A clutch with one 3.5 inch diameter friction disc turning 4000 rpm can easily accommodate 15 ft. lbs. of torque.  A two disc clutch, 30 ft. lbs.  A three disc clutch, 45 ft. lbs.  Why then, since the engine produces only 10 or 11 ft. lbs. of torque, would a two or three disc clutch be any advantage?

Heat Capacity 

While the clutch is slipping it generates heat.  Air passing around and through the clutch sheds heat.   Unfortunately for racers, clutches gain heat faster than they lose heat.  The heat must go somewhere, so it's stored within the drive plates that oppose the clutch discs.  As heat is added to drive plates their temperature rise.  Within limits that's okay.  But given enough heat, temperatures rise to a level where problems occur; friction materials lose their adhesive properties, metals lose their strength, wear rates become excessive.  

The temperature rise is inversely proportional to the mass or weight of the drive plates.  In other words, given the same amount or slippage or heat, temperature goes down as weight goes up.  A heavy clutch will have a lower temperature than a light clutch.  Three disc clutches run cooler than two disc clutches which run cooler than one disc clutches.  But there is no advantage to a cold clutch over a warm clutch.  Below a certain critical number, temperature has almost no affect on performance.

Weight

The light clutches spool up faster than heavy clutches.  The laws of physics are very clear about that.  But the clutch is bolted to a very heavy crankshaft and a very heavy flywheel.  A good dynamometer with fast, accurate sensors will measure a difference of only three ten thousandth (0.0003) of a second!  Energy calculations will confirm that number.

But don't forget that tiny, tiny acceleration advantage exists only if the light clutch is the same temperature as the heavy clutch.  Never, ever the case.  Over heated clutches can give back as much as three seconds per corner!  To finish first, first you have to finish.

Response Time

Clutches do not engage instantaneously.  It takes a tiny bit of time for each drive plate and each friction disc to overcome friction with its mating part (drive hub and basket) and move into position.  One disc clutches accelerate a bit quicker simply because there are fewer moving parts. 

A compromise between heat capacity and mechanical efficiency is called for.

Summary

The one disc clutch is fastest unless it gets hot. Helpful guidelines:

• Qualifying rounds should be run with one disc clutches.  Track time is too short for the clutch to get hot. 
• One disc clutches are recommended for long tracks i.e. #35 clutch sprockets with 16 or more teeth (#219 clutch sprockets with 19 or more teeth ).  The clutch has time to cool down between corners.
• One disc clutches are recommended for the junior classes.  Lighter karts put less thermal stress on clutches
• Two disc clutches are recommended for short tracks i.e. #35 sprockets with 15 or fewer teeth (#219 sprockets with18 or fewer teeth ) because there is too little time to cool the clutch between corners and the extra drive plate increases heat capacity.  
• Two and three disc clutches are recommended for Honda engines regardless of gear ratio because the engine develops maximum torque at low rpm.
• One or two disc clutches are recommended for road (enduro) racing.  Although the tall gear ratio causes the clutch to slip for a very long time and generate much heat, the straights are long allowing plenty of time for the discs to cool before the next corner.
• Two-stroke engines have a very high engagement rpm and consequently slip for a long time.  Use a three disc clutch in combination with a jack shaft.

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