Yardbent

recently bought a Peugeot P10 with 52/42 crankset

hilly surroundings [ and fitness. lack..!] means the 52 isn't used much with 14-28
and the 42-28 is still a little too high

a UK supplier is offering a new 50/34 but notes;
''- Requires 110mm square tapered bottom bracket''

mine is 122mm --- is there a conflict



still learning - so thanks....

Wilfred Laurier

because the bottom bracket axle
or spindle as it is commonly called
gives the crank its position on either side of the frame

if this position is wrong there are a couple of problems that can occur
the chainrings on the crank can be grossly off of alignment from the cogs at the back
called bad chainline

or the chainrings or arms can hit the frame

or the arms can be different distances from the centre line of the bike
which is undesirable if the mismatch is too great

Wilfred Laurier

response to your edit

there are two dimensions you need to be concerned with when selecting a bottom bracket

one is the spindle length
to which the given 110 mm spec refers

the other is the bottom bracket shell witch
or more generally
the dimensions of the bottom bracket
the dimensions on an older bike that are of interest are the width of the bottom bracket shell part of the frame
and the threading

most road bikes have a 68mm wide bottom bracket with english threading
that is 1.370 X 24 tpi
with a left hand thread on the drive side

but if you have a french made peugeot
and not a canadian made or other nationality
then you might have french threading
which is more rare

there are other sizes
Sheldon Brown's Bicycle Bicycle Bottom Bracket Crib Sheet

if you are lucky you might be able to find just a spindle to fit into the old bearing setup
but i wouldnt count on it

Yardbent

Yardbent

this is what i cannot grasp
i am not replacing the BB

my BB is in place
the spindle is in place
with an exposed length of 21mm....see pic

the chainset fits over this exposed piece --- doesn't matter how long/short the installed spindle is.??

thanks.......john

Yardbent

thanks for all the data/info....lots to learn

but i'm not replacing the BB so puzzled why i cannot just fit a lower ratio crankset

thanks..........

cny-bikeman

Different cranksets are offset differently from the end of the BB spindle, thus take a different length of BB spindle to put the chainwheels in the proper position. The chainwheels would be out too far - would not line up properly with the rear and front derailleur might have a hard time reaching the large chainwheel.

hueyhoolihan

don't be puzzled. there very well may not be any reason at all to replace your BB. in fact, it's likely that you should not have to, if the new crank is also a square taper. BB length is a consequence of frame geometry, for the most part, IME.

Wilfred Laurier

no it is not

bb length is what sets the relationship between the chainrings chain and cogs

frames are almost always symmetrical and use standard sizing for wheel and bottom bracket spacing

if a bottom bracket and crankset result in good chainline on one frame
then they will almost certainly be good for any other frame
provided the bottom bracket to frame interface
like threads and shell width and diameter
are compatible

Homebrew01

This, or the problem can go in the other direction. The chainring could rub against the frame if the BB axle is too short for that crankset. Crankset & axle need to match, or at least be "close enough" for the application. Not too close & not too far.

Bill Kapaun

Not sure where you come up with some of your answers???
IF what you say were true, why so many different lengths of sq. taper BB's?

Older sq. taper cranks "tend" to use longer spindles than newer.

Bill Kapaun

You MIGHT be able to get by with simply flipping the spindle.
It won't be exactly "correct", but maybe usable.
The left crank would end up wider than it should be.

Yardbent

the fog is clearing...

so on crank manufacturer A, the vertical centreline of the 2 chainrings [for want of a better tech term] could be nearer or further from the bike centreline compared to crank manufacturer B

if crank A is too far out from my chainline with my 122mm spindle - then i have to fit a 110mm spindle in MY BB to bring the chainline closer in

right..?

just measure the BB housing = 68mm i think that's a standard

BTW the serial number is GBTX54X
so it's a UK model
TX is Misty Blue paint code
54 is frame
X = ?

Yardbent

thanks... beginning to see the light.........

Yardbent

ahha ... so going for an older crankset may be the solution.?

Al1943

What is the BCD of your existing crankset? If it is the more common 130 mm you could replace the inner chainring with a 39 or probably a 38 giving you lower gearing. Chainrings are relatively cheap, it may be worth a try. You can refer to sheldonbrown.com for how to measure your crankset BCD.

Yardbent

?? i didn't measure the length of the drive-side exposed end ...but it looked similar

the 13.8 and the 13.0 refer to the width across the taper flats

thanks...john

Yardbent

thanks.............but it's a riveted job - worth about £5 on eBay-UK.........

bradtx

Yardbent, If the drive side BB cup is removable by turning clockwise, it's an English threaded BB. If the drive side cup is removable by turning counter clockwise, it a French threaded BB. There are modern cartridge style BBs available: Velo-Orange Grand Cru French Thread Bottom Bracket Set 110mm - Harris Cyclery bicycle shop - West Newton, Massachusetts . I don't know where in GB they're available.

Brad

Homebrew01

It gets more confusing because some models from the same manufacturer are not compatible. My Campy Chorus bb axle is too short for my Campy Centaur crank.

Yardbent

Brad
thanks
i had the BB apart to clean, insect and regrease and canna remember which way. doh
as i said the serial code is GB****** so it's prob a UK thread

3alarmer 

...if it makes you feel any better, about a year ago I replaced the crank and BB spindle on a Raleigh Carlton production
bike that had been done wrong before I got it, the BB shell was 71mm wide, the threading was standard, not Raleigh, and
it was old enough that whatever information on the original stuff is long forgotten.

I'm supposed to kinda know what I'm doing, and I still ended up installing and trying three different setups.

The whole spindle replacement/crank setup for best chainline thing is one of the mysterious dark arts of bicycle mechanics.

Yardbent

thanks everyone for taking the time

I don't have a LBS [country-living.!]
so will use Bill K's comment and source an older crankset

if that's a failure .......possible a new 110mm BB

stay safe....john

dsbrantjr

I think that by trying to find a crankset which fits your existing BB you are letting the tail wag the dog.

Find a crankset you like and then get the proper BB for it, they are often sold together at an attractive price.

Yardbent

probably very true

however a new starter here and the thought of finding the correct BB and fitting with limited tools and a kitchen table ''workshop'' is a bit daunting

but - have had the existing BB apart for a relube when i got the bike
so if i can tackle the 'fixed cup' side i could be OK

regards