3

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.

5
  • Why do you want to change the front chainring? What gearing do you have, and are you pedaling fast enough?
    – Batman
    May 8, 2016 at 20:22
  • I'll edit the question
    – Kemosabe
    May 8, 2016 at 20:26
  • 2
    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, 2016 at 21:34
  • Oh smart, forgot that you can change chain length! Thanks!
    – Kemosabe
    May 8, 2016 at 22:56
  • 1
    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.) May 9, 2016 at 11:47

1 Answer 1

5

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.

4
  • 2
    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, 2016 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, 2016 at 0:26
  • 1
    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, 2016 at 0:27
  • Also thanks for the response, and for the ideas from your other project!
    – Kemosabe
    May 10, 2016 at 0:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.