In article <
[email protected]>,
DiabloScott <
[email protected]> wrote:
> Tim McNamara wrote:
> > As I ponder this and the photos, I'm thinking that (1) this was a tubular wheel as it appears
> > to be an all-carbon rim and (2) that the tire was held on by tape rather than glue, which is
> > what you see in the photos after the crash (rather than a Velox rim strip). Carbon fiber isn't
> > much of a conductor of heat, so the glue shouldn't have gotten warm enough to allow the tire
> > to roll off. But tape simply doesn't hold as well, especially if Beloki ground through the
> > tread and casing while skidding- allowing the tire to pop with a bang.
>
> Since carbon fiber is a poor conductor of heat, the CF rims get HOTTER than aluminum rims (reduced
> ability to dissipate the heat of braking friction), therefore glue softening is a bigger issue. I
> think Todd's right, it's just a trick of the light reflecting on the glue - I just can't see the
> ONCE mechanics using tape.
From what I've read from the engineers in this newsgroup over the years, the fact that carbon fiber
is a poor conductor of heat would mean that the *brake pads* would get hotter, not the rim. And even
if the sidewall surface got hotter, the heat would not be conducted to the rim bed and therefore not
to the glue. Here's some of the things I've read over the years.
From Jobst Brandt in various threads, some time ago, talking about ceramic rims which are also poor
conductors of heat:
> Braking heat is generated in the softer medium, the brake pad, in this case. This heat must first
> enter the rim, which it cannot easily do.
> Ceramics are insulators, both electrical and thermal, so the braking energy converted to heat in
> the brake pad cannot transfer to the aluminum rim where it can be dissipated.
And about carbon/composite rims:
> Forget it. Braking on composite surfaces is a dud anyway, because the thermal conductivity is so
> poor that your brake pads will melt if you try to use them on a descent. The whole concept is for
> road and track TT's where you don't have to brake.
> Most carbon brakes, such as commercial airliners and F1 race cars use carbon on carbon because
> the temperatures are such that other friction materials would burn. Besides, the wear debris from
> carbon brakes is CO2. I'm sure your brakes won't get that hot, but the pads will melt if you
> brake hard.
> I don't see where any of the above reasons rule out rim brakes except for the rain. Disc brakes
> have too little surface to dissipate power at the rate that rim brakes do, but that will change as
> more people go to insulators for rims (aka carbon fiber).
From David Blake:
> The ceramic is an insulator, so that the heat from braking will be kept in the brake pad. Aluminum
> is a pretty good heat transfer agent, and the rims normally stay cool on long descents by
> convection. With ceramic rims instead, you've lost your heat dissipation capacity, and the brake
> pads will melt instead. The rim will not get as hot for a given amount of braking.
The same phenomenon of overheating brake pads was common in the days of wooden rims which were used
because they were poor conductors of heat (and, being composites, are strucurally closer to carbon
fiber than to metal), which resulted in burning/vaporizing brake pad material. How about that-
carbon fiber is just high-tech wood! ;-)
> This snippet from
http://www.zipp.com/BrakeBlocks/
>
> "Carbon composites have thermal capacity similar to aluminum, although their ability to transfer
> heat is much more limited.
Which is why CF composite is used as an insulator in heat shields. Note that the last half of Zipp's
sentence contradicts the rest of their paragraph.
> However, we are building carbon rims at roughly half the weight of competing aluminum rims, so we
> are faced with the challenge of what to do with the heat energy.
The heat energy which is in the brake pads, and does not transfer well to the rim.
> In theory, a Zipp 280 rim, because of its extreme loss in mass, will achieve roughly twice the
> temperature as an industry standard 30mm deep aluminum rim weighing twice as much.
But apparently they haven't measured- not that doing so would be at all difficult. Since CF is a
poor thermal conductor- an insulator- it will poorly transfer heat from the brake pad into the rim.
The rim won't get hot and it won't conduct heat to the glue bed.
> Furthermore, we have taken every measure possible to conduct heat down the rim sidewall and not
> into the tire bed where it may soften tire cement or potentially cause other problems, somewhat
> further reducing the rim thermal capacity. "
And what measures are these? The thermal equivalent of lightning rods?
Perhaps one of the engineering types can correct either my understanding or Zipp's.
