Thursday, 8 May 2014

Converting Bike from Single Speed to Three (3) Speed

Converting from Bike Single Speed to Three (3) Speed - Intro


I have a cheap folding bike (Giant Conway) a purchased 2 years ago which is single speed, as I live in a hilly area, I spend most of the time either coasting or pushing the thing up hills. I may as well use a scooter. There is no gear hanger, and the rear drop out spacing is narrow, so I opted to replace the exist hub with a sturmey archer 3 speed. A failed attempt at gearing using a mountain bike hub can be viewed here.

Converting from Bike Single Speed to Three (3) Speed - Wheel Build


This is the first 16" (305) wheel I have built, and it was 2 cross, 28 spoker. The 2 cross lacing confused me for a while. I used stainless 14 gauge spokes from SJS.


Converting from Bike Single Speed to Three (3) Speed - Ratios / Range


The original bike had a 14 tooth sprocket which was too high-a-gear uphill and too low down hill, so I assumed that keeping the sprocket the same size on migrating would give good gearing. But no.

Sheldon brown recommends adjusting gearing so as to use the top gear as you normal gear, keeping the two the gears for inclines / climbing. And this worked out right. I didn't have any sprockets, but a free alternative was to swap chainsets. The original was 46 tooth, and I dropped to 38! Which is about right.I was surprised how much smaller I had to go.

The MTB chainset used, is a shimano one, with 170mm crank arms, and is just the job. Why do they use such short crank arms on folding bikes?

Converting from Bike Single Speed to Three (3) Speed - Conclusion


Swapping to 3 speed?

Use Sheldon's Gear Calculator  my original set-up gave 3.5 meters of development.

After Installing the 3 speed hub this changed to:

4.7 meters is too high for practical use, so by decreasing the chain ring size I got:

If I had chosen the correct sprocket size I should have chosen 17 teeth (16 or 18).

So there we are, Happy tinkering.

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4 comments:

  1. Good stuff.
    I work out my gearing in inches, so i don't know how it measures up.
    My Cinzia folder (406 wheels) ran about 40/55/70 in a 3-speed SA, which I found pretty good for the modest hills around town,

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  2. In answer to why folders sometimes have shorter crank ...
    the BB tends to be at least an inch lower than a "typical" modern bike, and if you are an idiot with a good lawyer, you might try and do a steeply banked turn with the pedal down on that side, then sue the manufacturer for selling an "unsafe" bike.
    I have 170mm cranks on my Cinzia (came on the bike, which I bought new, but that was 20 years ago, and things have changed.), and other than the cornering issue, it is fine.
    I'm pretty sure Sheldon Brown has written about this, and how "modern" bikes have a higher BB than "classic" bikes.
    For serious off-road stuff, a higher BB has a purpose, but for general usage, it seems to be to make the bike more "idiot-proof"!

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  3. Mine is 27 / 36 / 48 in gear - inches.

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    Replies
    1. you like a "gentle" ride then, or you live near some BIG hills :-)

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