alex1234

Hi,

The 4-speed Brompton P Line has quite a narrow drivetrain range (163%) and after using it for some time I find myself frustrated when I can't take advantage of a downhill to reach full speed. Given that I need to keep my bike as lightweight as possible, I was thinking of the following options and I would really appreciate if you could provide answers to any of the related questions below:

1. (Ideal) Increase the number of sprockets (e.g. 5 or 6 total sprockets) to increase the range. I guess I would have to choose sprocket sizes that make sense, in order to cover the whole desired range (preferably similar to the 6 speed Brompton).
a) You think it would be possible?
b) Would it require to make any modifications to the rear triangle (e.g. widen it) ?
c) what would be the most lightweight way to do it?

2. (good) Keeping the number of sprockets the same (i.e. 4) but choose different sizes so as to increase the range.
a) Would that be possible?
b) and what kind of range do you think I could reach (without making gear shifting awkward)?
c) What would be the most lightweight option here?

3. (least preferable) If not possible to change the range without adding considerable weight or irreversibly altering the frame, perhaps I should shift its current range upwards?
a) Would getting a 54T chainring mean that I could go faster downhill but would have to put more effort while climbing uphill?


Thanks!

Ron Damon

[Channeling J!pe]
The problem with your purchase is the embarrasingly narrow range of the stock gearing coupled with the high price, weight and the proprietary nature and the inherent design compromises of the purchase item. By contrast, you could have, say, built yourself a 349er with a Litepro or FnHon frameset, or bought an Origami or Downtube complete bike for a fraction of the price and ended up with wider gearing, lower weight and an overall more competent bike when unfolded and ridden. Hope that helps.
[End Channeling J!pe]

Jipe

None of these bike provide the small folded size of the Brompton.

Schwinnsta

Check out. On my 2014 H model which I modded going from 12t 16t cogs to 12t 14t 19t cogs, I could go higher just by filing a small notch in the dropout. It would not compromise the integrity of the frame. I don't know if the B derailleur can shift higher or not. The answer on 3a is yes but no to getting the range of a six speed.

alex1234

Amazing, that's what I was looking for! Thanks!
BTW, do you think this shifter (https://www.sjscycles.co.uk/gear-shi...hifter-hsj206/) could work for a 5-gear conversion of the brompton p line?

Schwinnsta

I don't think that would be compatible. Google Bikegang he sells upgrade kits for Bromptons and Birdys and my guess is he will soon have complete kits and parts.

Jipe

There are already some upgrade kits for the P-line the question is (as always with these third party components for Brompton): does it really work ?

Schwinnsta

Quite right to be skeptical here. For that kind of money, it had better work. A lot of the stuff out there is just bling. A couple of things struck me about this. It jumps from 12t to 16t, so it is like the 2 speed Brompton with extra gears, That is a big jump. The spread from there, goes by three teethe, That is curious. Another thing, the one you linked to is for the T-line whereas they have another one for a little less money for the P-liine shown in link below. Why two different ones.

alex1234: Here is another option. This should work, and it has a nice spread, but it is limited because it's a manual or fancy footwork gear change. There are more options like bottom bracket 2 speeds which work and have great gear spread, but they are expensive, made from heavyum and are said to have a lot of drag in their added gear. Schlumpf and ATS are two companies who make them in both under and over drive configurations. Personally I would not do either of these options, but I live in Flatlandia and only have read about hills.

Jipe

Its just one option.

There are several brands/vendors selling similar things.

And indeed, different versions for T-line, P-line and also for C-line and previous years Brompton.

I am now always skeptical about Chinese third party parts for Brompton, I had so many disillusion with parts that do not work or/and are not reliable, even from expensive Taiwanese brands.

I prefer always standard parts from well known brands if possible like installing a real cassette with a real rear derailleur from Shimano, SRAM, Campagnolo... but it requires moving to a standard rear OLD and to find a solution for chain tensionning (as shown by other forum members, unfortunately, the excellent Birdy III solution cannot be mounted on the Brompton due to its special bottom bracket box).

Its really a pity that Brompton choose this poor 4s system instead of installing a real modern rear derailleur+cassette.

Schwinnsta

I would also prefer a standard derailleur system if it were available. However, for a 6-speed, going to an additional gear to get 9-speeds works perfectly well with a nice even gear range. That was my system originally. After I retired I stopped travelling as much and seldom used 1st or 3rd on the SA, so I changed wheels and went 3 speed for simplicity of changing flats rear tire and the weight and complexity reduction.

Jipe

On my Brompton, I enlarged the original titanium triangle of my superlight Brompton and installed a Rohloff what solved the problem without adding weight compared to the original 6s BWR Brompton solution and with the Rohloff its possible to use an original Brompton 1s/3s chain temsionner.

But its different on the P-line and even more on the T-line that have the new Brompton 4s derailleur system on which moving to a IGH will increase the weight.

12boy

I have a one piece BikeGang cassette deal with 11, 14 and 17 teeth sprockets on my 2010 Brompton. I also have both a 38 and 58 tooth chainrings and use the 38 infrequently so stopping and shifting for a steeper slope and later upshifting back to the 58 is not a big wow for me. That gives me about 85 to 55 gear inches on the 58 and 56 to 36 on the38. I also a rear wheel with the SA177% 3 speed with similar lows but 103 gear inch high. No one I've seen has addressed putting an SA IGH on a P line but it may be fairly easy. They are heavy, though. Still, a 58 and 38 pair of chainrings are fairly cheap and require minimum installation effort.

2_i 

So what about the wear on that one-piece cassette? Are you using your Brompton so little that you do not need to care? Are you swapping the cassette each time you swap the chain or are you waiting for the cassette to lose its teeth? The latter process may start slow but then complete in a blink.

12boy

I have ridden thousands of miles with the cassette and the wear has been neglible. I figure the 11 tooth sprocket will go first since I use it a lot but it hasnt happened yet. I do use waxed chains and rarely ride on wet and muddy road so that maybe helping. BikeGang gave me the sprocket as a tester BTW. Mighty nice of them. It would be nice to replace the one piece cassette with a different one which had removeable sprockets and use an 18 tooth for the low.