3 sprockets on a 2-speed Brompton

[Fastpedaller]

I've previously asked about fitting 3 sprockets on a Brompton, and understand this is possible with sprockets off a ten speed cassette and using the original Brompton freewheel. The issue I have is that my bike is a 2007 Superlight and therefore the chainring is too thick for a 10 speed chain, so unless anyone wants to buy the crankset off me (bike has only done less than 100 miles - it was mint and unused when I got it a few months ago) I'm stuck with the cranks. I don't think it would be possible to turn off a little metal even if I had a lathe, as the chainring could flex during the process. Anyway, back to the purpose of my note.......... Does anyone know if a Brommieplus freewheel (Brompton hub fitting of course) with the shimano lockring is wide enough to use 3 standard sprockets and 8 speed chain? This would mean I don't have to buy new cranks, as long as I could adjust the derailleur to suit.

 

[u_i]

Does anyone know if a Brommieplus freewheel (Brompton hub fitting of course) with the shimano lockring is wide enough to use 3 standard sprockets and 8 speed chain? This would mean I don't have to buy new cranks, as long as I could adjust the derailleur to suit.

Yes, three 8sp sprockets should fit comfortably on that freehub. Note, though, that the Brommieplus freehub does not come for free in terms of the pain. Thus, the wheel needs to be redished after that freehub gets mounted. Basically, the axle gets displaced into the left nondrive side with the Brommieplus freehub. Without redishing, the wheel begins to rub against the left stay and you need to move it relative to the axle toward the right drive side. You do it by slightly relieving the spokes on the left side and tightening them on the right side. You do it gradually and the whole process takes over an hour. You better do it with some dedicated equipment, but you may also give it to a shop to do. Changing the freehub takes some work too, but less in comparison.

A benefit of the Brommieplus freehub is that you can achieve a pretty flawless shifting over 3 cogs with that freehub, while the shifting over 3 cogs on the original freehub will struggle.

 

[Fastpedaller]

Thanks for the info. I'd certainly like to get a 'between' gear on the 2 speed, and to go for 10 speed sprockets and spacers would mean a new chainset and chain. Getting 3 gears with the eight - speed components means it's affordable. If (when) the chainring eventually wears out my plan is to replace the crankset with a Spa Cycles double 130 bcd, and use a 'dirty finger' change or fit a front derailleur as others have done. Looking at ratios that will probably result in 5 gears rather than 6 because of one duplicate. Redishing the wheel won't be a problem - I guess it's only about 2mm, and I've been building wheels for 40 years, so shouldn't be a problem as long as tensions L to R aren't hugely uneven. With a small wheel bike the only route to higher gears is a hub gear (I'm not a fan) which also adds weight, or fit a massive chainring. I'll just twiddle (and fit 165 or even 160mm cranks when I do the change). I've previously taken the rear wheel out and swapped the 12T sprocket for a 13 (so would probably refit the 12 if I could then go 14, 16. and at the time I removed the freewheel (very easy, and just hold the pawls in to replace it).

 

[u_i]

Indeed, the shift in redishing was not large - I do not remember the exact value, though, and cannot check as I am away from my base. There is another issue though that may affect you in that the outer cogs may get separated far enough with the 8sp spacing that the original Brompton pusher may lack sufficient reach to cover both of them. With this, you may need to replace the pusher with the one that Brommieplus offers @USD34. You may need longer pulley axles regardless of whether you stay with the Brompton freehub or move on to the Brommieplus one.

 

[u_i]

Oops, I see that you do not have the BWR - this makes a small difference, though, as far as I can tell in what you are trying to accomplish.

 

[Fastpedaller]

I'll examine the bike to try and ascertain how much extra movement is available. If I also find the widths of sprockets and spacers I can calculate the width they will occupy. Doing the same calculation for 10 speed spacers and sprockets may give some idea of available width - I think it's correct that the brommieplus freewheel is ok with 4 of the 10 speed sprockets, but I'll see what I can find via their website.
This is more involved than expected. I wonder why Brompton didn't just use a freewheel with the screw thread, but I guess it may have been cost driven.

 

[u_i]

Brommieplus normally use 11 spacers and chain with their freehubs. I started with 10 speed and still fitted them, but barely.