Tyre pressures vs contact patch area.

[Terry Junior Member Find all posts View Profile Close Terry]

Have you ever wondered how much tread is on the ground when you let your tyre pressure down?

Well today I thought I would measure the contact patch of tread at different pressures.

I started at my normal road pressure of 40 PSI and went down to 8 PSI which is the lowest I have used on sand.

My tyres are BFG AT 265/70-17. The test took place on concrete.

At 40psi the contact patch was 260mm at the width of the tyre tread and it only changed by 3 mm as the pressure went down.

The tread length was a different story...

40 PSI the tread length was 242 mm

35 PSI = 250 mm

30 PSI = 260 mm

25 PSI = 272 mm

20 PSI = 298 mm

18 PSI = 306 mm

16 PSI = 318 mm

14 PSI = 338 mm

12 PSI = 356 mm

8 PSI = 396 mm

Cheers, Terry.

[Guest Close Guest]

Very good info, and you have proven that tyres don't bag out but the contact patch area increases by changing the length of the 'footprint'.

Another way to do it is to find out the weight at each corner. The contact patch area is a function of the weight and tyre pressure.

Weight (lbs) / pressure (psi) = area (in2)

[Kapitan Member Find all posts View Profile Close Kapitan]

Thank you for sharing your measurement!!
I normally stop at 16 PSI for soft sand. Anything below I would leave as reserve to get out of trouble in case it is needed.

[Terry Junior Member Find all posts View Profile Close Terry]

Thank you for sharing your measurement!!
I normally stop at 16 PSI for soft sand. Anything below I would leave as reserve to get out of trouble in case it is needed.

My normal sand pressure is 14 PSI but I had to get down to 8 psi on very soft, hot sands on Fraser a few years ago.

Cheers, Terry

[AJ120 Out of control poster! Find all posts View Profile Close AJ120]

That's good info and reflects my experience perfectly as I run the same tyres and also have been as low as 8psi

On some SA beaches 16psi will see you sitting on the axles in seconds, some beaches I start at 12psi and go lower if needed.

Cheers Andrew