destroy the crank?
>
IMO, a shim of that sort is a real "last resort" measure. Spacer(s)
under the right side cup are the easiest/cheapest solution, assuming
the problem really does lie with the right crankarm.
Any reasonable LBS should have spacers, or buy online:
http://www.loosescrews.com
> Federico
Peter Cole
I've shimmed the crank, as Sheldon describes, on one bike where I had
the same problem. Worked fine -- lots of miles, big rider.
Ozark Bicycle
I'm curious: did you shim the crank to make a damaged crankarm usable,
or to compensate for a too short BB spindle?
Peter Cole
The latter.
Ozark Bicycle
Hmm....you found that method preferable to either installing the
proper length BB spindle or placing spacers under the RH cup?
Peter Cole
Obviously. I use shims all the time on various locations on bikes, I
don't see the problem.
Ozark Bicycle
The problem? A concern about distorting the crankarm/spindle
interface, making the crankarm more likely to have a problem fitting a
properly sized spindle at another time.
Werehatrack
As long as the inner sprocket adequately clears the chainstay when
pedalling, you may not need to worry about it. Unless the front der
is too close to the crank to shift properly, or you're in a situation
which necessitates that you must go with measurements and numbers
alone instead of getting on the bike and trying it out, I'd advise
riding it to see which front/rear sprocket combination you'll actually
use most. If those would be closer to being aligned by moving the
right crank away from the seat tube, by all means, either swap out the
BB for a longer one, or add a shim under the right-hand BB flange if
that works. (Sometimes the shim isn't useful; I've had 110mm and
113mm BB setups on which a shim under the right flange caused the left
crank to hit the chainstay or caused the left cup to bottom out on the
end of the shell threads before it clamped the BB.)
If the setup you've assembled does an adequate job, I'd use it and not
worry about the numbers.
--
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Words processed in a facility that contains nuts.
Luke
<de0b7849-87a0-45d3...@v4g2000hsf.googlegroups.com>,
Ozark Bicycle <bicycle...@ozarkbicycleservice.com> wrote:
Once I considered a spacer behind the RH cup but nixed the idea because
IMO it would have left too little of an overlap at the RH cup/BB shell
interface. My concern was that the minimal overlap could result in
stressed and worn right side cup/shell threads.
For a 75 KG rider with English Al BB cups/steel frame, is there a
minimum overlap recommended?
f.c...@gmail.com
> worry about the numbers.
Thanks for the detailed explanation.
Unfortunately the current setup doesn't work since the right crank is
too close to the seat tube and the front derailer isn't able to clear
the small chainring / big sprocket combination (which is really
useful!). Also I am afraid that, with such a low chainline, I will get
a lot of chainrub in the middle chainring / small sprockets
combination, which I use quite often.
So I really need to move the right crank further to the right...
Bye,
Federico
Peter Cole
Not going to happen.
Ozark Bicycle
Perhaps you're correct; I've only done this (shim on spindle) with
crankarms already having problems. I'm not inclined to use
(sacrifice?) a fully functional crankarm to find out.
Peter Cole
I was correct at least once, that's all I needed.