Airing down contact surface/patch size comparison and clearance loss

[mudking82]

If you have ever wondered just how big your contact patch gets when you air down, well today is your lucky day. I have been wanting to do this for a while and finally got an opportunity; and I want to share this for the benefit of all.

The test tire was a 285/75-16 Nitto Terra Grappler, load range D, 8 ply rating, with about 40% tread left. It was mounted on stock 80 series wheels which are 16x8. Mounted on a 1994 80 series Landcruiser with bumpers, lift, winch, sliders, etc. Tire was on the rear.

PSI---Length of Contact Patch---Clearance Loss from 50 psi
50------------6.75"----------------------0
40------------7.50"----------------------.125"
30------------8.00"----------------------.25"
20-----------10.00"----------------------.50"
16-----------11.25"---------------------1.00"
12-----------12.75"---------------------1.50"
10-----------13.50"---------------------1.75"
7------------15.00"---------------------2.00"
5------------17.50"---------------------2.75"

So for this tire size using 20 psi as a baseline the following is true:
16 psi yields 12% greater contact patch.
12 psi yields 28% greater contact patch.
10 psi yields 35% greater contact patch.
7 psi yields 50% greater contact patch.
5 psi yields 75% greater contact patch.

Using 30 psi as a baseline, the following is true:
20 psi yields 24% greater contact patch.
16 psi yields 40% greater contact patch.
12 psi yields 59% greater contact patch.
10 psi yields 68% greater contact patch.
7 psi yields 87% greater contact patch.
5 psi yields 118% greater contact patch.

It wasn't until below 12 psi that the sidewalls REALLY started to bulge. At 5 psi the sidewall was pushed out so much that it was only about one inch off the ground. Below 10 psi the ground contact patch really starts to grow fast with every 1 psi.

Assuming a 7000 lb. rig equally loaded so that all 4 tires split the load equally, the following is true on this tire:
50 psi in tires yields 29 psi ground contact pressure.
40 psi in tires yields 26 psi ground contact pressure.
30 psi in tires yields 24 psi ground contact pressure.
20 psi in tires yields 19 psi ground contact pressure.
16 psi in tires yields 17 psi ground contact pressure.
12 psi in tires yields 15 psi ground contact pressure.
10 psi in tires yields 14 psi ground contact pressure.
7 psi in tires yields 13 psi ground contact pressure.
5 psi in tires yields 11 psi ground contact pressure.

The above was calculated using the tread width contact patch of 9.00". That really helps you see how airing down can help you stay on top of sand and snow when wheeling.

I usually run at 16 psi off-road, mostly for a comfortable ride.

I would imagine that these numbers would run similar in similar size tires. Hope this helps someone!

 

[Qball]

Awesome information! Thank you for putting in the work and for such a great write up!

What is a safe psi for non bead locked wheel/tires? I am pretty sure 5psi will not be good, is 12 pr 16 psi optimal for no bead lock tires?

 

[mudking82]

Opinions vary. It also depends on how you are driving. Like if going really slow speed in snow with no sharp turns you can get away with a lot less psi then say bombing through the desert and taking sharp corners. Most people would agree I think that 12-16 psi is safe for most normal conditions.

 

[Fast Eddy]

Assuming a 7000 lb. rig equally loaded so that all 4 tires split the load equally, the following is true on this tire:
50 psi in tires yields 23 psi ground contact pressure.
40 psi in tires yields 21 psi ground contact pressure.
30 psi in tires yields 19 psi ground contact pressure.
20 psi in tires yields 15.5 psi ground contact pressure.
16 psi in tires yields 14 psi ground contact pressure.
12 psi in tires yields 12 psi ground contact pressure.
10 psi in tires yields 11.5 psi ground contact pressure.
7 psi in tires yields 10.5 psi ground contact pressure.
5 psi in tires yields 9 psi ground contact pressure.

Where do all the rest of the PSIs go?

 

[mudking82]

Huh??

 

[Qball]

I was thinking about this and wondering if the contact patch gotten wider also as the psi dropped. If so did you take that into consideration when calculating the contact pressure?

 

[rev80]

Where do all the rest of the PSIs go?

As the contact patch grows the weight of the rig is spread over a larger area, thus the amount of pressure on the ground drops over every extra square inch of contact area you gain.

 

[mudking82]

I was thinking about this and wondering if the contact patch gotten wider also as the psi dropped. If so did you take that into consideration when calculating the contact pressure?

It does, but only slightly. I used 9" in width for all calculations. At higher pressures the full width of the tread is not quite touching, but by the time you get to about 30 psi the full width is touching and stays that way all the way down low.

Edited to tread width of 9".

 

[Fast Eddy]

If there's xxx PSI inside the tire, there has to be xxx PSI pushing back from the other side.

It doesn't matter how much the vehicle weighs, or what tires you have, etc.

contact patch size = weight on tire / PSI

PSI inside = PSI outside

 

[cartercd]

Here is my version. By doing these experiments you learn how your particular tire responds to air pressure drops. In my case notice that dropping from 40psi to 30psi doesn't increase the footprint much (from 82 to 86). Dropping from 30psi to 20 psi, however, has a much bigger footprint increase (from 86 to 102). Going down to 20psi also reduces ground clearance by 0.6in.

 

[80t0ylc]

Great info & thanks for your work and posting the results for everyone. One concern that occured to me is the loss of ground clearance in the lower pressures (10 psi or less). 2" & approaching 3" loss around 5 psi in sand or snow is not much of a concern as in the rocks. And for most of us that are running solid axles front and rear, at least it's consistant and just need to adjust our judgement for obsticals. But woe to the independant front end rigs. They would really need to watch it (or have bullet proof skid plates) Might be something to watch and bring to attention of IFS folks on runs when you stop to air down.

 

[tmils470]

If there's xxx PSI inside the tire, there has to be xxx PSI pushing back from the other side.

It doesn't matter how much the vehicle weighs, or what tires you have, etc.

contact patch size = weight on tire / PSI

PSI inside = PSI outside

The thing you are not getting is that when you lower the tire psi the tire patch gets bigger. The tire patch "si" is larger therefore the pounds are smaller per "si"

The psi on the ground is not the same as the psi inside the tire.

 

[Fast Eddy]

The thing you are not getting is ... The psi on the ground is not the same as the psi inside the tire.

What you're not getting is that that is not physically possible. Newton's third law...

Unless the tire is so low it's being pinched by the rim, the sum of the area of the four contact patches times the pressure in the tires equals the weight of the vehicle.

If you're getting a different result, then your measurements are flawed.

 

[mudking82]

I don't know the laws of physics, but this can't be true. I just went out and measured my contact patch width, and I had used 11.22 for the width in my calculations, but that is the actual width sidewall to sidewall I just realized. My contact patch width is 9" at 30psi, which is the full tread width. So I am going to re-figure a few things. But I know the full 9" of tread is going to touch the ground from the 30 psi on down to 5 psi. So using the above contact patch length numbers I have, and using the width as 9", and using your calculation in your last post, it is telling me my rig weighs anywhere from 3150 lbs. at 5 psi, up to 8640 lbs. at 30 psi.

5psi= 17.50" x 9.00" x 4 tires x 5psi= 3150 lbs.
30psi= 8.00" x 9.00" x 4 tires x 30psi= 8640 lbs.

By your rule at 5psi my contact width would have to 19.4" which is not possible with a tire to get it to my 6800 lbs. I actually weigh.

So obviously this is not correct as you are stating. I am not trying to start a fight here, but one of us is obviously missing something, and I didn't think it was me, but I could be wrong. Let me know what you think. I like your idea of a nice linear measurement, but so far I can't see it working.

 

[cartercd]

Take a typical barbecue propane tank that weighs 38lbs full and sits on a thin steel ring. For arguments sake let's say that ring has cross-sectional area of 1 square inch. So the tank will exert 38lbs/square inch on the ground. Now let's look at 2 scenarios. Scenario 1 the tank is in the cool shade and the internal tank pressure is 120 psi. Scenario 2 it sits on hot asphalt in 120 ambient temperature and the internal pressure rises to 200 psi. The tank still weighs 38 lbs and the contact area is still 1 square inch. So it exerts 38psi on the ground whether the internal pressure is 120psi or 200 psi. Internal psi is not equal to external psi.

Your tire may not be as strong as a propane cylinder, but it's contact patch will not increase/decrease linearly with internal pressure.

 

[Golgo13]

Sorry, Eddy, but you should go back to physics class and pay attention before trying to make other people feel dumb.
Newton's third law deals with every action having an equal and opposite reaction. In other words, if you push on something, it is pushing back with equal force.
Pressure is force per unit area. In the case of tire pressure it is measured in pounds per square inch(psi) or kPa(kilo-Pascals). For the sake of being complete, a Pascal is 1 Newton of force per square meter.

Newton's law states that every pound of force the vehicle exerts on the ground(the vehicle's weight) is matched by the force the ground exerts on the vehicle. If the ground wasn't pushing back, the vehicle would sink at a constant rate, without stopping.

The pressure between the two objects does not matter, only the force and its reaction matter.

To the OP what you have done is absolutely legitimate, but if you want a more accurate measure of the ground pressure, find a way to measure the contact of only the tread. Its tough because of the irregular shape of the tread, but its doable.
In practice, there are a few ways to do this more accurately than chalk marks, but it is very time consuming. One method is to turn the tire into a giant rubber stamp. Set the pressure in the tire, lift the wheel off the ground with a jack, rub some ink on the tread, lower the tire SLOWLY onto a sheet of posterboard(paper might tear). Lift he wheel again and, viola, a perfect measurement of ground pressure.

I'm not sure what your original object was to trying this out, but, if you have a youngster in school, you could get this done for free by making them use it as a science project.

 

[tmils470]

Hypothetical examples:

4000lb vehicle aired up to X psi is 16 square inches of contact
this yeaids 250 lbs per square inch on the pavement/dirt (4000/16=250)

4000lb vehicle aired up to somthing less than X psi is 24 square inches of contact
this yeilds 167 pounds per square inch on the dirt or pavement (4000/24=167)

It does not matter what the NUMBER of psi is in the tire the only thing that matters is that it changes and when it changes the footprint of the tire gets bigger therefore pushing less psi on the ground even though the total pounds of all 4 tire patches are the same.

Remember bigger footprint equals less psi on the ground, that the whole reason for airng down.

 

[Fast Eddy]

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

 

[mypancreasbroke]

Great info guys, thanks for charting it out for a good generalization of psi and contact area. I might go out this weekend and run my own tests!

 

[fishEH]

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

Nope. Unless you're aired down to some ridiculously low psi where the sidewalls are actually touching the ground.