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[zap mc]

As we can see the range wars are hotting up, but can anyone with  a more fundamental grasp of the theoretical physics involve say whether a geared bike is more likely to get a better range than a non geared bike?

My initial reaction is that by using the gears you will use lower revs and hence less power and consequently go further, but is this true? and if it is will all manufacturers eventually be forced to follow brammos lead?

[gasdive]

I can't see how it would help, but I'm willing to be educated.
=:)

[Marshm]

It is an intriguing topic. I will throw out some thoughts and maybe we can arrive at some logical conclusions with other peoples help.  I hope you can follow my reasoning, for that matter I hope I can follow my reasoning. 

Power equals torque times rpm.  The power used must be supplied by the batteries.  So I ask myself, as the bike accelerates and picks up more rpm, what does the torque do?  If the torque stayed the same and rpm went up, ah, more power drain on the batteries.  So if I had some gears, I could shift, and then drop the rpm back down.  Hey that sounds great, less rpm, less power drain.  However, can I stay at constant torque? Do I need more torque to go faster?  It must take more torque to go faster, because cars and bikes can only pull so tall of gearing.  I think in that ideal frictionless world going faster would not take more torque.  What is it about going faster that requires more torque.  Whatever resists the forward motion I guess.  Wind resistance, must be more friction losses in bearings, drive chains and everything else that spins when they spin faster.  Could I be missing some other reason besides the friction, hmm.  So now, comparing both bikes going 30mph, would that not take the same amount of torque to push all that wind and friction? I would say same torque.  So do I conlcude that if i can go 30 mph at less rpm, my bike would be using less power than the bike at higher rpm?  Is it that simple?  Did the first part of my paragraph have no bearing on this conclusion, haha. 

Ah but wait, extra spinning things comprise a transmission, thus more frictional losses, thus just having a transmission will drain more power all the time, drat.  How can I take transmission losses into account?  If I assume 10% loss, then that gives the single gear bike an advantage of 10%.  The gear rations on my quad are from 1.2 to 1.4 change between gears, thus 20-40% faster each gear. So the transmission loses 10% but the shift gains me 30%, so I come out ahead by 20%. So even with transmission losses I still come out ahead? Based on those numbers anyway, which I think are somewhat reasonable.

Based on ride reviews that say freeway driving and aggressive riding have lower range, then I conclude those situations must be using more power, which is some combination of torque and rpm.  Agressive riding would be using more torque, trying to accelerate fast.  So that means non agressive riding can actually use less torque.  That is interesting.  So the electric engine can put out high torque at low rpm, but that doesn't mean it always does.  So in casual riding, low throttle setting, I guess the controller limits the voltage supplied to the motor?  Thus the motor puts out less torque, thus less power at any given rpm, thus consumes less battery.  The freeway riding is high rpm and maybe high torque also to push all that wind and friction.  That type of riding must consume a lot of power.

I must be missing something important, but it sort of looks like the transmission will help give more range. 

I am tired now.  You can't have too tall of a gear and keep the rpm super low, it seems like the taller gear would take more torque to spin. Maybe I can ponder that later.

[gasdive]

So now, comparing both bikes going 30mph, would that not take the same amount of torque to push all that wind and friction? I would say same torque. 

Same torque at the rear wheel, but the slower motor must be geared differently.  (otherwise it would be going at the same speed).  So the slower motor must produce more torque.  The increase in torque needed is proportional to the reduction in rpm so as the power is torque times rpm, it comes out the same.

=:)

[Marshm]

Ah I think I see your point.  In that case, the transmission losses are always there just hurting efficiency.  If we ever get to see the Brammo 6 speed, it will be interesting to find out how real world tests turn out.  Although you would have to compare the same bike with and without the transmission, which probably will not happen.  Even in light of this theory, it still seems like the transmission would help. 

The electric motor doesn't put out as much torque at high rpm.  So maybe the high speed riding would be more efficient with a higher gear?  Maybe not more efficient, given the torque rpm trade off, maybe what happens is that the bike with gears is just capable of going faster.  Hey, in this comparison the geared bike is going faster, so if it uses the same power and is going faster, then it will get more range.  Anyway, I think a real world test would be good in this case.  Although theory is good excersize for that gray matter i don't use much. 

[Marshm]

So far it looks like we were just thinking about power in verses power out.  That is not the whole story though.  With any motor you have to put in more power than you get out.  So the efficiency of the motor itself comes into play.  http://www.gizmag.com/antonov-3-speed-transmission-ev/19088/   Gizmag has an article of the Antonov 3 speed and they claim the transmission does help by 15%.  It is interesting to put a theory to how something works, but in this case it might be more beneficial to see some test results.  Even the experts in the industry cannot seem to agree on the topic.  Also, the rider would need to know where the motor is most efficient so they can drive it there, otherwise they might use a different gear and not gain any benefit. 

[Richard230]

Here is my comment: The motorcycle magazine editors consistently tell us that motorcycle drive-line losses amount to between 10 and 15% on an IC motorcycle. I imagine the majority of that loss is from the transmission gears meshing and spinning in their oil. My guess is that an efficiency loss of as much as 10%, assuming a typical motorcycle transmission with splash lubrication from 80wt oil, sounds about right to me.  At the current technology of electric motorcycles, that is significant.  However, a transmission would certainly improve take-off performance from a dead stop and overall acceleration.  I am not convinced that having a transmission will improve range, though.

[rider728]

In theory I have to agree with the last comment.  Transmissions are essentially converters for balancing the torque required to meet throttle controller demands and the rpms required to maintain peak efficiency in motor operation.  Thus, as in transmission uses on conventional vehicles, the varying gear ratios provide desired acceleration.  Conventional transmissions use the top gear as a 1:1 ratio.  So 1 motorshaft revolution equals one final driveshaft revolution.  Single speed ebikes are essentially always in this 'top gear' 1:1 state (excepting for setups using a gear reduction or increase between motorshaft and final driveshaft, but since this is a static ratio scenario it can be considered in the same 'top gear' sense).  Top gear 'overdrive' transmissions such as those in late model twincam harleys go beyond this 1:1 ratio (1 motor rev equals greater than one final drive rev) to provide lower motor rpms at a given steady cruising speed where not much rapid accelerative torque demand is expected.  Shifting gears in a transmission is for staying in the desired rpm range for optimal motor operation...since there's no stalling an emoto at low rpm, and no need to rev up to find the best torque band in the rpm range, I am very interested to see what ratios could be exploited to benefit the power output characteristics of emotors.  Current energy storage chemistry being what it is, increased range would be the obvious focus to work towards...anybody got some real world experimentation they need a test rider for?

Remember also that transmissions bring not only inherent output efficiency losses, but added weight, added maintenance, added mechanical complexity (aka moving parts that will inevitably fail over time), and added complexity of use for the inexperienced rider.  These are all parts of the vehicle design equation that must not be overlooked if the ebike is to maintain its inherent distinctions from and advantages over the conventional hydrocarbon models.

[Richard230]

This is an interesting subject and one that has been beaten to death on El Moto - without any conclusive results. Until Brammo finally comes out with the Empulse and we all get to see what happens when you install a 6-speed transmission on a production electric motorcycle and how that affects its price, range, weight, complexity and overall performance, we are all just guessing. There is nothing like seeing something work in the consumer market to validate (or not) a new (to EVs) concept like this. 

When even the major auto manufacturers are not installing transmissions on their all-electric cars, you have to ask yourself whats up?

[frodus]

The discussion about efficiency always ends the same. It’s unlikely that it would be much more efficient, if at all. With losses in the transmission, It's unlikely that the change in efficiency due to gearing a motor down so it's in a more efficient part of the curve is more than the transmission losses.

Let’s take this motor


Now look at the RPM range at 72V for different loads (torque along the bottom). The range of RPM for 10-70ftlbs of torque is about 2000RPM to about 4500rpm. The efficiency of the motor goes from 80% to about 89% and back down to about 85%. I doubt they're designing the transmission around an existing, as the ratios will be different. Let’s assume that we'd want to go in maybe 2000rpm increments. So 1st to 2nd, we'd shift down 2000RPM from 4500RPM, and you're around 2500RPM. Efficiency at 4500RPM is 80%, and efficiency at 2500RPM is close to 89%. So that's a gain of 9% efficiency, right?

But what is the efficiency of motor ---> primary ----> gear x ---> transmission output shaft. We assume the same efficiency for final drive.  Let’s say there’s 3 transfers of power within the transmission, with 3-4% loss each, that’s 9-12%. So either it’s got to be a super-efficient transmission, or the gearing has to be such that the efficiency gain is higher than losses.

I think the REAL gain in all this, is you get better performance out of a motor. Having different gear ratios helps a fairly low HP motor use it’s torque better than a single speed, thus improving performance up to the limit of the HP of the motor.

[rider728]

Great illustration and analysis Frodus!  Thanks for clarifying my clumsy and unscientific allegations.

Cheers brother!

[flar]

Here is the issue I have with that analysis.

If the question you are wondering is "if I can switch gears right now in this particular steady state to another gear that puts me in a more efficient part of the rev band, then is the cost of a transmission worth the gains from this one isolated single decision?"  And, you've shown that it is a wash.  OK, I didn't verify all the math, but I can believe that.

But, that's not the question at all.  The question is "if you have an electric motor that is small and light enough to fit on a motorcycle along with its battery pack and you can choose one single gear for it, what gear ratio do you pick and how does that affect a) the efficiency and power on the highway and b) the off-the-line grunt you will see from a stop light?"  And I think the answer to that is "you will not do well at both, and in fact if you want to do well at either then you will do very poorly at the other".  From my test ride on a Zero and reading the Zero forums on real world range I get the impression that their choice was to go with a gearing that gives safety margin power on the highway and decent (for a very green vehicle) standing start power, but nothing stellar in either category and it is very inefficient range-wise on the highway.  Their compromise does work and I could live with that for the green feeling and novel torque response that it provides (even if, on an absolute scale, it isn't breathtaking in performance), but in the end it is a compromise, not the be all and end all of solutions.

But, I think Brammo wanted more than that experience.  They seem to be looking for excellent off the line acceleration as well as decent highway legs.  You simply will not achieve that with a single gear.  Tesla can achieve that in their cars because they have different economies of scale - size and weight wise.  But the size constraints of a motorcycle just don't appear to let us get there.  They could produce a fun, liveable compromise vehicle, but they would rather produce a more complicated vehicle that is exciting and green.

In that analysis, all you've shown is that they can probably gain back the efficiency that they lost by deciding to put a gearbox on in the first place.  But your analysis isn't the goal they were aiming for when they did it, it's the reason they break even on those non-goals while they do it in pursuit of other "excitement" goals.  They can add the excitement of having better gearing for all (or at least more) conditions while not giving up much efficiency in the wash...

[Richard230]

However, from a business standpoint, you could argue that while you are attempting to design and develop the very best production electric motorcycle and the first one with a functioning gearbox, you are loosing sales and market penetration.  Right now, considering the cost and small market for electric motorcycles (at least in the U.S.), you would think that the smart thing to do would be to come out with a single-speed Empulse, sell as many as you can, then follow up in a couple of years with the 6-speed model and make a big splash. Then you could sell that advanced bike to the guys that bought the original bike, thereby doubling your sales. That is what I would have done. Meanwhile, Zero marches on with an electric motorcycle that works as a commuter and green transportation (that runs without needing high-priced gasoline or a lot of expensive maintenance). It may not be the best motorcycle that they could have made, but it is here now and selling.  

[flar]

Hi Richard, I'm not sure I understand your comment.  Isn't the Enertia single speed?  Was that a typo and you meant to say that they should come out with a single speed Empulse for starters and then later follow it up with a 6-speed Empulse+?  I would buy that if they didn't already have the Enertia selling to the single speed commuter/city crowd.  While the Empulse has styling that may appeal to a new crowd than the Enertia (with Zero S somewhere between Enertia and Empulse styling), if it provides the same riding experience as existing electric motorcycles then it will likely leech more sales from existing models than it will establish a new niche.  And, it may not sell well at all since those who might be attracted to a naked sport bike look may not be willing to put up with a less than stellar power curve.  For the near term they are providing the Enertia and Enertia+ which probably appeal to 80-90% of the crowd that would be looking at electric motorcycles in today's market and Empulse seems to be the first of a new generation.  Or, is there some angle I'm missing.

Also, I wasn't suggesting that their goal was the best goal, just that the analysis that tried to show that it was not a good idea was missing the point of why they are doing it.  The world may not be ready for a multi-gear electric motorcycle, but the reason to try it isn't to improve efficiency in isolated running states over a single gear design - so the analysis of that factor is not relevant to its raison d'être...

[flar]

And stepping back a bit, I notice that the title of the thread is, indeed, about efficiency.

But, my comments are still relevant since the gearing of a single speed bike is not chosen arbitrarily.  I am suggesting that all of the existing crop of electric motorcycles have a compromised highway range due to this consideration and the choice on the highway isn't "can we get from 80% to 89%", but rather they are likely much further outside their efficiency range and it is more like "can we get from 40% to 80+%".

The analysis to perform on that chart is to say something along the lines of:

- The break even, knee of the curve, point for efficiency at speed seems to be the 10lb/ft at 4500 RPM point.  If we drive the engine much faster then the efficiency takes a huge dive.

- If we gear so that 4500RPM gives us 55-65MPH, then what efficiency do we then have at 75-80 and what kind of power do we get off the line and in city traffic?

- If we gear for good power for excellent standing starts, what RPM do we have at 55-65-80MPH and what efficiency do we have at that RPM?

If you can find a single gearing answer that gives good results for both of those questions, then that is the analysis that would show that a gearbox is not worth it.

And, if you throw 100+MPH in to the equation (which I think they believe they must embrace to make this particular model appeal to its target audience) then it is even more likely that that speed will be unachievable with the torque/RPM that you end up at with a single gear choice.