CVT Rackmount with CVT?

Discussion in 'Transmission / Drivetrain' started by heathyoung, Sep 4, 2008.

  1. heathyoung

    heathyoung Member

    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.

  2. arceeguy

    arceeguy Active Member

    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. ;)
  3. heathyoung

    heathyoung Member

    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 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.


    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...
  4. arceeguy

    arceeguy Active Member

    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.
  5. heathyoung

    heathyoung Member

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

    loquin Active Member

    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)
  7. arceeguy

    arceeguy Active Member

    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!
    Last edited by a moderator: Dec 16, 2015
  8. sparky

    sparky Active Member

    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.
  9. heathyoung

    heathyoung Member

    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...
  10. duivendyk

    duivendyk Guest

    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
  11. sparky

    sparky Active Member

    I cheated and bought this folding bike from for $500. It came with the dual freewheel hub. It's a Chinese Tanaka knock-off --> 5:1 reversed, compact (Dax?) gearbox --> 11t sprocket --> 44t freewheel --> hub on 20" rims, dawg... for an overall ratio of 20:1. Next to the 44t freewheel is a standard 12t KMC freewheel which is unusually problematic, tho. I've only taken the back wheel off twice, and I'm pretty sure the KMC freewheel will just spin off... so I'm going to replace that very soon. The 44t freewheel is actually a freewheel that just uses the "clamshell" clamping method to attach any appropriately fitted sprocket (inner hole of sprocket must be proper fit and be able to accept 4 properly spaced nuts & bolts).

    Anyway, if you figure out the CVT, gearing, idler gear, etc... just call the guy at FiveFlags and he'll prolly be able to get you a 20" or 26" dual freewheel wheel already laced by itself.
    Last edited: Sep 9, 2008
  12. heathyoung

    heathyoung Member

    Thats uber cool. Exactly what I was looking for. With a CVT that would be perfect!
  13. sparky

    sparky Active Member

    Glad to hear it. Can't wait to see what you put together!!
  14. stude13

    stude13 Active Member

    hi; i dont get the math--help please. i have an occ chopper and a high zoot 47cc. i ordered a cvt. a range of teeth is availlable. question is if i install a 36 tooth on the rear what should the drive gear be to get 25-30mph.? mitch
  15. sparky

    sparky Active Member

    stude... do you know what RPM the engine is supposed to get?

    For example, if it gets 6000 RPM & have a 26" rim...

    6000 RPM / (3.15 * 36/x) = ___ RPM * 60 Min = ___ RPH * Pi * 26 In = ___ In per Hour / 12 In / 5280 Ft = ___ MPH

    If x were a 5t drive sprocket, you'd get 29.45 MPH, I believe.
  16. sparky

    sparky Active Member

    Oh yea, and to correct one of my previous posts... the compact gearbox is 3.11:1 not 5:1, so my overall gear ratio is 12.44:1 with 20" wheels. Not that it should matter too much in this thread, but oh well...
  17. stude13

    stude13 Active Member

    thanks sparky! now to work.
  18. heathyoung

    heathyoung Member

    The compact gearbox is the one on the 49cc 2 strokes that are kicking around on ebay, right? (ie CCW to CW) or the chain drive internals (CW to CW) - the CCW to CW is the one fitted to the drive of the CVT. Ratios still stand for the CVT though.
  19. sparky

    sparky Active Member

    Yes, if staring at a clutch spinning CCW, the gearbox turns the driveshaft CW. As far as I can tell, the ones on eBay are the same... but I remember the sprocket being a bit different from mine. Seemed like mine loked thicker or was for a different type of chain -- a BMX chain.
  20. heathyoung

    heathyoung Member

    Thats Ok, I'm looking at getting one of the "X2" CVT gearboxes (which have a 18 tooth sprocket) and one of the chain reduction boxes (which have the same pitch/keyway etc sprocket, but with 11 teeth) and a 54 tooth sprocket for final drive.

    The chain to suit is 6mm wide (internally) with a 11mm between links. Not sure what size that is compared to everything else...

    The 7 speed cassette I'll probably remove the 7th speed cog on a lathe or something, and modify the shifter so it wont go into 7th. The 54 tooth final drive will mounted as per the HT drivering (clamped to spokes with rubber isolators) but I will probably countersink the bolts into the drivering - just to reduce the possibility of the drivering interfering with the gear cassette. I decided against flywheel, mainly because I don't know how the CVT will respond, could be very laggy, or risk over-revving the motor.

    The mounts on the transmission appear to be rubber vibration isolating, and they appear also to be the engine mounts as well (mounted on the swingarm), so this is also a big bonus to cut down on vibration.

    The gearbox is also short, so the motor can be mounted closer to the tyre, to reduce the profile, probably will add a mudguard as well if there is room.

    Just need to find an Australian seller of these CVT boxes :( There is an ebay seller that will ship, but the box is $40, and shipping is $50 (EEEEEP!)

    Low range is 32.21:1, high range is 15.47:1 - Very nice :)

    With a 49cc 2Hp 2 stroke, this thing will fly. Pity about the stupid 200W rule. My current motor says 200W, I think I'll use it (31cc 4 stroke).

    It will be nice to get the back wheel into a bike rack, stupid gebe pulley doesn't fit the bike racks where I live...
    Last edited: Sep 11, 2008