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.

Really? A flexing tyre (and an under-inflated one) will deform and increase rolling resistance,
that's why we pump them up so hard. Any extra, or stiffer sidewall material would add
to stiffness and decrease RR.

With a tyre as small as a bicycle tyre, the deformation wouldn't be that great, so
you'd still get some braking, and with a flat, you wouldn't want to slam the brakes
full-on anyway. And given the choice between a flat tyre and a rim failure,
I know which one I'd choose. That's the point of the idea, moving the wear from
braking from an expensive, infrequently-replaced component to a less expensive
frequently-replaced one.

your argument is to favor frequent replacement of inexpensive tire, instead of rare replacement of expensive rim.


consider however, the average non mechanically inclined recreational rider,
who never inspects or maintains their bike, only takes it to repair shop after something breaks
traditionally, they retire the bike to the attic long before a rim wears out, so reducing rim wear has no benefit
conversely, decreased tire life and unexpected on the road blowouts (remember, they arent inspecting for wear) will be inconvenient or even dangerous

your concept might make sense to you, but the market is not you



besides, why save a rim from wearing out when spoke fatigue will eventually kill the wheel anyway? I'd rather replace it all at once with a new wheelset

Eh, I think disc brakes are probably a better choice than all this.

Is it possible to have some kind of "coaster brake" variant in a front hub?

M.

I said absolutely nothing about underinflating the tire. Pneumatic tires always deform to some degree to allow better grip and cushion the bike and rider from bumps. Otherwise we'd still be using solid rubber tires. Rolling resistance comes largely from the friction in the tire casing, yes, but making the sidewalls stiffer actually adds to the frictional losses, because the tire has to fight itself to conform to the road surface.

I can't tell if you're trolling or just haven't experienced a flat tire in a long time.

Actually this is not correct. Bias belted tires on cars of days gone by had stiff sidewalls, then radial tires were invented with thinner sidewalls and radial direction plies allowed the tire and sidewalls to flex increasing handling and decreasing rolling resistance. Bicycle tires do the same thing which is why you see paper thin sidewalls in tires like the Conti Supersonics, sure they run high psi but that psi is the same for thicker sidewall tires too! The thinner sidewall allows better handling and lower rolling resistance. However as sidewalls get thinner damage to the sidewall goes up so one has to be careful that they buy a tire like the Supersonics only to find them tearing within 500 miles of street use.

You are right about underinflated tire though, and an over inflated tire will lose traction plus increase rolling resistance on rougher surfaces.

If that's a serious comment, then absolutely not. I'm just continuing the debate by responding to comments about my idea, and trying to keep my thread on track in the face of diversions by spoon-brakers and truck racers.

So you haven't pressed on the sidewall of a flat tire before. There is absolutely no resistance available for braking. Please do not involve anyone else in your tests for this concept.

I would suggest that the 100-year-old spoon brake has fewer inherent flaws than your idea. You want to shift the war from a rarely replaced part to a frequently replaced part and increasing the frequency of it's replacement. This is going to require specially made tires, which if history is any indication are going to be ridiculously expensive, and you're going to be replacing them a few times per year. Great savings, there.

Taking this thing on any sort of large hill or mountain is suicidal, since a front blowout in such situations is very dangerous. If there is any sort of flat, there is no braking at all. Also fairly dangerous.

So far I'm spending a lot more to be less safe. Why am I buying this thing again?

I answered both those points earlier. The braking surface is designed to last as long as the road surface of the tyre (with a safety margin), so replacement is no more frequent.


We're not getting any useful discussion here any more, so I retire.

Were you able to keep your benefits?

Let it die

Lol

Can't really say I derived much benefit from the discussion.

Actually, I did. The idea of using dynamos to convert braking energy into electricity is promising, I think I'll look into it. My idea is to use the power to top up a battery for lights and/or an audible device.

There is nothing wrong with doing something differently then everyone else, this is how great ideas are born, is your's a great idea? I have no idea but there is no harm in trying it and see what happens. I applaud your efforts and hope it succeeds.