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Old 03-25-2019, 07:25 PM
Soloshot55 Soloshot55 is offline
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Default Mounting a hub and sprocket with no keyway

Hi everyone! My son has a little 2 cycle go kart that I would like to modify by mounting a 3hp predator engine. The stock rear axle isn't keyed. I need to add a new hub and sprocket but how can I do this without a keyed shaft? It is a 3/4 axle. Any help would be greatly appreciated!
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Old 03-25-2019, 07:47 PM
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It won't be a straight swap from two stroke to 3 pony Predator. The mounting and crank systems are different. Be ready to do major modification. And for less power too. 2 strokes are usually more powerful.
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Old 03-25-2019, 08:04 PM
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Originally Posted by JTSpeedDemon View Post
It won't be a straight swap from two stroke to 3 pony Predator. The mounting and crank systems are different. Be ready to do major modification. And for less power too. 2 strokes are usually more powerful.
Don't do it. I did it....it resulted in a pretty crap kart. They aren't designed for them and really shouldn't be given a 4 stroke. If you want to do it, go for the 6.5HP predator which will give you the power you need because of the poor gearing that will result.
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Old 03-26-2019, 07:45 AM
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Even then, I rephrase:
"It won't be a straight swap from two stroke to 6.5 pony Predator. The mounting and crank systems are different."
I'm pretty sure most 2 strokes have tapered crankshafts, so you would need to also swap out the driveline.
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Old 03-26-2019, 07:59 AM
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Lets see more pics of the kart and motor...
Off hand that looks like one of those crappy 49cc china engines for a weed eater..
That also looks like #25 chain... And getting a clutch of that type to fit a bigger engine is an almost custom job...
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Old 03-26-2019, 08:10 AM
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Originally Posted by JTSpeedDemon View Post
Even then, I rephrase:
"It won't be a straight swap from two stroke to 6.5 pony Predator. The mounting and crank systems are different."
I'm pretty sure most 2 strokes have tapered crankshafts, so you would need to also swap out the driveline.
No...those two strokes generally have an integrated clutch with a really tiny sprocket on it. Like REALLY tiny.

Also for the chain...some are 25 but most use T8F chain which is similar in size.
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Old 03-26-2019, 08:14 AM
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Hmm, Ok. I learned something new today!
Soloshot55, could you give us more pics and details about the kart?
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Old 03-26-2019, 03:46 PM
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Sorry guys long day! I understand this will not be a direct swap. I look forward to the challenges of modifying things to make them work. Yes this a Chinese 49cc engine. I have it worked out in my head but was wondering if anyone had any good idea's in regards to mounting the sprocket. The axle is not keyed. Here are some pics!

Also I want to use a new engine with the proper clutch, chain and sprocket. The 35 chain seems to be common? The 3hp predator would be a much more natural fit for this size go-kart over the larger 6.5. That said I don't want it to be a dog! I would like it to have decent acceleration with a top speed of at least 22mph. Can anyone suggest how many teeth the sprocket should be?
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Old 03-26-2019, 04:07 PM
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Ok you will need a jackshaft. No way to get around it. I think you should get one of these....it will probably be the best as it is a nice beefy motor mount and a place to shove a jackshaft. https://www.bmikarts.com/Motor-Mount...gs_p_1051.html

BTW: The 79cc motor is about 12" cubed while the 212cc is 14" cubed. Is that really that much bigger. 3HP is only good for about 15mph in most cases.
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Old 03-26-2019, 04:12 PM
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Thanks! So could you explain why I need a jackshaft? I was going to mount a new sprocket in line with the clutch on the new engine. Why won't that work?

Also good point about the engine size differences! I will be going with the 6.5 then.
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Old 03-26-2019, 04:35 PM
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I am sure there are cheaper/easier ways to do this, but our build is doing exactly what you are asking through a ton of custom fabrication. Predator 212, keyed rear axle, custom wheel hubs and lots of cutting and welding. Using a small 2-stroke would have been lots easier and cheaper, but that is not what we were looking for.

http://www.diygokarts.com/vb/showthread.php?t=40354
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Old 03-26-2019, 05:15 PM
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Quote:
Originally Posted by Soloshot55 View Post
Thanks! So could you explain why I need a jackshaft? I was going to mount a new sprocket in line with the clutch on the new engine. Why won't that work?

Also good point about the engine size differences! I will be going with the 6.5 then.
Because of the "large" sprocket on the engine, the ratio between the clutch sprocket and the largest possible sprocket directly on the axle. So you need a multi-stage reduction in speed from the engine...say if you wanted a 9:1 ratio, which is about right for a 3HP motor. With a 12T clutch, a 108T sprocket would be needed and that would be impossible to find and like a solid 3" bigger than your wheels. Instead, you put the 12T upto a 36T on the jackshaft, and put another 12T on the jackshaft and a 36T on the axle. Now you have the same 9:1, but it actually fits. A jackshaft also allows for easy tweaking of the ratio without pulling the whole axle off.

Now with 6.5HP, thats alot more power. You also can throw on a TAV which will help, its like a clutch and jackshaft but it shifts so you can have a nice high ratio for lots of starting torque to get you going, then shifts up so you can attain a decent speed. Also, even with a clutch, a 4.8:1 ratio will work fine...give you ~20mph with a governor on it and a 48T rear sprocket with a 10T clutch should be attainable. 48T sprocket is 8" in diameter, which I believe you have 9.5-10" tires so it would work but be close. With a TAV, you could get away with a 4:1 ratio and get more ground clearance and wicked acceleration(plus 30mph instead of 20)....and if you keep the 4.8:1 ratio you can make a total drift beast. Also the TAV means you don't need to make your own jackshaft, just incorporate room for it in your design.

And one other thing about getting the 212. The 79cc one doesn't have a throttle cable hook up and anything that is made for it is janky at best. The 212 has a proper throttle cable hook up.

Now its your choice on the approach you want and how you want to about it.
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Old 03-26-2019, 05:22 PM
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Quote:
Originally Posted by Tpdingo View Post
Because of the "large" sprocket on the engine, the ratio between the clutch sprocket and the largest possible sprocket directly on the axle. So you need a multi-stage reduction in speed from the engine...say if you wanted a 9:1 ratio, which is about right for a 3HP motor. With a 12T clutch, a 108T sprocket would be needed and that would be impossible to find and like a solid 3" bigger than your wheels. Instead, you put the 12T upto a 36T on the jackshaft, and put another 12T on the jackshaft and a 36T on the axle. Now you have the same 9:1, but it actually fits. A jackshaft also allows for easy tweaking of the ratio without pulling the whole axle off.

Now with 6.5HP, thats alot more power. You also can throw on a TAV which will help, its like a clutch and jackshaft but it shifts so you can have a nice high ratio for lots of starting torque to get you going, then shifts up so you can attain a decent speed. Also, even with a clutch, a 4.8:1 ratio will work fine...give you ~20mph with a governor on it and a 48T rear sprocket with a 10T clutch should be attainable. 48T sprocket is 8" in diameter, which I believe you have 9.5-10" tires so it would work but be close. With a TAV, you could get away with a 4:1 ratio and get more ground clearance and wicked acceleration(plus 30mph instead of 20)....and if you keep the 4.8:1 ratio you can make a total drift beast. Also the TAV means you don't need to make your own jackshaft, just incorporate room for it in your design.

And one other thing about getting the 212. The 79cc one doesn't have a throttle cable hook up and anything that is made for it is janky at best. The 212 has a proper throttle cable hook up.

Now its your choice on the approach you want and how you want to about it.
Wow! Thanks for all the great info! Seriously man I appreciate it. For what it's worth the tires are 8". Not sure how much that changes things though.
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Old 03-26-2019, 05:46 PM
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Wow! Thanks for all the great info! Seriously man I appreciate it. For what it's worth the tires are 8". Not sure how much that changes things though.
Ok so with 8" tires and the 6.5HP motor, I'd say 4:1 with a clutch and 3:1 with a TAV. Diameter of a 40 tooth sprocket is 6.7", so a direct hook up would work without an issue...and still have 1/2" ground clearance. Would recommend fabbing a sprocket guard though.

BTW, I'm doing all the measurements given you have a 10T 420 chain clutch and matching 420 chain sprocket. 420 chain is 0.5" pitch, while 35 chain is 0.375", so do note the difference when you purchase, and don't mess them up. 420 chain is more durable, but the bigger links give less room for adjustment because there are less teeth to add/remove.
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Old 03-27-2019, 01:24 AM
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Ok so with 8" tires and the 6.5HP motor, I'd say 4:1 with a clutch and 3:1 with a TAV. Diameter of a 40 tooth sprocket is 6.7", so a direct hook up would work without an issue...and still have 1/2" ground clearance. Would recommend fabbing a sprocket guard though.

BTW, I'm doing all the measurements given you have a 10T 420 chain clutch and matching 420 chain sprocket. 420 chain is 0.5" pitch, while 35 chain is 0.375", so do note the difference when you purchase, and don't mess them up. 420 chain is more durable, but the bigger links give less room for adjustment because there are less teeth to add/remove.
3:1 might still be a bit too high, even with small tyres, but I could also be wrong

And I think a jackshaft is probably the only way to go here. You don't want to set the whole thing up without one, and then realise the ratio is too high and now you need to get a jackshaft to sort it out. That would be double the work.

Just my opinion though. These numbers need to be tested in real life though to see if they will work
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Old 03-27-2019, 06:07 AM
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3:1 might still be a bit too high, even with small tyres, but I could also be wrong

And I think a jackshaft is probably the only way to go here. You don't want to set the whole thing up without one, and then realise the ratio is too high and now you need to get a jackshaft to sort it out. That would be double the work.

Just my opinion though. These numbers need to be tested in real life though to see if they will work
I got 3:1 to barely work on 10" tires and 3HP. I don't see why not 6.5HP and 8" tires wouldn't work fine.
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Old 03-27-2019, 06:51 AM
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I got 3:1 to barely work on 10" tires and 3HP. I don't see why not 6.5HP and 8" tires wouldn't work fine.
Like I said....I could be wrong
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Old 03-27-2019, 01:27 PM
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You may want to consider this engine over a Predator: https://www.homedepot.com/p/LIFAN-6-...-203277091-_-N

The 2:1 wet clutch it comes with will be better than any normal centrifugal clutch you will find and it will help with the ratios. Now you only need another 2:1 ratio...so you can have a nice and tiny sprocket on your driveshaft.....or a reasonably sized one and you can play with changing the ratios on the motor for easy ratio changing. a 6:1 ratio, 6.5HP, and 8" tires....well you will have a tire smoking drift machine!!
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