The Weakest Link
Between your Throttle and the Pavement
Ken "the Mucker" Sexton
 
 

               A number of riders have asked me why their troublesome clutch slips in the higher gears, 
          but not usually (or as badly) in lower gears. It seems obvious to them that, since acceleration 
          is strongest in the lower gears, then the clutch should slip more while accelerating through 
          them.  But if their clutch isn’t as healthy as God and Yamaha intended it, then it tends to
          slip more in ever higher transmission gears. Why is that so?

          The confusion is a result of applying an “obvious” concept to a complex situation.

          For purposes of clarity the following should be kept in mind:

       ·      *On most modern motorcycles the engine is more than it seems. What we see as the
                “motor” is an engine, clutch and transmission. The engine is just the part with cylinders,
                crankcase, pistons, connecting rods, crankshaft, and head(s).

              *The clutch is “upstream” of the transmission, meaning it receives the engine’s power 
                output directly from the crankshaft and then delivers that same power to the 
                transmission.

              *The torque and horsepower (HP is just torque applied to engine RPM’s) made by 
                 the engine doesn’t change with changes in gearing. The torque produced by the 
                 engine only varies with RPM’s because the engine’s efficiency varies according 
                 to RPM’s and their relationship to the essential design of the engine and it’s intake 
                 and exhaust systems. So the torque curve rises to a peak, then falls off as efficiency 
                 drops off.

              * Few dynamometers can measure engine output, as that would require removing the 
                 integral transmission from the “package”. So most engine dynos measure torque 
                 and HP at the transmission output shaft and a rear wheel dyno takes its measurements 
                 at the interface of the rear tire and a spinning drum. With both of theses common types 
                 of dynos the transmission (and rear wheel drive system in the case of the rear wheel 
                 dyno) plays a significant part in the resulting power measurements. 
                 There are other types of dynamometers, and variations of the above two, but these are 
                 the most common.

              * The torque measured at the rear wheel drops as gearing between the crank and the 
                 rear wheel rises (lowers numerically) and power is “wasted” through friction at each
                 transfer of power between internal components. With the engine in first gear, more 
                 torque gets applied to the ground (OR the dyno) than it does in any “higher gear”. 
                 As gear ratios rise (drop numerically), the mechanical advantage of the engine 
                 (read: measured torque on a dyno chart) over the rear wheel drops. 
                 That’s why you’ll see several roughly parallel lines on dyno charts published by 
                 most reputable magazines, when they test a new bike. Each line will represent 
                 the results of dyno-testing each transmission gear available in that particular 
                 motorcycle. And each successively higher gear will fall lower on the chart than
                 the gear before it, as a result of gearing effect (mechanical advantage) and energy 
                 losses. If a dyno chart shows only one curve, it should have a note stating in what 
                 gear the curve was generated, or if it is actual engine torque output, as measured 
                 by an engine-dynamometer, with the tranny removed.

            So why does a problem clutch slip worse in each higher gear, if the available torque 
            drops with each upward shift? The answer is because the clutch is “upstream” 
            of the transmission. The clutch gets the full brunt of the engine’s power output, 
            without the torque reduction that the transmission applies. Higher gear selection 
            increases the onset and severity of clutch slippage is because there’s another 
            factor to consider… traction.

            Traction is NOT just a concern between the tires and the road. Traction is the 
            friction between any two surfaces as they try to slide against each other. We usually 
            don’t want any loss in traction between our tires and the road. That’s generally bad. 
            But there are other places where we want varying amounts of traction. As we modulate 
            our clutch with our left hand, we’re adjusting the traction between the spinning plates 
            of the clutch to suit our needs at the time (pulling away from a stop, smoothly engaging 
            the next gear, etc.). The total available traction between the crankshaft and the road 
            is only as reliable as the weakest link in that entire chain of power transmission. 
            The torque made by the engine will be expended as forward motion or at any place 
            where it can overcome the available traction. If your clutch is healthy and you’re in 
            first gear, a twist of the throttle should yield strong acceleration (unless the rear tire 
            breaks loose and the engine’s efforts go up in wheelspin and smoke). If the traction at 
            the rear wheel holds, the bike goes forward (and/or maybe the front wheel goes UP!). 
            But, in each successive up-shift, your forward acceleration will be with reduced G-forces,
            because the engine’s power to the rear wheel drops with each successive up-shift 
            (and because air drag rises with increases in velocity, but that’s another factor not 
            relative to this discussion). If at any time the rear tire should engage a surface that 
            causes a significant drop in available traction then wheelspin may result. But while 
            wheelspin can occur in any gear, it’s more likely to happen in lower gears, because 
            rear-wheel power is strongest in the lower gears, as shown on the rear-wheel dyno. 
            More power equals more ability to overcome available traction. If the clutch has 
            worn out or suffered damage as a result of abuse or overheating, or simply expended 
            it’s useable life, it may slip and allow the engine to gain revs faster than the apparent
            acceleration would warrant.

           The reason a less-than-healthy clutch slips more in higher gears, than it does in lower 
           gears, is because the engine’s ability to overcome traction at the rear tire is reduced 
           in the higher gears, while the power applied to the clutch has remained constant 
           (relative to RPM). So, if the rear wheel traction holds, the clutch can become the 
           weakest link between the crankshaft and the road.

           Good Day
               Ken "the Mucker" Sexton.