Why shaft drive motorcycles have lighter handling
When people review motorcycles, they often mention how “light” certain bikes feel, even if they’re on the heavier side. BMW is a brand people bring up a lot; Moto Guzzi riders are super loyal to their bikes, and the Honda Gold Wing has a ton of fans too. Reviewers are also surprised by how the Triumph Rocket 3 handles, especially since it has a big 2.5L engine and weighs almost 300kg. Plus, a lot of older riders have great memories of those shaft drive middleweight bikes from the ‘80s.
What makes these bikes stand out? Aside from the shaft drive, they usually have engines with a longitudinal crankshaft (meaning the crankshaft runs along the bike’s front-to-back axis). Most other bikes have their engines mounted sideways, with the crankshafts sitting at a right angle to the bike. This difference isn’t just about the crankshaft; other spinning parts like the clutch, gears, transmission shafts, camshafts, water and oil pumps, and alternators have the same orientation. So, does this actually change how the bike handles?
I think it does! Spinning mass works like a gyroscope, and if you’ve ever played with tops or toy gyroscopes, you know that they resist tilting. The more mass you have, the further it is from the spin axis, and the faster it spins, the stronger this effect is. This gyroscopic effect helps keep stuff upright while it’s moving and has even been used in designs for stabilizing ships (like stabilizers). People have also suggested using gyroscopes to increase motorcycle stability!
So, I’m going to talk about how bikes move in three ways: roll, pitch, and yaw.
For bikes with sideways-mounted engines, the gyroscopic effect will make it harder for the rider to lean the bike over when turning (this means it resists roll). This can make handling feel “heavy.” There are also gyroscopic precession forces that might cause the bike to yaw, but tire friction usually works against that (unless something’s going on that reduces grip).
On the flip side, bikes with longitudinally mounted engines will have the gyroscopic effect resisting pitch motion. This could make riding more comfy, especially for the passenger since less pitch means less bobbing up and down. Plus, leaning into turns won’t be resisted, which can make the bike feel “lighter” to handle. But, when you speed up or slow down those spinning parts, it could cause the bike to roll (think of a “reaction wheel”). You can lessen this effect by having some parts spin in the opposite direction (“counter-rotating”).
You might wonder if popping “wheelies” is trickier on bikes with longitudinal engines since the precession force from changing pitch could make the bike yaw.
Just a heads-up: the gyroscopic effects I’m talking about are different from the well-known issue in shaft-drive bikes called shaft jacking, which manufacturers handle with suspension tweaks.
How big of a deal are these gyroscopic effects? I’m not really sure; I haven’t come across any research or talks about it yet. But it seems like it could become a bigger deal as bikes get larger and more powerful. More engine size means more rotating mass, and trends like more gearbox speeds mean more pinions. Higher power also means heavier parts, like clutches and driveshafts, and the added electrical demands from modern features (like heated grips) lead to bigger alternators. With today’s short-stroke engines revving higher, there’s increased angular momentum too. So, whether the engines are mounted sideways or longitudinally, the impact of flywheels and gyroscopic effects has probably grown over the years.
I think we should dive deeper into these gyroscopic effects. If upcoming research backs up my thinking, then it might be a good idea for motorcycle makers to explore longitudinal engine mounting and shaft drives in other types of bikes, including singles, electrics, and Wankel rotary engines.