Converting 2speed Brompton to 4speed similar to the new P and T lines

[l4dva]

i see this quite a bit on you tube, how easy is it? Has anyone done it and got any advice tips?

Thanks

 

[berlinonaut]

If you want it similar to P and T line you need a new rear frame (and obviously the 4-derailleur mechanism and shifter and possibly a new rear wheel). You can easily and cheaply convert the 2-speed to a 3-speed by adding a sprocket and modifying the shifter. You may be able to convert it to four speed with parts from Asian tuners (I think one of them offered a 4 speed-block), but as far as I remember it was not exactly cheap.

 

[u_i]

Sprockets wear out as quickly as chain on Brompton. If you reconfigure the bike for a proprietary cassette, you will need to replace that costly proprietary cassette each time you replace the worn out chain and the cassette's supply may thin out at any moment. In my opinion, switching to a proprietary cassette is a pointless exercise for anyone riding the bike on a regular basis. Still, there are so many people who hardly ever ride their bikes, while still enjoying upgrading, that it may not matter in any specific case.

 

[ukoldschool]

There is a company called Brombacher that sells a 4 sped conversion that fits the standard 2 speed wheel. Ive not tried it myself but was considering it as an option:

http://en.brombacher.kr/product/bro...4s-shift-kit/339/?cate_no=106&display_group=1

It uses standard shimano cogs which are readily available (so can be changed cheaply), in 11-13-17-21T.

 

[berlinonaut]

There is a company called Brombacher that sells a 4 sped conversion that fits the standard 2 speed wheel.

Not exactly cheap either:

It uses standard shimano cogs which are readily available (so can be changed cheaply), in 11-13-17-21T.

Hmm, normally with a normal Brompton 17t will fit, 18t needs in most cases already a light mod of the rear frame - I cannot imagine how 21t would fit.

It is always the questin what you want to achieve. I've changed my 2 speed from 12-16/54 to 12-16/60 (at least in the summer months) as I live in the flat and am more of a masher than a spinner. This allows me to reach 30km/h easily. Obviously, steep hills are not my favorite. But the 2 speed does surprisingly well (and even better with the standard gearing).
The Brombacher variant offers shorter gears, but still not short enough for serious hills:

For a real mountain-gearing setup you'd have to come close to 2m of development, for steep areas with luggage even below that. Thus (as with the original P-Line 4-speed) I am a little bit in doubt if there is really a relevant advantage in practice of a 4-speed over the 2-speed.

 

[ukoldschool]

I'm not sure I understand the issue around a 21t? why would it require a frame modification? surely it being the largest cog, and being the closest to the spokes it is pretty far from catching on anything? is it a concern of the chain hitting the rear triangle?

Video of it in operation here:

View: https://fb.watch/dM0scVrkOG/

 

[berlinonaut]

I'm not sure I understand the issue around a 21t? why would it require a frame modification?

Give it a try. Sprockets > 17t on a 2-speed Brompton do in most cases hit the rear end of the chain stay and require a little help of an angel grinder to fit. With a different rear wheel the situation is different - i.e. on one of my Bromptons I do run a S/A XRF8w, using a 25t sprocket - but this is not a 2-speed rear-wheel. It might be the case that with the Brombacher conversion due to the 4-speed the 21t is so much on the inside near the spokes that it is not an issue but I would not bet on that.

 

[T4tomo]

i see this quite a bit on you tube, how easy is it? Has anyone done it and got any advice tips?

Thanks

Expensive , difficult and not worth it mainly due to it needing a different rear triangle

the P/ T line 4 speed has 11-18 sprockets and 163% range, this is less than a standard 3 speed hub 177% and way less than the 6 speed 300%. this is the inherent issue with small wheel and derailleur / cassette set ups.

If you want a wide range of gears on a Brompton, you need one with a hub gear (or a dirty finger change on the front as well). clearly a hub gear adds a bit of a weight penalty

 

[Schwinnsta]

Is the Brombacher kit for the 2-speed SA Brompton? I doubt it. I think you need a different hub or wheel. Normally the lowest cog on the Brompton is 12T. Brombacher has 11T. I am running my 2-speed as a 3 speed with 12-14-19. The 19T cog barely fit. Overtime, having the wheel on and for flats, I can see that it has nicked the drop out. I would like an even lower gear, say a 20T or 21T. That would require filing a small notch in the dropout for clearance, which is not a problem for me or the Brompton. However, how big a cog can the Brompton reliably shift? Does anyone know? 19T is not a problem.

 

[berlinonaut]

Is the Brombacher kit for the 2-speed SA Brompton? I doubt it.

No need to doubt or ask if you can simply check it on Brombacher's linked offering page:

This product is not avilable on the stock 2s hubs.
You need a hub or conversion kit that can install 4 cogs.

PS: The 2-speed is in no way made by S/A.

 

[Schwinnsta]

True enough, and I do know but forgot that the 2 speed is not S/A.

Still interested if anyone knows how large a sprocket the Brompton shifter can reliably do.

 

[u_i]

Still interested if anyone knows how large a sprocket the Brompton shifter can reliably do.

In the pusher system, there is a short rug effect. The aftermarket tensioners put the inner jockey wheel farther away from the axle to allow for an increased size of the largest sprocket. However, the cost is of deteriorated (if still existing) shifting on the small sprocket end, if you go below two sprockets.

 

[l4dva]

Thanks for your input guys.

I guess I was hoping to achieve something like this you tube video below but looking at costs of all the upgrades and then the cost of a second hand brompton I'm starting to think I might as well buy a P line bike on cycle to work scheme.

View: https://www.youtube.com/watch?v=rz24HOdUctA


Ideally I want light weight as possible as I have a large hill to climb to get to my house. or maybe I need to hire a 6 speed bike for a few weeks to see how it handles the hill!

 

[Kell]

If you want a wide range of gears on a Brompton, you need one with a hub gear (or a dirty finger change on the front as well). clearly a hub gear adds a bit of a weight penalty

Or you could look at the Schlumpf Mountain Drive system.

Not exactly cheap, and I've no idea how useful it would be in practice, but I've always liked the idea of it.

It gives you whatever your standard gearing is, plus a 2.5:1 reduction gear ratio.

It's basically a two speed hub that lives in your BB and converts your front single chainring to a double - but with a much broader gap than you'd get with a genuine double. i.e. your standard 50T ring and the equivalent of a 20T.

Kick it into action with your heels.

http://www.schlumpfdrive.com/index.php/mountain-drive.html

 

[mitchibob]

If you want a wide range of gears on a Brompton, you need one with a hub gear (or a dirty finger change on the front as well). clearly a hub gear adds a bit of a weight penalty

Think James Houston is using a cleaner finger front derailleur, as well as standard 6-speed on his PCR warm-up before the TCR this year. He did say he'd share how he set it up. Not sure I've found that update yet. Dot-watching instead!

4-speed option does seem good alternative to 2-speed, with closer ratios. Perfect for a lot of London. I have to cancel certain roads out of my way home where I live now with certain amount of beers in the bag unless using lower hub gears, or grind to a stand-still, especially if knee accidentally knocks the gear up mid-ramp.