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Motorbike Macgyver
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379 Posts
Discussion Starter #1
I would like to know if it is possible to build a relatively simple 3 speed helical transmission. I am building a motorized bicycle to resemble a classic motorcycle, and a 3 speed gearbox would really help with the look and also make climbing hills a lot easier. The pto shaft is 5/8" counter clockwise. My current gearing is 13.58:1, d like to have that for a second gear, and have a first gear of around 15:1 and a third gear of around 11.5:1. I would also like to keep my automatic clutch if possible, and use a jockey lever for shifting. Can anyone tell me if this is possible, or if there is already a 3 speed gearbox that will fit a 5/8 counterclockwise shaft? Thank you.
 

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Motorbike Macgyver
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379 Posts
Discussion Starter #2
Well, I'm not sure if I will ever convert my little bike to multispeed, but if I do, it will most likely be as primary belt drive. There is a pulley unit that changes diameter and thus gear ratio with engine rpm, normally shifts automatically but its possible to control shifting manually by controlling the belt tension. I may or may not do it, if my bike runs well and does what I want it to do as a single speed, I may just leave it that way.
 

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American Legion Rider
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23,605 Posts
Well if you are going to stick with a belt then what I was thinking won't work at all. I was wondering why you don't go with a derailer like a 10 speed but make you're own super duty one. You could put as many or few gears as you want then.
 

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Motorbike Macgyver
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379 Posts
Discussion Starter #4
They do make these things called shift kits, where if you have a geared wheel you can hook up the shift kit, which is a jackshaft that transfers power from the engine to the bicycles gears. But here is my thinking on that. The type of classic motorcycle I am trying to emulate is a cafe racer. Really kind of obsessed with that look. Cafe racers didn't have their gears on the wheel, they had them attached to the engine via a gear box. Now, I know that a shifting belt drive doesn't look exactly the same as a helical gear box, but would be much closer to the look I am going for with my bike. I do appreciate the input and I know that you're trying to help, so thank you.
 

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American Legion Rider
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23,605 Posts
If I remember correctly, Schwinn made one with some kind of internal gear shifting of a 3 speed. I never did know how that actually worked but always thought it was slick. That seems like it would be a lot of work to make a hub that would do that for the torque you might be putting out.

Okay, I looked it up and I'm correct. Here's a link to a bicycle forum where they talk about it. And I'm right in that you would have to beef things up. But man that would be just the ticket.
 

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Motorbike Macgyver
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379 Posts
Discussion Starter #6
What you are referring to is an internally geared hub. They operate on a planetary gear system, and most of them won't stand up to engine power. The ones that do would cost about the same as it might cost me to build this shiftable belt drive I described, possibly more. And even the highest quality ones have self destructed under engine power. All in all, the shifting belt drive idea has so far been the least expensive, least difficult and most reliable way of adding gears that I could think of. However, I've also realized something else. It doesn't need to have gears to resemble, or indeed even actually be considered, a classic motorcycle. Now, I know that cafe racers did have gears, but those were racers. Some classic motorcycles were very basic, single speed machines and although I can't seem to find out for sure, I'm betting that there were some single speed motorcycles that looked a lot like the bike I'm building.
 
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