For those not sure, the Cush Drive (Cush meaning Cushion) is basically a shock absorber for the chain and lives in the rear wheel where the rear sprocket carrier fits into the hub. It's role in life is to provide a rubberised cushion for the rear sprocket to engage into the rear wheel.
It compromises of a series of rubber blocks sandwiched in between flat plates half of which are on the sprocket carrier, the other half are in the wheel. The rear sprocket carrier is then supported on the axle by a bearing and usually has a dust seal on its outer side. As power is applied to the sprocket via the chain the rubber blocks are compressed thus providing a smoothing of the power transfer to the wheel. A properly serviced cush drive helps to greatly reduce chain snatch and vibration and can greatly increase chain life.
I have just finished fitting a new chain and sprockets to a Triumph Tiger which had only recently been fitted with new tyres – so you might think the cush drive would have been carefully checked – but maybe not so much.
Upon removing the rear wheel, I found the inner spring on the dust seal had collapsed and the carrier bearing was quite worn. The bearing itself is retained by a circlip and is a light press fit into the carrier. So, after stripping out the old bearing and cleaning the whole assembly, I fitted a new double sealed bearing and a new genuine dust seal. A pair of new sprockets and a new chain and the overall effect was an obvious and marked reduction in chain vibration and a much-improved smoothness both under power and on a trailing throttle.
I know this stuff is not revolutionary, but these are the types of small things which take away that new bike feel and, over time, make your bike less reliable and less enjoyable to ride.
So spare a thought for the humble rear hub - and remember, cush drives need love too!