what has more surface contact area? also a lift question.........

hi y'all,

i currently have a re 5.5 long arm, with a 3/4 spacer up front for the weight of the warn bumper and 1461 long rear re springs. the front has a 3 inch re bumpstop as well as a 1 inch longer tower bumpstop from re and im running 33x13.50r15 toyos on 15x8 with 3 and 3/4 inch backspacing. heres the question: i want to go to a bigger tire like a 34 truxus or the new km2 35 inch bfgs, do i have enough lift and the proper bumpstops as well as the right wells? and what tire has more surface contact to make my jeep more stable, the 33x13.50 toyos i have or a 34x12.50 truxus mt or the km2 bfgs 35x12.50? oh i almost forgot, i have bushwackers too. thanks for the help!

Even though I don't run those tires, the surface contact patch of a tire is mostly dependent only on the air pressure in the tire and the weight it supports (sidewall stiffness will have some effect also). If you know the weight the tire is carrying (in pounds), divide by the air pressure (in psi), and the result is the contact patch area (in square inches).

That's the technical answer anyway (I'm an engineer...sorry!)

i appreciate it, i run my tires usually at 13-15 psi offroad. thanks again!

Flyfisher said:

Even though I don't run those tires, the surface contact patch of a tire is mostly dependent only on the air pressure in the tire and the weight it supports (sidewall stiffness will have some effect also). If you know the weight the tire is carrying (in pounds), divide by the air pressure (in psi), and the result is the contact patch area (in square inches).

That's the technical answer anyway (I'm an engineer...sorry!)

Click to expand...

does that included the weight of the rim as well?

yes, as the weight of the rim is being supported by the tire as well

Yes, the "formula" applies to all weight carried by the tire...including sprung and unsprumg weight. It will vary with weight carried by the structure of the tire (sidewall), but is a good guide.

Apply the formula...this is way airing down is good for traction...doubling contact patch in many instances. With my 33x10.5's...I routinely air down to 8-10 lbs...less for snow!

cool thanks!

I'm glad an engineer answered this question, because sometimes it gets some argument. It seems hard to believe, at first, that wider tires don't give you more ground contact. I know I had to go outside and measure different cars before I was convinced.

There are a couple more interesting points that come out of applying that formula. Flyfisher, correct me if any of this is scientifically inaccurate, but this is the way I understand it from past threads on this topic.

First, wider tires may be more prone to slip (lose traction entirely)because the contact patch is wider and shallower.

Second, think about the tread on your tires. Big knobby tires may tend to give you a wider purchase, especially on rock. Since only the parts in contact with the ground bear weight, the recesses between the knobs that don't touch the ground, don't count. Thus a contact patch of 15 square inches may be spread over a significanty larger area, depending on your tread pattern.

Third, think about how the weight borne by each tire changes as you move. When climbing, the front tires bear less weight and thus have smaller contact patches, if inflated the same as the rear tires. Thus, whereas most drivers tend to focus on putting the front tires on a good surface to pull yourself up, it's really more important to think about where the rear tires will wind up if you steer a given course.

Related to the third point, some people like to air down the fronts more than the rears, so they have more contact and thus more traction when climbing, when you need it most. Personally I like this strategy, but you have to be careful not to go overboard. Bear in mind that when descending the fronts will carry more weight, thus flattening out more and risking tire damage.