Gear ratio for my street bike

[Pellethuntr2]

hey guys im trying to figure out what gear ration to get for my street bike...I have a 12hp briggs and 14in tires...now because this is a strictly street bike I want it to top out at 50-60...im also using v-belts...my thought is to use a 9in on my tire and a 4-5in on my motor...im not a real big guy my bike and I will way just over 200lbs prolly does that ratio have enough bottom or do I need to go smaller on the motor??

 

[fowler]

go to the speed calc on this site and fiddle with the settings
set the RPM at 3600 presuming the engine is not modifyed

 

[Pellethuntr2]

Thanks Fowler I all ready did that...but it doesn't tell you how your bottom end is gonna be it just tells you top speed

 

[Kaptain Krunch]

2:1 is pushing it. i would go with at least 3.5 or 4:1. Your going to need a jackshaft.

 

[Pellethuntr2]

I might go with 3:1 and why do I need a jackshaft Krunch??? what benefits will I get???

 

[Kaptain Krunch]

Trust me, acceleration and belt life will be very low with 3:1, you need to gear it lower.

You will need a jackshaft to get that reduction. Minimum pulley size i would recommend is a 4", anything smaller and the belt will slip more often/easier. So with a 4" pulley on the engine, and a 13" pulley on the rear axle, you still only have 3.25:1 ratio.

 

[fowler]

yes i know the issue im still looking for a torque calculator

 

[Kaptain Krunch]

You dont have to take my advice, but asking 60mph in one gear is a lot. Especially when the engine only hits 3600rpm. most cars wont car faster than 40-50mph in first at 7 or 8 grand.

 

[Pellethuntr2]

what if I get a 2 on the engine and a 9 on the tire???

 

[Kaptain Krunch]

i wouldn't recommend a 2" pulley, i wouldn't go smaller than 4", more surface area = better belt grip.

 

[Pellethuntr2]

good point thanks Krunch

 

[robbie]

If you want to go that fast, maybe you should get a torque converter. I don't think you'll get it with just one ratio because you'll fry your belt trying to accelerate.