Gearing Conundrum

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Found an old single speed bike on the side of the road, and my friend has a ton of old parts left over from a build, so there is only one logical thing to do here. I think it is a 6ish hp engine w/ torque convertor, the bike is a 27.5". So what gearing should I run? 10 to 54? This will be an on-road only bike that has to be able to climb our hilly area. I'm very likely to do stage one mods (maybe not governor, it only has one rear brake).
 
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I just wondering what gearing I should run, I have access to a plasma table so I can cut out just about any size, I just need to know how many teeth I need.
 

Functional Artist

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Um,
...you want to put a 6HP industrial engine on a bicycle?

If so, most bicycle frames are not designed to handle that kinda power
...& the wheels &/or brakes are not designed to support &/or stop that kinda weight
 

karl

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I just wondering what gearing I should run, I have access to a plasma table so I can cut out just about any size, I just need to know how many teeth I need.

Handy tool on the main site

https://www.diygokarts.com/speed-calculator.html

If you keep the governer , type in 3600rpm, wheel size, and play with sprockets.
You can just ignore the torque converter overdrive to get close, or add 10% to the engine rpms in the calculator.

This will show you your top speed, you will want to pick something reasonable, probally 30-40mph.

I think you will find the sprocket requires on the axle will be massive, almost as big as the tire.

This is where compound gearing come in, jackshaft time.

https://www.bmikarts.com/Go-Kart-Gear-Ratio-Calculator-Jackshaft-Setup
Um,
...you want to put a 6HP industrial engine on a bicycle?

If so, most bicycle frames are not designed to handle that kinda power
...& the wheels &/or brakes are not designed to support &/or stop that kinda weight


Yeah that too
 

landuse

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What is your specific question? I will try to help you!
He has already mentioned that he needs gearing info.

So, with a 6HP engine you are going to have to beef up EVERYTHING on that bike...otherwise you are going to twist it like a pretzel. If you don't have an engine yet, I would suggest rather going for something in the 3HP range. As for gearing, you are going to HAVE to use a jackshaft, otherwise your rear sprocket is going to have to be as large as your back tyre. You want to shoot for at least a 7:1 ratio.....nothing higher (rather lower). That means if you have a 10T on the torque converter, you are going to need a 70T on the rear axle if no jackshaft is used.

As Karl mentioned, the brakes on the bike are going to be useless, and you will have to get something a little beefier to stop you
 

vegasboy

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A long time ago in a galaxy far away... I built a recumbent bicycle and my buddy installed a 212 predator motor with a 60 tooth rear sprocket on a 20-in tire. Had a top speed of about 55 and you could drag race any car off the line. Had so much acceleration it would throw you into the seat. And the most ever done to that motor was only a governor bypass with no extra mods. Like others have stated you definitely need to beef up your brakes and make sure your tires are in decent enough condition to be running those

speeds. We also had a 10-speed bike with a similar type motor and had no issues with it. Not only that, there are companies now building frames to purposely mount a predator 212 in the frame. And those work fine also, plus Google some images you'll find others designs that might help you out.
 
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I'm not worried about the frame, it's steel and I'll add gussets everywhere, and the axles are 3/8" bolt-on and won't pop out like a quick release. I am worried about the brakes, but if I can find a fork I have an old set of hydraulic disc brakes. I don't think I can put on a 70 tooth onto the bike without hitting the frame, and a jackshaft is going to be hard to fit. I could find a smaller rear wheel at the junkyard and slap that on there, run as big a sprocket as I can and see what happens. Worst case scenario is it doesn't work and I have $75 down the hole.
 

landuse

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Go for it and see how it performs. It might be overgeared, but it will still move. As long as you are ok with your TC belts burning up real fast, you should be good
 

vegasboy

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Landuse is correct on a torque converter belt. Torque converter belt do not like high RPMs on a consistent basis, they tend to vaporize. Gearing is everything. If you can't be satisfied with top end or low end going with middle ground it's your safest bet.
 
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