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[expeditionswest]

Sean K. previously said:

I dunno....I just read his "position" and frankly, it's a little too overly simplistic in trying to discuss a very dynamic process. It's a good basic guide and alot of it I agree with (the majority in fact in theory)....but he places so many stipulations and limitations on the topic that it isn't an accurate picture for every rig.

I believe the paper is not overly simplistic. It is appropriate to the intended audience and is meant as a primer, a position based on my time testing tires for the manufacturers and consulting on tire construction with tire engineers.

To go into greater detail would require additional testing and detailed review of the physics behind the process, including environmental variables, carcass variables, compounds, soil and geo-science, etc.

If you can ever corner a real tire engineer and ask what they think of overly wide "off-road" tires, it will be an enlightening discussion. They mostly just roll their eyes and say "that is what marketing wants"

The article is specific to expedition vehicle use, which I state clearly in the assumptions section:

This article is specific to heavy expedition vehicles with nominal lift and stock engines (as is desirable for extended travel reliability). The article is NOT about 1,500 lb. sand rails with high HP motors, or other competition platforms with one dimensional surface conditions.

Sean K. previously said:

To take the example to the extreme, why wouldn't a really tall, but very narrow (say 8" wide) be even better yet? B/c there's a point of diminished returns that he doesn't even seem to acknowledge.

I don't acknowledge that?

Here is a quote, directly from the Typical Questions section:

What is the limit of a narrow tires effectiveness? Tire load capacity and rubber tearing. At some point the tire becomes so narrow and the contact pressure so high that the tires rubber molecular bonding cannot sustain the tearing load created by the bonding and mechanical keying to the surface. Remember, the width of a train rail cars wheel is only 4”, and they have incredible traction on a very smooth surface due to the intense vertical load force.

I also include a chart with recommended section widths. I do not recommend narrower than 8.5", even for the lightest of vehicles.

There absolutely is a point of diminishing return, at both sides of the width equation. Performance values drop off precipitously for a 33" tire when they become narrower than 8" or wider than 13". There is a clear bell curve that defines itself in testing for 90% of trail conditions. For a vehicle in the GWV range of the FJ Cruiser, the sweet spot is right at 10".

Sean K. previously said:

Personally, as I said before, 10.5 vs. 12.5 tires don't constitute a huge difference IMO and when you limit your discussion to only 15 PSI, it doesn't give an accurate depiction of what you can really accomplish with a good tire and a set of beadlocks.

I clearly state the paper is not about air pressure, which would require volumes on its own. The paper is about tire selection for expedition vehicles, not rock crawlers. Rock crawling has a very clearly defined medium, which varies in its adhesion properties and surface properties, but the goal is essentially maximum deformation and adhesion, combined with sufficient track width for stability in cambered obstacles. Many rock trails are highly tactile, or highly irregular and air pressures are 10-15 psi lower (or more) than would typically be run on a heavy expedition vehicle. A completely different scenario and criteria. Competition rock crawling on man-made courses takes the criteria even further from natural terrain variables and present the driver and vehicle designer with a highly tactile terrain and consistent surface (essentially irregular concrete poured over dirt or foam or other medium). This favors adhesion over macro and micro deformation, which favors very low tire pressures and maximum rubber contact to the surface, in addition to very soft compounds, bead-locks, etc.

As you no doubt know, the specification for a vehicle and tires from rock crawling in Florence Junction to driving around the world is completely different.

Sean K. previously said:

He doesn't even talk about tire construction either. Clam shell molding vs. sectional molding makes an absolutely HUGE difference in how well the tire works as does (obviously) radial vs. bias. Yet neither is mentioned.

Carcass construction and rubber composition is out of the scope of this paper. The paper is specific to "The impact of tire width on traction" and does not go into detail on carcass construction, rubber composition, tread design, air pressure, etc. My SAE library has thousands of pages dedicated to tire theory. I specifically reviewed tire width.

In summary, running a 12.5"+ wide tire on an FJ Cruiser that is intended for exploration and general trail/DD duties will clearly create a negative performance, reliability and efficiency outcome. If you want to turn an FJ Cruiser into a dedicated trail machine and run a solid axle and the largest tires possible, then that would be an entirely different discussion.

Hope this helps.