Please explain late vs early apex.

[livi]

Does any given turn have a fixed geographic optimal point of apex? Or does it depend on the turns before and after it? I read about late and early apex. Early/late in relation to what? "Mid apex"? Any general rules for what is more effective in any given circumstance?

Help me out, please.

[widebody911]

Originally Posted by livi

Does any given turn have a fixed geographic optimal point of apex? Or does it depend on the turns before and after it? I read about late and early apex. Early/late in relation to what? "Mid apex"? Any general rules for what is more effective in any given circumstance?

Help me out, please.

Depends on the turns before and after. The optimal turn would be a straight line, and you can come out of a turn faster than you can go in; ergo, you want to set up for your turns such that they're as close to a straight line as possible.

Imagine running down a hospital corridor in socks, and you have to make a 90 degree left turn at the end of the hall. It would be easier to slow down, get up against the right-side wall (late apex), turn, and push off down the turn, than it would be to move over to the left wall and try to turn (early apex)

[cstreit]

It also depends on a corner, but in a general sense... An early apex is when you cross the apex curbing earlier in the corner and a late apex is when you cross the apex curbing later in the the corner.

[cstreit]

DARK BLUE is an early apex. LIGHT BLUE is a late apex.

THe light blue lets you get on the gas harder because you have less steering input mid-corner and therefore more grip available for acceleration since its not being used for lateral grip.

[IROC]

Late apex = slow in, fast out. Early apex = fast in, go off the track.

[Porsche-O-Phile]

This terminology is a bit confusing - the "early" apex actually occurs later in the corner than the "late" apex.

[Christien]

The part I've never understood is the straight line bit. Intuitively, it seems that you could carry the most speed through a corner by turning the smallest angle possible, which would be a constant angle from the initial turn in until you straighten the wheels. It seems to me that slowing down enough to safely make the sharp-angled turn to set your apex approach would be taking off significantly more speed than maintaining a less-sharp angle throughout the turn.

In practice, I know this isn't the case, and that the sharp(er) angled turn in with as straight a line as possible throughout the turn is the fastest route, but I don't understand why. Is it because you'd have to maintain a constant speed throughout the entire turn, with the constant angle, and the speed increase you can get with a straighter line, after apex, more than makes up for the slower entry speed?

[artplumber]

There are these things called slip angles, grip, acceleration (deceleration too, although that's just another kind of acceleration), runoff etc....

[IROC]

Originally Posted by Christien

The part I've never understood is the straight line bit. Intuitively, it seems that you could carry the most speed through a corner by turning the smallest angle possible, which would be a constant angle from the initial turn in until you straighten the wheels. It seems to me that slowing down enough to safely make the sharp-angled turn to set your apex approach would be taking off significantly more speed than maintaining a less-sharp angle throughout the turn.

In practice, I know this isn't the case, and that the sharp(er) angled turn in with as straight a line as possible throughout the turn is the fastest route, but I don't understand why. Is it because you'd have to maintain a constant speed throughout the entire turn, with the constant angle, and the speed increase you can get with a straighter line, after apex, more than makes up for the slower entry speed?

I think there are two factors at work here. The real-world requirements of exit speed and lateral grip.

Lateral grip is a function (all other things being equal) of the rate of radial acceleration acting on the car. After all, F=ma and for a given mass of the car, "a" can only be so big before you exceed the frictional capabilities of the tires. Since the equation for radial acceleration is :

a = V^2/R, where V is velocity and R is the radius of the turn,

You can see that the natural shape of a correctly-apexed turn maximizes the radius of the turn. By making R as large as you reasonably can, you decrease "a" for a given velocity. In reality, you always want to be "at the limit" so you're going to keep "a" as high as possible (back to F=ma again). This effectively means that as you make R bigger, V can be larger also and you are cornering faster.

The practical application of this becomes very complex with track geometries, weight distribution in the vehicle, driver skill, etc. so we end up with slight modifications to what would be theoretically the quickest way thru the turn.

[livi]

Thanks!

I know I could trust you guys with premier explanations!

So, late apex=slow in fast out=typical 911 territory? Actually, this is how I intuitively drive streight threw a round about. First far out to the right and when it opens up a rather sharp turn left and then it is streight ahead. I have noticed that I need not very high speed, yet the cars behind me lag far behind when we are out of the circulation. Theory match practice - great!

Thanks again!

[FastCarFan]

One more factor that I don't think anyone has mentioned. Often you "compromise" (i.e., choose a less than ideal line) the line through 1 turn to "optimize" the line/speed for the next turn. On the track you usually have to consider the entire sequence of turns, not any single turn in isolation.

And what is the saying? "Late is great, early is squirrelly"?

[artplumber]

Livi choosing the apex also has alot to do with your car (which has under then oversteer). You might choose a different line with a car that mostly understeered (eg/AWD). You might choose to induce oversteer to help you turn the car. Any number of things. Everybody starts at the outside edge of the corner on entry (assuming just one corner in question), and then chooses a combination of braking and turn-in to arrive at an apex (inside edge of corner), and then decrease the steering angle to exit - usually at the outside of the corner again. 911's tend to like (in my limited experience) late turn-in and more straight line braking - ie don't like trailing throttle for most of us average joe's

[Dantilla]

Originally Posted by Christien

In practice, I know this isn't the case, and that the sharp(er) angled turn in with as straight a line as possible throughout the turn is the fastest route, but I don't understand why. Is it because you'd have to maintain a constant speed throughout the entire turn, with the constant angle, and the speed increase you can get with a straighter line, after apex, more than makes up for the slower entry speed?

Our cars can brake much faster than they can accelerate- Look at 0-60 times compared to 60-0.

To compensate, we brake hard very late going into a turn, but use most of our steering input before the corner so we can apply throttle sooner, since the throttle is less effective than the brakes.

[artplumber]

The other thing is car position itself. The following car has to abide by both the laws of physics and what the car in front allows. Hence, while Porsche may not have the highest speed just before the apex, they (following car) can't physcially be where you already are (and need to brake more). OTOH, because the engine is in back, this gives Pcar some additional grunt to pull out of the corner (horsepower for horsepower) that improves the Pcar exit so you can create some distance before the next corner. You see this all the time in racing where a following car yo-yo's position because it is faster into the corners but doesn't have enough edge in exit acceleration or top end for the straights. In other words, that following car might dust you, if they started out in front and pulled away in corners while you caught up on straights (eg Mini or Lotus vs your car).

[kang]

Originally Posted by Christien

The part I've never understood is the straight line bit. Intuitively, it seems that you could carry the most speed through a corner by turning the smallest angle possible, which would be a constant angle from the initial turn in until you straighten the wheels. It seems to me that slowing down enough to safely make the sharp-angled turn to set your apex approach would be taking off significantly more speed than maintaining a less-sharp angle throughout the turn.

In practice, I know this isn't the case, and that the sharp(er) angled turn in with as straight a line as possible throughout the turn is the fastest route, but I don't understand why. Is it because you'd have to maintain a constant speed throughout the entire turn, with the constant angle, and the speed increase you can get with a straighter line, after apex, more than makes up for the slower entry speed?

That’s correct. You can “carry the most speed through a corner” by doing what you say. But remember, speed through a particular corner is only part of the picture. Racers are more interested in overall lap times than speed through a particular corner.

This has been addressed above, but say this corner leads onto a long straight. What’s more important for overall lap time? Speed through the corner, or speed down the long straight? Since cars brake better than they accelerate, getting on the gas as soon as possible, resulting in higher speed down the long straight, is more important to lap times.

You get a faster lap time by sacrificing speed through a given corner for getting on the gas earlier. A center apex gives you speed through the corner, but a late apex lets you get on the gas earlier.

With a late apex, you can get on the gas about where the light blue circle is. With a center apex, you can get on the gas where the green circle is. With an early apex, you can’t get on the gas until the dark blue circle.