Square taper spindle length.

[Cycleops]

I have an 2007 aluminium Trek road bike and the original triple chainset is worn out and can't find a similar replacement down here. My local LBS has a Chinese 52/42 chainset so I bought one of those for £8.50.
The existing spindle is too long giving a poor chain line. How do I calculate the best spindle length?

 

[ColinJ]

Measure the one you've got and buy one shorter than that...?

Maybe the taper on the cranks and BB axle don't match so the cranks are not going far enough on? (Sheldon Brown article.)

 

[GuyBoden]

The measurement from the centre of the rear hub to the centre of the middle sprocket gives an indication of a good chainline position.

Compare that measurement with what you already have on your inner front chainring.

This will give an indication of the BB spindle length you need.

 

[fossyant]

You probably need to chop a foot off that axel. Wowsers.

 

[Ming the Merciless]

Most FD are designed for a 45-50mm chain line. Measure your current chain line then calc from that. The wider chain line being for mtn derailleurs. If you know make and model of FD derailleur you can get the chain line it’s designed for from the manual.

 

[Cycleops]

Thanks all, that's all useful stuff.