Thread: Tyre Pressure Vs Load Rating

Slightly OT but I had some Goodyear AT/R 205 s (I think) fitted the other day & I mentioned that the LR recom pressures were, Front 28psi & Rear 38 psi. From the door sticker.

Vehicle has only general stuff, fuel, tools & misc. bits & pieces & not a lot more

He more or less told me that was bollocks & was for UK roads not OZ. Different cambers he said.

He recommended Front 35 & Rear 40.

Is that rubbish or what? Comments?

Vehicle still handles ok & the ride is not hard, & maybe absorption is being taken more by the suspension now & not the tyres.

Your tyre with a load index of 105 will carry less weight at max pressure than a tyre with an index of 113. Generally speaking the 113 index tyre has a rating of 8 or 10 ply "rating that is". The australian rim & tyre standards association manual has a Cross referance guide for tyre pressure v load. In theory your new 113 tyre will need less pressure to carry the same weight. Poeple buy tyres thinking they have an 8 or 10 ply tyre, infact most of these tyres have a rateing of 8 or ten ply read the side wall, if a tyre is indeed a paticular rating then it will be stated as 8ply 10ply.

The easy guide to on road pressure. Set your pressure at what you think is the right pressure. Drive for an hour, check the pressure. If you have an increase of more than 4 psi than you need more pressure. less pressure more friction = more heat and pressure. This is only a general guide as there are many varables to consider. But this makes a good starting point.
shsould your pressure not increase a little than you have to much pressure. The same check can be done if you load up for a trip.

Des your tyre man is basicly right, However the stickers for recomended pressure is set by A standards manual. All tyre standards are common, E11 is european, JATMA is japanese standards and DOT is US STANDARDS all of these use the same engineering standards which are the same as Australian standards. High pressure will help in tyre life but the down side is the tyre becomes more vunarable to impact fracture.

glen

Thanks Glen. I'll leave them up for now & see how they go.

The "Pirelli 4 psi" rule applies to running tyres at highway speeds and not necessarily AT or more aggressive tread patterns as these will generate higher temperatures because of the friction. But it is the best place to start.

Actually there isn't much science just a lot of experimentation.

A couple of psi either side of the ideal isn't critical as long as it is balanced all around and a few more in the rear when there is a load.

Yep - thanks for that. I will be buying them in the next couple of weeks, so I will try to 4psi rule. Will see what they tell me to start at, but otherwise I think I'll try them at the same 32/34psi.

Tyre pressures are always a compromise, as there are several (often conflicting) factors to consider.
Firstly, tyre life. The pressure needs to be set high enough to avoid excessive temperature rise in use - but this merely begs the question as to what is excessive and what use we are talking about - travel on the Stuart highway (no speed limit) with the air temperature around fifty is very different from driving round the suburbs in winter in Canberra for instance. The actual pressure needed to keep the same temperature rise will vary according to the construction of the tyre and the tread type and composition.

Secondly, vehicle handling. The rigidity of the tyre depends on the pressure, and too low a pressure allows the tyre to flex between the ground contact and the wheel, decreasing the precision of the handling. It is important to keep the same amount of flex front and rear (to avoid unexpected handling characteristics), which is one reason you need to increase pressures when loaded. Unless the suspension design is so bad that it needs to be compensated for by having different tyre characteristics front and rear, in general the higher the pressure, the more precise the handling.

Thirdly, ride. Generally the lower the pressure, the better the ride.

Fourthly, adhesion. The adhesion of a tyre depends on the tread making contact over the whole width and length of the footprint. This will not happen if the tyre pressure is too low or two high.

Fifthly, flotation. The amount that the tyre sinks into soft ground depends almost entirely on the tyre pressure. Not really relevant except offroad.

The actual pressure recommended by the manufacturer depends on their assessment of the above factors, mainly from experience. It is of interest that with my 110 the pressures recommended for the UK are superseded for Australia - and the Australian pressures are quite a bit higher, I'm guessing because of the higher ambient temperatures. There are quite detailed specifications for increase in pressure for both speed and load from 35/35 (speeds to 110 unloaded) to 60/65 (fully loaded, speeds above 130).
With non standard tyres pressures may need to be varied from the specified ones, but for normal driving the variation will be very little if any. Thicker walled tyres driven at high speed in high temperatures need higher pressures to limit temperature rise, and wider tyres will tolerate lower pressures, and may in fact need them to avoid a harsher ride.

However, as one_iota says, experimentation is the key to establishing the optimum pressure - but the placarded pressures are probably the best starting point regardless of the tyre size or type.
John