I just wrote a small explanation on the principle of a sprag clutch. (Available here)
I locked the topic as a tutorial should not be like a post where people ask or answer questions.
I like to have all the 'gurus' on this forum to have a look at it and send me by PM any correction or addition required.
It is very difficult for me to put myself in the shoes of somebody with very limited mechanical knowledge in order to explain the basic concept.
Thanks

Bert

Nice job, thank you very much.
Ned

Once again, a job well done Bert.

Bert, excellent tutorial, well done. I am no "guru" but I work with these devices in industry, we mainly see them as backstop devices typically in large gearboxes.
Just for interest, the over-run principle is the reason the starter can be safely engaged with the engine running. Conversely, never attempt to turn an engine backwards, as the result will almost certainly be -
(a) very difficult to achieve
(b) damage to the starter from over-speeding

Best regards,
Dennis

Dennis wrote:
Just for interest, the over-run principle is the reason the starter can be safely engaged with the engine running.
Dennis

This is a nice security feature, but the BMW engineers went overboard and also designed an electrical protection.
When the engine runs over 710rpm, the electronic ignition module shuts off the ground to the coil of the starter relay.

Bert

Brilliant write up well explained thanks Bert.

BIG D

BTW, here's one (says it's new!) on eBay for those who just might need one.

http://cgi.ebay.com.au/k100-k1100-starter-clutch-/170519118467?cmd=ViewItem&pt=AU_Motorcycle_Parts_Accessories&hash=item27b3bb4283