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Thread: Transmissions. yeah.

              
   
   
  1. #81
    Senior Member Athlon's Avatar
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    I'm not a big fan of transmission for EV , but sometimes they can save the day , on light well powered vehicles like bike there is almost no need to have a gearbox , but on commercial truck with some overload and very step hill to climb a transmission is the only way to climb on top with a chap motor , motor prices grow up faster than transmission prices when you go up with power , so for a 10Kw bike can be cheaper adding a second motor , but for a 50Kw truck can be cheaper adding a 2-3 speed gearbox

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    Senior Member harlan's Avatar
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    Quote Originally Posted by teddillard View Post
    troublemaker.
    Ha! Just thought we finally had some data points to analyze, maybe plug into the elmoto spreadsheet.

    As someone has said before, (Noah?), the only fair comparison would be identical bikes with/without transmissions. At a glance, it doesn't look like transmissions cause the dramatic efficiency/range boost that was eluded to.

    Maybe Brutus can chime in on the subject since he's the only one with any experience with trannys on an elmoto. Have you had the chance to measure your power consumption at different speeds? You have said in the past you were expecting to get about 100 miles on a ~4kWh pack which would certainly make a strong case for transmissions. Any data you can share? Does it give an efficiency advantage or is it just a fun factor?

  3. #83
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    A gearbox gives you the ability to use your full power at more speeds, so it allows you to use your battery faster if that is what you want.

    For example if you have a bike with a single speed, and it puts its peak power, say 30hp, down at 60mph. Now say you put a 2 speed gearbox with a 1:1 and a 1:2 ratio. now you have the ability to put that 30hp down at 30mph and 60mph if you want to.

    Another way to look at it is: if you get harder acceleration, that power accelerating you needs to come from somewhere, and on an electric motorbike, that is from the battery.

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    Quote Originally Posted by Biff View Post
    For example if you have a bike with a single speed, and it puts its peak power, say 30hp, down at 60mph. Now say you put a 2 speed gearbox with a 1:1 and a 1:2 ratio. now you have the ability to put that 30hp down at 30mph and 60mph if you want to.
    Or you could use an AC system (like the AC15, 80V) and get constant torque up to 30mph and the same power (about 30hp) at both 30 and 60mph with a fixed ratio. You'd have less gear loss and more space for battery.

  5. #85
    Not to be taken seriously DaveAK's Avatar
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    Quote Originally Posted by Brutus View Post
    Honestly even if a trans is useless for electrics it sure is fun to ride and that is what's most important to me.
    Bingo! We have a winner!

    But black? Really? That's so 2010.

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  7. #86
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    Quote Originally Posted by sparky_mark View Post
    You mean replace a $200-$300 part with several thousand dollars worth of kit?


    A gearbox on an electric bike allows you to select a gear to suit the riding conditions, and therefore give your batteries an easier life, doesn't it?. For example, if you have an electric bike with a gearbox that has a max. top speed of around 80mph but you were riding around town at no more than 30mph in stop/start traffic then you'd use the lower gears. This would give you very good acceleration from standstill, you'd still have reasonable top speed, and the load on your battery pack would be lower. If you then take the bike on the motorway/freeway you'd work your way up through the gears until you achieve top speed.

    I don't know if this is true, but it certainly sounds plausible doesn't it? That a gearbox would, in certain conditions, give your batteries an easier life and could extend range? Wether its true or not, I think this is one reason that the idea of a gearbox/transmission refuses to die.

    No. If your motor is able to turn faster, the BEMF is higher, and you have the ability to draw more power from the battery for any given phase current limit your controller does. When you have a tranny and you're in 1st gear and it's accelerating harder than if it was in 2nd gear, it's because you're able to draw more power in first gear because the motor is spinning faster.

    Likewise, though no amount of changing KV in a motor through changing the number of turns, changing how turns are grouped in series or parallel, changing from Delta to Wye etc, NONE of these things can give your motor the ability to have higher torque, higher continuous torque, etc (assuming the amount of copper in the slot is identical still).

    What it can do, is enable you to pull more power out of your battery because it's BEMF is higher. When you change from a higher KV wind to a lower KV wind, you now have the ability to hit the batteries harder for a given amount of phase current your controller is capable of, and it's this additional power you draw that makes higher torque for a given controller phase current limit. HOWEVER! The motor is not capable of any additional torque at all, it's capable of exactly the same torque as before, it would have just required higher phase amps from the controller to be feeding it the same amount of power that the lower KV motor can draw with less phase amps, because it's getting it's phase current is multiplied by a higher phase voltage (because BEMF climbs faster with a low KV motor, and phase resistance grows at the square of turns).


    Also, there are some massive misconceptions about motor windings and I^2*R loss. The misconceptions are written a few times in this very thread, and are seen in pretty much every thread related to motors anywhere that confused people talk about motors.

    If the copper fill is the same in the slots of the motor, you make the exact same amount of heat if you're a 1-turn winding or a 100-turn winding.

    The resistive loss grows at the square of current, and the coppers resistance grows at the square of turns, so if you have a 2-turn motor with 10mOhm resistance, when you make that a 4-turn motor, the wire has to get twice as long to make double the turns, and it also has to get half the thickness to enable it to fit in the slot, so the 4-turn version of the same motor is 40mOhm of resistance. If we put 100amps through the 2-turn motor, we only need to put 50amps through the 4-turn motor to make the same torque, the 10mOhm winding with 100amps I^2*R = 100watts of resistive copper loss (heating). The 50amps through the 40mOhm I^2*R = 100watts of resistive copper loss (heating). How about the power they both draw to make that torque? The 2-turn motor needs 1v of drop to draw it's 100amps, making 100watts of power needed to make x amount of torque, and the 4 turn motor needs 2v of drop at 50amps to make it's x amount of torque, also 100watts.


    Long story short, apply that expense and weight and bulk of the tranny towards putting a motor properly suited for the job rather than trying to band-aid an incorrectly sized motor, and you end up winning everywhere if your pursuit is performance and durability.


    If you're building something to be a utility tractor, and you want to crawl around at 1mph and pull stumps out of the ground with it, and then also be able to cruise along the highway at 80mph, then a tranny makes more sense than making a giant motor to do it. For situations like 0-200mph on a motorcycle, you're simply going to be most happy and have the best performance by filling that area with more motor rather than tranny. If you don't have the physics skills to see why, you can see excellent evidence of this in examples like Chip Yates bike getting to 190mph in the standing mile... Any addition of a tranny would have just slowed him down from shifting delays, and taken away space and added weight that could be going towards a higher power motor.

    http://www.youtube.com/watch?v=b-SYq8H4xnI

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    No. If your motor is able to turn faster, the BEMF is higher, and you have the ability to draw more power from the battery for any given phase current limit your controller does. When you have a tranny and you're in 1st gear and it's accelerating harder than if it was in 2nd gear, it's because you're able to draw more power in first gear because the motor is spinning faster.
    But what if you are in 1st gear and you aren't accelerating hard? You don't ride everywhere flat out do you? Surely if you were in 1st gear and the bike was only accelerating moderately, say at the same rate as if you were in 2nd or 3rd gear, then aren't you achieving the same rate of acceleration using less power?

  9. #88
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    Or you could use an AC system (like the AC15, 80V) and get constant torque up to 30mph and the same power (about 30hp) at both 30 and 60mph with a fixed ratio. You'd have less gear loss and more space for battery.
    30bhp to do 60mph? Is this a real world example? Do you really need 30bhp to do 60mph? My old 12bhp Honda 125cc could go faster with less....

  10. #89
    Moderator Nuts & Volts's Avatar
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    Quote Originally Posted by sparky_mark View Post
    But what if you are in 1st gear and you aren't accelerating hard? You don't ride everywhere flat out do you? Surely if you were in 1st gear and the bike was only accelerating moderately, say at the same rate as if you were in 2nd or 3rd gear, then aren't you achieving the same rate of acceleration using less power?
    Nope, to move a given mass, m, at a certain acceleration, a, you need the same force regardless of gearing.
    F=m*a

    This force equals the torque divided by the wheel radius. So for the same 'a' you will need to generate the same amount of torque with the same amount of battery voltage and current (ie power).
    Last edited by Nuts & Volts; 23 April 2012 at 1612.
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  11. #90
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    Quote Originally Posted by sparky_mark View Post
    But what if you are in 1st gear and you aren't accelerating hard? You don't ride everywhere flat out do you? Surely if you were in 1st gear and the bike was only accelerating moderately, say at the same rate as if you were in 2nd or 3rd gear, then aren't you achieving the same rate of acceleration using less power?
    Not at all. There is no free lunch. To cruise at a given rate, you need x amount of power. You can achieve that power by spinning your motor at a certain speed (say 1000rpm) and 20ft-lbs of torque (say this requires 200amps), or you can have twice as short of gearing, and achieve the same X power needed to cruise at your intended speed, now your motor is at 2,000rpm and you only need 10ft-lbs of torque (requires only 100amps), but requires DOUBLE the voltage, so you end up drawing exactly the same power of the battery as before. (Keep in mind, motor amps are NOT battery amps)

    Every transmission will always add some amount of friction and drag and weight, and require space to package on the bike, and not give any higher power levels or continuous power levels etc. It can just enable your motor to draw more power sooner while in shorter gears. If you just have a motor that can handle more power, you win everywhere.

    At least that's my $0.02

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