bottom brackets & chainline - 3 speed crank upgrade

[DxMachina]

Hi all

I'm edging closer to ordering parts for my 1987 Triumph 3-speed crank upgrade: A grinding bottom bracket and cotter pins that loosen themselves every 30 miles are trying my patience.

I'm less concerned about the threading on my b/bracket shell since I discovered the existence of unthreaded expanding bottom brackets. (option I will use IF the threading turns out to be raleigh-proprietary)

What's stopping me is the axle length. The current crank axle (cottered!) is 146mm long, with (obviously) a single 46T chainring

Cartridge Bottom Bracket axle lengths seem to vary but none seem to be that long. So clearly the cotterless (JIS Taper) cranks and chainrings overlap the ends of the axle by a fair way....

Problem is I have no idea by how far. Can anyone advise? I'm a bit lost here as I dont really want to start prising my other bike apart to measure the overlap in case I cant get it back together again....

This is the crank I propose to use: http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... _600wt_733
The seller suggested a 127mm bottom bracket axle length would be best for it.

Thanks in advance for any suggestions offered
Chris

 

[terryhfs]

Measure the BB shell - If it's 68mm then I'd go with the seller's recommendation. If it's a lot wider then clearance could be an issue.

 

[DxMachina]

b/bracket shell is 68mm....

 

[DxMachina]

my biggest problem is I don't understand WHY the b/bracket length should be 127mm and I'm not buying parts til it makes sense to me.

Is the axle length determined by the thickness of the chainring/crankset, by the desired chainline, or by the fittings on the bike itself? Is there a standard length for different types of bike? or an online calculator?

For instance, the Stronglight single chainset apparently requires a 113mm bottom bracket? is this because its made for a different type of bike or just because its a lot slimmer than the old-style crankset i am looking at?

any help or explanation greatly appreciated

 

[terryhfs]

Sheldon brown's site has a lot of detail, but essentially it's down to the chainset, not the bike - unless the bike is unusual.

The b/b length is part of the calculation along with the design of the chainset the ultimately determines the right fit. If the width is too little, the crank arm will hit a chainstay. If it's too wide you'll get an uncomfortable distance between the pedals (Q factor) and to much load on the bearings because of the leverage you'd be applying. Chainline is also a factor - fit the wrong BB and the front mech (if you have one) can't reach the rings.

Honestly - if it's a 68mm shell, go with whatever the seller suggests - it'll be the standard width for that chainset

 

[DxMachina]

Thankyou

That helps a lot

 

[DxMachina]

As it happens I made a mistake to trust the seller on this - the chainset's well out of line on a 127mm b/b. 122mm probably a better fit although it may still be on the wide side.

That said I dont really care - I'll now have a spare B/b in my toolbox.. and my bike is close to being rideable now and I'm certain of success in the upgrade project.

EDIT - no it isnt that far out, i just hadnt applied enough strength.