25cc gear ratio

hill climber

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Sep 11, 2008
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paso robles, california
can anyone let me know if 20.5:1 is enough gear for a 25cc motor on a 26" mountain bike. i am not looking for a lot of speed. i know 25cc is enough on a friction with a 3/4" roller, pulls from a stop to about 23mph. anyone have any idea of rpm on a craftsman 25cc hedge trimmer, 2 smoke of course.
thanks,
hill climber
 

5-7HEAVEN

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Aug 2, 2008
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No, the 20:1 ratio would not work. It is equivalent to a 1.46" roller.

If you say that a 3/4" roller scoots to 23mph, that engine is screaming at 11,648 rpm...

which is equivalent to 39.2:1 gear ratio at 11,648 rpm...

or a 33.6:1 ratio at 10,000 rpm...

or a 30.3:1 ratio at 9,000 rpm...

or a 27:1 ratio at 8,000 rpm.
 

5-7HEAVEN

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I totally agree.

Hill climber, maybe you're not going 23 mph with that 3/4" roller..boogy1a
 

hill climber

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Sep 11, 2008
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paso robles, california
thanks guys, i forgot to mention that the 25cc 3/4" roller is on a 20" bike, different motor but still 25cc. yeh, the 20.5:1 was not enough on the 26" bike. well the final sprocket was a 44 tooth and now im going to try a 56 tooth, should be 26.9:1 ratio. should be better.
 

5-7HEAVEN

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Aug 2, 2008
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We're still confused.

You must have a jackshaft. Can you describe your gear setup? I'm tring to figure out how you get 26.9:1 with a 56t sprocket.
 

hill climber

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Sep 11, 2008
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paso robles, california
ok, the trimmer had a gear box, 5.6:1, i added a 12 tooth sprocket to the gear box output shaft, that i fabbed, from 12 tooth spocket to 56 tooth sprocket, these are a 4.66:1. so let me know if i did this right. gearbox ratio times chain ratio, 5.6X4.66=26.096 , sorry misses the "0" before.
 

5-7HEAVEN

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Aug 2, 2008
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With 20.5:1 gears at 7700 rpm, the 25cc engine might've been sluggish at low end, and top speed was theoretically 29.1mph.:ride:

With 29.1:1 gears, same engine/same rpm pulls to a more realistic 23 mph with more low end torque.:bike2:

Sounds reasonable.
 

hill climber

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Sep 11, 2008
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paso robles, california
well that was fun till the plastic clutch housing broke. i have to pull the clutch/ gear box off and see what to do with it. maybe a gravity clutch friction setup. thanks for all the help. hill climber
 

comfortableshoes

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Jul 22, 2008
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Beverly, MA USA
your craftsman whacker 25cc had a gearbox? I have had 3 in my shop over the last 6 months. One an ancient 20 year old 32cc had a fantastic clutch set up, no gears though. The next also older, 21 or 25cc had no clutch and no gears, and the last one I've got is a 25cc slightly newer model, and by that I mean that it is from around the 90s and not the 80s like the other 2 has no clutch and no gears to speak of. I've also got a mid 90s poulan that has no gears nor clutch.