Every now and again, astute Gizmag readers come to the fore to keep us on our toes - and never has this been better demonstrated than with last Friday's D-Drive Infinitely Variable Transmission article. More than 40 comments and e-mails have flooded in over the weekend questioning the D-Drive's capabilities as a true IVT, and its potential efficiencies. Furthermore, an engineering report was made available on the D-Drive website that flat-out negates some of the key claims that were made in our interview video. So let's take another look at this device in the harsh light of engineering scrutiny.

Steve Durnin's D-Drive gearbox has spurred a lot of interest since it first came to public attention on the Australian ABC's New Inventors show earlier this year. But despite its winning the weekly invention challenge, the device was explained in only the vaguest of terms, giving technically minded folk little from which to draw any proper conclusions.

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I contacted Steve Durnin and arranged a meeting, during which we filmed the device from all angles and discussed its potential at length. I then put together a video story, touting the D-Drive's potential as an Infinitely Variable Transmission, talking about its potential efficiency advantages, and explaining how it works from my own (non-engineer's) point of view.

Steve, meanwhile, was working to put together a website for the device - which he made available to me just before the Gizmag article went live - and which contained a link to an engineering report that presented the D-Drive in much more sober terms.

What we got wrong

Firstly, the D-Drive as pictured in our video is not a complete infinitely variable transmission system. At best, according to the engineering report, it is a cheap, innovative and potentially very useful primary component of an IVT.

The key problem here is that the D-Drive's control shaft needs to be driven at variable speeds in order to effect the final ratio - so effectively, you need a variable drive motor attached to the D-Drive before it actually works. e3k's engineering report goes so far as to say the control shaft could foreseeably be driven through an external CVT, using a clutch - which of course introduces not one, but two friction components to the system.

One of the main advantages we spoke of in our original article was that the D-Drive got around the need for friction components and transmitted all power through gear teeth. Effectively, if mated with a CVT, the D-Drive outsources the friction components to the transmission of its control engine.

The next thing we failed to pick up on, but that several commenters have pointed out, is what happens when you run the D-Drive under load. With the control motor running to decide on the final gear ratio, the input motor's power would be transmitted to the wheels, where it would meet resistance under load. That load would then be passed back through the gears to the weakest point - which would be the engine driving the control shaft, if it wasn't up to the job.

So it's not possible to run the control shafts using a small electric motor as we said in the video - in fact, the engineering report is quite clear on the fact that the 'control' motor needs to be just as powerful as the 'input' motor: "Our designation of 'Input' and 'Control' shafts in this report is arbitrary in that both would conventionally be used to provide power. There is no inherent character of the mechanism that requires the input to be the dominant power-providing element. The torque provided by the control shaft will typically be of the same magnitude as the torque provided by the Input shaft... the Input and Control should be considered as parallel power paths rather than as 'power ' and 'control' elements respectively."

What you got right

You can certainly rely on Gizmag readers to think through an issue like this one - some of the technical discussion in the comments section of the original article - as well as the discussion threads on Slashdot, Reddit and elsewhere - was fascinating.

The D-Drive does indeed operate as an epicyclic gearset. It does indeed operate in a similar way to the Hybrid Synergy drive on the Toyota Prius - and this is a matter of some pride to Steve Durnin, who designed it with none of Toyota's considerable resources behind him.

It does require an external CVT or some other powerful drive component for the control shaft that will not yield to the torque of the primary input motor.

So where does this leave the D-Drive?

More or less where the engineering report concludes. The D-Drive is an innovative component that could be used in the design and manufacture of a true IVT for use in vehicles or anywhere else a variable drive would be required.

It's not a 'holy grail' and there's no rabbits coming out of hats - and what's more, it can't forseeably operate with no friction components between engine(s) and wheels, or at least something like an electric motor that can be smoothly varied in speed.

The D-Drive should, and will, be evaluated on its abilities inside this scope - and as such it may well still become a very valuable piece of intellectual property for its designer.

Thanks for calling us out

So thank you, astute readers and commenters, for calling bunk on this one. We saw an interesting and remarkable piece of emerging technology that hadn't been covered yet in enough detail to generate discussion. We certainly generated discussion, but we got some key information wrong - for that, we apologize.