# Re: Question - car technology

On Wed, 30 Jan 2008 23:08:42 +0000, Chris Whelan wrote:

John Greystrong wrote:

Duncan Wood wrote:
On Tue, 29 Jan 2008 17:15:13 -0000, John Greystrong
<johnny_g@xxxxxxxxxxxx> wrote:

Duncan Wood wrote:
On Mon, 28 Jan 2008 17:58:40 -0000, John Greystrong
<johnny_g@xxxxxxxxxxxx> wrote:

Ben C wrote:

If the tyre's a bit flat, the wheel's radius is effectively
reduced, so
it does more revolutions for a given vehicle speed.

No it doesn't. Unless somebody's taken a slice of rubber out of it
which would probably negate the need for a pressure sensor.
If you're going to be pedantic it's the tyres rolling radius that
is
reduced, & if it's dropped by more than 25% it's detectable.

I wasn't trying to be pedantic, honest. I'm not convinced the
circumference is going to change whatever the pressure so why would a
flat tyre rotate more?

The circomference of a loaded tyre does change,

The length of rubber doesn't change, there's dirty great big steel
belts underneath it. The tyre doesn't rotate around the rim.

How does having 20 or 30psi in a tyre change the distance the tread
travels per revolution?

John

Can I refer you to the post I made in this thread dated 27/01/2008?

There is a link there to a Wikipedia article about tyre pressure
monitoring.

A snippet:

"Indirect TPMS measures the air pressure indirectly by monitoring
individual wheel speeds and other signals available in the vehicle. Most
indirect TPMS uses the fact that an under-inflated tire has a slightly
smaller diameter than a correctly inflated tire and therefore has to
rotate more times to cover a specific distance to detect
under-inflation."

WRT the circumference of the tyre, think about a bicycle wheel with a a
big dent in it that has forced a point on the rim nearer to the hub. The
distance around the rim will still be the same, but because the wheel is
no longer truly circular the effective circumference will have reduced.

Only very slightly...

There's a report which you can see a snippet of by Googling for "rolling
radius change with pressure ieee" - it's the top answer entitled 'OCRed
document'. Can't see the full report without subscribing to the IEEE, but

"The effective tyre’s rolling radius is affected by tyre pressure only in
a small degree, namely about 0.5mm per. 0.5bar of pressure change"

0.5bar = 7.25psi

.

## Relevant Pages

• Re: Question - car technology
... it does more revolutions for a given vehicle speed. ... which would probably negate the need for a pressure sensor. ... The length of rubber doesn't change, there's dirty great big steel belts ... The tyre doesn't rotate around the rim. ...
(uk.rec.cars.maintenance)
• Re: Car tyre: theory that internal pressure is equal to pressure of contact patch on road
... defined to be the same as the pressure of the contact patch between the ... tyre and the road. ... Consider the case of a stationary car and look at a tiny patch of rubber say 1mm x 1mm in the middle of the contact area. ... After you have lifted the car and everything is stationary again the forces acting on that bit of rubber must again balance out to zero. ...
(sci.physics)
• Re: Question - car technology
... it does more revolutions for a given vehicle speed. ... Unless somebody's taken a slice of rubber out of it which would probably negate the need for a pressure sensor. ... The tyre doesn't rotate around the rim. ... How does having 20 or 30psi in a tyre change the distance the tread travels per revolution? ...
(uk.rec.cars.maintenance)
• Re: Question - car technology
... it does more revolutions for a given vehicle speed. ... Unless somebody's taken a slice of rubber out of it which would probably negate the need for a pressure sensor. ... I'm not convinced the circumference is going to change whatever the pressure so why would a flat tyre rotate more? ... The tyre doesn't rotate around the rim. ...
(uk.rec.cars.maintenance)
• Re: Tyre pressures
... round the fact that the correct tyre pressures are specific to the car not ... I'd have thought that a tyre has a specific "correct" pressure regardless of ... On the edge of the car door it gives a pressure to inflate the tyres to, ...
(uk.rec.cars.maintenance)