Figuring out gearing with a CVT

[sno-kart]

For our go kart, we are putting a 440cc snowmobile engine with a CVT onto a race kart style frame. We are trying to figure out the gearing, but it's difficult to do since the gear ratio for the CVT is never constant. Does anyone have any idea what the ratio of a snowmobile CVT tends to be when the engine is at max RPM? For example, our engine is out of a 1981 Kawasaki LTD 440. Max RPM is 8200. The thing we need to know is: at that point of highest RPM, what is the ratio from the driving clutch to the driven clutch on our jackshaft? I can't seem to find this info anywhere unfortunately and it makes it hard to choose sprocket sizes from our jackshaft to our axle. Below is a video of what the CVT looks like in a snowmobile very similar to ours (jump to 0:36 in the video to see the engine revving.) From the video, it looks like at max RPM, it is close to a 1:1 ratio, but it's really hard to tell.

Any help figuring this out is greatly appreciated, thanks!

http://www.youtube.com/watch?v=NnHnFDb2Jxg.

 

[Doc Sprocket]

at that point of highest RPM, what is the ratio from the driving clutch to the driven clutch

That's really only half of the equation. It's not just the highest RPM, it's the highest speed, too. Remember that both the drive unit and the driven unit have variable effective diameter, and the driven unit (basically) responds in direct correlation with road speed.

Now, that having been said- What you need to determine is the overall high ratio (say, 1:1) and gear for that. In the absence of a spec sheet, you maya have to partially disassemble things. With the springs 'n' things out of the units, you can visually determine the effective diameters. Manually adjust the driver unit to its largest effective diameter, with the belt wrapped at least halfway around it. Measure the outer diameter of the belt. Now, manually place the driven unit into it's smallest effective diameter with the belt wrapped around it, and measure that diameter. Dividing the two numbers will give you a high ratio, which you can then factor into your overall ratio.

 

[mysteryboy28]

1:1 is all you need to know. ignore the CVT ratio. it's not important. all it does is change you from torque to speed.

you mentioned in an email to me that you were thinking about ditching the chaincase. bad idea, as it really DOES affect your gear ratio! they usually have around a 29:17 gear ratio, so once you've got a 45 tooth sprocket on your axle the motor doesn't try to take you to 200 mph in a week and a half.

if you utilize the motor mount from the sled, then you can simply weld a 10 tooth spocket to the jackshaft coming from the bottom of the chaincase. you will probably need to lightly grind down the hex edges of the jackshaft to the point of there the 10t sprocket will fit perfectly over it. then line it up with your axle sprocket PERFECTLY and weld it into place on both sides (using a slow wire speed and hi temp for max penetration). then hit it with a grinder polish wheel (60 grit flap disk) to smooth out the welds so the chain doesn't hit them.

 

[Doc Sprocket]

1:1 is all you need to know. ignore the CVT ratio. it's not important. all it does is change you from torque to speed.

you mentioned in an email to me that you were thinking about ditching the chaincase. bad idea, as it really DOES affect your gear ratio! they usually have around a 29:17 gear ratio, so once you've got a 45 tooth sprocket on your axle the motor doesn't try to take you to 200 mph in a week and a half.

if you utilize the motor mount from the sled, then you can simply weld a 10 tooth spocket to the jackshaft coming from the bottom of the chaincase. you will probably need to lightly grind down the hex edges of the jackshaft to the point of there the 10t sprocket will fit perfectly over it. then line it up with your axle sprocket PERFECTLY and weld it into place on both sides (using a slow wire speed and hi temp for max penetration). then hit it with a grinder polish wheel (60 grit flap disk) to smooth out the welds so the chain doesn't hit them.

Curious: do ALL sled CVT's have a 1:1 high ratio?

 

[mysteryboy28]

prolly not. but i don't think that the ratio of a torque converter is really something to confuse a newbie with. the chaincase will have more of an overall impact than the CVT will... maybe... lol.

i gear my buggies for around 9:1 (more or less depending on tire size), without factoring in the CVT, and the gearing is awesome. tons of speed and torque!

 

[Doc Sprocket]

No argument there. I personally would verify the ratio and factor it in. I did not get into how you could play with the numbers, to avoid confusion.

Problem is, I don't play with sleds, so I have no idea what the CVT ratio range may be. However, if it significantly away from 1:1, it can have a profound effect on calculations.

 

[exenos]

Typically sleds have a 10-20% overdrive so .9:1 to .8:1 final ratio. The initial ratio is around 3:1-4:1 depending on the manufacture or clutching.

If this is going on a racing kart frame then I would ditch the sled frame and run the engine on a custom mount because it will be much lighter and more compact. Just make sure you learn about aligning the clutches properly, if they are out of alignment you will chew up belts.

 

[mysteryboy28]

yeah, so pretty much 1:1, not enough to make worth the hassle and confusion. i'd just focus on the chaincase gear ratio and the jackshaft and axle sprockets. that 10-20% overdrive isn't even going to kick in till you've got plenty of momentum anyway.

 

[Doc Sprocket]

Maybe not, but it gives you a 20% margin to gear lower for tire-frying torque while still hitting the same theoretical top speed as a 1:1 ratio would net. Maybe it's just me, but I'm not sure I'd consider a 20% OD insignificant.

 

[OzFab]

What you need to determine is the overall high ratio (say, 1:1) and gear for that.

I have a question:

When using a CC, we recommend a final gear ratio around 6:1 as, any higher & you lose torque & risk damaging the clutch. Now, with a CVT, there's a lot more "wiggle room" so you could gear a lot higher.

Hypothetically, let's say some nut gears for 3:1; how would this effect the low end drive of the CVT?

 

[Doc Sprocket]

Well- that's further than I'd push it. You're into belt-burning territory. I'm talking about a few percentage points. 10, 15% at best, not 50%.

 

[exenos]

The cvt would not shift out, it would force the engine to stay at a low rpm. There would also be excessive belt wear.

EDIT: Ninja'd

 

[Doc Sprocket]

The cvt would not shift out, it would force the engine to stay at a low rpm. There would also be excessive belt wear.

EDIT: Ninja'd

:ninja: LOL

 

[mysteryboy28]

when you've got 7000-8000 RPM to work with, 9:1 or even 10:1 will give you crazy torque AND good top speed.

 

[r97]

I'm going to agree with toystory here. If you are trying to calculate a ratio, you should really find out what numbers you are dealing with. Knowing what your chain-case's ratio is, and the max over/under drive of your CVT can make a huge difference. Let me use mysteryboy's setup as an example. I'm assuming a:
-7500rpm engine red-line
-17:29 chain-case ratio
-10:45 final ratio
-22" OD drive tires

Now if I say that the CVT can only achieve a 1:1 high ratio, your theoretical top speed would be about 64mph. If I say that the CVT had a .8:1 overdrive, your theoretical top speed would be boosted up to about 80mph... that is not a negligible difference. Calculating this ratio correctly the first time can save time and money with fine tuning your ratio later.

Math:
Eng rpm/((chaincase bottom sprkt/chaincase top sprkt)(axle sprk/js sprk)*CVT)(tire OD")*.00297499304=mph
7500/((29/17)(45/10)*1)(22)*.00297499304=63.9452526989
7500/((29/17)(45/10)*.8)(22)*.00297499304=79.9315658736mph

 

[mysteryboy28]

i agree.

 

[sno-kart]

1:1 is all you need to know. ignore the CVT ratio. it's not important. all it does is change you from torque to speed.

you mentioned in an email to me that you were thinking about ditching the chaincase. bad idea, as it really DOES affect your gear ratio! they usually have around a 29:17 gear ratio, so once you've got a 45 tooth sprocket on your axle the motor doesn't try to take you to 200 mph in a week and a half.

if you utilize the motor mount from the sled, then you can simply weld a 10 tooth spocket to the jackshaft coming from the bottom of the chaincase. you will probably need to lightly grind down the hex edges of the jackshaft to the point of there the 10t sprocket will fit perfectly over it. then line it up with your axle sprocket PERFECTLY and weld it into place on both sides (using a slow wire speed and hi temp for max penetration). then hit it with a grinder polish wheel (60 grit flap disk) to smooth out the welds so the chain doesn't hit them.

Two things I don't understand in your comment above. Probably because I'm new to this, but...

Why would one ignore the CVT ratio? It seems very important; if the ratio from the drive converter (on the crankshaft) to the driven converter (on the jackshaft) is not 1:1, the jackshaft is spinning slower or faster than the crankshaft and that must be factored into the gearing from the jackshaft to the axle. If it is 1:1, then it makes the math easier since the jackshaft will be spinning at exactly the same RPM as the crankshaft (8200 in my case). Then, using a regular speed calculator such as this one, http://www.csgnetwork.com/kartpotentialspeedcalc.html, one can put in the diameter of the drive wheel, the max RPM, and the number of teeth on each sprocket to figure out the maximum speed. In my case, with 12" tires, a max RPM of 8200, and a desired top speed of 70mph, I could use a 13 tooth sprocket on the jackshaft and a 54 tooth sprocket on the axle, which is not unwieldy. Perhaps this would not work for heavier vehicles with bigger diameter wheels, but it seems it would work fine for a race kart application such as ours.

The second thing I don't understand is the importance of the chaincase. I took mine apart and it looks like it does nothing more than apply gear reduction (as you said, about 1.7:1) from the jackshaft to the drive axle on the snowmobile which runs the tread. When applied to a go kart however, you need another chain from that drive axle to the rear axle. So to me that seems like an unnecessary chain, sprocket and axle, all of which add weight, take up space, and cost money. This means the kart has a belt, 2 chains, and 3 axles when it could have 1 belt (from drive converter to driven converter on jackshaft), 1 chain (from the jackshaft to the rear axle), and 2 axles (jackshaft and rear axle).

Again, I am new to this so I may be missing something important or not considering certain factors so I'm looking to learn... thanks!

 

[r97]

I think you are spot on with what you are saying. The ratio of the CVT does matter (As I discussed in post 15). I also believe that the either the chain-case or the secondary chain would be unnecessary for your application. One thing I am concerned about is the power you will have, it might be more than you would want in a race kart. It might be worthwhile to gear for a higher top speed, even if you don't need it, just to keep from burning up tires every 30min of driving.

 

[sno-kart]

Good point r97... I was concerned about too much power as well but I'm starting to think the kart is going to be fairly heavy unfortunately... around 350lbs without a driver probably :/

And thanks for doing the math in post 15... very helpful!

 

[Doc Sprocket]

Two things I don't understand in your comment above. Probably because I'm new to this, but...

Why would one ignore the CVT ratio? It seems very important; if the ratio from the drive converter (on the crankshaft) to the driven converter (on the jackshaft) is not 1:1, the jackshaft is spinning slower or faster than the crankshaft and that must be factored into the gearing from the jackshaft to the axle. If it is 1:1, then it makes the math easier since the jackshaft will be spinning at exactly the same RPM as the crankshaft (8200 in my case). Then, using a regular speed calculator such as this one, http://www.csgnetwork.com/kartpotentialspeedcalc.html, one can put in the diameter of the drive wheel, the max RPM, and the number of teeth on each sprocket to figure out the maximum speed. In my case, with 12" tires, a max RPM of 8200, and a desired top speed of 70mph, I could use a 13 tooth sprocket on the jackshaft and a 54 tooth sprocket on the axle, which is not unwieldy. Perhaps this would not work for heavier vehicles with bigger diameter wheels, but it seems it would work fine for a race kart application such as ours.

The second thing I don't understand is the importance of the chaincase. I took mine apart and it looks like it does nothing more than apply gear reduction (as you said, about 1.7:1) from the jackshaft to the drive axle on the snowmobile which runs the tread. When applied to a go kart however, you need another chain from that drive axle to the rear axle. So to me that seems like an unnecessary chain, sprocket and axle, all of which add weight, take up space, and cost money. This means the kart has a belt, 2 chains, and 3 axles when it could have 1 belt (from drive converter to driven converter on jackshaft), 1 chain (from the jackshaft to the rear axle), and 2 axles (jackshaft and rear axle).

Again, I am new to this so I may be missing something important or not considering certain factors so I'm looking to learn... thanks!

You've pretty much nailed it!