| Author | Message | ||
     Blake |
Jeff, >>> As Bill alluded to, the motor's power (consumption) is a function of voltage and current, not RPM and torque. First, for these complex servo motors, that is a pretty big oversimplification. Second, with the "consumption" clarification, both are in fact true; just factor in the electrical to motive transformation efficiency and torque and RPM will give accurate energy consumption. What was the original point that started all this? I thought y'all disagreed that an electric motor obeys the HP = T * RPM / 5250 rule. They do. That's all my point has been all along. Seems mythology has been permeating the electric bike scene concerning their slow speed performance. Without a variable ratio drivetrain, they are relatively slow to accelerate from low speeds compared to what a typical sport bike is capable of. I think some confused constant torque with constant power. Just wanted to try to help correct that misunderstanding. Now Bill has made my brain tired and sore. | ||
| Sifo |
They aren't constant torque though. Torque drops off with RPMs. See the graphic I posted much earlier. | ||
| Notpurples2 |
Not many stock bikes can dip under 3sec, Blake. http://www.superstreetbike.com/features/1009_sbkp_ horsepower_shootout_bmw_s1000rr_vs_world/viewall.h tml Reported XB9SX times around 3.6 (XB12R got 3.5, 1125R got 3.0 on same site) http://motoprofi.com/motospecspictures/buell/light ning_cityx_xb9sx-2005.html I looked around on that site and found some 4.5ish bikes: Ducati 900 Monster: 4.7sec Buell M2: 3.9-4.4sec (edit: '97 is 4.4, newer is around 4) (Message edited by notpurples2 on November 10, 2011) | ||
| Hootowl |
"HP = T * RPM / 5250 rule" Absolutely true as it relates to mechanical energy, though not necessarily as it relates to power drawn from the battery. Which as you pointed out is true of fuel consumption in an IC engine as well. I guess I don't know what my point was, other than to say that there is no clutch, there is a single gear ratio determined by the front and rear sprockets, and that they don't appear to need anything else. If they did, we'd probably see it on the latest model. Oh, and that I have serious reservations about one of these beating an XB to 60, or even 30 MPH. | ||
| Reepicheep |
The original point was that a transmission would make a huge difference in an electric bike. Sifo, your plot was actually really useful... thanks! I think it shows that there is an efficiency difference in the motor based on RPM. So you add a transmission to keep the motor in a (pretty broad actually) effective RPM band, not to get more or less effective rear wheel horsepower. So I am still with Mr. Hootowl... a transmission would help an electric bike, but not a lot, provided you have it geared right in the first place. | ||
| Blake |
Bill re your AC versus DC motors comment: Regardless of type, they all obey the same rule concerning power output: HP = T * RPM / 5250 No matter how they achieve that, whether by voltage or by current or by staged windings or by fancy flux vectors and frequency and waveform manipulation, the output shaft looks the same to the dynamometer. It cares not a wit how the torque and RPM are generated. Me neither! LOL! | ||
| Reepicheep |
(reposting Sifo's plot here)  So if you geared it so your operating range is basically between 10,000 and 25,000 RPM, your peak available power is only varying by 15%. So adding a transmission to stay very close to that *magic* 20,000 RPM where power is max and efficiency is still high, will only gain you 15% better RWHP max. How much power does a tranny eat? 15%? What might be more interesting is making it a 2 position CVT... if you look at the plot, there would be a gearing for an engine RPM with "max power" and a gearing for an engine speed for "max efficiency". | ||
| Hootowl |
Clarification: I think it would be cool if it did. I love the idea of an electric bike or car. Just don't make me subsidize their existence or purchase, or try to con me in to believing that they will be the salvation of mankind.  | ||
| Hybridmomentspass |
Blake "To launch hard, you need a clutch. Just no way around it. 0-30 mph for an XB is under 1 second. " No way. I really dont see it happening with a stock XB. I'd be surprised if it happened with a stock 1125 too. As for your 12sec time pretty common though And I definately agree with your comment about people trying the dragstrip out its a lot of fun for little money and gets you better acquainted with the bike | ||
| Reepicheep |
(which doesn't explain why this thing looks geared so high...)  And dang, is that bike growing on me.  | ||
| Hootowl |
"there would be a gearing for an engine RPM with max power and a gearing for an engine speed for max efficiency" That may well be what Tesla has done with their two speed transmissions. | ||
| Reepicheep |
Hey! Cool! The swingarm pivot is on the same axis as the drive motor (I think). Nice for zero lash drive train (Hi Abe!), but I wonder how they pulled that off. Hollow shaft on the drive motor? Probably more likely a hollow pivot on the swingarm. Dang. I'm "engineer bonding" with that damn thing. This could get expensive... | ||
| Hootowl |
30.8 to 33.1 seat height depending on the options. I think it looks higher than it is because it's a smallish bike. | ||
| Blake |
Bill, We agree that the motor is providing max torque through it's entire rev range, yes? Plot the HP curve 0 to 88 MPH? Zero HP at 0 MPH via straight line to 53 HP at 88 MPH. Plot the torque, 50 LB-FT constant across the chart. Where do the two curves intersect? Okay, now add a two speed transmission dividing the range into half and re-plot that same chart. Assume speed shifting, no clutch. Now as you approach 44 mph you have twice the power compared to the single speed version, an extra 26 HP. Not a big difference??? If you plot speed versus time, you will see that you've gotten to 44 MPH in half the time compared to the single speed version. The first half of the HP vs MPH plot for the two speed version will have twice the slope compared to the single speed plot. The second halves will be identical. Add more gears, get even better performance by keeping the motor spinning nearer it's peak HP output. | ||
| Hootowl |
There you go Bill  | ||
| Blake |
Bill, That is not a constant torque motor characterized by that plot. It's peak output torque varies widely versus speed. Not same animal. | ||
| Froggy |
You mean the 2 speed transmissions that it never got to work without turning into rolling grenades? | ||
| Reepicheep |
Wow. Without a shaft through the middle of that swingarm, that's a heck of a load if you hit a bump leaned WAY over...
Errr, no. I think the torque is proportional to load, which changes the EMF strength (Electro Magnetic Field), which changes the current draw. So I think the motor can produce more torque at lower RPM's, and less torque at higher RPM's. (Sorta, with efficiency changes which are probably pretty significant at the corner cases). (Message edited by reepicheep on November 10, 2011) | ||
| Hootowl |
Froggy - No idea. I can't afford one, and wouldn't buy one if I could, so I don't really follow. Perhaps if I had, I would have known that there was an electric car out there with a two speed transmission. I think it is safe to say that if I had the money for a supercar, Tesla wouldn't get it. Audi might. Definitely Aston Martin  | ||
| Reepicheep |
Gotta stop looking at that bike.... Aaack! | ||
| Hootowl |
I thought EMF was electromotive force? There's EMF, counter EMF, counter-counter EMF. Related to transformers. Same same? | ||
| Froggy |
Reepi, keep it in your pants, something tells me there will be something else on the market soon from another company that will also get your jollies going | ||
| Blake |
Bill, All along I've been taking about WOT performance only, maximum motor output capability for maximum acceleration. Seems your talking about something else entirely. I've also only ever meant power output, never power consumption unless specified. When I read Jeff's statement that "the motor's power is a function of voltage and current, not RPM and torque", I didn't understand that he meant "power consumption." It looks like we were talking about two different things. One other benefit of some gearing would be a top speed better than 88 MPH and that being only short term.  If we were willing to put-up with similar performance our Buells might have just two gears, what is now 1st and 3rd. Engine wear would be a huge issue though. | ||
| Fast1075 |
I already have an EV . It's not fast and will only go about 25 miles on a charge, but I can ride it back and forth to work for less than a dollar a week . And I even have a designated medium speed road that takes me from home to shop. The Zero rocks, and with the optional "low" seat, I can get around the "you have to be this tall to ride this" sign. Me Like!! | ||
| Blake |
Need to check on the motor. My understanding is that they provide full torque capacity through their operational range. That's how the servo motors on the fancy satcom antennas work. Full torque is available no matter the speed. If torque output capacity drops off with speed, then the torque curve sucks! That would be horrible! Yuck! | ||
| Reepicheep |
Interesting. I'll wait. I'm not even sure what I am talking about, I won't even speculate on your topic. I'm just trying to understand in my own head why e-bikes and e-cars lack transmissions. I think I kind of have it. Motor speed is a function of frequency. Motor power at that speed if a function of EMF (which any idiot knows is Electromotive Force  ), which throttles current. So the bike can effectively make peak horsepower at the rear wheel at most RPM's. (except for minor efficiency losses at the corners, which aren't enough to justify the losses, weight, and cost of a transmission). I'll keep watching... I'd love to see Zero and another neat American motorcycle company going at each other hammer and tongs building electric bikes that the rest of the world can only dream of. | ||
| Blake |
Bill, The DS is my favorite too. Dyno chart courtesy of Motorcycle.com...  Not what I was expecting, but the 2012 may be an entirely different deal. The above Zero S dyno chart indicates near constant torque up to 45 MPH. Then it takes a dump. | ||
| Fast1075 |
The motor is an electrically commutated 3 phase device. Rpm and power are proportional to pulse width and duration. Absolute power output is a function of the voltage and the programming variables set in the controller. It is a balancing act between power and efficiency. Regen captures some lost energy, but must be carefuly tailored to the rider's preferences. A little is ok, but personally, I prefer the ability to coast freely and use the brakes to stop. | ||
| Blake |
E-vehicles can forego a transmission since running an electric motor at max speed for long term is fine, and they are not aimed at high performance enthusiasts as the above plot clearly demonstrates. Acceleration is poor, and top speed is dismal. That will change over time. Count on it. I am! I still think that for around town an electric bike with 100 mile range is very enticing! Good call BTW on the missing structure. Likely the motor bolts in there and does a two for one job? I sure hope so. I wonder how much a spare battery pack weighs? | ||
| Blake |
Purple, Thanks for the info on 0-60 mph times. 3 seconds. |
