# Tire Physics!!! Surface area question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ChadH82, Oct 13, 2001.

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Ok... I might be way out there but I've been thinking about the width of tires compared to the weight of your vehicle and how it effects traction. Say you have an 39.5x18 tire. The weight of your vehicle is going to be spread out over a larger surface area. So would this result in less traction than say a 38x11 which has a smaller surface area or would you be losing traction because 38x11 is too small. I just want to know what the best tire size to weight ratio is for the most traction on hard surfaces. Is there a science to this or is it just guess work? I'm not trying to get into a "what's the best tire discussion" just the science of it so leave your opinions in the trash can next to you. J/K.

2. ### shaggyk51/2 ton status

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If i remember some stuff from physics class.....(ok, here goes) you get into the trade off of pounds per sq. inch of tire surface, and the coefficient of friction. the less contact area there is, the more weight is on the contact patch (per sq. inch) BUT, at the same time, the Greater contact area given by larger tires, gives more friction btween the tire surface and the road surface. Findng the ideal tire size is finding the balance point between the two sides.

Or at least i think thats how it goes....its been a while since physics......

--matt
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3. ### michaelm1/2 ton status

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if the surface is hard the square inches of tire contact wont be as different as you think. but the shape will change. soft surface would be different

4. ### Triaged1/2 ton status

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f = Nu
f = friction
N = normal force (in this case the weight of your truck)
u = static coifficent of friction

Notice that there is no mention of area. It doesn't matter (on hard surfaces).

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5. ### shaggyk51/2 ton status

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well are you going to give us the formula that doesnt mention area or what?

--matt
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6. ### Triaged1/2 ton status

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It is right there at the top of my last post

f = Nu

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7. ### shaggyk51/2 ton status

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oops. I expected something a little longer, shot right past that one. sorry.

Been a while since physics, (11th grade i think?)

--matt
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8. ### Triaged1/2 ton status

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No prob. I deal with that stuff almost every day (I am a Mech Eng student at Cal Poly Pomona).

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9. ### RedDwarf1/2 ton status

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77Chev is right. The contact patch will be the same, assuming identical tire construction and flexibility. The Skinny tire will have a longer tread contact patch which is preferable to the wide, short contact patch of the wider tire. This is because you want to travel forward or backward, not sideways. The tread can wrap up and over obstacles better, like tracks on a tank sort of.
Also preferable in the mud because the skinny profile pushes up less mud in front of the tire.

Of course in real life, tire construction usually isn't the same so there are some differences, and what pressure you run affects the performance and contact patch of any tire.

Wider isn't always better.

Still Poundin' "pavment" after all these years!!!

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Alright... what I got from the F=Nu is the heavier your vehicle the more traction you are going to get? But then if you have a heavier vehicle it would take more friction to go up a slope? I got kind of lost in that static coefficient stuff but I get the general idea I think... maybe. Thanks for the answers.

11. ### white_knight1/2 ton status

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Wouldn't part of this discussion include tire pressure?

As they said when airing down- The bigger the footprint the softer the stuff you can travel.

Depends on the type of wheeling you do, correct?

In sand- big wide tires and horsepower rule.
Mud- tall thin tires work best
Rockcrawling- I would think wide aired down tires would be it

With wider tires- more traction- you spin tires less, it also increases your moving power.

Or... am I way off base here?

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In this case N is (more or less) equal to the weight of the truck only on a level surface. Otherwise, N is the component of force (weight if you will) perpendicular to the surface. So, N will be some fraction of the total weight when not on a level surface. The remainder of the weight is actually fighting against your efforts to move up the hill. That is why you eventually loose traction when the angle of incline becomes too much. Basically you have less "N", leading to less friction (see the simplified equation above), and to make things more interesting, the portion of your truck's weight not contributing to friction is working to pull you back down the hill.

Yes, this is a gross over simplification but I think it is close enough to see the overall effect...

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Sounds dead on to me. There is no simple equation for all needs which is why there are so many different tires with different proportions. There is allot more to choosing the correct tire than just simple friction.

In addition, when in the rocks (or any hard, irregular surface) wide, large diameter tires with low air pressure will sort of wrap around stuff and "grab" hold of any irregularity which provides much more bite than the same tire on the same surface without irregular features. Even though this is still due to friction, the calculations become much more complex contact surface is not flat and parallel to direction of motive force.

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Soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.

14. ### Triaged1/2 ton status

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When you look in a table for the coefficent of friction there are 2 numbers. One is the static (not slipping) and the other is kenetic (sliding). The static is always larger than the kenetic which is why you stop quicker if you don't lock up the wheels.

And yes N is only the weight of the truck when it is on flat ground (otherwise I would have had to include nasty sin/cos stuff).

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15. ### 84SierraRegistered Member

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Something very important was overlooked in the physics lesson here. The friction equation is correct. However, anyone will tell you that a wider tire has more traction then a thiner one. What was over looked is that the friction coeffecicent takes into consideration surface area, materials, structure, and every other factor. Different tires will have different friction coefients. The same tire will have a different friction coeffiectent on different surfaces. Calculating frictions coeffiectents is possible but not worth doing. They are almost always found by experimentation. There are charts for certain matterials which give the friction coeffienctent per area. Unfortunatly you don't learn these things in physics. You have to wait untill machine design and dynmatics. Hope this helps. BTW a wider tire will have more traction.

Matt

16. ### 84_Chevy_K10Banned

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In mud you don't necessarily want narrow tires. If there's a bottom to be found, you do. But if there's no bottom (or before you find it, your axle will find the ground) then skinny tires are worthless.

In my opinion, for mud, you need a tire that has a width proportional to the weight of the truck. Too wide and you'll float and slide all over. Too narrow and you'll dig and bury yourself. It's always better to be a little on the wide side of things as long as you know how to drive.

Tim
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17. ### KrebsATM021/2 ton status

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So all in all, what would ya'll think would be the best tire, and width. I know taller the better, but only if your not over doing your lift so that you get top heavy. Meaning cut the hell out of your fenders so you can fit the biggest tire with minimal lift. I got ssr's that are 14.5 inches wide, and sometimes i think they are to wide, but i really think it is just the tread that dosen't bite enough.

Doug Krebs

18. ### white_knight1/2 ton status

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You are correct. If there's a bottom the tall skinny tire will win out. A stock 350 on 41" tractor tires beat me in a bog race. I'm running 44" boggers with 550 ponies.

For all around wheeling I choose the wider tire. If there was a 46" bogger I would run it. Tractor tires would suck.
I;ve run 38-1/2 by 11 boggers and the 44's outperform them without question. But you need the power and you also need the ground clearance. I've got about 15" of ground clearance at the moment.

The truck is stripped with 44"boggers.

Light body ,horsepower and wide tires are a winning combination.

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19. ### 84_Chevy_K10Banned

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Winning combination? As in, competition, or just wheelin'?

I personally don't care if my truck is fast through the mud, as long as it always makes it, and it does, usually!

Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles

20. ### white_knight1/2 ton status

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Winning combination overall.

Obviosuly if you want a race truck, it will be built for racing only, period.

Kinda looking for the mid point where you can have a good wheeling rig and take in the occasional bog.

Like I said a midpoint

I like the bogs but I prefer wheeling. It's a lot more fun. I also like having a truck that will do fairly well

Kind of got off the topic a bit here.

There are so many factors in choosing a tire.

My opinion still goes to the wider tire

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