Why have disc brakes become so popular?

[reohn2]

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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?)

You wear out disc rotors in two years,what sort of mileage?
I've yet to wear out an Avid rotor in Oh,probably 20,000miles even on a tandem and they only cost about a tenner and 15minutes to swap over

[amediasatex]

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?

If bicycles or rims cooled primarily by radiation (they don't) then it would depend on how emissive they are in the thermal IR part of the spectrum, which is not necessarily related to how they absorb or reflect light in the visible part of the spectrum.

Since the bicycle cooling is primarily conduction and convection driven what you want to consider is how conductive* the material is, and then convection, ie: how quickly you can get the heat away from the object using airflow.

It's also worth remembering that whatever happens the energy has to go somewhere, if you take brakes for example, if you have X joules of kinetic energy and you want to stop or slow down to Y joules then you need to dissipate Z joules, where Z=X-Y.

Which basically means 'something' needs to get hot** to the tune of Z joules. If the energy doesn't get into your rims it will stay in the much smaller pads (and calliper etc.), this can have a more detrimental effect that hot rims. Basically to brake quickly you want to heat something up, rims or discs are the prime candidate (as opposed to the pads/calliper) as they can also cool quicker. If they don't get hot then either something else will, or it will take you longer to stop.

This is why they go to such great lengths to keep discs cool by making them bigger/Alu sandwich/drilled/Alu spider etc., it's not to stop them absorbing heat in the first place, it's to get rid of the heat. Current high end brakes from Shimano (and others) use finned*** and Alu*** backed disc pads for the same reason, to get rid of the heat that they must absorb in order to work.

*a coating/paint of any colour might actually make the situation worse by acting as a thermal insulator and reducing conduction.
** or make a noise, or move/vibrate, or even emit light, although that outcome is highly unlikely!
*** to improve convection
**** to improve conduction

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.

A bit of a mild myth there as 'Blackbird' was actually still quite radar visible despite their best efforts, it was however much better than other planes. The paint did contain elements to help with radar signature but again that’s a different bit of the spectrum to visible. The nearly black colour they were painted was (partially) heat related but it was to do with emissivity in thermal IR ranges, it's mostly coincidence that the material appeared black in visible, and this is the root of a lot of the misconception.

Things that are 'black' in the visible spectrum are often also good at absorbing IR, and highly emissive in thermal IR ranges, but it's not because they are black in the visible range and it is entirely possible to have highly emissive things that appear white (or any other colour).

[roubaixtuesday]

Supersonic flight generates a *lot* of heat.

As stated above, radiation of that heat is dependent on properties in the infra red, not the visible.

Note that Concorde was white.

[Mick F]

Something I DO know about.
Radar.

Blackbird could indeed be seen on radar ............. but, this the main thing ................ the radar returns were far less than the size of the aeroplane would you would think. Not that is was "radar invisible" but that the size of the return belied the size of it and the detection range severely reduced.

Same as warships these days.
The angle of the ships' superstructures subtended from the right angle to 70odd degrees. The ship-builders and designers went, and go, to a lot of trouble NOT to have a right angle anywhere on the upper deck.

Ships suitably designed would and do still produce a radar return, but this is - by design - kept to an absolute minimum. Many of the upperdeck areas are coated in RAM ....... Radar Absorbent Material. No doubt Blackbird has RAM too.

This means for Blackbird and warships like that that you can't detect them as far away as you would be able to do if they were normally designed.

The design figures and parameters for all this are Top Secret.

300px-Royal_Navy_Type_23_Frigate_HMS_Argyll_MOD_45153839.jpg (21.87 KiB) Viewed 525 times

This was my last ship. HMS Argyll.
Note the angles of the upperdeck superstructure.
Also, you can't see it, but the funnel exhausts are cooled to reduce the heat and therefore the IR signature. Also, the hull is soundproofed and all the internal equipment very much silenced, to reduce the sound signature.

Lots of other things to tell you about, but if I did, I'd have to kill you!

[[XAP]Bob]

SR-71 had some RAM - but it basically was just too fast, and too high, to be in danger.

Various times pilots saw a SAM launched and just outran it. It took a SAM doing Mach5 a minute or so to get to altitude, and when it dod it couldn't manoeuvre much - there wasn't enough air.. So all the SR71 pilot had to do was change course a bit...

"The SR-71 was an expensive aircraft to operate. The most significant cost was tanker support, and in 1990, confronted with budget cutbacks, the Air Force retired the SR-71. The Blackbird had outrun nearly 4,000 missiles, not once taking a scratch from enemy fire."
My emphasis

[Peter W]

It's surprising what you learn from wandering threads like this. I'm all for it! Thanks for all the info.

But to brakes - I was blissfully unaware that the Ice Tech rotors on road bikes were composites. It was Brucey who mentioned the minimum allowable wear limit of 1.5 mm. It was only after that I actually checked the instruction book that came with the bike. You know, 26 pages on how to ride a bicycle which everyone ignores, with somewhere amid all the dire warnings the actual wear limit of the rotors. (It was also stamped on the rotors themselves which I hadn't looked for.)

Reohn, Theses Ice Tech rotors obviously wear much faster than others, but it seems to be the price to pay for more efficient cooling and safety to prevent excessive heat build on long fast descents. (The pads have fins also.) I remember reading an account from a tester of the earlier road bike discs, with him being thrown off while braking very hard at a corner, on a long fast descent, when it all drastically overheated. I thought then, no way to trust them on a road bike! (O.K. om MTB where high speed braking is not usually prolonged.)

But I can say that theses clearly more developed brakes do seem to have largely overcome earlier issues. I can only conclude that in a few years time discs will become almost universal, and rim braked die-hards will be the equivalent of steam engine buffs! (Everyone knew that a Dutchess could piddle all over a stupid diesel while climbing Shap! It was all a plot to scrap 'em all, wasn't it?

[Spinners]

Supersonic flight generates a *lot* of heat.

Only at higher supersonic speeds - I think Mach 2.5 is the threshold above which heat becomes an important factor in design and materials.

[reohn2]

..... Reohn, Theses Ice Tech rotors obviously wear much faster than others, but it seems to be the price to pay for more efficient cooling and safety to prevent excessive heat build on long fast descents. (The pads have fins also.) I remember reading an account from a tester of the earlier road bike discs, with him being thrown off while braking very hard at a corner, on a long fast descent, when it all drastically overheated. I thought then, no way to trust them on a road bike! (O.K. om MTB where high speed braking is not usually prolonged.).........

No bicycle brake is put to extremes more than tandem brakes especially in the hills.
We've made the rotors on our Cannondale road tandem blue with heat on North Wales descents but never wore out a rotor.
In fact they never showed any signs of wear in the 7+years we owned it(now owned by RickH's sight impaired friend)and AFAIA it's still running on the same rotors.
I'm aware the newer hydro road bike disc rotors are of a sandwich construction but a I've personally never felt the need for anything more exotic or powerful than the BB7's n that bike and on my Salsa Vaya and my Genesis Longitude and I do like to descending fast.
That's not a criticism of your discs but I was surprised at how quick your rotors wore out.

[roubaixtuesday]

Supersonic flight generates a *lot* of heat.

Only at higher supersonic speeds - I think Mach 2.5 is the threshold above which heat becomes an important factor in design and materials.

Depends what you mean by a *lot*, I suppose.

This meets what was in my mind:

[amediasatex]

Disc rotor wear is heavily influenced by pad choice as well as actual rotor material. I’ve got some rotors almost a decade old with minimal wear, and likewise some which are nearly dead at 26 months.

The lower end Shinano rotors specify resin pads only, they’re obviously softer and if you use metallic sintered pads with them you can destroy them in no time at all.

[reohn2]

Disc rotor wear is heavily influenced by pad choice as well as actual rotor material. I’ve got some rotors almost a decade old with minimal wear, and likewise some which are nearly dead at 26 months.

The lower end Shinano rotors specify resin pads only, they’re obviously softer and if you use metallic sintered pads with them you can destroy them in no time at all.

I've no experience of Shimano disc brakes,I use sintered pads exclusively on all three bikes with BB7's and either G2CS or the waves rotors that came with them,all 160mm.The Cannondale was also on 203mm G2CS and sintered pads.I quickly found organic pads didn't last on muck roads and can remember wearing out a set of them on one ride on mucky rainy day on Lancashire lanes

[Mick F]

Supersonic flight generates a *lot* of heat.

Only at higher supersonic speeds - I think Mach 2.5 is the threshold above which heat becomes an important factor in design and materials.

Depends what you mean by a *lot*, I suppose.

This meets what was in my mind:

Very interesting.
Thanks muchly!

[Spinners]

Compared to Concorde, the SR-71 Blackbird is 'only' 50% faster but gets three times hotter.

[Peter W]

All this heat barrier science reminds me of George Gamow's famous book of Mr. Tompkin blundering through the bizarre relativity effects, if the speed of light was only 20 m.p.h..

If the speed of light was only that fast we cyclists could join in the fun of warping time by our furious pedalling while appearing to age relatively less quickly than our couch potato neighbours. Not sure if there would still be any heat issue at the limit (on the bike, if not the cyclist trying to push against it) but I still find it difficult, despite Gamow's explanation, to visualise how it would appear to others if a passenger on a train travelling at the light speed limit of 20 m.p.h. ran up the corridor at 15 m.p.h.? Supposedly, he would appear to others as a paper thin relatively squashed object, kind of like, in our real world, a cyclist with rim brakes in the wet who rams into the back of a suddenly stopping bus!!

P.S. My keyboard has developed a curious effect. (Past coffee spill.) When I type these, it often adds an extra s on the end. (Theses.) And no, I don't believe in ghosts.

[rmurphy195]

I did "pop back" to this thread to see how the disc brake debate was going. But I can't find it - a prime example of thread hijacking to the detriment of those who want to follow the original discussion perhaps? Maybe the moderators should do something about this one!