The Weakest Link Between your Throttle and the Pavement

Reprinted by Permission from Mr Tidy's Tech Tips

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.

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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.

Comments (6)

drivetrain problem Written by driveshaft, on 09-29-2012 14:05 my 2006 roadstar the middle shaft and pulley splines stripped. my bike makes a metalic noise when clutch is engauged.The problem I found is the middle shaft and the pulley is not going all the way into the pulley.Over 50% of the spline not going into the pulley.I installed a new middle shaft and pulley for my 2006 roadstar and the unit fits fine.Yamaha installed the wrong shaft and pulley ,won:t stand behine they workman ship.

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