What width Bottom bracket?

PenguinWithHair

Pictured below is a new crankset (Sturmey Archer FCT244B) I've put on my bike (Genesis Day 01). My old set was a Sugino RD2. The red lines are roughtly where my old crankset used to sit, the new ones sit approx 3 to 4mm outward of that.

I'm riding it like this for now (it's my commuter) but I'd like to fit a new bottom bracket so the cranks sit more snug with the frame, can anyone briefly explain bottom bracket lengths to me please? I'm confused about what the different lengths are actually refering to (spindle length?) I'd like to know how I can work out what a suitable size/fitting/thread type (or what ever else) would be.

Many thanks!

alfablue

If a BB is say 68x113 that means the first figure is the shell width (typically 68 or 73mm) and the second number is the length of the spindle from the end of each taper. So you need one with a shorter spindle (second number). Don't go too short or you may have problems with the chainline and fouling the chainstays with the chainring.

Have a look here http://www.sheldonbrown.com/bbsize.html

nicklouse

well have a look on SA webby and they list the BB sizes needed for their cranks.

but a FCT24 with a 43.5mm chain line needs a 103mm axle what chain line do you want on your bike?

Berk Bonebonce

For sure, BB's come with axles of different lengths.

However, is your new chainset of the correct taper?

PenguinWithHair

Hey, thanks for all your feedback.

To be honest I'm not sure what chainline I want, although I can tell you it's a single speed..? I'm not sure if how it is now is the same chainline as it had, or whether it's 3mm more outward - but either way I think I can probably get away with 3mm?

It's all JIS square taper.

To be honest I'm pretty confused. Sturmey Archer state the optimal bottom bracket is BBS15 - what ever that means. The ebay seller states it's compatable with any axel width but 113 or 115 would be ideal.

Looking at the specs for my old crankset (Sugino RD2) confuses me even more! 103 single, 110 double, 113 tripple. My chainset only had one chain ring, which I would've guessed means it wants a 103 - which would mean in theory my bottom bracket should be too small!

I mean, I pushed these cranks on, and tightened the bolt up pretty good - is there anyway I could've done something wrong while fitting them? As far as I could tell, the spindle of the bottom bracket was seated in as much as it wanted to be.

Thanks again!

nicklouse

time to get measuring then. check the axle length.

but if does look like the taper could be different IF the axle length is 103mm

chain line is center of seat tube to center of teeth. and should be as above I guess.

topdude

The cranks don't look like they are fully seated on the axle, too much taper showing :?:

PenguinWithHair

I'm pretty sure it's all JIS square taper so it should all be matching.

I know it looks dodgy right? For some reason these new cranks are a lot 'shallower' than my old ones where they fit in (possibly becuase I downgraded somewhat). I'm sure I checked the spindles were seated in as far as they could go.

Sorry just to be clear, when I'm measuring the spindle, I'm measuring just the part that's square?

To help determin whether the chainline and postition of the left crank is 'as stock' as it's pictured now - what sort of clearence would you expect from the chainstays from the inside of the crank?

If it turns out, the cranks are positioned exactly where they should be even with some taper exposed - is there nesecerally any reason why I couldn't just run it like this? The only reasons I can think of is that the bottom bracket internals are more vunerable to dirt and getting wet, and there's is possibly a dodgy leaverage effect which could put stress on the bottom bracket. Any thoughts?

I am currently riding it like this be it rather gingerly, it does look a little off putting!

PenguinWithHair

By the way thanks again for your input folks!

nicklouse

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


Chainline Measurement-Front

Chainline is measured from the centerline of the frame to the center of the chain.

You can measure the front chainline directly with a simple ruler. Simply hold the ruler against the seat tube or down tube and measure the distance to the middle of the chainring teeth. In the case of triple chainwheel sets, measure to the middle chainring. In the case of doubles, measure to the halfway point between the two rings.


and D


and makes no difference to being vulnerable to dirt.

PenguinWithHair

Apologies for digging this thread up again, but before I take this to a bike shop (it's started creeking every so often which is a little off putting) I'm going to ask what's possibly a very silly question.

Is it at all possible that tightening the crank bolt up isn't applying enough pressure to seat these cranks into place? My reason for assuming they're properly in place is that tightening the bolt will pull them in. However my old cranks were proper proper tight to get off, I had to take the whole lot to my lbs after the handle fell off my crank extractor!

Before I go getting a shorter bottom bracket I thought I'd ask! I suppose my actual question is - "Do I need a rubber mallet?".

keef66

No, just tightening the crank bolts should pull the cranks on to the taper. Quite a high torque usually. Then tighten them again when you've ridden them a while.

Did you ever get round to measuring your BB axle length??

PenguinWithHair

Uh... This is a little stupid of me but I've just realised it's got B110 engraved onto the non drive spindle. I am a little confused now.