Cycling UK Forum

I have 6 speed Brompton 2017 version.

It as 3 speed wide range SAhub..

It came with 13/16 spockets and 50 chainring.

I have now changed it to 13/18 spockets and 44 chainring.

It gives gears 83 top to 25 bottom.

I suggest you look at sheldon brown gear calulator.

With hub gear there are trade offs.

The gear I got is ideal for me. Does anybody need higher than 83 gear for touring partlicley with the bag on the front. I rode up green hill in high wycombe with 2 breathers on the B. The hill has arrow ie it 11 3% grade

I have 6 speed Brompton 2017 version.

It as 3 speed wide range SAhub..

It came with 13/16 spockets and 50 chainring.

I have now changed it to 13/18 spockets and 44 chainring.

It gives gears 83 top to 25 bottom.

I suggest you look at sheldon brown gear calulator.

With hub gear there are trade offs.

The gear I got is ideal for me. Does anybody need higher than 83 gear for touring partlicley with the bag on the front. I rode up green hill in high wycombe with 2 breathers on the B. The hill has arrow ie it 11 3% grade

Thanks Philip, that's very helpful. I like to climb with very low gears, and I'm used to a bottom gear of about 18" (and even with that I'd be walking up anything that said 11.3%!). It was mentioned upthread (and I can see for myself) that a large sprocket may foul the chain tensioner; when you fitted your 18t sprocket, was that the maximum possible, or was there clearance for a bit more?

When Brompton were prepared to deal with Spa we regularly used to fit an additional smaller chainring on the crank side of the existing chainring, to be changed by hand.
Its my recollection that the original Brompton chainset (at that time) had drillings which would accept a standard smaller chainring, eg Stronglight

Been in shed: I was slightly wrong before. I added a 39t ring to that 2005 Brompton 50t chainset. It camewith spare bolt holes at 122mm & 86mm BCD.

Stronglight made the Brompton chainsets.I then replaced the 50t with the above 44t chainset, which I presume to be the same as the Brompton badged 44t chainset, but without the chainguard (??)....which I presume would use the spare bolt holes, which are at 122mm BCD.

Looks pretty straight forward to drill bolt holes in this type of chainset... so maybe search on ebay to see if there's a cheapo 2nd hand 44t and add a lower ring.

My recollection is that with the 50t and 39t that I had to fold the bike on the 50t...I think the chain tensioner snagged?...and there will be a limit on total tooth difference the chain tensioner can handle. Those who are running doubles and triple chainsets currently may be able to advise further

nirakaro wrote: ↑23 May 2022, 8:41am Thanks Philip, that's very helpful. I like to climb with very low gears, and I'm used to a bottom gear of about 18" (and even with that I'd be walking up anything that said 11.3%!). It was mentioned upthread (and I can see for myself) that a large sprocket may foul the chain tensioner; when you fitted your 18t sprocket, was that the maximum possible, or was there clearance for a bit more?

Now I am in front of my laptop I give exact figures.

My gear ratio are as follows
CR 44
13= 34.7, 54.4,85.3
18 = 25.1,39.3,61.6

I think you may be able to fit a 19 tooth just? see photo

I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

ANTONISH wrote: ↑23 May 2022, 5:06pm I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

I understand it is not recommended to us sa on tandems

Philip Benstead wrote: ↑23 May 2022, 7:07pm

ANTONISH wrote: ↑23 May 2022, 5:06pm I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

I understand it is not recommended to us sa on tandems

I know a local CTC member who did. It didn't last long and the planets lost teeth.

rogerzilla wrote: ↑24 May 2022, 7:42am

Philip Benstead wrote: ↑23 May 2022, 7:07pm

ANTONISH wrote: ↑23 May 2022, 5:06pm I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

I understand it is not recommended to us sa on tandems

I know a local CTC member who did. It didn't last long and the planets lost teeth.

They can always be replaced, consider them consumables?

Use xldev.co.uk to calculate current gearing and to play with possible lowering.
My 6 speed runs a 38/34 up front on a standard hub. Manual/greasy finger change. Didn't even change bottom axle length, doesn't affect folding and never dropped the chain.
Cost c. 50 quid as best I recall.
By far easiest route to sub-25" bottom gear!

ANTONISH wrote: ↑23 May 2022, 5:06pm I can see the benefits of lowering the gears but is there a ratio where the torque at the hub can become excessive ?

There is a subtle difference between the torque transmitted through the hub and the gearing ratio, torque is a function of the balance between applied load at the pedals and load resistance at the back wheel’s rim.

IIRC Brucey advised that overloading the hub was less of an issue with small wheels. I was building a 6 speed Brompton wheel using an old pre NIG AW 3 speed hub and the discussion was whether I should change to the stronger axel used in current NIG AW's. I had indicated I'm often out of the saddle giving it welly in low gears going up hill. Brucey's view being as above....i.e. probably not s problem on 16" wheels.

Small wheels make more revolutions per second for the same speed and gear, so torque at the hub is smaller.

531colin wrote: ↑24 May 2022, 6:48pm Small wheels make more revolutions per second for the same speed and gear, so torque at the hub is smaller.

I'm using that reason to slightly under-gear the Rohloff in the 20" back wheel on my bent.
I'm at 1.8:1 (38/21) vs the minimum recommended 1.9:1 (40/21) chainring /sprocket ratio.
I've run it like that for over 12 years now with no apparent damage.

So .........