A CVT For Every Application

When the subject of CVTs or continuously variable transmissions comes up, the chances are that most readers will think of the various motor vehicles they’ve appeared in. Whether it’s a DAF, a Ford, a FIAT, or a Chevrolet, most major manufacturers have tried one at some point or another with greater or lesser success. The automotive ones inevitably use a variation on a V-belt or metal band between variable separation conical pulleys, but this is by no means the only CVT configuration. Serial tinkerer [Robert Murray-Smith] takes an in-depth look at the subject as part of his ongoing fascination with wind turbines.

What caught our eye about this video isn’t so much the final 3D-printed design he selects for his experiments, but the history and his look at the different CVT designs which have appeared over the years. We see the V-belts, as well as the various cone configurations, the disk transmissions, the hydrostatic ones, and even magnetic versions. His transmission uses two cones with a rubber coating, with of all things a movable golf ball between them. We’re guessing it will appear somewhere in his future videos, so watch out for it.

Meanwhile, this isn’t the first time we’ve seen a CVT, [James Bruton] used a hemisphere to make one on a robot.

36 thoughts on “A CVT For Every Application

          1. MartyK:

            Nissan’s CVTs (Jatco) typically go 30k.
            If you’re lucky miles, if unlucky km.

            It’s an absolute shit brand now. Their best car is their electric…That’s just pitiful.

            Beware other Japanese brands with Jatco CVTs.

  1. It’s interesting to see how far back the CVT was actually used. Steam engines were actually an early example of the epicyclic CVT.

    One type that he didn’t mention are the electrical CVTs. Diesel-electric engines and the Toyota Prius are an example of this. Another is the Ward-Leonard system, used in elevators, cranes, and other applications for moving heavy loads.

  2. Too bad most manufacturers insist on simulating shifts. I’d love a transmission that lets me dial in for max torque (or HP?) and keeps the RPM’s at that point under full throttle as the vehicle accelerates.

    1. I never saw a Ford do that. I owned a freestyle for many years and the cvt was one thing that never needed service and never did any kind of simulated shifting.

      There was a mantra in a forum that I for that car, if your cvt is still going strong at 100k don’t dare open anything on it. Leave the original transmission fluid in and drive it till the wheels fall off. We donated ours with over 200k (miles) on it and it was still running fine. Never a lot of power but it was reliable.

    2. Honda CVTs don’t use simulated shifts, or fixed ratios, under most loads. I do most of my stoplight-to-stoplight accelerating under 2000 RPM.

      I would also like more control over throttle/RPM ratios like you described.

    3. You add a set of springs connected to a hydraulic dampener.
      You add bypass ports along the length of the dampener with one way valves and have springs of different stiffness, to alter the shifting profile over the entire range. Perhaps using micro channels and Tesla valves in a closed system, no moving parts, no maintenance.

    1. I insisted! he put a watermark in all future videos under moany of his uploads when that other channel “King Mechanical” ripped all his content as soon as it came out… As well as flagging that other guys stolen content.

      And then some day he finally did it.

    1. That is sort of a contradiction in terms.
      “Rigid toothed gears” implies set gear ratios, How could it possibly be a “Continuously Variable” Transmission?

      My sarcasm detector did flash a reading while I was going over your comment…

    2. gears always have an integer number of teeth, so can’t be varied without reversing direction with unreliable ratchets. You could use a twin engine design, so that one engine takes over while the other is changing gear. That would be smooth and uninterrupted, unlike the famous dual clutch transmission.

    3. The superimposing gearbox is as close as you can get. It is a gearbox with in-line input and output shafts and a rotatable gear case. If the input speed is fixed, changing the speed of rotation of the gear case varies the output speed. I could say a lot more.

    4. You can make a CVT using hydraulics without any flexible parts – in a single housing, build two variable displacement gear pumps hooked into each other. When you move a lever or whatever to increase the displacement of one while decreasing the displacement of the other, the “gear” ratio changes. That’s got nothing a regular transmission doesn’t have – no hoses, just steel and oil pretty much. Add a planetary gearset to make the setup hydromechanical where the power that bypasses the hydraulics by going through the regular gears improves the total efficiency. You can probably plan things out so that e.g. highway cruising is primarily mechanical with very little hydraulic contribution.

    1. Summary to save time otherwise spent watching a video: two variable-displacement pumps and a planetary differential are combined so that the engine’s input is split between a direct path and an adjustable-ratio second path through the hydraulic system, which also allows for reversing. The hydraulic path is less efficient, but it can be made to incur no losses at higher speeds by zeroing out the motor so that the pump locks in place and the engine just drives the geartrain completely normally. The result is that the engine can run at whatever rpm is optimal, and at higher speeds along the ground it can efficiently transfer its power through a simple set of gears, but you have fully controllable speed including variable reverse speed with no gear changes like any other hydrostatic transmission.

      The mechanics look like it could also be used to provide an overdrive taller than whatever main mechanical gear ratio was chosen, and the efficiency would taper down the further away in either direction from that ratio you chose to operate. It’s very elegant, but if you want to complicate it you could think about hydraulic accumulators for regeneration, or extra gears to help cover more speed ranges efficiently while remaining continuously variable.

  3. Reminds me of a film I first watched in High School during Film Appreciation week called ‘World Safari’ where two Australians drove a ‘car with a rubber band’ across Africa.
    The vehicle was a little European-made DAF and it had a CVT. Although the car had numerous breakdowns I don’t recall if any were the cause of the CVT itself, but it’s been a long time since I saw the movie last.
    It was quite a journey when they did this back in the 1970s:

  4. Everyone argues about torque vs. horsepower, but acceleration – especially for a car with a CVT where you can mostly tune out transient response – is mostly a function of torque at the wheels minus horsepower required to overcome aerodynamic drag. The acceleration equals out on properly geared torquey and high-revving cars with the same peak horsepower after you take the torque multiplication of the transmission gearing and final drive into account.

    The numbers are pretty easy to calculate: power to overcome drag is a function of velocity cubed times frontal area times coefficient of drag (both of which you can usually find online).

    1. Horsepower = Torque * RPM * (unitsconversionfactor)

      Which is why the Torque/HP curves always cross at the same RPM (one RPM for freedom units and one for SI).

      The main argument Torque vs. HP is guts vs revs. Big V8s put out massive torque at low RPMs, windup motors have no guts (power below 3K), but make massive HP when spinning.

      1. If you have enough speeds in your transmission, then you can keep a small engine in its power band. And if you intentionally try to, you can probably keep a turbo spooled fairly well too. You just aren’t going to do that while trying to get good fuel economy on your commute – you generally don’t want to use the engine like that all the time. And as long as you are engine limited (not enough power to break your tires loose) in first, then you may have somewhat worse than the maximum acceleration from a stop until whatever speed your power band starts.

        The real world acceleration advantage of guts now that we have 8+ speed transmissions is that you can run around at lower rpm and still quickly provide more power when needed. Especially in the naturally aspirated engines that have cylinder deactivation, because you can open the throttle and double the number of active cylinders in an instant, while building boost takes time and downshifting time is mostly independent of the engine – or maybe longer for the smaller one if the transmission is a type which takes longer to skip gears. That’s a nice property for hills and passing on highways and such.

  5. Given this is HackaDay I suspect this will be old news to most.

    Never buy a car with a Jatco CVT!
    That’s all the Japanese brands except Toyota and Honda. Likely some Frog cars as Peugeot owns Nissan, which owns Jatco.
    The last there is likely unneeded, we’re engineers and techs here. Nobody cluefull and sane would buy a frog car, under any circumstances. Not even if you lived there.

    Jatco CVTs are just plain awful. The first will be replaced under warranty, but they second will be on the owner. Jatco slush boxes aren’t great either.

    Don’t do it, run away! There is only pain down that road.

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