Re: Why do soccer cleats cause friction?

Sports shoes allow athletes (or those of us pretending to be athletes) to 
run, stop quickly, cut sharply, or maintain balance.  For basketball or 
tennis shoes we envision the force between the shoe and floor or ground as 
frictional because it derives from two surfaces rubbing against each 
other.  

Cleats are a little different.  Cleats actually penetrate the ground.  It 
is not friction but mechanical force/torque that is at work.  The surface 
of the ground and the surface of the shoe actually interpenetrate.  
Scientists and engineers might even use words like “slip-stick” which 
indicates that the two interacting surfaces are rough and little surface 
spikes and valleys actually block motion between the surfaces.  The cleats 
act as the spikes and the holes created in the ground act as the valleys.

The weight of the athlete is distributed over the tips of the cleats 
instead of the sole of a flatter soled shoe.  The surface area of the 
cleats is typically a lot less than half of the surface area of the sole 
of the shoe.  Therefore, the pressure of the weight on the cleat tips is 
usually enough to puncture the ground, unless the ground is very dry.  The 
two surfaces are prevented from slipping up to an amount of force that 
depends on the length of the cleat into the ground, the number of cleats 
in contact, the softness of the ground, and a lot of other properties and 
characteristics.

One way you might imagine it is a shoe with cleats on the ground is a lot 
like stabbing a piece of meat with a fork.  The deeper the fork penetrates 
the meat the less independent movement the fork can make relative to the 
meat.  A deeply penetrated fork cannot be rotated or twisted without 
damaging the food.  A lightly penetrated fork can be used to scratch the 
meat, sort of like a claw.  The more tines the fork has the better grab it 
will have on the meat.  The tines do not need to be extremely sharp to 
work, but the force required to stab the meat successfully is related to 
the sharpness (or really the surface area of the points).

So when the cleats “bite” into the ground, the shoe will not move without 
chewing up the ground (or breaking).   As the cleat digs in deeper, more 
force can be generated to resist slipping laterally.  But there are a 
negatives.  If the cleats dig in too deep then the athlete has to work to 
get his/her feet un-stuck.   This can cause leg fatigue pretty quickly.   
Other problems like accumulating a layer of mud on the bottom of the 
cleats adds weight to the shoes and can actually defeat the whole purpose 
of cleats by making the shoe effectively a flat sole.  These sorts of 
considerations, as well as, the anticipated behavior of the cleated 
athlete enter into shoe design.  Cleated shoes for sports like golf, 
baseball, soccer, and football are all slightly different.  For example, 
golf cleats are not very deep because the golf shoe is designed to give a 
sense of stability for a reasonably static stance and to avoid damaging 
the golf course, especially the putting greens.  Soccer shoes, on the 
other hand, are for rapid acceleration (change of direction, stopping, 
starting) and need to grab firmly, but they need to allow the athlete to 
kick the ball as well without catching the ground too much.  Of course, 
some of the difference is more for style than function especially between 
different makes of the same type of shoes.

Sincerely,

Tom “Spike Marks on the Greens” Cull