Offroad Tire Pressure with Lower Profile Tires

[Bob]

I got to wondering about the appropriateness of the usual air down pressure advice on lower profile tires, like the 265/60-R18 on my WK2 Grand Cherokee. Typically, one hears 15 PSI as a starting point for offroad, or alternatively some use a sidewall height of no less than 75% of highway pressure.

So, I made some measurements of tire sidewall height as a function of pressure on the heavy front of my WK2. I used a combination square with an integrated level to ensure accurate readings, and an accurate electronic pressure gauge. Here are the results (in mm and calculated inches since my square happened to line up that way):

Front	265/60-R18
PSI	Sidewall, mm	inch	Percent
35	113	4.4	100
30	110	4.3	97
25	105	4.1	93
20	100	3.9	88
15	94	3.7	83

(For this size tire, Jeep recommends 33 PSI at max GVW; the Toyo tire pressure table indicates 30 PSI for a more normal weight.)

It's clear that 75% will not be achieved until 10 PSI or less, but even at 20 PSI the sidewall already had a very large bulge that I fear could easily result in pinching the tire or cutting a sidewall since the rim is only about 4 inches from the ground. That danger wouldn't be much of a factor on typical higher profile tires on a Wrangler for example. The good news is that clearance only changes slightly, about 0.7 inches even from 35 to 15 PSI.

I think what this shows is that reducing offroad pressure on lower profile tires should be done very cautiously and only to the reduced pressure required to get adequate traction and ride comfort.

I'd appreciate any comments on this.

[FINOCJ]

I like what you are thinking here. Another way I see people discuss how much to air down refers to the size or area of the contact surface between the tire and flat pavement - I think the old rule of thumb is you are looking to double the contact patch (this can be checked with chalk). It would be interesting to compare the contact patch of low profile to a more standard off road tire.

From my own thought process...Low profile would have effectively stiffer sidewalls than the exact same tire construction with a full profile height (my thought is like the 'flexibility" of a steel beam - short beam doesn't flex much, long beam is a noodle). That would indicate maybe a lower than normal pressure - but not sure how that additional 'pooch' or sidewall bulge would affect the bead - might make it more likely to come off.

My final comment: I think for many alspplications - you can (and maybe should) air down more than you think. I realize my cj is an extreme version, but here is some other 'professionals' personal thoughts and pressure. looks to suggest that low profile tires might need lower pressures than comparable full profile to due short sidewall, but that they would also be more susceptible to popping a bead.
http://www.fourwheeler.com/how-to/15...nder-pressure/

I think you sum it up here: I think what this shows is that reducing offroad pressure on lower profile tires should be done very cautiously and only to the reduced pressure required to get adequate traction and ride comfort.

Of course that is not as easy to quantitatively figure out as it is to say! Thinking I might go even lower on my jeep - right now I run 9psi. Thinking maybe 7 for RedCone!

[Spieg]

Agree that you need to be cautious. In general terms, most low profile tires tend to have widths closer to the wheel width (opposed to most truck tires tend to be 3-4" wider than the wheels), which means you can really put the beads at increased risk of impact or even wheel damage when pressure is lowered.

[Jim]

FINOCJ previously posted:

[newracer]

Another consideration is as a tire gets older the sidewalls flex more.

[TJS86TOY]

Bob previously posted:

[Java]

Tire width is barely affected when airing down, even with a large sidewall. It's all about the length of the contact patch. There is a graphic in the link below:

"As can be seen from the side wall photo, the width of the tread actually touching the ground remains unchanged, although the side wall does flex (if it actually touches the ground) it may help a little with reducing the pressure of the whole tire footprint – but only marginally when compared to the increase in tread length in contact with the ground."

blog.arbusa.com/?p=255

It also makes you much less likely to puncture a tire, I'd worry more about flats if you didn't air down than did.

[TJS86TOY]

Paul previously posted:

[Bob]

Thanks for the comments.

I hadn't thought about rim cracking risk but it could certainly happen with them close to the ground. If nothing else they are more likely to get scratched and scuffed. Unfortunately there is probably a fair risk of that even without airing down with this wheel/tire combination.

I thought about measuring the ground contact patch but didn't want to put that much effort into it.

Besides bottoming out the rim on the inside of the tread, I worry about the much more acute sidewall bending- almost a "U" at the lower pressures. I seems that could eventually cause a failure by itself even though speed is low. Considering a sharp rock touching the sidewall, at some point the flexibility to bend around the rock with lower pressure must be offset by the bending tension making the bulge quite stiff.

Well, I guess I'll not air down so much next time and see how the tires perform. I can always stop and let more air out!

[Jim]

From your last comments - I have an idea that you think your sidewall will be WAY bending to cause the rim to contact the ground (when hitting a rock). My thought of what's in your mind is that - just a thought. But, you could air down to a lower-than-you-go level, put the tire on a rock that you think would cause the condition - and get out and look (or spot someone to drive your jeep so you can see the overall motion). My thought is what I think you're thinking - won't be in effect. I think you will have more "typical" (not crazily bent) sidewall shape.

I agree with what you're saying (in the thread - overall) but I think there might be an area where real-world testing might be a "huh" moment.

If you wish to make a trail visit to specifically test air pressures / tire deflection over rocks, I'd be happy to join. Good science!

[Bob]

Jim previously posted:

[Jim]

Nice test & pic.

[Bob]

I came up with an easy way to approximate the tire contact area without inking the tire and jacking the tire up and down on paper for each data point.

Lay a ruler on the ground snug against the tread and parallel with the direction of travel. Then slide an index card between the ground and the tire on each side until they stop and read the distance between the two on the ruler. Change tire pressure and repeat. The width of the tire footprint changes very little, so the contact length is proportional to the contact area.

Here is the data for the same tire again, and plotted on the same graph as before. The contact area gets larger as pressure decreases and the rate of change really increases below 25 PSI. Interestingly the sidewall height above a narrow obstacle is almost a mirror image of the contact area for the particular obstacle I chose.

So unfortunately a big improvement in contact area requires a big loss of sidewall height and increased risk for these lower profile tires, and tires with taller sidewalls are clearly better for reduced pressure offroad use.

Contact Length
Front	265/60-R18
PSI	Distance	inch	Percent
35	182	7.2	100
30	194	7.6	107
25	203	8.0	112
20	225	8.9	124
15	263	10.4	145