Holding back the gears

by Simon Hargreaves

The relationship between gear ratios, torque, thrust and 'retuning' is never an easy one

Honda's new CTX1300 is, at first glance, technically unremarkable. It is, essentially, a stripped-down, half-faired Pan European with sharper steering, a wider rear tyre, a stereo and, somehow, more weight. It has the same 1261cc transverse 90° V4, 'retuned' for more midrange.

But the spec shows this is more, or less, than a retune: a stock Pan makes 125bhp at 8000rpm and 92 lb.ft at 6000rpm. Honda say the CTX makes 83bhp at 6000rpm and 78 lb.ft at 4500rpm. That's a massive 40% less peak power and 16% less peak torque!

Doubtless the curves are reshaped, but it's still a big deficit. Why Honda has done this will be revealed at the launch, as will how – probably mild cam timing, and modified engine management, intake and exhaust (the exhaust has two catalysts, one for each cylinder bank; a good way to soak up power. And add weight).

But the technical challenge will be making the CTX not feel underpowered. Which is where gearing comes in.

We're familiar with power and torque. Torque is turning force at the crank and power is the rate at which it's produced. But as lines on a graph they don't tell you what the engine feels like to use. Give or take, a power or torque curve looks the same in first gear as top. It doesn't take into account the effect of gearing (the measurement that does is called thrust. Thrust is a reaction force at the back wheel – as the wheel pushes the bike forward, it's also trying to spin the Earth. It's measured in pounds; a pound of thrust accelerates a pound of mass at 32ft per second per second – the same as gravity. The reason it's a good way of showing how an engine feels is because it translates torque into actual forward motion, through the gears).

Gears are like levers. A long lever (think of a spanner) amplifies the effort at one end, but it trades it off against speed (of turn) at the other end. Gears do the same: they take the torque appearing at the crank and trade it against turning speed. First gear trades speed for torque because a bike needs a lot of force to accelerate from rest. But once you're rolling you need to swap some of the torque for more speed. So you go to second gear, and so on. Unfortunately you can't go on indefinitely because drag from aerodynamics and rolling resistance eventually overcomes engine torque (which you've been busily trading off for speed).

So gearing adds flexibility to an engine's torque output, letting a manufacturer tailor thrust characteristics to suit the bike: for acceleration, or top speed, or relaxed cruising.

With the 'retuned' CTX1300, you'd expect Honda to lower gearing to pep up acceleration at the expense of top speed. But they haven't. Instead, they've used the same internal ratios as the Pan and raised the final drive ratio, 'lowering engine rpm and adding to the tranquil, enjoyable ride.'

With 40% less power, it sounds like the CTX will indeed be tranquil. Anything more would be, technically, remarkable.

Navy Boy
Offline
Joined: 12/02/2009

Sprocket changing was something we'd often do on our off road bikes when I was growing up. However I've never felt the need to do so on any of the road bikes I've owned.

Having said that there are a fair few that would have benefited from it. The last couple of Ducatis I owned spring to mind.