Re: Why does low pressure cost mileage?

Phil W Lee, I don't think that's the case now - the cars are stationary for only 3 seconds or so. And, the cars have tire temperature telemetry (IR sensors) that monitor tire temperatures in real time. Kerry

More or less. I didn't mean the overall outside diameter of the tire

+wheel (i.e. roughly 27") but the diameter of the tire's cross section (roughly 1").

But my "something like 15%" is merely working from dim memory of what the tire literature said. Given that I'm not sure of the exact percentage, the difference between sidewall height and cross section diameter isn't very important.

- Frank Krygowski

You may have misread my "decrease" as "increase."

- Frank Krygowski

If it doesn't decrease, does it increase, or stay the same?

Maybe the difference gets lost in the noise ;-)

James considered Wed, 08 Feb 2012 17:44:22

+1100 the perfect time to write:

Sorry, yes. To clarify, overpressure in car tyres will get you further on a given amount of fuel, but wear out the tyre faster, owing to all the wear being on a smaller contact area. This also reduces grip for all purposes - acceleration, braking and cornering.

This is only true of car tyres because the contact area is so much greater than the normal surface irregularities which cause the problem for cycle tyres - if those irregularities were scaled up to match the increased tyre contact area, exactly the same effect would be observed.

My uncle Ralph claimed that you could never overfill a tire, that you should just get a pump, and pump air in until you can't get any more in there and that was the right amount of air.

Surprisingly enough, he was not killed in a car accident.

--scott

Per Scott Dorsey:

That one went right over my head.

Can somebody explain it for the temporarily humor-impaired?

Is there any imaging of this phenomenon? I would not expect the thin/flexible walls of a typical narrow road bicycle tire to contribute much of any support simply through their own shape.

I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much to the rolling resistance since they suffer from tread squirm at all pressures. While a higher pressure results in a smaller contact patch, the tread squirm decreases in size (due to the smaller contact patch) but also occurs at a higher pressure.

What tread?

DougC, Are you saying that tread squirm is a much greater contributor to rolling resistance of non-belted bicycle tires than the tire inflation pressure? For all values of inflation pressure? For all tread depths? For all road surfaces? If not, please clarify. If so, then do you contend that a silk track sew-up on smooth concrete will not have much difference in rolling resistance if it is inflated to 1 bar or if it is inflated to 8 bar? Thanks, Kerry

Dear Doug,

Yes, measurements of the contact patches of bicycle tires confirmed that they act like car tires, where the non-linear relationship between pressure and area is well-known. Tire patches were 20~30% smaller/lower than predicted at high/low pressures. In other words, the tires stubbornly try to remain at the sweet spot in the middle, where the sidewall effects are minimized. Tom Schmitz put my measurements of a single tire to shame with a wide variety of much more carefully-done tests--and the curve of his average measurements overlaid my single-tire tests:

The scissors-jack effect doesn't depend on the thickness of the walls of the tire--a good deal of force is needed to distort the curve of the sidewalls, which leads to the halo of high pressure at low pressures.

As for the halo of low-pressure at high pressures, that's easier to understand. Assume a 150 psi inflation. Pressure at the center of the tire patch is 150 psi. Near the edges, the tire starts to curve away from the ground, until the 150 psi tire's pressure on the ground drops to zero.

In the end, fairly simple physics explains the easily reproducible results. Just pump a bicycle tire to 100 psi, ink it, lower it with a

100 lb weight onto paper (or glass as Tom did), measure the two dimensions of the ellipse, and then repeat at 10 psi increments for a range from 40 to 150 psi. As I mentioned, the effect is well-known in automobile tire engineering.

Cheers,

Carl Fogel

snipped-for-privacy@panix.com (Scott Dorsey) considered 8 Feb 2012 12:00:14 -0500 the perfect time to write:

But by an exploding pump?

Got nothing to do with humor unless you have the uncle. It's about uncles with bad ideas, which they thought was common sense. If somebody would argue with Scott's uncle about that air pressure, he might say something like, "You don't know much about equalization, do you? Common sense." I had one like that. I was his favorite nephew, because I fished with him, and usually brought the beer. And I liked his jokes. Man, he could tell jokes. First time we went together in my car, he was totally shocked when he saw me fasten my seat belt. Seat belts were pretty new then. Started yelling at me, "What the hell you doing that for??!!" I told him it was safer. He looked at me like I as stupidest man alive, and said, "Safe?!! What if you go in the river and can't get loose?!"

Anyway, you have to be there.

--Vic

Dear Carl,

I imagine that with the aid of some integral calculus, the contact pressure variation and patch size can be explained. It would be interesting to see the results. One day maybe I'll try tackling such a problem, unless someone beats me to it.

Being here is often enough.

DR

Precisely. And, it's possible that all he ever used was a foot pump so there was some limit to the pressure he could get in the tire. But he harbored a pretty serious delusion and it is a miracle he wasn't killed by it.

I knew better than to even try.

--scott

Flexing the rubber.

More flex, more energy lost.

Alan Baker wrote in news:alangbaker- snipped-for-privacy@news.shawcable.net:

no one has stated the largest one yet!!!! the lower the pressure the smaller the actuall dia of the rotating tire, lowering the overall ratio, less distance traveled per rev. less milage. KB

bull. tire size has no more to do with mileage than transmission gearing does - some cars use /more/ gas in overdrive than they do the next gear down.