Talk to me about Brompton gear upgrades ...

[CopperBrompton]

Ok, thanks. That one would be too low for me with that particular setup - I currently have 47/63/84 and am looking to get a little lower and somewhat higher.

 

[Rohloff_Brompton_Rider]

Yeah, I emailed him but no reply

Sorry, forgot to say..he's on holiday...oops.

 

[Rohloff_Brompton_Rider]

5 speed Sturmey hubs are still in production.
Have a look at www.sturmey-archer.com/

Yeah, I meant already built wheels, sorry I should have been clearer.

 

[CopperBrompton]

Sorry, forgot to say..he's on holiday...oops.

Ah, ok :-)

 

[rualexander]

Ok, thanks. That one would be too low for me with that particular setup - I currently have 47/63/84 and am looking to get a little lower and somewhat higher.

Ok, well you'd just have to change your chainring and/or sprocket size then.
For example, a 50 tooth chainring and 14 tooth sprocket would give you a range of 36 - 92 gear inches, or with a 13 tooth sprocket 38.6 - 98.9 gear inches.
Sheldon Brown gear calculator is here http://sheldonbrown.com/gears/
Note, the Sturmey Archer 5 speed x-rf5(w) seems to 36 holes so you probably need to change the rim too.

 

[CopperBrompton]

The Sheldon and Brompton calculators seem to give totally different results <Confused>

 

[Night Train]

Note, the Sturmey Archer 5 speed x-rf5(w) seems to 36 holes so you probably need to change the rim too.

I don't know if you can get 36 hole 16" rims. I've tried looking for some to build QR trailer wheels and haven't managed to find any.

 

[rualexander]

Yes 36 hole 16" rims are available, St John Street Cycles sell them, Sun CR18 model
I'd post the link but for some reason this website doesn't let me paste into posts when using the browsers on my tablet.
Just Google 349mm rim 36 hole and it will come up.

 

[Night Train]

Yes 36 hole 16" rims are available, St John Street Cycles sell them, Sun CR18 model
I'd post the link but for some reason this website doesn't let me paste into posts when using the browsers on my tablet.
Just Google 349mm rim 36 hole and it will come up.

Cheers.

 

[rualexander]

36,spokes between a SA hub and a 16" rim would leave very little space to get a pump onto the valve.

Shouldn't be a problem if the wheel is laced properly with the valve hole in the right place, see this picture of a 36 hole Brompton rim with an electric motor front hub,

 

[CopperBrompton]

Thanks for all the input on this. The gear calculator was wrong about the SA-8 - it actually gives 33-107 gear inches with a 50T chainring, which gets me everything I wanted: greater range at both ends, and smaller gaps, and with my preferred twist-grip shifter.

The upgrade would have cost a little shy of £300, but Tiller had a ready-converted brand-new one on offer. Buying that one and selling my existing one meant a premium of only £150 to get a brand-new bike, so decided to treat myself. :-)

 

[Messenger88]

Hi Trikeman,

I wrote the Brompton Gear Calculator, so would like to know where it went wrong with your SA-8 calculations. Could you tell me how many teeth you have on your rear sprocket, and let me know the model number of your hub? I'm keen to iron out all the bugs.

Many thanks

 

[CopperBrompton]

Hi Messenger, I'm rather non-mechanical, but I believe the rear sprocket is whatever is standard for a 2013 M3L.

The hub is, from memory, the SA SRX-8, but if that doesn't make sense I can check when I get home.

 

[Messenger88]

Hi Trikeman,

Thanks. I can't find any information for the Sturmey Archer SRX-8, and my calculator only has data for the wide-range Sturmey Archer 8-speed hubs. However, all the 8-speed Sturmey Archer hubs I know of have direct drive in gear 1, so with a 50 tooth chainring and a 13 tooth rear sprocket, the approximate 1st gear ratio is easy to calculate: wheel diameter x (chainring / rear sprocket) = 16 x (50 / 13) = 61.5 inches.

However, I don't think the Sturmey Archer hubs support the standard Brompton rear sprockets, so to get your gear ranges you must have a larger rear sprocket - perhaps 25 teeth? I don't know what's possible, but that's the combination required to get your gearing.

If you've got a completely different hub, please let me know.

88

 

[CopperBrompton]

It is definitely a Sturmey Archer, but it would probably make sense for you to talk to the man who knows: Graham at Tiller Cycles. http://www.tillercycles.co.uk/page6.html