Author Topic: Brompton chainline (derailleur)  (Read 2194 times)

rogerzilla

  • When n+1 gets out of hand
Brompton chainline (derailleur)
« on: 18 June, 2021, 06:14:41 pm »
I know this is a big ask, but can anyone with a 2- or 6-speed see if the chainring actually lines up halfway between the two sprockets?  Mine is definitely lined up with the smallest (outboard) sprocket.  The sprocket stack is correct and the chainring is already perilously close to the frame, to the extent that I'm going to fit a 2mm longer BB to shift the ring out by 1mm.

It probably doesn't matter much with a 3/32" flexy chain, but I suspect the rear triangle, like most steel full size frames, is 2-3mm off.  Maybe I'll just rearrange the sprocket spacers and ignore the instructions.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

LittleWheelsandBig

  • Whimsy Rider
Re: Brompton chainline (derailleur)
« Reply #1 on: 18 June, 2021, 06:31:08 pm »
My 6sp B's crankset is biased outboard but I can't currently measure by how much, given that it is on a different continent to me.
Wheel meet again, don't know where, don't know when...

rogerzilla

  • When n+1 gets out of hand
Re: Brompton chainline (derailleur)
« Reply #2 on: 18 June, 2021, 06:36:30 pm »
My 3 speed one is straight. but that's totally non-standard (hub, BB, crank) and I moved washers all over the place to make it right.  Had a look at the 6-speed sprockets and they can't go any further out anyway.

I've left it on the 113mm BB for now, as the chainline is only about 2mm off with that.  There is clearance, if not much.  I had to fit the hidden chainring bolt on the XD2 crank the same way around as the other four as the allen key head is fractionally taller than the slotted head, and hits the rear triangle when folded.  It is very tricky to fit the allen-headed part between the crank and spider, and get it properly tight!

The exact same BB and same (model of) crank gave far more clearance on the 3-speed.  Square taper, eh?
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.