In a message dated 1/24/98 12:39:22 AM Eastern Standard Time,
gardner7@pilot.infi.net writes:
> Here's his question. If the coefficient of friction is only
> dependent on the two surfaces in question and the weight of the
> object being moved, and not the amount of surface area in contact
> with the ground, why do drag racers use such wide tires?
Scott,
I just happen to know the answer to this one! Several years ago ('57 -- '58),
Hot Rod magazine, much to their later embarrassment, did an article in which
they calculated the maximum speed possible in the quarter mile. Amongst the
factors they used in the calculations was the coefficient of friction for the
tires. Their answer? Approximately 150 MPH, less than half the current
record.
Their fallacy? The tires are not limited by friction! The tires on a
dragster are very soft, and are run at very low pressures. The drag strip
surface is very rough, as you can imagine, being either concrete (at the
starting line) or asphalt (the remainder of the course). This combination
allows the tires to "grip" the surface, the soft tires actually "interlocking"
with the strip surface. In effect, the road/tire interface is almost like a
geared interface, the little teeth on the tires interlocking with the little
teeth on the strip. The wider tires allow them to apply much more torque
without ripping up the tires, by distributing the load over a larger area.
Another factor to think about. At rest, the tires are very wide, but at
speed, the tires grow, reducing the width while increasing the diameter. The
diameter increase is quite dramatic, as can be observed in the burnout box
when they heat up the tires. The back end of the cars rises significantly
when they spin the tires. In effect, they have a variable ratio rear end --
low gear for max acceleration off the line, and a tall rear end for max speed
through the traps.
Drag racers are some pretty clever people!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
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