When it comes to transmissions, a geared continuously-variable transmission (CVT) is a bit of a holy grail. CVTs allow smooth on-the-fly adjustment of gear ratios to maintain a target speed or power requirement, but sacrifice transmission efficiency in the process. Geared transmissions are more efficient, but shift gear ratios only in discrete steps. A geared CVT would hit all the bases, but most CVTs are belt drives. What would a geared one even look like? No need to wonder, you can see one for yourself. Don’t miss the two videos embedded below the page break.
The design is called the RatioZero and it’s reminiscent of a planetary gearbox, but with some changes. Here’s how the most visible part works: the outer ring is the input and the inner ring is the output. The three small gears inside the inner ring work a bit like relay runners in that each one takes a turn transferring power before “handing off” to the next. The end result is a smooth, stepless adjustment of gear ratios with the best of both worlds. Toothed gears maximize transmission efficiency while the continuously-variable gear ratio allows maximizing engine efficiency.
There are plenty of animations of how the system works but we think the clearest demonstration comes from [driving 4 answers] with a video of a prototype, which is embedded below. It’s a great video, and the demo begins at 8:54 if you want to skip straight to that part.
One may think of motors and gearboxes are a solved problem since they have been around for so long, but the opportunities to improve are ongoing and numerous. Even EV motors have a lot of room for improvement, chief among them being breaking up with rare earth elements while maintaining performance and efficiency.
TLDR: Sadly, it’s just another complicated ratchet mechanism.
with a wobbly output – and still needs a friction component in the one way bearings (or a ratchet/pawl in which case it is not continuously variable)
you don’t need friction in a one way ratchet, you need a spring load. not even remotely the same thing.
you also clearly didn’t watch far enough into the video to see the speed-equalizing elliptical gears.
and no, a ratchet and pawl has absolutely no effect on the contentiousness of the gear ratio. that is entirely determined by the leverage of the drive arms.
even on a spherical-cow theoretical level, the ratchet and pawl means discontinuity of drive motion, not gear ratio. for this a classic solution exists: a flywheel.
Yes. The oval gears are a neat trick, but it’s still a ratchet at heart
you make it sound like the ratchet is the whole deal. it isn’t. it merely requires a ratchet, in the same way it required ball bearings and shafts and screws.
nobody’s ever done this before. requiring a ratchet as one small element is hardly a high price to pay.
Video implies the ratcheting mechanism is specifically for the bicycle application, allowing the rider to coast instead of having the tires force the rotation of the pedals. Same functionality as existing 8- or 9-speed cassettes.
Yes, but as everyone pointed out multiple times in the comments of the video, the ratchets are mandatory for this design. It being on a bicycle has nothing to do with it.
“Same functionality as existing 8- or 9-speed cassettes.”
They’re out to 13 now, although I’ve stopped at 9, partly for reasons of chain & cassette durability. In a typical 12-25 (or 26) road cassette, the only difference between 9- and 10-speed is the 16-tooth cog between the 15 and 17, and otherwise all the jumps are the same size. They have 12-13-14-15-17-19-21-23-25 (or 26), versus 12-13-14-15-16-17-19-21-23-25 (or 26) which for me, is close enough to being a continuously variable transmission, and the chain drive can be up to 98% efficient. The “clicker” however is in the freehub body that the cassette slides onto, or in older technology, the freewheel. Some brands of modern freehub bodies are extremely loud when coasting. “Electronic Eel” here in the comments mentioned the 14-speed Rohloff Speedhub which is apparently an excellent design and very reliable, and each gear is something nearly 14% higher than the last; but its weight, and perhaps to a lesser extent its cost, have kept it from becoming very popular. FWIW, I have over 100,000 miles’ road cycling experience.
Unfortunately, this transmission, being CVT, does not provide continious output rotation. Output rotation speed and torque is pulsed few (equal to number of “satellites”) times per output rotation, and working principle does not allow creation of steady output.
Interesting thing, really, but unuseable for transportation.
My first thought was “surely some gearhead in the last few centuries designed something like this already.”
That said, even if it was designed something that worked on paper, the technical limits of the time may have made it impractical so it never saw the light of day.
the best anyone’s come up with was the thing with cones and a belt. seriously.
not really, ratcheting cvt’s seem to have been around for several years before this thing
Very surely, something similar was in popular (something) magazine about thirty years ago.
It seems the first application the developers are aiming for is bicycles. This means there is pretty though competition regarding efficiency and weight. Traditional chain gears have pretty good efficiency. Even designs like the Rohloff Speedhub have a hard time competing there and were just able to conquer a niche market over the years.
The prototypes that he showed in the video looked to me like they still have to go quite a way until they get their efficiency to a reasonable level.
i’d be very surprised if this ever got popular as a transmission for pedal bicycles. (although it might find a niche with e-bikes.) but that’s likely not the intention; bicycles (and e-bikes) would be a good, quick way to get practical experience with making these transmissions work on actual (if low-power) vehicles and in real use. a few thousand kilometres of road testing with such will likely teach them valuable lessons for when they scale up towards cars or motorcycles.
I would have to see some real data (Efficiency, power to weight ratio, endurance) before I believe in this thing.
I’ve been reading a bit in:
https://www.ratiozero.com/241203_Crowdcube_Ratio%20Zero%20MY24_Q4.pdf
One of the prototypes (In 2018) could not handle enough torque for a bycicle. Understanable for a prototype of course. The problem here is that only one gear in the planetary gear system is active at any one time, and thus the load is not distributed over the gears such as in a normal planetary gear.
I like the idea of the oval gears to reduce the ripple in the transmission ratio, but also wonder why they chose for simple oval gears. Ideally the form of the gear would be the inverse (reciprocal?) of the torque ripple to further smooth it.
They also claim an efficiency of 95% but they do not state whether this is just for the gearbox, or if it includes the chain drive too.
And they have an “Exit Strategy”. They intend to sell their company for USD 80M over 5 years. Sigh. Instead of giving this “invention” some time to develop it further and mature their intention is to get as much money out of it as possible, and as quickly as possible. Even if this thing can be made to work good (enough), extracting too much money too quickly may well bankrupt the company.
This is a variation of an idea I had when I was young. The basic idea is that a piston on a crankshaft will move more or less depending on the length of the crank. In fact, when the crank is 0 length, the piston doesn’t move at all. However, a piston gives reciprocating linear output. In order to turn that back into rotary output, you can use a rack and pinion, with the rack attached to the piston, and the pinion having a freewheel so it only engages its output shaft in one direction. Now, since the rack only turns the pinion during half the stroke, you have to add one or two more of these mechanisms at different phases in order to keep turning the output shaft continuously.
This new mechanism “simply” arranges all the racks in a circle. Quite clever, actually.
The transmission isn’t truly continuously variable due to the freewheel. The adjustment needs to reach the next click on the freewheel ratchet before a change happens. Of course, having several freewheels creates more adjustment points.