Racing Lawn Mower/ Go kart with 200cc dirt bike engine tachs out to early

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Ttali211

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Ok so I recently was given a old craftsman riding mower for free. I was determined to build something badass! So I sourced a dirt bike engine and found a 200cc Chinese 4-stroke motor for crazy cheap! So I turned this mower into a awesome shifting kart. Here’s my problem, I have found that when I’m running my Hoosier slicks on the rear that the motor tachs out way to quick and it only gets up to 30mph. The Hoosiers are 11x9x6, I swapped them out for some wheels I have from another project that are 18x8.5x 8 and tested it and man it went through all gears smoothly and got up to 50 mph easily. I’m running a 18t sprocket on the motor, to a 18t sprocket on the jackshaft In, a 18t sprocket on the jackshaft out, to a 48t sprocket in the rear. Which gives me a gear ratio of 2.667 which should not be taching out quickly. Can anyone explain why this happens? I’ve tried searching about tire size for gearing and can’t find squat :surrender:
 
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Kansaskart

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It tachs out fast because it takes less torque to turn the small tires and dont think i need to explain why its slower.

A racing mower is on my project list. Can you post pics would love to see it! Stock or upgraded steering?
 
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itsid

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get a bigger JS-out sprocket .. 29 or 30T should give you about the same performance on 11"
as you have with the 18" wheels.

(wheels are an essential part of gearing.. since the motor needs to push not against a sprocket but the road in order to move, right ;))

So whenever you feel like changing the wheel diameter you can just include it's effective diameter into the ratio,
as long as the scale is the same for both wheels (metric or imperial) it'll cancel out
and just tells you which way to adjust the gearing to have the same performance with the new wheelsize.

current-ratio / current-wheeldiameter * desired-wheeldiameter = new-ratio

in your case: 1.63:1 for a 48T axle sprocket that'd be 29.45 on it's driving sprocket..
tough.. so 29 or 30 (30 is a bit faster, 29 offers a bit more torque)

'sid
 
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Thanks for all the info. It’s definitely appreciated! I’m new to building go karts and what have you, but I really enjoy it. Especially cuz it’s cheaper then building cars! But awesome info and again thx for the quick reply’s! I will post my pics to my photobucket and then post them tomorrow. It’s not painted and a little rusted from not sealing the welds but it will get done all in due time.
 
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Ttali211

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Kansaskart said:
It tachs out fast because it takes less torque to turn the small tires and dont think i need to explain why its slower.

A racing mower is on my project list. Can you post pics would love to see it! Stock or upgraded steering?
Click to expand...

Here are a few pics of the build.. again I’m new to building stuff like this so don’t be to rough on me, lol. But I upgraded the steering so it goes over the motor instead of the barstool look.

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