Stumped on gear ratio

[Thor]

Hi, friends. New user/lurker from Houston here. I'm in yhe middle of the slow process of building my first cart.
I started off following the Arachnid plans and have veered off into my own ideas. I picked up a wrecked pit bike with a 125cc motor for $60. A couple hours after buying it I had the thing purring like a kitten.
My question is about the axle sprocket. The motor has a 15t sprocket and I'm running 20" rear tires. What tooth axle sprocket should I run for a happy medium between speed and torque. I'm 205lbs. myself. Any input or help is greatly appreciated. You guys have created the best karting community on the web and I deginitely couldnt have gotten this far into my build without DIYkarts so thank you guys.

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[Thor]

A few pics

Sent from my SAMSUNG-SGH-I437Z using Tapatalk

 

[Thor]

A few pics

Progress

 

[rocketkart]

you plan on using a jackshaft set up if not i would go with say a 40T axle sprocket to start with not noing the transmission gear ratio on the engine. is it a auto transmission? if not how many gears? what size sprocket did it have on the bike? and what size tires did it have on the bike? try to get it as close to the same gear ratio as the bike and maybe go a lil smaller on the axle sprocket seeing you are going to be pulling more weight.

 

[Thor]

you plan on using a jackshaft set up if not i would go with a 36T axle sprocket to start with not noing the transmission gear ratio on the engine.

Thanks, Rocket. I dont plan on using a jackshaft. I know the rear sprocket on the bike was 41t and it had 14" tires on the rear.

I tried the gearing calculators and was surely doing something wrong as I was coming out with top speeds of over 100 mph. Thanks for the help.

 

[firemanjim]

Well hello, fellow Houstonian! What side? Im near Pearland, 521@2234....
Hey, we need to know if its a standard (got gears?) or automatic (no gears,just gas n go...) . With that we can tell you a good number....
EDIT...... Im thinking, auto trans youre gonna want at least a 60 tooth axle giving you 4:1 . With the bigger tires n more weight, maybe even 68-72ish teeth.... 4 speeds, you can keep it as is...... your gears make up the difference in speed n torque......

 

[Thor]

Hey, Jim im actually just north of Houston in Humble 1960/59. And to answer your question its a standard 4 speed. The motor came off an Apollo pit bike.

 

[firemanjim]

Hey, Jim im actually just north of Houston in Humble 1960/59. And to answer your question its a standard 4 speed. The motor came off an Apollo pit bike.

Your good to go.... MAYBE drop to 46-52 ish teeth to compensate a little for more weight n bigger tires....

 

[Thor]

Your good to go.... MAYBE drop to 46-52 ish teeth to compensate a little for more weight n bigger tires....

Excellent. Exactly the info I needed. Thanks alot. Love Pearland too, great part of town btw.

 

[OzFab]

Anyone?

Bumping after an hour & a half? You have to understand, although it's based in the US, this is an international forum; of the three active moderators, none of us are in the US, which is a reflection of the member base.

Members are signing in & posting 24/7 from different parts of the world so, sometimes you may have to wait a while for an answer but, you will get one. If you don't recieve any response after a day & a half, then, by all means, bump your thread

I tried the gearing calculators and was surely doing something wrong as I was coming out with top speeds of over 100 mph. Thanks for the help.

The one drawback with our speed calculator (& all speed calcs for that matter) is it's merely theoretical &, as we all know, there's a huge difference between theory & practice...

If you setup your kart using the numbers you put into the calc, it probably will have a massive top speed but, the thing it doesn't mention is it will probably never move because the engine simple doesn't have the torque to make it move

Increasing a gear ratio to suit larger tyres is all about percentages. First you need to figure out what percentage you increased the tyres by, then add that percentage to the sprocket:

14" to 20" gives you a 30% increase: 20" = 100%.
20/2=10x10=100 therefore
14/2=7x10=70%

41t + 30% = 58t
41/70=0.5857x100=58.57

 

[Thor]

Bumping after an hour & a half? You have to understand, although it's based in the US, this is an international forum; of the three active moderators, none of us are in the US, which is a reflection of the member base.

Members are signing in & posting 24/7 from different parts of the world so, sometimes you may have to wait a while for an answer but, you will get one. If you don't recieve any response after a day & a half, then, by all means, bump your thread



The one drawback with our speed calculator (& all speed calcs for that matter) is it's merely theoretical &, as we all know, there's a huge difference between theory & practice...

If you setup your kart using the numbers you put into the calc, it probably will have a massive top speed but, the thing it doesn't mention is it will probably never move because the engine simple doesn't have the torque to make it move

Increasing a gear ratio to suit larger tyres is all about percentages. First you need to figure out what percentage you increased the tyres by, then add that percentage to the sprocket:

14" to 20" gives you a 30% increase: 20" = 100%.
20/2=10x10=100 therefore
14/2=7x10=70%

41t + 30% = 58t
41/70=0.5857x100=58.57

Thank you for that. That explains it to me very well. Appreciate the help.

 

[Doc Sprocket]

When using a motorcycle-derived engine/gearbox combo, the common gearing calculators do not take a major factor into consideration- primary reduction. There is usually an internal reduction ratio of maybe 3:1 between the engine and the transmission. So if you calculate using the engine's RPM, the high gear redux ratio, and the sprocket ratio, you're going to be off by quite a margin.

 

[Thor]

When using a motorcycle-derived engine/gearbox combo, the common gearing calculators do not take a major factor into consideration- primary reduction. There is usually an internal reduction ratio of maybe 3:1 between the engine and the transmission. So if you calculate using the engine's RPM, the high gear redux ratio, and the sprocket ratio, you're going to be off by quite a margin.

Ok I see so those are generally supposed to be used with industrial motors and such. I knew something was way off even factoring in variables such as weight the numbers were just way off.

 

[Thor]

Bumping after an hour & a half? You have to understand, although it's based in the US, this is an international forum; of the three active moderators, none of us are in the US, which is a reflection of the member base.

Members are signing in & posting 24/7 from different parts of the world so, sometimes you may have to wait a while for an answer but, you will get one. If you don't recieve any response after a day & a half, then, by all means, bump your thread

Sorry about that, Fab. Didnt know how bumping worked. Wasn't aware commenting would jump my thread to the top. Duly noted, buddy.