New Sprocket Help

[kcordova56]

Hey so I have a go kart with an ATC90 engine on it and tearing it apart to fix it up. But also I wanted to change the gearing cause when it did run the top speed was like 10 mph.

Does anyone know how gearing works and what sprocket size I'd need??

I have an 11" ish wheel size with a stock 49T sprocket on it.

I want to get a really fast top speed, i had a go kart with a predator 212 on it and it went 45 mph. So I wanted at least like 60 but hopefully more.

I know nothing about gearing but do you guys know what I'd need for fast top speed but also able to do burnouts in it?



Sorry I know this is a 3 Wheeler forum but was hoping to get help since I didn't know any other place to go

[Knobby125m]

Although I'm no expert, I would guess that 60 mph is not possible with that setup regardless of sprocket size. Even if you change the sprocket size to get higher top end speed it's gonna take u forever and a day to get there. Just my assumption tho and I could be wrong.

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[kcordova56]

I mean I had an engine that put out 6.5hp with no gears and it goes about 45 so I thought this having gears over just a centrifugal clutch. Plus I heard online (I could be wrong) but this one puts out 7 plus having gears.

But I'm totally new to this so I could be totally wrong and missing something

[ps2fixer]

60mph takes a fair amount of power, the ATC90 engine isn't very big, it just has a lot of gears. If you want to do burn outs, you need low gearing, if you want high top speed, you need high gearing. You'd be looking at putting like an ATC350X engine on it or a ATC250R engine on it to get both goals (25hp+ I'd guess). The air drag for higher speeds is an exponential resistance, meaning the power to go 60mph is very much greater than 30mph, not just 2x like most people would think. I'd guess if you geared up an ATC90 engine pretty high, you could hit 40mph. A 200cc should be able to push it to around 50, and a 350cc I'd guess around 60mph. The stock 200 and 350 can hit those numbers, but that's with 3 wheels (less rolling resistance) and I'd guess the air drag on them would be better than a gokart.

The ATC90 stock tire size was 22in and you're running effectively 1/2 the size of tire with roughly the stock gearing so easy math says your top speed should be about half the speed as the stock machine. If it could go 35mph stock, you're looking at around 17mph. To get back to stock gearing, you'd have to make the rear sprocket half the size (25t), or increase the front sprocket 2x (there's not enough room though). You can modify both sizes, but you have to do math for that to see the effect. Keeping everything else the same and assuming the engine would have enough power, to hit the 60mph you'd need a tiny ~15t to have just the gearing for it. I don't think they are made quite that small but I could be wrong.

A quick google search says the predator 212 is 6.5hp (212cc), the ATC90 is about 4.9hp (90cc). Even though the hp is similar, the torque curve (the actual push of the engine) is less on the atc90, it's just a better built engine to rev out to achieve the higher hp number. Torque is the "true" measurement of power, horse power is a measurement of torque over time. Like if you have a given engine with x hp with ideal gearing and such another engine with the same hp will have the same top speed (work over time), but getting to that top speed can be very different. Like a built v8 car on a drag strip just about does a wheelie taking off, while a similar built 4cyl (similar times) won't be doing wheelies at the starting line. The v8 has more torque pure in design, so it has the bottom end/torque to launch harder, but the 4cyl has less weight in the engine so even though it has a slower take off, it revs out faster (generally) and can catch up and potentially pass the v8. Also a smaller engine is generally easier to get to rev higher.

I don't know engine mods super great for the atc90, but people used to race them back in the day. I think the atc110 cylinder fits on them to bump the size up, add a high compression piston, port and polish the head, bigger carb, etc and you might be able to get the power up enough to gear closer to your goals.

A note on burn outs, if you have an open diff or the drive wheel is only one of the tires, it's much easier to break the tire loose than if both tires are tied together. That's why posi track or limited slip is more desirable, or for the people that mud bug, they like to weld up the spider gears (the ones that makes the tires able to spin at different speeds).

Here's some guys messing around with a v twin Honda engine, a whole lot more engine and even though they have slicks on they can't burn the tires, they just wheelie, and it looks like the stop speed is about 45mph (gearing could be higher probably). They do have the solid axle between the two tires else their wheelies wouldn't go as straight, or maybe the tire would break loose and do a burn out at that point.



Personally, I'd say to get the gokart back to around stock gearing that the ATC90 was and have fun with it, it should have reasonable low end power, but still go pretty fast. I'd assume no suspension, so 35-40mph is pretty much the max you'd ever want to go. You can use your phone with a GPS app to get the real top speeds with out guessing. 40mph in a gokart feels like 100mph vs driving a car 40mph so it's hard to gauge the speed with out some sort of measuring system.

[Jim mac]

there's speed, gear ratio calculators on line. if you put in your tire diameter gear ratio probably divide sprocket teeth to get the final drive ratio it should tell you how many rims your motor needs to spin to get a certain mph. jim

[kcordova56]

Ok thank you guys For your response I got a good idea of how this all works

[kcordova56]

Hey I also put in the values onto a calculator guessing on the engine rpm being at max 4000 and it said top speed was 40 mph. Could that not be taking into account for something? I don't know much about this topic.???

https://drive.google.com/file/d/1D17...ew?usp=sharing

[Shep1970]

Remember- ok lets say 4000rpm, but 4000rpm in 1st gear will be a different speed in say 4th gear. Your output shaft is not spinning at 4000rpm the "gears" in your transmission reduce that significantly for torque and or speed before it hits your sprocket combination. You need the fourth or 5th (whatever the engine has) gear tooth count to even start to figure the output shaft rpm. I dont think you'll be happy with the rpm you actually have.
Hope that makes sense-
shep

[kcordova56]

Ahh ok so I would need to open it up to check the sprocket size? And what would the rpm's of the inside sprocket be about?

[ps2fixer]

The sprockets are external to the engine, that's what the chain goes around. The stock ATC90 had a chain cover to keep dust off it and such.

Internal in the engine there's a couple different sets of gears. The primary gear reduction takes the engine rpm and drops the revs a little going to the input shaft of the transmission section. The transmission section is where you get the different gears. Low gears means several turns of the input shaft vs output shaft, high gear generally is near 1 to 1, like 1 turn of input shaft to 1 turn of output shaft. The chain goes on the output shaft (front sprocket), and is smaller than the rear sprocket, so again is a gear reduction making the revs drop again to the axle/wheels. The actual tire size dictates how far you travel per turn of the axle, so half the size of tire means you travel half the distance.

This gear ratio stuff is probably easier to show you in person than in text sadly. I used to be pretty terrible understanding ratios and such, but eventually I tore down an atv engine and got to see what goes on inside first hand and the hands on experience really helped me learn it. I kind of did the same thing with electronics which I know most people don't like to poke around with too much. Once you get the core concepts, it becomes pretty simple.

I could do all the math to give the rpm of each shaft and such, but the critical one is the axle rpm and tire size for your setup for the max engine rpm speed (top speed). Burn outs are a lot trickier to figure out. I'm sure there's math on how to figure out a given gear ratio/tire grip etc and calc how much hp is required to break traction, the atc90 isn't a big engine, so you likely won't get much of a burnout machine.

Either case, good luck with your project.