The chart shown here is an Excel chart using data from an AiM Solo lap timer; there are quite a few similar products that you can use to obtain your own data, from any one of several available GPS-based lap timers to full data acquisition systems. This data is from a lap of Autodrome St-Eustache by Jodi Christie on his Jodi Christie Racing / Honda Canada / Accelerated Technologies CBR600RR during practice for the Mopar Canadian Superbike round held there earlier this year.
Displayed in the chart is a section of track beginning on the back straight, through the left-hand corner at the end of the straight and into the following right-hand turn. Here we are concerned with braking at the end of the straight and entering the corner, and getting an idea of how much Jodi is trail braking into the turn. Speed is shown in black at the top of the chart.
When Jodi gets on the brakes at about the 340-foot mark, the braking G trace (in red) ramps up to a maximum of about 1g; from there, it gradually decreases through the braking zone and into the turn. As Jodi banks into the turn, shortly after he begins braking, cornering G (in green) gradually increases. In the middle of the turn, at the 781-foot mark, Jodi is completely off the brakes; speed is at a minimum, and cornering G is close to maximum.
The area shaded in blue shows where both braking and cornering are happening at the same time, and its shape is characteristic of what we are looking for entering a turn. As braking G smoothly decreases, cornering G gradually increases, giving the trail braking area its triangular shape.
In this section of the track, we can see that Jodi brakes at a maximum of about 1.1g, and corners at a similar maximum of 1.1g. As I discussed in my last blog, when braking and cornering forces are combined, they add using vector addition because they are at right angles to each other. Thus, we can calculate that theoretically the shaded area should have a height of .85g. Jodi reaches close to the theoretical maximum on this lap, combining .8g of both braking and cornering at the peak of the triangle.
While that shows trail braking for just that one point on the track, we are interested in what is happening through the whole entrance to the turn - from the point where the rider begins the turn to the point where the brakes are completely released. This can be accomplished by calculating the vector sum of cornering G and braking G, and plotting the results - shown here as the dark blue trace.
Again, because we know that Jodi can brake and corner at about 1.1g in this section, the theoretical vector sum of braking and cornering G - which is usually called total G - should also be 1.1. And that is close to being the case: Through the entire entrance of the turn, the total G trace is between 1.0 and 1.1g, indicating that Jodi is close to the theoretical maximum for trail braking right from the point where he begins arcing into the corner to the apex.
While combinations of corners, changes in altitude or camber, and other factors can significantly change how the traces appear, the shape of the trail braking triangle is usually consistent and a good indicator of the rider's trail braking technique and progression over time.