| Author | Message | ||
     Sifo |
I had this discussion before months ago in another thread. Someone posted a power curve for the motor versus motor speed. It started at zero and went nearly linearly to it's max, then yes in the overspeed region it went limp. Point being, the torque in the lower 70% to 80% of the motor speed range was flat. Flat torque produces a linear power curve. I'm pretty sure the someone you are talking about was me. I couldn't get you to see what I was talking about then either. Look at the torque curve for an electric motor. It's a pretty linear drop off with RPM. Note that peak efficiency comes well past peak power with an electric though. The are happy spinning fast, but produce torque at low RPM.  Like I said earlier, it isn't that there's no benefit to a gear box. It's just that there are aspects of the compromise that make a gear box not worth while for most applications. | ||
| Glitch |
I'm sure it's the internal combustion in my head, but that chart looks backward to my electric motor ignorance infested brain. Looks like it gets more efficient as the power drops off, as it uses less current?  | ||
| Sifo |
Looks like it gets more efficient as the power drops off, as it uses less current? That's exactly right. Conversely, they will produce huge amounts of torque at very low RPM, but they are not efficient, i.e. they draw very high amp loads. Stall it completely and it will draw max amps while doing zero work, until you let the magic smoke out anyway. They also cool better at high RPMs just because the armature acts as a fan. Low RPM they get hot, the windings gain resistance and generate more heat. That's a bad cycle. I guess if you wanted to tow a heavy load, a gear box would make a lot of sense. I don't see much of that happening with electrics though. | ||
| Sifo |
Or you can make a motor with multiple windings, that's capable of multiple torque curves. "Shift" between the windings, and no mechanical losses. I was thinking a bit more on this sort of idea. It would be a simple matter to make a motor with adjustable timing to maximize either power or efficiency at various RPMs and loads. | ||
| Fast1075 |
Pulse width modulation makes EV motors a plausible reality. It allows ample power for acceleration and reduced current draw for cruising. I like the hub motor for utilitarian use where unsprung weight is less of a factor. I have a scooter that uses a hub motor. It is crude, using lead/acid batteries and a relatively low voltage of 60 volts. It has a cruising range of 25 or so miles at 30 mph or less. Perfect for someone who lives within a mile or two of their normal shopping, etc, IF they have low speed vehicle designated roads such as the suburban centers in Florida. With a 5 stage gang charger system, it draws an initial 2.5 amps at 115v, which tapers as the batteries charge. It takes 3 hours to recharge at a calculated average cost of 9 cents. It is currently down with a controller casualty due to poor quality control, but the concept is solid if used appropriately. | ||
| Blake |
Sifo, >>> Look at the torque curve for an electric motor. Wow, 34,000 RPM and no performance data below 3,000 rpm! Not any kind of typical electric motor that I know! What is that motor used for? Is it a good representation of the type of motors used on an electric motorcycle or automobile? Talk about gyroscopic effect! Just spin up the motor and you don't need a kick stand.  Given that Zero motorcycle's drive ratios for their street and dual sport bikes are around 5:1 +/- 6% with top speeds between 70 and 90 mph, we can figure that the motor is limited to around 5,900 rpm, less than 1/5th that of the motor your plot shows. Looks to me like torque is constant for the Zero bikes up till motor overspeed, somewhere around 4,000 rpm, where it then starts to steeply decline. You can look at Zero's own acceleration performance plot and see that the motor torque is not dropping off much if at all with speed like your plot would indicate.  The other manufacturers Brammo and NativeS do seem to suffer immediate decline in torque all through their motor speed range. It's an old plot though...  There certainly are valid reasons for not implementing a gearbox on an electric bike: cost, weight, added complexity and maintenance. There are also valid reasons for including one, namely a major improvement to performance. The current crop of retail e-bikes offers poor performance at lower speeds. Instead of 14 HP at 30 MPH a small gearbox would allow double that. And you get to enjoy the kick in pants and just plain fun when working through the gears. Not having a clutch would seem to bug the heck out of me anyway. On slippery or dicey terrain, I REALLY want to be able to disengage the drive-train from the rear wheel. But I'm ignorant of the technology as you so easily revealed. Maybe the motor controller in effect performs that if the rear brake is not being used? (Message edited by Blake on April 05, 2012) | ||
| Blake |
>>> I guess if you wanted to tow a heavy load, a gear box would make a lot of sense. I don't see much of that happening with electrics though. Why wouldn't a gearbox make sense if you just want to accelerate more aggressively? Cause no motorcycle rider wants more than 15 RWHP at 30 MPH? | ||
| Blake |
I feel like I've been transported into bizarro universe. People on a sport bike forum arguing against doubling or better the performance of a motorcycle. | ||
| Blake |
Old plot of Zero X motor performance...  Thanks to www.motorcycle.com for that. | ||
| Reepicheep |
Not me! After spending *another* several hours on a piece of junk KX-60, trying to chase down some sort of high RPM bog, I was thinking about how to convert it to an electric bike. I was trying to figure out how to keep the tranny and clutch, and just remove the top end. Oh, and I guess the stator and exhaust could go... | ||
| Mr_grumpy |
Blake, This has always been the bizarro universe. Why do you think the ET's never stay? Beam me up Scotty, it's time to go home!  | ||
| Sifo |
Blake, Just because it is a small motor doesn't mean it isn't typical of electric performance. Want to see IC engines that will run near that RPM range? They are commonly available. You keep talking about torque and showing HP data. Here's the thing... Can we agree that 100 ft. pounds of torque at zero RPM will yield zero HP? Correct? No actual work is being done. Correct? So when a stopped electric motor is given power, how the heck does it actually get the vehicle moving when the motor s turning zero RPM? As usual just looking at HP data doesn't give you all the answers. So when you compare the HP curve from my chart that goes up to 34K RPM to the larger motors of the bikes, is the shape of the curve all that different? I don't think so. If anything the more linear HP curve of the Zero would mean that the initial drop off in torque would even more severe, dropping a bit less as RPM builds. All show a good build up of HP with RPM with a well defined peak and quick drop again. Why no data below 3000 RPM? It's a small motor designed to spin fairly fast. You wouldn't source that motor for work at low RPM. At least I wouldn't. That doesn't mean that the torque curve is likely to change going down to zero RPM though. | ||
| Mr_grumpy |
How about this? http://litmotors.com/c-1/how-the-c-1-works/ | ||
| Blake |
Sifo, >>> is the shape of the curve all that different? Yes, wildly so. The Zero bike's motor is constant torque from 0 to about 4000 rpm, then torque drops off in a major way heading to the 5,900 rpm motor limit. This is a dyno plot of one of their motors from back in 2007. Newer models have much improved performance, but the shape of the torque curve doesn't appear to have changed much if at all. Constant or near-constant from 0 to a bit past half speed, then torque plummets. | ||
| Blake |
>>> It's just that there are aspects of the compromise that make a gear box not worth while for most applications. On an e-bike, that could only be true if the customer is not eager for significantly improved performance. | ||
| Jaimec |
Maybe eventually, Blake. Right now, I picture the typical eBike rider the kind of person who'd buy a scooter; just looking for practical, inexpensive transportation from here to there and back again. Once they catch on, then maybe you'll see more attention paid to performance. | ||
| Mr_grumpy |
See my link above Jaimec. | ||
| Blake |
Effect of three speed gearbox, very roughly illustrated...  Not desireable? Really? Heck, I'd want a clutch anyway in order to be able to completely disengage the rear wheel from the drivetrain. | ||
| Sifo |
Blake, do you not see something odd about that dyno chart? It looks like there are 3 points of data, and nothing more. NOTHING I've ever seen has that kind of torque "curve" with a single sharp kink right in the middle. That looks far more like marketing's representation of what the power is supposed to be. Where did that even come from? OTOH, it might just be a result of limiting the amps that are available in the lower RPM range. This might make sense, though I doubt you could ever control it as precisely as the chart shown though. That would indicate limitations in the power delivery more than actual capabilities of the motor though. | ||
| Sifo |
Effect of three speed gearbox, very roughly illustrated... The thing is, these bikes, performance wise stack up against what? Bikes in the 125 to 250 cc range. They are already over priced and overweight in that class. You are talking about adding even more cost and weight into the equation. There is a far easier solution... Just give them a stronger motor/battery combo. Electric cars like the Tesla did very well in that regard without the need for a gearbox. People loved the torque off the line and they had good top end. Just don't "brick" the battery! | ||
| Sifo |
Just look at KillaCycle. Direct chain drive from output shaft to rear wheel. 155 MPH in 8.168 seconds! Gears? We don't need no stinkin' gears! http://puregreencars.com/Green-Cars-News/Other-Gre en-Car-News/KillaCycle-SuperDrag-Electric-Bike.htm l Seriously. If gears were the answer, a bike like this would use them. | ||
| Sifo |
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| Froggy |
The Tesla Roadster did at one point have a 2 speed transmission, but they couldn't get it to work without blowing up due to the high torque and high RPMs. They ended up selling the car with the transmission locked into high gear, and later replaced the transmissions under warranty to a better single speed unit. It isn't a direct drive transmission either, with a 8.27:1 ratio and a 14,000RPM motor limit that will let it hit 125mph. | ||
| Jaimec |
Ah yes... the Killacycle. I remember that one... | ||
| Mr_grumpy |
Ah yes, hold my beer and watch this.... | ||
| Blake |
Sifo, You're not getting it. No matter what motor power, the gearbox always significantly enhances performance. If the electric drag bike had a gearbox, it'd be even quicker in the quarter. Weight and cost of small gearbox compared to larger mother and batteries? Not an obvious winner in that trade off comparison. The motor Dyno plot is theoretical for motor folks I think. If you look at the Zero Dyno though, it is very close to that. | ||
| Mr_grumpy |
I'm not sure that you're correct in the Drag bike case Blake. The advantage of the direct drive is that there is no power loss through the box nor any weight penalty, plus the fact that the power is on constantly with no pause for shifting. Anyhow a gearbox isn't a prerequisite for a dragster, EJ Potter didn't use one. http://bangshift.com/blog/the-bangshift-interview- ej-potter-aka-the-michigan-madman.html | ||
| Glitch |
I dunno, I'm just trying to wrap my mind around how all this works. I know I need to quit trying to compare ICEs to Electrics, but it's hard for me as I've never really considered how all this works. Mainly because I've not faced anything powered by an electric motor that would benefit from a transmission, you know, like a vacuum cleaner. I admit my ignorance so please be patient with me, my questions may seem like simpleton stupidity, but I think all this is worth conversation. Now am I comparing apples and oranges when I ask... If an I.C.E uses a transmission to get to a 1:1 ratio (or a little better if it has an overdrive) and an electric motor starts off and remains at a 1:1 ratio or better, what's the need for a transmission if you're not trying to go faster than legal speeds? | ||
| Sifo |
No matter what motor power, the gearbox always significantly enhances performance. REALLLY? You need to let these guys know about that...  They seem to be stuck in a one speed world. http://www.universalauto.com/worldsfastestcars/top fuel_dragster/dragster.html
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| Glitch |
No matter what motor power, the gearbox always significantly enhances performance. The transmission is a one speed forward with reverse. Yeah but that doesn't really make Blake's point invalid, it only means that for what they're doing they only need one speed. They have a programmable slipper clutch to get the car from a 0:1 to a 1:1 ratio. I'd like to know some of the stuff these guys have tried before coming to their end product. I mean for all we know they have tried a transmission and for whatever reason they found it ineffective, inefficient, or something along those lines. As far as I know, none of us in this conversation know one way or another. It seems we're arguing over how we'd start out with our experiments toward an electric bike? Or are we just taking it for granted they just went to a straight jack shaft and never considered a transmission? |
