Re: Why does low pressure cost mileage?

Friction losses within the tire's material is certainly one way. I think if you lowered one of your car's tires to, say, 20 psi and drove on the freeway for a mile, you could easily detect that the tire was hotter, indicating that it's sucking up energy. Admittedly, I don't plan to try that test myself.

Think about dribbling a basketball. If the ball's soft, it doesn't rebound nearly as well. Obviously more energy is lost. It's probably going into the rubber.

Also, I think there would be more relative movement between the pavement and tire, due to a sort of scrubbing action. Tires that are consistently run with pressure that's too low tend to wear faster, mostly toward the edges.

What seems to be different about bikes and cars is that, AFAIK, a car's gas mileage doesn't decrease if you have tire pressure too high, at least within reasonable limits. But a bike can take more energy to move if you have too much pressure. The difference seems to be related to lack of mechanical suspension on a bike; the bike tire is typically the only suspension, and too much pressure makes the bike a rigid, unsuspended vehicle.

- Frank Krygowski

I did not know more tire pressure = more resistance.

From here, the difference seems to be your ability to imagine you know what you're talking about. -----

- gpsman

Generally, more pressure reduces rolling resistance.

It is possible that above some pressure, and depending on the road surface roughness, that more vertical energy is transmitted to the rider, who absorbs vibration energy and does not return it to forward momentum.

I don't know that there has been an awful lot of conclusive work done in this field due to the variability of roads, riders and conditions, that make measurements of this kind difficult. We are only talking very small amounts of energy. Maybe 1% of the riders output? (3W in

300W to hazard a guess.)

Tyre rolling resistance tests are performed on a smooth steel drum, so that the manufacturers can have a standardised test and end users can compare, however few of us ride on smooth steel drums or on boards.

larger perimeter of deformation of the tyre and more abrupt change of angle lead to higher hysteresis loses which are found as heat.

In the threads of the carcass, in the latex or rubber between the threads, in the rubber tread and any sales bits and bobs the manufacturers can reach a bit deeper into your wallet with.

Your premise is false: Single cause fallacy.

No. You infer fallaciously as well.

Demonstrate your understanding by quantifying your assertions. Then, maybe I'll have a chance... -----

- gpsman

You first, gpsman. If you have a better explanation of suspension losses, hysteresis losses and power requirements, give it. So far, you've said nothing meaningful. Quit sniping from the sidelines.

In fact, you were confused right out of the gate. When I said "But a bike can take more energy to move if you have too much pressure" you seemed to misinterpret that as "more tire pressure _always_ yields more resistance." At least, that's what your statement "I did not know more tire pressure = more resistance" seems to say.

- Frank Krygowski

Red herring. Thanks for playing. -----

- gpsman

Here's an experiment you can do. Dribble a basketball. which approximates an underinflated tire in terms of its coefficient of restitution. Then try to dribble a basketball-sized rump roast (thawed), which approximates a human body in terms of its coefficient of restitution. Which one absorbs a larger share of the kinetic energy of its fall to the ground? Does the meat bounce back higher when dropped directly onto the floor, or when dropped onto the basketball?

The tire's suspension characteristics can save more energy losses than they can possibly incur, even on comparatively smooth surfaces. That's why Dr. Dunlop's business prospered so marvelously.

The tire pressure at which rubber losses exceed suspension losses depends on a lot of factors including surface unevenness, tire width and construction, road speed, riding style, etc, etc. But it's both observable and broadly acknowledged that for any given set of conditions, there is a point at which higher tire pressure results in less, not more, speed per effort.

Chalo

It's not absolute, but Heine's data shows a 'sweet spot' above which resistance increases for any given section tire.

So, excessively low or high pressures should probably be avoided but the actual numbers vary from one setup to another.

wow, it didn't take long for your b.s. detector to go off!

truth is, there ain't much krygowski can teach anyone. the guy's not just a fraud and disgrace to his faculty, but an ignorant fraud to boot.

"The engineering ignorance of Frank Krygowski" has become a meme - and has been running for many years over on r.b.t, without my assistance!

ISTR engineers from Michelin saying the optimum (for bikes) was a certain percentage deflection under load - something like 15% of tire diameter.

- Frank Krygowski

Per gpsman:

I'd think it depends on the surface being ridden on.

Taking it to the absurd, my 55-55's roll much better over rough ground at 35 psi than they do at 60 psi.

OTOH, running the same comparison on smooth blacktop, 60 psi rolls easier.

Dear Frank,

For what it's worth . . .

Here's a fairly short thread with actual test data on measuring drop versus pressure and replies with more comments about various tests, including Jan Heine:

If you browse far enough, you'll find that the changes for a tire drop of 15% are so tiny that they can hardly be measured for ordinary pressure changes.

On the test 700c tire, a 15% drop (as opposed to 10% or 20% drop) covered a huge range of roughly 40 psi, 70 psi to 110 psi, and a tiny range of drop from 0.126" to 0.174", roughly 0.050 inches, less than a sixteenth of an inch.

There's a link to a suspiciously smooth graph that doesn't seem to show the predicted divergence from linear changes as tire pressure increases or decreases from a sweet spot:

Briefly, tires don't actually work in the over-simplified way that we often assume in which contact patch area increases and decreases linearly with pressure. The contact patch stubbornly refuses to expand as much as expected when pressure is reduced, and it's just as reluctant to contract when pressure is increased.

The reason is that the pressure is not even on the contact patch, due to the sidewall acting somewhat like a scissors jack when its curved shape distorts.

At low pressures, a halo of high pressure forms around the edges of the contact patch.

At high pressures, a halo of low pressure forms around the edges.

Only in a narrow range of medium pressures does the contact patch spread out at an even pressure to match our expectations.

Cheers,

Carl Fogel

ISTR engineers from Michelin saying the optimum (for bikes) was a certain percentage deflection under load - something like 15% of tire diameter.

- Frank Krygowski .+^+.+^+.+^+.+^+.+^+.+^+.+^+.+^+.+^+.+^+.+^+.+^+.

Might you mean 15% of tire sidewall height?

Frank Krygowski considered Fri, 3 Feb 2012

13:58:03 -0800 (PST) the perfect time to write:

Actually, it does, but grip decreases as well, so nobody will admit to it, out of a perfectly reasonable fear that the stingy will go around with their tyres pumped up so hard they double their stopping distances :)

No - it's to do with the difference in contact area. If you used a motor vehicle on a road with surface irregularities that were proportionate to what exists for a bicycle's contact patch (which would be a truly atrocious road surface - something like 5-6" lumps with gaps of 8-10" distance between them), you'd see the same results.

That's why vehicles designed for such terrain have huge soft tyres.

Race teams use a tyre thermometer to ensure the tyre pressure is right for the load and conditions. If you watch an F1 pitstop closely, you'll see the guy with the probe stabbing the tread as soon as the car stops.

gpsman considered Fri, 3 Feb 2012 15:19:57

-0800 (PST) the perfect time to write:

Only above a certain point, when the tyre becomes so hard you waste energy bouncing from one bump to the next. Most often seen on road bikes with 23mm tyres on coarse chip & seal roads.

gpsman considered Fri, 3 Feb 2012 21:58:52

-0800 (PST) the perfect time to write:

Then you use 23mm road bike tyres off-road then?

Only because you refuse to think about it.

I'm confident on 24mm tubulars but don't go all out over the roughest bits. If I thought about it, I'd probably put a little more air in for the rocky stuff. It's not an ideal situation but it is more than workable.

You need to be specific and say high-pressure covers. Tubulars cope well with coarse chip[ings at least at appropriate pressure.