GTA

Guys:

During the past 2 years, I have done a couple of experiments on replacing Brompton's original gear shifting system with standard rear derailleur system. Now I want to share with you my experience.

I think we all love Brompton, but I believe not all of us really like its primitive gear shifting. It is slow, blurry, soft and noisy, even compared with the cheapest "modern" rear derailleur system. So let's do something.

Thanks to the various third-party components, I was able to install a dropout onto the rear fork, and a shimano derailleur onto the dropout.
This significantly improved the shifting quality and speed.

However, there was a problem with the chain tension. Without the original pulley arms, the chain was very loose when you fold the bike.
There were some solutions to solve this. For example, add an extended bracket on the lower pulley (like Birdy). But it is not very stable, sometimes the chain still comes off the chainring.

In the end, I came up with an idea ---- OSPW （oversized pulley wheel）system!
Because the 2 big wheels can "eat" quite a lot of chain links when folded, so it is possible！

Then experiments started. I encountered some issues in the beginning, but after optimizing chain length/ pulley arm tension/ B-tension/ flywheel teeth, finally I got decent results: the derailleur shifts perfectly while the whole system always keeps a proper tension when folded and unfolded; the chain remains on teeth and never come off.

So far,I have done this on 3 of my bromptons(and clones, of course).

I will keep you guys posted if there are any new mods.

P.S. : The new Brompton P-line and T-line have a new gear shifting system, which is somehow "similar" to my solution. But mine was earlier than them. From my point of view, mine solution is still better, and probably cheaper. :-)

P.S.: According to my experiment notes, in order to work perfectly, the derailleur and OPSW have to be chosen from a couple of center models. If you are interested, I will talk in detail in a separated thread amybe.

2_i 

Looks good. My comments are as follows: When you put additional chainrings in the front, there is more slack for the tensioner to absorb. In my case, I go from 50T down to 20T. The standard Brompton tensioner can take that 30T off with nearly no grumbling and your arrangement will not. Another issue is of winter riding. When you ride with the tensioner in the low position into a pile of snow, problems develop. I am not even sure the carbon cage could take it. Finally an alternative to the oversize pulleys could be 3 small pulleys, likely riding a bit higher over the ground - in the snow/debris issue any cm matters. The derailleur hanger looks like Brombacher, right?

GTA

2_i, thanks for your comments.
I have no plan to use more than 1 chainring on Brompton. Actually I think 4-5 gears in the rear is already enough.
At least for me, brompton = city commuting bike.
3 pulleys sounds interesting. Maybe I will try it, too.

2_i 

My Brompton is primarily a city bike too and cities vary - the piles of snow are high on my consideration list during the season. However, at times the Brompton an excursion bike and then, e.g., tall grass comes into consideration. Below is a link to 3 pulleys going along a similar tangent to your work:

Schwinnsta

Nice. Are you using stock rear frame without stretching?

Jipe

Did you consider to use the parallelogram type of chain tensioner mounted on the bottom bracket of the Birdy III ?

GTA

Yes, I tried something similar. As you can see from the following pictures. In this case you can put on a standard rear derailleur.
However, I don't like this solution. because is it is not very stable, sometimes chain comes off; and when folded, the chain is completely locked, you can hardly turn the crank.
And personally, I always prefer to avoid putting on any "additional" part.

2_i 

Was the Birdy tensioner's clamp grabbing tightly the Brompton's bottom bracket shell in the setup? When I looked at my shell, there was quite some buildup of the brazing alloy there. Maybe it could be partly removed with a dremel tool, but without any action it looked like the clamp might not mount properly. Given the uncertainty in the solution working, I feel hesitant going with a dremel tool there just for a trial.

GTA

Not sure if you can see the posted .GIF
Check it out.

Jipe

OK, it works with the bike in the animated .gif and bikes of the previous pictures.

But all those bikes seem to have relatively small cogs with a small difference of size between the smallest and biggest cog of the cassette.

Do you think it would also work with a much bigger difference like with the original 9-32t cassette of the R&M Birdy Touring, i.e. with 23t difference between the biggest and smallest cog?

tcs

Sun Tour did this commercially about 35 years ago. It works, wrapping up lots of chain with a shortish cage.

GTA

Hi Jipe,
technically, the allowed cog difference is only related to rear derailleur's capacity.
If you want to try my solution on your birdy, then you just need to decide which gear the chain (and RD) stays when you fold it. I suggest to put it to the slowest gear (biggest cog), and adjust chain length to have a suitable chain tension during folding/unfolding. Another thing is to avoid collision between the OSPW cage and the ground.

Jipe

On the Birdy with derailleur, there is no choice to fold the bike, the chain must be placed on the smallest cog to also be at the extreme right side of the cassette.

hihi2u2

What is the rear cassette and chainring on this experiment ? Have you tried adding a OSP on this rear cassette configuration?

I'm looking to put 11-28T on the rear with 56T in the front, if I can get a 9-32T cassette then maybe with 52T at the front.

Sepp

Very interesting - now I am building a clone with carbon rear triangle and fork. I would like to use Ultegra 11-speed rear mech with 11/12/13/14/15/16 cogs. The biggest pulleys I could find are 13 upper and 19 lower.

Can I expect this to be big enough to take the slack out of the chain when folded?

What are the disadvantages of such a shifting system vs the original derailleur?

Jipe

The original 2s and 4s Brompton derailleur are crap. A real derailleur like the Ultegra is just not comparable to the one of Brompton.

But no standard derailleur can take the slack of the chain when the rear triangle is folded.

The only tensionner that really work is the concept of the above picture which is derived from the Birdy tensionner: a tensionner arm around the chainring, a fixed arm works, but the best one is a spring loaded deformable parallelogram arm like on the Birdy 3..

Sepp

My question is about oversized pulleys on a standard road derailleur, like shown on the pictures above, and from what I see they DO take up all the chain slack.

So I would like to know how big is big enough for those pulley wheels, as to NOT require any tensioner arm around the chainring.

Jipe

You mean the one below with carbon derailleur cage I guess?

Its working because the teeth difference between the smallest and biggest cog is very small and because the chain length is adjusted to its minimal possible length with the derailleur cage to its extreme front tilt.

But with a useful, much bigger teeth difference with a 9-32t cassette ideal for small wheel bikes (like on the nickel plated Brompton with Campagnolo derailleur), it doesn't work and an additional tensionner is needed.

2_i 

Location under the axle reduces the arm of the tensioner. A large diameter reduces the angle over which the tensioner arm can swing. No, it cannot be as effective as the Brompton's design. They had decades to think about it.

Sepp

Thanks Jipe,

that sound reasonable, but I also stated that I want to use 11-16t cassette, which is plenty for me. It is flat where I ride, and for commuting rather short distances.

I just found bigger pulleys with a total of 34 and asume that it will do for my purpose.

Have you found the birdy tensioner to work ok at all? I may add it in the future if I see fit for a different gear range...

Jipe

Yes, the Birdy 3 tensionner works perfectly which wasn't the case of the previous one that was a derailleur cage extension.

But it cannot be mounted on the Brompton due to the very special shape of its bottom bracket box (the sides of the Birdy bottom bracket box are round, the tensionner is attached by a clamp around the Birdy bottom bracket box).

Sepp

OK, kind of glad it won't work with Brompton, as I prefer to avoid those things

I guess I will have to try/buy, and fail/succeed whichever comes first. LTWOO is an option, but I am not convinced at this point...even though I can do without many gears, I prefer the sleekness and functionality of a Shimano rear mech.