possible idea for variable speed belt drive

Discussion in 'Transmission / Drivetrain' started by butterbean, Jan 26, 2014.

  1. butterbean

    butterbean Well-Known Member

    98cc 4 stroke mini bike engine with agk jackshaft. Replace jackshaft output sprocket with belt pulley and install gebe sheave on rear wheel. Use existing chain tensioner (kit tensioner converted to spring-loaded) and attach some sort of pivot-arm such as a jockey shifter to increase/decrease tension on belt. That's my possible idea. Here are my questions: one, will it work the way I think it will (varying the tension on the secondary belt will change the speed/gear ratio even though the primary drive is still chain)? Two, what size pulley would I need for the jackshaft output in order to maintain current gear ratio (realizing I am attempting a multispeed bike so there won't be any fixed gear ratio until I control it with the shifter, but still wanting the pulleys to closely match current setup for a baseline ratio) (current ratio is 12.44:1, 11t at crank, 28t input, 9t output and 44t at rear wheel, the 44t at rear wheel will be replaced by a gebe sheave and I'm not sure how to calculate what size pulley to closely match current ratio). Thought about doing a shift kit, but I think a belt and a jockey shifter would be cheaper and simpler. Thoughts, opinions, and most importantly, ACCURATE info/advice?

  2. SunkyWorks

    SunkyWorks Member

    Russell has done this with a notched (AX,BX) V belt.

    Cruiser with a BTR flavor

    This would be cheaper and stronger than Gebe IMHO.

    If you want to do a shift kit on the cheap, look at an old Shimano Integer FF - Front Freewheel System. I bought a bike for $20.

    Attached Files:

    Last edited: Jan 26, 2014
  3. butterbean

    butterbean Well-Known Member

    Actually, a thumb shift lever could control the belt tension, i.e shift down to increase tension, shift up to decrease. Just need a way to hold the cable in place and attach it to the tensioner. Lessen the slack on the shifter cable, it should increase tension on the spring that is attached to the tensioner. Increase the slack on the cable, decrease slack on the spring and increase tension on the belt.
  4. SunkyWorks

    SunkyWorks Member

    Russell's design shortfall is the shift lever required for the torque to change the size of the variable pulley.

    I think Gebe's belt is a cogged flat belt. I am not sure how you would change the ratio with it?

    Attached Files:

  5. butterbean

    butterbean Well-Known Member

    Ok, currently I have a spring-loaded chain tensioner. The amount of tension on the spring controls the amount of tension on the chain. Switch the chain for a belt and add a shifter cable to control tension on the spring, and you've got multispeed. The tensioner has a bracket that pivots and a spring to apply tension. With a shifter cable attached to the pivoting bracket where the spring is also attached, you could control the tension on the spring, thereby controlling the tension on the belt. Unless there is something I am missing, varying the tension on the belt would vary the gear ratio. This is the basic premise of russel's design, though he controls belt tension with a lever rather than a cable. I could use a selt of brake calipers to hold the cable, but just don't put pads on them and make the calipers stationary so they don't move when I shift.
  6. butterbean

    butterbean Well-Known Member

    Oh I get it. You need a non-toothed belt for variable ratios? Then I suppose I would need non-toothed pulley and sheave as well.
  7. grinningremlin

    grinningremlin Active Member

    Very cool, and surely stronger than GEBE, but what GEBE has on all others is lack of weight.If you are lightly loaded you don't need super strength or gearing, except the bikes.Whizzer-like stuff is intriguing though.
  8. butterbean

    butterbean Well-Known Member

    Basically the most important part of my question is this. If I'm running a primary chain and secondary belt with the clutch on the crankshaft, will simply varying the tension on the belt vary the gear ratio? Or are my assumptions incorrect?
  9. SunkyWorks

    SunkyWorks Member

    The only way to get an adjustable ratio with a flat belt I know of:

    If you find the right kind of centrifugal clutch to expand the diameter of the motor pulley ...

    The clutch will expand causing the tensioner pully to rise up and keep the belt taught.
    The tensioner spring will be just strong enough to keep the tensioner pully in check.

    An extra heavy guard will be required if the clutch explodes.

    Attached Files:

    Last edited: Jan 26, 2014
  10. butterbean

    butterbean Well-Known Member

    I'm not sure if you,re understanding my idea. The shifter cable would control tension on the tensioner spring. I would start with medium tension for a baseline ratio, basically close to the ratio I'm running now. Down-shifting would give the belt more slack, which would give the engine more torque for climbing hills and getting off the line (at least I am assuming). So perhaps this wouldn't be a true variable ratio, more like a clutch that loosens the belt. Now from what I understand about belts, less tension = torque while more tension = speed. I still haven't gotten a straight answer as to whether this is correct or not. This is the foundation of my idea, if it's wrong then I won't pursue this any further.
  11. butterbean

    butterbean Well-Known Member

    Your previous illustration shows that I'm right. But I'm now thinking of a two-speed, using a locking clutch lever. Pulling the clutch lever in would move the tensioner pulley away from the belt, thereby decreasing the tension obviously and putting the engine into a lower gear ratio and allowing for more torque i.e hill climbing and getting off the line. Leaving the clutch lever in the "engaged" position would leave the engine in top gear, or normal cruising speed.
  12. butterbean

    butterbean Well-Known Member

    Got it all figured out, 9t gear on crankshaft, 22t on jackshaft input, clutch with belt pulley on jackshaft output, Whizzer sheave on rear wheel. Locking clutch lever with cable attached to tensioner pulley pivot arm to control belt tension. This will be a 2 speed, clutch lever in locked position puts bike in hill climbing/takeoff gear, clutch lever released puts bike in cruising gear. Locking the clutch lever will loosen the tension on the belt, releasing it will increase the tension. My cruising ratio will be 12:1, really not sure how to tell my hill climbing ratio, but the bike doesn't struggle up hills much as it is so it should be even better with this setup. It's also going on a 1955 Roadmaster frame (current setup is on a repop schwinn).
  13. Jax Rhapsody

    Jax Rhapsody Member

    are you trying to build a variator?
  14. keatonx

    keatonx Member

    Slipping the belt doesn't give it more torque, but it will let the engine get into it's powerband on take off. Just a normal clutch. If you drop a car in 5th and feather the clutch out you still won't accelerate as fast as you would normally. Not to mention clutch wear (or in this case you would wear flat spots into the belt).

    If you use a variator (like on a torque converter/CVT) as your drive pulley and had a spring loaded belt tensioner to pick up the slack, then you would have a variable ratio system
  15. SunkyWorks

    SunkyWorks Member

    A number of mopeds use this system, except the the motor pivots to provide belt tension.

    As cheap as CVT's have gotten on Ebay it would be interesting to try.
  16. keatonx

    keatonx Member

    I'm surprised somebody hasn't already replaced the engine pulley on a GEBE setup with a cheap pocketbike variator
  17. BigBlue

    BigBlue Active Member

    There have been builds that use pocket bike CVT, but they just don't hold up without modification.

    AKA: BigBlue
  18. HeadSmess

    HeadSmess Well-Known Member

    simply, with no jargon


    you require a "variable pulley" which consists of two discs, held together by spring tension. belt rides on the outer diameter of discs for high gear (if this pulley is the DRIVER) and the discs spread apart on the shaft, against that spring, which then lets the belt ride near the shaft, or the inner diameter of the two discs, which will result in LOW GEAR.

    the ideal setup is TWO such pulleys, as one gets larger, one gets smaller, and no belt tensioner is required! using only one variable pulley will require a belt tensioner, and also, the belt will experience some offset if only one disc moves. if both move an equal distance from the centreline, the belt will remain on the centreline.

    you could use a single pulley, and vary the ratio by changing the belt tension, as long as the spring holding the two discs of the pulley together, is slightly weaker than the tensioner...

    i used jargon, didnt i?

    im sorry :(

    simply reducing the tension on the belt alone will act like a clutch. and when it slips, you will burn your belt out. smoke, heat, squealing... think a car when the belts loose/worn out and its steering is turned to full lock. the belt will HOWL!

    you NEED a variable pulley. be it the two discs that move together or apart (commercially available) or an expanding drum type (um....never seen one...)
  19. HeadSmess

    HeadSmess Well-Known Member

    i was beaten to it. :iagree:
  20. butterbean

    butterbean Well-Known Member

    Ok, two more questions. One, if I use a variator pulley, can I manually control the speed by manually controlling the belt tension with a lever-actuated idler pulley (same idea as above, only in conjunction with variator pulley). Two, anyone know where to find a variator pulley that can fit a 5/8 keyed shaft?