| PF are harder than '100A' (100A is max of the A-scale, 104a is an extrapolation that can't be measured with the durometer in A-scale mode....), so they can be rated on the B-scale only. For different brands (or even 'millesime'), you have different grip for the same hardeness, so the shore index (A ou B scale) is only one among many other parameters that keep busy on rainy days....
Some interesting ints from Powell forum:
"the PF is our Powell version of the SPF. we use different terms to differentiate between the two brands, Powell and BONES."
"DTF's are 95a, STF and SPF wheels are harder than can be accurately measured by the A scale, but this is the only scale that skaters have ever gotten “calibrated” on and understand, so we had a choice of calling them 104A as we have in the past, or taking a different direction and stating what they were designed for, street or park. Getting into how we came up with the number 104A is an interesting story, and part of the Bones legacy, but perhaps more technical than most skaters care about. Also, I think if the kid reads the wheel as 104, he’s gonna think it’s too hard. But the urethane that we use—it needs to be that hard to ride the way it does. So we just use STF Street Tech Formula and SPF Skatepark Formula as the hardness. I think what we need to do is educate the kids on the B scale so that they understand it. It’s gonna take some time, but if everyone gets into it, we can have a way more accurate reading on wheels. My 101A wheel could be completely different from somebody else’s 101A wheel just because of the urethanes they use and because it is a guesstimate. There’s too many variables involved there."
Source:
http://www.skateone.com/forum/comments.php?DiscussionID=5
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