What's my line?
Is "the line" the same for everybody?
How and why do I adjust my line?
Welcome back! Thanks for taking a break from Cinco preparations. If you recall, we
began by discussing how we can modulate the balance of the car to our advantage to
improve its handling characteristics. In our last column, we explored how to control
traction while cornering. This month I'd like to discuss how and where to get around
a track, or driving the line.
When driving on the streets, we are restricted to driving on only part of the paved
surface, lest we dare a dangerous encounter with on-coming traffic. In Driver's
Education, we utilize the entire width of the track to our advantage. Racing, which
we aren't doing, involves an entirely different path and strategy to pass and prevent
others from passing.
Instructor Wendy uses the late apex technique at turn 3 at Pinal Air
Park. The geometric center of this corner is about 1-1/2 car lengths earlier
than where the cone had been placed. At this point the engine would have been
at full throttle for a full car length or more. Photo Credit: Rink Reinking
Let's begin by defining some common terms. For purposes of the driving lines, a
straight away is really any segment of the track where you can accelerate or brake,
and actually includes many gentle curves (like turn 6 at Arroyo Seco, and even the
turn 6-7 area of Pinal Air Park). The braking zone is the stretch of track just
before we turn the steering wheel to begin the turn. Obviously this is where brakes
are used to slow the car to the desired speed for the turn. Turn-in is the point
where we leave the edge of the track and the vehicle begins rotating. The apex is
the point on the turn where the car approaches closest to the inside of the track.
Finally, track-out is the point at the end of a corner, where you reach the far side
of the track and hopefully (!) have straightened out the car.
Common sense and past experience tells us the sharper the corner, the slower we must
negotiate it. By modifying our path around the corner, we can open up the curve to
drive a straighter path. Straightening out the curve means higher speed, right?
It's not quite that simple, but it's a start.
In the 2 diagrams shown below, the direction of travel is from left to right, turning clockwise
around the curve (apologies for the simple diagrams ? my graphics options are limited).
The first illustration is called the geometric line, and is simply the largest radius arc
from turn-in to track-out. This is the fastest line around the curve, because it is the
straightest path. It is not, however, the fastest way to drive the track. First of all,
if you've misjudged your entry speed, you'll need to slow at the exit to prevent running
off at track-out. Think about drivers in town who cut corners on left turns at traffic
signals (i.e., an early apex) ? where do they hit their brakes? They brake at the end
where they have a sharper kink to complete the turn. Further, if you recall from the
first column, when you are using all available grip for turning, you've got no grip left
to accelerate. This means you can't use the throttle until you straighten the car out at
track-out, and the following straightaway is where you really generate the speeds.
Start the straight segment faster, and it carries through the entire stretch. This is
what is meant by "slow in, fast out."
In the second diagram, the driver has gone deeper into the curve, moving turn-in into
the curved portion of the track. He must slow more than in the first diagram, though,
because he must then negotiate a much tighter turn than in the first example. But our
goal was to go faster, right? Isn't this counter productive? Here's the payoff. Once
the driver makes this tighter, slower turn, he has an even straighter, gentler turn to
complete the curve?essentially lengthening the following straightaway. The apex has
been moved from the midpoint of the arc of the track to a point about 2/3 around the
curve. This is what is meant by a "late apex" turn. Since the car is following a
gentler path, all the available traction is not being used up by turning, meaning
there is grip left for acceleration. Acceleration shifts more weight off the front
tires, diminishing the turning effectiveness and the car unwinds and drifts out to
the track-out point (remember throttle steer discussed in the 1st Ground School?).
In this second line, the car enters the straightaway after the turn at a much higher
speed. Higher entry speed means the entire straightaway will be faster, and this is
where this driver opens a wide lead over the first.
But wait, there's more! Not only is a late apex turn faster, it is safer. In the case
of a geometric line, if the car is going too fast at turn in, either the car will not
be able to turn at the desired radius and run off the outside edge of the track, or the
driver will try to slow the car (is anyone remembering Rink's Mantra ? "Don't Lift in a
Turn") and risk spinning out. Either way, this becomes "fast in, slow out". The later
the apex, the larger the margin of safety.
Ok, back to our lead-off questions. Is the line the same for everyone? Some organizations,
like the BMW Car Club and NASA might lead you to believe this, mandating their instructors
teach "only the Club Line." This line has been pre-determined, and is a safe, late apex
approach to the track. For most cars, it will be fairly fast, but more importantly, it is
a safe line and reduces those organizations exposure to adverse events. While I would never
dissuade anyone from driving these "club lines," they are not, by any stretch of the
imagination, the best line for every car, every tire, and every track condition. It
shouldn't come as a surprise that a 914, a Miata or a Mini Cooper will out corner the tires
off larger cars. Similarly, when reaching the straightaway, an M5, a GT2 or a Viper will rip
the doors off most mortal cars. Clearly the best line for these road rockets is not the same
as the line for the small momentum cars. Even just considering a single car and a single
driver on one day, the line will change throughout the day. Cold tires and overheated tires
don't grip as well as a hot tire. Tread wears as the day goes on. Dust and sand (and even
rain at our last CAC event) can blow onto the track and reduce traction. So what's a driver
to do?
First of all, this is one time where an experienced instructor is invaluable. There is
no substitute for having one sit next to you as you learn the track, then adjust your
line. When I go to a new track, I try to get a ride-along with one of the locals before
I drive it myself. When working up your line on a corner, start with a very late apex
(look for tire marks on pavement) to determine where your turn-in point must be to reach
it. Your braking zone is determined by where you turn in, since essentially all your
braking and downshifts must be completed before this point. Pick a landmark or
(hopefully permanent) feature to begin braking. On subsequent runs, you can push this
braking zone later, as you become comfortable, to slow to the speed at which you are
happy making the turn. Turn in should be swift but smooth to minimize disturbing the
balance of the car. Try to hit your desired apex on each run. As you pass the apex,
roll on the throttle and accelerate out of the turn as you unwind the steering wheel.
Accelerate as early and as hard as you can comfortably maintain without running off the
track. At Arroyo Seco turn 14 in the clockwise direction, I'm at full throttle as I
pass the apex. I'm told Formula 1 drivers can repeated hit within an inch or two of
the apex every time. I've never ridden with one, but the Indy test car driver with
whom I rode was VERY consistent. In any case, until you can consistently hit your
mark, trying to adjust it is futile.
Ok, so let's say you've reach a point in your driving where you can hit braking zone,
turn-in and apex repeatedly. How do you know where your line should be, and how do
you get there? The key is track-out. If you accelerate and unwind the steering wheel
as you leave the apex, and you end up in the middle or the track (i.e., you're not using
the entire track), you can move your apex earlier. This means you'll be on the accelerator
earlier, and carry more speed out of the corner.
On subsequent laps, move the apex earlier by a few inches at a time. Your track-out
will approach the outside edge of the track as you reach the ideal apex for your conditions.
The goal is to drive safely, but make use of the entire width of track. If you apex too
early, you risk tracking out beyond the edge of the track, i.e. running off track, a
condition to be avoided (particularly at Turn 12 at Arroyo Seco where there's a 4-5 inch
drop off!). For momentum cars (Miata, etc), the apex will be earlier. For the heavier
road rockets (Shelby Cobra, etc), the apex may be very late. For most of our cars, the
optimal apex likely lies somewhere in between. When your tires are cold or over heated,
when the track feels greasy or slippery, or when there is water on the surface, the apex
moves even later. Recall as we discussed previously, anything that reduces traction means
your cornering speed and effectiveness of your brakes decreases. As most of our March CAC
Autocross drivers will attest, though, it still makes for a fun drive!
I strongly recommend the Street Practice Guide. Check it out when you have a
chance. Have a safe and enjoyable Cinco de Mayo!
Greg
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