I stand at the end of the causeway looking at the sunrise on a glorious
morning. Life is good. I get into the truck and begin the haul down to the
pits. There I unload my gee whiz bang vehicle. Today will be the day! This
year, I brought enough horsepower to kill the doggone record!
I get the car inspected, the most technically astute car in the place, well
almost, Flatire was ahead of me in line and it is the most technically
innovative and costly to boot, I might add. The inspectors, however, oohh
and ahh'd over my car as well, though. I get back to the pits and begin the
preps for making that important qualifying run. Nope, nothing amiss anwhere
that I can see. I fire up to warm the motor and the guys and gals from the
next several pits ove come by and comment on the powerful sound of the
motor. Oh Louise was there and she mentioned that the motor was chirping
nicely. Chirping? Hell, this is a fire breathing hell bent for leather REAL
motor...ain't no chirping here! Barking growling snarling...but not
chirping, for crying out loud....
Off to the ERC fuel trailer. There. Got my tank of gas, tank is sealed, off
to staging. Yikes, look at that line! Four hours later, I am close enough to
suit up. Another hour and I fire the motor to rewarm the engine. At the
start line...butterflies are flying everywhere, especially in the tummy.
But, I've done this before, I say to myself. Quit worrying. The starter
tightens my safety belt...jeeze, I cant breathe... and gives me to the
signal to go and have fun, the course is mine All right!
Off I go. Oops, little fish tail there in first gear. Ok better now shift
into second...more fishtailing....rapidly coming up to speed...into third
gear more slipping an sliding there goes the 2 and a quarter mark...wonder
how fast I was...into final gear.... motor sounds strong and healthy, cones
are flying by just blossoming up from the salt as I whiz by. Car feels good
albeit a bit squirmy..rpm is maxed out. Killed the record, eh! Pull the
'chute now...there it is yikes my eyeballs almost popped out hard hit on
the chute. off the gas now...slowing.....slowing....slowing.....there is the
turn off...stopped...
What a ride....out of the car, picked up the chute, got the top half of the
safety suit off because it is HOT!. Here comes my crew...
How'd I do? I yell as soon as they were close enough ...I just knew it
wailed on the record and am anxious to head for impound. That was a good
solid qualifing run, no doubt about it...in my mind..
WTF? I didn't even clome close! Nah, couldn't be...gotta be a mistake...Glen
screwed up in the timing tower... bummer.
Back to the pits. What the hell went wrong? Motor sounded good, went through
the gears like a banshee. Musta hurt the engine. Pull the plugs. Read the
plugs...no, they look great! Look at my data from the daq. No anomalies
there. Just hit the peak rpm and there it stayed. My dream wheel says that
at that rpm I should beat the record by 10 mph. What happened?
Does this sound familiar?
a little analysis and conjecure. That tire slipping at the start cooked the
tires a bit and made them unresponsive to the salt. the car was not weighted
on the corners correctly for best traction especially at top speed. Now wait
a minute you say. How do you do that and why is it important for top speed.
No matter if you have a bazillion horsepower or just enough, if you ain't
got traction at the end, you ain't gonna qualify. If you know the
coefficinet of friction for the surface, then with a little tiny bit of
math, the kind you used in the 8th grade (maybe) then you can tell how to
weight the drive wheels to assure you are getting all that power to the
ground. Here is what can happen. As you reach terminal speed, the motor must
overcome aero drag and mechanical drag. It does this through the only
interface it has, the tire to the salt interface contact patch. Look at your
dyno sheet rear wheel chassis dyno curve (or if you used the engine dyno,
multiply that torque at top speed rpm by the final drive ratio) and find the
amount of torque being delivered at that rpm you just failed to make the
record at. Now divide that number by the radius of the drive wheel/tire
dimension, in feet. See, I told you this was 8th grade math...now the number
you have is the amount of force that you are appling to both tires at the
salt interface, if you have a posi trac or spool. An open diff is worse
because all that force is transferred to the wheel that has traction
momentarily (in a slippery condition this bounces back and forth between
wheels that get traction...not a great system for top speed, IMHO). So now
what? Ok, you just the coefficient of friction from (insert name here). So
now I know what my applied force is and the coefficient of
friction....aaahhhh...I see now...cof equals the applied force divided by
the corner weight on the tire and wheel... Well, jeeze...I only have about
80 percent of that needed down force (either by weight or aero aid like a
wing). So As I get to speed, the motor runs up against the aero and
mechanical drag load and the tires begin to spin or haze at the top speed.
This means that while you are at the top rpm of your motor and you have all
the power needed, you are spinning the rear tires because the down load is
in sufficient to allow all that power to be applied to the salt without
slipping. So weight the corners with ballast, provide some aero aid for down
force.
I suspect there are a lot of you out there in draggy cars and bikes that fit
the above situation. If you don't want to wait until the COF is found then
assume a number like 0.5 and weight you car or bike accordingly. Or get a
bigger driver. But don't just add power without making some sort of a
change....
This is how COF can make a better day...
mayf
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