Conclusive proof gearboxes are awesome.

[Arlo1]

There is one argument for having a multi speed gear box as I think about this. In some cases the losses with 1 speed VS multi speeds will be the same. reason is they both will need gear reduction and if you use a gear box with oils and similarities to a multi speed gear box the drag or losses will be very close to the same depending on design. This still doesn't make up for the added weight and the loss of acceleration during shifting and the lower reliability.

I use one stage of reduction done with a chain and sprockets when I can for motorcycle/bicycle stuff with RC type motors but once you get to a higher power level you will have to use something stronger although the Nitro bike I help on makes around 1000hp and the chain lasts a few runs lol.

 

[turbo1889]

Well, I didn't read the entire thread just skimmed but I think I get the basics of the thread. Single gear ratio drive vs. multi ratio drive. My take on the whole discussion is pretty clear at least in my mind:

First of all, adding multiple gears may or may not decrease the mechanical efficiency of the drive. If the drive is not direct drive in the first place (direct drive wheel hub-motor or similar) then you have to go through some sort of mechanical gearing ratio change anyway. Gear box, chain drive, belt, and/shaft drive you usually have to use something unless your putting the motor right in the wheel. Adding multi gearing ratios to that combination may or may not decrease the efficiency and if so not by much, a good example is the multiple sprockets and chain-rings on a mountain bike with a derailer system. Still the same chain running between two sprockets, the only additional drag in the system being in the two idler sprockets in the rear derailer arm and the condition of the chain makes more difference to the efficiency then the small drag in the two idler sprockets at least if their bearings are in decent condition. So, yes, adding multi ratio capacity to the drive system can certainly decrease the mechanical efficiency but the provided its done correctly its marginal.

Secondly, lets talk electric motors, in my opinion a whole lot of people are abusing their motors. Certainly if you allow unlimited current on the motor end of the controller you can get impressive amounts of torque in an electric motors bottom end but by doing so you are literally using your motor as a self destructing electric heater that is heating itself at its core potentially to damaging levels if allowed to do so for any extended period of time. Next the motor is running at incredibly bad efficiency levels at its bottom end even if it does have the torque. At a dead stall an electric motor has exactly 0% efficiency with 100% waste in the system. Even running your transmission dry without lubrication your going to get better then 0% efficiency and you don't have to drag a motor down into its lower RPM range very far to get some pretty nasty efficiency levels. For example a 3-Kw scooter hub motor I've got a specs sheet for that has an 85-87% efficiency when it is in its top end power and efficiency range in the 410-510 RPM range but if you drag the RPM numbers down to 300 RPM by loading the motor down the efficiency drops below 50%. That means that over half the power your putting into the motor is just heating the darn thing up trying to burn itself out not to mention that this also means that if you climbing a hill and your speed gets dragged down as a result so that the motors RPM is reduced to that level then you now have less then 1.5Kw of usable power to help you climb the hill not the full power available from the motor which is better then 2.5Kw of usable power at the road contact surface after you account for the motors efficiency when its running in its efficient RPM range and hasn't been dragged down too low. Even if you build a transmission between that motor and wheel that only has a 70% mechanical efficiency (a very bad inefficient design) you will still come out ahead of the game and have a better overall efficiency and more power at the wheel when climbing that hill then if you are running the motor direct drive.

Long story short, if all you care about is the performance at top speed and he aren't ever intending to climb a hill or fight a headwind then, yes, going direct drive with no multi drive ratio capabilities is your best bet for maximum efficiency. But if you happen to live in the real world where there are these things called hills and this thing called wind and you want to be able to deal with them effectively for a small reduction in your top end performance then having a multi speed transmission (or more correctly a multi drive ratio transmission, speed really hasn't much to do with it, its a matter of loading).

Here are some spreadsheet calculations I did a few days ago that shows the difference in performance between a single ratio direct drive ea-bike and a 3-speed (three front chain-rings with front derailer only with no rear gear spool eliminating the inefficient and easily damaged rear derailer arm and replacing it with a single large diameter tension idler mounted half way back on the chain stay gently pushing up into the bottom of the chain loop to keep its tension and keep it higher up out of the road grime and improve the over-all chain geometry with more contact arc on both the rear sprocket and front chain rings) ea-bike using the exact same motor with the exact same motor specs on the exact same bike under the exact same sets of conditions with the only difference being one is single ratio (more commonly called single speed) drive and the other has three selectable drive ratios via the front chain rings with a chain tensioner.

I did take into consideration the inefficiency of the gearing setting the mechanical efficiency of the large reduction (13.5:1) double stage roller chain reduction between the motor and the crank at a total cumulative 90% with the main chain driving at 98% efficiency due to the improved tensioning mechanism using only the three front chain rings for gears since the front derailer only adds drag and inefficiency when changing gears and none otherwise. These are realistic figures for quality well lubricated off the shelf bicycle components.

I set three different test conditions, first set is climbing a 5% grade hill with an extra 40lbs of cargo weight on the bike, second set is climbing a 1% grade against a 10mph head wind, third set is running flat with no extra cargo weight or a head wind or tail wind.

I did open up the motor side current limit on the controller to 30A on the direct drive bike in the spreadsheet simulation instead of the 17.5A motor side limit I normally have on this motor with a 15A battery side current limit. Setting a limit on the motor side of the current as well as the battery side is really a nice thing to do so as not to torture your motor in the lower end where you get current multiplication due to the reduced RPMs but it also limits its torque in the low end and I had to raise the limit for the direct drive otherwise it wouldn't even climb the hill in the first set of conditions:

Single Drive Ratio (1:4.5 motor ratio, 3:1 pedal drive) =

Three Drive Ratios (1:8, 1:6, & 1:4.5 motor ratios, 1.6875, 2.25:1, & 3:1 pedal drive) =

As you can see by both the increased speed and the increased peak "sweet spot" range results for the first two situations having just three drive ratios instead of only one greatly improves the performance when climbing hills and bucking head winds. It does so by keeping the motor in its higher efficiency range more of the time and not having it get dragged down into the very inefficient bottom part of its RPM range. Granted the top end performance and range are not reduced in that simulation due to the fact that I was simulating a situation where I had to use a mechanical gear box with its mechanical inefficiencies to gear down the motor to the wheel anyway so I wasn't adding any noticeable inefficiencies to the system by simply having three different drive sprockets for the motor available. A little extra weight yes, but nothing beyond that. With higher power more complicated builds obviously that isn't always the case especially if we are talking an automobile type transmission, especially an automatic hydrolic type which usually can't even come close to touching anything above 80% efficiency even in the best of circumstances so obviously something of that scale where we aren't talking just moving a chain around on different sprockets with a front only derailer assembly where the only added constant inefficiency is in a single chain tensioner idler sprocket mounted mid-way on the chain stay pushing gently up into the bottom of the chain to hold its tension, that's a small price to pay for having the smaller chain rings available to have the lower gearing ratios for climbing hills and bucking headwinds to much better effect.

The over all results get even better with only a small decrease in the flat land, no wind, no extra cargo "sweet spot" range if you run the calculations for a full set of gears including a rear derailer and rear gear spool as well as the front chain rings. I've got those calculations as well but I don't have them for this same motor as I used for the single speed cross comparison so it wouldn't be an apples to oranges comparison to include them as well in this post.



Long story short, everything I have seen in my spread sheet calculation analysis shows me that provided you can build a clean high efficiency transmission to give you multiple gear ratios (mutli-sprocket spools on opposite ends of a single roller chain loop with derailers to move the chain up and down the spools with a chain tensioner n the non-load leg of the loop systems are among the most efficient possible often with mechanical efficiency in the 95-98% range) where you loose less in the transmission gears then you would if you allowed the motor to be dragged down below its efficient RPM range having multiple gear ratios in the system is an excellent thing that improves performance and extends range under real world conditions. For light vehicles, especially if you can use roller chain on multi sprocket spools to achieve the multiple gearing ratios that isn't hard to accomplish, especially when you consider that with the efficiency curve on many of the motors we use you don't have to drag the RPMs down very far at all before you end up in the sub 50% efficiency range on the motor. As I said the scooter motor I've got a full specs sheet for that was intended and designed for a peak efficiency and power output in the 400-500 RPM range only has to get dragged down to 300 RPM or less before its efficiency is in the sub 50% range. I think a whole lot of people don't realize how quickly the efficiency on electric motors goes to ^#%#%^#@ when you start dragging them to much less then their designed power band under load which is exactly what happens when you start climbing hills and bucking headwinds with a single gearing ratio electric vehicle.

 

[Hillhater]

same debate happening in the "Tesla vs Viper drag race" thread.
a few points to comment on..

Top fuel cars have no issue running one gear ratio.....!

One ratio AND a complex "slipper" clutch that effectively gives them a CV drive with full lock up about 300yard down the track !

...
For ultimate purpose-built EV bike/motorcycle performance or EV performance etc, a transmission will never have a place

Ultimate performance..? drag strip maybe ?
That has yet to be demonstrated.
The 2 fastest EV drag cars last year both ran 2 speed transmissions.

I find it significant that so many of the new builds on this forum are focused on mid drives, through the rear cassette , .. which presumably people are finding suits their usage better than a fixed gear DD hub motor ?

 

[Punx0r]

I'd agreet that adding a clutch to an electric drivetrain just seems perverse

Shifting times on modern sequential gearboxes are so short as to be negligable.

 

[jonescg]

Here's a simple test.

Jump on a bike with a 7 or 9 speed cluster at the back. Normal bike, not an e-bike.

At a set of lights or a stop sign, start your stopwatch and take off in the lowest gear and as quickly as you can, bang your way up the gears until you are in the highest gear, and pedalling at top speed and stop the stopwatch.

At the next set of lights, stay in that highest gear. Repeat, and stop the watch when you are pedalling at top speed.

Assuming you had a good rest between take-offs, you will probably find that the geared take-off wasn't really all that faster than the non-geared takeoff at reaching top speed, but you sure felt better doing it as the forces on the chain and your legs were noticeably milder.

So from a motor stress perspective, yes, gears definitely stop you from working your motor as hard. But the space and weight taken up by that gearbox could easily be taken up by a more powerful motor, and you're back to where you started.

And for some, belting through a gearbox just feels fun

 

[Arlo1]

One ratio AND a complex "slipper" clutch that effectively gives them a CV drive with full lock up about 300yard down the track !

This is not even close.

The clutch in a Nitro car/bike turns the power into heat if its at a 50% slip its 50% heat. The nitro bike I help on has to get water poured on the clutch at the end of every run.
The reason they need the Clutch is because a ICE Nitro engine included can not run at 0rpm and can not self start off the line at 0 rpm and makes 0 ftlbs torque at 0rpm.
The clutch lets the motor stay in its power band as it slowly engages. A cvt actually changes the ratio in which the motor is geared to the wheel but with 0 slip so when set up poorly very little losses to heat.

I understand how some can think a slipping clutch is like a CVT but its not even close.

 

[bearing]

Even if the motor isn't saturating, increasing phase currents make for terrible efficiency.

If the efficiency during acceleration is only 50%, it's not going to affect the range very much, unless you are racing.

The higher RPMs in the lower gear will cause no load losses. Those losses will probably add up to more than the I²R losses if the rider stays in a low gear while cruising.

 

[gwhy!1]

I think there are a couple of things to think about when it comes to multi gear setups for use with electric motors, Most peeps are thinking along the lines of a ICE transmission setup and using some sort of synchromesh system to enable quick selection of gears , but what most peeps are forgetting is that we do have to deal with all the draw backs of a ICE so the GB can be something that is much more efficient, faster shifting and stronger than a 'normal' gear box. A slipper clutch is nothing like a CV as Aro1 has said , you will need one monster of a slipper clutch to handle any real amount of power/torque/heat and I will not be really viable on it own, you can incorporate it with a CV and this then makes it a very good option, I think a slipper clutch is only really needed for reliable brushless motor , sync startup so it can be replaced if using a sensored startup and a very good torque control controller.

 

[Chalo]

Seems like if the motor has enough torque capability over the entire speed range of interest, the fundamental problems you might try to address with a mechanical transmission could all be taken care of with a suitably sophisticated controller. It's just that most of us don't have one of those.

I wonder how much cost and efficiency loss would be incurred by a controller that could vary both current and voltage to the motor independently? One that would limit target current as a function of throttle position (in an e-moped) or crank torque (in a pedelec), but then correlate wheel speed and torque (current) demand to a motor-specific map to set the instantaneous voltage? Less cost and loss than a good reliable mechanical transmission, I bet. I'm imagining a controller that for every combination of current demand, motor speed, and particular motor, has a reference value for voltage (up to the system's limit) to deliver optimum efficiency and performance. It would not be a bad idea to integrate some kind of temperature sensing into the system too, to protect the motor automatically without just shutting it off entirely.

This is a less sophisticated control problem than what cars and motorcycles have for their ignition systems these days, because there are fewer relevant variables to correlate. Seems like it would just take some relatively fancy power electronics and a little more state sensing and motor mapping than what we use now.

At Active Power, we programmed FPGAs to do this kind of real-time control on the flywheel energy storage machines I used to prototype. Those machines' control systems had their work cut out for them. For each flywheel, the device would accelerate the rotor at a steady power until the target rotor speed was reached, then maintain that speed until there was a blip in the power supply. In that case, within milliseconds it'd turn up the field coils and draw enough kinetic energy from the rotor in the form of wild AC to support up to a 300kW load for as long as the rotor's momentum held out. And that was only the flywheel side of the system. On the supply and load branches, the machine would take in whatever noisy power came in from the utility (or the flywheel), rectify it, and then electronically synthesize a nice clean waveform with steady frequency, even compensating for changing and noisy loads on the line. The largest of the machines I worked with had four flywheels, and could condition and support 1.2MW of line power. Those were about the size of 2-3 commercial kitchen fridges standing side by side. But unlike fridges, they had to be bolted directly to the building's foundation! (Reaction torque, y'know.)

I reckon a better, smarter speed controller is the second lowest hanging fruit, improvement-wise, in the e-bike and light EV world-- after getting some reasonable batteries (dammit). I sure wouldn't mind having a bike that smoothly, quietly and efficiently added three or four times as much power as I make at the pedals, irrespective of speed or conditions and without active management on my part. Add a strain sensor to the motorized wheel's brake (maybe embedded in a brake pad), and it could work the same kind of invisible magic with electrical braking.

A speed controller like that would be fully transferable among different vehicles, too-- only the input device to set current demand (and the individual motor's load/speed/voltage map) would change.

 

[amberwolf]

If the efficiency during acceleration is only 50%, it's not going to affect the range very much, unless you are racing.

Or riding in stop-and-go traffic.

 

[grindz145]

And for some, belting through a gearbox just feels fun

After all of the valid technical discussion, I think this will be the last say in the success of a gearbox. Whichever one is more enjoyable to ride.

For my money I think I'd rather have the reliability, but if pound for pound a gearbox prove to be more enjoyable, that will determine it's fate.

A single speed properly sized motor, may just be more fun than an equivalent vehicle with gearbox though...

 

[turbo1889]

If the efficiency during acceleration is only 50%, it's not going to affect the range very much, unless you are racing.

Or riding in stop-and-go traffic.

Or climbing a hill steep enough or bucking a head wind strong enough that your motor can't ever spool all the way up into its power range and is "stuck" constantly trying to accelerate at 50% efficiency just to maintain speed. For an ea-bike intended for 25-mph top end speed with a single gear ratio that only takes about 3% hill grade or 10-mph head wind to start pushing the motor down into its low end. By the time you hit hills steeper then 5% or head winds of 20mph or more you are seriously messed over if you don't have the ability to gear down on that level of performance ea-bike and your going to be making a toaster oven out of your motor and draining your battery pack in short order.

 

[Hillhater]

One ratio AND a complex "slipper" clutch that effectively gives them a CV drive with full lock up about 300yard down the track !

This is not even close.

The clutch in a Nitro car/bike turns the power into heat if its at a 50% slip its 50% heat. The nitro bike I help on has to get water poured on the clutch at the end of every run.
The reason they need the Clutch is because a ICE Nitro engine included can not run at 0rpm and can not self start off the line at 0 rpm and makes 0 ftlbs torque at 0rpm.
The clutch lets the motor stay in its power band as it slowly engages. A cvt actually changes the ratio in which the motor is geared to the wheel but with 0 slip so when set up poorly very little losses to heat.

I understand how some can think a slipping clutch is like a CVT but its not even close.

Define CV trans..

A continuously variable transmission (CVT) is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the input shaft to maintain a constant angular velocity over a range of output velocities.

..And what exactly does a slipper clutch do ??...lets the "motor" stay at optimum rpm whilst the wheelspeed slowly increases.

Sure a lot of the power is wasted as heat.. ( but dont all transmissions , including CV's, have friction losses that end up as heat ?) so are you saying that because a slipper is not very efficient, its not effectively acting as a variable trans ??

Oh, and obviously you missed that .." effectively " .. prefix ?

Maybe i should have used the analogy of "Torque converter " ?

but what ever you call it ,..its enables the motor to be coupled to the drive train at different rpm ratios as required !
Doesnt that define a " variable transmission" ?

 

[toolman2]

nope, arlo's right a clutch can never output any more torque than its input, all it can ever do is waste some motor power away to heat

a gearbox/cvt have the ability to multiply torque (ideally by the same amount the speed is reduced by, and no power is lost)

and a torque converter, although mostly clutch like, has the bizarre ability to multiply torque also via vanes and fluid coupling, but is still pretty wasteful.

-we have to care a lot more about lost energy than a 4000kw drag car that prolly uses 20 to 30 000kw of energy to do it

overall i reckon its the perfect debate, cos at times it appears that a gearbox or no gearbox is clearly the best way to go, but to me theres no simple answer and it completely depends on what the machine is used for..

 

[Punx0r]

It's definitely an interesting debate.

Does anyone know the towing capacity for the Model S? I can't find it on Google.

OK, it's not a typical car you'd use for towing a caravan etc.

I'm figuring an average family car can pull something around it's own weight up a 25% grade at less than 20mph (steepest hill you're likely to find, using first gear).

 

[liveforphysics]

It's definitely an interesting debate.

Does anyone know the towing capacity for the Model S? I can't find it on Google.

OK, it's not a typical car you'd use for towing a caravan etc.

I'm figuring an average family car can pull something around it's own weight up a 25% grade at less than 20mph (steepest hill you're likely to find, using first gear).

A model S can rip the traction of sport compound wide 20" rubber apart while throwing tiny bits of asphalt up into it's rear wheel-wells (which you can hear the little pebbles hitting stuff, because it's so dead eerily silent while it's doing this). It's putting more torque to the ground than most any family car I've ever driven by a large magnitude. The only things comparable that can be crudely considered "family cars" would be like a new 560hp twin turbo v10 BMW M5 which can put down similar force against the ground from a stop, though the Tesla still edges it out in 0-60mph acceleration.

A model S would NOT have any trouble pulling anything reasonable for any other car can pull, it would just have it's range decreased obviously by the amount of added energy you're requiring to be dragging a trailer through the wind and accelerating it's mass when you take off etc.

Once you've pinned the throttle in a model S with the sport package, you will stop any form of doubting it's performance capabilities. It's pretty over the top fantastically fast for a street car. It's so fast, in my racecar, if I were to race one and miss a gear or something, I don't think I could come back from it and win, and if I botched a launch I don't think I could win, and the Tesla driver has nothing in the world to worry about when racing, just pin it try not to crash and it's going to deliver all the power the tires can handle to the ground all the way with no interruptions, just a silent smashingly powerful silent thrust that's hard to describe.

That brings up something extra odd to the table on this topic as well. The Tesla can't be making much power when you're taking off from a stop (because motor RPM is near 0). Mathematically it can't be making much more than say 30hp when you're below say 5mph with it. This might seem like a huge disadvantage vs a gasoline engine with a clutch in respect to launching the car from a stop. HOWEVER! The gasoline car with a clutch, as I've learned from dyno'ing them plenty of times and watching the power I put out starting the dyno rollers from a stop or nearly stopped and slipping the clutch to be in the powerband and launching the car, you don't put down jack-squat for HP with a gas car when it's nearly stopped either. That energy is just wasted as heat in the clutch, and when your tire speed is like 5mph, your dyno number, even on my racecar that's insanely fast, is still only like 10-15hp while at 5mph, and only slightly better, maybe 20-25hp while revving it up and slipping the clutch. This is because a clutch is NOT a CVT at all, and doesn't behave like one and should not be modeled like one. A clutch is more correctly modeled like a pull-down resistor, it can not MAKE or multiple torque, it just gives you a system to burn that speed difference between a zero RPM shaft and a thing that can't make torque at zero RPM (your gas engine), so you can rev that engine into an area that makes more torque, and then let the clutch slip, burning all that extra HP you made as frictional loss/heating in your clutch plates (I run a multi-plate dry clutch in my racecar), so your engine doesn't die. This is why even though a car like the BMW M5 has more HP and torque and RPM and it's lighter, it STILL can't beat the Tesla in a 0-60 race, because it doesn't really matter if your engine is capable of generating 560hp while the car isn't even moving just by holding the throttle down, because it comes at an RPM so far off from your wheel drive speed, all you're doing is turning that extra power into heating your clutch or transmission fluids (if you're a slush-box). You're not putting down any more power than the Tesla is with it's electric motor starting out at 0rpm, and it doesn't have the time delays of shifting to literally just be wasting it's time it could have been spent accelerating.

 

[Arlo1]

Here is a picture of a BWS scooter with a pipe and big bore I did for my fiance and the run was just last night as I am getitng it ready for the year. You can see what Luke is pointing out at 0 rpm you make 0 hp This is with BOTH A SLIPPER CLUTCH AND A CVT (that's how most ICE scooters work). Hp is a measurement of work being done Torque is a measurement of twisting force you can calculate one from another but never confuse the two for each other. Without movement of the wheels you will always have 0hp The slipper clutch on the scooter will slip until about 4000 rpm then engage this sudden engagement is shown by the burst of torque in the graph its there but only for a split second. I would try harder to smooth that out but I choose a clutch with heavier shoes to make it grip better because the last racing clutch I built for her was slipping at full rpm!

 

[toolman2]

yep ive seen this too, even the regular tesla here in oz had this bizarre silence flat out off the line accept for the bits of tarmac pattering the rear guards.

real impressive, and it would have no problems towing, but its an induction motor and a bit like a series wound dc brushed type -is unlikely to need gears, because they can do true field weakening and get to very high rpm without much loss, the tesla motor does over 10 000rpm (i think) and thats like revving a turnigy 80-100 to well over 20 000rpm.

for us its a different story cos on a bike, to match a good 4kg bldc motors torque an induction motor would have to be around 10kg and even then none appear suited to us.
we are a bit stuck with our need for the highest torque/kg density motors (bldc) that have a more limited efficient rpm range.
-if we had ironless brushless motors that can rev high enough, then our need for gears would lessen.

 

[Hillhater]

nope, arlo's right a clutch can never output any more torque than its input, all it can ever do is waste some motor power away to heat

a gearbox/cvt have the ability to multiply torque (ideally by the same amount the speed is reduced by, and no power is lost)

and a torque converter, although mostly clutch like, has the bizarre ability to multiply torque also via vanes and fluid coupling, but is still pretty wasteful. ..

Well i agree the slipper ( and fluid torque converter) cannot multiply the torque like a gearbox ...BUT..
Think what it is doing on that motor. ?
It is enabling a motor that makes jack squat torque at low rpm to rev up near peak power/torque and transfer a portion of that to the drive train , at a lower rpm.... So you are getting much more torque "at the wheels" than you would without the slipper.
So its serving a similar purpose .. IE:-rpm reduction and torque increase ( above what it would be otherwise)

Back to efficiency and power..
.should we not be talking "torque" ? ...since that is what motors produce, dynos measure..and what really counts.
Power is just a calculated number.

..The Tesla can't be making much power when you're taking off from a stop (because motor RPM is near 0). Mathematically it can't be making much more than say 30hp when you're below say 5mph with it

.. but if the driver has pinned the pedal, what torque is being produced, and what is the current draw on the battery ?..
.....at the limit set by the controller maybe,..IE:- maximum ??
If so, the motor may not be making much power...but it is using a lot.

EDIT.. here is the "S" vs the M5 drag race
http://green.autoblog.com/2012/10/09/tesla-model-s-vs-bmw-m5-drag-race-video/
..I think the M5 driver was sleeping on the line ! :lol:

Actually, there is something odd about that video ??
its a 0-100mph test,.. but the BM is clearly going much faster way before the end flags.
Other tests have clearly shown the M5 to be quicker and faster than the "S" over the 1/4
http://f10.m5post.com/forums/showthread.php?t=744322
http://www.autoblog.com/2013/01/28/tesla-model-s-sets-quarter-mile-record-for-quickest-production-e/

 

[flathill]

Real world
Street
the tesla crushes all opponents
Reaction time of the driver is one thing
Reaction time of the drivetrain is another
A combustion motor seems awesome
Until you drive a model s you dont even realize the lag time
between input and output

 

[John in CR]

Hillhater,

You trade more torque at the low end for less torque at the high end and add losses, weight, and failure prone complexities into the equation. Invest that extra weight into more motor and you come out ahead. What leads you guys to the incorrect conclusion is that you've yet to use a properly sized properly geared motor. Using motors with the power and torque belonging on a low speed kids toy of course leaves you wanting a transmission just to get started better. The part you miss is the horrible acceleration you'll have as soon as you shift gears. Trading to get a few seconds of more performance on the launch to kill performance the rest of the ride is a terrible compromise.

Your problem is your lack of motor, not a lack of a transmission. Yes, I use a bigger heavier motor than most want, but my motor is also pushing a heavy load at almost 400lbs all up. It's also geared higher than most want with a top speed of 65mph, yet I still have more torque on launch than I can really use, and once I get out of the hole riding is pure pleasure.

That's with a single speed. A proportionately smaller load can get away with a proportionately smaller motor. The same goes for speed, ie A proportionately lower top speed can be geared proportionately lower and get away with a proportionately smaller motor. These proportionately smaller motors, need to be of similar high quality and efficiency to get those results. That's where the typical motors most guys use fall flat on their face. Their cheap construction results in crap efficiency which limits performance when cold and limits them even more once warm, so they end up using motors of almost the same weight for far lower performance despite pushing a far lower load. To make matters worse the cheaply constructed motor cost more. You don't need a tranny. You need a better motor for the job.

 

[liveforphysics]

Hillhater- Its not using a lot of power to be ripping up the asphalt right from a start.

Its likely getting 20:1 or 30:1 leverage on its phase current to battery current used as its starting out. This is because its only loss is the same loss its having when its making this amount of torque in its peak efficiency range, only now it has even less losses, cause its not having all the additional core losses of turning while its making this torque.

Thats the whole thing with an electric motor. Youre going to be using the least energy ind doing whatever has the least losses. If you're doing something crazy like reving a motor up to its peak efficiency RPM to take off from a start, and then slipping a clutch or running through a lossy CVT or something, you're just going to be adding losses over just generating the torque you needed at the motor from the lowest RPM it can make the torque from


This is why a solar car is a low RPM direct 1:1 drive hubmotor. Because a slow turning direct drive motor can be both the most durable and mechanically simple, but also the most efficient.

But let's not forget what efficiency is in a motor and why we talk frequently about getting it into its "efficient range" and all that. Let's look at a PM motor, because that's what 99% of us run here. To generate a torque, you just n3ed to pay the copper loss of the tiny voltage across your stalled rotor to generate the phase current across a winding with 20mOhm or whatever. Say its a controller thats going to pump 100A into that 20mOhm phase lead to make the torque we want. Thats 2 volts of resistive drop, times 100A, so 200W of power to make this torque we need at a stalled rotor and 0% power out (no rotation, but a the force we wanted).

Say I rev the motor to its most efficient range (which is the point core losses = copper loss always). Now you slip a clutch and couple this shaft to your stationery non-rotating axle. Now you've got the motor at its 95% part of the curve rather than its 0% part of the curve, but now core loss equals copper loss, and you're spending exactly twice the energy from the battery to generate the exact same torque at the stationary wheel to take off, AND now wasting as much power feeding core losses as are actually generating any torque.

Remember, the relationship between torque and efficiency and core loss is one that means, if you simply reduce a core loss (lets say changed out bearing for a lower drag one), then you now just shifted the peak efficiency point of the motor to be at a lower torque output level. This doesn't mean its any less capable of making torque, its actually more capable now that its got some tiny tiny bit less heating adding to it from some bearing drag, but now the peak efficiency point has shifted to be at a lower torque output level, so going by peak efficiency motor tracking, it would now recommend using less torque from the motor, even though you're using less energy and making less heat than it did before.

Motors are amazing torque generating machines. Size them appropriately for the task and let them make big torque right at the useful speed for you to drive something like a wheel with, and use it directly. Thats always going to be the ultimate least energy consumption way to do something, even if at any point of looking on the peak efficiency curve it looks like its in the dumps all the time. Its only in the dumps because its not wasting a bunch of energy on core-losses to make the math balance. It doesn't mean you're not making torque very effectively (not power perhaps, because it's turning too slow, but torque yes, and at low speeds you don't need or use much power anyways, even if everytime you want to launch youre ripping up asphalt with your tires, youre still not using jack-squat for energy to do it. If I want to see 660phase amps to my motor from a stop on deathbike, it costs me under 20A off my 100V pack to do it. My efficiency is zero, but its hardly any power that Im using to create the torque I need, just a mere 20A to generate 660A of torque creating huge force at the motor with basically zero added core losses while Im still at 0rpm.

 

[Hillhater]

You trade more torque at the low end for less torque at the high end and add losses, weight, and failure prone complexities into the equation. Invest that extra weight into more motor and you come out ahead. .

Well John, you could of course use another gear to reduce the rpm at higher speed to keep the motor in the higher torque range.
(You have seen how the torque drops at increased speed ?)
And most of my comments are related to "large" car sized EV's ..the "warp", 9" -13" brushed motors. as used by the EV drag racers, where they are using 3-400v and 2000+ amps, dual motors, AND transmissions.
Infact some of those guys use the trans in the way i described above,.. as an "overdrive" to give a higher top end speed.

 

[circuit]

IMHO, gearboxes are good, but only if they can shift quickly.

 

[toolman2]

agree with that circuit, ive ridden an espire (i think is the same one that Hollywood idiots like to be seen on for $70k) and with a top speed of 45kph in 14th gear (rollof 14speed hub) so this gives you a whopping 3.2kph of speed PER GEAR, and it just gets better, cos if you didn't lift off the throttle for each one it would soon strip the rollof. so accelerating to just 22kph requires 7 careful lift off's/change/back on it.. its funny to watch a 6 year old kid peddle a bmx and leave it for dead.

Motors are amazing torque generating machines. Size them appropriately for the task and let them make big torque right at the useful speed for you to drive something like a wheel with, and use it directly. Thats always going to be the ultimate least energy consumption way to do something, even if at any point of looking on the peak efficiency curve it looks like its in the dumps all the time. Its only in the dumps because its not wasting a bunch of energy on core-losses to make the math balance. It doesn't mean you're not making torque very effectively etc

-this is pure gold,
im just not 100% on this bit luke. 'Thats always going to be the ultimate least energy consumption way to do something'.
cos you know that if you lost say 3 teeth off the motor sprocket, the machine would be way more efficient (like double) at climbing steep tricky hills.
and you know that if you gained 3 teeth on the motor sprocket it would have a higher top speed and cruise more efficiently.

-so we normally compromise with gearing set somewhere in the middle.

what if instead of compromising, you could have either ratio with roadbike shift speed and 97% efficiency, would it still be all bad?