Left 2 into Right 3 minus - what exactly does it mean?

21.03.2016. / Knowledge

In the last post we've started to introduce the reasoning behind rally pacenotes. We've tried to explain step by step how one would end up making pacenotes that would sound as they are coming from some PC rally game. However, we've stopped at the sentence "left 90 into right 45 opens" which indeed sounds right, but not completely.
Using degrees to describe sharpness or curvature of the corner is plainly misleading, as we've seen comparing hairpin turn and very long half-circle corner. Obviously, the speed which a car can carry through these two different corners is completely different, although their sharpness in degrees is 180° in both cases.
Now, which measure shall we use instead of degrees to describe corner's sharpness? In racing, the goal is to drive through corners as fast as possible. But, is there a single measure which can uniquely describe top speed through the given corner? Luckily, no. We've said "luckily" because that's the exact point of racing - being fast ultimately depends on driver's ability to decide at fraction of a second how fast he can go through the corner in front of him. The top speed through the corner depends on grip level, weather conditions, car's suspension, tires and many more factors. Of course, top speed fundamentally depends on corner's curvature, or, to be very precise, top speed inversely depends on radius of corner's curvature. Basic physics say that when the grip force through the corner is equal to centrifugal force, then car drives at the maximal cornering speed. However, the grip itself is the problem; there is no practical way to measure such thing and driver must do his best to estimate grip level at every corner.
The maximal cornering speed therefore depends on two ingredients: corner's curvature and grip limit. Curvature is measurable but grip limit is not. Up to the some exceptional cases, pacenotes do not contain information about grip - the fun part is left to driver's judgement. Curvature is, however, exactly what pacenotes do contain and that's the meaning of the number 2 in phrase “left 2”.
Radius of corner's curvature is expressed in meters (see figure). It is perfectly reasonable then to use descriptions like "left 43.5" in pacenotes. But, such literate approach is not used in reality. First, it is very impractical and virtually impossible to measure radius of curvature with such precision. Second, it doesn't really matter if the exact radius for the given corner is 40.3m or 42.7m since the grip difference between such two corners would typically be much larger than the difference between centrifugal forces in these corners.
Rally drivers use a very simplified scale for describing radius of corner's curvature, typical plain numbers ranging from 1-7. The range used also varies, so some drivers prefer only numbers from 1-5 while others would use 1-9. To answer the question from this blog's title, phrase "left 2 ..." means "left corner with radius curvature of 2 on my scale ...".
For iPACENOTES project, we want to fix certain scale and stick with it. This is the first tradeoff which we have to make in order to achieve our goals. We could, for example, allow anyone to select any scale that sounds natural to him or her, but we set iPACENOTES goals slightly different. We want pacenotes created by iPACENOTES software to be shared among people, so that somebody creates pacenotes for, say, Nurburgring track and share it on Internet. Others should simply download these pacenotes and drive Nurburgring straight away. But for that principle to really work, we must stick to some common "pacenotes language". Therefore, we have to fix corners scale.
We've selected scale from 1 to 7. It is detailed enough even for professional rally drivers and, on the other hand, it is simple enough for enthusiasts. The following list summarizes iPACENOTES corner's numbering scheme:
1 - fastest corner
2 - almost as fast as 1 but probably requires downshift
6 - slowest corner, almost a hairpin
7 - Hairpin
Many rally drivers use "reversed" scale in the same range 1-7, but with the exact opposite meaning: 7 is the fastest corner and 1 is the slowest. The reasoning behind such reversed scale is that it almost naturally says which gear should driver use to drive through particular corner. So, "left 3" would mean "left corner with such curvature that in ideal conditions and perfect grip you should drive in third gear". There's nothing wrong with this approach, but for iPACENOTES project we wanted a scale whose values can be mapped to corners in more deterministic fashion. In other words, whoever makes an estimation that given corner could be driven in third gear is actually making an "educated and experienced" guess. iPACENOTES is designed for enthusiasts and much less experienced people, so we had to select scale which is easier to map to real corners.
The exact number of a corner in iPACENOTES scale can be measured using steering wheel and few stickers. Picture below is taken inside Hayden Paddon's car during reconnaissance (process during rally race when drivers are making pacenotes). Hayden Paddon is, at the time of writing this blog, professional rally driver who drives for Hyundai factory WRC team, and he's one of the most promising young drivers (note that Hayden also uses “reversed” scheme).
In the next blog post we’ll describe iPACENOTES corners’ scale using the same Hayden’s tools and we’ll also explain in detail how you can start to measure corners for pacenotes.

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