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[Spoonman]

Right so I kinda touched on this in the intro post but I figured it might as well be thrown up here as well for more "in depth" discussion.

Gear ratio's - do you need more than one?

So we all know that the Zero, Evolve, and Agility favour the direct drive route as it saves on weight, space, complexity, and cost.

However, the much hyped Brammo Empulse sports a standard (although clunky apparently) 6-speed box and a clutch; the Lito Sora [..concept? has anyone seen one yet?] promises a CVT; the Brutus (yes, I aware of how much the bloody things costs) lists the transmission as "optional"; and the Agility Saietta R has "Drive-Torque Geometry-Control"... which speak towards CVT to my ears.

So - with all of the ...top shelf offerings incorperating some form of on-the-go drive ratio tuning - is it worth the effort?


I crunched some numbers based on the mass and aerodynamic specs for the NC24, with allowance for an 80kg (~170lb) pilot, deriving figures for the net accelerative force available* at the rear wheel for the two relevent scenario's:
- NC24 stock engine and 6-speed box
- AC15 motor at 96V and 650A, employing a 5:1 drive ratio (14F, 70R)

The graphs show that the acceleration of the AC15 drive matches to slightly below peak acceleration in 4th, and slightly above that of 5th.

Now that's not bad, and particularly when you consider that it's available all the way out to about 90km/h (55mph) before it starts to tail off and eventually tops out at about ~160km (~100mph), with peak power being produced from about 90km/h out to about 120km/h (55-70mph).
 
However, the gearbox has two distinct advantages:

- the first, is that peak power from the engine is readily accessible all the way from 45km/h out to 180km/h (30-110mph), and peak acceleration in 1st, 2nd and 3rd, are 250%, 180% and 120% respectively, of that available from the direct driven version.

- the second is that even if you don't want to acclerate 12m/s, 1st gear means that you don't have to use 650A in order to accelerate at 4.5m/s. So there are tangible benefits for the batteries as well.

Personally, my only concern about the introduction of a clutch and gearbox is to what ends it will affect the control of regen.

Now as you've notices, all of my numbers are on paper (or in excel in this case) and none of it is from the actual use of a fixed gear EV motorcycle in real life - so I am more than happy to hear about how anybody who rides an EVMC finds the performance - similarly, I'm interested in hearing the devils adovocate's on the notion.



*being a function of torque, mass & drag and neglecting rolling and final drive resistance - all numbers are approximate but reasonable, I was far more interested in the profiles of the relative values than the accuracy of the absolute ones.

[protomech]

I don't see weight for the saietta R, but 96 HP should be good for much more than 80 mph if using a transmission. They probably weigh in the ballpark of 450 pounds; 3.9s 0-60 is about right for direct drive.

Brutus has moved away from a transmission.

Lightning and Mission and Empulse RR are all direct drive, but they employ very expensive motor controllers.

Sora and Empulse are basically the only production-intent bikes using a transmission currently, and that's being somewhat generous to Lito. There there's these guys:

http://www.motori24.ilsole24ore.com/Moto-Novita/2013/04/R6E-Vercar-Moto.php

I'd love to have a two speed transmission on my 2012 Zero, even if it was only selectable while stopped. But given a choice between a two speed transmission with the 2012 motor and direct drive with the 2013 motor, I'll take the 2013 motor.

The Empulse is significantly quicker to 60 mph than the 2013 Zero - perhaps 4s vs 5.5s - but it's also running a sevcon size 6 controller vs a sevcon size 4. I guess a sevcon size 6 would bring the bikes to near parity in acceleration and top speed.

[protomech]

Same acceleration at the same wheel speed means basically the same power draw from the batteries, regardless if the motor is doing 40 Nm at 1000 RPM or 20 Nm at 2000 RPM.

Hand-wavey numbers, but you'd have something like 4kW from batteries = 40 A. Motor current would be around 200 A vs 100 A at double the voltage and RPM.

Transmission allows you to decouple motor RPM = motor voltage from wheel RPM. So now you can adjust motor current and voltage for a particular power draw or wheel torque requirement. Depending on the efficiency map for your selected motor or your operational requirements, if you primarily operate the motor in low efficiency edge conditions - ex very low rpm / high torque or very high rpm .. then a transmission is probably a winner. Otherwise it's a tossup IMO.

[Richard230]

My 2012 Zero really struggles trying to get moving from an uphill stop.  If it could carry a passenger, I don't know how it would get going under those conditions.  I would be worried that the motor would overheat trying to move out on a steep uphill grade with a full load, so I think a transmission would have some real benefits under those conditions.  But for my usage, I still would prefer more batteries and the lighter weight, greater efficiency, reduced cost and simpler maintenance of direct-drive compared with a transmission.

However, I sure would like someone to give a CVT a try.  That sounds like something that would work well, although power losses through the transmission, the likelihood of the transmission sucking up even more space on a vehicle and the additional complexity would likely be negatives.  But it would still be interesting to see how well it would work in practice.

[frodus]

CVT's are pretty inefficient... they use a belt under tension and it robs power. Plus, they're made for the torque curves of an ICE machine, unless you get one that is externally actuated (like some of the Suzuki Bergmans).

I like my Empulse R and it's transmission, but consider this: It was designed and built around the Parker Hannafin motor. I don't know what your skills are for fabrication, but unless you can design and build your own, I don't know how you'll aproach mounting an AC15/20 motor to an existing motorcycle transmission.

One of the big issues I see here is that you said you want to put 10kwh onboard. I just don't see you fitting 10kwh AND a transmission AND an AC15/20 on any bike you've listed. Even the TTXGP/IOMTT racers haven't fit a transmission AND 10kwh+ on a bike. They opt for a larger motor that has better performance, and go direct drive. A transmission, as discussed on several forums, allows use of a smaller motor on a bike that would suffer from that same motor with direct drive. If a larger motor was used (and maybe larger controller), you can meet and exceed the performance of a smaller motor + transmission.

After doing all the research I've done, and designing and building my own bike, I'd suggest:
Go direct drive, get a slightly more powerful motor (AC20) and jam as many kwh of batteries as you can on the bike.

[Spoonman]

I agree on the CVT's personally - wouldn't be my preferred choice.

I still have to measure up the space in the frame but I think if I get to the point where I'm considering direct driving a bigger motor, then the '20 would definitely be on the cards, and maybe even the '35 although the increase in diameter might make it too awkward.

As I said there's nothing set in stone yet but I have a theory for the gearbox solution which would require some machining alright but has other benefits beyond the aformentioned. I'm gonna turn out a timber mockup out of the '15 in the next few weeks in order to do some measuring up and see if it's at all feasible - also have to do performance testing on these NCA cells and then come up with an optimal fitment and mounting arrangement for them assuming they don't decide to detonate on me.

Volumetric contraints were always gonna be a serious issue but both the TLS and the TLR give you good bit of room to play with so we'll see what happens.

...'tis all still very early days yet.

[NoiseBoy]

Would there be any sense in having a single fixed gear ratio a la direct drive but with a clutch?  Clutch slip would alleviate the issue of hill starts and allow you to dump power quickly to get off the line, but then in normal riding you would just leave the clutch engaged and notice no difference.

[protomech]

Electric motors have very little rotating mass. Unless you add a flywheel, you'd really gain little from a clutch dump at start.

[___]

What about a 2 speed dog clutch? It would give you a "starting" gear and a "cruising" gear. Would that lose as much power as the CVT?

[BSDThw]

Unfortunately I don't have the ability or friends to built it but I would like to but a planetary gear in the rear hub.

It is a bit hard to explain by words but you can set it for "first gear" and the planetary parts do the reduction. If you block the whole gears, it will work like the standard sprocket no gear friction "second gear"

The only disadvantage is a bit more weight in the hub!

Maybe this will explain my idea:

If you go to this link you will see a planetary gear and above there are 3 switches. Activate the middle one "Sonnenrad festhalten" => press the "Start" button.


If you think the center sprocket is locked to the swing arm, the rear wheel sprocket is fixed to the ring gear (Hohlrad) and the hub is fiexed at the "planetary gear" (Planetenrad). You can see the hub spins slower.

Now you open the locked center sprocket and lock it to the planetary gear mechanic. Now the hole gear system is just a rotating sprocket like we have already installed.

[Spoonman]

That might not be much additional weight, but at that location it's unsprung.

Might work well on pushbikes and ok on mopeds, but adding unsprung mass is something you want to avoid if at all possible on motorbikes.

[BSDThw]

That is what I want to say with adding weight (unsprung mass was the term  I didn't know).

But a hub motor will be much more worse and is often used.

If I could get something like that, I would really like to test it

[Spoonman]

Right - so I was crunching a few more numbers, and in doing so noticed that I'd input torque in ft.lbs rather than Nm in one of my original sheets. That corrected I went ahead and intruduced a 0-100km approximation as a frame of reference in order to get a real world idea of how the performance would pan out.

With the AC-15 coupled at 5:1 and running 96V 650A, you're looking at acceleration in the region of 6m/s out to 4500rpm (assuming bike and pilot mass of 270kg, and a 190/50R17 rear wheel), and tailing off to about 2.5m/s by 7500.

That equates roughly to the peak of 4th gear performance on the TL1000S, or 3rd gear performance on the NC24.

It also comes out to a smidge over 3s 0-100km/h

100km/h arrives at ~5500 RPM (right in the peak power bracket) and continuous motoring at that speed will require somewhere in the region of 9kW between drag, drivetrain losses and rolling resistance meaning that the range to 80%DoD for the proposed pack would be aroundabout 100km on open roads.

Contrasted against the gearbox drivetrain, I'm now starting to sway towards the direct drive alright. Barring 1st and 2nd, the benefits over the NV24 motor are negligable - and if I can match the performance of that then I'll be delighted.

[protomech]

Yep, I think once you go big enough on the motor then a multi-speed transmission offers at best a small amount of straight-line acceleration benefit for the increased operator attention, material cost and upkeep, weight and volume.

I think it's already marginal at the 40 kW level. Empulse is marginally faster than the 2013 Zero at 0-60, comparing 380 pound ZF11.4 vs 470 pound Empulse R.

I suspect Zero would be just as fast, maybe faster than Empulse with the same Size 6 controller Brammo uses (Zero currently uses Size 4).

[hamr1]

Just go direct only rb7s need a seven speed