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[Richard230]

I keep getting my Empulses mixed up with my Enertias. I corrected the typo in my previous message.

I was not making a point about efficiency (for which I am really not qualified to say much), I was just grousing that Brammo should have come out with their single-speed Empulse and sold those first, before modifying it with a gearbox.  I waited for 18 months as an Empulse pre-order customer, before finally giving up and buying a Zero, but I would have been happy to own a direct-drive Empulse.

P.S. I owned a 1997 Funduro for 9 years and my "230" comes from my Chain Gang membership number.

[CliC]

Hmm. Six speeds? I'd find it hard to believe that many gears would be necessary on an EV, with electric motors' wide powerbands and high torque at low RPM. Didn't the original Tesla gearbox only have two speeds?

I suspect any efficiency gains made by gearing the electric motor (taking into account motor improvements and mechanical losses in the transmission) are not significant enough to outweigh the cost and complexity of a gearbox in most cases.

C

[flar]

I don't know much about how the IET differs from a standard gearbox, but if they add the complexity for any gear change, then adding a few more gears wouldn't really change the complexity much so 6 shouldn't be much worse than 2.  Also, given the price of these bikes, if they can at all use a stock shifter pedal and gear set then they save having to fabricate even more custom parts and 6 speed gearboxes are probably a dime a dozen.

Comments have been made that the bike will be able to run around in any one gear, just like the single gear bikes, so with 6 to choose from you could probably better pick a single gear that fits your immediately foreseeable (as in "this morning" or "the next 50 miles") circumstances, but with the option to always go into "fastest 0-60" or "highest highway mileage" modes at the flick of a foot.  In my S4, which had a 6-speed manual transmission and a twin turbo engine with the widest power-band I've ever seen in an ICE car, I often found that I could find a single gear that would keep me happy through miles and miles of twisties, but it wasn't always the same gear that worked depending on the nature of the road and my mood.  The car almost didn't need a transmission (I'd commonly go 1st, 2nd, 6th and would even use 6th gear (good to 150MPH) at city speeds), but it was nice having the fine tuning of 6 gears to choose.

With one gear on the Empulse you'd most likely end up with lackluster 0-60 and poor highway mileage.  With two you could have both of those fixed, but in the twisties you might find that either gear "works", but you might not find either gear optimal, like golfing with a driver and a putter.  With 6 it will likely be a dream to ride in all conditions with no constant shifting once you figure out the right gear for the situation.

[protomech]

The original single-speed Empulse has a maximum design speed of 100 mph, and pinkyracr claims she hit 91 mph on the 2012 S in a tuck downhill.

70 mph should be WELL below either motor's maximum rpm and efficiency should be pretty good - maybe a bit lower than at lower speeds, but probably not enough to cover the added losses from a gearbox.

I don't think efficiency drives the selection of a gearbox over a direct-drive transmission.

[flar]

@protomech

I'll admit up front that I don't have hard and fast facts to back up the following points, but the impressions are worth noting as they are not really addressed in the point you raised.

First, my impression of most ICE motorcycles is that the mileage differential from going 70MPH vs. 30MPH is not huge.  You might see 20-30% difference.  Am I off there?  With an electric bike, on the other hand, the figures being quoted show a differential of well over 200+%.  Either I'm way off on how much I expect the mileage of my ICE bike to vary by speed, or there are some serious efficiency issues with existing single speed electric motorcycles at highway speeds.

Second, it may be that existing electric bikes are within a reasonable efficiency envelope at highway speeds for these motors, but they don't really have exciting off the line performance.  If what you say is true then that means we can have the same kind of highway mileages we tend to be seeing in today's bikes, but we'll never get breathtaking 0-60 times.  If you want to include the wow factor on the low end performance, then the gearing will have to change and that new gearing will threaten the existing highway mileage.

[Hmm.  The following fails to take into account that when you switch gears, you multiply the power and torque by a new multiplier before it reaches the rear wheels - so, if you can halve the motor speed then you might be at a part of the curve where the HP/torque are higher, but you also divide those numbers by 2 due to the gear changing the mechanical advantage.  I'll include it for posterity with a strikethrough to indicate that I withdrew the points...]

Another issue with looking at these graphs is that they aren't measuring what is really going on.  They are measuring maximum capability at a given load/RPM.  But, a bike is not running at maximum load on the highway - if it was then you wouldn't be able to roll on the throttle and accelerate.  Instead, the bike is running at some percentage of its maximum load.  The question is, what percentage is it running at.  If you were held to that speed by, say, wind resistance of a head wind or steepness of a grade being so high that the motor needed to be pegged to stay at that speed - then you would hit the point being measured on those graphs.  But, you are not.  Instead  you are running at some percentage of the available torque and power - just enough to overcome the grade and the wind resistance.  The question is - what percentage is that?  If the motor had huge reserves of power and torque at those speeds then you would only have to feed in a very tiny amount of electricity to maintain that speed.  But, the motor is nearing the maximum speed it can maintain - both because as you speed up the power and torque available are declining and also because the power needed to maintain that velocity are rising.  If you can change the gearing so that you have more torque and power available then you can let off the throttle and pump in less electricity for the same speed.

Your arguments all assume (insofar as they redirect to those graphs) that the motor is constantly running at maximum output - but it is not!

We do not need to know what the engine "can" do at a given RPM, what we need to know is where the potentiometer is at a given highway speed.  The more open the potentiometer, the faster you are draining the battery.  You may have another 20MPH that you can achieve, but if all of that is at 80%+ throttle then it is a very inefficient setup.  If you can regear and swing the motor into a part of the graph where it has lots of torque at that speed then you can bring the throttle down to 50% and get much longer range out of the battery.

[flar]

I wish the chart above was linear in RPM, it would make this analysis much easier without having to do double cross referencing of every figure.

If you look at the motor at 2000 RPM you see that it is capable of 25HP and 65 torque.

If you look at the motor at 4000 RPM you see that it is capable of only 10HP there and 12.5 torque.

If you riding along at 4000 RPM and then you throw a 2:1 reduction gear on, then you will shift the motor into a speed where it can produce 25 HP which is halved by the gear to generate 12.5 HP at the rear wheel, torque is also reduced by the gear from 65 to 32.5.  You've gained 25% in power and have more than double the torque with that gear change.  For a 10-15% loss in having a gearbox, that's still a net gain, though.

At 5000 RPM it is capable of something like 7 HP and 7.5 torque.  The 2:1 reduction gear would take it to 2500 RPM where it gets about 20HP, reduced to 10HP, a gain of 42% - and around 42 torque, reduced to 21 which is almost triple the torque.

It would be nice to project the curves back to the 1000RPM area and see how those figures are modified, but the graphs don't go that far.

I'm pretty sure that miles/kwh are more related to the relative HP figures than the relative torque figures, but I presented both for completeness.  If so, then a gain of 42% over a loss of 10-15% isn't going to break new ground.  It might mean an extra 5-10 highway miles on the larger batteries?

I'm falling back to the value of the gearbox being more in providing exciting 0-60 times - without sacrificing highway mileage - than being solely about highway mileage.

[Richard230]

I'll let protomech respond to your electrical power questions, flar, but I can say that IC motorcycles seem to get their best mileage at around 50 mph, when in top gear. Riding at 80 mph, also in top gear, typically drops the fuel mileage (range) by 20 to 30%. 

My initial thought is that IC motorcycles have been optimized over a great many years to perform well in their top gear, while the lower gears are just used to get them to that top gear without straining the motor or drive-train and to allow them to work well at lower speeds that are outside their normal operating range, as they just don't have the power flexibility of an electric motor.

[protomech]

flar,

You're right that ICE motorcycles generally don't use much more fuel for highway riding vs city riding. My GS500 is faired, it gets about 60 mpg at 30-40 mph type speeds and 50 mpg at 70-80 mph type speeds (+20% higher consumption). The Zero is around 80-90 Wh/mile at 30-40 mph speeds and 180 Wh/mile at 70 mph (+100% consumption).

Two things working here:
1. Gas engines are really inefficient at light loads typical of city miles. Engine efficiency improves a bit under moderate load see highway miles, even as the total power required goes up due to drag. Take a Honda EU2000i gas generator as an example - at 1.6 kW rated load it can run for 4h (5.8 kWh / gallon) on 1.1 gallons, at 25% load (400W) it can run for 9.6h (3.5 kWh / gallon).
2. The GS500 is faired, the Zero is not. Motorcycles have really terrible aerodynamics, especially naked bikes. The Lightning $40k electric bike claims 100+ miles at freeway speeds from a 12 kWh pack (< 120 Wh/mile, no speed claim but probably 70 mph), and it has a pretty nice aero fairing (not clear if that range figure is with the LSR fairing or the race fairing).

Not unique to the Zero. The Tesla Roadster (theoretically) consumes 160 Wh/mile at 35 mph and 300 Wh/mile at 70 mph - also rather unaerodynamic, but well below its 120 mph top speed.


You're also right that a gearbox provides significantly higher performance at low speeds while the motor to operate in an efficient band for top speed. The Zero is geared for maximum rpm at 90-100+ mph in order to have (relatively) useful power and efficiency at 70 mph.

But it's a tradeoff. You could perhaps use the gearbox cost, weight, and volume for more batteries - that'd definitely give you more range. You could use it for a bigger more powerful motor - might give you similar power at low speeds as a smaller motor + gearbox, and more power at medium speeds.

The implementation is important. Once the Empulse is out we'll be able to see how well it works - the Empulse could have a much longer highway range than the Zero, which would indeed point to Zero's motor operating in an inefficient region at highway speeds.

[flar]

So I guess we should be looking at this as a glass half full thing.  The glass half empty would suggest that these bikes suck at highway efficiency when you compare it to city mileage, but the glass half full approach would suggest that the highway mileage is as efficient as that motor is capable of and it's the amazing city mileage that is the plus side, not the norm to compare to.

I believe that Brammo was saying that they were doing the gearbox to give more of the performance and feel of an ICE bike.  That reads to me like the intent is more to focus on bringing back the excitement.  Also, while gearing might not make a night/day difference in highway mileage, it might be able to get more top end out of the engine without sacrificing low end grunt, no?  In other words, my previous comment about the tradeoff being highway mileage vs. low end grunt isn't exactly right, but you could say that if you want 100+MPH top end, then the low end grunt is going to suffer even more without gearing.  So, to give us 100+MPH and also exciting off the line acceleration - like an ICE bike would typically have - then they need gears.  But, the highway mileage is already about as good as we can expect from existing single speed bikes.  Is that a better summary?

And, once you've bought into the need for a gearbox to be able to get both 100+MPH and fast starts, you might be able to do slightly better at highway mileage just to keep on the win/win side of things.

Also, I'd still like to see that graph recast as linear in RPM.  The efficiency curve looks pretty flat on that graph, but it is a plot of efficiency vs. torque and we are more interested in how things graph as the speed of the bike (and thus the speed of the engine) varies.  The RPM plot is a curve and right where the efficiency falls down you see the RPM curve start to take off - meaning the efficiency is flat, but over a deceptively smaller range of RPMs than the graph suggests due to its nonlinear RPM distortion.

[protomech]

Yep. A multi-gear transmission gives you whatever top speed your peak motor powerband can support AND good initial acceleration. A single speed direct drive transmission requires a compromise between the two.

For a commuter bike? Dead-simple to use, higher reliability and lower cost. Don't need speeds > 80 mph, initial acceleration is "okay" but not good. The 2012 Zero S is a bit faster than the 2011 bikes, despite a 40 lb weight gain (ZF9) and 30% higher top speed..

For a race bike? Also fine, speeds < 60 mph will only be seen at the very start of the race. Top speeds aren't super-important either, see aerodynamics.. most races > 30 miles will be energy-limited.

For a sportbike? Needs a transmission or a bigger motor. Watch the 1.5 year old video of the Empulse RR prototype track test - indicated 10-60 takes about 6s. (granted: may not have been WOT, may have still been using the perm 156 motor, speedo may not be accurate.. so take it as an illustrative point)

I think the Empulse is going to have a top speed closer to 120 mph than 100 mph when it shows up.

[CliC]

I believe the aero drag goes up with the cube of the speed, so to up the top speed any decent amount, they are going to have to have significantly more motor power output (and hence less run time per charge at the upper end). The tranny isn't going to get them around that.

What the tranny will do, though, is provide some torque multiplication at the low end, so they can get better low-end acceleration. Maybe that's how they aim to achieve superbike-level performance, at least up to 80 or 100 mph.

With the flexibility of the electric motor relative to IC, though, I still question why they need 6 gears. As protomech pointed out, though, we'll see when their implementation hits the streets.

[Richard230]

My guess is that the ITE transmission has been adapted from an existing IC six-speed transmission that was bought off-the-shelf and that is why it has 6-speeds. Designing a completely new motorcycle transmission from scratch would be a daunting task for any company that didn't have a lot of experience with motorcycle gearbox design and manufacturing. There are a whole lot of things that can go wrong (and have in the past) with a gearbox and it can take years to get the bugs worked out of a new design. Using a proven, off-the-shelf gearbox would be the only smart thing to do for the company that is making the ITE transmission.

[protomech]

SMRE does seem to have a fair bit of design experience with gearboxes. I would not assume it's an adaptation of an ICE gearbox.

1416 on brammoforum mentioned that Tesla dropped their two-speed transmission because of failures resulting from the large rpm swing on shift, and suggested the six-speed transmission from Brammo was a compromise towards increasing durability.

ttxgpfan talked some with BrammoBrian on this topic, I recall that the gist was basically the same thing. Interview is in the archives, it's worth a listen to. http://esbk.co/ttxgpod-podcasts/

[Neal_Saiki]

Let me add a perspective from someone who has designed many of these drive systems.

As far a range is concerned, no gears are better because with a really good electric motor (not some off the shelf unit) you can get near 90% efficiency through a broad speed range.  It's tricky to read these constant voltage motor graphs because EV's use variable voltage for speed control.  For most speeds, you can operate from 85% to 90% efficiency.  Adding gears just wastes ~5% efficiency.

The problem is how to have both good top speed and standing start torque.  The reason that the car guys can do it is because the have very high motor voltages which gives the motor top end RPM and then they have a better gear ratio for starting torque.

For electric motorcycles, you really need well over 100 volts to have something that does both well.  So far, most of them just haven't got there yet, but they will.

Now I'm not saying a gearbox isn't fun.  It is fun, but not a necessity.

Range has to do with battery capacity.  The battery, motor, controller and drivetrain are all very efficient (90% or more) if they are designed correctly.  Check out my article on range 

http://www.ntsworks.com/Range.html

Neal

[Richard230]

Thanks for the link to your article, Neal. That was very a interesting and clear explanation of electric motorcycle range for the layperson It conforms to my experience riding my Zero around on a daily basis.  I absolutely agree that the industry needs to come up with new range standards such as you propose otherwise the public is going to think they are being hoodwinked by the industry claims and nothing but bad press and poor expectations by the public will be the result, similar to how people feel about the "highway mileage" claims by the auto industry.  Or the mileage claims by Honda for their Insight that resulted in a class-action lawsuit, which was widely reported in the press and no doubt did nothing for the sales of that model car. The budding electric motorcycle industry does not need a similar issue to arise due to "inflated" range claims.