working out spindle length for bottom bracket.

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petzl
Posts: 176
Joined: 20 Jul 2007, 8:44pm

working out spindle length for bottom bracket.

Post by petzl »

I have trawled through this forum and the web only to finish more confused than when I started, and yes, the subject has taken a bit of a thrashing. Can someone advise.

For a bog standard touring bike I need to find out the spindle length of a new bottom bracket for a triple chainset , 135mm wide rear drop outs running an 8/9 speed cassette on conventional hubs with rim brakes.

The nearest I have got is an article from the Bicycle Quarterly and Compass Bicycles stating: The chainline is the centerline of the chainrings in relation to the centerline of the bike taken on a line through the centre of the frame bottom bracket housing and centre of the drop outs For a triple it’s measured on the middle ring. At the rear, the chainline is measured at the middle of the cassette. The chainline for 130mm drop outs is 43.5 mm, measured from the centre of the seat tube to the middle chain ring.

I therefore presume that for 135mm dropouts the chainline is 46mm (43.5 + 2.5) taken from the centre of the seat tube.
I am aware that choice of cranks and final fit will have to be taken into consideration and that near enough is good enough however, I need is a start measurement.
Last edited by petzl on 25 Aug 2015, 3:13pm, edited 1 time in total.
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NATURAL ANKLING
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Re: working out spindle length for bottom bracket.

Post by NATURAL ANKLING »

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Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: working out spindle length for bottom bracket.

Post by Brucey »

a foolproof method is to

a) get your cranks,
b) try them on a ST BB that you think might be about right (this can be a scrap unit from the LBS)
c) check for chainline, clearance, front mech access etc
d) choose your new BB length

No-one can give you 'a starting point' because it can vary by over 20mm for different types of triple crank.

Also as you suggest the chainline itself may vary. I quite like to run a ~43mm chainline onto a 135mm hub for touring, because I reckon the top gear is one I'll use least, so the bad chainline in that gear won't really matter that much. But if you are trying to use (say) an MTB front mech, this won't work at all, and many chainsets will run out of clearance well before that too.

cheers
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Valbrona
Posts: 2694
Joined: 7 Feb 2011, 4:49pm

Re: working out spindle length for bottom bracket.

Post by Valbrona »

What crankset is ? We know it's a triple, but which make/model?
I should coco.
petzl
Posts: 176
Joined: 20 Jul 2007, 8:44pm

Re: working out spindle length for bottom bracket.

Post by petzl »

Valbrona wrote:What crankset is ? We know it's a triple, but which make/model?



A TA Zephyr triple.
I am well aware that much has been discussed as to the Zephyr being JIS or ISO and there being doubt as to when any change took place.
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CREPELLO
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Joined: 29 Nov 2008, 12:55am

Re: working out spindle length for bottom bracket.

Post by CREPELLO »

So you need to establish BB standard as well as BB length. Follow Brucey's suggestion, but first note the donor BB's standard. If your cranks are JIS, an ISO BB will tend to bottom out. Conversely, if your cranks are ISO and the BB is JIS, the cranks won't slide on as far - there will be more of the flats of the cranks exposed.

I've run a JIS BB with ISO cranks, although I forget how much of the flats were exposed (maybe 8mm?). Although it didn't look quite right, I felt it wouldn't ruin the cranks as I'm not an immensely strong rider.

Once you've established BB standard, follow Brucey's procedure. But I have to caution deriving the correct length, once you have your sample measurement. There seems to be some confusion about measuring BB's viewtopic.php?f=5&t=78892 - certainly for me at any rate :oops:
Valbrona
Posts: 2694
Joined: 7 Feb 2011, 4:49pm

Re: working out spindle length for bottom bracket.

Post by Valbrona »

I would ask people that speak fluent TA, like Spa Cycles, a TA distributer or even TA themselves.
I should coco.
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