Looking for a bottom bracket for a fixie frame with a 68mm wide BB shell. What length? Nothing super expensive just now.

http://www.merlincycles.co.uk/Bike+Shop/MTB++Parts/Transmission++Gears/Bottom+Brackets/Shimano+UN54+Square+Taper+Bottom+Bracket_19.htm

Thanks for the link. Any idea of the most suitable axle length?

I use these Miche jobbies on track and fixie bikes. Depends on chainset but most square taper track chainsets use a 107mm axle length.
This Miche one also allows you to adjust the chainline as the lockrings aren’t shouldered.
http://www.parker-international.co.uk/1822/Miche-Primato-Track-Bottom-Bracket.html

whats your chainset?

I also use a Miche chainset but the tapers are the same profile as Shimano eg UN54

Chainset? – an old 80’s Specialized.

Merlin list three Shimano UN54 bottom bracket lengths. 107, 110, 113mm

My chainset is the one below. It is a triple and I will be mounting the ‘ring on the outside. I’m assuming that a track chainset will have the shallowest offset to reduce flex. I’m thinking I’ll perhaps go for the 110mm spacing.

Any advice would be welcome.

going to assume:

1) 120mm rear OLN
2) those are modernish track cranks designed for 120mm OLN

the chainline should be somewhere around 42mm with the chain in the outer position

Stronglight, the closest i can find here http://sheldonbrown.com/chainline.html and http://sheldonbrown.com/bbsize.html

would have a 107mm BB axle to get the right chainline on their track single.

So, you may find that you need either 107mm or 110mm. But unlikely 113mm.

Cheers Stoner. Its going to be a bit of guess. The frame has a 130mm spacing and I’m using a 126mm screw on freewheel hub with an adaptor.

I’ll go for the 110mm, the 3mm over the 107 might be enough with the wider crank.

Have you a frame with BB you can set it all up on to see? Otherwise it’s total guesswork unless you are running a derailleur chain.

Those take 122 for a triple set up.

The frame has a 130mm spacing and I’m using a 126mm screw on freewheel hub with an adaptor.

then start by calculating your chainline at the rear,

Next, work on the assumption that 107mm puts your crank at 42mm and adjust the BB length you actually want to buy accordingly.

i.e. if your 126mm respaced hub in the dropouts actually has a chainline of 45mm, then use a 113mm BB.

As a slight bodge – which I know will offend Stoner(!) – if you use 3/32 sprockets and chairings and a 1/8 chain, you’ll allow yourself a bit of play if the chainline isn’t exact. Has worked for me on a much more out of line chainset.

I saw the oddest chain yesterday on a fixiet**t at Oxford train station – it had a captive outer link – i.e. it wrapped the chainwheel/sprockets on both sides and the outside.

not sure it would work well in mud 😉

stoner 122.5 will give 47.5 chainline…does that add up to your figures?

are they triple cranks as opposed to track cranks then?

In which case 47.5mm chainline on 122mm (middle) is equivalent to about 54mm ish in outer position on 122mm. So take 54mm less the hub chainline multiply by two and take it off 122mm.

So if Hub chainline is 45mm, then take 122-(54-45)*2 = 103mm.

OR, put the chainring on the middle position so it goes:

122-(47.5-45)*2 = 117mm

As I said, start with the hub and then work it out from there.

Yes, triple. Sounds right

I went for the 110mm, I’ll let you know how it turns out.

The chain line worked out pretty well. 🙂