REFLECTOR: Insulation?

[Al Gietzen]

Let me put my 'heat transfer engineer' hat on; and give you my take on the
heating/protecting of the gear leg issue. (you knew I would; didn't you).
It is obviously a complex configuration with different potential paths, so
this is not based on any detailed analysis. 

 

First of all, the large majority of heat generated by the friction goes into
the rotor.  Some will be transferred to the caliper, but - the brake pad is
an insulator, the contact area to the puck is not large, the fluid is a
relatively poor conductor, the mass of the caliper is relatively large; etc.
So the caliper will heat up, but not nearly as much as the rotor.

 

How does the heat get from the rotor to the gear leg? Three basic path;
radiation directly across the gap, conduction through the air across the
gap, and conduction from the rotor to the wheel - to the axle, to the leg.
Let's look at these in reverse order:

Conduction from the rotor to the wheel is effective but somewhat choked by
the limited area of contact where the bolts are; it then has to heat up the
wheel, then transfer to the bearings, through the balls (limited contact
area), to the axle and on to the gear leg.  A very torturous path - so my
thinking is not much goes this way.  And if it did, the first thing we would
see would be damage to the tire at the bead, because that would be the first
place to get hot.  I haven't heard of heat damage to the tires.

 

Conduction through the air from caliper to leg: as already pointed out, air
is a very good insulator, and there is nearly an inch of intervening air.
And this air is not stagnant, it is moving - a lot in the case of RG.  Not
likely a significant transfer path.

 

That leaves radiation.  Radiant transfer goes as T (absolute) to the fourth
power, times the emissivity (yes, and don't forget Stephan-Boltzmann and
their constant).  I'd guess that if you did a hard stop at night and looked
at the rotors; they'd be glowing - so probably over 1400F.  The emissivity
of the rotor is relatively low on the shiny part (maybe .15) and higher on
the rest (maybe .4 - .5) and the absorptivity of the leg is high.  So; based
on some actual calculations, the amount of heat going into the leg opposite
the rotor may be 75-100 watts.  I'd say that much heat going into a small
area of the leg over the period of time for a stop, and then some more
braking for turning, and maybe dragging . . is definitely significant.

 

So, here's the deal, too.  Whether it becomes a problem or not is critically
dependant on how hot the rotor gets; if the temp is just 200 degrees higher
one time, the amount of radiation heat transfer goes up by 1.5 times.  So
the biggest factor is how you use your brakes.

 

Supposing that the above evaluation is somewhere close to reality, then the
biggest gain can be made by protecting against radiation heat transfer.  By
far the best way to reduce it is to interpose a layer of shiny material
(something with low emissivity). One baffle of shiny metal, particularly if
it is cooled by the air, can reduce the radiant transfer many fold.  Clean
SS may have an emissivity of 0.05. Similar for aluminum. The foil coated
insulating wrap can also be effective.

 

Personally I'm not that impressed with the 1/8 of high density
'bakelite-type' material. It's emissivity/absoptivity is likely relatively
high, and clamped against the leg, it will transfer a significant part of
what it absorps.  And it's not the greatest conduction insulator.  No doubt
it will reduce the heat transfer so that's a good thing.  

 

Probably in most cases, no protection is needed - as long as you are careful
about minimum use of the brakes. But doing something to limit the radiation
part of the equation can surely be helpful.

 

OK, this got long, and I won't even guarantee that I've got it right:-).

 

Al

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