The sand won't have much effect. The drum diameter is what?, 7"?. Not very large. To get much effect from the sand, you would need all of it right at the 7" (3 1/2" from the center of rotation) but of course, most of it is a lot closer to the center of the drum where it is doing virtually nothing to add inertia. (The grindstones that coast, the old pedal driven stones, have diameters like 2-3' (or more). The inertia for rotation is based on the square of the radius. Your roller is (say) 3 1/3". The grindstone is (say) 30". ( 30 / 3.5 ) ^2 = 15.6 That grindstone has nearly 16 times the inertia effect and it has no belt to drive, no front roller and no front wheel. (True, you have bearings for all rollers and hubs vs steel on steel for the roller axle and pedal rods but it has solid rock, not ground sand so that probably balances out.
Ben