I've always been led to believe that airing down doesn't alter the width of a tyre (well very little) it's the length of the footprint that changes and gives extra grip.
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OK, I will read more moggy but I’ve already read a lot. I’m always ready to learn, but I’d prefer to learn why - rather than just take someone’s word for it, simply because I’m more stupid than others because I’ve taken the trouble to ask and question why the obvious is wrong.
Chas’ diagram makes 100% sense, and theres no rocket science in that scenario. It’s impossible to make a tread wider by airing down unless you run flat to bring the side walls down to the ground, and then you’d probably be lifting the Centre of the tread anyway and defeating the object.
But, and here’s the big but that makes me ask the question...
From left to right, 50 psi down to 15 psi, let’s presume a footprint area (length) increase of 80%.
If we also presume that the tread footprint in the diagram is from a 235mm wide tyre, just to put some numbers to it, then replace that footprint with a 305mm wide tyre, you’d gain 30% more footprint in the first 50 psi step, before you start.
Then you’d also gain the same 80% by airing the 305 mm tyre down from 50 psi to 15 psi assuming that the diameters of the tyres are the same.
Now that’s not rocket science either, so I’ve obviously missed some magical bit of math or maybe some hidden physics that’s printed in the only article that I haven’t read on the subject, because nowhere and I mean nowhere in the dozen or so articles I have read have ever answered the above question.
I do understand that penetrating mud can or may somehow increase grip, and I’m ready to accept it, but I’m buggered if anyone has explained why it might be the case.
There’s more suction when you sink which is detrimental and there’s more of a ridge for the tyre to try to “climb” out of which is also detrimental.
So, accepting I’m stupid because my mathematics contradicts the skinny is better, please someone give me an answer that makes sense.
Maybe it’s about weight per square cm, because a bigger footprint will reduce ground pressure, that’s physics, and that will suggest that a mud type tread won’t be getting a full bite into the surface.
Maybe this is the answer, but in a 3.3 tone truck, 4 footprints on 305mm tyres compared with 4 footprints on 235mm tyres, isn’t going to be so catastrophically different to cause total loss of grip.
Answers on a postcard please, I’m really at a loss on this one.