Almost everyone who wants to go faster attacks the corner entry first, because the entry is where speed feels like it is being lost. It is the wrong end of the corner. This note puts numbers on why, using arithmetic anybody can redo, and then looks at what happens to a line once a hundred laps have gone through it.
The arithmetic, stated before it is used
Two riders leave the same corner onto the same straight. They accelerate at the same constant rate. The only difference is exit speed. The time to cover a straight of length L, starting from speed v with constant acceleration a, is the standard result of uniform acceleration.
The desk uses a of 0.30 g, which is 2.94 meters per second squared, as a plain working value for a bike accelerating out of a corner on dirt. That number is an assumption, not a measurement of any machine. What follows ignores air drag, gearing, wheelspin, suspension movement and the rider's body position, all of which matter in reality. The point of the calculation is the size of the difference, not a lap time.
| Straight | Exit at 22 mph | Exit at 24 mph | Difference |
|---|---|---|---|
| 60 meters | 3.865 s | 3.707 s | 0.158 s |
| 100 meters | 5.553 s | 5.368 s | 0.185 s |
| 150 meters | 7.293 s | 7.089 s | 0.204 s |
Now do the same for the braking point
Take a hundred meter approach, arriving at 50 miles per hour and needing to be at 25 miles per hour at the corner entry. Braking at 0.6 g uses 31.8 meters and the whole approach takes 4.949 seconds. Braking at 0.7 g, which is roughly seventeen percent more braking effort, uses 27.3 meters and the approach takes 4.881 seconds. Braking at 0.8 g uses 23.9 meters and takes 4.830 seconds.
So a seventeen percent increase in braking force buys 0.068 seconds. A thirty three percent increase buys 0.119 seconds. Two miles per hour more at the exit buys 0.185 seconds on a straight of the same length, and it does so without moving the point at which the front wheel is asked to do the hardest work of the lap.
Why does the exit difference compound?
The braking gain happens once, in the few meters where you are still travelling fast. The exit gain applies to every meter of the straight, and it also changes the speed you arrive with at the next braking point, which changes nothing if the next corner is slow but changes a good deal if it is fast. On a fifteen corner lap, an exit gain of 0.15 seconds per corner is more than two seconds a lap. Nobody actually finds it at every corner, which is exactly why the corners where it is available are worth identifying and practicing.
A corner does not keep one line
Traffic turns one line into several. The first riders take the geometrically obvious line. Their tires remove loose material and expose whatever is underneath. Once that line is polished or rutted, the next riders find that a slightly different entry gives better drive, and a second line appears beside the first. By the middle of a busy day a well used corner commonly carries three: an inside line that is short and slow, a middle line that is deepest and most worn, and an outside line that is longer but holds a berm.
Which of the three is fastest changes during the day, and it usually changes in the same direction: the busiest line degrades first. That is the subject of rut corners, and the shape that decides whether the outside line survives at all is in berms and corner speed.
How do you test a line without spending the day on it?
Testing lines badly costs more than it gains, because each experiment is one corner ridden slowly. Three rules keep the cost down.
- Change one corner at a time, and only on alternate laps, so you always have a recent lap on the old line to compare against.
- Judge a line by where it puts you on the straight, not by how it felt in the corner. A line that feels smooth and drops you two feet from the edge of the track has cost you the drive.
- Give a new line three laps before deciding. The first lap is spent finding it, the second is spent worrying about it, and only the third is representative.
The short version
Brake where you can still turn, and spend the effort on getting the bike straight and driving earlier. The exit gain is the one that repeats down the whole straight.
What this arithmetic is not
It is not a claim about any rider, machine or property. It is uniform acceleration arithmetic with an assumed rate, which means it overstates the gain on a long straight where drag matters and understates it where a good exit lets you use a taller gear. It also assumes both riders reach the same corner exit under control, which is the whole difficulty. Use it as an argument about where to spend practice time, not as a prediction of a lap time. The reading that tells you which corners actually have a straight worth driving onto is in how to read a track.
Sources checked for this note
Every figure in this note is arithmetic performed by the desk from the standard constant acceleration relations, with the assumed values stated in the text: 0.30 g out of the corner, 0.6 to 0.8 g under braking, and no allowance for drag, gearing, wheelspin or suspension. No measurement of any track, rider or machine is claimed, and no external source is cited because none is needed for a calculation the reader can repeat.