Ford S-MAX (2006 - ) - 07 2.0 Probably silly tyre question

Honest John Back Room » Technical matters
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Sat 26 Jul 2008 08:50
07 2.0 Probably silly tyre question - ajsdoc
Feel a bit silly asking this as probably shows basic lack of knowledge. However, does it matter if the car is not on flat ground when checking tyre pressures? Have a sloping driveway - have to move it to check oil levels etc but does sloping ground affect tyre pressures?

Reply | Report as offensive | Link
Sat 26 Jul 2008 10:33
07 2.0 Probably silly tyre question - Simon
No.
Reply | Report as offensive | Link
Sat 26 Jul 2008 10:34
07 2.0 Probably silly tyre question - BB
Nope it doesn't make a difference if it is on a sloping driveway.....and don't feel silly :)
Reply | Report as offensive | Link
Sat 26 Jul 2008 12:57
07 2.0 Probably silly tyre question - L'escargot
Nope it doesn't make a difference if it is on a sloping driveway ..........


I'm not so sure. Does the tyre pressure depend on the load applied to the tyre at the contact point with the road/drive? The greater the load the more the tyre is deflected and (possibly) the smaller becomes the internal volume of the tyre, even though the mass of air in it remains the same. On a sloping drive the load on the contact point will be less than on a horizontal drive. The pressure difference may be small, but I think it may be finite.

Edited by L'escargot on 26/07/2008 at 13:00

Reply | Report as offensive | Link
Sat 26 Jul 2008 14:23
07 2.0 Probably silly tyre question - Simon
>>On a sloping drive the load on the contact point will be less than on a horizontal drive.

I disagree. The contact patch of the tyre to the surface will be exactly the same whether the ground is perfectly horizontal or whether it slopes upwards at say 45 degress.

The only time the contact patch of each tyre differs is say when you park on uneven ground so that the weight of the car is distributed differently depending on how each tyre touches the surface.
Reply | Report as offensive | Link
Sat 26 Jul 2008 16:07
07 2.0 Probably silly tyre question - L'escargot
>>On a sloping drive the load on the contact point will be less than on
a horizontal drive.
I disagree. The contact patch of the tyre to the surface will be exactly the
same whether the ground is perfectly horizontal or whether it slopes upwards at say 45
degress.


I was talking about contact load (which is not related to contact area) and not contact pressure which is related to contact area. On sloping ground the contact load will be the weight on that axle multiplied by the cosine of the angle of the slope.
Reply | Report as offensive | Link
Sat 26 Jul 2008 15:08
07 2.0 Probably silly tyre question - Number_Cruncher
>>but I think it may be finite.

There is a finite effect, but not one you would measure with typical tyre inflation apparatus.

Reply | Report as offensive | Link
Sat 26 Jul 2008 20:08
07 2.0 Probably silly tyre question - caine
Tire pressures are actually a messure of gases contained at sea level or one atmosphere.greater changes than your concern occur at temp or outside air pressure variations .manufacturors decide on given numbers taking into account local variations of conditions.I dont have any doubt programs exist for formula one teams to take into account the geographical height of the track and adjust accordingly.that aside,if tested on a slope wouldnt it depend on whitch way the car was facing?Which tires would you adjust to compensate for uneven distribution of mass.
Reply | Report as offensive | Link

From choosing the right tyre to what to do when it goes wrong - we have the answers.

Tyre News And Advice