Is Steve Durnin’s D-Drive the holy grail of infinitely variable transmissions?

Ready for a bit of a mental mechanical challenge?

Try your hand at understanding how the D-Drive works. Steve Durnin’s ingenious new gearbox design is infinitely variable – that is, with your motor running at a constant speed, the D-Drive transmission can smoothly transition from top gear all the way through neutral and into reverse. It doesn’t need a clutch, it doesn’t use any friction drive components, and the power is always transmitted through strong, reliable gear teeth. In fact, it’s a potential revolution in transmission technology – it could be pretty much the holy grail of gearboxes… if only it wasn’t so diabolically hard to explain. We flew to Australia’s Gold Coast to take a close look at the D-Drive – and it looks to us like Durnin has pulled a rabbit out of his hat. Check out the video after the jump and see if you can work out if there’s a catch.

Geared transmissions – a useful compromise

In basic terms, the idea of a gearbox is to create the ideal level of mechanical advantage between a motor and its output. Motors of all kinds have a speed of maximum efficiency, and a speed of maximum power, and you use a gearbox to decide what engine speed you’re running compared to the output speed.

In a car, that means you want a low gear at low speeds or for quick acceleration – because in lower gears, the engine revs harder and produces more power. Cruising on the freeway, you want a high gear that lets you trundle along using the minimum practical engine RPM so you don’t waste fuel.

So most gearboxes offer a compromise – manual, semi-auto and auto transmissions offer you a set number of gears you can choose to find one that’s close to the ideal ratio for what you’re doing. But there’s efficiency losses in between gearshifts as you disengage the engine using a clutch – or in the case of an auto, a torque converter. And although some geared transmissions offer lots of gear choices, every set gear ratio is a compromise.

And the clutch itself is a fairly crude device – when you’re engaging a clutch, you’re basically pushing a set of plates together, some of which are coated in high-friction material, which grab the other plates and force them to spin. This approach is inefficient and prone to slip and wear under large power loads.

Variable transmissions – very good, but not quite

Then there’s Continuously Variable Transmissions, or CVTs. The CVT is in theory a much better solution, because it allows a constant range of gear ratios between low and high gears. Scooters use them, as do some cars nowadays – with a CVT, the engine can sit at its most efficient or powerful RPM, while the gear ratio constantly adjusts itself to match wheel speed.

But most CVTs have a limited range of ratios they can work through – so while you can transition all the way from low gear up to high, you can’t go all the way down to neutral. So they need to use a friction clutch or torque converter to get them started from a standstill – and what’s more, in order to achieve variablilty in the gear ratio, they’re almost always built around some sort of friction drive too – like belts pulling on conical rollers, or rollers being mashed against toroid shapes.

All these friction components cause troubles when you start trying to put high power and torque through them – they start to slip and fail, they wear and generally contribute to inefficiencies in the drive train. That’s why you tend to go back to gears when you’re designing a high-powered machine. Gear teeth are reliable – the bigger the teeth, the more power they can handle.

‘Wind is cheaper than new coal’

The D-Drive – infinitely variable, no friction components

If all this gearbox talk seems like a long setup, it’s kind of necessary to understand the problem when you’re looking at the solution Steve Durnin has come up with.

Because at the heart of it, what Steve has managed to do is create a gearbox that:

requires no clutch at all;
is infinitely variable – from top gear through neutral and even into reverse; and
doesn’t use ANY clutches or friction drive components – instead, the power is ALWAYS transmitted from input to output through gear teeth.

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