Bottom Bracket Length?

[andylaw79]

I posted a couple of months ago about converting the hybrid to an Alfine hub for winter commuting and due to having to get boiler repairs done at home that's gone out of the window.

I've been commuting on a ss mtb but have recently been feeling the limitations of it in strong winds. I'm now looking at stripping down the hybrid and just having a single chain ring on the front and an 8sp cassette on the back as a lower maintenance (than the current 24sp set up) option.

What length bottom bracket would I need for the new single crankset. When I converted the MTB to ss I used a 108mm BB, would the same be Ok for this or would I need a longer one to allow for the gears on the back?

 

[MartinC]

If you're just going to convert your triple crankset to a single by removing the outer and inner chainrings then keep the same width BB as you have now. This should put the chainline in the middle of the block (assuming it was OK before).

If you're buying a new single chainset then the manufacturers specs should tell you what the chainline is for a given BB width. Normally their choice will give you a chainline pretty much in the centre of an 8 speed cassette. It's only when you're going to Fixed, SS or a hub gear that the chainline gets to be an issue.

Keep the front mech on, remove the cable and shifter and adjust the stops so that it's centred over the chainring. This will stop the chain coming off at the front - it sometimes happens with a single chainring and a wide cassette.

 

[andrew_s]

BB lengths depend on what chainset you are using.
If you have a single chainring you want it to line up with the sprocket of a singlespeed or hub gear, or the centre of a cassette. However the position of the chainring relative to the axle end depends on the shape of the spider - some are flat, some slope back towards the bike. The chainset manufacturer will quote a BB length for a "standard" chainline, but if you are using an unusual setup you may have to use a different length.

 

[andylaw79]

Thanks for the replies,

Martin - will have a look at keeping the current cranks and just removing the outer and inner chainrings this weekend, just ordered some shorter bolts. If that fails the Alfine cranks come with a BB supplied so should hopefully be no issue.

Had a look round for solutions to keep chain on front ring instead of having the front-mech on and found a a little gadget from Deda called a Dog Fang. Little plastic widget that cyclo-cross riders use to stop chains skipping off when shifting.