Brompton transmission modifications

[citoyen]

I'm considering tinkering with the Brompton's transmission - to be precise, I'm thinking of replacing the 3-speed rear wheel with a singlespeed and swapping the 50T chainring for a 54T. The current rear hub is knackered and the chainring is worn out, so both could do with being replaced anyway.

First question: anyone know where can I buy a suitable singlespeed wheel for a Brompton? Neither SJS nor Bikefix acknowledge the existence of such things on their websites, though I dare say Bikefix could sort me out if I asked them.

Or failing that, where can I buy a suitable hub so I can build my own? According to my tape measure, the OLN is 113mm. Would any singlespeed/fixed hub of the right size do, or do I need one specially for small wheels?

Next question: with a 13T rear sprocket and a 50T chainring, the "middle" 1:1 gear on the current 3spd set-up is about 65" according to my calculations. If I swapped to a 54" chainring, it would be more like 70". Does that sound too high? I find the current gearing slightly too low for my liking, but I don't want to push it too far the other way. Anyone use a similar set-up and care to comment on how they find the gearing?

cheers,

d.

[peterh]

It's probably an expensive option, but there's one guy who has built up a fixed gear Brompton with a Phil Wood hub

[rogerzilla]

50/13 is a shade over 60" with bigger tyres (Marathon Pluses are about as big as you can get in the frame).  I use 50/14 for them, but 50/13 for the Stelvios.  On a singlespeed you generally go for a lower gear than on a fixed (where 70" or so is usual) because you don't have the problem of getting down hills.

If you want a rim and spokes, I have some unused ones.

Any bolt-on hub with the right axle diameter and an OLN of 120mm can probably be spaced to fit, but you can't guarantee a dishless wheel; the sprocket has to end up in the right position wrt the dropout because the chain tensioner determines its position (even if you change BB length).  If you have a friendly LBS they might let you play with a few freewheel hubs to see what would work best - a set of vernier calipers is recommended.  Ideally you want the flanges to be centred between the locknuts when the sprocket is the right distance from the locknut and the spacing is 112mm - then you have to hope the standard length spokes fit, or you'll be paying £££ to get them cut down and rethreaded.

The Brompton freehub is about £55 if you want something that Just Works.

[citoyen]

Ooh, that is lovely! But a Phil Wood hub is possibly a bit out of my price range...

He seems to have it set up as a fixed. Is that a good idea with a Brompton, bearing in mind it requires a chain tensioner? I was thinking of a single-speed freewheel, but a fixed Brommie is certainly an appealing notion...

d.

[rogerzilla]

People have fixed Bromptons, but it can't work well unless you completely lock out the suspension, seeing as the pivot is not concentric with the BB.  The plastic chain tensioner would be destroyed the first time you let the bike do the driving.

[citoyen]

50/13 is a shade over 60" with bigger tyres (Marathon Pluses are about as big as you can get in the frame).  I use 50/14 for them, but 50/13 for the Stelvios.

I used to use Marathons but I use Stelvios all the time now - cos they is quick.

On a singlespeed you generally go for a lower gear than on a fixed (where 70" or so is usual) because you don't have the problem of getting down hills.

Interesting - I hadn't thought of it like that. Good point. But I do find the standard 3spd gearing too low, so I'm still tempted to go for the bigger gear.

If you want a rim and spokes, I have some unused ones.

Yes please! I'll PM you to discuss.

Any bolt-on hub with the right axle diameter and an OLN of 120mm can probably be spaced to fit ... The Brompton freehub is about £55 if you want something that Just Works.

Yeah, the latter sounds like a much better idea for me.

d.

[citoyen]

OK, so I've found where they were hiding the Brompton freehubs on the SJS website... Next daft question:

What sprocket does the freehub take? Looking at the picture, it seems to require a splined sprocket rather than the standard hub gear sprocket. SJS have a set of two splined sprockets - is the freehub designed solely for a two-speed set-up?

d.

[rogerzilla]

This sort of implies it's the same hub:

http://www.brompton.co.uk/content.asp?p=206&l=1&s=2#

but I'd try phoning SJS.

[LittleWheelsandBig]

The Brompton freehub takes Shimano cassette sprockets, either one or two.

[rootes]

i have a 2 speed brompton on a 54 chainring and a 12/16 rear

just swapped from marathon to stelvio - the latter has lowered the gearing slightly but has made the bike mush quicker - now just awaitng the first puncture! lol


was thinking of going single speed perhaps 54/13


BMX rear hubs have the correct 110 OLN spacing - looking into using one perhaps or you coudl just use the brommie 1/2 speed rear hub

[rogerzilla]

Or a Sturmey-Archer TC.  If you can find one.  No derailleur required - it's a 2 speed epicyclic hub.

[rootes]

still need a 'derailleur' to tension the chain when the bike is folded