bottom bracket advice

Ben Fitzgerald said:

Hi,

My bottom bracket is knackared. The axle on the side of the front
sprocket (left side) seems to have shortened meaning my front sprocket
doesn't have enough clearance from the frame so it rubs against my
derailer cable causing it to wear out and snap.

I need to replace the bottom bracket. It is +not+ a new bike.

Here are the specs:

Make: Shimano BB-CT90
Other info: VIA 68 SL
BC1 37x24
YL 116

The model is mentioned on Sheldon Brown's site:

http://www.sheldonbrown.com/bbsize.html

Slightly worring entry:

Altus CT90 Triple 116-121 Square 47.5-50 Recalled! Do Not Use!
See Shimano dealer for free replacement. Compact 1995

I have two questions:

* anyone know where I can get one these?
* how can I know if a different model is a suitable replacement? is
this a good idea?

What matters is:
- the thread size (probably on the BB codes above, and guess its standard
UK spec),
- the axle length ( 116-121mm on the above),
- and the method of connecting to the cranks (older common designs use
square taper, more modern have splines. On older bike stick with taper).

With the above information, get new bits. A sealed decent Shimano bottom
bracket is about £15 with taper ends. eg. UN52 or UN72 (the 72 being
slightly better quality).

The crank. If there is nothing wrong with it before the failure, just re-use
it. If worried get a fairly cheap aftermarket complete replacement. Eg.
Stronglight units from Spa Cycles in Harrogate (advertise in cycle
magazines, decent mail order service) are often good value. What matters is
the numbers of teeth on each crank ring. You probably want to swap for the
same teeth numbers unless you want to change the gearing.

- Nigel

--
Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/

Ben Fitzgerald said:

Hi,

My bottom bracket is knackared. The axle on the side of the front
sprocket (left side) seems to have shortened meaning my front sprocket
doesn't have enough clearance from the frame so it rubs against my
derailer cable causing it to wear out and snap.

I need to replace the bottom bracket. It is +not+ a new bike.

It is a standard 68mm wide BB with a 116 mm axle, English threading.
This is the standard BB size for bikes in UK.

Go to your local LBS and tell them you need a new BB. A standard
UN52/UN53 or if you want metal lock rings, a UN72/73 should be fine.
one of these
http://wiggle.co.uk/Default.aspx?Main=ProductDetail.aspx&W=0&Manufacturer=&UberCatName=&Cat=cycle&CategoryName=Bottom%20Brackets&ProdID=434278922&UberCat=0

http://tinyurl.com/copf9

should be fine. Get the 68 shell, 118mm length, or get your LBS to sort
it for you.

...d

Quoted message said:

Here are the specs:

Make: Shimano BB-CT90
Other info: VIA 68 SL
BC1 37x24
YL 116

The model is mentioned on Sheldon Brown's site:

http://www.sheldonbrown.com/bbsize.html

Slightly worring entry:

Altus CT90 Triple 116-121 Square 47.5-50 Recalled! Do Not Use!
See Shimano dealer for free replacement. Compact 1995

I have two questions:

* anyone know where I can get one these?
* how can I know if a different model is a suitable replacement? is this
a good idea?

Ben Fitzgerald said:

I need to replace the bottom bracket. It is +not+ a new bike.

Here are the specs:

Make: Shimano BB-CT90
Other info: VIA 68 SL
BC1 37x24
YL 116

I would get a Shimano UN73:
from LBS or www.chainreactioncycles.com/Models.aspx?ModelID=49 (115mm
should be ok)

Tools (if not using LBS):

www.mwdyason.ltd.uk/shop.asp?no_category=23&text_company=TOOLS

- Crank remover
- Bottom Bracket Tool for Shimano

Also a good idea to refit the cranks with a torque wrench. Some
recommended torque values are on the Park Tools website.

~PB

Pete Biggs said:

Ben Fitzgerald said:

I need to replace the bottom bracket. It is +not+ a new bike.

<snip>

Quoted message said:

- Crank remover
- Bottom Bracket Tool for Shimano

Also a good idea to refit the cranks with a torque wrench. Some
recommended torque values are on the Park Tools website.

Many thanks to all who replied. I had the necessary tools ready so I
did the replacement myself. I went for a slightly longer axle to make
the crank sit clear of my rear derailer cable and that seems to have
done the trick. Just had a quick ride up + down the road and it felt
good. I would have gone for longer but it's raining!

The /real/ test comes on the commute tomorrow.

Many thanks to all who responded for the good advice. This is the kind
of thing I love news groups for. ;-)

Cheers,

Ben.

--
Registered Linux user number 339435

"Ben Fitzgerald" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

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.

michael adams

....

Quoted message said:

meaning my front sprocket
doesn't have enough clearance from the frame so it rubs against my
derailer cable causing it to wear out and snap.

I need to replace the bottom bracket. It is +not+ a new bike.

Here are the specs:

Make: Shimano BB-CT90
Other info: VIA 68 SL
BC1 37x24
YL 116

The model is mentioned on Sheldon Brown's site:

http://www.sheldonbrown.com/bbsize.html

Slightly worring entry:

Altus CT90 Triple 116-121 Square 47.5-50 Recalled! Do Not Use!
See Shimano dealer for free replacement. Compact 1995

I have two questions:

* anyone know where I can get one these?
* how can I know if a different model is a suitable replacement? is this
a good idea?

If I have left out information please let me know and I'll do my best to
get it.

Thanks,

Ben.

--
Registered Linux user number 339435

"Ben Fitzgerald" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

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.

michael adams

....

Quoted message said:

meaning my front sprocket
doesn't have enough clearance from the frame so it rubs against my
derailer cable causing it to wear out and snap.

I need to replace the bottom bracket. It is +not+ a new bike.

Here are the specs:

Make: Shimano BB-CT90
Other info: VIA 68 SL
BC1 37x24
YL 116

The model is mentioned on Sheldon Brown's site:

http://www.sheldonbrown.com/bbsize.html

Slightly worring entry:

Altus CT90 Triple 116-121 Square 47.5-50 Recalled! Do Not Use!
See Shimano dealer for free replacement. Compact 1995

I have two questions:

* anyone know where I can get one these?
* how can I know if a different model is a suitable replacement? is this
a good idea?

If I have left out information please let me know and I'll do my best to
get it.

Thanks,

Ben.

--
Registered Linux user number 339435

michael adams said:

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.

It is nonsense. The crank side is the fixed cup and is tightened up
completely. Undoing this will move the crank out, not in.

...d

"David Martin" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

michael adams said:

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.

It is nonsense. The crank side is the fixed cup and is tightened up
completely. Undoing this will move the crank out, not in.

..d

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.

michael adams

....

David Martin said:

michael adams said:

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.

Quoted message said:

Quoted message said:

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

Quoted message said:

It is nonsense. The crank side is the fixed cup and is tightened up
completely. Undoing this will move the crank out, not in.

Shimano have made cartridge bottom brackets that insert from the left
(non-drive side). Ben's BB-CT90 is one of those:

http://bernd.sluka.de/Fahrrad/Shimano/TM/BB-CT90_1995.gif

So it seems likely that Michael is correct.

James Thomson

michael adams said:

"Ben Fitzgerald" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

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!

Ben.

--
Registered Linux user number 339435

James Thomson said:

David Martin said:

michael adams said:

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.

Quoted message said:

Quoted message said:

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

Quoted message said:

It is nonsense. The crank side is the fixed cup and is tightened up
completely. Undoing this will move the crank out, not in.

Shimano have made cartridge bottom brackets that insert from the left
(non-drive side). Ben's BB-CT90 is one of those:

http://bernd.sluka.de/Fahrrad/Shimano/TM/BB-CT90_1995.gif

So it seems likely that Michael is correct.

I sit corrected..

...d

Ben Fitzgerald said:

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" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

michael adams said:

"Ben Fitzgerald" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

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/Shimano/TM/BB-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

....

Quoted message said:

Ben.

--
Registered Linux user number 339435

michael adams wrote:
[snip]

Quoted message said:

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