Transmissions: To Be, or Not To Be?

Crimping connections vs. soldering, the benefits of regenerative braking, and the use of transmissions seem to be the Holy Trinity of crap-that-gets-discussed/argued/wars-fought-over until Hell Freezes Over on just about any group you care to look at.  We just got through a doozey on ElMoto, but after all was said and done (well, it still continues, to tell the truth…) I’ve come to some pretty basic conclusions.

To wit.

My personal conclusions are that a transmission is basically for distributing a narrow powerband. The more the powerband broadens, the less a transmission is necessary. When the powerband is in the realm of an electric motor, a transmission becomes almost academic- that is, there may be an advantage, but it’s very small at best. Some motors will benefit more, because of the nature of their powerband, and some, not at all- and it’s really not a point of discussion, it’s demonstrable by mathematical models testing, as we’ve seen (see the thread for those numbers).

Add to that the mechanical problems of designing, building and installing a transmission, and the slight benefits get even slighter. What we saw here is that in actual use, it was heavy and inefficient. In theory, at best, it adds weight, takes up space, and saps energy.

Not the route I choose to go, but god love ya if you can make it work (better).

There seems to be a lingering belief that you can make the best out of a weak system with a transmission.  My vote is to apply the fix directly to the problem.  For example- My bike is light- 250lbs- and is running a healthy controller and motor- Mars ME0709/Alltrax AXE 7245. I need more power from my batteries to go faster, quicker. My top speed is also slow, at 65mph. I can gear it up, to get 85mph out of it, but again, because my discharge is limited to 2C, the motor will not get enough to pull well in the lower end… it may not get enough to even pull at the high end.

In my specific case, the simple way to get my bike to perform better is to upgrade the batteries. Not put in a transmission.

The best example of how a transmission benefits you  is a bicycle. The source of power- you- on a bicycle, has a very narrow RPM range at peak power (known as your “cadence” to bike racers). Thus, the gears. They keep you running at your optimum cadence. The flatter your power curve, the less you need the transmission, and the more your parasitic power loss cuts into the benefits of it. My bicycle had eleventy million “speeds”. I ride in the city, on flat roads, so I removed 2/3 of my gearing because it was not being used. (And I’m a simple **** and they confuse me.)

Here’s your powerband, on a bike, at the rear wheel with a single-speed.

With a three-speed transmission, you’re able to spread that narrow power peak over a much broader RPM range:

Great.  Now I’ve probably incited all the fixed-gear bike guys, too.

So the flatter the power curve (it’s about HP, not torque) the more critically you have to look at the tradeoffs of running a transmission. It’s not about getting more power out of the system, it’s about getting the power you want out of where you want it.  As the differences in result get smaller, the mathematical models, first, and real world testing, second, will become more important. (It’s easy to tell the difference between a 250 and a 600 with the seat of your pants, but not so much the difference between, say, a bored and ported 600 than a stock bike, without running it and timing it- or using a dyno, right?)

The bottom line comes straight out of Physics class.  Anything in the system that doesn’t contribute more energy, by either supplying it, like batteries, or making the system more efficient, is going to reduce the output of the system. That will, ultimately, be demonstrated by reducing the range, or, all else equal, reducing the top speed.

Now. If a transmission lets a drive system work more efficiently, then it will save energy and increase range, regardless of the comparative acceleration (that is, compare two systems accelerating equally). …and heat is how the system loses energy. If the motor or batteries are heating up, the system is losing energy. If the tranny keeps them cool, under the same load, then it’s paying the rent. Whether it’s paying for it’s entire overhead is the question, I guess.

That is, is the energy saved (from loss as heat from overloading) equal or greater than the energy it takes to carry it.

ummm…

However, I can’t help but keep hearing Zoe Rem’s interview in my head as I’m reading all this:
“However, I think the e-bikes feel most like a very low powered two stroke, especially with the lack of engine braking. The power band is really torquey and either it’s all right there, or you’ve got nothing, like a 125, when you’re not on the pipe, there’s nothing. And while there’s nothing you can do to the e-bikes like you can by keeping a 125 high in the revs, it’s still a similar kind of feeling to me of having all or nothing power.

Clearly, it sounds like the ability to downshift and spin it up may address this- although it was the first time I’d heard someone say it, and it frankly didn’t make a whole lot of sense to me. (I wondered if the controller was de-tuned somehow to keep the ICE racers from tapping the cells too hard.)

Now.  Let’s say you want to tackle the idea, you think  it’s worth it, and you are starting to try to figure out how to build a strong enough, reliable enough, and small and light enough transmission to make your mark in EV motorcycle-building history, and silence all the critics.  I give you: the D-Drive!

(Vintage Vincent transmission graphic via TheVincent.com)


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