The impact of tire width on traction

I often get asked about tire selection in regards to tire sizing. This is one of the best articles out there I have found on tire size selection, and I thought I would share.

http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html

 

I don't fully agree with the article. The author left out too many variables, and I felt misrepresented/misunderstood what he was siting. Example the friction clause, " The greater the contact pressure, the greater of effectiveness of the friction elements". This refers to the weight of the vehicle pressing the tire down, not the pressure in the tire, which by the way he does say is constant through out all sizes of tires. If the following statement were true, "on a perfectly smooth surface, traction is consistent despite width" Drag cars would not have huge back tires, and cirlcle track cars would not have wider tires to maintain contact (friction) with the ground. The biggest flaw I see in his argument of wider vs narrower tires is that he maintains the same air pressure in the different size tires when in order to exert the same amount of outword force on the tire the bigger volume (wider) tire needs less air pressure than that of the smaller volume (narrower) tire. The following is a chart for racing bycicle tires but the principle is the same. It shows the relationship of air pressure vs. tire width to maintain the same pressure at the tire wall.



What this means is that tire wall pressure is more important than tire air pressure, (you can read the whole article and see the math equations here http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html ).


Based on the author of the article in question the above picture is true at the stated 15psi but the wider tire will need a lower air pressure to maintain the same outward pressure thereby creating more contact with the ground creating more traction (friction) with the ground.

A real world example of this is the Ford F350. In order to carry a full load, the factory E-rated Narrow tires need to have 90 psi, and the air pressure sensor is set to illuminate a warning light on the dash if it is under 90 PSI. When one purchases new wheels and bigger tires for good looks, the new tires will be E-rated, but because of the larger volume of air they only need 65psi to carry a full load, and now you have the "Dummy" light to deal with.

My take is that wider tires give you better traction than narrower (which is the bases of the article), but there are other things to consider when it comes to offroadability, like how wide is too wide before you start losing turning radius and so on.

One more thing. Most my wheeling buddies run 12.50 wide tires at 12 psi in order not to break a bead. I purchased 14.50s because I got a good deal on them, and I run 9 psi without breaking a bead. We all have about the same squish, and I tend to slip less climbing obstacles.

 

Interesting, and I agree with a lot of it, but in the midwest I'm often faced with deep mud so I like a wide tire to keep me from sinking down too deep. Tall skinny tires work very well, just look at youtube videos of cars in the 1920s going through terrific amounts of mud. For this to work, the tires have to be tall enough to keep you from getting hung up in ruts. Not knowing the rut depth you either need to go wide or VERY tall. Side hills and unexpected steering input (handling) are another reason to go wide. Tough to really pick one over the other unless you limit where/how you will drive.

 

I agree with the Mud scenario, wide to float, or tall to dig. We have mostly mud down here around Houston. It seems that in Moab, in the rocks, most run 12.50 wide. My JKU that I keep at my Parents in Moab has 40x12.50x17s. If I go wider I loose turning radius on an already challenged Jeep. My CJ7 that I run here in Texas I run 14.50s only because I got a great deal on them, and it being a CJ with a narrow axle compared to all of the following Jeeps, I was pleasantly surprised with the handling of the wider tire.