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butch
10-04-2007, 07:59 PM
Does anyone know how to use the gearing charts with a cvt? Im trying to figure out what size front sprocket to run.

Pacman
10-04-2007, 08:32 PM
Does anyone know how to use the gearing charts with a cvt? Im trying to figure out what size front sprocket to run.

Huh?

CVT has a drive clutch and a driven clutch.....where does the sprocket go?

butch
10-04-2007, 08:46 PM
Sprocket on the jack shaft. I thought you were smarter than that!

minibajaman
10-04-2007, 08:57 PM
I have a chart that will let you play with the gearing for a CVT...but you can only put in the total reduction after the CVT, so you'll have to decide sprocket sizes once you have a reduction figured out.

butch
10-04-2007, 08:59 PM
Thank you very much.

Pacman
10-04-2007, 09:05 PM
Sprocket on the jack shaft. I thought you were smarter than that!

I am...........If I was to use a CVT it would be mounted to a FNR Box.......no sprockets! Your original post didn't mention that you where mounting to a jackshaft....so the assumption is that it is going to be mounted to a FNR Box....hence....no sprockets!

hummerman
10-04-2007, 09:10 PM
sled engine with reverse drops the need for a FNR box........

that make sense pacman?

butch
10-04-2007, 09:14 PM
No fnr box. I probably wont even use the chain case thats in the sled. It doesnt have reverse so its just extra weight. the question was how to use the chart with a cvt, would a fnr box make that much difference?

LiveWire
10-05-2007, 07:57 AM
I have a chart that will let you play with the gearing for a CVT...but you can only put in the total reduction after the CVT, so you'll have to decide sprocket sizes once you have a reduction figured out.

I was just running these same number manually yesterday. So this is nice so that I did not have to make up my own spreadsheet.

I think your actual speed cell is not a good calculation. It is assuming top speed will be 70% of calculated top speed disregarding engine power. If the engine has enough power after losses to push the vehicle at the calculated top speed, then they would be the same.

You might find this interesting to play with: I attached a spreadsheet that I had downloaded during the 150MPH busa rail discussion. I did not know the HP curve so I just put 170 at all RPMs in order to disregard the fact that if geared too high, you'll actually lose top speed due to making less HP at the RPM of your actual top speed.

Butch, I think the high ratio of Arctic Cat clutches are 1:1 which is one of the values you'll need for that chart.

Pacman
10-05-2007, 09:16 AM
sled engine with reverse drops the need for a FNR box........

that make sense pacman?

Yes it does.....but down here in Virginia.....no such thing as snowmobiles! So.....in my world view....it doesn't come up as an option.

minibajaman
10-05-2007, 09:17 AM
I think your actual speed cell is not a good calculation. It is assuming top speed will be 70% of calculated top speed disregarding engine power. If the engine has enough power after losses to push the vehicle at the calculated top speed, then they would be the same.

Correct. *I had made this for the Baja SAE team, and I came up with the 70% figure based on the actual speed of the vehicle with that CVT and gearing setup. *We took an old car with similar gearing, and put in a 18hp Briggs instead of the 10hp and that bumped the top speed by close to 10 mph, so the losses were closer to 80%. *There is a lot of aerodynamic drag, esp with a full firewall behind the driver that really slows the SAE cars down.

So assuming the engine has enough horsepower to overcome the losses, then the "theoretical speed of wheel" will be the ground speed. *But the higher that number becomes, the less likely it is to actually be achieved.

Pacman
10-05-2007, 09:21 AM
No fnr box. I probably wont even use the chain case thats in the sled. It doesnt have reverse so its just extra weight. the question was how to use the chart with a cvt, would a fnr box make that much difference?

No difference.....just no sprockets.....gear ratio is set by the gears in the FNR box. I have a buggy with no reverse and for me it sucks big time......woods riding with tight trails can be a real pain when you have to get out and push your buggy backwards. But then again you save $2500 without the box....for me I'd rather work and make the $2500 and have reverse.

butch
10-05-2007, 09:49 AM
I have 3 kids, 2 of which will be getting braces soon. A fnr is not an option at this time. Not to mention the fact that my entire build will cost less than $2500.00.

Thank you livewire.

LiveWire
10-05-2007, 10:33 AM
[quote="LiveWire\";p=\"46702":1d092x96]I think your actual speed cell is not a good calculation. It is assuming top speed will be 70% of calculated top speed disregarding engine power. If the engine has enough power after losses to push the vehicle at the calculated top speed, then they would be the same.

Correct. *I had made this for the Baja SAE team, and I came up with the 70% figure based on the actual speed of the vehicle with that CVT and gearing setup. *We took an old car with similar gearing, and put in a 18hp Briggs instead of the 10hp and that bumped the top speed by close to 10 mph, so the losses were closer to 80%. *There is a lot of aerodynamic drag, esp with a full firewall behind the driver that really slows the SAE cars down.

So assuming the engine has enough horsepower to overcome the losses, then the "theoretical speed of wheel" will be the ground speed. *But the higher that number becomes, the less likely it is to actually be achieved.[/quote:1d092x96]

While I'm sure you don't know the actual drag, you could probably fudge it based on the actual speed vs the theoretical for use with the spread sheet I uploaded. If you combined that spread sheet with yours, you could come up with a pretty good guess as to how much HP you would need to achieve the theoretical speed or what gearing to be used with a bigger engine.

If you cannot attain your theoretical top speed, then lowering the gearing will improve acceleration without hurting top end.

butch
10-05-2007, 11:04 AM
If i did the math right i should have a theoretical top speed of 66 mph using a 12t sprocket on the jack shaft and a 60t on the final with 23 inch tires at 7000 rpm.

butch
10-05-2007, 11:29 AM
I found this calculator to. http://www.juric.org/mopar/tools/topspeed.php
It gave me the same speed as the other chart.