Vehicle & Technical > Defender

Tyre Pressures

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Saffy:
common theme this on all motorsport forums -  "what tyre pressure on my non standard rig with non standard tyres?"  With all sort of answers and suggestions.

How do the vehicle manufacturer calculate what tyre pressure you should have for the tyres they supplied? Guessing they didn't pluck that figure out of the air. Doesn't the vehicle weight on the wheel/axle,  manufactures max pressure and the load index of tyre come into it somewhere?   Though I never worked it out (or put my modified truck on the scales) so go with what feels right and doesn't wear the tyre, ball park figures like most others do.

Also....(I may of dreamt this so bare with me )... I thought there was a magical measurement of the width the tyre (at the bottom, the bulge for want of better description) which can be obtained from the tyre manufacture (it's not written on sidewall). You inflate/deflate the tyre until that bulge is of the correct dimension and you will pretty much have the correct pressure for that installation (as long as max load/max tyre pressure has not been exceeded).
(Like said I could be making that up as I can't now find info about it  :lol:)

Update Edit:
"The bulge for want of better description" It's the result of Deflection, that's the official term. Find what the manufactures recommend deflection  is for a particular brand/model of tyre (Static Loaded Radius figure all that needed?) and you can use it to figure what the pressure should be for a given install.


--- Quote ---Many people think that deflection describes the bulge at the bottom of a loaded tyre. Actually, this bulge occurs as a result of deflection. Deflection really describes the change in the tyre’s radius when a normal load is applied. The radius is measured from the centre of the axle/hub to the ground.
Unloaded Radius is measured with no weight on the tyre. The tyre is mounted on a rim and inflated to working pressure.
Static Loaded Radius is measured with the weight of the vehicle and payload on the tyre. Static means that the tyre is standing still.
The deflection is the difference between the Unloaded Radius and Static Loaded Radius. This is the same distance that the axle lowers when the vehicle is fully loaded. Deflection is extremely important. Engineers design tyres to operate at a certain percentage of deflection. Operating with too much or too little deflection lowers tyre life.
--- End quote ---

topless matt:
You are indeed correct about the 'bottom of the bulge  :lol:' but that would require altering tyre pressures everytime something changed to achieve that at all times. eg after you fill up with fuel, or carry anything, let alone tow a trailer

DIL:
I set them at 30psi when i fitted them and they seem to drive fine, just dont want them to wear or crack to quick. I will have  a look what the back ones look like when i have a trailer on and probally put a bit more in.

clbarclay:

--- Quote from: topless matt on October 21, 2009, 12:48:58 ---You are indeed correct about the 'bottom of the bulge  :lol:' but that would require altering tyre pressures everytime something changed to achieve that at all times. eg after you fill up with fuel, or carry anything, let alone tow a trailer

--- End quote ---

By rights the tyre pressures should be adjusted to suit theses changes in loads anyway, but instead we just pick one pressure as a compromise.


Tyre manufacturers do publish pressure tables for agricultural tyres, though I've never seen an equivalent for car tyres. These tables show the maximum load a tyre can carry for a given range of pressures and maximum speeds.


I have come across 2 similar methods for setting car tyre pressures. Start by going for a short drive to get the tyres up to temperature and then chalk a line across the tyre tread. Then drive a short distance and keep inspect the chalk line as it wears off. If the line wears of evenly then the pressure is about right. If it wears of in the middle first then its over inflated. If it wears off at the edges first then its under inflated. If you get any other wear patters, such as wearing off on one side first then go and get you wheel alignment sorted first.

The second, slightly more high tech version is to use a temperature probe to measure the tyres temperature across the tread. Just go for a drive to warm the tyres up and then measure the temperature. The temperature should be the same across the tread. If over inflated then the centre will run hotter, if under inflated then the edges will run hotter.

jadatis:
For on the road you can use my spreadsheet.
It works with the formula of the European  ETRTO wich gives saver advice then that of the American TRA.
http://cid-a526e0eee092e6dc.skydrive.live.com/browse.aspx/.Public/Recalculating%20tyre-pressure
In this map always take the newest version and there are examples.
This formula is used from P-tires up to those for trucks.
So if the answers are low to your opion, you can still trust them. And where there is a chanche something calculates wrong it is warned for in the sheet.
If there are questions tell it here.

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