Can anyone confirm which spindle length Shimano UN54 BB fits the Brompton chainset? As I understand it using a JIS type spindle should mean taking about 4mm off the length? The UN54 or an equivalent older model can sometimes be found very cheaply, good to have an alternative.
According to this thread (hard to figure out via google translate (which is better than my French)) there's only a 0.3mm difference when installing an ISO crank on a JIS BB, which would mean that a 118mm UN54 (or equivalent) BB would produce the correct chainline:
Yes, not so easy to follow, even in french, this discussion about the compatibility JIS/ISO between a crankset and a bottom bracket.
What I could translate for you? Perhaps the experiences of Fredo.
Fredo wrote (source: his original french-speaking post) that with a crankset Tiagra (JIS axle), there was not a problem on a ISO axle.
I translate his last posts (24th March 2010) - I hope that it will be more understable than the translation of a boot.
"I measured several combinations of mounting:
I mixed the original bottom bracket with, in the first case, the original Brompton's crankset and, in the second case, a Shimano Tiagra crankset. I repeated the operation [with the both cranksets] with a bottom bracket Shimano BB51.
The axis is fitted on the crank to a depth of:
On the Brompton's axis [ISO]:
Brompton bracket [ISO]: 16.4mm
Tiagra crankset [JIS]: 17.6mm
With the Shimano axis [JIS]:
Brompton crankset [ISO]: 15.3mm
Shimano Tiagra crankset [JIS]: 15.6mm
Note, the "input" [not found the right term fot my translatation] of the square on the crank Brompton is chamfered, not the Shimano. So the "contact" is longer (+ / - 1 mm) with the crank Shimano)
The difference of the "fitting" is very small in fact, between 0.3mm and 1.2mm (considering the same bottom bracket).
The best value is obtained with a mixed mounting: 17.6
I tried to measure the size of the square on the end of the axis but a chamfered axis of the Shimano prevented me from taking the measurement axis end ...
Note: I tightened the cranks on the axis so as regularly as possible but I have not used a torque wrench: these values are not measures "lab" but reflect a certain reality."
And in the second post:
"I mention the conclusions of "Berthoud":
JIS crank on a ISO crankset, clamping is possible, there is even a margin of a few milllimeters.
ISO crank on JIS axis: the "loss" of penetration is only 0.3mm compared to a fitting JIS / JIS."
And my answer about the UN-54:
I used a UN-54 (Shimano Tappered square) of 118 mm (of course BSA 68 mm). Not a problem with the original Bromton's crankset (model 2009) to fix it on the axis and the chain line was correct (visual observation). Sorry, I did not measure the chain line and Q-factor (+ delta) of this mount
Parenthesis: anyway, It's possible that the delta Q-factor is not good originally, today, on a Brompton. Of course with flat pedals, no matter. And so, we know also that a part of the production (before the Mark IV - pre-2005-, if well understood) of the Brompton bicycles, that the frame was not exactly "straight" looking it on a measuring table. I have 2 Mark I and perhaps soon an early Mark II, I'm curious to verify this point, the bikes are yet in (very) good condition and not modified.
To return to the UN-54: not really a great bottom bracket. I prefer other BB if I have to chose a sealed cartridge for less than 25 euros.