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So I have a bicycle that has a single front gear, but many back gears with a derailleur. I am wondering if I change the size of the front gear (like significantly), meaning if I get a bigger front gear without changing the back gears or derailleur, will the derailleur will still function and not cause the chain to go slack? If not can I adjust the rear derailleur to a larger front gear? I am trying to make a generator out my old bicycle, and in order to get a more efficient power output I need to have a higher front gear (since it is an old bike for smaller people so it has a rather small front gear). I am pedaling too fast on it as is right now. I do not know what gearing I have right now.

  • Why do you want to change the front chainring? What gearing do you have, and are you pedaling fast enough? – Batman May 8 '16 at 20:22
  • I'll edit the question – Kemosabe May 8 '16 at 20:26
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    All you will need to do is adjust the chain length. Normally a bigger chaining ring will mean a longer chain is required - so unless you have spare chain the same as the original, a new one will be needed. – mattnz May 8 '16 at 21:34
  • Oh smart, forgot that you can change chain length! Thanks! – Kemosabe May 8 '16 at 22:56
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    Yes, you'd need to adjust the chain length. Otherwise, if the new ring physically fits, you should have no problem. (You could in theory have a problem with chain angle if you installed an 18-inch diameter ring, but I assume you're not considering that.) – Daniel R Hicks May 9 '16 at 11:47
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For a generator you will probably want two sets of gears. Cyclists normally pedal at about 90-110rpm, and getting a small generator to work efficiently normally means spinning at 1000's of rpm. I used a "drill pump" to make a pedal powered pump once and that was designed to spin at 1800rpm. But it used 500W or more at that speed, so I spun it at about 1000rpm using about 200W and that worked fine. Unfortunately I lost interest in taking photos after building the first, single-stage version.

Most of the bicycle generators I've seen use the bike wheel as the second gearing stage, typically having a generator drum 100-300mm in diameter for a ratio between 2 and 6 to 1. The bigger the drum the lower the ratio, but the less tyre deflection so the higher the tyre/drum efficiency.

Depending on what tools you have (mostly, do you have a lathe) you can either use the large cog on the cassette to drive a second cassette, or make an adapter to mount a full size chainring on the cassette. If you get a steel cassette and chainring you can just weld the chainring on, and it will probably not deform too much.

Doing that lets you have a road bike first stage with a maximum ratio of 50T:12T or ~4:1. If the cassette is a wide range one off a mountain bike it could be 12-36, say, giving you a 36:12 second stage or 3:1. That's 12:1 overall, so your 100rpm input becomes 1200rpm output.

You can get the same ratio with a single stage but it will be big and expensive. Assuming a 12T small cog, you will need a 144T chainring to get the same 12:1 ratio. There's one on ebay for $US500. A friend of mine sells large custom chainrings in Australia and could make you a 144T one, but a 90T one costs $AUS200 (about $US150 in April 2016).

I think the important thing is to start with the generator you want to use and work out its ideal spin rate. Then work backwards to the gearing. I suggest leaving at least one derailleur in your final build so that people can vary the gearing a bit to get the most comfortable combination of pedalling speed and load. Varying the generator output has much the same effect if you can do that.

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    Right now I have a regular bike set up, but instead of a tube on the rear wheel I have a belt going around the rear wheel and a pulley around a motor (26" wheel, 2.25" pulley). I have access to a community bike shop with tons of old parts for free, so either I'll do the chain thing like @mattnz said or another derailleur (whichever's cheaper). – Kemosabe May 8 '16 at 22:59
  • Using the rim as a pulley is smart, that should make everything simpler. Just watch for slip between belts and pulleys. – Móż May 10 '16 at 0:26
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    Slip hasn't been much of an issue, though I added a hinge on the motor and a spring pulling it down to account for shaking of the bike on the temporary frame (final project should be more rigid) – Kemosabe May 10 '16 at 0:27
  • Also thanks for the response, and for the ideas from your other project! – Kemosabe May 10 '16 at 0:27

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