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N i 11. N .... '" ~ OF FAST RIDING ~ z ~ w Z AND OTHER SEMI-LITERATES------ ----------l '--~o(--'Q FIG. 3--Force diagram of a rider heeled over at speed. In engineering mechanics you analyze this setup by imagining the machine pivots around the tire contact patch (which it does). It's got two forces trying to rotate it around this point. Engineers call these forces "torques," and they are measured in units of force times distance. In the diagram HF" times ub" is one, and "w" times "a" is another. As a law of physics the bike will remain in a stable banked attitude if the two torques are equal and opposing. Since W-a works counterclockwise and F·b works clockwise, the system is stable if F-b equals W-a. Now if our- rider is pulling one g by definition F equals weight, W. So for the torques to be equal, a must ~qual b. This automatically makes theta 45 degrees. OK, we're on the home stretch. Assume you know the theoretical limits of your tires and the theoretical bes t line. You're still not out of the woods because the chasses and s.uspension characteristics of your own cycle can cos t you precious speed. Again, let's take a .simple idea flrst. What determines the angle you have to lean your cycle at a given speed in a given corner? To figure this out it helps to visualjze the problem as in Fig. 3. In a nutshell all forces of acceleration and gravity act through the C.G. of the man/machine unit. So at any given speed in a given curve there's a certain angle (theta) at which ~entrifugal force due to cornering and weigh t will cancel cach other out, yielding a stable system. Now if you've got trig tables handy, it's not hard to figure this angle for any given case. To free ourselves from dependence on trig tables, let's take a special case and use it for most of our examples. At one g, the cornering force "F" equals weight "lA'," so "a" must equal "b"- and (wow) theta is 45 degrees. (Now you see why I chose one g to work with earlier.) OK, if you ride pretty much stra;ght up or tucked in along the centreline of the bike, the C.G. will be pretty much in line with the wheels. This is important because, by defimtion, all weight and centrifugal forces act through the C.G. So our angle (theta) is of necessiry measured on a line between the C.G. and the tire contact path. So if you ride centered over the machine, at W ..J tJ > tJ one g cornering the bike must lean at 45 degrees-or else. If sometrung on the machine drags bround before reaching 45 degrees, tough. You can't exploi t the full power of your tires without courting disaster .unless you do something with your body to alter the situation. Now you know why many r~ad racers clamber all over their mounts like a monkey on the back of a charging bull. If a rider hikes his own weight well to the inside, he can shift the overall C.G. of bike and rider to 45 degrees without the machine having to lean that far. Most of the smaller riders do it by crawling from one side of the motorcycle to the other. The bigger guys just hang out their low side knee and shift their weight a little. Still others corner completely tucked in with supreme confidence that nothing is going to ground before the limit of adhesion is reached. The riding sryle must swt both bike and rider, so each man (after a little intelligent thought) must f"md rus own. That's part of the beaury of this cafe' road racing thing. You can theorize like crazy in your off hOUTS, bu t the theories only prepare your mind. They don't solve the problem for you. The solution is in your own instincts, your own reflexes, your own creativity, your own gut feel for a particular motorcycle and a particular patch of road. So go riding and try to dig what I've sa;d, so far. 111 come back and zero in on tires with Part n. riG. 4--Typical example of cornering clearance limitation. Classic single at left with. narrow engine cases and well tAcked in exhaust easily pulls one g limit cranked over at 45 degrees. Bulky multicyclinder machine at right grounds exhaust system at 37 degrees. By geometry, this works out to 3/4 g in a stable bank. To better this, see Fig. 5. right within the limits of your own traffic lane and who can say you're breaking the law? (Especially since many states have already put laws on the books giving a motorcycle the right to an entire lane.) Nobody with a car can match this, since a car just doesn't have the play room within a lane's width .. Let's hang numbers on the idea, .takin!!, the simplest case. Imagine a 90 degree circular comer with a 100 foot radius in the middle of its 12 foot traffic lane. Now any dummy with a ruler and a compass can generate and measure the theoretical max' radius (minimum curvature) path available to a rider. Then to the graph in Fig. 1 to obtain the maximum speed allowable for a given g limit. I've done this in Fig. 2. By trial and error it turns out that a i33 foot radius can be inscribed in our sample curve without running out of road. Going to the graph in Fig. 1 the 100 foot and 133 foot curves yield 56 ft./sec. and 64 ft./sec., respectively, at one g. In fig. 2 I've translated these figures into mph speeds and elapsed times through the comer. So by f"mding the 133 foot radius through this comer our imaginary rider has picked up just over 5 mph coming out and shaved just under 4/10 of a second off his E.T. These things add up; and you've done it without an appreciable loss of safery margin. Remember, our figures for both touring and racing lines assume the rider ·is cranked over at a full one g. So by using your head and picking the righ t line you ride no harder; you jus't go faster. Neat, huh? _ Now, bear in mind that the foregoing example is an extremely simpleminded ."ase. We've been talking about a perfectly flat, unbanked -surface, . constant width, constant radius. Few if any real roads are like this. Surfaces ripple, camber changes, the road widens or narrows in .spots and the radius may not be constant. So don'~ figure that you can run down to your friendly neighborhood computer center to program the fastest line through your favorite stretch of road. Nevah hoppen. You're still going to have to ride it yourself using your own judgement, your own instincts, your own reflexes. I'm just hoping that an understanding of the simple case will help you ride with a clearer head. / FIG. 5-Our multi rider gets his full one g by shifting his weight well to the inside. Though the bike still sits at 37 degrees the overall C. G. of the man/machine now lies t 45 degrees from the tire contact patch. This allows a stable force balance at one g without laying the machine any farther over than its ground clearance will permit.