Thread: Tire efficiency
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Old 01-11-2020, 12:11 PM   #4 (permalink)
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I’ve now watched the video. Here are my comments:

First, the video is discussing tires used on racing bicycles. Not only is racing of any sort a peculiar activity, racing bicycles is even more peculiar and has only a small cross application to street cars.

Burst pressure: The guy got it backwards. The standard comes first, then the tire is designed to that standard. ETRTO (European Tire and Rim Technical Organization) is one of several tire standardizing organizations in the world. The US counterpart is TRA (The Tire and Rim Association). TRA does NOT publish bicycle tire standards because no one manufactures bicycle tires in the US.

While it maybe true that the burst pressure of bicycle tires is about twice the max pressure listed, that is NOT true for car tires. The value is 3 times or more the max pressure. – and, No!, the burst pressure has nothing to do with the max pressure. The burst pressure is dependent on the fatigue strength of the car tire at the rated inflation – and for Standard Load Passenger Car tires, that is 35 psi – and those tires will be labeled with a max pressure of either 35, 44, or 51 psi.

Further, tires are NOT labeled with a recommended pressure. They are labeled with either a MAX pressure or a rated pressure (as in Max load XXXX at YY pressure). That is true for ALL tires, regardless of what their intended use is – meaning that even tires for farm tractors will be labeled with either the max or rated pressure, never a recommended pressure.

Footprint: Because bicycles lean in turns, the tread surface is rounded. That produces a footprint that is narrow and long – and that is true even when the bike is cornering.

By contrast, a car tire doesn’t lean into the corner. It tends to lean more away from the corner, so it is common for cars to start off with negative camber (leans in at the top), which results in a vertical stance (sort of!) when cornering. Also, most cars are designed NOT to be cornered severely, and have to be adjusted if the car is adapted for racing.

The fact that the bicycle tire footprint is small and narrow is more affected by the macrotexture of the pavement than car tires. (They used the term “negative space” where I am using the term “macrotexture”). That means that racing bikes can benefit from changes in inflation pressure due to road texture – much more than street cars. The idea that a racing bike should use less inflation pressure on more coarse pavement has merit, but that does not apply to car tires – unless one is racing a car. Further, the racing bike tire needs much less fatigue resistance than a street car tire – and higher inflation pressures help fatigue resistance.

Latex vs Butyl Rubber: Butyl rubbers are used where air retention is needed. In a car tire, that’s the innerliner – the innermost layer of rubber. In a bicycle tire, using an innerliner adds weight, so some bicycle tires are made of butyl throughout to reduce the weight – BUT – a racing tire (both bicycle and car racing tires) don’t need to hold air pressure for very long – just enough to finish the race – so they may not even use butyl at all.

And there you have it!

Again, be aware that these guys are discussing tires on racing bicycles. That's quite different than car tires used on the street.

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