Trail-braking is a subtle driving technique that allows for later braking and
increased corner entry speed. The classical technique is to complete braking
before turn-in. This is a safer, easier technique for the driver because it
separates traction management into two phases, braking and cornering, so the
driver doesn't have to chew gum and walk at the same time, as it were. With the
trail-braking technique, the driver carries braking into the corner, gradually
trailing off the brakes while winding in the steering. Since braking continues
in the corner, it's possible to delay its onset in the preceding straight
braking zone. Since it eliminates the sub-optimal moments between the ramp-down
from braking and the ramp-up to limit cornering by overlapping them,
entry speeds can be higher. The combination of these two effects means that the
advantage of later braking is carried through the first part of the corner. In
many ways, this is the flip side to corner exit, where any speed advantage due
to superior technique gets carried all the way down the ensuing straight. The
magnitude of the trail-braking effect is much smaller, though: perhaps a car
length or two for a typical corner. Done consistently, though, it can accumulate
to whole seconds over a course.
When I was taught to drive in the '80s, not all the fast drivers used trail
braking and instructors usually gave it at most a passing mention as an
optional, advanced technique. The reason was probably a risk-benefit analysis:
it's a small effect compared to the big-picture basics, like carrying
speed out of a corner, that everyone must learn early on
it's difficult to learn, so why burden new students with it?
mistakes with it are ugly
Another reason may have been that my instructors hadn't got their butts
kicked recently by a trail-braking driver. It was not a commonplace technique
back then, so one might drive a whole season of club racing without getting
spanked by trail braking. Since not everyone used it, not everyone had to
develop the skill.
Nowadays, however, the general level of driving skill has increased to the
point where it's no longer optional, unless you're content with fourth place.
As with most driving skills, it's difficult to get a feel for the limits
without exceeding them from time to time. However, exceeding the limits at trail
braking has some of the worst consequences one can invite on a race track,
typically worse than those from mistakes at corner exit. It's definitely a big
risk for a small effect, justified only because it accumulates. Blowing it
results in too high an entry speed. You get:
inappropriate angular attitude in the corner
immediate probing of the understeer or oversteer characteristics of the
car
surprise, pop quiz on the driver's car-control skills
missed apex and track-out points
a looming penalty cone, gravel trap, tyre barrier, concrete wall, tree,
etc.
when you bounce back from that impact, you can hit other cars,
spectators, corner marshals, berms, etc.
anything else that can go wrong in a blown corner
That's one of the reasons I have not, in the past, singled it out for my
personal driver-development work - it's hard to do at all and harder to do it
consistently and just didn't seem worth it. Another reason is that the kinds of
cars I like to drive let you get away without it much of the time. I prefer
ultra-powerful cars because they're fun and loud and attract a lot of attention.
Paradoxically, though, such cars can lull one into becoming a lazy driver. With
a lot of power on tap, you can often make up for an overly conservative entry
speed on the exit.
However, when the cars are equalized, as in spec races, showroom stock, or in
a lot of Solo II car classes, trail braking takes a prominent role. It can be
difficult to spot it as an issue in Solo II, where drivers are alone against the
clock. All else being equal, a Solo II driver without trail braking may just
find himself scratching his head wondering how in blazes the other drivers can
be so much faster. Go wheel-to-wheel on the track with equal cars, though, and
the issue becomes instantly and visually obvious. You may be just
as fast in the corner, coming out of the corner, down the
straight. You may have perfect threshold braking. You may have perfect turn-in,
apex and track out points. But that little extra later braking and entry speed
will allow the trail-braker to take away several feet every corner. Corner after
corner, lap after lap, he will gobble you up.
I recently completed a road-racing school at Sebring International Raceway
where this is precisely what I saw. In identical Panoz school cars, the drivers
who were faster than I were doing it right there and nowhere else. My ingrained,
outdated style did me in, and even though I had much, much more on-track
experience than the rest of the students, and even though they weren't faster in
top speed than I, and even though their cornering technique was not nearly as
polished as mine, three (out of twelve) of them had better lap times than I.
The instructors were as surprised as I. One even said he would have bet money
that I was the quickest from watching me and riding with me (instructors did not
ride in the wheel-to-wheel sessions). The clock doesn't lie though, and we were
scratching our heads and I started swapping cars. Once we went wheel-to-wheel on
the third day of the program, I spotted it, right there the first time into turn
2: the three quicker drivers took a car length from me on the corner entry. They
did it again in turn 10 (Cunningham), at the Tower turn, and turn 15 approaching
the back stretch: all the turns requiring full braking and downshifts. I made up
a bit at the hairpin, which is an autocrosser's corner if there ever was one,
and I knew the importance of not missing the apex by more than an inch or two if
possible. They also couldn't beat me entering turn 17, which has no straight
braking zone: instead, the best technique is to brake partially after turn in
(at 115 mph, this is big-time, serious fun). Thus, turn 17 did not trigger my
old-fashioned "braking-zone" program, and I was able to use my
high-speed experience to coax a bit more than average grip through it. So, in
sum, my conservative turn-ins on the slow corners added up to about half a
second per lap, which is about 65 feet at the start-finish line where we're
going about 90 mph =132 fps (90 x 22 / 15). Ugly.
I was doing it the old-fashioned way: get the braking done in the braking
zone and get your foot back on the gas pedal and up to neutral throttle before
turn-in. That little tenth of a second or so where I'm coasting and they're
still braking is the car-length they were taking out on me. It was small
enough that the instructors couldn't feel it or see it. But electronic
instrumentation would have picked it up. When I go back to the Panoz Sebring
school next year, I will take advanced sessions in fully instrumented cars,
where the instructors go out for some laps at 10/10s, then the students go out
in the same car and take data. Back in the pits, the charts are differenced and
the student can see precisely what he needs to do to come up to the instructor's
level (most of the instructors have years of experience on the track, and hold
current or former lap records in various cars on the course, so it's quite
unlikely that a student will be as quick out of the box).
The following is a picture of the course snipped from the web site at http://www.sebringraceway.com, so you
can see the bits of the course I'm talking about:
Let me say a few things about the school. The three-day program consisted of
solo exercises in braking, skid recovery, and autocrossing
detailed in-car instruction as driver and passenger over several lapping
sessions
racecraft including passing and rolling starts
wheel-to-wheel sessions on the full open course
It's a great program, easily better than spending the same amount of money on
the car: highly recommended.
Sebring is large, exciting, lovely, complex course with a deep history of
sports-car racing. It is currently 3.70 miles in length, though it has been as
long as 5.7 miles in its history. Let's do some dead reckoning, that is, math in
our heads without even envelopes to write on. We'll see if we can cook up some
data, from memory, to justify the intuitions and explain the results above.
There are 2.54 centimetres per inch: that's an exact number. Therefore, there
are 2.54 x 12 = 30.48 centimetres per foot. The number of centimetres per mile,
then are 30.48 x 5280 = 30 x 52 x 100 + 30 x 80 + 48 x 52 + 48 x 80 / 100 =
156000 + 2400 + (50 - 2)(50 + 2) + 3840 / 100 = 158400 + 2500 - 4 + 38.40 =
160,934.4. Thus, a mile contains 1.609344 kilometres, which we can round to
1.61, which is, conveniently, 8/5 + 1/100. So 3.70 miles is 29.637 / 5 = 5.927
kilometres or just about 6. Now, there are 5280 / 3 = 1760 yards in a mile, so
we have 3700 + 2590 + 222 = 6,512 yards, which is consistent with 6 kilometres,
so we've got a check on our math. In fact, we can be a little more sanguine
about it. Another number we remember is that there are about 39 inches per
metre; that's a yard and three inches, or 13/12 yard. So, if we have about 6,000
metres, that's going to be about 6,000 + 6,000 / 12 = 6,500 yards. Amazing,
isn't it? Finally, this is 6,512 x 3 = 13,036 + 6,512 = 19,048 feet.
Big Track. Nice.
A record time around the course in the Panoz school cars is 2 min 28 seconds.
The students were doing 2:40 to 2:45. I believe I uncorked a 2:36 somewhere
along the way, but my typical lap was 2:40 and the quicker guys pulled about 65
feet on me at the start-finish every lap, which I reckoned before to be worth
half a second. What's the average speed at 2:40? That's 3.70 miles in 160
seconds. The average speed is 19,048 / 160 fps ~ 1905 / 16 ~ 476 / 4 ~ 119 fps,
which is 119 x 15 / 22 mph, and that is (1190 + 595 ~ 1785) / 22 = 892.5 / 11.
It's hard to divide by 11, so lets multiply instead. 80 mph by 11 would be 880,
and that's not enough by 12.5. So, if we go with 81 mph by 11, namely 891, we're
short by 1.5. A tenth of 11 will take care of some of that, so 81.1 by 11,
namely 892.1, leaves us close enough. Now, doing the same distance in 2:28, or
148 seconds, yields an average speed of 19,048 / 148 ~ 4,762 / 37. Another tough
divisor. Let's try 130 x 37 = 3700 + 1110 = 4810, too much by 48. But, we lucked
out, it's obvious that 48 is about 1.30 x 37, so we get 130 - 1.30 = 128.7 fps.
Now multiply that by 15 / 22: (1287 + 643.5) / 22 ~ 1930 / 22 = 965 / 11. 90 x
11 would be 990, too much by 25, which is a little more than 2 x 11. So 90 - 2 =
88 x 11 would be 880 + 88 = 968, too much by 3, so we'll reduce 88 by 0.3 x 11
to get 87.7. The average speed of a record-setting lap is 6.6 mph faster than
our pitiful student laps! The difference is 12 seconds, so, as a rule of thumb,
a second at 85 mph average is worth a little more than 1/2 an mph.
But, before we wander too far off topic, let's compare 2:40 to 2:40.5, since
my contention from the beginning of this note is THAT difference can be
accounted entirely to trail braking in four corners of this course: 2, 10, 13,
and 15. Well, at 119 fps, average speed, half a second is about 60 feet, which
is about 4 car lengths. Yep, there you have it: one car length per significant
corner due to trail braking. Darn it, looks like I'll just have to go back there
and keep trying, over and over again.
