While the aluminum rims on bikes do dissipate the heat fast, carbon fiber rims do not.

i imagine the spoon brake left the party when the pneumatic tire showed up.

Yes, except not alone, it ran away with the dish brake.

I'd like to purge this thread of the spoon brake comparison, which is a red herring. The spoon brake was designed to work on the road-contacting surface of a solid, slick tyre. Because the tyre picked up grit, and the spoon was made of ordinary metal, it heated up too much, wore out quickly.

I am suggesting a brake pad made of a more more suitable metal (or other substance), designed to work on the sidewall of a pneumatic tyre made with e.g. extra thickness, a different compound. If the sidewall is designed to wear at a slower rate than the tread, they will both be replaced at the same time.

The advantage is the same as with a disc brake (not compromising the rim) plus the lack of the disc mechanism with all the extra weight and adjustment/maintenance issues.

As for stopping with a flat, the pads would still exert enough pressure to slow the bike, and you have another brake on the other wheel as well.

Build and market it and see how it fares, a significantly better system will sell.

The spoon brake comparison isn't a red herring at all. While there's a difference between the tread and side of a tire as regards road grit, the issue of heat is the same.

Mechanical braking is about converting kinetic energy to heat through friction. The amount of heat generated is very significant, and must be dissipated or absorbed by a "heat sink" otherwise the surface temperature will rapidly rise to well beyond the melting point or rubber.

Since rubber is an insulator, the heat will flow into the metal part which if it isn't large enough or cooled somehow will heat up quickly leading to failure. It's not an accident that all brakes use small pads of friction generating material against larger moving heat conducting rotors.

Feel free to experiment, but you might save some time by doing some research into the physics of mechanical braking.

Additionally, as the tyre heats, the air inside of the tube will expand.

M.

This is much less of a problem than one may imagine.

First of all the relationship of pressure to temperature is based on absolute zero, (-460F) so going from 75f to 125 F would mean less than a 10% rise in pressure.

Also consider that rubber is a lousy heat conductor, so the temp on the inside surface of the tire will be slow to climb. Then consider that air is also a lousy conductor, so there'll be a long delay before the air inside matches the temp of the tire's inside wall.

Over all, by the time the air starts to warm in any meaningful way the braking cycle will be over and everything will be cooling off already.

Oh, well, then. Learn something better every day.

M.

As FB points out this idea is inherently flawed, but it got me thinking...

So here's a neat idea for a commuter bike: instead of a traditional rear brake (which most competent riders can go without pretty happily), you could have the rear brake lever actuate an old-school dynamo and use it to charge one of those nifty USB backup batteries used to charge phones and whatnot (some electrickery would likely be required to get the voltage right).

You wouldn't be able to lock up the wheel with it, but it'd probably suffice pretty well for most normal rear braking... if you squeeze the lever hard enough!

You could even have two pinching the tyre, or better yet, one of those nice ones that rolls on the outer circumference.

I notice you didn't say 'could heat the spoon'; have you performed the experiment? :eek: :p

I agree. I use to ride and race in the mountains of Calif, and a lot of that riding temps would be over 95 degrees, combine that temp with road surface temps and braking coming down fast descents you have quite a heat situation, and yet I never blew a tire from heat. I have heard of the rare occurrence of tandem riders that happened to, but it was even more rarer for a single roadie to have that happen to. I was taught when I first started training and racing mountains was to brake hard for about 3 to 5 seconds then release for about 3 to 5 seconds, and even though that 3 to 5 seconds of release seems short the air rushing past the aluminum rim does cool the rim enough. It was called stab braking...semi trucks do the same technique for the same reason! Problem is with CF rims is that they don't cool down like aluminum so the heat just builds and builds to the point either the tire blows or on cheap CF generic rims the rim can delaminate.

I assume you'd need to have special tire sidewalls: as others have noted, a rubbing brake pad can damage a tire sidewall relatively quickly. Tire sidewalls are also one part of the bike that you definitely don't want to risk wearing through. Brake pads wear consistently and visibly, and the failure mode isn't likely to lead to a sudden loss of control: in the worst case you'll get some horrible noises and impaired braking performance, and should have a second brake to use. Whereas if breaking wears at the sidewall, you're risking a blowout and sudden loss of control

Isn't it just a consequence of the evolution of bike design? The wheel came first (and was designed to carry the bike, not to dissipate braking heat) so the brake pad was designed to act on the wheel as it then existed.

Sure, but these points can now be taken into consideration with a modern design, e.g. brake pad material with a high heat capacity (some early ones were wooden),larger brake pads, heat sink or cooling fins to dissipate heat.
Now that I like. I've been wondering about a way of harvesting braking energy, and that seems like a simple, workable method. Maybe it should be a new thread.

Hmmm, doesn't the Copenhagen Wheel use regenerative braking?

Yes, but it's estimated cost is $600.

Regeneratve braking is fine for long gradual braking, or speed control on descents, but not for hard braking and short stops. The size and weight of a motor that could provide that much torque is more than anyone would accept on a bicycle. Even hybrid cars that use R-braking still use traditional mechanical brakes for hard braking.

A great many ebike drive systems incorporate regenerative braking.

Or to look at it another way, the motor is going to be about as powerful braking as it is accelerating. But in fact you need and want to be able to stop much harder than you go. Also in the usual bicycle arrangement it's on the rear when you need the most braking power at the front.

Here one on my local C.L. right now

http://cosprings.craigslist.org/bar/4145184181.html

I never realized this until now, but it makes a ton of sense. Thank you!

No, he pointed out the flaws in a 100-year-old spoon brake.

A couple things come to mind: the sidewalls of good tires are as thin and flexible as possible to lower the rolling resistance. Adding enough rubber to serve as a braking surface would turn the bike into a slug, negating any weight savings from the braking system. And trying to implement such a system with regular tires would lead to blowout city.

With a flat tire, sidewall pads will just push into thin air -- I don't think you've thought that part through all the way. ;)

I really wish we had a "Bicycle Mad Science" subforum, though. It's good and fun to think outside of the box, even if it doesn't go anywhere productive.

I doubt that you'd reach the point where the spoon would be red hot. If you made the spoon out of steel, low temperature "dark red" where you can see a red glow in low light is from 425 C to 600C. The melting point of rubber is around 400C. The rubber would melt first. But you probably wouldn't reach that point either if you were using pneumatic tires.

I had thought of aiming jets attached to water reservoirs to the rear brake pads of tandems to attenuate heat on long descents. But I think a better way to prevent heat-induced blowouts is to just fill the tire with nitrogen, which does not expand with heat. I believe this is what they do on F.1 cars.

Aside from heat buildup, the big problem with spoon braking is its inefficiency. Someone pointed out that they don't work very well, and in an age where we're used to having very firm, responsive braking, even to using compressionless housing, think of the brake lever feel of pushing a spoon into a pneumatic tire, even if it is fully inflated. Even if you had a spoon on each side of the tire, the tire cross section would deform to comply with the forces imposed on the sidewalls. You'd not only get unresponsive brakes, you'd likely bottom out the levers under hard braking!

Luis

Is Nitrogen somehow exempt from Boyles Law?

All gasses increase in pressure proportional to the temperature Kelvin. I don't know if nitrogen is a poorer heat conductor than air, which may be a factor, but given that air is already 80% nitrogen, I can't imagine that it would be very different.

I think what filling a tire with nitrogen (or CO2) from a tank accomplishes has more to do with keeping out water vapor than any special properties of those two gases.

Helium!

M.

Helium leaks like mad...

No, he pointed out the inherent flaw in anything resembling the notion.

Viz, the prime consideration in designing any braking system is heat dissipation. Do I need to continue by further restating FB's incontrovertible points?

This was a first approximation of a brake, like what you find on a billy cart. It deserves no further evolution.

(cue perverse steampunk gleam in more than one reader's eye)


...Actually, I'm reminded of a penny-farthing I saw once rocking a pair of Aerohead rims.

Got me thinking, what would a racing highwheeler look like if designed today... some sweet pneumatic tyres on those Aeroheads would be a start. And I guess there might be a place on such a machine for a nifty finned aluminium-backed brake spoon... certainly a dual-pivot caliper would be total overkill.

I guess spoon brakes were appropriate for highwheelers, since anything more than the most moderate braking equalled a header.

Since the volume is constrained, the question should be "is nitrogen somehow exempt from Gay-Lussac's Law?". That's the law that describes the relationship between pressure and temperature. But your point is correct. Nitrogen is used where heat becomes a factor in tires because it deviates less from the ideal gas law. Water has strong intermolecular interactions that cause it to deviate from ideality. Nitrogen's pressure curve vs temperature is closer to linear while water's pressure curve is exponential.

Since bicycles don't see the heat load that tires like those used in race cars do (passenger cars don't really see those kinds of heat loads either), using dry nitrogen would have little effect. To bring the discussion back to spoon brakes, putting that kind of heat load on a tire would make nitrogen beneficial. Putting that kind of heat load on a bicycle tire would be stupid but nitrogen would help a little in that situation.