Tyre pressures

I have those tyres. Towing a camper on gravel I set the front and the trailer tyres to 28 cold and lower the rears until they look similar because they always take the most load. I have found the expensive way that any higher and the tyres will chip out on sharp, recently graded rock sections etc. - assuming there are any. That’s what works for us, but stand by for many different answers.

Whatever pressures you run, you have to reduce speed. I never go over 70k/ph with anything less than 32 psi.

Hi Gramps. The time this thread finishes your head will be spinning...lol.


Id be more worried about axle housing failure on those roads, with that weight. (depending on vehicle)


Cheers,

Hey Gramps, I reckon it depends on the tyre. My BFGs were different to my current Mickey Ts. The latter bag out more and don't chip. It also depends on the severity of the corrugations. 28psi is about right but amazingly 2 psi either way can make a difference. Then you get the heat effect. In my experience it is suck it and see. Start at 30 psi then adjust down as required. The real pita is when the corrugations are severe then they aren't then they are etc.

I guess we've ended up with 30 psig cold all round.

Speed, load and temperature are all interrelated.
A little less pressure means a lot less speed or you risk destroying the tyres because they get too hot.
Luckily for me, Michelin tell me that relationship, so I don't have to guess. This chart is derived directly from the published Michelin technical data.

EDIT: The Blue line is highway speeds, the red line is 65kph, the yellow line is 20kph.
Cheers,
Peter
OKA196 motorhome

That’s a fairly easy one to answer.

Good gravel road like the GRR between 26-28 psi but you must remember that you must not exceed 80kph.

The next easiest one is the likes of the Anne Beadell.........

18psi as speed will be impossible as most times you will be sitting between 20 to 40 kilometres per hour.

As with any heavy corrugations, make sure you stop regularly to let your shockers cool down and do not overheat.

Funny, when I first started looking at this forum a decade and a half or so ago people were still debating the need the reduce tyre pressures on corrugations.

On the CSR in 2019 I was down to 18psi front and rear, but probably not carrying as much weight.

Worrying about tyres overheating from speed was not a factor.

On good gravel starting point (on advice from 4wd trainer and guide) is 10 to 15 percent lower than usual highway pressures. And watch speed.

As road turns to track just keep going lower.

Gramps
It all depends on a few factors. Some tyres heat faster than others due to compound of tyres. The tyres are the FIRST line of suspension and suitably inflated for the conditions, speed aside, greatly reduces the amount of repetitive axle movement. RIght tyre, right pressure = less axle movement and shock heat generated. Large wheels used to have MORE air under the rim, ie, high aspect ratio, which can be traded for ride comfort and still carry the load at lower speed. Often, the vehicles today have larger dia rims but less aspect ratio to keep overall dia much the same and so those wheels/tyres if in your situation will be unable to perform as well, can't be deflated as much, may run hotter and still give the shocks far more action than previous models of the same vehicle. Combine a little outback dried sticky mud to the shocks to make sure they overheat through limiting heat dissipation and the shocks will suffer and fail. Only by careful monitoring, via stopping and testing heat of suspension components with a hand held infrared gun can you achieve what speed, inflation for the load suits your rig. One of your rear shocks and or tyres may be copping heat from muffler because of proximity, especially at lower speeds and so that side might be heat elevated above opposite side. We can all offer advice but it may not hone in on what you need.

Gramps
I find the old 4 psi rule gives a pretty good guide
If the pressure rises by more than 4 psi from cold, your pressure was too low.
If your pressure rises by less than 4 psi from cold your pressure is too high
Give it a good half hour to hour before checking and adjust accordingly.
Science tells us this system does not work, but, has always been around the ballpark for me.
Murray

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I have seen a number or recommendations for using some instrument measuring device to monitor the temperature of shock absorbers and tyres but I have yet to see any guidance as to what temperature is acceptable. I would also imagine that any temperature limit would vary with manufacturer and model.
I have seen non-authoritative comment on temperature limitations but without substantiation.
Just complying with someone's expression of arbitrary temperature limit may not be adequate as reliable advice.

So, just where would one find such data to enable prudent monitoring. My brief search of shock absorber manufacture websites produced no result.

At an earlier time I tried to determine temperature limits for operational tyres without success. Manufactures may make expressions of the hazard of excessive temperature but seem unwilling to put a number on it. The makers of TPMS units seem to commonly set their high-temperature alarms at 80C although I do not know where they obtained their wisdom. In any case, their valve-mounted sensors are incapable of gauging anywhere near accurate tyre temperature.

Hi Gramps,

We crossed the Anne Beadell Highway a couple of years ago in a convoy of 4 Subaru Foresters and had no problems. One of the things we did was recommended by Beadell Tours was to use your hand to check the temperature of the shock absorber, if it was too hot to touch - stop and let it cool down. At the same time check the temperature of the sidewalls of your tyres, again if they are too hot stop and let them cool down.
It’s a bit of a balancing act to get the tyre pressures right for the speed you are travelling. If the tyre pressure is too low the tyres will overheat and if too high the shockies will overheat.

Leave them pumped up and let the suspension do the work the way it should if your travelling at speed on gravel roads with the usual corners dips etc the tyres will still be flexing a lot so you don't want to damage them when cornering I've never had issues with tyres or suspension doing that in the past 30 odd yrs. If you plan on running at lower speeds on rough tracks then around 30psi might suit your vehicle still remembering to be cautious on corners as the tyre will roll a lot more. Avoid letting them down too much like some of those 4wd magazine or vides guys do they bit a bit carried away some times.

The best thing I've done lately is get a set of tyre pressure monitors. The last trip in the North Flinders it saved me a brand new tyre. It shows tyre pressure and temperature and for around $150 on ebay it's money well spent in my opinion.