Schwinnsta

Any problem with changing the 2 speed rear cogs from the 16 to 13 stock setup to 16 and 12?

Joost

The 2 speed has a 12-16 sprocket as a factory default. As far as I understand these are the minimum and maximum. You can fit anything between 12 and 16 as an alternative yourself.

Or am I misinterpreting your question?

berlinonaut

You are right, that's what's possible with the parts offered by Brompton. Plus you can normally go up to 17t w/o having to modify the rear frame, using a cog from a Shimano cassette. From 18t on modifications are necessary.

Schwinnsta

I suppose that I will have to pull the wheel off and count the teeth. Time consuming because I have the 6 speed. However I thought I had the 13 and 16 because that is default on Brompton gear calculator. Hub Gear Calculator and that matches with the 2012 Brompton brochure. I attached the relevant page.

fietsbob

My Mk 2 frame a 16t hit the ends of the chain stay tubes, so I used a 15t... AW3/BSR..


54:15.. maybe Mk4+ is different the 54:15 works good, so I didn;t change it.. with new bike needing lower.. I kick in the Mountain drive reduction geared crank..

consider., putting a 39 on instead of the 44t? Current spider crank uses a common 130bcd chainring,,


your chart is for those who cannot count teeth or do maths?




...

Joost

The 3 and 6 speed have a 13t low sprocket. The 2 speed has a 12t low sprocket. I thought your question was about the 2 speed, but you might have just meant 2 speed as in '2 cogs'.

Schwinnsta

Thanks. I did not realize that.

reppans

With the six speed and SA IGH, you'll want to keep a 3 tooth differential between the 2-cog derailleur. That is because the IGH steps are about the equivelant of a 6 tooth cog differential, so the OEM 3 tooth derailleur differential provides even-ish half-steps between IGH shifts. A 16/12 cog setup would leave a lumpy wide-ratio shift (equiv of 4 tooth shift) followed by a narrow ratio shift (equiv of 2 tooth shift) as you sequentially shift though gears.

However, the 3 cog 16/14/12 derailleur mod divides up the 6 tooth equivalent IGH shifts evenly providing a 9 speed with narrow ratio 2 tooth equiv. shifts.

berlinonaut

The AW3/SRF3/BSR is a totally different beast cogwise. We were talking about the 2 speed and 6 speed. Th 3 speed has a different driver and 15t is the maximum possible afaik.

Nobody was talking about Schlumpfs here and other than that though they are good on the Brompton they came out of fashion long ago, thanks to the BWR. Saves money and weight.

Why would one do that? A cog is far cheaper than a chainwheel plus with one or two teeth difference you can stick with the same chainlength...

badmother

No need to take the wheel off to count, I did it last night. Turned the bike upside down (or hang it up or even park it in kickstand mode). I used a pencil with colour, miy choice for the evening was orange . Scraped off some colour on one tooth and slowly rotating the rear wheel and counting the teeth of both cogs with no problem.

I curently use 16 and 12 on my 5 speed hub, this is what I had available at the time. Have been playing with the gear calculator to see if 13 was better.

Good to know that 17 is possible! thanks for that info.

Schwinnsta

If I had to do it over again I would purchase the 2-speed -19% instead of the 6 speed -12%, and have saved the weight. As it is nearly all my riding is in 3rd (45.6") and 4th (56.2). I often want a little higher gear but 5th is 71.5 and with out a good tail wind it is hard for me to hold. Nearly all my riding is flat terrain. If I go to 16 and 12 teeth, I will have 45.5 and 60.7 about the equivalent of the 2-speed -19%. The cost and effort is rather trivial and easy to change back again.

berlinonaut

again: ... on the 2-speed and 6-speed. Regarding the 5-speed the question is wether we are talking about the old Sprinter or the modern SRF5w. The latter can be brought to use 2 (and possibly 3 cogs) using the driver of the BWR. Sometimes wheels with this mod already applied are sold via ebay or directly from Asia (i.e. by brommiplus). I'd assume that the 17t would be a fit here as well but did not try it. On the sprinter (at least on some models) you can use 2 cogs (but not three as far as I know), but I do not know which sizes are possible then.

reppans

I hear ya... around your relatively flat area, those sound like good ratios, and I too would prefer something around 60-65" as my favorite flat ground/windless crusing pace... 71 is a little too heavy and 56 a bit too light for me on the -12%. How about just buying a 2nd rear wheel with 16/12 cog to swap in/out? Then you lose the dead weight of the IGH, except a few oz in the extra shifter/cable (unless you want to permanently delete that). Might be a neat option, with easy swapping in case of travel/vacation to hill country, and not a whole lot of money.

Schwinnsta

Thanks, good thought.

badmother

We are talking the modern (but our of production) SRF5W