PSI and Tire Patch

[Peter iNova]

I've measured my 18 psi tires as having a tire patch total for the HT and me of about 5 square inches. Our combined weight is about 250 lbs. Hmm. That means each square inch exerts 50 psi onto the ground which makes the inevitable conclusion that the tire pressure doesn't account for the whole psi of tire contact patch.

250 lbs at 18psi would require about 14 square inches of contact to the pavement and that is unreasonably large.

So how hard are the sidewalls of these tires? And does anybody know where this particular model is made? Are the knobbies made there too?

-iNova

http://www.glidewalk.com

[stevew]

I've ridden through water and looked at the HT's tire tracks. The full width of the tread ( about 2.5" is in ground contact. so with maybe 2" of circumference in contact that would be about 10 sq in of total contact. How did you measure your contact patch? Since the tire has some inherent stiffness and considerable thickness that must be supporting some of the weight. I quess you could reduce pressure to near zero and see how much contact you still have.

[Peter iNova]

I actually measured it with a "footprint" from the tire on paper. The oval contact patch is 1.3 x 2.5 inches. 2.55 sq inches of patch. I'm sure more area is involved with surfaces that give way, but this was on hard floor.

-iNova

http://www.glidewalk.com

[gclark7]

Hi,
The weight of the HT plus rider pushing down on the contact patch of the tires can not be used to determine the tire pressure. G

[BruceWright]

Peter, I wasn't sure what you were getting at there.

Tire pressure and the weight of the device are two different things.

An 85 pound Segway with flat tires still weighs the same as one with full tires, minus about a 10th of an ounce of air.

Imagine the air in your tires as spokes instead of air. It's not doing any pushing of the Segway up, The Seg's just sitting on the air.

-Bruce Wright

Segway: Vehicle of Dream

[stevew]

There is a confusion between tire air pressure and ground contact pressure. The ground contact pressure in psi must be equal to the total weight of the ht/rider in lbs divided by the area of the ground contact patches in square inches. This is what Peter was getting at. The 'patch' is that
area of the tread in intimite contact with the ground -- its footprint. If the tire were infinitely flexible (like a balloon) the ground psi would be equal to the internal air psi.

[toybuilder]

quote:Originally posted by stevew

There is a confusion between tire air pressure and ground contact pressure. The ground contact pressure in psi must be equal to the total weight of the ht/rider in lbs divided by the area of the ground contact patches in square inches. This is what Peter was getting at. The 'patch' is that
area of the tread in intimite contact with the ground -- its footprint. If the tire were infinitely flexible (like a balloon) the ground psi would be equal to the internal air psi.

Ah - I was wondering about that. I think I understand now -- I was going crazy trying to find some write up of the physics behind this.

As I understand it now -- the air pressure inside the tire has to be equal throughout the tire, and the ground contact force must equalize against the air pressure force in the tire.

So, a tire at 18 psi on an unloaded Segway would gain (approximately) an additional 200 lbs. / (4 inches * 60 inch circumference) = 200 lbs. / 240 sq. in = .83 psi of additional pressure when the rider steps on it.

Although the added weight would make the tires want to collapse, the air pressure acting throughout the tire resists that change in tire shape, essentially giving the sidewalls strength to keep the tires up.

http://www.pasadenasegway.com

[Peter iNova]

quote:Originally posted by BruceWright

Peter, I wasn't sure what you were getting at there.

Tire pressure and the weight of the device are two different things.

An 85 pound Segway with flat tires still weighs the same as one with full tires, minus about a 10th of an ounce of air.

Imagine the air in your tires as spokes instead of air. It's not doing any pushing of the Segway up, The Seg's just sitting on the air.

-Bruce Wright

Segway: Vehicle of Dream


Cars often measure the square inches versus car weight and come up with compatible numbers with just a little being contributed by the stiffness of the sidewalls. 133 sq in at 4000 lb car = 30 psi pressure being an example not of my making.

But as pressure goes down, more and more of the weight is handled by the sidewalls but the Segway must be shouldering 2/3 of the weight by rubber stiffness, not air pressure within.

The 5 square inches of tire patch is real, and the 250 pound payload is real, so on the average each square inch of tire patch presses down with 50 pounds. And according to tire patch measure, psi downward is the correct nomenclature here, too, as well as use in expressing pressure.

The three questions remain:
How much IS due to sidewall or rubber stiffness? Is my estimate real?
Where are they made?
Are the Knobbies made there, too?

-iNova


http://www.glidewalk.com

[Peter iNova]

quote:Originally posted by toybuilder

quote:Originally posted by stevew

There is a confusion between tire air pressure and ground contact pressure. The ground contact pressure in psi must be equal to the total weight of the ht/rider in lbs divided by the area of the ground contact patches in square inches. This is what Peter was getting at. The 'patch' is that
area of the tread in intimite contact with the ground -- its footprint. If the tire were infinitely flexible (like a balloon) the ground psi would be equal to the internal air psi.

Ah - I was wondering about that. I think I understand now -- I was going crazy trying to find some write up of the physics behind this.

As I understand it now -- the air pressure inside the tire has to be equal throughout the tire, and the ground contact force must equalize against the air pressure force in the tire.

So, a tire at 18 psi on an unloaded Segway would gain (approximately) an additional 200 lbs. / (4 inches * 60 inch circumference) = 200 lbs. / 240 sq. in = .83 psi of additional pressure when the rider steps on it.

Although the added weight would make the tires want to collapse, the air pressure acting throughout the tire resists that change in tire shape, essentially giving the sidewalls strength to keep the tires up.

http://www.pasadenasegway.com


I think you guys are on the right track.

No matter what, the tire patch supports the whole weight of what is above. Its measure is strictly square inches divided into total weight.

And if you put the air in your Seg while you were riding it (not comfortable to do) you would be putting less air (fewer total molecules) in because the deformation due to weight would be part of the internal pressure and when you stepped off, that pressure would now show up as being lower. And not by a trivial amount.

I wonder if Michelin--used to cars and heavy vehicles--didn't err in their 22 to 30 psi legend inscribed on each tire? 15psi --as stickered near the valve stem-- might become 22psi with a person aboard!

-iNova

http://www.glidewalk.com

[JohnM]

quote:Originally posted by Peter iNova
I wonder if Michelin--used to cars and heavy vehicles--didn't err in their 22 to 30 psi legend inscribed on each tire?


Historical footnote: Michelin got their start in the pneumatic tire business in 1891 making bicycle tires and have been at it ever since. They know what they are doing when it comes to tires for light vehicles. The silica-based compound used in the Segway tires was introduced in their bike tires in 1996 and the tubeless construction appeared in their mountain bike tires in 1999. Michelin has always been on the cutting edge of bike tire technology. Not surprising that DK went with them.

[wheels]

quote:Originally posted by JohnM

quote:Originally posted by Peter iNova
I wonder if Michelin--used to cars and heavy vehicles--didn't err in their 22 to 30 psi legend inscribed on each tire?


Historical footnote: Michelin got their start in the pneumatic tire business in 1891 making bicycle tires and have been at it ever since. They know what they are doing when it comes to tires for light vehicles. The silica-based compound used in the Segway tires was introduced in their bike tires in 1996 and the tubeless construction appeared in their mountain bike tires in 1999. Michelin has always been on the cutting edge of bike tire technology. Not surprising that DK went with them.





The factor not mentioned thus far, is the fact that our tires are 'RADIAL' not bias. (Michelin invented the radial tire [1947]). The stiffness contributed by the sidewall on a radial tire is negligible. The stiffness (suspension) is determined by the amount of air pressure in the tire.
The 'Paw Print' is NOT appreciably changed by changing the air pressure. 'Sidewall flexing' ie 'heat' and 'rolling resistance' are the determining factors for air pressure. Not a major factor on an HT.
Conclusion:di
Lower pressure= softer ride, slightly less range
Higher pressure= firmer ride, slightly more range



One way..... SEGWAY ! !