Airing down contact surface/patch size comparison and clearance loss

[nimrod712]

Yea, the fault in my reasoning is that some of the weight of the vehicle is carried by the sidewalls themselves.

I could be missing something because it's been a long time since I took physics, but I tend to disagree. The OP is talking about two different "PSI"s here: the pounds of air pressure exerted on any given square inch of the inside of the tire versus the pounds of pressure exerted by the vehicle onto the ground. By your reasoning it would seem that if you laid something on top of the tire, it would be exerting the same PSI as what is inside the tire...
Another factor in the equation is that your contact patch is a lot smaller due to the voids in the tread, so the PSI numbers for the contact patch would technically be a good bit higher than what they are...

That said, for the purposes of this thread, I think the calculations are sufficient. Good information for comparison. Good job, mudking!

 

[mudking82]

Another factor in the equation is that your contact patch is a lot smaller due to the voids in the tread, so the PSI numbers for the contact patch would technically be a good bit higher than what they are...

That said, for the purposes of this thread, I think the calculations are sufficient. Good information for comparison. Good job, mudking!

I never thought of that! Although in sand and snow that would probably not apply due to the material going up through the tread and still contacting the tire. Thanks, I really only did it to see contact patch size, the other stuff was an after thought.

 

[grinchy]

I ran a similar test on some Duratracs with 7mm tread left. Size 275/65r18 load e 123. Wheels 18x8.
Pressures were 42, 27, 20, 15, 10.

Note that even at 10psi the sidewall tread wasn’t fully engaged.

I used a couple right angle squares which had 1/8 thick sides.

Using inches and psi here

Psi — contact patch — ground to hub center
42 — 9x8.5 — 15
27 — 9.25x10 —14.375
20 — 9.25x11.25 — 14.25
15 — 9.375x12.75 — 14
10 — 9.375x15 —13.25

I went ahead and deflated-both rear tires so level left right would be the same.

 

[gaijin]

I ran a similar test on some Duratracs with 7mm tread left. Size 275/65r18 load e 123. Wheels 18x8.
Pressures were 42, 27, 20, 15, 10.

Note that even at 10psi the sidewall tread wasn’t fully engaged.

I used a couple right angle squares which had 1/8 thick sides.

Using inches and psi here

Psi — contact patch — ground to hub center
42 — 9x8.5 (76.5 in2/100%) — 15
27 — 9.25x10 (92.5 in2/121%) —14.375
20 — 9.25x11.25 (104.1 in2/136%) — 14.25
15 — 9.375x12.75 (119.5 in2/156%) — 14
10 — 9.375x15 (140.6 in2/184%) —13.25

I went ahead and deflated-both rear tires so level left right would be the same.

Great work, thanks!

To make your data easier for me to understand, I added contact patch area in square inches (in2) and using 42psi as 100% the contact patch area as a percentage (100%):

Psi — contact patch — ground to hub center
42 — 9x8.5 (76.5 in2/100%) — 15
27 — 9.25x10 (92.5 in2/121%) —14.375
20 — 9.25x11.25 (104.1 in2/136%) — 14.25
15 — 9.375x12.75 (119.5 in2/156%) — 14
10 — 9.375x15 (140.6 in2/184%) —13.25

This tells me that at roughly 50% of Recommended Cold Tire Inflation Pressure, 20psi in your data, there would be about a 36% increase in contact patch area.

Very helpful!

 

[Ridgemaster]

A thought: the contact patch measurements are based on a static vehicle on a flat concrete floor. Moving down a trail, the contact patch would fluctuate due to variable terrain influences and the accompanying forces. How much it fluctuates is going to be affected by tire psi and ground shape and by speed/mass which will produce a whole mess of momentary spikes in measurements such as contact patch or even contact pressure(say tire is on a rocky point).