Mud-club
Vehicle & Technical => Discovery => Topic started by: Jonsey on March 09, 2008, 17:59:44
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Hi, can any one tell me what pressure i should be putting into my BF Goodrich MT 235/70/16 tyres for road driving and also what pressure should i drop to for off road driving?
Cheers,
Chris
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Have a look on the sidewall, that'll tell you the max pressure. I run mine at 35psi for both on and off road.
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Crikey.
IIRC they should be 26/35, 235/70 are a "standard" size on the Disco (unlike the RRC) and any 235 tyres should run at the same pressures.
You can drop 10psi safely off-road, any more and I'd suggest suck-it-and-see.
On the RRC the tyres are 28/35 and off-road 15/25 IIRC.
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Crikey.
IIRC they should be 26/35, 235/70 are a "standard" size on the Disco (unlike the RRC) and any 235 tyres should run at the same pressures.
You can drop 10psi safely off-road, any more and I'd suggest suck-it-and-see.
On the RRC the tyres are 28/35 and off-road 15/25 IIRC.
Don't forget, BB's boots are 33 x 12.5 x R15 and after 20000 miles still have half the original tread left
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Have a look on the sidewall, that'll tell you the max pressure. I run mine at 35psi for both on and off road.
I also run 35psi front and back on-road but usually drop them to 30psi when off-road ~ they're AT's BTW.
My Colway MT's ~ which are the same size ~ are at 27psi all the time. It makes them a bit wallowy on tarmac but improves their grip in muddy conditions.
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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.
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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.
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 ;)
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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
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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
Cheers mate :lol:
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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
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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It's down to weight distibution. The fully loaded front axle carries the same as an empty back axle, there is a massive difference in their loaded weights and if you look at the Disco side-on you'll realilse that the bulk of the body is balanced over the back axle, the engine sits behind the front axle, so the weight is not evenly distributed.
the key element in tyre pressures is that the tread is held evenly against the floor, giving best grip and performance. the height of the sidewall is incidental to that.
I think that tyres are a bit more comlicated than some of you realise :-.
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The whole tyre/rubber technology thing is very complicated. Manufacturers have to give a best compromise pressure after testing which should work across a broad range of conditions. Calculating the presure for the new tyre is initially simple maths. I did have a link ti a 'chav' cruiser site that gave a siplish calc - they tend to go the other way - lower profile tyres which then need higher pressure. Will try and find the link and post here.
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You guys are sooooo last year,
AIR IN THE TYRES??????
my mate owns a garage and stuck some NITROGEN in the tyres, hate the game not the player :cool:
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Yeah, then we can all carry a bottle of nitrogen round with us when we go off-road :roll:
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Yeah, then we can all carry a bottle of nitrogen round with us when we go off-road :roll:
I though we were all carrying one of them anyway for the air shocks :?
There was a rule of thumb posted on here a while ago (by thrasher IIRC) that was along the lines of drawing a chalk line across the read of the tyre*, then driving it for a few miles. The idea is to adjust the pressure so that the line is worn off evanly. If the chalk in the centre wares out first then reduce the pressure and if the shoulders ware out first then increase pressure. Then repeat the test untill you have evan ware. If you have uneven ware then thats probably your tracking or camber etc. thats incorectly adjusted and will need sorting out first.
*as conditions change so does optimum pressure, so best done in the same conditions that you want optimum grip, typical this is a dry day once the tyres have been driven a few miles first to warm them up.
I've also heard another technique involving checking temperaures of the tyres, though can't remeber that one too well.
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Interesting :-k
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usually stick the disco's at 35 front and rear.. when we picked it up it has 47 all round :shock:
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i run my 235/70/16 tyres at 32 all round
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can someone confirm if im right on this or not?
my car weighs 1130kgs front and 1130kgs back which is 2260kgs that converts to 4972lbs
max weight of each tyre is 1160kgs which is 2552lbs
i cant find a max pressure but it says the max weight is 1160kgs at 35psi so i assume thats it and went with it
and fom my calculations following the link above came up with 17psi a corner
it sounds a bit low even for a 35x12.5x15 what do you think?
cheers
kev