Tom Steele

Ok,

Let's open a can of worms. I am curious why bigger tires make for better traction. Or do they - in every direction?

At first glance, it seems obvious that fatter tires would yield more traction, since they would put more rubber on the pavement, right?

Wrong, from the way I understand it...

Here are the numbers the way I understand them. We'll assume a 3,200 lb car, with 50/50 weight distribution, not too far at all from a C5.

That works out to 800 lbs per tire.

Now, the laws of physics state that for every action, there is an equal and opposite reaction.

So the ground is "pushing" back against the tire with 800lbs of pressure.

Now comes the fun part. We have 30 pounds per square inch of pressure in our tires.

So, that means that since we have 800 lbs of force pressing against the pavement, and 30 pounds per sq/inch. Then we have 26.67 square inches of contact area, right?

And that won't change, regardless of the tire size.

If we have a 12 inch wide tire (approx on Z06) then we have about a 2 inches deep by 12 inches wide of contact area.

Now if we were only using a 6 inch wide tire, then we would have approximately a 4 inch deep (front to back) patch of contact area.

So, I then arrive at the conclusion, that wider is better not because of more contact area, but because the wider tire creates a longer vector against the lateral force of a car trying to slide sideways.

That would lead me to think that for drag racing, you'd be better off with the 6 inch tire...

What am I missing. Anyone had college physics more recently than me that can tell me where I am going wrong here?

Thanks,

Godspeed

I can attest that my 335x30x18's make a world more of traction that the stockers because of a larger contact area. I'll someone else prove it with calculations. :cheers:

alanh

I know this part doesn't hold up:

So, that means that since we have 800 lbs of force pressing against the pavement, and 30 pounds per sq/inch. Then we have 26.67 square inches of contact area, right?

800/30=26.67

If this were accurate, you could let the pressure down to 1 lb. and have 800 sq. in. of contact area :eek: :eek: :eek: and that just aint going to happen :smash:


[Modified by alanh, 9:05 PM 8/1/2002]

AP

Your theory sounds correct ... but when it comes to all out dry traction area does matter ...

Your theory is correct that for traction in non-ideal situations a narrower tire will help, in a snow or rain situation a 6 inch tire would be better than a 12 inch tire ... but in drag racing you want a warm sticky tire and you want the largest sticky part you can get ...

FrankLT4CE

Tom,
Your right this will be a can of worms. I don't think the calculation of tire contact area is as simple as you made it. Tire construction characteristics need to be considered. Also the coeffecient of friction between the tire and the road surface need to be taken into account. I believe in general that assuming the same rubber compound and the same thread design that a wider tire will offer more traction on dry pavement with a high coefficient of friction; and a narrow tire will offer better traction on wet or icy pavement where the coefficient of friction is low. For example on ice think of how much better your corvette would do with narrow tires, the contact pressure is higher because the same weight is distributed over a smaller contact area and you have better traction in this condition of a low coefficient of friction surface. The opposite is true on dry pavement with a high coefficient of friction. All of the above is my opinion, sorry no equations to prove it. Its been too long since college for me also. FrankLT4CE :seeya

Miles in Michigan

Yes, because at some point the tire is completely flat and resting on the rim. So it isn't air pressure holding it up anymore, instead it is the rim itself (and the rubber in the tire, but not the air).

As another example, the argument wouldn't hold true with runflat tires (I am guessing), because the sidewalls are so stiff.

The 'constant area' argument would definately be true for a balloon, for instance, but I think anything with a non-negligible stiffness would cause discrepancies. I couldn't say what would qualify as non-negligable in this discussion, though.

ScreaminDemon

I'm simple minded here but these facts are missing. Your tire is going to expand and get narrower with the explosive torque off the line, so you would want to go as fat as possible but still keep the axle as narrow as possible in relation to the vehicles geometry. You would want a smaller rim and a tire with a thin sidewall for flex, so you get a better grip, and a lot less than 30pds for a bigger contact patch. Also missing from the equation is the transfer of weight. It's 50/50 down the track but not off the line.

bierbelly

Yeah, this is a fairly correct analysis. Traction has to take into consideration the differences between the coefficient of friction on wet and dry pavement. On dry pavement, you're in great shape with wide tires, but on wet, you're screwed. That's why snow tires are never 10-12inches wide. You want more weight to the ground per square inch. But really, as I recall my physics, the weight per square inch doesn't really make all that much difference on wet pavement, unless you're talking weight on the order of a tractor trailer. That's why almost all cars risk hydroplaning above about 55 mph in a heavy rain.

BTW, anyone have a line on snow tires for the Vette? ;)


[Modified by bierbelly, 8:09 AM 8/2/2002]

JoesC5

I recall that in the late 50's, when nitro burning dragsters were running in the 8's and at 140MPH someone with a sliderule stated that is was impossible to accelerate from a dead stop to 150MPH + in a quarter mile. It was in all the hot rod mags. So much for sliderules and pocket protectors.

rgregory

The reason bigger tires can be better is that the Coefecient of friction for some tires varies with load. The less load the higher the coef. of friction this makes the lighter loaded wide tire better.

Now for race tires say in an autocross where you don't have much run time bigger isn't always better because those tires perform much better at temperature so a smaller tire will heat up faster and give you more traction. When I was in school competing in formula car competition we used the narrowest tire available to us. Our car was the best handling (not just for the tires) at the competition.

marks99frc

Here is my take on this. The rubber compound will determine traction and sticky soft compounds will be the best. For sticky soft compounds to work well they need lower air pressure, as higher pressures will defeat the "soft" compounds effect. Since you need less pressure (and still must hold up the car) you must use more air, and to use more air you need a bigger tire.

Think of a bicycle. A race bike will have very skinny tires and use 60psi or more to hold the bike and rider while a bike with big fat tires can do the same job with 30psi or so. The bike with the fat tires with less pressure will have better traction.

I think thats correct.

Tom Steele

I THINK I can defend that part. At some point, the pressure in the tires is going to get low enough that the rims are actually going to be contacting the pavement (via the tires) and the PSI exerted on them is going to be much higher than 30psi, thus requiring much less contact patch.

In other words, you sometimes see those crazy TV shows where someone lets a car run over their stomach with one tire. The reason is that they have the load spread out over the contact patch.

But you don't see them doing it with the rim, because the PSI on those thin metal rails would be probably more than 800 psi!

Taking the situation to an extreme is a good way to look at it though.

I think if you had 10psi and that were enough that the rim still had SOME air in between it and the pavement, I suspect what you would have is an UNEQUAL distribution of weight, with only a small part of the weight of the car being supported by the air-pressured portion of the tire, and a LARGE portion of the weight being supported by the tire-sidewall, which would be handling much higher than 30psi.

Hope that makes sense.

Tom Steele

Some interesting stuff here. I am surprised that there isn't a tire engineer somewhere who can explain this stuff straight out. Come to think of it, I suppose I could do some google searching and come back with what I find.

I still think that vectors may factor into all of this. If my original theory holds up, then with a wide tire, you get an approximately 12x2 inch contact patch with the long side being in the "sideways" direction of the car. So when the car tries to slide, rather than roll, it encounters a LONG, THIN contact patch.

On a drag race start, it encounters a very SHORT, WIDE contact patch with a wide tire...

I'll do some looking.

Tom Steele

All right, here is the first stop on my path, looks like my basic assumptions weren't too far off. They add some interesting details about the fact that the contact patch is not homogenous. There is more pressure in the middle than the edges...
http://www.howstuffworks.com/question506.htm

Tom Steele

Shoulda thought about tirerack...

Here is a discussion of contact patch shape...
http://www.tirerack.com/tires/tirete...al/contact.htm

Picture yourself driving along the highway, (just slightly over the limit, well, more than a little) and the sports news comes on the radio. The announcer mentions Shaquille O'Neal. Now, that guy has pretty big feet, but does he put more rubber on the ground with his Reeboks than your tires put on the road ? Hard to believe, but Shaq puts more rubber down than most cars do. The contact patch of most tires is about the size of your hand and has to handle a lot more weight and force than those big Reebok's do.

The shape of a tire's contact patch or "footprint" greatly influences its performance and is dependent on its profile or "aspect ratio". Low profile tires (most performance tires) have a short and wide contact patch that is effective in converting the driver's input into very responsive handling, cornering stability and traction...especially on dry roads.

High profile tires (light truck and most passenger tires) have a long and narrow contact patch which helps to provide predictable handling, a smooth ride and especially good traction in snow.

You may want to check this link as well, it is a little off topic for this discussion, but interesting as well.
http://www.tirerack.com/tires/tirete...l/pressure.htm



[Modified by Tom Steele, 5:19 PM 8/2/2002]

bierbelly

Physics-ly speaking, things get pretty complex. As I recall, the traction of your tires is dependent upon a number of things, including but not exclusively the contact patch size. Like I posted before, the coefficient of friction (there are two types) is proportional to the contact patch and the nature of both the tire compound and the surface upon which it rests. But then, there's the weight of the car and how it's distributed over the contact patch, and more importantly, the coefficients of rolling friction and sliding friction (i.e. when the tire lets go, as in hydroplaning or panic braking or even breaking the tires loose when you accelerate). Oh, now my brain hurts... :rolleyes:

pburant

I don't think you've opened a can of worms. I think you've forced some people (including me) think. Then again, perhaps that's the definition of opening a can of worms.

The balloon reference was a good one. A balloon would behave according to the theory in the original post, however, a car's tire will not behave like that for several reasons, the tire having a solid center not being the least of these reasons.

Every tire/wheel combo has a minimum and maximum footprint. This is due in part to the rim being solid, but also in part because of a tire's limited flexibility. A tire will only deform so much before it maxes out, at which point the internal pressure begins to rise as more weight is applied to the wheel. The pressure also becomes less and less evenly dispersed about the contact area.

Runflats are an extreme example of limited deformation characteristics - air pressure only has a small effect on the shape of runflats compared to regular tires, which is one of the reasons they don't grip as well as regular tires, either in a straight line or laterally. With a regular tire, as more force is applied to the tire, more tread is put into contact with the road. With a runflat, the additional tread that is applied to the road surface during a loading condition (steering, accelerating or braking) is negligible due to the very limited deformation characteristics, so runflats have a distinct performance disadvantage compared to regular tires.

The one factor that people have addressed indirectly is the friction characteristics of rubber on pavement. Rubber on dry pavement behaves differently with regards to friction than most materials. With most materials, for any given weight, the less surface area you have between the two surfaces, the greater the amount of friction. With rubber on dry pavement, the exact opposite is true. This is why wide tires are best for dry traction, whereas skinny tires are better on snow.

As for wide not being a good idea for drag racing, I'd have to completely disagree on that for the simple reason that more rubber for any given vehicle weight equals more grip on dry pavement. As with any 'real' physics problem, drag racing tires are complex. Drag tires are designed with tall sidewalls that allow a lot of deformation. As the wheel first applies force to the tire and the tire to the pavement, the sidewall will actually begin to bunch up, putting more rubber on the pavement. Because the sidewalls on drag tires are tall, the amount that the tire's contact area is allowed to expand during acceleration is greater than a tire with a shorter, stiffer sidewall. A short, stiff sidewall doesn't allow any energy to be stored in the tire, and thus is more apt to spin (this is again one of the disadvantages of runflats).

Tom Steele

How do you mean more rubber? If the (mass over the tire) / (lbs per square inch) formula is true, then the amount of rubber touching the pavement (sitting still) is a constant for a given weight and tire pressure. Size makes no difference on the amount of rubber touching the road, just the SHAPE of the contact patch.

I would agree that more rubber is on the pavement, but not because of bunching up. The bunching just provides a "shock absorber" like effect (which we don't get much of with 35 series tires) and helps you not to shock the tire past c/f as fast.

I think more rubber on the pavement at launch is true because when you transfer weight back on the car, you are increasing the effective mass over the back tires momentarily.

I agree with that.

Let me clear up a couple of things. I have a pretty good physics background, but it has been 14 years since I was in college and I didn't go into a physics oriented career. So I haven't been practicing...

So... I understand coefficient of friction and I understand that I am simplifying things by not taking into account sidewall deformation and some of the dynamic changes that take place during driving.

I think that is ok though. I think you can throw those things out for a moment and look at the simpler questions:

Why are wider tires better for lateral handling?

The interesting thing is that what I once believed to be common knowledge is not turning out to be true. I always thought it was because there was more rubber on the pavement. I don't think that is the case now, at least not to any appreciable amount. I think that wider tires handle better because of the different SHAPE of their contact patch, not any difference in size.

I would go a step further and point out some pretty decent handling cars, with surprisingly small tires... Supra TT, some Porches, I'm sure there are more...

The second question, that I think can still be answered even if you leave out things like sidewall deformation (they can be made constants if you work at it, so they wash) is:

Is a fat tire better for drag racing?

If so, then why?

If it helps for me to go ahead and explain why the sidewally deformation can be made to wash, I will try.

Let's assume two different tires: One is 24 inches tall, 6 inches wide. The other is 24 inches tall, 12 inches wide.

The aspect ratios and rim sizes are different in a way that leaves them both with the same amount of rubber between the tire and rim. Let's say 4 inches from bead to tread.

Now, with my extremely unusual tires, we have identical sidewall properties. They are the same overall height, they have the same amount of rubber between the rim and the tread. The only difference is that one is twice as wide as the other and they have different size rims.

So we have washed out everything but width.

And we want to know: which one is better for drag racing and why?

One last note, take a look at indy cars. They run unusually skinny and tall tires... wonder why?

I am not preaching and I don't have these answers, I am asking questions and trying to figure them out, or even simply find a good resource with the answers.




[Modified by Tom Steele, 4:48 PM 8/2/2002]

Tom Steele

Here is another good link... I'll copy a small portion of the page as well...
http://www.totalcontrolproducts.com/tire_info.html


Are bigger wheels better?

Increasing the size of the wheel while decreasing the height of the tire is a simple and effective way of improving the handling performance of your car. A lower profile tire decreases sidewall deflection and enables a faster turn-in and improved cornering stability. This increase in handling responsiveness may be less noticeable with factory suspension components as opposed to a performance suspension system. (see article) Of course there is usually a trade-off for performance over ride comfort. A lower profile tire gives better handling but at the cost of increased road noise and vibration transferred on to the suspension.

Is wider better?

Your tires contact patch is actually the only part of your vehicle to make contact with the road. A wider contact patch is usually matched with a shorter length contact patch. This gives you better handling characteristics on dry roads but does not work as effectively on extremely wet or snow covered roads. This wider area will actually allow the vehicle to ride on top of the snow. A narrower tread pattern while not a responsive on dry roads will actually be better suited to extremely wet or snowy conditions. The contact patch is narrower but longer which in a sense cuts through the snow better. Again the trade-off issue.




[Modified by Tom Steele, 5:15 PM 8/2/2002]

Tom Steele

http://www.ida.his.se/ida/~kim/OORARS/Tires.html

The tire serves two primary purposes: First, to provide a relatively low friction interface between the vehicle and the track; and also to provide a means for modifying the vehicle's velocity vector. At times these two functions are at odds with each other, therefore the needs of the two must be balanced based on the track configuration.

The interface between the tire and the track occurs at the bottom of the tire in what is called the "contact patch" or "footprint". The size of the contact patch is a function of the size of the tire itself, the air pressure and the load on the tire. The larger the contact patch, the more force that can be generated for turning, acceleration and braking. However, the relationship between the various components and the force available is a non-linear function.

Steering is accomplished by changing the orientation of the tire with respect to the tire's velocity. The angle between the tire's orientation vector and the velocity vector is called the "slip angle". Slip angle is somewhat of a misnomer, as very little of the contact patch is actually slipping under normal conditions. The force along the velocity vector distorts the tire, deflecting the tread around the middle of the contact patch. At the leading edge of the contact patch, the tread "grips" the road. As the tread moves through the center of the contact patch, it is deflected relative to the centerline of the tire. This deflection applies a torque through the tire to the suspension system, and from there to the car mass, creating a yaw moment, which causes the car to turn. As the tread leaves the contact patch, the grip of the road is lost and the tread returns to its original position.

At low slip angles, the relationship between slip angle and lateral force available is more or less linear. At high slip angles, more and more of the contact patch begins slipping, which means that dynamic, rather than static friction rules apply. This lowers the force available for modifying the vehicle vector. Eventually, the entire contact patch begins slipping and traction force decreases. For bias ply tires, this tends to be a smooth transition, while with radial tires, the transition can be quite abrupt.

Higher loads on the tires generate higher traction forces, but the relationship between load and traction force is non-linear. In fact, if the data is normalized by dividing the traction force by the load to get a friction coefficient, we find that increasing the load on a tire decreases the coefficient of friction, which is known as "load sensitivity". To compensate for this effect, the center of mass of the car may be moved to redistribute the load in the corner. For example, cars that run on oval circuits generally have their standing weight biased to the left side of the car so that when the car turns to the left, the loads across the left and right side tires are more evenly distributed. The term used in racing is called "weight-jacking". Cars may be biased front-to-rear, known as "rear bias" to reduce sensitivity to braking or acceleration. Cars may also be biased from the left-front corner to right-rear corner, known as "cross-weight" or "wedge". All of these forms of weight jacking have one aim in mind, which is to redistribute the weight of the car to improve tire performance.