Q: Why is the theoretical line not the fastest line through a corner?

Q: “Following on from Podcast 61 with Ann Morey in which she talked about drawing arcs on track maps to work out ‘the line’, I tried a similar thing for my local kart circuit. The corner I picked is a combination of two fast clockwise corners that need to be treated as one. I plotted the largest diameter circle I could that joined the following three points: 1) the outside edge of the track at turn-in; 2) the apex/clipping point; 3) the outside edge of the track at track out. I also superimposed three karts onto the circuit at these three points. This corner sequence is preceded and followed by decent length straights so I see no obvious reason to compromise/prioritize entry or exit over one another. My question is this: Why is this theoretical line not the fastest, and if it is, why do people not tend to drive it? I see people make roughly that clipping point and that track out, but I don’t see many/any people turning in that late and missing the first apex by that far.”

A: With the limited information that I have, here are a couple of possible answers to your question (which is very interesting):

  1. The line that has the most grip usually is faster than the theoretically ideal line. So if everyone takes a more shallow line entering the corner, it will likely have more rubber down there, and therefore have more grip. And grip usually wins (but not always).
  2. Looking at the arc you’ve drawn, it does add distance to the track. Often, a shorter distance – even if the line is not as “ideal” – will result in less time taken to get from one point on the track (the start line) to another (the finish line). This seems to be more important in lower-powered cars (like karts). If you watch NASCAR Trucks run at Daytona they take a completely different line than the Cup cars do – they just drive around the bottom of the track. That’s the shortest distance, and since they don’t have the same higher power/speed that the Cup cars have, they don’t need to use all the track.

So if I had to guess why drivers don’t drive the line you’re drawn, I going with grip and distance as the two likely reasons. Usually, given enough time, at least one driver will try a different line and find out of it’s faster. If it is, others will follow. So I suspect that someone has tried the line you’re drawn and found that it’s not as fast.

There are always exceptions to the rules…

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