From: grr@cbmvax.commodore.com (George Robbins)
Newsgroups: rec.skate
Subject: Re: Wheel lore
Date: 31 Dec 92 06:07:55 GMT
Reply-To: grr@cbmvax.commodore.com (George Robbins)
Distribution: na
Organization: Commodore, West Chester, PA

In article <C033JA.3MJ@csn.org> pae@teal.csn.org (Phil Earnhardt) writes:
> In article <38196@cbmvax.commodore.com> grr@cbmvax.commodore.com (George Robbins) writes:
> >In article <C01Apx.Lt9@csn.org> pae@teal.csn.org (Phil Earnhardt) writes:
> >> A number followed by an "A" is a durometer, or rebound, rating. [...]
> >
> >Uh Phil...  Durometer is simply "hardness", i.e. the distance the indicator
> >plunger will depress the material with a given spring force.
> >
> >Simply put, rebound == bounce.  High robound is generally a good thing, since
> >the energy absorbed from impact/vibration is returned, however various
> >additives might be needed for durability which would detract from the
> >rebound.
> >
> >>Typically, for rougher road surfaces, you'd like to use a lower
> >>durometer number. [...]
> >
> >this is still correct anyway...
> 
> Can you talk about the relationship between hardness and rebound? My
> assumption was that the reason that one wanted to use 74As on rough surfaces
> was that their rebound was *lower* than the higher-durometer wheels.

I'm not sure that there is any direct relation.  If you've seen one of
those things that bounces ball bearings, you can see that even hard
objects can give good rebound.  Whether rebound really matters or not
is a good question, testing to see if a wheel bounces would tell you
it's "live" and not 85% clay filler, but not much more.

> I guess my point is that skaters may be more concerned about the rebound than
> the literal hardness of the wheel, even if that isn't what the durometer
> measurment is directly reporting.

Could be, but hardness also matters, at least to the point of finding a
soft enough wheel that you don't get that buzzing sensation.

> Also, can you tell us what the meaning is for durometer ratings greater than
> 100?

The durometer scale is completely arbitrary, so I would suspect that someting
like 102 or 105 simply means proportionally that much higher than the nominal
100 top of scale.  It's not that 0 would be liquid or 100 like steel.  I'll
have to look at the picture of one of the durometers in a instrument catalog
to see if they are actually graduated past 100.

> The other Fun Fact I know about durometer, from the former head of R&D at
> Kryptonics, is that the hardness of a wheel is highly temperature-sensitive.
> A 10-degree F shift in temperature will radically change the performance/
> handling characteristics of wheels.

I would expect this to be the case of any plastic, though you'd need a chart
to relate the temperature to the change in physical properties.

> Finally, Kryptonics has a disclaimer on its new wheels warning people that
> polyurethane wheels are not intended for speeds over 30MPH. Does anyone know
> anything about this? What is their concern, or is this a fiat to avoid
> liability? If nobody knows, I'll ask around...

Hard to say.  It could be the point at which the wheel starts to show the
effect of centrifugal force, but more likely it's just a notion of what
constitutes a maximum skating speed where might fall without a guarantee
of serious injury.

In some cases, the manfacturer of any part of the equipment could be named
in an injury lawsuit, even if it appeared that the injury was due to user
error.  Such suits are often tossed out, but "rating" their wheels for use
not to exceed 30 MPH could be an intended first line of legal defense, not
to mention a simple safety warning.

-- 
George Robbins - now working for,     work:   to be avoided at all costs...
but no way officially representing:   uucp:   {uunet|pyramid|rutgers}!cbmvax!grr
Commodore, Engineering Department     domain: grr@cbmvax.commodore.com
Newsgroups: rec.skate