Two speed concept.

[5-7HEAVEN]

2speed, did you know there's a pocketbike transmission with a 5:1 gear reduction? They're inexpensive, dependable and bolt directly onto 76mm clutches. You could incorporate this tranny in your quest to build a dual pulley system.

 

[Deleted Member 4613]

I know. I've already tested the 5/1 gear box. The problem with it is it has
to be mounted with a double right sided drive which I did. It provides
good power but it jerks a little when power is applied. It's not a transmission
though, just a one ratio gearbox. It's just too difficult to drive the
bike and the bicycle chain from the right side.

 

[FNTPuck]

While not applicable for friction drive, you can use a dual centri clutch jackshaft setup with freewheels and make a 2speed transmission. It will increase drivetrain loss of course so top end will suffer, but increased low end and fully automatic. Here is a design from a minibike forum members build for a simple setup based on an old Sears minibike. Other pic is the inside of my kitted Tomos engine with the setup which uses an internal version that works almost the same but with both clutches on the same shaft. Top clutch is 1st gear, bottom engages 2nd, and by playing with spring pressures you can tune when it shifts.

 

[mark20]

looks pretty good for a prototype!

woundering if the same thing could be made with a single roller and a nexus 3 speed hub as the friction roller (just gear it down before the clutch, and it should compensate for the size) then you have 3 gears for speed or torque!

 

[2speedboi]

Update: I need a higher density tire with no knobs, the sand belt has hardly worn down. What I have noticed is the tire is flat across the tread now as a result and I have excellent grip with less pressure. Still waiting to order a smaller reduction ring.

I have maybe 50 miles on the roller, I am having my local shop order some gator hardskin tires and I'll get some real miles on it. Thinking of trying a smaller grit as well. But we will see.

 

[5-7HEAVEN]

I know. I've already tested the 5/1 gear box. The problem with it is it has
to be mounted with a double right sided drive which I did. It provides
good power but it jerks a little when power is applied. It's not a transmission
though, just a one ratio gearbox. It's just too difficult to drive the
bike and the bicycle chain from the right side.

Semantics.
That gearbox as well as everyone’s jackshafts and shift kits are transmitters/transmissions/power transfer units. You don’t need two ratios to be called a transmission on an mb.
For some people, it’s not difficult to drive power thru the right side. That gearbox makes it much easier to get the correct gear ratios.
What’s double right sided drive?
I’ve run the gearbox. I’m unaware of the jerk; I’m simply amazed of the engine transferring power to the wheel.
We can agree to disagree.