What are the pacenotes and why are they required?

08.03.2016. / Knowledge

Broadly speaking, pacenotes describe road shape and (sometimes) road conditions immediately in front of a racing car. Now, why there's a need for pacenotes in rally and not in the other types of motorsport racing? Rally races are driven on public roads temporarily closed for racing purposes. A typical rally "track" is about 10-15 km long and it consists of public road sections with lots of corners, intersections and crests. Such rally track is called "special stage". A full rally race consists of 5-20 special stages (not necessarily different) whose total length is about 100-150 km (these numbers are here for orientational purposes since actual special stages may vary in length from 1km to monstrous 80 km; however, a modestly difficult rally would have about 10 special stages whose total length would be about 150 km). To make things more interesting and difficult, there's a general constraint that the average speed during rally should not exceed 120 km/h. Average speed constraint, however, is not racing car limitation. It is rather a guideline for rally organizers which dictates how to select roads for special stages so that it would be impossible to drive even the most powerful car faster than 120 km/h on average. 
To reduce average speed, rally organizer pick roads for special stages with lots of corners, tricky and changing road surfaces and roads as narrow as possible. Of course, this is kind of upside-down view on rally racing. Actually, rallying is all about drivers' skills about being fast on the most difficult road conditions there exist. Famous Colin McRae once grasped the spirit of rally racing in one sentence: "Straight roads are for fast cars, turns are for fast drivers."
Given the average length of special stage to be about 10-15 km, the number of corners on a typical special stage is very large. It could easily climb to 200 corners. Now, if there are 6 different special stages on a rally, the total number of corners on a complete rally could be larger than 1000. To put this number in a perspective, consider that there are exactly 18 corners on Silverstone Circuit Formula One racing track. This simply means that rally and Formula One racing require completely different drivers' skills, even when putting aside the fact that rally and Formula One cars are very different.
To be fastest possible on a racing track (either on a rally special stage or a Formula One track), a driver must follow optimal racing line through corners. However, to determine racing line through corner, driver must know in advance the shape of a corner (we'll describe in very detail how to determine racing line in one of the following blog posts). The word "advance" here could be slightly misleading. Namely, when driver approaches a corner, he must know whether that particular corner is fast or slow, should he brake hard or not, where is the apex of the corner and so on. All this knowledge is essential before driver enters the corner. That is the meaning of the word "advance" here. 
Now, recall that there could be easily 1000 corners in a typical rally. By the previous observation, a rally driver has to know in advance the shape and conditions for all 1000 corners. That's virtually impossible. It is possible, however, to remember everything about 18 corners on Silverstone Circuit. The huge difference between the number of corners in rallying and Formula One is actually the only reason why rallying needs co-driver and Formula One doesn't.
Co-driver does not remember the whole track either. Instead, co-driver has a booklet with track description, corner by corner, straight by straight. The trick here is the compactness of descriptions. Consider, for example, what would be the natural, human understandable description of two connected, S-like corners: "You are approaching a combination of two corners. The first one is left corner and it turns about 90 degrees. It immediatelly transforms to right corner, which is slightly wide at the end and its turns 45 degrees." As a reader, you got pretty good picture of such corner combination. However, during the race the literacy of this kind is unacceptable. The car runs very, very fast and the time required to read these sentences is far too long. Therefore, a more compact notion is required but without jeopardizing semantics.
If we just strip superflous information from the above description we'll get the following: "Two corners combination. First left 90 degrees then immediatelly right 45 degrees which is wide at the end.". This is much better, but still too long. If we analyze further, we'll see that sentence "Two corners combination" is also superflous because in the second sentence we have phrase "... then immediatelly ..." which implies that there is a combination of corners. So, we don't need first sentence at all. Moreover, the phrase "... which is wide at the end." can be replaced with more simple one: "opens.". Let's take a look at description now: "First left 90 degrees then immediatelly right 45 degrees opens.". Let's compact it even further by removing word "first" which is redundant since we are describing corner flow always from first (closest to the car) corner to the last corner. Also, let's change phrase "then immediatelly" with "into", keeping in mind that "into" would always means that there is corner combination when first corner transforms immediately to second corner, without straight line between the corners. Our sentence then becomes "left 90 degrees into right 45 degrees opens". Now the instruction becomes short enough to pronounce it during a race.

But we can do even better. Word "degrees" which determines the corner sharpness is also superflous since it always comes after the number. We may simply say "left 90 into right 45 opens". The sentence now becomes quite familiar to those who have played Richard Burns Rally or similar rally simulation. However, in the real pacenotes number of degrees as a measure of corner's sharpness is not descriptive enough. Turn degree is, by definition, the total angle between incoming and outgoing lines. But, it does not say anything about the length of the road between turn-in and turn-out points. The hairpin corner, for example, has turn degree of 180 and very short length between in and out points. At the same time, imagine very large corner that looks like half of the full circle whose length is, say 200 meters. The turn degree of such corner is again 180 but obviously the corners are completely different. Remember, the goal of pacenotes is to give enough information to driver so that he can visualize the road in front if him to the best possible level of detail.
We'll not describe the solution to that problem here. We're give the solution in the next blog post because it is quite a difficult problem which, as we'll see, professional drivers solve in different ways.

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