bottom bracket advice

"Ben Fitzgerald" wrote:

I suspect you may be right. The right hand peddle crank does
sit well clear of the frame now, which may well be because the
axle has migrated to the right. I will try this and let you know.

The axle can't migrate within the cartridge. Whichever side of the unit
contains the bearings and axle (the left side in the case of your old
model, probably the right in the case of the replacement) should be tight
in the frame with its flange firmly against the face of the bottom bracket
shell. The other side, just a supporting ring, should then be snugged up to
support the cartridge from the other side.

If you've replaced the original unit with a longer-axled model that fits
from the right, you can't adjust it leftward. If the crank is too far from
the centre line of the frame, the axle is too long.

James Thomson

"Ben Fitzgerald" wrote in message
in...
On Thu, 8 Dec 2005 01:36:38 -0000, michael adams wrote:

"Ben Fitzgerald" wrote in message
in...
Hi,

My bottom bracket is knackared. The axle on the side of the front
sprocket (left side) seems to have shortened

...

Bottom bracket axles can't shorten. What's hapened presumably is that
the bottom bracket cartridge has rotated on the threads inside the
shell and moved across.

If the above is total nonsense, then presumably it will be corrected
in short order.

If it's correct then nothing necessarily needs replacement. Only
adjustment. Unless the contact with the derailler wire has caused
serious damage to the chainring.( front sprocket) Which can be
replaced on its own in any case.

Just remove the cranks, and loosen the lockring on the bottom bracket
cartridge and then thread it through so the correct amount of axle is
showing on the chainring side - just try a loose fit to check maybe
and then tighten the lockring.

I suspect you may be right. The right hand peddle crank does sit well
clear of the frame now, which may well be because the axle has migrated
to the right. I will try this and let you know.

Thanks!


Having looked at the diagram helpfully provided by James Thomson -

http://bernd.sluka.de/Fahrrad/Shiman...-CT90_1995.gif

( it was guesswork on my part up until that point ) what appears
to have happened with the original cartridge, is simply that the
left-hand, non-drive side started working loose. i.e unscrewing
itself. Bringing the cartridge, and enclosed axle with it.
On a traditional cup and bearing bottom bracket, any such looseness
on either side would immediately show up as play in the axle. However
with cartridges one side fits in a sleeve inside the bottom
bracket. And it's the sleeve, rather than the actual cartridge that
threads into the bottom bracket shell. And so what happened, is that
the whole cartridge including the axle simply slowly unscrewed itself
and moved to the left.
So all you need do is replace the original cartridge and make
sure the left hand side is screwed on tight. ( I won't confuse
things or chance my arm with any further with reference to torque
wrenches, thread direction, or lock-rings etc.)

michael adams

....





Ben.

--
Registered Linux user number 339435

michael adams wrote:
[snip]
Fair enough.

So how is it possible to account for the fact that the bottom bracket
axle has "shortened" ?

Has it really shortened as the O.P claims ?

Otherwise how is it possible to account for the fact that it presumably
functioned o.k up until recently - the front chainrings were correctly
aligned - and now they're not?

Just asking.

The most likely cause is that the BB bearings were sufficiently goosed
that they allowed the BB spindle to rock backwards & forwards. At the
edge of the chainring, this backwards & forwards motion is rotated to
give a side to side motion, allowing the chain ring to hit the rear
stay under the appropriate load.

The Luggage