Hysteresis

 What is hysteresis?  In racer's terms: (his-ta-'ree-sis) is the difference in engine speeds when the clutch engages and when it disengages.  For example, an average clutch on an accelerating engine will engage at 4000 rpm.  When slowing down from high speed the same clutch will disengage at 3500 rpm.  This clutch has 500 rpm of hysteresis (4000 rpm - 3500 rpm).

Alternate explanation: hysteresis is a phenomenon in which the response of a physical system to an external influence depends not only on the present magnitude of that influence but also on the previous history of the system.  Expressed mathematically, the response to the external influence is a doubled-valued function; one value applies when the influence is increasing, the other applies when the influence is decreasing.

Why is hysteresis important?

If you take your engine’s torque curve and your clutch’s torque curve and plot them on the same graph you will see one angular speed (rpm) where the two curves intersect. This is the engagement rpm. Some call it the stall speed.  The clutch and engine are in equilibrium.  For angular speeds less than engagement rpm, the clutch can absorb less torque than the engine produces, consequently the clutch "slips" and the kart does not move.  For angular speeds greater than engagement rpm, the clutch can absorb all the torque produced by the engine and then some, consequently the kart moves forward.

Example of a typical four-stroke kart engine and clutch.  Engagement rpm occurs at 4000 rpm - peak torque rpm.

A clutch should be adjusted so that its torque curve intersects the engine's torque curve at its highest point.  This occurs at 4,000 rpm in the graph above.  At "peak torque" rpm the engine's crankshaft is twisting with its greatest possible force.

If the engine and clutch rpm stabilize at the peak torque value until slipping is no longer required, you get the maximum possible acceleration.

Stabilize is the key word.  At engagement rpm the clutch discs are squeezed and released rapidly in order to extract energy from the engine.  It happens faster than the eyes can see and faster than tachometers can indicate.  Each time the clutch squeezes, the engine slows down and each time it releases, the engine speeds up.  But it slows down more than it speeds up!  The engine does not stay at the ideal rpm for very long - it's unstable.  The engine moves down its torque curve where it produces less torque.  Potential acceleration is lost.  Hysteresis is to blame.

Eventually the kart develops enough speed where clutch slip is no longer required.  Engine rpm can then climb to a much higher value.  If the engine stays above engagement rpm for the duration of the race, hysteresis is not much of an issue.  But if slow starts, restarts or slow turns are part of the race, then hysteresis is very important.  Clutch slip lasts for several seconds.  Multiply that by the number of times it slips during a race and the total slip time is significant.

All kart racing clutches exhibit hysteresis.  Rim clutches, cone clutches and disc clutches have it.  Shifter kart clutches and axle clutches have it too.  Some clutches have more hysteresis than others.  A lot more.  "Coarse dynamic shift feel" or "Hard hitting" clutches have far too much hysteresis.  Clutches with little hysteresis have a smooth "dynamic shift feel".

In summary: minimize hysteresis to maximize acceleration.

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