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shaun.mobile

Member Since: 12 Dec 2011
Location: England
Posts: 156

2011 Defender 90 Puma 2.4 XS CSW Fuji White

Checked your tyre pressures recently?

I have a 2011 2.4 Puma XS 90 with 18 inch Silverline wheels running 285/60 TR18 Cooper tyres.

I only ever use this on the road.

I have been running at 40p.s.i. with no apparent issues for the last 18 months. tread wear appears to be even and apart from the ride being a bit crashy on bumpy roads all is good. I average over 35 m.p.g. with this setup.

just been to my local garage to check the tyre pressures (first time in a few months) and the first one showed 50p.s.i.

I let some (an awful lot) of the air out to try to get back to 40p.s.i. but the gauge on the airline seemed a little inconsistent.

bought a digital tyre gauge (Ring @ £12.50) and the pressure is indeed 50p.s.i.

does the hot weather really make that much difference?

just been on Wikipedia to remind myself of my o level physics and found Gay-Lussac's law which states

P1/T1 = P2/T2 where P is pressure and T is temperature.

by my maths that means that P2 = P1*T2/T1 and when I put some numbers in 40psi*30°C/22°C = 55p.s.i.

put another way 40p.s.i.*23°C/22°C = 41.8p.s.i. i.e. for each °C rise in temperature the pressure rises by 1.8p.s.i.

this all assumes that the tyre has not expanded, which it clearly will have but that probably explains why the pressure is 50p.s.i. and not the calculated 55p.s.i.

thoughts please.

19th Jul 2013 12:28pm

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6482

2015 Defender 90 Puma 2.2 XS CSW Montalcino Red

You need to convert the temperatures to Kelvin rather than use deg C. To do that, add 273 to the deg C value.

So P2 = P1 * T2/T1 = 40 * (273+30)/(273+22) = 40 * 303/295 = 41 psi (2 sf)

1988 90 Td5 NAS soft top

2015 D90 XS SW

19th Jul 2013 12:41pm

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6482

Also, the tyre (and its air) may be warmer than the surrounding air, which is measured in the shade.

A tyre in the sun will be hotter than that

1988 90 Td5 NAS soft top

2015 D90 XS SW

19th Jul 2013 12:43pm

theorangebandit

Member Since: 03 Jun 2013
Location: Stowmarket
Posts: 862

heat makes a huge difference to all working components on vehicles. I check my tyres weekly but I rack up plenty of miles.

easiest way to watch the pressures is check them on weather changes, drastic ones I mean, like snow then heat, then rain, then warm, and soo on

this will give you, in England about 3 weeks between checks lol

19th Jul 2013 2:19pm

shaun.mobile

Member Since: 12 Dec 2011
Location: England
Posts: 156

Thanks for the reminder Martin, I had forgotten all about absolute zero.

Doesn't help explain the 50p.s.i. Though

19th Jul 2013 11:48pm

william8

Member Since: 03 Apr 2013
Location: Motherwell
Posts: 259

2008 Defender 90 Puma 2.4 XS CSW Zambezi Silver

theorangebandit wrote:
this will give you, in England about 3 weeks between checks lol

And you dont get time to drive it if your in Scotland, too busy checking the tyres

William

20th Jul 2013 4:35pm

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