Narrow vs. Wide Tires

Roger had this note here:

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My wife prefers narrow (Pizza Cutter) tires (9.50 or 10.50) on her Jeep and she usually gets better traction than I have on my Jeep using wide tires. (12.50). She has better luck in both deep sand and on loose dirt.Β The article below seems to agree that for most off-road situations we often encounter, narrow tires are better. That is even though they don't look as "Macho".Β  - Roger
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I took a closer look at that paper. Here are my comments.

It is a supposedly technical examination of the benefit of wide vs narrow tires. It was written by Scott Brady.

The original (but now dead) link to the paper was:
http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html

A copy can be found at:
http://www.4x4brasil.com.br/forum/attachments/forum-geral-4x4-brasil/232729d1278073628-pneus-finos-x-largos-trilhas-com-atoleiros-e-rosoes-tire-selection-fat-x-thin.pdf

The title is "Tire Selection for Expedition Travel: The impact of tire width on traction".

He summarizes his "position" as: "A tall, narrow tire is a superior choice for all off-highway surface conditions with the exception of deep, soft sand or mud."

Note that he says narrow tires are not better for soft sand.

Reading the article I find it to be 90% bogus. It is a confused mess of contradictions, malapropisms, tangents, and unnecessary technical-ish jargon.

He calls it an empirical study. There's nothing empirical in it.

He repeatedly brings up that a narrower tire will fit a vehicle when a wider tire won't. What's the point in bringing up a case in which one doesn't have a choice?

At about 1/3 down, after much introduction he says: "For the sake of the following details, assume that the test vehicle is 5,000 lbs., and a narrow tire would be considered a 33x10.5 R15, and a wide tire would be considered a 33x12.5 R15, both run at 15psi for trail use." Then he calculates contact pressure at 40 psi. Bogus. Contact pressure actually ends up being vary close to the inflation pressure. He has it at more than twice the inflation pressure.

He mentions "without equating the secant". What the hell is that? It is apropos of nothing. He just tossed in some random trigonometry term to sound scientific.

He simply states: "The wider the tire, the less the rubber will deform to the surface irregularity given the same vertical pressure." There is no support or explanation given. I see no reason to believe that.

With assumptions he gave the contact pressure and the contact area are nearly identical for both narrow and wide. Equal inflation pressure will give about equal contact pressure. With that and the equal vehicle weights you get equal contact area. The contact patch for the narrower tire will be narrower but longer than that of the wide tire. The area should be nearly the same for both cases.

Later he says: "A narrower tire generates greater vertical load on the rubber and the tread..." 
No, that is not true under the assumptions he gave.

He says: "A narrow tire presents less rolling resistance on improved surfaces..."
I've found several articles from bicyclists that say the opposite. The longer contact patch requires more flexing of the rubber. They say that increases resistance.

He says: "A narrow tire will cut easier through mud, snow and sand than a wide one (due to resistance)."
But doesn't that contradict his "position" at the beginning?

"The smoother and wider a tire is, the better it will perform in sand"
OK. Not exactly scientific but it sounds reasonable.

Overall that article is a mess.

-Steve  (12/2/2018)

Author: Roger Mullins

Owner/Founder/Moderator of JustRuns 4 Wheel Drive Club. justruns.com