Vehicle & Technical > Discovery
Tyre Pressure
glaggs:
Tyre pressure always seems to cause a big debate and bring a lot of opinions out of the woodwork. Firstly the tyre pressure given on the side wall is a max safe limit for that tyre and has nothing to do with the pressure that your tyre on your disco should be run at. Second, larger tyres don't need the same pressure as smaller tyres on the same vehicle. Take a look in your owners manual - two tyre sizes are given in my 300tdi hand book, 205 r16 and 235/70 r16. The smaller tyre has recommended pressure of 28psi front and 38psi rear. the bigger tyre is 26psi front and 34psi rear. Why - because the mass of air required to carry a given weight is constant. The same mass of air in a larger space = lower pressur. Different vehicles fitted with the same size tyre will have different recommended pressure because of the difference in the vehicles weight.
Air has two jobs to do in your tyre. 1. it helps the tyre keep its shape and 2 it controls the temperature of the tyre. More air = cooler running - the reason that you add a couple of psi for high speed use or higher loads
Off road pressure depends on the surface and will always be a compromise, but generaly you will be looking to increase the contact patch of the tyre and as usually you won't be traveling very fast the tyre won't be over heating. The rule of thumb here is that the tyre should be deflated so that where it comes in contact with the grond the side wall measures approx 3/4 the hight of side wall when measured at the point furthest away from the ground.
SteveGoodz:
--- Quote from: glaggs on March 10, 2008, 01:12:54 --- The rule of thumb here is that the tyre should be deflated so that where it comes in contact with the grond the side wall measures approx 3/4 the hight of side wall when measured at the point furthest away from the ground.
--- End quote ---
Hi Glagg, that's an interesting rule of thumb that I haven't come across before.
When you talk about the side wall are you actually referring to the bit of the tyre between the rim and the start of the tread, or are you including the tread as well? Just want to be sure for when I check the ratio on my MT's at the weekend. Just out of curiosity you understand ;)
glaggs:
Its only a rough guide - but it includs the tread. Here's a link that gives a bit more info
http://www.4x4mag.co.uk/novfeatures/novtyres.htm
SteveGoodz:
--- Quote from: glaggs on March 10, 2008, 09:04:29 ---Its only a rough guide - but it includs the tread. Here's a link that gives a bit more info
http://www.4x4mag.co.uk/novfeatures/novtyres.htm
--- End quote ---
Cheers mate :lol:
Wireless:
--- Quote from: glaggs on March 10, 2008, 09:04:29 ---Its only a rough guide - but it includs the tread. Here's a link that gives a bit more info
http://www.4x4mag.co.uk/novfeatures/novtyres.htm
--- End quote ---
Yeah, explains things part way, but then we move back to the original question of why the tyres on the front are a lower pressure than those on the back.
I realise it's down to steering, braking, and weight distribution of the body work of the vehicle (otherwise why would the maximum axle loading weights be different as well), maybe someone is able to provide the equation that allows a proper calculation of tyre pressure in relation to a change in tyre size from standard fittings?
Ok, I'll work it out for myself, it's only maths.
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