I know how to calculate gear ratios and development-in-meters and all that, but I'm aware that the same development on different bikes feels... different.

So, has anyone ridden the same Brompton with a 50 tooth chainring vs. a 44 tooth one? I live in a hilly area, and my 6-speed Brompton is fine until I hit the really steep hills - the ones I can do on the low gears of my "big" bike, but not on the Brompton.

I've tried to get an intuitive feel of which gear in my "big" bike corresponds to each gear in my Brompton, and things sort of make sense. And I think, well, if I had the 44t chainring it *should* feel like this or that gear on the big bike, but end up doubting whether changing the chainring is worthwhile. I seldom use the highest gear anyway; just to give myself an extra push when I'm already going fast or downhill, but I could just spin faster on the 5th gear.

Any insight is appreciated.

My 1st Brompton was a 2012 S6L and came with a 50T chain ring/wheel.
Swapping the one piece crank/chain ring with a 44T helped me on the hills:
https://www.youtube.com/watch?v=hYo8...6zPoymgKaIoDLA

My 2nd and current B'ton is a 2013 S6L-X that came with a 44T chainwheel.
Swapping it for a 39T chainwheel helped me climb the really steep parts in my area:
https://www.youtube.com/watch?v=5AT6...IoDLA&index=11

Spinning out in the flats or downhills is frustrating but i never think it's as frustrating as walking up a hill.

Wow, down to 39T? Now we are talking!

I just ran the numbers and this would give me roughly the same development as the middle-ring/big-sprocket combination on my big bike, which is as low as I go normally, without going into the granny ring.

Thanks for the idea; somehow I hadn't thought of looking for smaller chainrings than what Brompton offers.

The video shows that the chain wasn't shortened, but the tensioner handled it just fine... how does it feel that way?

I find the 44T too slow when riding on flat roads while 50T is good.

As an alternative, some people keep the 13/16T sprockets but add a third one to go up those hills. Apparently, it's not that difficult to do if you're into things mechanic:

Brompton 9 vitesses - Page 3

The yellow line shows the meters-in-development you get with a 50T chainring and an additional 18T sprocket, and the percentage between each speed.

I had the opportunity of trying a bike like this a bit, and the smaller difference between each speed makes the bike smoother so might get someone to perform the surgery. A lower gear inch would really be a good thing when riding in hilly areas.

I went the other way. took off the 50t, put a 15t cog on the BSR, and bought a Schlumpf Mountain drive 2 speed crank. which came with 54t on a 130bcd spider.

In Low Range the crank turns faster than the chainring, acting as if a 21.6t

hub high, in crank low range, is the next lower gear. 6 ratios, all individual, no overlap. ~17" low gear .. so almost 1:1 with the little 16" wheels






...

For my first Brompton, I was convinced by the shop to stick with the standard 44T instead of paying for the upgrade to 50/54T. I thought Singapore, with no mountains and a scant number of challenging long climbs, would make the 44T redundant.

Turns out I was wrong - the 44T is simply more fun. I've currently using a 50T and I can feel some exertion on certain hills that was never there. I guess from a long term viewpoint, it is better exercise.

On my 2-speed B, the 39T is absolutely perfect for short range rides imo. I'm most comfortable doing 15-20mph, so the spin outs don't bother me. The bike handles slopes very nicely.

9 spline 2&6 speed hubs take the 12 T cog & 13, 15, 16, the BSR takes a 3 spline cog from 13 , to 15 (I tried a 16t on my Mk2 bike,

it hit the frame tube ends , the 15 cleared ..

It is Hilly here .. many I still have to get off and walk the Bike up them. some streets have stairway sidewalks.

With Speed Drive, you can even down to 27T (SD250 will get you 27*250% = 68T equivalent overdrive)

I have the Brompton chart showing meters gained and gear inches. (see below) I've looked but can't find a table or conversion chart that shows speed as a result of gear inches and pedaling RPM. I'd love to see the entire six speed range and I personally am most concerned with the six speed standard and -12 gearing.

What I want to know is what will my speed be in sixth gear at 80 RPM, 90 RPM, fifth speed at 80 RPM, 90 RPM, etc.

Thank you.

The chain was shortened after a few rides; can't remember if I had to cut 2 or 3 links.
Never had any problem running out of gears on the flats; only downhills.
As you can see in this video; no issues keeping up with other riders(actually passed a lot)
https://www.youtube.com/watch?v=lmFU...6zPoymgKaIoDLA

if you go with 12t and 15t you get 307% with nearly equal spacing:

You speak of and show 2 different products..
High Speed drive is the inverse of the mountain drive a 2.5X overdrive.. (unique chainrings) The speed drive uses conventional 110 bcd chainrings and is a 1.65X overdrive

speed drive, shown with a 34t chainring acts like a 54t in overdrive range .. a bit more practical..

I live in hilly area and commute on flat roads. 44t on 6 speed BWR is a pleasure. Fast enough and no walking up the hills. All 6 gears get used. :-)

I have a single speed to which i added a 177% 3 speed SA hub, with 14 T cog and 54T chain ring. I had replaced the stock chain ring with a Sugino RD2, and while this yielded a low of 48.2 gear inches, fine for most riding around here, there are times when the wind and/or hills make that a little high. having lots of spare stuff lying around and since I had mounted the 54 on the inside position on the chain ring it took about 5 minutes to put a 42 T chain ring on the outside. this gave me a low of 37.5 which I find to be low enough. It takes a few seconds to change chain rings, but I only need the ultra low gears about 10% of the time. The extra chain ring is very light, and this affords me a chance to experiment with gearing options with very little trouble. I may wind up with a 12 T cog in back and a 48/42 combo in front but for the moment this is fine. Is this more trouble than flipping between 2 rear cogs as regular 6 speeds have it? Sure, but the change of 12 teeth give a wider range. Cheap solution for experimentation, and since my nearest Brompton dealer is 210 miles away i don't have to buy things by mail so much and wait for parts to arrive.

NB: BSR 3 spline driver wont take smaller than the 13t, only the 9 spline BWR hub driver takes a 12t

So I took the plunge and got a 39T chainring. It did feel great on the one short uphill I tested it on - need to do a real test!

At first I didn't shorten the chain. The derailer took up the slack nicely, but the overlap made it impossible to backspin the crank while folding the bike to attain the proper orientation of the crank arms - unless you fiddle with the derailer by hand.

Hilarity ensued thusly:

1. I reasoned, 50T - 39T = 11T, so I need to remove 10 links.

2. With 10 fewer links, the bike folds, but doesn't unfold!

3. Realized that I'm an idiot and I should only have removed 5 links.

4. Unhook the master link.

5. Accidentally let go of one end of the chain. Have the derailer whiplash back, sending the master link flying off under the wood pile in my workshop.

6. Sigh. Add back the links by hand, close the chain by hand.

7. I'll wait until daytime, or a day when I'm feeling energized, to hunt for the master link under the wood, paint cans, and assorted detritus in *that* corner of the workshop.

I have this sort of thing regularly. The only difference lies in step 6, where I don't sigh. I say *content would have been censored by moderator if I had actually posted it*.

Did a real test today - with the 39T chainring, the bike rides *GREAT*. It's going to be a much more useful bike this way, in this hilly town. I heartily recommend it to anyone who lives in a mountain area.

My Brompton 2015 model. I want to do 39. Which brand and model do I need to purchase?

In my video on post #2 ; I used a Shimano Dura Ace for 10 speed gruppo. You'll need to shorten the chain. When my chain wore
out; I also swapped to a 10 speed - Shimano Ultegra.

2013 Brompton DA chainring Ultegra chain by 1nterceptor, on Flickr

Image is a 34t on a 110 spider SD, with TA crank arms ...
And a single speed hub rear..



yea, a 34 acts like a 54 when SD is in overdrive 1.6X,
the High Speed Drive , over drive is 2.5X.. (your 27, 67.5)
(Mountain Drive is a 2.5 reduction gear , so like a 50-20)

Recently produced; Efneo GTRO is a 28t with 2 overdrive gears
functionally a 40 & 50t,
Shift, a cable to lever on the bars..

Patterson Metro its another 1.6 overdrive, 28:45.. also cable shift..



Schumpf, their overdriven gears use a chamfered BB shell edge, and friction,

The internal torque load for the Reduction gear needs a solid torque transfer,
& because its climbing hills that way too..
for Brompton it's a lever with a knob on it, sitting on the frame shelf, between
BB and rear section Pivot axis/bushing ..
[ I found it a simpler retro-fit ]






...

any 130bcd 39t will do, Surly's Stainless steel 39-130 will last/wear longer, than the aluminum ones..