CVT Rackmount with CVT?

heathyoung

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Hey all - has anyone done this yet?

Using a 76mm clutch CVT gearbox as designed for the cheap chinese ATVs/pitbikes, with a HT/pitbike rear sprocket?

The gearing looks OK...

CVT - Low range = 6.56:1, High range 3.15:1
Stock output pulley is 18 tooth, but you can get 11 tooth that fit.
Rear sprocket - 54 (minibike) or 48/44 (HT)

So... with an 11 tooth on CVT, 54 rear sprocket - 4.91:1

Combined, Low range is 32.21:1, high range is 15.47:1.

Doable? Ratios look pretty good, or is my maths bad?

Even with a 48 tooth HT (4.36:1), ratios are still good for top speed + low end
Low range is 28.6:1, high range is 13.734:1

The high range assumes that the motor reaches high enough RPM, if it bogs the ratio increases to compensate.

The metalwork is reasonably simple as well, and the extra weight of the gearbox on the other side of the motor will fix up the bad weight balance issues of the bike.
 

arceeguy

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This is exactly what I am working on with my homebrew project.

Stumbling blocks are:

Output of CVT is on the "wrong side" of the rear wheel, so an idler shaft is needed to move the sprocket to the "drivers side" vs. the "passengers side" :p

Pocket bike sprockets use a different chain size than "HT" sprockets.

It really isn't all that difficult, but it does require some basic metal working tools and a welder. For now, I am building a couple of HT powered bikes and the CVT project is on hold.

Also - rather than "cheap Chinese ATVs/pitbikes", you could have just said "cheap ATVs/pitbikes". Adding race to the description is unnecessary. My friend owns a Volkswagen that was built in Mexico. It is a POS, it burns oil like a two stroke and is literally falling apart at 4 years old. But it is not a Mexican POS, or a German POS. I don't describe the engine as a mexicangine because it is a POS built in Mexico. A POS is a POS, regardless of where or who built it. ;)
 

heathyoung

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I was thinking that it would be rather a problem with the reduction gearbox on the CVT being CW rather than CCW, I did some reading and the box is a reduction drive using cogs rather than belts.

Drivers vs passengers side is confusing - esp considering LHD/RHD :)

The seller's own description on https://www.ebay.com.au actually includes "chinese pitbikes" etc. Its probably a cultural thing, I dunno, but you need to differentiate somehow. Its not really worth getting excited over.

Hmmm....

Other possibilities...

Swap the 7 speed cassette for a 6 speed, machine a single speed freewheel to fit a pitbike sprocket (probably machine the pitbike sprocket so it fits over, then weld)

Stick the 54 tooth freewheel sprocket behind 6 speed cassette... Or don't bother with a freewheel, machine 1 sprocket off a 7 speed cassette, make up an adaptor as per HT mount with countersunk screws for clearance - don't need to use jackshafts etc.

Thats probably the easiest way to do this I think...
 
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arceeguy

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......The seller's own description on ebay.com.au actually includes "chinese pitbikes" etc. Its probably a cultural thing, I dunno, but you need to differentiate somehow. Its not really worth getting excited over...........

Just because others use the term, doesn't make it right. (notice the eBay seller didn't say CHEAP Chinese pitbikes, but used Chinese pitbikes as a generic term without the pejorative "cheap".) I'm not getting "excited" over it, just pointing it out and asking people to think before they trash.

As far as having both chains on the same side, I think it is a little too risky IMO. If one chain jumps, it will probably get tangled with the other and things can get pretty messy from there. That's why I am going to use an idler shaft.
 

heathyoung

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Hmmm.... Good point, but I've had chains jump on normal mountain bikes - without a chain guard you smash spokes :(
 

loquin

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Per the Comet folks, a new belt is about 95% efficient at transferring power from the input shaft to the output shaft of the CVT, with a new belt. As the belt wears, this efficiency gradually drops, so you need to be sure to perform regular maintenance on the CVT to ensure that the belt is not wearing too fast, and to replace the belt as needed. (Our small motors don't have too much leeway in that regard)
 

arceeguy

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Hmmm.... Good point, but I've had chains jump on normal mountain bikes - without a chain guard you smash spokes :(

This is what makes those jackshaft "shift kit" conversions a potential problem. A dérailleur system wasn't meant to shift under engine power, and it can be a hazard. The CVT solution gives us variable ratios without putting engine power through the dérailleur system. I think we could be on to something here!
 
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sparky

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As far as having both chains on the same side, I think it is a little too risky IMO. If one chain jumps, it will probably get tangled with the other and things can get pretty messy from there. That's why I am going to use an idler shaft.
That's how my bike's setup, and the engine chain has popped off several times without tangling up in the spokes or with the other chain. As a matter of fact, I don't see how it could possibly get caught up with the other chain at all.

It could possibly get caught in the spokes if it fell just right, but I don't see how it'd be any different than having the sprocket on the left side. The only way for me to adjust the alignment is to move the back wheel until both of the chains are just right. I think this actually causes my wheel to get out of alignment, just for the sake of aligning the chains. I could align all three of them, but I'd have to fix the way the chain tensioner is setup. I'll need to pull out a non-standard tool that isn't in my tool bag for that, tho... and I'm always reluctant to grab the metal shaping tools for some reason.
 

heathyoung

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How did you get both chains on the same side?

Machine off a gear set on the cassette? Freewheel or solid? Makes it look like an interesting solution if it could be persuaded to work.

Using the bike's gears with that much torque is a bit risky methinks. There are a few conversions out there, but the big pain is to get hold of a freewheel crank with two freewheels. There are a few asian sourced ebike kits that use these, would love to know what the ratios are though...
 
D

duivendyk

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The ratios are in the ballpark:at 6k rpm from 13.7 to 31.5mph.The deraileur based setup has some potential gotchas,the crossing chains don't bother me as long as there is adequate spacing and no excess lateral slop.The dinky 3/32 chain to the deraileur leads a challenging life,but with a clutch you don't need to shift under power and the peak torque is not likely to be any higher than with pedaling.Chains are not expensive,just replace it as part of a regular maintenance routine
 
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