ehhh not exactly.

For instance:

2.9/7.9= 2.72

Now lets add the .3" of diamater the ribs add on both sides of the pulley
3.2/8.2= 2.56

 

Yeah, I thought about this after I posted. Thanks for the example.

 

I'll though in my 2 cents, lol. I'd wrap a belt around the pulley and then measure to the back side of the belt. That way you would have a true representation of diameter where the belt is riding in the pulley for a measurement.

And different belts can ride at different heights on the pulleys depending on construction.

 

So I measured the belts I have at home, and the actual rib depth is only about 0.05", which is shallower than I was expecting, but you never want the ribs to bottom out in the pulley. I had my rib spacing about perfect from eyeballing it which I was pleased about. Overall belt thickness was about 1/8".

 

I did some reading and what you should be looking for is the pitch diameter of the pulley. It is at the fibers in the belt when it is wrapped around the pulley. So you can cut a belt and measure from the back of the belt to the fibers then subtract twice that from the diameter of the pulley with the belt wrapped around it. That should be the effective pulley diameter.

 

To me it would make sense to measure from the top of the ribs of the pulley, which is the same as the bottom surface of the belt. I think this is the same thing as you described. Who knew measuring a pulley was so hard!

 

Lol. Well, I don't know if the belt fibers are at the bottom surface, I would think they are somewhere in between the top of the ribs on the pulley and the smooth belt surface. The top of the ribs is probably a close enough measurement that it won't make a huge difference though. I will have to cut a belt that I have lying around and measure where the fibers are. Maybe this weekend.

 

I mean I measured a belt this morning, didnt have to cut it, just bend a portion back so you can see it. I measured total height of 0.125" and rib height from the bottom of the belt at 0.05", so belt thickness w/o ribs is 0.075". The valley in the pulleys best I can tell is around 0.15" or almost triple what the actual belt rib height is.

 

Check out this waterpump i ran across

Wide pulley LS3 water pump - for 10 rib belt drive

 

Way to steal my thunder there champ