belt drive conversion

I would suggest studying the whizzer section of the forum to get familiar with what is involved. Also look up the builds of msrfan. He is very knowledgeable about belt driven bikes. Good luck!
SB

If you are going to run a CG it is already geared down to the point where a Whizzer rear sheave would probably be too big. Would make a slow bike that would climb like a mountain goat! I converted a Honda 70cc clone engine to a belt drive(primary) by simply welding a well centered pulley to the engine drive sprocket. Should work in a similar way on a CG. I would think a rear pulley could be fabbed pretty easily from a sheet steel pulley like from a riding lawnmowers trans input pulley. Open a hole with a hole saw big enough to clear the hub and figure a method to clamp it to the spokes, not horribly difficult. The belt will have to be well tensioned though to pedal start the GC.
A belt drive is worth the effort IMO if properly done because they are quiet and very smooth.

You need to establish your current ratio. Divide the # of teeth on the rear sprocket by the # on the front. Lets just say you have a 10t front and a 44 rear. That's a 4.4/1. To get a similar ratio would require something like a 2" front pulley driving a 8" rear or any combo that gives a similar ratio. A 2" front is about as small as I would use as a driver due to belt area. An idler placed close to the pulley and keeping the belt wrapped into 180 degrees of the pulley would be good both for driving and pedal starting. If the idler is made moveable you could release the belt for nearly drag free pedaling.
I had a build that had a hand engaged belt idler that easily started a 5hp engine.

Heres a link to another build of mine with a belt drive showing the idler set up. Works really well nothing to adjust.

Also what is the internal reduction of the CG? If its not over 4:1 then a Whizzer sheave would work. I had a couple of CGs years ago and just remember the gearing looked pretty low, but never counted the teeth to know. Might work just fine as is. http://motorbicycling.com/showpost.php?p=269173&postcount=48

I am trying to get the pulley size figured out. I am going to run a belt from my 79cc predator to a jackshaft with an 18t 20" rear wheel #41 sprocket on the output side of the jackshft and rear wheel of my stingray chopper bike and a 2" x 1.55" pulley on the input side of the jackshaft. So what diameter pulley do I need on the motor to get a decent amount of torque to the rear wheel and approx. 20 to 25 mph at the rear wheel top speed?

John76 said:

I am trying to get the pulley size figured out. I am going to run a belt from my 79cc predator to a jackshaft with an 18t 20" rear wheel #41 sprocket on the output side of the jackshft and rear wheel of my stingray chopper bike and a 2" x 1.55" pulley on the input side of the jackshaft. So what diameter pulley do I need on the motor to get a decent amount of torque to the rear wheel and approx. 20 to 25 mph at the rear wheel top speed?

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If you ran a 2.5"/6" pulley combo(or any combo that gets a 2.5:1 reduction) on the primary you would have an overall ratio of around 13:1. This would get you where you are wanting to be. Good hill climbing and a top in the low to mid 20s@ 4000rpm which the little Preddy will easily do. I wouldn't go any smaller than 2.5" on the front pulley due to belt area considerations. Most cent. clutches have a 3" pulley if you are using one of them the secondary pulley will have to be bigger. This can be a problem as the rear pulley needs to be 7.5". You can run the 6" but the overall ratio will drop to 10.28:1 Should be ok with the 20" well but will loose a bit of hill climb ability. You can juggle the sprocket sizes also to get the ratio lower. A good middle of the road ratio is 12:1 how ever you get it.