Tires


Tires are the other key component when considering an off roading adventure. The type of tires you get are unlike your typical street tires as you will not be traveling on normal paved surfaces. The right choice in tires can make the difference between a good day and getting a ride home. When you are driving around on paved roads the friction coefficient between your tires and the road remains more or less constant and consistent. The surface is even and very solid so a tire with lots of smooth flat contact patches is very beneficial. When you are not on the pavement however the surface can vary greatly even as little as a foot away. You are driving on loose terrain which is not very solid and in most cases has a much lower coefficient of friction. This means that you will need more surface area to make up for the loss of friction. The main ways to do this are larger and wider/narrower tires, airing down, and tread choice.

Tire Size

By using a wider and larger tire on your off road vehicle you are increasing the contact patch between the tire and the ground. With this larger contact patch you are increasing the area in which the friction forces can act upon the tire. You are also increasing the weight which is acting down on the the ground. This is a very complicated part of the choice as the type of terrain dictates what you should get as far as going with a wider or narrow tire. A wide tire is good for surfaces like sand and mud where you want to stay above the surface of the terrain. By having a wider tire you are allowing the weight of the tire to be spread over a much greater surface area which means less lb/in^2 acting down on the ground. A very handy experiment you can use is pushing a pencil into a sand box, then repeat the experiment but place a quarter flat on the sand then push down on the quarter with the pencil. By putting the quarter down you increased the surface area and it required a significant amount more effort to push it into the sand right? This is true with tires as well. The larger tire will have a harder time sinking into the sand.
Another way to further increase the surface area of your tire is to "air it down" or let the air out so that the pressure in the tire is lower. The pressure in the tire is what keeps the tire inflated and round. By letting some out you allow it to sink an bit and the contact patch increases even more. This allows more grip and as a bonus the tire can now deform more when impacting rocks and other objects and form around it and contact more surface area.


Wide
(Taken From: http://www.expeditionportal.com/forum/threads/42186-Question-about-BFG-BAJA-T-A)

The second argument is narrower tires which being in Alaska and working for tire sops I have always heard "run narrow tires in the winter" because it increases the force the tires exert through a smaller contact patch. Which using physics you can determine like above the smaller the contact patch the larger the lb/in^2 increase. On hard low friction surfaces such as ice or rocks this can come in handy as you exert more force on the smaller patch the traction is much greater then you would get with a wider tire. This is somewhat hard for me to explain as you would think the larger surface area would apply here and more is better but somehow it isn't.


Skinny
(Taken From: http://bayourenaissanceman.blogspot.com/2011/03/convertibles-on-ice.html)


Tread Design and Compounds

The design of the tire and the compounds that it is made out of greatly affect the performance of the tire in different situations. First the compound of rubber is a big one that applies to both street and off road tires. The compound of the rubber basically affects the friction that the tire itself has when acting against other surfaces. Many race cars on rock crawling buggies use a soft and sticky compound because of the greater coefficient of fiction which allows them to apply more rotational torque to the surface and not slide around as much as a harder tire. On a rock buggy they want the rotational force to be higher in order to pull a heavy buggy up a slippery rock slope and using the sticky rubber can make up for the lack of friction on the rocks. The other part is the design of the tread, most off road vehicles use a tire called an mud tire or for some an "extreme traction" tire. The mud tire is designed with larger voids between the big chunks of tread which is perfect in an off road environment because the loose surface can squeeze between the voids and further increase the contact patch as well as use the surfaces friction against itself by dragging it along between the tread blocks.


Muddy
(Taken From: http://www.intercotire.com/tires.php?id=13)


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