possible idea for variable speed belt drive

butterbean

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

Cruiser with a BTR flavor
http://motorbicycling.com/showthread.php?t=44397

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.
 

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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.
 
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?
 

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

Cruiser with a BTR flavor
http://motorbicycling.com/showthread.php?t=44397

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.
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.
 
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?
 
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.
 

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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.
 
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