Wide Tires

(Created prior to 1993)

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