Gearing for minibike [HELP]

A

AussieBikeBuild

New Member
#1
#1
I have a question about my proposed gearing for my minibike



After my calculations I concluded that if my engine was spinning in the middle of it's power band at 3600rpm, I would be topping out at 37MPH. Now I'm not sure about you but I want to get at least 45mph from an 8HP engine. I have jetted the carb and removed the governor so assume this engines top RPM is 4000. Hopefully someone with knowledge of gearing can help me out. My question to you is, how big should my current 18 tooth sprocket be for a nice balance between speed and acceleration. Here are some math details you might need

Tire Radius: 0.15m
Mass with me on it: ~100kg
Current Gear Ratio: 3.33
Engine torque: 11.67 ft/lbs @ 3600 or 15.822 Nm

And for those who just want to see this thing, please remember this is not finished yet and I have yet to weld the jackshaft on, weld on the new engine base plate on and get some more fuel line.

 
misfit

misfit

Member
#2
#2
3.3 gearing with your tire and engine at 4000rpm should get you 42mph, a 20t (3:1 ratio) would get you 46mph. Id try the 18 and see if the motor has the nads to get you going, it should. You might want to go smaller than 18 and go for more rpm and power, depends what you like, power or speed.

Does your clutch have a #40/41 sprocket or #35? Does it have 10t??
 
A

AussieBikeBuild

New Member
#3
#3
misfit said:
3.3 gearing with your tire and engine at 4000rpm should get you 42mph, a 20t (3:1 ratio) would get you 46mph. Id try the 18 and see if the motor has the nads to get you going, it should. You might want to go smaller than 18 and go for more rpm and power, depends what you like, power or speed.

Does your clutch have a #40/41 sprocket or #35? Does it have 10t??
Click to expand...
My clutch is a #40 chain 10t but here is the catch. The 18t and the 60t are #35 chain, not sure if that makes a difference or not. Thanks for the help, I'm assuming the engine has enough grunt to pull at a reasonable pace since I put new rings on it and compression tests show consistency around 75psi. It's my first time building a bike with this engine so I'm not really sure what it's capable of, would an 18t sprocket still give me the "umph" to go up a sort of steep hill?
 
A

AussieBikeBuild

New Member
#6
#6
Alright, I just filled out the gearing worksheet Motra kindly gave to me and here is what I have for 18T.
With my limited knowledge on the subject, is 86 lbs of thrust enough?

 
Last edited:
deejaaa

deejaaa

New Member
#9
#9
AussieBikeBuild said:
Thanks for telling me but I'd rather get some input on whether or not 86 lbs of thrust is adequate considering the bike with me on it will weight at least 200lbs. I think I will just go with a 15t sprocket just to be safe.
Click to expand...
it's because the pics didn't show up at work and it didn't make sense. carry on!
 
joshua. c.

joshua. c.

Well-Known Member
#10
#10
i think that engine will do more than 4000 rpms un governed in stock form. i manually revved one of these engines wile tunning it and it went way over the govenor limit. just remember the govenor it there for a reason. without a billet rod you take a chance of blowing the engine apart and you will be sitting right above it.
 
You must log in or register to reply here.
Top