Pressure distribution

[pompelmo]

I'm wondering, how tire pressure changes and distributes itself (linear, quadratical,...) in a tire while spinning, depending on a velocity of spinning???

Not just in "pressure in a tire for dummies" style, I need in a scientific way (equations, formulas, articles,.....)

Tnx a lot guys , and sorry for my bad english

[sharkie17]

im not an engineer by any means but i would think that tire pressure should be constant under spinning condition... only under loads...
i.e, cornering, would pressure change...and since tires are constructed to deform not much at all, spinning wouldnt effect the shape much...

[riff_raff]

pompelmo,

A very large amount of the total suspension movement in an F1 car is due to the tire sidewall deflection. Just compare the aspect ratio of an F1 tire versus a high performance road car tire- the F1 tire has a very tall sidewall in comparison.

The F1 tire's inflation pressure and sidewall construction determine it's spring rate. Thus the tire has a very significant effect on suspension performance with regards to suspension travel.

If you want to see an extreme example of race tire sidewall deflection at high rotational speeds, take a look at a top fuel drag race tire.

http://www.nhra.com/gallery/action/Top_ ... dison.html

The tire diameter actually increases so much as the car accelerates down the track (to over 320 mph), that it functions effectively as the car's transmission. As the car goes faster, the tire spins faster. The tire increases in circumference thus changing the effective gear ratio between the engine and rear axel. It's kind of like a CVT. If you've ever had the pleasure of seeing a top fuel burn out, you'd swear that the rear tires seem to double in diameter as they spin up.

Regards,
Terry

[Monstrobolaxa]

riff_raff don't forget that Top Fuel dragsters beside the "tire transmission" also have a clutch transmission...that is built up by pre-load weights that as the speed increases they engage and reduce the clutch slipage to a minimum....though this kind of transmission is only working for under 1 second before reaching minimum slipage.

There was an article on this in the Racecar Engineering magazine a fw months ago.

[tempest]

Hey Guys, long time no post

I think the change in diameter of the Dragsters tire has more to do with the weight of the trie casing than the (minimal) weight of the air inside it. Correct me if Im wrong guys but the pressure change in the tire is due to the air inside heating up. So pompelmo, i think you need to be able to work out the rate at which the air heats up and to do this you need to know something about the tyre characteristics, i.e. how much movement you are getting and how much energy does it take to make that movement, and how much gets returned to the system.

I think

[DaveKillens]

The air inside the tire has very little mass compared to the tire. So when tire deflection is observed, it is as a result of external forces. Just think of those new vacuum cleaners, they rely on the difference between the mass of air and dirt to separate the dirt, by centrifugal action.
And some tires can deflect like crazy, that is the cause of internal heating, and one of the more important factors in tire longevity, and performance.
In dragsters, that deflection we observe is a direct result of wheel rotation. They run very low pressures, approximately 4 psi, and since the tire is basically slipping and spinning after the first part of the acceleration run, that tire is rotating at easily over 300 MPH. Maybe 400, I wouldn't be shocked.
Since at launch, the tire pressures on a dragster are low, and even if the air was heated and expanded in the short six seconds of a run, it's effect would be very slight.

http://www.nhra.com/anatomy/index.html

[tempest]

Since at launch, the tire pressures on a dragster are low, and even if the air was heated and expanded in the short six seconds of a run, it's effect would be very slight.

Remebering that this is an F1 site, i think that the question was probably more about the tyre heating up over a long period of time due to the compression and movement of the air inside the tyre than the casing of the tyre heating from wheelspin, which is a fair bit less common in F1 than in Drag racing.

[Monstrobolaxa]

Dave is right....the tire diametre "change" is caused by the external forces....the pressure inside the tire doesn't have a significant role cause those tires do heat up but it takes time to heat up the inside of the tire.....when we see smoking tires it's caused by the rise of the tempreture of the tire surface...the tempreture inside the tire does rise but slower due to the heat difusion in the rubber from the surface to the inner part.

[DaveKillens]

I respectfully agree tempest, we sort of wandered off the F1 theme when describing dragster tires. I was including them in my post because I was trying to emphasise that the inertia of the tire carcass, and the physical forces on it determine deflection, and that fundamentally, inflation pressure is relatively constant (it does change with temperature, but it takes time), and equal inside the tire. This generalization applies to all tires in all forms of motorsports.

[inside line]

I'm wondering, how tire pressure changes and distributes itself (linear, quadratical,...) in a tire while spinning, depending on a velocity of spinning???

Not just in "pressure in a tire for dummies" style, I need in a scientific way (equations, formulas, articles,.....)

Tnx a lot guys , and sorry for my bad english

tire pressure changes with temperature.
temperature in a tire affected by (in random order):

wheel spin in acccelleration
braking, wheel lockup and proximity to the hot disc
hysterysis in the sidewalls due to cornering loads and compression of the tire from car's mass, downforce and braking loads
hysterysis in the tire surface due to cornering loads and compression of the tire from the car's mass, downforce and braking loads
stiffness of the sidewalls
durometer and compound of the rubber
coeficent fo friction with the track surface in cornering and accelleration
tire pressure
ambient temperature
relative humidity
cooling from air flow
track topography
driver style
weight distribution
bumps in the surface
suspension settings
tire roundness tollerances

depending on the conditions that you wish to study some or all of the variables may fit into your equation. ie: in pure straight line rolling, one wouldn't need to take any cornering, accellerating, braking loads and driver style into account.

pip

woo hoo... first post
awesome forum btw been lurking for a couple of years

[DaveKillens]

Welcome, guest. Jump in, the water is nice. Great place, great info, great people.

[inside line]

Welcome, guest. Jump in, the water is nice. Great place, great info, great people.

thanks dave,

i'm a fellow canadian, vancouver.

there's lots of great peeps here i've noticed. its refreshing to follow a forum that eschews petty insults and infighting. if i want that i'll follow parlimentry debate. science and technology are nothing to fight about as they are empirical. understanding the underlying science is paramount.

i've got a bunch of ideas and questions so i'll register and try it on.

cheers,

pip

[Divia]

As far as i know the tires in F1 are inflated with Nitrogen. Road cars can be inflated with Argon and Neon, gases which are resistant to temperature change. So the pressure is mostly affected by weight transfer while accelerating, braking, cornering and so...

Nice to meet U guys!!

[riff_raff]

pompelmo,

In response to some of the previous posts:

-race tires are usually filled with dry nitrogen. Compressed air contains a significant amount of moisture. Race tires run at temperatures above 212degF, which would cause the water vapor in the compressed air to turn to steam, significantly increasing the pressure within the tire.

-road race tire spring rates are about 2500 lb/in (very gross approximation).

-a 1 psi change in inflation pressure equates to a change of about 40 lb/in spring rate in the tire (again, a very gross estimate).

-a tire sidewall becomes very stiff, due to centrifugal force, when rotating at 200 mph vehicle speeds. The radial tension forces in the tire sidewall increase at the square of the rotational speed of the tire.

[scarbs]

Bridgestone now use dried air for tyre inflation, the expense of shipping all those cylinders wasnt worth the effort....