Centrifigal clutch modification.

I am interested in modifying a Staton heavy duty centrifigal clutch to replace the factory Sabaru 35cc clutch. The factory clutch idles at 2400 rpm and engages at 4200 rpm, almost half of the throttle capability of the engine which is too non-responsive and slips too much for my tastes.

Staton warns that the engine must be capable of idling at or below 2000 rpm, which mine is not comfortable idling at without stalling intermittently.

The heavy duty clutch weighs 200 grams while the factory comes in at 95 grams but both use different release springs so engagement comparisons are not possible due to spring tension differences.

I plan on drilling lightening holes in the heavy duty clutch in order to retard its engagement openning timing, shooting for a 3000 rpm engagement.

How much weight do I need to remove from the heavy duty clutch to get a 3000 rpm engagement? Has anybody done this before or have a means of ballpark calculating the amount of weight removal?

Contacted Staton about this but he had no idea about the needed weight removal.

Thanks, Don in Redondo.

Photo of clutches for Sabaru 35cc, factory on the right, heavy duty replacement on the left to be drilled with lightening holes;

Erich_870 said:

That's an intriguing question! If you have an accurate way to measure RPM, the weight of the clutch and the spring tensions, you should be able to work up an equation to get close to your 3000 rpm goal.

What are you capabilities to measure those three variables?

Erich

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I think I have reasonable measurements of the three variables;

RPM= Engagement rpm= 3,000 Idle rpm=2,400

Clutch half weights= 100 grams (3.5 oz) per side or 200 grams (7.0 oz) total

Spring tension= 15 pounds force retracted and 21.5 pounds of force at .125"
or 1/8" extended about the engagement distance of the
clutch shoes when contacting clutch drum as measured
using a postal scale

My gut feeling is that it is going to take about 20-30 grams of weight removal per side or 40-60 grams total to achieve the 1,000 rpm engagement delay that I am looking for. If I cannot come up with a ballpark estimate I will just have to start out conservative at say 15 grams removal per side, re-assemble, test, and re-drill. Problem with this trial and error method is that dis-assembly of the engine, gearbox, and clutch is a long drawn out procedure that I dont want to have to repeat too often...............Don

Just finished drilling, weighing, installing, and testing lightened heavy duty clutch using the trial and error method.

Drilled twelve 1/4" dia. holes at outermost diameter area of clutch halves, reducing clutch weight by 25 grams total from 200 grams to 175 grams. Material drilled easy, appears to be a very fine grained cast iron.

Tachometer shows that the clutch now engages at 2,750 rpm from 2,300 rpm idle, this heavy clutch runs exceptionaly smooth on road testing. Wish the engagement was up over 3,000 rpm for a little more separation from idle speed. Will test ride for a period of time over various trail conditions before deciding if further modifications are necessary...............Don

rustycase said:

Great project !

Chamfer a pair of opposing holes to lighten it a bit more and it will probably engage at the higher rpm you wish... Not? chamfer two more...

Biggest difference will be at end of shoe farthest from the hinge point.
Fine tune adjustment closer to the hinge.

Best
rc

Click to expand...

That will be the plan if needed................Don