A Thought on Tire Size

[Synister]

Quote from another site:
"I'm running 285's because I like as much rubber on the rocks as I can get."

I had a thought while reading this. Do you actually have more rubber on the rocks? If a 255 and 285 are both filled to 35 psi or whatever (as long as they are equally pressurized) and lets say weight distribution is nominally 1k lbs per corner, wouldn't both tires exert equal pressure on the ground by deflecting enough to support the weight? So a wider tire would deflect (flatten out) less than a skinny tire but the actual rubber contact patch would be the same for equal inflation, would it not? Unless you air waaaay down Im thinking the actual physical rubber contact patch must be the same for equally inflated tires regardless of width, simply to support the weight. One contact patch would be more square while the other more rectangle, but with equal areas. Thoughts?

 

[03_4x4Runner]

I feel that it is the larger diameter that makes the most difference in rocks. A larger diameter simply rolls over the rocks easier.

 

[Synister]

Right, should have qualified my original post that the tires in question are 255/75r17 and 285/70r17, they are less than 1" difference in diameter and are 2 popular size options for moderately lifted rigs.

 

[Acidmeld]

It depends on the other dimensions of the tire but the saying is more or less true.

285/70/17 would be a 285mm x approx 5in patch 285x127mm= 36192 sqmm
255/75/17 would be a 265mm x approx 4.5in patch 265x114mm= 30210 sqmm

I know my numbers arent exact so dont fault me on my math but i think you get my point. If anyone has an exact formula for this i would love to be able to use it.

 

[DRC]

Interesting thought. Without having a degree in tire engineering I would tend to agree with your assertion that a 285 set at pressure P would have relatively the same contact patch as a 255 set at pressure P. Assuming the tire diameter, rim size, compound and sidewall construction are all equal. This also assumes that the load is equally distributed across all 4 tires. The 285 would simply displace across the width whereas the255 would displace across the length. The displacement should be the same to support the load thus the contact patch should be relatively equal.

However it’s the ability of a tire to deflect and conform to the surface that gives you traction. So I would guess that the wider 285 tire having a wider contact patch would deflect less and therefore give you less forward traction then the narrower 255 tire as it has a longer contact patch that would deflect and conform more to the surface thus giving better traction.

 

[Synister]

My math looks something like this:

1000 lbs supported by 35 psi = how much area

1000/35 = 28.57 sq in

Well, we know a 285 tire is 285 mm wide which converts to 11.22 in. That means the actual physical contact patch of a 285 wide tire inflated to 35psi to support a 1000 lb load will be 11.22 x 2.546. The load and pressure are still the same for the 255mm wide tire which converts to 10.04 in. So the contact patch of a 255 tire under the same load and pressure would be 10.04 x 2.846. Now lets say you air down both tires to 15 psi

1000/15 = 66.67

The 285 tire contact patch is now 11.22 x 5.94 and the 255 tire is now 10.04 x 6.64, again, same total area but more elongated dimensions.

 

[Ross]

I feel that it is the larger diameter that makes the most difference in rocks. A larger diameter simply rolls over the rocks easier.

An issue here is loss of gearing, a small change may not be noticeable. The larger diameter your tire the less gear ratio you will have. You will have less "power" with larger tires. Width really won't affect this. With a lower ratio you won't be able to crawl. You will have to use more gas and this can make controlling your vehicle and staying on line more difficult.

 

[thmastr]

so judging by the previous posts forward traction would be the same with the difference being the shape of the area that makes up the contact patch... more square, more rectangular...

would that mean that a 255 @ pressure P give the same grip than the same tire in 285 @ pressure P in rain conditions, or dry, on paved roads?

Again this is assuming that the tire is allowed (by its design) to deform enough. not a tire expert here

if that is the case then why not go with a 255 since they tend to be cheaper, slightly less unsrpung weight? less weight to make it go and less weight to make it stop...

 

[techno]

I like the technical side of things, but sometimes its easy to over-think them.
285 and 255 are the section widths of the tire. Therefore a 285 tire is wider than a 255. It clearly has more contact area with the road at equal inflation pressures.

There are many articles on the web debating the fat vs skinny tire question and their pros and cons. Like this one Wide Vs. Narrow Tires | eHow.com

 

[Synister]

@ techno

Thats my point, its easy to say, ok, yeah, wider tire = more contact. Yeah its wider left to right, but its shorter front to rear (contact patch I mean). Its kinda like how there are a lot of different shapes of beer glasses but they all pretty much hold a pint. There are a many ways to support a specific load, in my example, 1000 lbs. If the total pressure exerted on the ground is 1000 lbs (how can it not be) and the pressure inside the tire is fixed (at 35 for this example) then the only variable is the dimension of the area that must support this weight. We know one of the variables (285 or 255) so the other is simple math. The total physical area is the same, just shaped differently.

There is a formula (developed by the aviation industry) to calculate the speed at which a tire will begin to hydroplane. You take the square root of the tire pressure and multiply by 9. Notice, this is completely independent of weight and tire size. So a 45000 lb aircraft with 90 psi tires will hydroplane at the same speed as a pickup truck running load E tires inflated to 90. How? This formula, and those using it, understand that for a given load and tire pressure, a certain surface area will be required to support it. The larger the load, the larger the surface area required at a given pressure and, in the hydroplaning example, a greater surface area for the water to act upon for the heavier load, hence the equal speeds. Wide tire, narrow tire...they all have to support the same load so the physical dimensions can be different but the total area of the contact patch must be the same.

 

[techno]

The total physical area is the same, just shaped differently.

Only if you change the inflation pressure. If you don't, the physical contact area can vary a lot.
It is the contact pressure (force in PSI) that the tire exerts on the road surface that changes.

Wider tire = less psi
Narrower tire = higher psi

Lets assume that the vehicle mass is 4000 lbs and is evenly distributed to all four wheels. Each tire is supporting 1000 lbs.
If at the present inflation psi the contact area is 20 sq/in then the tire is exerting 1000/20 = 50psi on the road. Each square inch of tire is carrying 50 lbs.
Now, if you lower your inflation pressure, the contact area will increase and the contact pressure will decrease. For example, if the new contact area is 40 sq/in per tire, then the contact pressure becomes 1000/40 = 25psi.
The same applies to increasing/decreasing the width.

At similar inflation pressures, with any tire width, the front-to-rear dimension of tire contact area is usually the same and this is where the debate starts as to which is better under certain conditions. In sand for example, a narrow tire has higher contact pressure than a wider tire and will be more likely to sink. However, sometimes a narrow tire is better because it might sink down to a firmer surface and then achieve better traction.