This is the EZ part, over here, Harry... Armchair quarterback!
I'll try to explain better...
You have built a semi-circle 'shoe' that pivots at the front, and hold a sheave at the rear which applies pressure to the belt, tightening it up against the two drive pulleys.
The amount of pressure applied to the belt will be determined by spring pressure, (which I haven't yet seen in your design), or by a lever, above that pivot point, which you will operate by hand.
The amount of pressure put on the handle determines how much is applied at the belt by the sheave.
If the handle above the pivot is the same effective length as the 'shoe' that hold the sheave, it's a 1 to 1 ratio, and quite a bit of pressure might need to be applied at the handle. Double the length of the handle, it will take only half the amount of necessary pressure.
Gosh I hope that explains better, what I see from my rocking chair! LoL
IMO, the length of your 'shoe', which holds the sheave, is somewhat longish.
It will require significant pressure at the handle end to get suitable tension applied to the belt so there is a minimum of slip once you need to motivate on down the road.
I do think you will find this to be true. Eventually...
Problem could also be resolved with an over-center cam action in which effective spring pressure forces that sheave against the belt so it will not slip excessively... once the slipping clutch action is no longer needed in a start from a dead stop.
The old harley tank shift motorcycles used a foot clutch. In part, because a traffic cop could always achieve a 'neutral' and quickly stop to direct traffic.
The harley straight lids on the transmissions were not real positive when selecting a gear... and sometimes a neutral could be found between gear selections, but the tranny might jump into a gear on it's own, from the general vibration of the machine.
With a harley clutch, the engagement was accomplished by multi-plates and spring pressure within the clutch hub on the transmission. The foot clutch control moved a lever that it was connected to by a rod.
Old design.
Next in line was the mousetrap...
(which is also a very interesting looking device you might wish to look into.
Glad you're having fun, Harry!
I enjoy your posts very much and appreciate you posting the pics of your progress.
Tnx
rc