I would not think the surface area would make that much difference, but I have not built that type of engage / disengage drive pulley by this method. Hope there are other people to respond with specific experience!
I did just do two days of work and got the stuff I wanted to do without a welder. I am putting it off getting a Hobart capable of 1/4 inch till I have more of a need for it and get it as MIG ready with all except the cylinder when I do. I've seen them on Amazon for about $375.00
With the pulleys that I used with the belt I have, I felt I had to make the belt tension tight enough that I could hold one pulley still in one hand and try to turn the other. I was not able to turn the other and did not have any slippage, it was that tight.
I added one link in this belt and had is slip just with enough pressure that it would not hurt my hand, but it did slip. It seemed to be about ½ inch longer with the one link added.
I took the link in the belt back out as I felt necessary to have it be sure I could climb hills. The thing that had me initially wanting to make the belt looser was that the Rear wheel would turn freely on the bearings with the belt off, but not with the belt on. I must have forgotten that that is the way belts on pulleys are supposed to be. The two bearings on my jack shaft freely roll the shaft, but not so easy when the belt is on the pulley.
Thinking of your situation I would think you need to have the belt that tight too. The amount of tightness that you can then loosen too, maybe the range is not good enough.
If that is the case you may see if you can get more movement to get a greater range. Possibly too, you may add some way of getting mechanical advantage, where you move the clutch handle and it is like geared up to move twice or more the distance at the pulley.
Now that I think of it, the hinge point on what you have does the opposite where it is like gear down. The fulcrum is down below with the longer portion going toward the handle.
One other thing, I am not sure I have ever heard of, but the way the pulley type centrifugal clutch I have for v-belt (which is not in my current build design), gains traction with the belt by squeezing from the sides. The sides then being at an angle then naturally have the belt pushed away the center of the pulley and essentially create a larger effective pulley. If this is something that is a possible method to have the belt tighten with a lever, maybe some scheme of that has already been in use, and you may do that if necessary.
I would think what you have as it is should work as I have heard others doing the same. Do some searches on this MB site to see if they had any issues that you have with this system.
I am soon to get the tank with gas on the frame and give it a test. I have not made a motor bike using both chain and belt with jack shaft, so I'm hoping it goes well.
This guard / guide that you may make for the situation you found, how does this exactly work? Is there some place that I can see pictures of how it looks?
Thanks
Measure Twice