Why have disc brakes become so popular?

[Brucey]

You're claiming the black coating restricts the heat from escaping.It doesn't.

well with all due respect it certainly can do; a painted rim (of any colour) provides an insulating layer that helps to make the rate of heat loss less. The layer of paint is only thin but it is made of a material that may conduct heat over 1000 times less well than aluminium.

cheers

[reohn2]

You're claiming the black coating restricts the heat from escaping.It doesn't.

well with all due respect it certainly can do; a painted rim (of any colour) provides an insulating layer that helps to make the rate of heat loss less. The layer of paint is only thin but it is made of a material that may conduct heat over 1000 times less well than aluminium.

cheers

Please tell how black paint stops heat dissipating from a rim and by how much.
I'm sure it can be measured but I'm also sure it's as near as makes absolutely no difference in any practical sense.

[roubaixtuesday]

Well I knew I'd be wrong.
How wrong am I?

You may be wrong on your interpretation of Mick, though it's hard to be sure.

Mick said

It got very hot inside as it conducted the heat inside better.

You quoted him as

You're claiming the black coating restricts the heat from escaping.It doesn't

However, on

You're claiming the black coating restricts the heat from escaping.It doesn't.

it's actually impossible to say if you're right or wrong with some more data, and assumptions. It depends on
(1) The thermal conductivity of the coating
(2) The thickness of the coating
(3) The convective heat transfer coefficient from the rim to the atmosphere (which depends on the shape of the rim, and its speed)
(4) The degree to which the emissivity of the rim is changed by coating it (likely it will go up)
(5) The temperature of the rim (radiative heat transfer depends on the fourth power of *absolute* temperature, whereas convective heat transfer depends on the temperature *difference* between the rim and the atmosphere)

At a guess, it will make little difference: Some figures

(1) Convective heat transfer
A 1mm coating of epoxy will have a heat transfer coefficient of 170W/m2K. (1mm thickness, 0.17W/mK conductivity).
A typical value for the heat transfer coefficient between the rim and the atmosphere in forced convection in air is ~200 W/m2K
I'd guess that 1mm is extremely generous, so at a first approximation, convective heat transfer dominates over conduction through the coating so the coating makes only a little difference to how much heat is conducted away - there will be a small drop.

(2) Radiative heat transfer
Let's say our rim is at 60C, or 333K (this is just about the point at which it is painful to touch)
Radiative heat transfer is 5.67 x 10 -8 watt per meter squared per kelvin to the fourth, which gives us about 700W/m2, assuming perfect emmissivity. The is the *maximum* possible improvement.
Comparing this to convection, with air at 20C, so 40C difference, convective losses are 200*40 = 8000W/m2
So convection clearly dominates over radiation, and there's not going to be a big change even if emissivity is hugely improved.

(1) decreases, (2) increases, neither by very much, so overall no meaningful change.

[caveat: just dashed this off, may well be an arithmetic error buried within]

[Mick F]

I reckon it was the same reason that my black rims got very hot whereas the silver ones do not.

Your reckoning, alas, is in direct contravention of the laws of physics.

So ................ why did they overheat?

[Brucey]

approximate thermal conductivities are given upthread; it isn't a terribly difficult calculation, but it'll vary with the thickness of the paint film and how aggressively the surface is air-cooled, amongst other things.

But let's put it this way, if the thermal conductivities differ by a factor of ~1000, the heat will find it about as difficult to conduct through 10mm of aluminium as it would through 0.1mm of paint. This would make the contribution of the outside of the rim (all of it that isn't braking surface) much less significant vs a similar polished aluminium rim.

cheers

[reohn2]

Story alert
I used to own a white LWB Transit van
I then swapped it for a mid Blue coloured LWB Transit van on a hot day the blue van was by a large degree hotter inside.
No surprise there.
The question is,if the weather suddenly went from hot to freezing which van would lose the heat from within it the quickest?

[reohn2]

I reckon it was the same reason that my black rims got very hot whereas the silver ones do not.

Your reckoning, alas, is in direct contravention of the laws of physics.

So ................ why did they overheat?

Because you dragged a poor brake.

[roubaixtuesday]

So ................ why did they overheat?

Brucey gave some suggestions above.

Mine off the top of my head would be:
(1) mass of the rims was different - light rims will heat up quicker
(2) conditions were different - heat loss will depend on temperature
(3) the amount of energy put into the rims was different - even a small difference in speed can massively change the proportion of energy dissipated in air resistance vs frictional braking. Likewise things like air pressure and body position affect this to a surprising extent.
(4) confirmation bias - you perceived what you expected to find (this is not a criticism, but a well known problem corrected for instance by double blind trials)

Only suggestions. There are doubtless other possibilities.

[Mick F]

I still have the rims, and I could possibly rebuild the wheels with them, and try a long steep downhill and experiment.

............. but I won't.

I believe what I believe.
I believe that black rims get hotter than silver rims, and having poor bendy brakes exacerbates it.
Remove either one of those things, and the issue wouldn't have arisen.

[roubaixtuesday]

I believe what I believe.
I believe that black rims get hotter than silver rims, and having poor bendy brakes exacerbates it.

You are, of course, entitled to believe in whatever you wish, be it tooth fairies, spoon bending, or the existence of magic paint which creates energy.

The first law of thermodynamics, however, is a cruel mistress and I confidently predict she will remain entirely unmoved by your beliefs, magical or otherwise.

Energy can neither be created nor destroyed. It can only change forms.

[[XAP]Bob]

:lol:
I still have the rims, and I could possibly rebuild the wheels with them, and try a long steep downhill and experiment.

............. but I won't.

I believe what I believe.
I believe that black rims get hotter than silver rims, and having poor bendy brakes exacerbates it.
Remove either one of those things, and the issue wouldn't have arisen.

Was it a sunny day, so the black rim was 'pre heated' by the sun?

[Mick F]

No.
Without looking them up, the latter one happened quite early in the day. I was on my way to meet up with Chat Noir on his ride round the SW peninsula coast. I met him east of Boscastle and we rode together up to Hartland Point. I probably left home 6am or so.

[amediasatex]

:lol:
I believe what I believe.
I believe that black rims get hotter than silver rims, and having poor bendy brakes exacerbates it.
Remove either one of those things, and the issue wouldn't have arisen.

Mick, you're clearly quite an intelligent chap but in this case you seem to be wilfully refusing to educate yourself.

This isn't a philosophically un-provable concept that requires faith, it's demonstrable and explainable with well understood Physics. Rims do not get hotter under braking due to being black. They might get hotter due to being of different construction, or due to being of different material or having coatings, or having different environmental or input conditions, but the blackness* is not a influencing factor.

Explaining the theory is clearly not working, so just out of interest what (if anything) would make you change your beliefs?

* as I mentioned on the other page 'black' is just a description of how an object responds to EM in the visible part of the spectrum. Visible light is not an influencing factor during braking, nor is an objects behaviour to visible EM any indication of how it will heat/cool under conduction or convection (or even radiation if it's not visible light!).

Your rims are heated due to friction, mechanical energy/vibration, not due to absorption of visible EM radiation. They will cool via a mix of radiation in thermal IR (not visible), conduction and convection (listed in increasing order of effect), none of these are influenced by (visible) colour

[Mick F]

Ok.
End of.

Wish I could experiment.

Yes, clearly quite intelligent.
I was on the point of joining Mensa some years ago having passed their tests. Never took it up though.
I am well down on the autistic spectrum, probably Aspergers. I have difficulties with letting go of conversations. Maybe why my post-count is so high!

[Peter W]

While it is true (I would think) that no cyclist, however small their wheels, descends quickly enough to generate air friction heat, I can't help wondering what colour rims, or even bike, would best aid heat dissipation? (After all, in cowboy days the baddies wore black hats and we all know how hot headed they became in the heat of the midday sun!)

The titanium Lockheed 'Blackbird' Mach 3 plus aircraft were painted (or coated?) a nearly black colour which must have been for heat related reasons? It couldn't have been for disguise since they were almost invisible to radar, and were so fast and high that they couldn't be caught anyway.

By the way, new discount price Shimano Ice Tech rotors arrived for road bike (measured at 1.75 mm) so after two years of perfect braking performance from the old, it works out a great deal cheaper than the replacement cost of two expensive new wheels every couple of years for the rim braked bike. Nice one Mr. Shimano! (Remains to be seen how long the rest of the braking system lasts?)