Possible 6 speed for the Empulse!

[FreepZ]

I have to admit, when I first heard about a gearbox on an electric bike, I thought that it would be a gimmick with little real benefit.

I'm sure that everybody knows that electric motors have the highest torque at 0 rpm (compared to an ICE, which has highest torque at highest rpm), so what's the use of a gearbox?

Then I saw this website, which does a pretty decent job of explaining electric motors. Here's an interesting graph from that site:

I realized that while the torque is highest at 0 rpm, the efficiency (i.e. the power you get out, compared to the power you put in) is highest at 75%-90% (depending on the motor) of peak rpm. The more efficient the motor can be, the more range you will get for the same battery power.

I'm not sure how the efficiency vs rpm curve for an ICE looks like, but I don't think that it's the same.

I wonder if an electric bike with a gearbox would need to have some sort of efficiency indicator, to let the rider know when they are running at peak efficiency. I guess that this could be a simple rpm counter, but with the optimal efficiency zone marked out.

[Brammofan]

Excellent find, Freepz!.  The idea of an "efficiency indicator" is a good one, too.

[Richard230]

I have been wondering how the rider is going to know when to shift (assuming that the shifting is not done by an on-board computer). With my IC motorcycles, I shift by either listening to the engine sound and/or by feeling the engine vibration. While I do have a tachometer, I prefer to keep my eye on the road and not on the meter. But with an electric motor, both the sound and vibration would appear to give little indication when you would need to shift gears. Personally, if a 6-speed transmission is integrated into the power system, I would prefer that the motor controller perform the shifting for me.

[Liv2xplore]

I'd rather do the shifting myself, but an efficiency indicator is a great idea. Most people are accustomed to the powercurve of an I.C.E. along with sound to know when to shift. An electric does taper of in power at the top end and at this time you're in reduced efficiency. Keeping the motor at the optimum 4800 rpm (or whatever it turns out is best) would probably take at least an indicator to gear up.

To me, the act of shifting is part of the experiance on the bike, but that isn't saying that you have to do it that way. Car's converted with manual transmissions often only use 2nd and 3rd gear for most of the driving, including 3rd gear off the line. That's the advantage of the 100% torque at 0 rpm.

[oobflyer]

This cannot be a coincidence:

http://www.litogreenmotion.com/

[Brammofan]

This cannot be a coincidence:

http://www.litogreenmotion.com/

We've discussed this bike before on this forum.  I don't think it has anything in common with the Empulse.

[oobflyer]

To clarify:

Before Lito Green Motion unveiled their Sora bike, which has a 12kWh battery AND a CVT transmission, Brammo made no mention of using transmissions in their drive train.

But apparently the range, using the same battery back, can be nearly doubled if the power is controlled through a transmission. This seems like a logical evolution of the electric drive train, just as it happened with internal combustion drive trains.

I meant no disrespect to Brammo, quite the opposite. If this is the direction that the technology is headed Brammo is smart to delay production of the Empulse to incorporate a transmission into their drive train. This will allow them to not only compete, but to stay ahead of the game.

The Sora is not going to be an option for most people anyway - as it's priced out of the range of those in the middle-class.

Imagine a 12kWh Brammo Empulse with a 6-speed (or CVT) transmission - now imagine me riding one around next year.
 

[HighlanderMWC]

But apparently the range, using the same battery back, can be nearly doubled if the power is controlled through a transmission.

I don't see how this is possible. A transmission isn't going to change the amount of energy required to go a certain speed (assuming the motor is geared to go that speed in its efficiency band). A transmission should just affect the speed off the line (or acceleration near top end if geared for fast off the line).

[oobflyer]

I don't pretend to understand the physics of it - but you probably hit the nail on the head with

assuming the motor is geared to go that speed in its efficiency band

The graph on this thread may have the answer - the efficiency appears to peak at about 2500 RPM - why not keep the motor spinning at this rate and maintain speed via a transmission?

[Richard230]

My guess is that a transmission would reduce the torque required by the motor to drive the motorcycle up hills and away from a stop, thereby reducing power consumption and extending the range of the vehicle, as well as requiring a lighter and less powerful motor to propel the vehicle.

However, my feeling is that if 6-speeds is good, how about using a CTV transmission? That way you could keep the rpms at a very narrow range and shifting would depend upon load. Many IC motor scooters use this type of transmission and they appear to be relatively cheap to make and design and also seem to work fairly well, although perhaps the less sophisticated of the type are not as efficient at transmitting power as a mechanical gearbox.

[HighlanderMWC]

I don't pretend to understand the physics of it - but you probably hit the nail on the head with

assuming the motor is geared to go that speed in its efficiency band

The graph on this thread may have the answer - the efficiency appears to peak at about 2500 RPM - why not keep the motor spinning at this rate and maintain speed via a transmission?

I just don't see how it would be possible to double range unless the motor was chosen specifically to generate this result and the range described as being almost purely accelerations. The Enertia is near its highest efficiency at about 35mph. When I commute I spend a fair bit of time at this speed and I don't see how a different motor + 6 spd transmission would have any chance of increasing that range significantly, never mind doubling it. What kind of Wh/mile does that work out to? I bet that would work out to be less Wh/mile than an e-bike can get (with full aero canopy).

If the motor has such a narrow band that it makes that much difference it would be a better candidate for a CVT than a geared transmission.

Again, performance I can buy...range extension not so much.

[oobflyer]

This may have been posted elsewhere already - from Brammo, May 2:

 We had a tough decision to make recently: Deliver the Empulse this riding season, or integrate our latest technology and deliver a superior bike in 2012. After much deliberation, we decided to be true to our values and build the best bike we can. We still have a good amount of work and testing to do in order to bring you the world's greatest electric motorcycle. We believe it is going to be worth the wait.

[emphasis mine]
It seems clear that the Empulse will have a transmission... we just have to wait another year   

[Shinchan]

Hey Brammo.  Yeah, you over there with visions of exciting new tech making your eyes glaze over...please please PLEASE release a version of the Empulse without a multi-speed transmission.  One of the things that really excites some of us about electric bikes is the simplicity and lower cost-of-ownership.  Transmissions fail.  I'm not saying that other components don't fail, but a multi-speed transmission is just another thing to worry about.

I'm perfectly happy with a 60-100 mile range and 100mph top speed...

[Gavin]

Good point...You also don't want a ton of different models, but really having a single speed bike is a selling point for some. Gets Brammo some new riders (which I thought was one of their priorities) ...gets more scooter riders to move up to a motorcycle maybe, adds more female riders, more older riders...Americans tend not to like shifting in cars, some won't like shifting on bikes.

Me? I could go either way....my car is a stick, one scooter is a shifty, one a cvt...so i'm fine either way...I will say that my scooter shifty is different than a motorcycle shifty (all in the left hand) so I have actually never ridden where I shift with my foot...I'm sure I could learn pretty fast, though I've always loved the simplicity of the Vespa shifty...pull clutch and twist hand.



But having options is good.

That said, Brammo will have to weigh the benefits and downsides and come up to their own conclusion. More options could be good, but too many models could be bad??

I imagine the Enertia and Plus will always have a single speed option, maybe a clutch, maybe not. The Engage and Encite might only be shifty...the Empulse probably should have both??? who knows...but simple is often best, less parts to break is often best...though the gear heads won't mind a tranny that breaks now and then...gives them a reason to get the hands greasy.

Gavin


ps...and I still think it behooves Brammo to get bikes out now/soon. I know they plan to make some big changes and they don't want to get this years riders mad...but they could sell some now and still make big changes....just spell it out to deposit holders. "Hey Brammo fans (no, not Brammofan), we are making some big changes...we think for the better. So we plan to delay the Empulse to 2012. Next years bike will have a 6 speed transmission and have more speed and longer range. We think it is an amazing bike you will love. That said, if you don't want a 6 speed tranny and prefer a single speed, non shifty sports bike, well have we got the bike for you. We will sell the Empulse 2011 this year as a single speed bike. You get to choose. A great bike now that if fast and fun with one gear, or a great bike in a year that shifts."

[theentropykid]

I don't know much about transmissions on electric motors (haven't looked into the Tesla specs) but I kinda agree with Shinchan. Mo' mechanics = mo' problems. I think I'd rather have the simplicity of the current model with 100 mph/ 100 mile distance than squeeze out another 20 miles and a lot more moving parts. But hey, in Brammo we trust!