hI I have a jack shaft kit on my mountain bike and am looking at the gear ratios for the front crank end. I am currently running the standard sprokets that come with the hd jackshaft kit. I think they are 36 tooth driving the bike chain to the rear 8 speed cassette but I can only really use 4 or 5 gears as my motor does not seem to have the power to pull the higher gears on the flat so the only time I can use them is going down hill. can I change the gearing on the crank from a 36 tooth to a 44 tooth ? would this help? also that would mean I would be running 44 and 44 tooth on the front set up?...will this work? im sure their is someone out their that knows more about these things than me
jack shaft gear ratio ? is this possible
- Thread starter poloste
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No that will not work as you will be geared way too tall and only be able to use one or two of your eight gears. You would want to drop the 36t down to a 34t or so.
With the 30t you won't have as high of top speed as the 36t, but you will be able to use all eight speeds of your gearing. I wouldn't use the 24t though....too low of gearing.
I run a high-rpm 2-stroke engine thru a shift kit. It's not the China engine, but the principle is the same.
When gearing a shift kit, shoot for the lowest gear ratio (highest numerically) in the next-to-last gear in the cassette. That way, you can still have throttle response while zipping down the highway in the next-to-last gear. Final gear will be very useful and let the China engine cruise @ its highest torque range.
Unsure what the optimum rpm and torque for your engine. For sake of discussion, let's say 5,000 rpm @ 45mph. The easiest way to get there is the lowest-possible gears and cassette available. Not to worry, because sprockets, chainrings and rear cassettes are relatively cheap.
I'll work some numbers and return.
If you can arrange gears to get 9.7:1 in 8th gear, that's like a 23-tooth single wheel sprocket for a China engine setup. At 5000rpm engine speed, bike speeds to 39.9mph. At 7000rpm, bike speeds up to 55.6mph (theoretical).
In 7th gear, ratio is 11.47:1. That's like a 28t rear sprocket. At 5000rpm, bike speeds to 33.7mph. At 7000rpm, bike speeds to 47.2mph.
In 5th gear, ratio is 14.99:1. That's like a 36t sprocket, a popular single wheel sprocket. Speeds of 25.8mph @ 5000rpm and 36.1mph @ 7000rpm.
To get these ratios, it's very simple. Start with 4.15 China engine gear ratio. Use a 10-tooth engine sprocket and left-side 17t jackshaft sprocket. Right-side jack sprocket is 10t, chained to a 45t chainring. All these gears and sprockets calculate to 31.75:1.
Use a 36t inside chain ring and an 11t-34t rear cassette. First gear is 29.98:1, which is good for flat ground and medium hills. If you'd like a gear ratio to climb very steep hills, I can also figure that out for you.
Good luck.
In 7th gear, ratio is 11.47:1. That's like a 28t rear sprocket. At 5000rpm, bike speeds to 33.7mph. At 7000rpm, bike speeds to 47.2mph.
In 5th gear, ratio is 14.99:1. That's like a 36t sprocket, a popular single wheel sprocket. Speeds of 25.8mph @ 5000rpm and 36.1mph @ 7000rpm.
To get these ratios, it's very simple. Start with 4.15 China engine gear ratio. Use a 10-tooth engine sprocket and left-side 17t jackshaft sprocket. Right-side jack sprocket is 10t, chained to a 45t chainring. All these gears and sprockets calculate to 31.75:1.
Use a 36t inside chain ring and an 11t-34t rear cassette. First gear is 29.98:1, which is good for flat ground and medium hills. If you'd like a gear ratio to climb very steep hills, I can also figure that out for you.
Good luck.
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