spinningmagnets said:
Since motors have a very broad and fairly flat torque curve (not like ICE), I would say that it depends on the application. If I was converting a motorcycle to electric, I would leave out any transmission if I was staying off the highways 45-MPH top speed). But...If I spent most of my time on city street stop-and-go, but on rare occasions I needed 70-MPH on the highway...I would want a 2-speed (possibly even a 3-speed, with first gear for steep uphills with a passenger only).
If geared for 70-MPH top speed, and I spend most of my time at 0-to-35 MPH on city streets, I would constantly be at half throttle, also using more amps than necessary compared to a 2-speed when accelerating. Battery volume is restricted on a motorcycle frame, and I must strike a balance between volts and amps, keeping in mind the C-rate of the chosen chemistry.
The Tesla originally was designed to have a 2-speed (city/highway), but it was causing delays right at the moment when they needed to show investors that they were able to actually produce...
FrankG converted a dirt bike using the stock transmission, but he said he only used 2 of the gears (6-speed?), and also he found the frequent shifting to be annoying (to get at the 2 best gears and back).
This is filled with conceptual fallacy and conclusions drawn from experiences with poorly suited drivetrains for the applications.
This isn't your fault, most EV conversions and things we build here involve taking most anything that can spin when it receives electricity and apply it towards moving a bicycle or something similar. This ranges from DC forklift pump motors to RC helicopter motors to industrial induction motors, even really pathetic single-phase motors like a table saw motor being driven off a 12vdc to 110vac 3000w inverter has been seen here.
Virtually nobody on here has actually gone through the exercise of making the ideal bicycle application motor and then also built it. This is because it's really expensive, either in time or skills or resources to do it (lebowski recently showed you CAN make your own no-compromise large diameter low RPM coreless motor on the cheap if you are highly stocked with enough time and skills alone).
I was just looking over some design stuff from the EV1. They went through every last detail to make this car as light and aero as it possibly could be, including taking it to boarderline crazy extents like electrically actuated aluminum brakes and huge styling compromises and interior space compromises and angling the passengers back more to lower the roof to make the body as slippery it possibly could be.
The development program had access to not only just every best of the best transmission in the world, they had access to all the racing development programs transmissions with special friction lowering coatings used in exotic race trannys and dry-sumped oiling trannies to minimize windage loss from the gears running through oil drag and bearing drag etc.
They were hunting the pinnacle of economy because they were starting out with some shitty lead acid cells they had to haul around that weighed more than the rest of the car, and so any reduction in this amount of cells to drag around obviously had huge benefits both in cost, as well as performance benefits and range benefits etc.
What did they conclude with for a drivetrain after all this pretty impressively un-restricted budget of development for making the uber-ev car (at a time when battery tech just sucked too much for EV cars to make sense really).
They went direct drive, no tranny, induction motor.
Same as other people who actually go through the exercise of seeing what's possible to do when you take the restrictions away of "I gotta make this forklift pump motor that I have laying around work!" or "These RC aircraft motors are soo cool and tiny! Let's put them on bicycles, they have the power to do it!" So many of the comments in this thread are just people re-stating, "look at my example of using the wrong motor for the job and how much better it would be with a tranny band-aid", which doesn't help people to see the ultimate solution would never involve adding more frictional losses and additional stages of complexity and things, it would involve directly generating the force you wish to have, over the speed range you desire it, and THIS is the pinnacle of EV design. This is the end-goal for EV's to arrive at. Fortunately, it's been nice to see that most programs who actually go through the exercise of seeing and testing what does work best in the end, and what enables them to get the most range per unit of battery cost (because batteries are often like 70% of the cost in an EV, so it's CRITICAL to use what you have as efficiently as possible), end up with a single motor fixed ratio drivetrain, because it is the ultimate solution over the useage range that includes everything to do with normal transportation or sporty commuting, and SPECIFICALLY a dedicated race vehicle, which it's always going to be the optimal solution for when you want to deliver lots of power for a long time, and have high average power levels while in use. When you grow your level of difference in power needs between max acceleration and cruse beyond a certain point, then PM motors core loss (and hence the $$$ ways to reduce it) start to be a bigger factor, and the move to a direct drive induction motor becomes the best choice.
If you actually go through the exercise of looking at what's possible vs real application situations, you will find direct drive with an electric motor IS the most efficient path, over a very wide range. A range so wide, I think a good example is the bulldozer that also wants to be a top speed vehicle, but remain functional as a bulldozer that can still push dirt proportionately hard to perform work in an undiminished performance as a bulldozer of that power level, yet also be a land-speed record vehicle. If it was merely a land-speed record vehicle, or merely a bulldozer, it would function best and most efficiently with no transmission, just building a direct drive motor for the application for either of those applications, BUT, for situations so far outside the normal operating range of things where you want to be a dirt pushing champ bulldozer AND a land speed record holder, then I agree, a tranny is going to make a lot more sense to add than trying to make some crazy motor to suit such a wide range of parameters with a single speed, and do it more efficiently than could be done without a transmission, because it's not that it couldn't be done with a single motor and direct drive, it definitely could, but it would involve a motor with such a large radius that the core losses and things while doing the land-speed record breaking would just get silly huge, and a transmission would make sense with respect to system efficiency in a big way for that extreme of an application.
I believe all the calculations for writing this up as a mathematical proof, as well as mathematically finding where that ratio limit (like the LSR/bulldozer combo example) are all capable of being drafted. Further, I believe this elaborate exercise including all the oil drag calcs on the tooth a gear and the oil sheer film friction and surface displacement over the moving tooth/tooth pressure zone, even the tiny bearing drag calcs that are load dependent by a fairly insignificant amount and the shafts flexing causing bearing misalignment and additional bearing heating and/or bearing case mounting failure from this flex (common in Honda gear-boxes, and often fixed by sacrificing a gear set in exchange for an extra mid-shaft mounted bearing support called a "hand-cuff"). All this stuff has been around long enough know. You can get good tranny calcs because it's leveraging many centuries of calculation and development, so that I've got entire books I haven't opened for years that just say something like, "practical formula related to mechanical gear effects", which will inevitably then load you with pages of the least practical calculations you've ever seen for making fairly accurate calculations on things like shifting the weight of oil in a gearbox, and how that's going to effect heat production or whatever. It's a well known, very painfully tediously documented subject.
Someone can write the proof set to show that over a range that extends well passed what a sporty-commuter, or a dedicated race vehicle operates in, that nothing involving adding a tranny in any situation is a higher efficiency alternative vs building a motor to directly generate the desired torque over the desired speed range you wish to have. This is why wind-turbines are direct drive. This is why the EV1 was direct drive, even in a car where they were willing to sacrifice everything for ultimate efficiency, even human safety things. This is why a solar car team racing in a situation where if you can trim just ounces off parts, you pay thousands of extra dollars to use that exotic material or whatever to do it, because you're willing to do whatever it takes to make the ultimate highest efficiency system, still uses a direct drive hubmotor at its highest level.
None of these examples really help it sink in though, the math example if somebody (maybe even myself if I ever find more than 20mins of freetime again) will do the math example of modeling an optimized motor over the whole range of a given application vs adding a tranny and it's losses to shift and trade some of your own losses etc (which I'm sure has been done by Tesla and GM and Nissan by now, and the conclusion is obvious, it's not that they didn't have access to transmissions...). Maybe somebody will be convinced by just the pure logic of realizing that if you're going to convert some electrical potential energy into making something move around, thermodynamically, it will be impossible to beat just directly using the energy to create the force you require vs creating something other than the force you require, and then converting it into other forms, and the inherent associated loss with ANY power transfer etc.
However, none of those above things taught it to me. What taught it to me, was having deathbike do violent snap-up and over power wheelies on command if you ask for them, and STILL being able to go to a higher top speed than I'm even willing to reach on a bicycle chassis, and get there while just modulating the throttle to try to keep the front end only a few inches off the ground, and try to keep some front tire speed up so when it touches down for braking at the corner, it doesn't upset the chassis so much from just riding the wheelie the whole way, and having like 5mph front tire speed when you touch the tire down at like 70mph and it has to buck and jerk as it gets the wheel up to speed before you can get traction to start braking. That's what finally opened my eyes and made me realize, ohh! Hey! Getting the right motor/controller combo for the application f*cking rocks in all ways!!! And it's more reliable and quieter from having less sh*t moving and less shit to go wrong.
