Re: Hot rubber...cold track

[Bruce Augenstein <Bruce.Augenstein@xxxxxxxxxxx>]

richard.d.birkenstock@domain.elided writes:

>From what I've managed to pick up hanging around the pits at Indy...as the
>track warms up, the speeds go down - significantly.  Seemed totally
>counterintuitive to me so I had to ask somebody why that was.  A race
>engineer told me that it is quite simple - as the track heats up, all the
>bullshit (oil and other gunk) that has become soaked or embedded into the
>concrete rises to the surface and makes things quite slippery.  Therefore,
>while it is advantageous to have warmed up the rubber, you'd actually want
>to be running on a colder - not warmer - surface.  Gotta believe the same
>principle holds true at the dragstrip.

The simple answer is: Nope.

First of all, my understanding is that traction at Indy is a whole bunch
more complicated than warmer track = more slippery. *Colder* track = more
slippery, as well. The fact is, track compound and tire compound(s) are
optimized for a warm track. *Not* a really hot track, and *not* a cold
track. A warm track can probably be defined as something in the neighborhood
of 100 degrees Fahrenheit - maybe a little more. When track temperatures get
to 130 or thereabouts, traction is down, and when the surface is at, say, 70
degrees, traction is also down - perhaps even more significantly. The
*primary* reason for this is that the tires are designed to operate within a
fairly narrow temperature range (maybe 275-300 degrees at the tread
surface?), and a hot or cold track surface directly affects their operating
temperatures.

Your race engineer (whatever that is) was sort of on track :-) with the
statement about oils coming to the surface on a really hot track. However,
this is mostly oil that is part of the asphalt mix (Indy isn't concrete).
The asphalt begins to break down slightly as temperatures get past 130
degrees or so, and it gets "greasy". However, tires get greasy, too, as the
rubber begins to break down slightly when it gets past its optimum operating
temperatures, and as higher inflation pressures (from the heat) reduce the
size of the footprint so that loading per square inch goes up, compounding
;-) the problem. Anybody who's done some "warm" laps in succession at a
driver's school can testify to this, especially if they've bumped cold tire
pressures significantly in an effort to improve handling under duress.

Somebody with more training and knowledge than I have may want to be more
specific about Indycar tires and track surfaces. I know I'd benefit from
some words on this.

The second key point is that the starting line at Indianapolis Raceway Park
(for example) is a whole 'nother critter from the 2 1/2 mile oval at
Indianapolis Motor Speedway. For one thing, it's probably concrete, since
most of the better drag strips are now using concrete pads out to 200' or
more. However, even if it's asphalt, operating conditions are unique on a
drag strip starting line.

For best traction, you need a good, thick coating consisting of layers of
rubber and traction compound (typically pine tar and an emulsifying agent
that evaporates). These layers tend to work best (from a traction point of
view) when they are warm, and, as with the oval, warmer is better. On a
really hot, clear day, traction may suffer, largely because the thick
layering (more than a quarter inch) may tend to soften up and be more apt to
tear away under duress. Even so, lining up in the tracks of a race car that
just launched ahead of you is still the hot tip for a street wienie (such as
myself), because that section of track is a known quantity.

Even on extremely hot days, a Top Fuel dragster will do a burnout across the
line to heat the slicks to optimum temperature, and if you should be so
fortunate as to get to the starting line right after he or she made a pass
(and after you've collected your senses), lining up in those tracks will get
you a launch you won't ever duplicate otherwise.

Walking around on the starting line at a good drag strip makes you look as
if you are walking in gravity boots. You have to physically pull your foot
up as if you had stepped in some sort of glue. It typically isn't until
ambient temperatures get well over 90 degrees Fahrenheit on a dead clear day
that traction fades.

Bruce

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