Re: Royal Enfield Queries The sprag clutch and the engine clutch are totally different things. The only thing that is common to both is they both transmit rotary power from one place to another place.
The engine clutch transmits power thru it or it slips and transmits no power depending on the position of the clutch lever.
The engine has a chain that transmits power from the crankshaft to the engine clutch.
The power rotates the clutch, usually at about 1/2 of the engine speed.
Depending on the position of the clutch grip lever, this power either stops at the clutch (lever pulled towards the hand grip) or, it transmits the power to the gears in the transmission.
The sprag clutch also transmits rotary power but in one direction only and in the case of the Royal Enfield, the sprag clutch is located on the engine crankshaft.
It is driven by the electric starter motor thru a series of gears.
If a sprag clutch is turned in one direction it locks and transmits the power to the crankshaft. If the crankshaft is rotating faster than the sprag clutch, the clutch releases and no power is transmitted thru it.
Putting it a different way, when the electric starter motor is energized and it starts to run it rotates the gears that drive the sprag clutch.
If the engine is not running, the sprag clutch locks up and rotates the crankshaft to start the engine.
When the engine starts, the crankshaft begins rotating faster than the sprag clutch so the clutch unlocks and allows the crankshaft to continue to rotate while it comes to a stop (after the user releases the starter button and the starter motor stops running).
Getting into the way a sprag clutch works, there is an outer, driven ring, a bunch of elongated steel "sprags" and an inner ring.
If the outer ring begins rotating the sprags become tilted and because they are longer than the distance between the outer and inner rings, they wedge between the inner and outer ring. This transmits the power thru the clutch.
If the inner ring is rotating faster than the outer ring, the sprags rotate slightly and rather than wedging in place they tend to lay flat and just lightly skid on the inner ring allowing it to rotate freely.
While were discussing sprag clutches, one could ask, what happens if the inner ring rotates in the wrong direction? Will the sprags become wedged between the inner and outer ring and cause the clutch to "lock up"?
The answer is yes.
If the inner ring rotates backwards, the clutch will lock up and transmit the backwards movement of the crankshaft to the starter gears and starter motor.
The shock of this happening can be great enough to shatter the hardened steel sprags, ruining the clutch and depositing bits of hardened steel sprags throughout the engine.
Engine "kickbacks" that happen when the engine is being turned off and the crankshaft reversing direction due to the piston compressing the air/fuel in the cylinder just before the engine totally stops running caused the ruination of many of the sprag clutches on older Royal Enfields with electric starter motors.
That is why the recommended way of "killing" the older electric start Royal Enfields is to apply the compression release and only turning off the ignition key after the engine has stopped running.
To get around this problem, when Royal Enfield redesigned the engine to the newer UCE style, they installed an auto-decompression device. It releases most of the engine compression to prevent "kick back" due to compression as the engine is coming to a halt.
This feature protects the sprag clutch while the owner simply turns off the ignition switch and the engine stops running.
If the sprag clutch is defective, it will not transmit power from the electric starter motor to the crankshaft.
If you hit the electric starter motor button to start the motorcycle, a defective sprag clutch will cause the electric motor to accelerate to full speed but the engine won't rotate at all.
Before anyone freaks out, if this happens on a very cold day, it is possible the thickened engine oil that is coating the sprags is not allowing the sprags to rotate and lock up.
(This has happened several times to me after the motorcycle sit overnight at temperatures below freezing (0° C).)
The answer to this is to stop pushing the start button and let the motor stop running. Then, try it again.
If the engine oil is warm or hot and the electric starter motor runs without turning the engine crankshaft (No choof...choof...choof), a bad sprag clutch is the first thing to suspect.
Last edited by ArizonaJim : 27th December 2016 at 04:19.
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