chagzuki

I've not been on this forum for a long time. I used to post a lot in the days when I was tinkering with folding bikes, often trying to improve the ride quality in whichever way I could. I settled on a custom 5 speed hub for my Brompton, but the efficiency loss always bothered me a bit, and the weight is less than ideal, so I always thought a well-designed derailleur system could be preferable. It seems that Brompton have finally gone down that route.

Has anyone here ridden one yet, and have an opinion on the performance, ratios etc? How does it compare to aftermarket solutions, of which there used to be a few and probably are many more now?

childcarepro

I’m also interested in your wisdom on this question. Thanks in advance!

Jipe

It works well but the range is too small.

Why did Brompton reinvent the wheel with this limited range obsolete 4s derailleur instead of using a standard cassette+derailleur ?

2_i 

With BWR and cogs, I currently have 360% gear range in the rear. I do not see how you can safely pack this kind of range and properly take off chain slack in the physical space of 16" wheels with a derailleur system alone. Of course there may be people willing to trim the range and/or not riding in snow or off-road.

willydstyle

Shimano Zee or Microshift Acolyte can do it.

2_i 

Your experience must come from the folding bikes with a midframe hinge. The folding alone on Bromton requires taking off about 15T of slack, before you turn to any slack from differences in cog size. After you take away 15T from 25T Zee capacity, there is not much left for the cogs. Besides the plain capacity, there are issues of chain management after the bike is folded, that have been discussed here in the context of Birdy, Brompton and Brompton clones. Finally, there is the issue of proximity of the cage for the standard derailleur to the ground. Various problems can be solved at different levels, but in the end you need to ask whether you arrive at a more practical product. With the BWR, cogs and the standard Brompton tensioner you have comfortable room for putting double crank and a front derailleur without any special chain provisions.

Jipe

On my Birdy which rear swing arm folds similarly to the Brompton and has ETRTO355 wheels (only 6mm more that the Brompton) I have a 9-34t cassette = 378% with a Shimano RD-R8000GS derailleur and I do ff road riding with it.

The BWR solution with 6s and external 2s derailleur doesn't like snow, mud, dirt at all !

If you want a solution dirt/mud/snow proof, you need a wider range IGH like the Alfine 11 or Rohloff (its what I have on my Brompton).

The derailleur solution is for people who want a lightweight more sport oriented solution which is what the P-line and T-line are supposed to be.

2_i 

It is normal for optimization problems to yield many minimae and broader criteria may favor picking one or another. A 9T cog venture into ridiculous in terms of inefficiency and wear, also of the chain. The beauty of the chain is that it allows for many teeth on a cog or ring to work together in sync. However, that depends on the angle between the subsequent chain links and deteriorates rapidly when the angle is increasing. While on a 12T cog, that I normally use as the smallest, coarsely 4 teeth work in sync, that drops to 2 teeth on a 9T cog. The Brompton tensioner puts a flexible plastic tensioner arm near the ground, not a metal one of a standard derailleur that can bend or crack.




I manage and I have a wider range due to 20T smallest ring in the front.

That is correct.

tcs

Other paths to take: the Sturmey-Archer X-RF4 hub that BikeFriday and Hummingbird use offers a 210% gear range.

Jipe

Almost all Brompton users had problems of non working 2s derailleur due to dirt. It s a well known problem of this Brompton proprietary simplistic derailleur. It requires very regular maintenance to remain functional in difficult conditions.

The multiple chainring front are unpractical and cause problems of chain tensionning while folding the Brompton. Triple chainring systems is something of the past and now double chainring systems are also declining. Most cyclist do not like triple at all and many prefer single chainring systems.

And the BSR/BWR hubs are also an old obsolete design, almost only used still by Brompton.

No other folding bike manufacturer use a transmission like the one of the Brompton (excluding Brompton clones of course).

Now if you like it, its your choice, many people like antiquities.

All major bicycle transmission manufactures use now 10t cogs without problems, one has 9t cogs. Your opinion is in contradiction with what the rest of world is doing/using, for the old fashioned 3s SA IGH and for the cassette cogs !

2_i 

My Brompton rear derailleur is now 3s. I hardly ever clean it and it works fine. I have now absolutely no problems with Brompton folding when on the smallest ring. My level of care about what the trends are is whether they are along what I want to do or not. Incidentally in my observation everybody and their brother in TdF rides double rings, like in the past, and the single ones seem to be there for weekend racers. I ride a quad on my main full-size bike. It is nice when I can get parts I need from the industry, but when that is not possible and it is with the realm of what I can do I remake the part or make one from scratch. The inefficiency of 9t cogs is plain math, not opinion. Is Brompton overall less efficient than a full-size bike? Yes, it is, but overall it is also good enough - you get something for something.

Jipe

I agree with you that the efficiency of the Brompton is lower, its partially due to the poor original transmission but not only. My upgraded Brompton with Rohloff cannot reach the efficiency of other bikes including ones with a Rohloff like the Birdy with a Rohloff and this is due to the Brompton frame.

The efficiency of 9t cog is not only math. In real conditions it depend of many factors because the real efficiency of a derailleur transmission in real condition is lower than what it is in math. In real conditions, the 9t cog is only seldom used for high speed like any longest gear inch so, its not very important in global efficiency and when using high quality steel, it last long enough. Compared to the BWR solution, the longest gear inch of it uses the longest overdrive setting of the BWR that has also a low efficiency.

Besides the price, for me the best solution is the Rohloff, with some small tweaks it weight almost the same as the original BWR 6s (probably less than what you have with your triple chainring), it is very durable, doesn't need maintenance besides changing oil once a year (and even this isn't really mandatory), has an excellent efficiency and a large gear inch range.
Its the same for the Birdy excepted for the weight were a derailleur+11s 9-32t or 9-34t cassette weight less (these cassettes are high end with a low weight, the Ultegra derailleur is also low weight and the XDR hub from Tune or Hope are lightweight too, 11s KMC chain very lightweight and durable).

For the two chainrings of road bikes, you are right but its a compact double with optimized sets of chainrings, these work much better than any triple chainring and also a double without an optimized set of chainring.

But gravel and MTB almost only use single chainrings.

2_i 

Yes, frame response of course matters and tire deformation too. As to the cogs, other factors contribute too, but at the edges of the range, one factor may stand out and for a small cog, the drop in torque that each subsequent tooth contributes goes like cos(360/N) where N is the teeth number. Even the first working tooth will have a drop in torque/efficiency that is between that factor and 1. So with N=9, this is 0.77 and with N=12 this is 0.87. This translates into half as many teeth working together for 9T cog as for 12T. A 10T cog is closer to 9T than 12T. Yes you use such a cog less.

Rohloff obviously is an option, but not not enough range for me. I spend 1 month per year in steep mountains where the bike is my main means of transport and locally the winters can be tough due to the humidity from Great Lakes. Also the width/size in Rohloff/Birdy options matter quite a bit for me. I know that you fly little but over here you really have no choice. Even without flying, when moving in multimodal manner across Europe I saw that few cm in width mattered in a packed train, where your only option was to place the bike in the corridor, the level of disturbance that the bike caused depended on those few cm.

As to the front rings, I modify derailleur cages, so that shifting depends little on ring features. I do not bother matching the rings and can make the shifting smooth even when there are no ramps or pins. Is that as smooth as the rear? Not quite but very close. I often rent bikes, at times high end, so I have some experience with various, but, if I can get more gears, I take them.

Jipe

Rohloff on the Brompton provides a range from about 1.5m to slightly about 8m with 54x13. Its possible to even reduce the lower with a bigger chainring since for many people 8m is too much.

I can climb and have been climbing anything with this range, the limiting factor is not the gear inch but the grip of the rear wheel on the ground that with 1.5m requires a good surface, as soon as it is slippery, its impossible to restart because the rear wheel is slipping due to the very high torque with 1.5m gear inch.

2_i 

This is very close to what I had with two rings in the front and I am sure it should be enough for most people. However, in the end, two rings were not enough for me. In my steep area, it is common for large gradients to be there even within a yard surrounding a single building. My lowest two gears yield 1.03 and 1.17m of development, respectively. Just to ride out to the street from my workplace, I need a combination of those two (there is a lower gradient midway where I can shift). Around my base, in winter I may use one of those lowest gears to get out of an iced up ditch and you need such low gears to move very slowly to prevent slippage. During winter I may have many days when I constantly stay at 20T in the front.

For me, the worst in riding steadily uphill in low gears is the practical inability to shift gears. In practice, when you can still shift, you need to estimate the lowest gear you will need and switch to that gear, resigning yourself to the fact that you will be riding at a lower gear than necessary over portions of the climb.