I have no personal direct experience with the Vivax mid-drive although I do admit it does look like a very cool set-up so what I have to say on the subject of mid-drive vs. hub-drive is from experience with simpler cheaper systems that usually attach by adding an extra chain loop to the bike by either mounting the motor between the pedal crank and rear wheel of the bike (usually on a long-tail bike) and running one chain loop back from the pedal crank to the motor and then another chain loop from the motor back to rear wheel with a double-stage freewheeling common shaft assembly on the motor such that either or both the pedals and motor can drive the rear chain loop back to the motor, or by using a freewheeling crank with a second chain loop running from the motor to the freewheeling crank.

Anyway, long story short using such cheaper not as fancy mid-drive set-ups and having experience with several different type of hub-motors as well I can say that for a lower powered motor assembly (at or below 500w) I most certainly prefer to use a mid-drive set-up and that for higher power set-ups (500w or above, especially anything 1,000w or above) I prefer a hub motor set-up at least for a normal bike. Specialized heavy cargo bikes are a different matter since if your loaded down with cargo you might need to gear down even a 1,000+ watt motor to climb the hill hauling or pulling heavy cargo weight. But if you leave cargo bikes out of the equation and stick just to regular general purpose and commuting bikes those are my preferences.

I will note that not all mid-drives are created equal, a good mid-drive should provide strong smooth power with very little losses in the motors power-train that connects it to the bikes primary drive chain and no noticeable additional drag when proceeding under pedal power alone. Not every product out there, at least the ones I've worked with provides that. Additionally the mid-drive system to be an effective human-electric hybrid MUST have the motors output RPM closely geared and matched to the human pedal cadence rhythm. You don't want the motor on a mid-drive to run the chain faster then you can comfortably pedal to keep up nor do you want it to run it too slow and slow down your cadence so you can't spool up to full pedal cadence in the human power zone sweet spot.

For the cheaper simpler mid-drive set-ups that connect to the bikes drive system through the chain drive (usually with a second chain loop) a single tooth count difference on the sprocket ratios between the pedal crank and the motor can make all the difference in the word. Most electric motors have two sweet spots in their power curve. One sweet spot will be the RPM at which that electric motor produces its greatest power output, at faster or slower RPM the total power output of the motor will be lower with the same throttle setting. The other sweet spot is the point of maximum efficiency. Almost with all electric motors the maximum efficiency sweet spot is a little slower RPM the the motors free spinning no-load RPM and the maximum power output sweet spot will be at a little slower RPM then that. The trick with a lower power level mid-drive e-bike is to set up the gearing ratio between the human pedal crank and the electric motor such that at full throttle (with a low power electric mid-drive most of the time you run the bike with the throttle at full) when you are "spooled up" on your pedal cadance and pumping hard, fast, and smooth on the pedals (usually between 85 and 100 RPM cadence for most people, but depends on the person) the gearing between the pedals and the motor are such that the electric motor will be running in its maximum efficiency sweet spot. With the gearing set-up that way when you are accelerating and you shift up to the next gear your pedal cadence slows slightly with the up-shift and this pushes the electric motors RPMs down from the efficiency sweet spot into the maximum power output sweet spot and you get a burst of additional power out of the motor to help you "spool up" to speed in that gear and accelerate without having to do anything with the motor throttle except for just keep it pinned on full. Then as you "spool up" in that gear and increase your pedal cadence back up into your human sweet spot the electric motors RPM also rise as well and transition from the motors maximum power sweet spot into the motors maximum efficiency sweet spot and thus as you spool up in each gear you take some of the load off the motor and take more of the load yourself while bringing the motor it into its maximum efficiency sweet spot. With the ratio's set-up just perfect like that you and the motor work together like a dance and you have a true human-electric hybrid vehicle with a smooth feel and experience and that encourages you to continue to do your part on the pedals and run strong and hard in the highest gear that the combined power of you and the motor can push into for the terrain and conditions.


With all that in mind my personal primary question I would have about the Vivax and similar integrated mid-drives where I can't adjust the ratios by swapping out sprockets would be if those who designed it took what I'm talking about into consideration and tuned its gearing ratio between the motor and the crank for the sweet spots of the motor and the human power element to line up with each other like that to provide that kind of feel to the ride.