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So in doing research on building my car, I came across some interesting facts about tires: INCREASED WIDTH DOES NOT INCREASE TRACTION. Wait, WTF?!

Now read on before flipping out as your world view crumbles (as mine did).

Somewhat-relevant background: I'm in the engineering phase of building a rwd tadpole 3-wheeler I designed and was trying to figure out what width of tire I would need so that I didn't turn the thing to smoke every time I tapped the gas. So, having the general layout of the vehicle settled, I figure I have enough info to rough up some numbers on tractive force at the rear. With a terrible worst-case scenario and figuring on a .7 tire coefficient of friction, I come up with 403.8 lbs. of force. Fairly acceptable, but I'd like better for aggressive launches and acceleration in the wet or on marginal surfaces.

If you work the equation to find the tractive force, the value that causes the greatest change by far is coefficient of friction of the tires. So, interwebs, what width tire should I go to for an increase in C of F while maintaining a tread I can use on the street and hoping I don't have to go to something near 300 mm or more (for multiple boring design reasons as well as cost)?

Answer: doesn't really matter! The calculations don't even factor it in as the only things that matter (essentially, and I use the formulas for a locker rear differential—open diffs take into account a bunch of other BS—because I only have one tire anyway) are vertical load & C of F of the tire and surface. The only improvement you can get is purely from the tire compound, tread, pressure, and sidewall strength. The advantage to the width increase is that the sidewalls can better support the tire under repeated stress, it's insurance to keep a greater percentage of tread on the road on uneven surfaces, and they have the capacity to dissipate more heat than an otherwise identical skinnier tire, but do not actually grip any better on an even surface. However, the reality of it is that the higher C of F tires tend to be wider.

So what does this mean to you? Probably not as much as most cars since Subarus generally have 4 wheels of traction for acceleration and the wheel arches don't easily accommodate wide tires, nor is the suspension well designed for maintaining a consistent contact patch. But going to a higher C of F tire before going wider will get you much better results than immediately trying to jam something huge under the car, so exhausting the skinny possibilities first is the best plan. So how do you get a higher C of G tire? That's the trick—the number's seldom published and I don't know if there's even a standard anyone uses to determine it, so comparing between brands might be pointless.

Still, I thought it was an interesting bit of knowledge.

Wow, that's interesting... so those huuuge rear wheels Porsche fits to the 911 Turbo are essentially window dressing? I gotta think contact patch size would factor in to traction somehow, but maybe it's simply for better cornering grip...

Okay but,
One would tend to think that (more) contact area between tire and road/vehicle weight would (positively) affect the C of F.

epicfail wrote:Okay but,
One would tend to think that (more) contact area between tire and road/vehicle weight would (positively) affect the C of F.

F=uN : that is, force = coefficient of friction times the weight applied to the surface. If you increase the weight applied to surface (say, with a spoiler to apply downforce) you'll get more force, but the friction does not change. Notice how nowhere in this equation does contact area come into play. The coefficient of friction depends entirely on the two surfaces that are in contact, meaning the composition of the rubber, and the condition/type of road being driven on.

The other fun thing to note is that the only real effective way to change the size of the contact patch is to change the pressure in the tire. You have a constant weight of the vehicle. So imagine a car that is 1000 lbs, and tires that are filled to 10psi. 1000/10 = 100, so the contact patch is 100 square inches. Nowhere in that calculation does the width of the tire come into play. Change the tire pressure to 20psi, and you get a contact patch that is 50 square inches.

Wider tires allow for softer compounds to be used because they change the shape, not the size, of the contact patch (to reiterate: contact patch size depends on the weight applied to the pressure in the tire, nothing to do with width). A narrow tire means that the contact patch spreads out front-to-back more than side-to-side, meaning the sidewall has to deform more to achieve the same contact area, which causes heat; a front-to-back distribution means that a larger length-wise section of the tire is compressing/decompressing with each cycle, so each piece of the tire acting as the contact patch has less time to cool down before it comes into contact with the surface again. With soft rubber, this will cause the tires to fail rather quickly. Now, real world data is much more complicated, and it turns out that the width of the tire does have some affect on contact patch area, but it is much less than you would imagine, and is not the main reason to run wider tires.

wider tires generate grip with lower slip angles. That's a major reason for staggered sizes and the real benefit of wider tires.

there's also a curve of CF dependent on load (and temperature and pressure). So having the right tire size means you will have more grip.

brand wrote:The other fun thing to note is that the only real effective way to change the size of the contact patch is to change the pressure in the tire. You have a constant weight of the vehicle. So imagine a car that is 1000 lbs, and tires that are filled to 10psi. 1000/10 = 100, so the contact patch is 100 square inches. Nowhere in that calculation does the width of the tire come into play. Change the tire pressure to 20psi, and you get a contact patch that is 50 square inches.

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