Re: Why do tractors in a tractor-pull puposely spin their tires?

Rick,

Sliding friction coefficients are generally lower than the static 
coefficients for the same object/surface as illustrated in introductory 
physics classes.

Regarding tractor pulls in general:  The competition occurs generally on 
dirt.  The load on the sled is dynamic - it provides increasing resistance 
as time elapses.  According to the American Tractor Pullers Association at  http://www.atpapullersonline.com , it involves: 

"Weight Transfer -otherwise known as the sled, it is hooked to the pulling 
vehicle by a chain. As the competitor takes off down the track the load 
will get heavier to pull. The two major components of the sled are the box 
and pan. The pan is sliding on the ground without much resistance at the 
start of the pull. The pan will increase friction with the track as the 
box, which is chain driven comes towards the front of the sled. Inside of 
the box are lead blocks... Once the box stops the friction of the pan is 
greatest and the pulling vehicle slows to a stop."

Basically, the concept of spinning the tires for traction is kinda complex 
but analagous to the type of thing you see NHRA dragsters doing on 
dragstrips.  

Because the sled is lightest in the beginning of the pull this is where 
the tractor puller wants to both gain distance and momentum; as the load 
increases the puller will lose momentum and eventually stop.  The quicker 
they leave the line the farther they will pull.  What makes pulling a 
challenging sport is there is no set formula to apply all the horsepower 
these tractors can develop to the ground.  I said above it is complex and 
this is because there is a balance that must be achieved for the given 
conditions - too much power and the tractor will sit in one place and the 
tires will spin very fast with little effect ("buzzing the tires" see  http://www.outlawpulling.com , go to glossary and see 'miss the balance') -
 too little power and the tires may just begin to dig a hole in the track 
or even kill the engine (though not likely!).  A correct power balance 
allows the tractor to kinda float on the hard packed dirt 
without "damaging" that surface that the tires are using for traction - 
this is referred to as "hooking up".  "Shaved" tires are generally used 
which present a less aggressive tread pattern (more akin to a racing slick 
but not quite) to aid in hooking up.  While the static coefficent of 
friction between the tire and hard packed dirt may initially be high, this 
is what actually leads to the destruction of the dirt surface and starts 
the tires spinning in the first place.

Long and very complex story made short - the sliding friction coefficient 
between the hard packed dirt and the shaved rear tires of the tractor 
turning at (very) high rpms provide more tractive force for pulling than 
in trying to control wheelspin - the dirt surface simply wont withstand 
the forces exerted by the tires.  That being said there may be tracks and 
conditions where pullers will try to start with minimal wheelspin 
(difficult with in excess of 2000 hp).  It just depends on the conditions 
and how the tractor is set up.

Compared to the weight of the sled and tractor, it is unlikely that much 
benefit is gained by imparting momentum to the dirt being kicked out by 
the rear wheels.  Most likely this actually would lead to a negative 
effect because the dirt moved would either accumulate on or in front of 
the sled being pulled creating even more resistance.

And of course none of the above applies to the mud bog folks who use the 
viscous forces created by their spinning tires for forward momentum.

Hope this helps.

Sincerely,
Steven Miller
smiller@kahuna.sdsu.edu
Bachelor of Science, Mechanical Engineering
San Diego State University (Alumnus)