Understanding tire traction involves many factors and aspects. I don't want to condense it too much as to loose valuable information, so I decided to create a multi part series. In this Part B I will mostly focus on the practice of 'Airing Down' and how it really works.
Airing Down would be if you decrease your tire pressure to 15 psi or below. If you go from 40psi to 20psi you are not really doing anything besides increasing ride comfort. If this is your reason why you spend 15 minutes at the trailhead letting air out of your tires, go for it!
With that said, now let's look at 'Airing Down' as it relates to offroad traction.
As you can see in the illustration above, the contact patch of the tire gets significantly larger at 15psi. Please note that the increase comes mostly from the lengthening of the contact patch and only minimal from widening. The larger contact area (at the same weight of the vehicle) reduces ground pressure and that aids in 'floatation'. BUT, reduced ground pressure also reduces mechanical keying (big part of traction).
So: Airing Down can reduce traction!
As everything in this world, it is a trade-off. If you understand that, you can make good decisions and get further down the trail without getting stuck.
Let's look at specific scenarios:
In deep, soft sand, airing down is mandatory, no doubt! The floatation helps to keep you on top of the sand and moving.
In snow you have to consider additional factors. Ground pressure helps to force the tire tread into the packed snow under your tire and increase mechanical keying. Reducing the pressure by airing down, reduces your achievable traction.
If the snow is deep enough to high center your vehicle, you have to get as much floatation as possible to keep going -> Air Down!
If the snow is not deep enough to high center your vehicle -> Don't Air Down! The smaller contact patch let's you cut through the snow and get to hard packed surface. The traction gained outweighs the drag resistance of the snow you sink into.
Hard packed snow -> Don't Air Down! Same as above, the higher pressure of your contact patch get's better traction as it pushes into the snow.
Slush -> Don't Air Down! The higher contact pressure cuts through the slush and contacts the trail/road. You don't want to float on top of slush.
Mud is very similar to snow:
If the mud is deep enough to hit your differential or bumper (you would start to plow), you have to get floatation not to sink as deep -> Air Down! It still decreases your traction, but at least you are not pushing 400lbs of mud ahead of you.
If the mud is not deep enough to high center your vehicle -> Don't Air Down! The smaller contact patch let's you cut through the mud and gets the tire to hard packed surface under the mud/water. The traction gained will keep you going.
Tires-on-rocks-traction is a different beast.
It is closely related to on-road traction. The rubber compound adheres and keys with the rough surface. For this scenario, the more contact - the better -> Air Down!
If you find yourself rock crawling (not just running over some pebbles on a trail) airing down also allows the tire to conform to the rock and thereby increasing the mechanical keying.
Be aware that a soft, ply-able sidewall (i.e. aired down tire) is more likely to get pinched between a rock and the wheel which leads to an ugly cut. You have to be more careful where you place your tires.
Since most trails are a mix of surfaces, there is no one-fits-all approach. The 2 things you should stick to when Airing Down are: Do it before you get stuck, Air Down as low as possible, because: do or don't, there is no try.
If you take all factors, their connected nature and resulting trade-offs into account, the avid offroader's tire choice should be driven by the following:
Taller tires increase the possible contact patch size (when aired down) and reduce the ramp angle in front of the tire (lower roll resistance) compared to smaller tires.
Wide tires increase the contact patch size, but at the expense of increased roll resistance compared to a narrow tire -> not as beneficial.
In conclusion, tall, narrow tires are preferred in pretty much all circumstances if you want to increase your off-road traction, unless you are climbing glaciers.
I want to finish this post with one of my favorite videos of 2WD vehicles off-roading. It demonstrates how narrow but tall tires get through some serious mud, where your 12.5" wide 35s in 2WD would get stuck.
I will probably create a Part C in this series to address any questions or requests that I receive. Don't be shy, let me know what you want to see/read about in regard to off-road traction. Thanks!