4 speed Superlight Brompton

[Dogtrousers]

TBH - 9 speeds to me is only achievable in a few ways.
[...]
3x3. Where they add two more chainrings AND an additional sprocket and use a single speed rear wheel. This actually makes more sense than the 1x9 to me. Or at least feels more achievable.

Maybe dumb questions but ...
Where would you put the front mech?
Would you need a longer chain tensioner?
Would folding be affected?

Giving the BWR a wider range would mean impracticable gear sizes internally. You'd need a very big sun and, given that the OD of the shell is limited, the planets would be too small - the ratio of a single epicyclic is given by the teeth of (sun+gear ring)/gear ring. The BWR already pushes it about as far as it can go.

You can use cascading gear trains but they are pretty inefficient. You lose at least 3% in an epicyclic with perfect tooth shape and lubrication, but over 6% in a two-stage hub.

Interesting. As I don't know anything about hub gears my proposed solution to this kind of problem is generally to point at the hub and say "do more magic inside there".

 

[berlinonaut]

The simplest 9 speed by far would be the BWR hub with, say, 12 14 16 or 13 15 17 sprockets. 9 gears and a slightly wider range.

Giving the BWR a wider range would mean impracticable gear sizes internally.

The slightly wider range in this setup is not a result of a change of the BWR but of a wider spread in sprockets. From factory the BWR is equpped with 13-16, in i.e. my 9-gear setup I use 12-14-16. So one tooth more spread, resulting in a slightly wider range than the original BWR-based 6-speed.

 

[berlinonaut]

Maybe dumb questions but ...
Where would you put the front mech?
Would you need a longer chain tensioner?
Would folding be affected?

As usual on the seat tube. Solutions exist for more than 20 years from various sources over the years. Today you can source a clamp for the front mech on Asian marketplaces. Typically no need for a longer tensioner though - depending from the difference in chain-wheel size - in many cases it is not recommended to fold with the chain on the smaller chainwheel. For that reason I don't think Brompton would go down that route from factory - it adds complexity as well as cost and is not foolproof enough for an innocent user.
Apart from that a double chainwheel-setup works very well in practice. I do currently use one as a granny-gear setup on my two-speed (ATM 53-39/12-16).

Interesting. As I don't know anything about hub gears my proposed solution to this kind of problem is generally to point at the hub and say "do more magic inside there".

That's the proper classical management approach!

 

[12boy]

A front derailleur has not proven necessary for me as I rarely use the smaller chainring. When I do use it is a granny gear setup for long steeper grades and/or stout headwinds. The tensioner works just fine, BTW ,except folding needs to be done with the chain onthe big chainring.
There is an SA 4 speed hub that is close to the 3 speed in weight with a 210% range and with direct drive in first gear. It has a 120mm OLD. Having first as the direct drive would allow for some pretty tall high gears with a smaller chainring. I have no idea where to buy such a hub, though.

 

[berlinonaut]

There is an SA 4 speed hub that is close to the 3 speed in weight with a 210% range and with direct drive in first gear. It has a 120mm OLD. Having first as the direct drive would allow for some pretty tall high gears with a smaller chainring.

Interesting. Possibly it could fit the Brompton rear triangle after minimal modifications to the hub w/o spreading the rear triangle. This is the case with it's bigger brother X-RF8 in 120mm. Still I am not sure if the X-RF4 would be desirable. The X-RF8 also has the first gear as direct gear and in practice it lacks efficiency in higher gears (at least subjectively) plus mechanically I would not call it first choice. I had it in my Brompton for a couple of years and have moved it to the Brecki (where it is a surprisingly good fit as this is a more relaxed bike anyway).
Anyway: With two chainwheels the X-RF4 offers 315%, so not much more than the BWR and less than the X-RF 8. Also the spacing is kind of ok but not too nice: https://ritzelrechner.de/?GR=SAX8&KB=54,36&RZ=19&UF=1330&TF=75&SL=2.6&UN=KMH&DV=development

When looking for an alternative for the 3-speed hub I'd rather point to the S-RF5 that superseeded the sprinter. It it solid, has a 256% spread, not heavier than the three-speed, available in 112mm OLD/28 spokes and can even be tweaked to take two sprockets. For unknown reasons is has vanished from S/A's webpage already years ago but used to be available i.e. at SJS until recently (though currently n/a, documentation still there) and you can get it as a 5-speed or 10-speed hub or prebuilt wheel via Brommiplus: https://www.brommieplus.com/portal_c1_cnt.php?owner_num=c1_343119&button_num=c1&folder_id=44131

Also not my first choice and with 2 sprockets you'll have a hard time figuring out a really good setup: https://ritzelrechner.de/?GR=SAX5&KB=44&RZ=16,12&UF=1330&TF=90&SL=2.6&UN=KMH&DV=development

With a single sprocket however it is a direct and simple replacement for the three speed and does a good job with an enhanced spread 177% -> 256% with a single shifter. The gear jumps are however pretty high and adjustment of the hub is a bit sensitive (but not as bad as with the X-RF8).

 

[TheDoctor]

Having crunched the numbers, my personal bet for a 9 speed Brommie would be the BWR and 13/15/17 sprockets.
That gives a gear range of 30, 34, 39, 47, 54, 62, 74, 84 and 97 inches, and next time I replace the transmission I'll probably go down that route.
For comparison, my M12R currently has BWR, 50/44 chainset and 13/16 sprockets.
Gear range of 32, 39, 50, 62, 79 and 97 inches on the 50T ring, which is standard.
The 44T ring gives 28, 35, 44, 54, 69 and 85 inches, which suits me better for touring. I got up a 1:7 iirc with luggage on that setup, doing a Hadrians Wall ride from Carlisle to Newcastle.

 

[berlinonaut]

I got up a 1:7 iirc with luggage on that setup, doing a Hadrians Wall ride from Carlisle to Newcastle.

If Hadrian had had a Brompton back then to patrol his wall he'd probably have gone for the same setup!

 

[TheDoctor]

I hope he'd have done better than me. I was utterly cream-crackered by the time I got to my overnight stop at Heydon Bridge. I struggled with my main, only just managed to finish my chocolate fudge cake, and quite possibly left some beer. Most unlike me!