Gear Ratio??

SpiderSpartan

New Member
Jul 29, 2008
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Cincinnati
-So I have a 25cc Homelite trimmer and I'm planning on doing the direct chain drive thing. I've found couple of clutches and sprockets for the system, but I'm not sure what a good ratio would be.
-I figure the engine will spin at about 8000 RPM and I have 23" tires. Using a gear ratio of 6:1 I will get a top speed just under 30MPH.
-Do you guys think that this setup will have enough torque to get me started or should I try to go 10:1 which would give me a top speed just under 20MPH?
Thanks.
 

deacon

minor bike philosopher
Jan 15, 2008
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north carolina
Sorry I can't help you I don't know anything about gears. Someone will come along I'm sure. Just didn't want you to worry that you were being ignored.
 

ocscully

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Jan 6, 2008
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Orange County, CA
If you compare the various kits available from vendors you will find that the total reduction is in the 18.75:1 to 22:1 range. I think you will find the 6:1 reduction you are considering not near enough. I haven't actually run the #'s but I'm guessing at 6:1 you will have to pedal the bike to at least 15mph or more before the engine can do you any good.

ocscully
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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If you were to adapt this engine to Staton's chain drive with 18.75:1, then your numbers would jive. At 8,000 rpm, your 23" bike would be traveling at 30 mph.

With 6:1 at 8,000 rpm, top speed is 91 mph. But of course your clutch would burn out before you could get the bike rolling.
 

SpiderSpartan

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Jul 29, 2008
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Cincinnati
Dang! Thanks for pointing that out to me. I was definitely calculating my speed using the diameter instead of the circumference of the wheels. Looks like I'll have to add in a jackshaft to work out the ratio. So much for keeping the pedals.
 

5-7HEAVEN

Well-Known Member
Aug 2, 2008
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You are so close, Spider. Keep all your calculations and simply multiply by 3.1416.
 

SpiderSpartan

New Member
Jul 29, 2008
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Cincinnati
ok, so I got a 10 tooth clutch off ebay. I think the easiest thing to do now will be to replace the pedals with a jackshaft to get the right ratio. Here's what I'm thinking. 10 tooth clutch to 41 tooth jackshaft = 4.1:1 and then 10 tooth on the jackshaft to 46 tooth wheel sprocket = 4.6:1. Which comes to a total of 18.86:1. Is that right?
 

comfortableshoes

New Member
Jul 22, 2008
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Beverly, MA USA
I think your might find that clutch you ordered doesn't fit your weed eater. the clutched on weed eaters are around 2.5 to 3.25 inches in diameter, and for a go-cart, they are much larger. I don't think your clutch will ever engage- your clutch simply cant spread out far enough to hit the sides of anything larger than it's own clutch.

Does this come with have the internal parts of the clutch too?

I hope it works for you.
 

SpiderSpartan

New Member
Jul 29, 2008
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Cincinnati
The clutch I got is a complete assembly with the clutch and bell with a 10 tooth sprocket attached.

The description of it said that it engages at 2000 RPM, so hopefully that should work with my weed eater. It does have a 5/8 inch center bore, but that's ok, since I was planning on making a custom drive shaft anyway. Now the next step is getting the rest of the gears. I'm going to run this to the original pedal sprocket that has 52 teeth, giving me 5.2:1 ratio. The original gears on the back wheel have a max. number of 28 Teeth. If I could find a 6-8 tooth sprocket with a 7/8 inch bore to fit on my pedal/jack shaft that would give me a total ratio of 18.2-24.2:1, which would be perfect. Anyone know where I could get a gear this size with 1/2 inch pitch for about $10? All I've been able to find are 10 tooth with 3/4 inch bore.
 

Austritch

New Member
Apr 16, 2016
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NC
Hey! Just checking in with my 150cc GY6 fat bike build. I am beginning the final stages with doing my gearing. So far I have (on paper) a 5:1 (12 on the clutch and 60 on the axle) gear ratio on a 26 inch drive wheel. What do you think about this ratio? Will it eat up my centrifugal clutch? Questions? Comments? Concerns?