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    The Physics of Racing, Part 5: Introduction to the Racing Line

    Math and Physics

    Myopic Rhino
    physicist and member of
    No Bucks Racing Club

    P.O. Box 662
    Burbank, CA 91503

    (C)Copyright 1991

    This month, we analyze the best way to go through a corner. "Best" means in the least time, at the greatest average speed. We ask "what is the shape of the driving line through the corner that gives the best time?" and "what are the times for some other lines, say hugging the outside or the inside of the corner?" Given the answers to these questions, we go on to ask "what shape does a corner have to be before the driving line I choose doesn't make any time difference?" The answer is a little surprising.

    The analysis presented here is the simplest I could come up with, and yet is still quite complicated. My calculations went through about thirty steps before I got the answer. Don't worry, I won't drag you through the mathematics; I just sketch out the analysis, trying to focus on the basic principles. Anyone who would read through thirty formulas would probably just as soon derive them for him or herself.

    There are several simplifying assumptions I make to get through the analysis. First of all, I consider the corner in isolation; as an abstract entity lifted out of the rest of a course. The actual best driving line through a corner depends on what comes before it and after it. You usually want to optimize exit speed if the corner leads onto a straight. You might not apex if another corner is coming up. You may be forced into an unfavorable entrance by a prior curve or slalom.

    Speaking of road courses, you will hear drivers say things like "you have to do such-and-such in turn six to be on line for turn ten and the front straight." In other words, actions in any one spot carry consequences pretty much all the way around. The ultimate drivers figure out the line for the entire course and drive it as a unit, taking a Zen-like approach. When learning, it is probably best to start out optimizing each kind of corner in isolation, then work up to combinations of two corners, three corners, and so on. In my own driving, there are certain kinds of three corner combinations I know, but mostly I work in twos. I have a long way to go.

    It is not feasible to analyze an actual course in an exact, mathematical way. In other words, although science can provide general principles and hints, finding the line is, in practice, an art. For me, it is one of the most fun parts of racing.

    Other simplifying assumptions I make are that the car can either accelerate, brake, or corner at constant speed, with abrupt transitions between behaviors. Thus, the lines I analyze are splices of accelerating, braking, and cornering phases. A real car can, must, and should do these things in combination and with smooth transitions between phases. It is, in fact, possible to do an exact, mathematical analysis with a more realistic car that transitions smoothly, but it is much more difficult than the splice-type analysis and does not provide enough more quantitative insight to justify its extra complexity for this article.

    Our corner is the following ninety-degree right-hander:


    This figure actually represents a family of corners with any constant width, any radius, and short straights before and after. First, we go through the entire analysis with a particular corner of 75 foot radius and 30 foot width, then we end up with times for corners of various radii and widths.

    Let us define the following parameters:

    _8119_tex2html_wrap113.gif radius of corner center line _8119_tex2html_wrap115.gif feet

    _8119_tex2html_wrap117.gif width of course = 30 feet

    _8119_tex2html_wrap119.gif radius of outer edge _8119_tex2html_wrap121.gif feet

    _8119_tex2html_wrap123.gif radius of inner edge _8119_tex2html_wrap125.gif feet

    Now, when we drive this corner, we must keep the tires on the course, otherwise we get a lot of cone penalties (or go into the weeds). It is easiest (though not so realistic) to do the analysis considering the path of the center of gravity of the car rather than the paths of each wheel. So, we define an effective course, narrower than the real course, down which we may drive the center of the car.

    _8119_tex2html_wrap127.gif width of car _8119_tex2html_wrap129.gif feet

    _8119_tex2html_wrap131.gif effective outer radius _8119_tex2html_wrap133.gif feet

    _8119_tex2html_wrap135.gif effective inner radius _8119_tex2html_wrap137.gif feet

    _8119_tex2html_wrap139.gif effective width of course _8119_tex2html_wrap141.gif feet

    This course is indicated by the labels and the thick radius lines in the figure.

    From last month's article, we know that for a fixed centripetal acceleration, the maximum driving speed increases as the square root of the radius. So, if we drive the largest possible circle through the effective corner, starting at the outside of the entrance straight, going all the way to the inside in the middle of the corner (the apex), and ending up at the outside of the exit straight, we can corner at the maximum speed. Such a line is shown in the figure as the thick circle labeled "line _8119_tex2html_wrap229.gif." This is a simplified version of the classic racing line through the corner. Line _8119_tex2html_wrap229.gif reaches the apex at the geometrical center of the circle, whereas the classic racing line reaches an apex after the geometrical center-a late apex-because it assumes we are accelerating out of the corner and must therefore have a continuously increasing radius in the second half and a slightly tighter radius in the first half to prepare for the acceleration. But, we continue analyzing the geometrically perfect line because it is relatively easy. The figure shows also Line _8119_tex2html_wrap207.gif, the inside line, which come up the inside of the entrance straight, corners on the inside, and goes down the inside of the exit straight; and Line _8119_tex2html_wrap215.gif, the outside line, which comes up the outside, corners on the outside, and exits on the outside.

    One might argue that there are certain advantages of line _8119_tex2html_wrap207.gif over line _8119_tex2html_wrap229.gif. Line _8119_tex2html_wrap207.gif is considerably shorter than Line _8119_tex2html_wrap229.gif, and although we have to go slower through the corner part, we have less total distance to cover and might get through faster. Also, we can accelerate on part of the entrance chute and all the way on the exit chute, while we have to drive line _8119_tex2html_wrap229.gif at constant speed. Let's find out how much time it takes to get through lines _8119_tex2html_wrap207.gif and _8119_tex2html_wrap229.gif. We include line _8119_tex2html_wrap215.gif for completeness, even though it looks bad because it is both slower and longer than _8119_tex2html_wrap229.gif.

    If we assume a maximum centripetal acceleration of 1.10g, which is just within the capability of autocross tires, we get the following speeds for the cornering phases of Lines _8119_tex2html_wrap207.gif, _8119_tex2html_wrap215.gif, and _8119_tex2html_wrap229.gif:


    Line _8119_tex2html_wrap229.gif is all cornering, so we can easily calculate the time to drive it once we know the radius, labeled _8119_tex2html_wrap189.gif in the figure. A geometrical analysis results in _8119_tex2html_wrap109.gif and the time is _8119_tex2html_wrap110.gif

    For line _8119_tex2html_wrap207.gif, we accelerate for a bit, brake until we reach 32.16 mph, corner at that speed, and then accelerate on the exit. Let's assume, to keep the comparison fair, that we have timing lights at the beginning and end of line _8119_tex2html_wrap229.gif and that we can begin driving line _8119_tex2html_wrap207.gif at 48.78 mph, the same speed that we can drive line _8119_tex2html_wrap229.gif. Let us also assume that the car can accelerate at _8119_tex2html_wrap199.gifg and brake at 1g. Our driving plan for line _8119_tex2html_wrap207.gif results in the following velocity profile:


    Because we can begin by accelerating, we start beating line _8119_tex2html_wrap229.gif a little. We have to brake hard to make the corner. Finally, although we accelerate on the exit, we don't quite come up to 48.78 mph, the exit speed for line _8119_tex2html_wrap229.gif. But, we don't care about exit speed, only time through the corner. Using the velocity profile above, we can calculate the time for line _8119_tex2html_wrap207.gif, call it _8119_tex2html_wrap209.gif, to be 4.08 seconds. Line _8119_tex2html_wrap207.gif loses by 9/10ths of a second. It is a fair margin to lose an autocross by this much over a whole course, but this analysis shows we can lose it in just one typical corner! In this case, line _8119_tex2html_wrap207.gif is a catastrophic mistake. Incidentally, line _8119_tex2html_wrap215.gif takes 4.24 seconds _8119_tex2html_wrap217.gif.

    What if the corner were tighter or of greater radius? The following table shows some times for 30 foot wide corners of various radii:


    Line _8119_tex2html_wrap207.gif never beats line _8119_tex2html_wrap229.gif even though that as the radius increases, the margin of loss decreases. The trend is intuitive because corners of greater radius are also longer and the extra speed in line _8119_tex2html_wrap229.gif over line _8119_tex2html_wrap207.gif is less. The margin is greatest for tight corners because the width is a greater fraction of the length and the speed differential is greater.

    How about for various widths? The following table shows times for a 75 foot radius corner of several widths:


    The wider the course, the greater the margin of loss. This is, again, intuitive since on a wide course, line _8119_tex2html_wrap229.gif is a really large circle through even a very tight corner. Note that line _8119_tex2html_wrap215.gif becomes better than line _8119_tex2html_wrap207.gif for wide courses. This is because the speed differential between lines _8119_tex2html_wrap215.gif and _8119_tex2html_wrap207.gif is very great for wide courses. The most notable fact is that line _8119_tex2html_wrap229.gif beats line _8119_tex2html_wrap207.gif by 0.16 seconds even on a course that is only four feet wider than the car! You really must "use up the whole course."

    So, the answer is, under the assumptions made, that the inside line is never better than the classic racing line. For the splice-type car behavior assumed, I conjecture that no line is faster than line _8119_tex2html_wrap229.gif.

    We have gone through a simplified kind of variational analysis. Variational analysis is used in all branches of physics, especially mechanics and optics. It is possible, in fact, to express all theories of physics, even the most arcane, in variational form, and many physicists find this form very appealing. It is also possible to use variational analysis to write a computer program that finds an approximately perfect line through a complete, realistic course.

    converted by: rck@home.net
    Thu Sep 29 14:05:26 PDT 1994

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