jwest
Full Access Member
You can't let enough air out for good traction. With a fully inflated tire, you have no where near the traction of a tire at 20 psi. It's night and day different. A deflated tire can 'wrap' over objects which not only provides over 50% more contact area of the tread to surface/object, but that also allows the tire to deform away from dangerous objects that can cause a puncture.
Consider the amount of force a basic balloon requires to pop against a knife point. A super tight balloon can simply drift in the air onto the point and pop. No pressure at all really, hard to even feel. However, take a balloon with almost no air, in your hand, press on knife. It's actually going to take some intentional pressure to break the rubber material. Kind of like your hand if it's moving quickly vs gentle pressure. For either, a lot more pressure is endured before puncture. Pretty much the same on tires and sharp stuff off road.
So the soft tire wins huge on traction and puncture resistance but there's more...
The ride is **** for hours at 40-45 psi. The ride is like a freakin magic carpet dream at 20 psi. This is nice for sure but it also allows the tire to become an active part of the suspension, not just an extension of the rigid wheel.
The added deflection in a soft tire, plus more sidewall, actually starts to appreciably increase the effective articulation of the suspension and overall range of 4 tire traction. This is because on the same obstacle, a stiff inflated tired vehicle will left tires off the ground, like a diagonal situation, a lot sooner than mine will with a full inch more sidewall plus 2-3" of actual tire deformation. The two tires that deform so much allow the other 2 to stay in contact and therefor help drive through the spot.
Basically, the whole 20" vs 18" thing is not just one aspect. There are several things that all come into play making the whole system so much better than simply comparing a tire size.
People taking 20's off road much eventually screw up the wheels and switch which is way more expensive than just going the practical route up front.
I like 20's, they look awesome, and on highway trips they can feel a little snappier but my current big trip set of Michelin MS2 are set to firm psi and running 80-90 in canyon interstate sections is as stable as ever. Even those can be aired-down to mid 20's for long days on gravel forest roads. Sometimes the coolest places are just beyond the basic gravel road. I am not a fan of changing tires even at home, I am much less a fan of getting a flat off road, in the sun, let alone at night, in rain, snow, mud, bugs flying around, whatever.
A lot of big trucks roll around on 20's, they are also rolling a 35" or bigger tire so there is still a lot of sidewall. The lr3/4rrs has a limited tire size even with the mods and can actually go into much more rugged areas than a full size truck will ever see.
Can you imagine bringing a girlfriend along and you dork up a 20" wheel having to change it trailside for a spare and she's thinking "what a short sighted man chose an inappropriate wheel and hauled me out here knowing that" LOL
