1-, 2- and 3-Cone Autocross Turnarounds- Part 3

This post presents a real-world data example of a 2-Cone Turnaround from the Bristol Fall Champ Tour just last weekend. Same car, two different drivers.

Figure 1 shows the GPS path plots of two different runs, Red and Blue. I’ve added the approximate location of the last cone in the preceeding feature at “A” and the entry and exit cones that define the turnaround. I’ve also annotated the paths with some other information from my 20Hz GPS data device that may be interesting.

Figure 1

Similarities: Both cars pass point “A” at an identical speed of 55.0mph. Both cars were very close to the entry cone as verified by video and both drivers downshifted to 1st gear without difficulty or incident. The minimum speeds for both cars are very similar, as are the peak lateral G figures recorded by the data device. Both cars trail-braked effectively right into the geometric center of the corner though Red’s data shows a slightly more “perfect” brake release than Blue. Blue’s path appears more circular and Red’s path closer to a spiral as a result. Both cars began to smoothly add throttle at almost exactly the same location which was near the “23.8mph” arrow. Both cars reach their maximum lateral G figure at almost the same location around the turn at which location a slight slope began that assisted turning. (Otherwise the G-peaks would have been closer to the minimum speed locations near the center.) Finally, at point B both cars reached exactly the same speed of 48.5mph. Frankly, I’ve never seen two sets of data by co-drivers look so similar in so many respects.

Differences: The key difference is that Blue enters and exits the turn with essentially zero angle on either the entry cone or the exit cone. Red has made sure to obtain at least some angle on entry, about 7 degrees as close as I can measure. (This cost Red an initial loss of 0.03s which was quickly made up.) Red also doesn’t try to be perpendicular to the line between entry and exit cones when he passes the exit cone… he allows an exit angle. This allows some of the turn to be completed after the exit cone where turning while simultaneously accelerating fully in 1st gear was still possible. (Gotta love those Yokohamas!) Of course the effect of the 7 degree entry angle is the tighter line achieved by Red all around the turn. Neither driver even considered trying to add width to the entry. There simply wasn’t sufficient time or space to do so. Neither driver tried to add width to the exit. The geometry of the next feature favored exiting right on the cone, though Blue slightly missed it, as shown in the GPS path trace and as also confirmed by video.

Result: The tighter line achieved by Red resulted in 24′ less traveled and a stunning 0.38s time saved over the distance from A to B, almost all of which was gained in between the entry and exit cones.

I think this is a good example of how constrained we are so much of the time in autocross. The approach speed and pinched entry and exit geometries made it impossible to add width to either the entry or the exit of the turnaround without paying a severe price. In effect, the usual advice to “use all of the track” had no applicability in this case, which I suspect is quite common in autocross. On the other hand, making the effort to achieve a relatively small angle on the entry cone and allowing an angle to remain at the exit paid great dividends.

Note: Post was updated 9/11 to include distance traveled information.

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