On to definition 1: In the simplest of terms a late apex means the car touches the inside of the corner somewhere beyond the geometric midpoint of the corner.
The figure below is intended to show a perfect 90 degree corner and a typical path through it. The borders of the corner are intended to be circular with the center point of the inside arc indicated. A 45 degree line is drawn through the corner.
In this perfect representation of a corner (which never actually occurs in nature) the geometric midpoint of the corner is point B where the 45 degree line intersects the inside edge of the track. (Of course, we have no track edges in autocross. We’ll get to that later.)
The dashed driving line shown starts on the outside at A, makes an apex at B and tracks out to C. Most of us know that something like this is the fast way through such a corner. It’s assumed that the car approaches A at high speed and must brake. Let’s not quibble just yet about trail-braking, spiral entry arcs or whether the length of an ensuing straight makes any difference. Keep it simple.
A late apex line is, per definition 1, any line where the car contacts the inside edge of the track beyond point B, as shown below. The new apex is at B’ and it is “later” than B. Simple as that.
So, when the road racing expert says “You want to late-apex almost every corner on this track” I think what he means is probably definition 1. Why you should late-apex the corners is left unsaid (other than it’s faster) and many assumptions are being made when he says that.
Assumption number one is that the expert is talking about a range of similar cars. Most cars that don’t have extensive aerodynamic downforce, but have relatively high power to weight ratios need to late-apex most corners at most every track on the planet. Does the expert know why? No way for me to know, but his statement is basically correct anyway. This is because if such cars apex at B while cornering at the limit and then try to apply full power as they unwind the steering wheel they will go off the track somewhere around point C. In Brouillard’s terms, they are unable to optimize the exit. To stay on the track they are unable to use their excess power to decrease time spent in the corner. A late apex “fixes” this issue.
By the way, I tend to roughly define a high power to weight ratio as about 10 lbs/hp or lower. A stock 1999 Hard S Miata at 2200 lbs and 120 hp does not qualify at 18.33 lbs/hp. My 2000 Corvette at 3100 lbs and 345 hp barely qualifies at 9.0 lbs/hp and it will need a slightly late apex. A Mustang GT350 at 3800lbs and 526hp is at a power to weight ratio of 7.2lbs/hp and needs an even later apex. If all three cars drive to the same geometrically centered apex at B the Miata wins.
For the stated Miata, for example, late-apexing at B’ is not the fast way through this corner. That car wants a nearly circular arc that apexes very near the geometric center. The same for a Formula 1 car. In spite of their huge power, F1 cars have proportionally even more downforce, and therefore cornering power, so they take cornering lines more like the Miata. Watch any F1 race and this will be evident. What Brouillard made really clear is that it’s the ratio of power to grip that determines the proper line through the corner for any particular car.
What happens when the “corner” is defined only by an entry gate, a single cone “apex” and an exit gate, like in the figure below? That’s what we’ll talk about in the next installment.