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 sis 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. |