Choosing a bottom bracket

[4F]

I have recently converted my old 1970's Dawes to fixed running off the smaller front double chainset and decided that I like it but need to up the chainset size as the 40 ring is just too small. It's not a spider so I cannot change the ring so I have just puchased this RPM chainset that has come off a Fuji Track Bike. 48 tooth 1/8th single ring PCD - 130 170 length cranks

Never having replaced a BB before what measurements do I need to look for to make sure I get the right one ?? I have measured across the outside of the BB and come to 6.8 cms.

Thanks

 

[e-rider]

Yes the BB shell is usually 68 or 73 - you have measured yours at 68 - this is the most common size and are available from all shops.

Is it a square taper or ISIS axle?

Now you just need to get the correct length axle - this is not so easy.
If it's a single ring chainset you'll need a fairly short axle to get a decent chainline but it depends on a few things. Trial and error is the usual approach. I'd guess at about 107mm or maybe a little shorter.

See what other posts you get!

 

[e-rider]

oh yeah, you might want to check the thread but it's probably english

 

[e-rider]

You should check whether these chainsets use a JIS standard or not

 

[4F]

It's a square taper and I am assuming English as all the original components were non Italian ie sachs huret mech and weinman brakes.

No idea what a JIS standard is ??? (OK just looked this up on sheldon)

 

[PpPete]

oh yeah, you might want to check the thread but it's probably english

If 68 mm shell it's definitely English thread.
I have no idea how you check whether a square taper on a crank or axle is JIS or ISO standard.

I would guess that a chainset off a Fuji would want a JIS taper.

One approach would be to pick up the cheapest nastiest Shimano compatible BB you can find on fleabay (I got one for 99p once!)
Pop that in - and see how far out your chainline is, deduct or add miilimetres accordingly then replace with a decent BB of whatever length you arrive at.

 

[4F]

My current BB is square taper so I suppose by offering up the new chainset I should be able to deduce if it is JIS or not and then try and measure from this how far I am out with regards to axle length. I assume a JIS will not fit onto a standard square taper ?

 

[jpembroke]

Bound to be JIS isn't it? JIS is Shimano (and most other makes). ISO is Campag. Your good 'ol Shimano square taper UN53/54 is JIS.

 

[Landslide]

I assume a JIS will not fit onto a standard square taper ?

JIS is a standard square taper, as is Campy's ISO. They're just slightly different standards.

FWIW, I've run a Campy chainset on a Shimano BB (ooh, the heresy!!!) without any problems, but I know some people have had issues.

Have you tried (loosely) fitting the new chainset on your current BB axle? You may find that it sits there quite happily, giving you a usable chainline.

 

[4F]

JIS is a standard square taper, as is Campy's ISO. They're just slightly different standards.

FWIW, I've run a Campy chainset on a Shimano BB (ooh, the heresy!!!) without any problems, but I know some people have had issues.

Have you tried (loosely) fitting the new chainset on your current BB axle? You may find that it sits there quite happily, giving you a usable chainline.

aaah OK thanks learning more and more as I go along, not got the chainset yet so will await its arrival and offer it up and see where I am at.

 

[jpembroke]

I built a fixie for a mate using an RPM chainset. It did need a JIS bottom bracket. I think I used a 110mm UN54, which cost about £15.

 

[4F]

Excellent cheers for all the help guys.

 

[NickM]

One thing to be aware of: Shimano 110mm and 107mm BBs both have the same drive side overhang - the NON-DRIVE SIDE is 3mm shorter on the 107mm BB.

So you just select whichever of 107 and 110 gives reasonably symmetrical crank/chainstay clearance.

Next (Shimano) size up is a 113 - and just for kicks, the difference between 110mm and 113mm is 3mm all on the DRIVE SIDE

 

[jpembroke]

One thing to be aware of: Shimano 110mm and 107mm BBs both have the same drive side overhang - the NON-DRIVE SIDE is 3mm shorter on the 107mm BB.

So you just select whichever of 107 and 110 gives reasonably symmetrical crank/chainstay clearance.

Next (Shimano) size up is a 113 - and just for kicks, the difference between 110mm and 113mm is 3mm all on the DRIVE SIDE

Cool! I didn't know that.

Gold star.

 

[dudi]

FFFF, I've got a 110mm one and i'm a little out on the chainline.

If you can ge a slightly shorter one, then go for that... but by the sounds of it, a 107mm one own't help much