VPI Classic tweak

No different with the SDS, different pulley positions are different diameters and will yield different speeds. You can verify it with the friekert app and test disc. The different pulley positions are for those that don't have the SDS to get as close as they can to 33 or 45 since without the SDS you have no control over AC frequency. With the SDS you don't need all the different pulley diameters since you can control AC frequency and optimize any of then for perfect speed. Adding another belt at different pulley diameter just upsets that balance.

Interesting. I'd think that the pulley is manufactured to very tight specifications and differences, if any, between different positions in the same speed diameter would be too insignificant to affect the speed. I attributed the small variances in speed among the different positions to the differences in the pull on the platter and bearing caused by the different height relative to the bearing. I would think that without the SDS using two belts would result in a certain speed that could then be dialed in precisely with the SDS if necessary, rather then the belts fighting. In other words, whatever the result of their "fighting," it could be adjusted perfectly by the SDS.

I don't agree. Check it with the friekert app and test disk, each position is a different diameter resulting in a different platter speed. Whether you use a SDS or not, the 2 belts will be spinning at 2 different speeds.

VPI does that for enthusiasts like me that don't have SDS,and no control over AC frequency, so we can obtain correct speed by changing belt position on the pulley, to go faster or slower.

Captain_winters,
I don't quite get how you reached the conclusion that "each position is a different diameter." I agree that each position might result in slightly different speeds, which two belts might or might not even out without the SDS. But why couldn't you dial it in with the SDS whatever the effect of using two belts? Regardless, it seems to me that the effect of using two belts would be more complicated than reducing it to an assertion of "fighting," assuming the belts are of identical length.