Mud-club
Vehicle & Technical => Defender => Topic started by: DIL on October 20, 2009, 12:39:16
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Hi,
Does anyone know the recomended tyre pressures for 265/75 R16 Grizzly Claws on a Defender 90, for on the road??
Thanks Mark.
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When i used to run them on my old disco i had them at 45 psi front and back and they where fine but i think they are rated at 55psi/60psi
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When i used to run them on my old disco i had them at 45 psi front and back and they where fine but i think they are rated at 55psi/60psi
Not saying thats wrong but 45 psi :shock: seems alot, ive got 35x12.50x15 cooper stt's and i run them at 28 psi for the road. in my own experience mud tyres behave better at slightly lower pressures on the road, when landrover had my truck for a service there tech man put the tyres upto 50psi and they where awful it very nearly caused me to spin out.
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The best pressure ive found is 28 to 32 psi. You go too high, your kill the centre of the tread.
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From LR experience for 265/75R16. Unloaded 25 front 27 rear. Loaded 27 front 35 rear.
Hope this helps
Rich
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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.
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.
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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
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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.
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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
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.
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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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On the side wall of the tyre there will be 2 bits of information
Max weight
Pressure
Say the max weight is 1000kg and the pressure is 50psi.
You've 4 tyres, so that's 4000kg at 50psi.
If the truck weighs 2000kg, then divide 4000 by 2000 that gives you 2.
divide 50 by 2 and that gives you the rated pressure from the tyre manufacturer for your vehicle.
For the above that would be 25psi per tyre.
If you want to get fancy, work out your axle weights and adjust the pressures to suit. That's why the information is stamped on the tyre, cus it's actually useful.
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sorry to be a killjoy but the best thing you can do with grizzly vlaws is put them in a skip bloody dangerous if you dont believe me do a search on here and other sites iirc there not approved for road use mine exploded off the rim at 70 check out ebay there allways for sale in 3`s common problem i believe
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Unloaded 26 rear 30-32 front
Loaded 32 rear 30 front
You want good tread contact yet flexible strong side walls with good even wear!