Motorcycle shaft final drive, that is.
Modern production motorcycles come in three different flavors of final drive: chain, shaft, and belt. Previously on “Party In The Back,” we discussed chain drives. This time, we’re all about getting the power from the engine to the rear wheel with a shaft.
First, it is what it sounds like: a metal rod. A motorcycle’s engine has to transfer power to the rear wheel somehow, and in shaft-drive motorcycles, that power goes through the transmission to the geared shaft, into the shaft hub and then onto the rear wheel. On modern bikes, this all happens inside a housing.
He Ain't Heavy
If you’re already saying “but Kate, isn’t all that awfully heavy?” Yes! Congratulations, you’ve just hit on the first “con” when it comes to shaft-drive motorcycles. That shaft, gearing, and housing is, in fact, all very heavy. Compare it to the two-sprockets-and-a-chain system, and you’re hanging a lot more weight onto a bike. That’s why you’ll find shaft drives on motorcycles that are already pretty big, and whose main purpose is not going very fast (like big ADV bikes, big cruisers, and tourers). Modern race bikes are all chain-driven for this reason: the weight savings is key.
Many A Winding Turn
Not only is the whole assembly heavy, but it is more complicated to design and build than a pair of sprockets and a chain. Shaft drives are more expensive to design into and install on a new motorcycle, but one could successfully argue that they are, in fact, less expensive to the motorcycle owner over the lifetime of the motorcycle (provided said shaft system is cared for and does not fail). Who’s going to be buying a new chain and sprockets every year or two? You are! Unless your bike is a shaftie.
The Road Is Long
That brings us to the very big “pro” of a shaft drive: maintenance. All of that gearing is bathed in oil, and as any good motorcycle owner knows, if it’s a fluid in a motorcycle it’s going to need to get changed out once in a while. Those of us who have shaft drive motorcycles swap this oil out at about every other oil change. Some of us who are a bit more anal about maintenance will pull the whole shaft assembly and grease all the fittings (you can insert your own “lubing your splines” joke here) every few years. As far as maintenance, that’s it. Sounds way better than swapping a chain and sprockets on the regular, right? It is, especially for those of us who really pile on the miles.
A shaft drive, by design, keeps its internals clean. A chain picks up all kinds of road grime which ages it pretty quickly. Grit in moving parts is bad for longevity, after all. Everything inside the shaft drive is protected from the elements and dirt, which means wear on parts is significantly reduced.
The oil inside a shaft hub is hypoid gear oil, and that is because the gears on a shaft are hypoid gears. Put simply, that means they’re shaped like a spiral instead of straight lines. This is so that the wheel-end gear can be as large as possible inside the hub, and the force of the engine spreads over all the little pieces and is never on any single tooth all at once. Since that whole assembly needs to move up and down along with the rear suspension, the shaft is jointed, and where that shaft is jointed the housing is jointed, too.
The wear points in a shaft are those joints, both inside and outside. The housing joints are covered by rubber boots that allow the housing to move. The shaft joints are “universal” joints, and they look quite a lot like that jointed socket extension you have in your tool kit: they can spin while easily holding an angle. If you ever disassemble your driveshaft, be careful that the two universal joints stay “in phase,” that is, correctly aligned with one another; if they are not, that will cause premature wear and can turn into a very expensive repair.
Some poorly-suspended shaft-drive motorcycles can experience a phenomenon called “shaft jacking,” which is when sudden acceleration makes the rear end of the bike pop up. It’s more pronounced on some shaft-driven bikes than others; I have felt it on exactly one motorcycle ever, and have ridden a lot of shaft drive motorcycles. Don’t let these stories scare you away from the nearly-maintenance-free glory that is a shaftie, especially if the person telling you the stories has a pretty short riding resume.