Why does low pressure cost mileage?

On Tue, 7 Feb 2012 08:10:17 -0800 (PST), Frank Krygowski
wrote:

On Feb 6, 11:26*pm, AMuzi wrote:
gpsman wrote:
On Feb 3, 4:58 pm, Frank Krygowski wrote:
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.

I did not know more tire pressure = more resistance.

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.

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

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.

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

- Frank Krygowski

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:

http://groups.google.com/group/rec.b...ba9253cce51d67

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:

http://bp1.blogger.com/_d-Yj0VDKhWQ/...ationgraph.jpg

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

"Frank Krygowski" wrote in message
...
On Feb 6, 11:26 pm, AMuzi wrote:
gpsman wrote:
On Feb 3, 4:58 pm, Frank Krygowski wrote:
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.

I did not know more tire pressure = more resistance.

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.

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

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.

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?

On Feb 7, 7:53*pm, Phil W Lee wrote:
gpsman considered Fri, 3 Feb 2012 21:58:52
-0800 (PST) the perfect time to write:

On Feb 4, 12:44*am, Frank Krygowski wrote:
On Feb 3, 6:19*pm, gpsman wrote:

I did not know more tire pressure = more resistance.

if the road has any
roughness at all, there will be some tire pressure above which power
requirements will be greater due to greater suspension losses. If that
weren't true, we'd dispense with the air and go back to solid rubber
tires, as hard as possible.

False premise; fails to consider weight. *We needn't delve into the
relative "roughness" of road surfaces.

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

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.

On Feb 7, 7:51*pm, Phil W Lee wrote:
gpsman considered Fri, 3 Feb 2012 15:19:57
-0800 (PST) the perfect time to write:

On Feb 3, 4:58*pm, Frank Krygowski wrote:

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.

I did not know more tire pressure = more resistance.

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.

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

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

- gpsman

"Phil W Lee" wrote in message
...
Frank Krygowski considered Fri, 3 Feb 2012
13:58:03 -0800 (PST) the perfect time to write:

On Feb 2, 9:34 pm, RichD wrote:
Low air pressure in tires retards fuel milege, but why?

The obvious answer is, greater friction. But even though
the tire is flatter, still it rolls, there's no relative movement
between the pavement and tire, hence no sliding friction.

So where and how do the losses occur?

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.

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

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.

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.

- Frank Krygowski

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.

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

On Feb 7, 12:05*pm, "Noone" wrote:
"Frank Krygowski" wrote in message

...
On Feb 6, 11:26 pm, AMuzi wrote:



gpsman wrote:
On Feb 3, 4:58 pm, Frank Krygowski wrote:
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.

I did not know more tire pressure = more resistance.

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.

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

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.

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?

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

On Feb 7, 2:49*pm, Phil W Lee wrote:
Frank Krygowski considered Fri, 3 Feb 2012
13:58:03 -0800 (PST) the perfect time to write:


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.

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

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

- Frank Krygowski

On 08/02/12 16:19, Frank Krygowski wrote:
On Feb 7, 2:49 pm, Phil W wrote:
Frank considered Fri, 3 Feb 2012
13:58:03 -0800 (PST) the perfect time to write:


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.

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

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


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

Maybe the difference gets lost in the noise ;-)

--
JS.

Phil W Lee wrote:

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.

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

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Per Scott Dorsey:
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.

That one went right over my head.

Can somebody explain it for the temporarily humor-impaired?
--
Pete Cresswell