Lowering the gearing on a Brompton

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nirakaro
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Joined: 22 Dec 2007, 2:01am

Lowering the gearing on a Brompton

Post by nirakaro »

I'm looking at buying an old 6-speed Brommie off my neighbour, primarily for touring. Taking it for a test ride, the only issue I have with it is the rather high – for me – gearing. The lowest gear, as near as I can measure, is about 30", which won't work for long leisurely climbs.
So, first question: how do I establish the ratios of the 3-speed SA hub, so I know my actual ratios in gear inches?
Second question: is there any reason why I shouldn't replace the 130BCD 45t chainring with a 38 tooth ring, which would drop the lowest gear to about 25"?
Third question: would it be possible to fit a larger rear sprocket? Are they standard sprockets?
Fourth question: am I right in thinking that the square taper chainset is not radically different from a normal chainset? Could I replace it with a double? No shifter of course, but I don't mind getting my fingers greasy occasionally.
All advice much appreciated.
Carlton green
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Re: Lowering the gearing on a Brompton

Post by Carlton green »

The SA AW has ratios of 4/3, 1/1 & 3/4.

See SJS Cycles for suitable sprockets. SA are available to 22 teeth.
Don’t fret, it’s OK to: ride a simple old bike; ride slowly, walk, rest and admire the view; ride off-road; ride in your raincoat; ride by yourself; ride in the dark; and ride one hundred yards or one hundred miles. Your bike and your choices to suit you.
rogerzilla
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Re: Lowering the gearing on a Brompton

Post by rogerzilla »

As you say it's old, is it a Sachs hub? You are very limited with what you can do at the back, as the chain tensioner will foul larger sprockets.

Typical Sachs setup is 50T front, 13/15 rear, giving a range from 40" to 83". The gears are quite closely spaced. If you have 45T, which I don't think they ever sold, the range is 36" to 75".

If you want a gear lower than 30", you will be spinning out when you get a tailwind and down any hill.
nirakaro
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Joined: 22 Dec 2007, 2:01am

Re: Lowering the gearing on a Brompton

Post by nirakaro »

rogerzilla wrote: 21 May 2022, 2:51pm As you say it's old, is it a Sachs hub? You are very limited with what you can do at the back, as the chain tensioner will foul larger sprockets.
No, it's an SA.
Typical Sachs setup is 50T front, 13/15 rear, giving a range from 40" to 83". The gears are quite closely spaced. If you have 45T, which I don't think they ever sold, the range is 36" to 75".
The chainring's 44t (not 45, I miscounted), sprockets 13/16. The range seems to be better than that - as near as I can measure, it's about 30" to 87"
If you want a gear lower than 30", you will be spinning out when you get a tailwind and down any hill.
Yeh, but I'd much rather freewheel down hills than walk up them! :lol:
thirdcrank
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Re: Lowering the gearing on a Brompton

Post by thirdcrank »

... Could I replace it with a double? ...
I'm no Brompton expert but AFAIK, there's none with a double clanger and there never has been. (Always open to being corrected.) I doubt if that's because of problems fitting a front mech. A small-wheel folder of the Brompton type inevitably has quite a short drive train making it easy to unship the chain, especially if the rider back-pedals. Thus, Bromptons etc are generally fitted with a chainguard. That may help keep trousers clean but the main purpose AFAIK is to keep the chain on

PS If you are looking at fitting a smaller chainring, make sure a suitable chainguard is available
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Tigerbiten
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Re: Lowering the gearing on a Brompton

Post by Tigerbiten »

You should have the SA BWR hub and it should be marked as such on the hub.
It has a 57% step between the ranges or double the 25% step between the 13/16 sprockets.
I'd use https://www.sheldonbrown.com/gear-calc.html to workout your gear inches and how you can alter them.
But the small wheel size makes it very sensitive the tyre size.

Luck .......... :D
rualexander
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Re: Lowering the gearing on a Brompton

Post by rualexander »

I modified my Brompton to take a triple chainset and front derailleur and also three sprockets on the SA hub.
I now have a gear range of 16"-100".
rotavator
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Re: Lowering the gearing on a Brompton

Post by rotavator »

I put a double chainset on mine with 48 and 40T chainrings and no front derailer. I leave the chain on the big ring unless there are long, steep ascents coming up, in which case I stop and use a twig or spanner to shift the chain down. With 12 and 16T sprockets and the BWR hub gear I have a range of 27-105 GI which is OK for all but the steepest hills. Using a 13T sprocket instead of a 12T would give more even gaps between gears.

The modification was easier to do than I expected.
hoogerbooger
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Re: Lowering the gearing on a Brompton

Post by hoogerbooger »

search this forum and google otherwise, as there is quite a lot previously discussed on gear options.

As said above look at your hub as it should say what it is then you can be sure of the gear steps. Then use this calculator to consider the effect of changing rings and sprockets:

http://xldev.co.uk/bgc.html

Should be quite possible to shunt down the gears with a smaller chainring. My old style brompton 44t (42?) chainset had bolt holes in it and I added a 38t stronglight ring and used it as a low ratio and high ratio by manually derailing. Or play around with old chainsets and BB's to find a combination with similar chainline to put a single smaller chainring on. This is your cheapest option, particularly if you have a bicycle bits box, and worth trying first.

My recollection is that 17t is the largest that will fit, or 18 with a bit of filing of the frame. I've not tried above 16.

If you have a BWR hub you are stuck with big gaps between gears, but the benefit of a pretty wide range. You should be able to get up most hills, but the gaps are not ideal for touring. Ultimately a Brompton is not ideal for touring, particularly on rougher roads. It can however be very useful and fun, used in combination with public transport to skip bits, or in combination with other modes of holiday, but with a few days of touring. ( Not to say .......there are hard core Brompton tourists out there)

If not a BWR and it's a BSR with the standard range then you will have narrower gaps...but a much smaller range. In that case you will have to decide on where you want to clip the range high or low. ( Like you I am happy to freewheel down hill....but I suggest you compare gears from the calculator with those you use on another bike.

I actually have a lower gear on my Brompton than I use on my proper tourer as I find the position on a Brompton makes it harder to "get the benefit". This is after adding small bar ends which greatly help in comfort, position, and purchase/push....in my view.

There are various ways to increase the number of gears and range.......varying from pretty to very expensive.

Anyway doo some googling & look at videos on Youtube .
old fangled
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

rualexander wrote: 21 May 2022, 6:44pm I modified my Brompton to take a triple chainset and front derailleur and also three sprockets on the SA hub.
I now have a gear range of 16"-100".
I also put a triple crankset on and a front mech and now have a span from 1.03 to 8.39 meters of development. You only need to change the crank on the right side. As was already advised before, I also suggest getting a crankset with a guard. I found mounting and tuning the front mech to be a challenge and would suggest starting without one and shifting by hand, again as was suggested before. To maintain a proper fold with a triple crankset, the left stay of the front mudguard needs to be deformed.
rualexander
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Re: Lowering the gearing on a Brompton

Post by rualexander »

2_i wrote: 21 May 2022, 9:06pm To maintain a proper fold with a triple crankset, the left stay of the front mudguard needs to be deformed.
Not on mine it didn't.
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

rualexander wrote: 21 May 2022, 9:20pm
2_i wrote: 21 May 2022, 9:06pm To maintain a proper fold with a triple crankset, the left stay of the front mudguard needs to be deformed.
Not on mine it didn't.
You bend the stay, so that it stays away from the crankset.
rualexander
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Re: Lowering the gearing on a Brompton

Post by rualexander »

2_i wrote: 21 May 2022, 9:23pm
You bend the stay, so that it stays away from the crankset.
Nope, didn't need to.
2_i
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Re: Lowering the gearing on a Brompton

Post by 2_i »

rualexander wrote: 21 May 2022, 11:03pm
2_i wrote: 21 May 2022, 9:23pm
You bend the stay, so that it stays away from the crankset.
Nope, didn't need to.
Without a chainguard, there may be indeed enough space left for the stay there.
nirakaro
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Joined: 22 Dec 2007, 2:01am

Re: Lowering the gearing on a Brompton

Post by nirakaro »

Thanks to all for a lot of helpful information. I'll keep you posted!
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