Does vehicle weight affect how far you can drop tyre pressures?

Reader question:

Hello Robert..
Ive just watched your Sand Driving tyre pressure vid on YouTube. Excellent demonstration. Thank you..
I have one question for you?
Does the weight of the vehicle determine how much you can drop your pressures?

If you have a box trailer at home running say 30psi in the tyres empty.. if you go and get half a ton of Sand in it.. will you be required to put more air in the tyres to drive safely?
This being said, your theory of dropping by percentages is far more accurate then dropping by numbers..
So many people I see trying to help people out of bogs and they drop straight to 15psi is incredible, no regards to the weight in the back of the vehicle. Or anything..
I run a Fuso Canter on 37s..
I run 70psi on the road and drop to 35psi on the sand.
With 5ton of weight that’s a lot of weight for the to roll off the rim, even with just the slope of some of the beaches.
Interested to hear back from you.

Yes, the weight of the vehicle affects the amount you can and should drop your tyre pressures offroad. The heavier a vehicle, the more a tyre will deform for a given pressure. And the heavier a vehicle is, the greater the contact patch needed to be effective on a compression terrain like sand. The two factors work against each other so the end result is, roughly, the same – always the way with tyres which just get weirder and weirder the more you look at the detail. Coming back to your specifics; talking to AAV4x4 who are the experts on this subject you’re fine to go down to 20psi, even lower if needs be. You may well find 35psi is too high on the sand.

However, the more a tyre pressured is lowered, the more it deforms, and the slower you must drive to stop the tyre overheating. The deformation is also governed in part by the diameter of the tyre; a 37″ tyre won’t deform as much for a given contact patch as a 35″ tyre, so it won’t heat up as much, and it also has a larger amount of tyre exposed to the air for cooling.

Sound complex? Yes it is. I’ve only just scratched the surface of this topic!

Re the box trailer; yes, if you add weight to the trailer you must add air to the tyres, and in this case the difference in weight between a trailer empty and filled with sand is significant, so the tyre pressure difference must also be significant. Same way when you put a car on a car trailer you should increase the tyre pressure.

Also, the recommended tyre pressures you see for 4x4s typically don’t apply to light trucks which run maybe 60-80psi on road whereas 4×4 run 35-42. So you can take them as the principle, but not the actual numbers.

Cover image courtesy AAV4x4

And here’s the video the reader referenced!