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Name: Unknown
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Date: Around 1993

If friction is surface area independent, then why do dragsters have wide tires?

The force of friction that the tires experience is independent of the tire size, certainly. However, what a dragster does not want is for the tires to slip - so the tires are spinning and the car is going nowhere. What determines when the tires will start to slip is the point at which static friction gives way to sliding friction. That force must of course increase with the area of the tires, and so the bigger the tires, the bigger the force you can use before you start slipping, and so the faster your dragster can accelerate.

A. Smith

Wide tires for drag racing tires also come in a variety of hardnesses or durometer ratings. The softer the tire, the more initial traction is provided. If you will notice, too, one of the biggest reasons for these wide tires has nothing to do with traction: When the car is sitting at the line, the diameter of the tire is relatively small. When the tires are spinning, the tires are constructed in such a way to allow the centripetal force to expand the diameter of the tire. This has the same desirable effect of changing the final drive gear ratio to allow for higher speeds. Racers very carefully size tires to allow for the optimum change in diameter over a given rotational speed of the wheel. This allows the racer to take advantage of the very narrow torque curve of their engines without changing gears too many times. This reason is more so taken into consideration than the width/friction reason.

Eric Peterson

The respondents do not answer the question properly. The reply by Smith claims that bigger areas make bigger forces without giving any reason. The reply by Peterson ignores the width question and instead focuses on diameter growth.

There are several issues that must be considered when choosing a dragster tire. Friction is surface-area independent in only a few ideal examples. The real world is more complicated. Especially for tires that are made of rubber. You want to choose a width, height, and tire compound that gives the best friction for the duration of the race. Top fuel dragsters have one-speed transmissions and slip the clutch during the run. In engineering it is commonly thought that the friction force is proportional to the force pushing the two surfaces together. This is only correct over a certain range of conditions and materials. The constant of proportionality is called the "coefficient of friction." The coefficient of friction depends on the material and condition of BOTH of the surfaces being rubbed together. It is small for DuPont's Teflon (TM) rubbing on DuPont's Teflon (TM), larger for DuPont's Teflon (TM) rubbing on wood, much larger for wood rubbing on smooth concrete and very high for wood rubbing on rough concrete.

However, if the surface becomes actually sticky, then conventional 'friction' theory simply does not work. It is possible to have large friction forces in the absence of a force pushing the two surfaces together. In fact, because the surfaces stick together when you try to pull them apart, a negative static friction coefficient is possible. I made a phone call to a company that makes dragster tires and their racing expert (Les Garbicz) provided me with some information. For most dragsters, certainly top fuel, the tires and track are sticky like scotch tape or flypaper. The tires may be inflated to only 7 psi and are fairly large. Thus, the contact area between tire and track can be a couple of square feet. (Each tire is 17 inches wide and the contact length is as much as 10 inches front-to-back). This enables acceleration to be up to five times that of gravity. The contact area decreases as the speed goes up.

The flypaper analogy is a useful image to illustrate the stickiness mechanism. However, the tire is not a flat surface sticking to a flat track surface. It is a rotating ellipsoid-shaped surface being compressed onto a flat unmovable surface. These ‘flypaper’ boundaries are localized on the surface and are made and broken as the tire rotates through the footprint. The rubber compounds that are used have the property that friction is low when cool. (Not really low, just lower than when hot). The friction increases with increasing temperature, even including the temperature when the rubber starts to melt. During a burnout, there is some melting of the surface, but the tires do not actually get runny and slippery. Prior to the race, the driver does a “burnout.” This short burnout liquifies a thin layer on the surface of the tire. This only makes the tire tacky and cleans the surface. This clean tacky surface grips the track very well.

A wing is attached to top fuel dragsters that produces a downward force. The downward force can be as much as 8000 pounds on a 2000 pound machine when traveling 300 mph. Thus tire slip is not a problem at high speeds. Centripetal force at high speeds keeps the tire from being squashed by the downward force of the wing. Increasing tire diameter and tire width increases the contact area. But there is a limit - a very large tire would not be well matched to the engine or axle ­ the torque becomes impracticable.

When slip occurs between tire and track, the slip is not like a normal automobile tire where the tire slides on the road. Race tracks that are a quarter of a mile long are built of concrete and asphalt. The first 330 feet or so is concrete. The rest is asphalt. A new fresh track has a liquid rubber primer sprayed onto it which then dries. It is then mechanically abraded by a tractor pulling old tires across it. A second coat of primer is sprayed on. The result is sticky. During the races, more sticky rubber is transferred to the track. This, as well as the sticky nature of the rubber, accounts for the tremendous friction.

Rubber is made into a useful tire by the process of curing at the factory. At a temperature much above 400F the rubber reverts to its uncured state, and becomes almost liquid. Obviously, the tire will fall apart if the body of the tire becomes that hot. If a dragster tire is abused or under inflated, the internal temperature can get very hot during the race, and racers make sure not to do that. The internal temperature is different from the surface temperature. When the racer does a 'burnout' prior to the beginning of the race, this liquifies a thin layer on the surface of the tire for good traction. The inside stays cool, and the clean, tacky surface is ready to race.

Aside from the friction issue is the “abrasion” factor. If the load is too high, the tire surface starts to form shavings instead of smoothly getting tacky. The shavings act like little bearings. Thus friction plummets. The rate of this friction drop seems to be related to the “recipe” of the compound, also is related to its hardness (modulus). A a softer compound may become “greasy” on the track, leaving thick black lines on the surface while slightly harder compound may abrade into shavings with tearing. Fracture lines across the tread surface is called “graining” by engineers. These effects describe and explain the sliding coefficient of friction zone identified with high tire slip.

To make sticky surfaces adhere, you need to push them together. That is why you push down on sticky tape. Thus, there is a controversy about what happens when the leading edge of the dragster tire slams down onto the strip as the tire rotates. It has been argued that this 'push' downwards causes the tire to grip much better than if it were gently rolling along. Overall this is why tires need to be wide: a tire that is too narrow will abrade (which is bad) instead of getting tacky (which is good).

Bob Erck

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