Ironless axial flux Halbach project - 170mm, 36 pole, 44mm bcd disc brake mounting

Discussion in 'Electric Bicycles' started by madact, Jun 12, 2010.

  1. madact

    madact Member

    Here's the concept - simple build, motor cost in the ~$500AUD range (plus elbow grease and/or machinists fees). Built on the pattern of the CSIRO motor (, but with a somewhat lower operating frequency (about a quarter, in fact), as I can't be bothered mucking about with litz wire...

    - Twin 36-pole Halbach array (144 magnets to mount :sweatdrop:)
    - NIB N42 1/2" x 1/4" x 1/4" magnets
    - Standard 6-bolt brake discs as both stator mounting and magnetic backing (Halbach array isn't perfect, there's still a few stray field lines to catch)
    - Mounts directly to a disc brake hub
    - O.D. of magnets is ~170mm, meaning the magnets may need a touch of grinding into wedginess, to fit nicely... or I just let them hang out a couple of mm...
    - Pole to pole gap ~5mm
    - Aluminium spacer to keep the poles apart, vented at the periphery for cooling

    - Double-wave wound (to keep the flux nicely confined) around an inert core about 2mm thick - putting the conductors in the airflow and near the magnets, and leaving the middle of the gap (where flux is weakest) for the core...
    - Mounted to hub via a nice big bearing which fits outside the disc brake bolts - I'm thinking a 6812 or thereabouts. Large diameter bearing should make it easier to eliminate play and shorten the air gap :evilgrin:
    - Held stationary to the frame by a bolt-on arm, which also allows for some frame flex.

    Magnets (and rotors) are already on the way :whistling: so it's a bit late to change the pole spacing, but I'm interested to know anyone else's experience in building this manner of motor...
    Last edited: Jun 13, 2010

  2. madact

    madact Member

    Some diagrams etc.

    A cross section of the rotor, stator and spacer (very much not to scale)

    The layout of the Halbach array:

    A FEMM flux density simulation (with N40) at the outer edge, where the magnet-magnet gap is greatest:

    Attached Files:

    Last edited: Jun 12, 2010
  3. safe

    safe Active Member


    You should try the same simulation, but change the backing material from steel (brake disc) to aluminum or anything else that is inert. You will find that steel backing actually doesn't help much and in fact can even slightly reduce the magnetic flux.

    Have you done the math on a 36 pole motor?

    Will the stator cover 100% of the disc?

    My design is to use about 44% coverage and a wider gap and add some core material back in to enhance the performance a little. (at the expense of cogging)

    I like the fact that others are experimenting with the Halbach Design.

    Also... another thing... I just got done cutting the steel disc out of my project and replaced it with a smaller more rigid base. For a disc motor you really don't want the flex that these discs allow. It's pretty easy to just cut and drill a flat plate and use it instead. I'm doing mine in fiberglass, but carbon fiber would be better.


    How much power are you planning to run?

    And I'm still not completely clear on how you are going to mount everything. The stator mounts to the frame right? The disc carries the magnets and turns right? Or no?
    Last edited: Jun 12, 2010
  4. madact

    madact Member

    Actually, I've run it both ways, and found the reverse to be true:

    It seems what's good for the goose (dual-pole) is good for the gander (Halbach)... though there are a couple of caveats:
    - With a Halbach array, too thick a backing will, yes, actually reduce the flux, particularly on the outer edge of the rotor as in this case, where there is a gap between the magnets. With large gaps and thick backing, you lose quite a bit, but the simulations above are for my outer edge worst-case scenario, and I'm still showing a decent gain. I'm basically aiming to use unmodified or minimally modified stock parts wherever possible, so I won't be ultra-optimizing the backing thickness - "better than nothing" is good enough for me here.
    - If you put steel in the gaps between adjacent magnets, you kill the air gap flux right quick :ack2:.
    The academics like to not use any backing because this gives them theoretically better energy density etc. (if the magnets have a steel backing, they have to count it in their 'magnetic materials' mass :rolleyes7:), but in the real world we have to make some engineering tradeoffs, and consider that the magnets don't float in space by magic, and have to be held there somehow - a bit of steel here isn't going to break the weight budget.

    There are another couple of other reasons for having a steel backing, too:
    - this design calls for a pair of N42 Halbach-enhanced 1/4" magnets to be held North to South within 5mm of each other. IMHO, holding them in place with only epoxy on the 'sides' and 'back' is a recipe for crunch failure in a real-world application - and putting fiberglass or aluminium on the 'front' - in the gap - is somewhat counterproductive. The pull force of the magnets onto a steel backing significantly reduces the tension and shear loads the epoxy has to bear, if not eliminating them completely.
    - Catching those stray field lines makes me less likely to wipe my credit card when I check the pressure on the rear wheel.

    Not a great deal of it :thinking: - but I have worked out I'm going to have to drive the coils at 200Hz (real bandwidth in the region of 1kHz, using pseudo-square excitation) to hit 80 km/h, which is a good reason for ironlessness right there, and means I'll be building my own controller... I'll probably use 3-phase wave-shaped excitation (matched to flux profiles) for the low speed regime to get decent, smooth torque, and single phase pseudo-square for the high-speed regime, so as to only 'hit' the coils when they're in the maximum flux region, to maximise efficiency...

    I'll admit to having done a fair bit of "looks about right" in coming up with the basic specs :whistling:, based on experience with model car and plane motors...

    The coils will cover upwards of 90% of the surface area, but will be <50% of the volume - they will be wound around an electrically inert core, probably fiberglass PCB material. I was thinking about carbon fiber sheet, but the eddy current losses would turn it into an inductive heater...

    Seems like the best-suited tech for the problem, and the axial construction means the number of parts requiring use of a lathe or other specialized equipment is minimal...

    Congrats... BTW, carbon fiber isn't as expensive as you might think - take a look on ebay, search for "carbon fibre cloth -vinyl", but be sure to include "-vinyl" or you'll get a thousand stick-on decals :grin5: (this stuff might be suitable - it should be fine on the magnet mount, and as long as you don't put it in any gaps with varying flux, it should be fine for eddy currents, too.

    Only in the 250W region, which is what's legal here in South Australia without a license & rego.... This is why I'm not too concerned about the disc brake rotor flex, and the chances of me exceeding half the rated torque of the disc brake rotor are non-existent - this is only a power-assist motor, after all, if I get 2Nm out of it I'll be quite happy. I will certainly be seeing what the motor as built can do before cooking the epoxy, though, and higher power outputs at higher speeds are certainly on the cards with a suitable controller...

    More-or-less. In the engineering terminology, the stator is static and the rotor rotates, and you can put your coils and magnets wherever you like :grin5:. However, the stator, while bolted to the frame, will be mainly mounted to the rotor through a big ol' bearing, the attachment to the frame will be designed to only transmit torque (and wires, of course) - it will be rigid in the direction of wheel rotation, but able to freely swivel left and right a bit.

    Attached Files:

    Last edited: Jun 12, 2010
  5. safe

    safe Active Member

    I'm having a hard time imagining how the bearing interacts here... I'm still not getting it.

    Maybe by comparing what I have in mind you can tell me the different way you are doing it by contrast.


    What I'm doing is making the rotor/disc be made of that steel base part and then fiberglass on top of it. Out on the ends of the fiberglass will be a "U" shaped groove that faces OUTWARD and in that groove will go the Halbach Arrays in parallel with each other. (same as yours) My stator will be attached to the frame and penetrate the groove from the outside facing inwards in the radial direction. You can see the stator mount I've already prepared on the frame. The stator would "insert into" about 44% of the disc's "U" groove. I don't require any added bearings this way.

    From the drawing it looks like you are doing a reversed "U" facing inwards and then I sort of lose where you go next.

    Explain more... I've read all the CSIRO stuff and they place the magnets on the wheel and then hold the stator stationary inside the wheel... my guess is that you are more closely imitating the CSIRO somehow? But where does the bearing go?


    And I agree on the steel plate issue... it's normally not a big difference either way unless you push iron in between the magnets.


    Another question...

    How do you plan to get the level of precision required to achieve such close tolerances?

    Part of my design effort was to admit to myself that I do not have the high tech tools to create a narrow gap and that's in part why I went with the extra cores because it relaxes the tolerences needed a great deal. Unless you have professional shop equipment it might be hard to achieve such high precision.


    I ran a quick simulation on a spreadsheet I've developed using my best guess about what you are up to and arrived at something that looks like this:


    ...I'm sure that with more tweeking you could get more accurate predictions and better results, but it does seem to suggest that you are in the ballpark. To get those last few percentage points you really need to run a lot of simulations because getting a configuration that is just slightly off means that you are in effect running the wrong effective gearing. The odds of just "getting lucky" with this stuff is pretty low I think. No doubt you know that thickness of wire and number of turns defines the effective gearing.

    I didn't realize at first you are using the 1/2" x 1/4" x 1/4" magnets. I got the 1/4" cubes so what you got is a better starting point. It takes twice as many of the smaller cubes to equal your "doubles". If I were to start over I'd go for the "doubles".

    Attached Files:

    Last edited: Jun 12, 2010
  6. safe

    safe Active Member

    Didn't manage to follow up on this previously...

    Yes, I see your point on this. I found that I could setup a Halbach Array on my brake discs and they wanted to stay in place without any need for adhesive. It has it's advantages that's for sure.

    What I'm planning to do is have 1/8" gaps between the magnets and you would think that it would harm the magnetic field strength (and in fact it does a little) but it also INCREASES the disc radius at the same time because of the added spacing. The advantage of the larger radius is when you get to the torque calculation it increases and so it pays for itself. Plus, as an added bonus, the extra space allows for more copper and that's the path to improving the efficiency. In a "perfect world" you would have an unlimited budget or selection as far as magnets, but I was more or less designing around 1/4" cubes because you can get 100 of them for something like $25 on ebay. I got 300 for $75 so I can do 72 poles and the number comes to 288.

    [​IMG] for assembly I plan to epoxy all the north / south poles in place first, then epoxy the halbach cubes in the gaps afterwards. That way the Halbach cubes are trapped in a box and the 1/8" of spacing means that there is more epoxy in place to keep things tight. Also, it's probably a good idea to use some sandpaper on the magnets to dull the coating a little. Epoxy should glue better if it has more to grip.
    Last edited: Jun 12, 2010
  7. madact

    madact Member

    So yours would have the stator in red here (if half the rotor were cutaway)?

    Got it in one - whereas your stator pushes in from the outside, mine will push out from the inside. Here's a more complete schematic (once again, very much not to scale - I need to get me some 3D CAD software working...)


    The bearing mount (lime green) and the rotor (dark green) bolt onto the hub to the right. The fiberglass stator core (black) bolts onto stator mounting (orange). The stator mounting (orange) then has an arm attached to the left, which attaches to the frame, much like the arm on a coaster brake.

    The beauty of axial flux is that all the really close tolerances are in the axial direction - in theory, I could make everything I need by plasma cutting it from sheet steel and rounding it off with a file with the parts in a drill press, given a certain amount of laissez-faire on the subject of limb retention :goofy:. One could centre the bearings using bolts with plastic spacers - a small amount (i.e. a couple of mm) of radial wobble is fine, it's axial wobble which is the killer here, and squeezing a bearing race between two bits of sheet steel gives a pretty flat rotation in the plane of the sheet steel...

    It would be nicer to use the right tools for the job, though, and machine the bearing mounts in aluminium... I have the machining skills, the trick is getting access to a lathe. I have at least one friend who has a suitable machine - I'm keeping the machining requirements to an absolute minimum, though. Also, getting the magnets all lined up nicely may require milling the spacer on a rotary table - I may have to renew some old ties with machinists at the local uni for that... otherwise, it's a dead simple job as milling goes, there are plenty of small engineering shops that could do it without costing an arm and a leg...

    Attached Files:

    Last edited: Jun 13, 2010
  8. madact

    madact Member

    I'd be interested to grab that spreadsheet off you - I presume that's using sinusoidal voltage 3-phase excitation? I'm planning on doing something much fancier with power delivery (I iz mode-D-amplifying with a buck convertor in u'r statorz :grin5:) - the waveform used will depend very much on the simulated and measured EMF curves of the system, but will probably end up being close to a 30-50% duty cycle square wave - say, for 40%, 20 +ve / 30 neutral / 20 -ve / 30 neutral. No point pumping current through wires with no back-EMF, right? Also, it's a touch unrealistic to feed the thing >250W all the way from a standing start, that's just asking to turn the wheel into spaghetti and/or burn out the coils, whichever comes first... I'd be feeding it a curve more like 40W at 5mph, 80W at 10mph, 120W at 20mph, 260W at 40mph.

    I have a baggie (a few tubes, really) of silly-low-resistance silly-fast-switching trench MOSFETs lying around (P50N06L - 50A, 60V, 0.022 Ohm), which are nice enough, but not ideal for voltage handling - this transistor choice also requires smaller turn count / lower voltages / higher currents - a limit of about 13S (with over-engineered transistor protection), probably 10S, realistically. Smaller turn count gives more room for fast switching, though. The geometry constraints mean I have to pack all my conductors into 8.5mm per phase, too. That graph is for 22 turns per pole, yes? Not going to fit, I don't think...

    My current plan is for a paired wave-winding of around 4 turns per winding (8 turns per phase), probably something like two strands of 22AWG in parallel, and relying on the transistors to step-down the voltage and step up the current - I'm going to need a decent inductor on each phase, too... (I've seen someone try this without an inductor - HF thermal shock and skin effect powdered the insulation of the magnet wire in the motor in no time flat - not pretty). As you say, the maths has a ways to go yet... I may just need to widen my gap to fit some more copper in there :thinking:.

    Yes, I am familiar with turn count & effective gearing - I was actually considering two windings run through a circuit breaker to switch from series to parallel (while suspending the drive current momentarily) - an electric gear change with a cha-chunk :evilgrin:.

    Mainly 'cos I really don't like what the magnetic fields of the square blocks do on the inside and outside edges in this configuration. Still, they do cost a bit more - I paid almost $120 AUD for 150 of em, including shipping.
    Last edited: Jun 13, 2010
  9. safe

    safe Active Member


    And now I understand what you are doing. By adding an extra bearing you do slightly increase friction, but it's mostly being used as a sort of alignment tool... as long as the bearing is in alignment the stator will be too.

    The trick on mine is that since the stator is external and the wheel needs to be able to be moved around to set the chain tension that I'll need the ability to adjust things in three dimensions in order to get it all to work. So it's not going to allow for a quick change if I get a flat. (would have to spend ten minutes getting everything realigned)

    Less drag though...
  10. safe

    safe Active Member

    Sounds like you have more experience with designing the controller part. I'm probably going to try to pick your brain on that when I get to doing mine. Since Single Phase produces a tighter copper packing (lowers the kv) all I need to do is make a single H bridge to implement a chip that I've found for doing such a thing. The chip has current limiting built in as a feature and that's something I felt was important. But I'll get to that after I finish the disc fabricating and the stator winding and mounting.

    Take a look at the spreadsheet. Keep in mind I've made certain assumptions based on the parts I'm using and the Single Phase design. In the upper right hand corner there is a section where I do FEMM compliance. (lower left are the results) The idea is that you enter a known current value into your FEMM simulation then get the "Force Via Weighted Stress Tensor" value and run that through the spreadsheet to arrive at the "actual" average magnetic flux. As you were mentioning some controllers use full "on" or full "off" current and others use sine waves. When I reintroduced cogging with small cores it skews the data so that it was harder to be confident of the results. It's nice to be able to go from the FEMM simulation directly to a graph and see the results. (it's a bummer that FEMM is not able to simulate a motors motion very well)

    Attached Files:

  11. madact

    madact Member

    I figure being able to tighten up the air gap is a good tradeoff. And the extra drag introduced by a practically unloaded ball race... has a pretty small care factor.

    To be honest, I haven't actually built that much, I tend to get caught up in 'paralysis by analysis' too often... much experience designing & simulating, little experience seeing if said designs let the magic smoke out :rolleyes7:. Most of my experience with power switching circuits is from building your average buck and buck/boost topologies and rebuilding a couple of PC power supplies to deliver ~10A at 14.5V (handy trick for testing car amps, RC gear etc, but I have a 'real' off-the-shelf DC regulated supply now) - I'm still a noob at motor controllers. I'll probably do my initial testing with a sensorless RC plane motor controller, and see if it can pick up enough feedback to fire the phases in the right order, then get into the controller design once I know I inserted all the magnets the right way 'round (or once I know the space ESC won't do the job)...

    Thanks for the file :2thumbsup: still working out how to use FEMM for moving coil simulation... I really want to do fixed speed, varying current on the coils (treating it as a linear motor), and integrate force, but the example scripts I've tried so far crash with lua errors (could be a wine thing) - will keep trying...
  12. safe

    safe Active Member

    In FEMM there is the ability to select areas and then get the "Force Via Weighted Stress Tensor" value from it. If you have coils in your simulation (or silicon steel cores like I do) you can select them and you will get a resultant force value. Since the coils are all identical you just multiply the force value times the coil count and can arrive at an overall force.

    Torque = Force * Radius the bigger the disc (rotor) the better leverage you get.

    Once you know the simulation results you can go back to the actual magnetic flux number and fine tune it. In mine I'm able to maintain about 1 Tesla while having room to lot's of copper. Without the cores I'd be forced to use very tight tolerances that I don't I could achieve given my fabricating abilities. I've done a lot of things to adapt to low priced magnets and low precision instruments. A professional shop could do this so much better than I can.

    In the spreadsheet:

    "Torque"="Current"*$Radius*$Depth*$FieldStrength*$TurnsPerCoil*$Series*$Parallel your case "Depth" is 1/2" or 200% of my 1/4".
    Last edited: Jun 13, 2010
  13. madact

    madact Member


    safe: Nothing around here that can open a .wks file :( can you save it as something else? A .xls would do the trick...
  14. safe

    safe Active Member

    Sorry about that... I'm on an old computer and that's an old Works format.

    When I convert to Excel it loses the Charting and the Range Names. In some ways that's a good thing... just go around the spreadsheet and define names for things as you go and that way you can figure it out.

    Things like RpmPerVolt once defined makes reading things easier...

    Attached Files:

  15. madact

    madact Member

    Yeah, I spent a bit of time the other day following the cells up down and sideways but it all makes sense now.

    I did promise some preliminary results... here's a scenario with 6 16AWG wires per phase (in 2 sets of 3) and an ~0.8mm air gap. At 12A per phase, 60% duty cycle bipolar, FEMM gives me an average of 2.73 N for 2 poles (manually accumulated over 10 points)
    which works out to 49.14N at an effective radius of 78.75mm, that's 3.87Nm.
    This is in roughly the right range - anything over around 5N and I'll be in serious doubt of my rotor and stator integrity
    Copper losses are around 25W at this current level. Makes me wish I could get square magnet wire in 16AGW...

    Long story short, with a 700C/25 tyre, this is giving a delivered power of 192W at 60 kph, efficiency around 88.5% ... still some optimization to go, there...

    Indeed you do - but perhaps you miss my intention... this motor is being designed as a high-speed cruising power-assist for a fully faired velomobile on the open road: 192W at 60kph should be far in excess of the total power required at that speed, if I get my aerodynamics right; this is the start of my envelope, not the end of it. At 60kph, typical motor loading would be 50-75W, and 'Long range cruising speed' will be ~ 90kph, with the motor operating at 100-125W (with the rider supplying another ~100W). At 90kph and 125W delivered power in this configuration, copper losses are only ~5W, for an efficiency of... 96% or so :grin5:... at 90kph / 200W, copper losses rise to around 13W for an efficiency of around 94%, and at 250W, maximum theoretical efficiency is still in excess of 92%... All that said, I haven't worked out how to model end effects on the inner and outer edges of the rotor yet, I'm just hoping simulation with N40 and no end-effect modelling will hold for real N42 and real end-effects...

    Next phase will be getting things running from octave to automate testing of stator parameters (conductor count and size, particularly) - I've looked at an example script, it doesn't seem too hard. I'd really like to do it with one layer of copper, as construction would be greatly simplified, but I'll just have to see how it goes.

    An interesting effect I noticed is that force does not vary linearly with current in the simulation... turns out I had a small offset in my force calculations with 0 current in all circuits. Looks like reducing mesh size in the airgap tightens it up nicely.

    In other news, a pic of the rotors which will be used in construction of the... rotor:
    Promax 180mm (DT-180C) - the brake surface has I.D ~145mm, OD ~180mm at the pointy bits, so it should be just about right... the drilling happens to have 36 holes in each ring, too - which is serendipitous for lining up a large number magnets at 5 degree increments: the poles and halbach magnets will line up with the peaks and troughs of the outer edge...

    Lastly, I've realised another factor which may make tight tolerances more bearable - if the coil timing is slightly advanced under heavy load, the windings will pull towards the centre of the gap - the side closest to the rotor will be pushed away more strongly, producing a magnetic bearing effect which hopefully might keep things apart if they start flexing at high torque. Conversely, if the timing is slightly retarded, the coils will pull the stator towards the sides of the rotor. The opposite is true under braking. Methinks I shall definitely be building my own controller, with sensors - this is not the kind of thing I trust to an off-the-shelf controller to be designed for (a sensored controller with an adjustible hall effect sensor might do the trick, but I'd rather not risk it).

    Attached Files:

    Last edited: Jun 18, 2010
  16. safe

    safe Active Member

    Looks like we lost a few postings.

    It's so darn hot here these days it's hard to do anything past 11 am. It was 87 degrees and about 70% humidity around that time this morning. The weather in the midwest is great for crops, but terrible for humans. (so I can't do much on mine when it's like this)

    About all I can do is get an early morning ride on my other bike and then that's it.
    Last edited: Jun 18, 2010
  17. madact

    madact Member

    Yeah, I updated my last post with a quote from one of yours that got nuked, and a reply... just in case you missed it...

    Pfft. When I went to school, hot weather go-home-early temperature for primary schools with un-airconditioned weatherboard classrooms ('portables' were quite common until recently) was 97F. That's about the point at which we start calling it a 'hot' day - it's a heat wave if it stays like that for a week. Hottest recorded temperature was 115F in Adelaide last year, but inland it can get hotter, I used to live out in the country, and it wasn't regarded as 'hot' until it hit at least 105F...

    Mind you, it's dry heat down here in Adelaide, I hear Darwin is far more humid - closest I've come to hot & wet is when I visited Seoul during a heat wave in '99, 95F/90% - it wasn't too bad after a couple of days to get used to it, but bizarre to see condensation on the windows of air-conditioned buildings, and we got caught in a bit of a deluge and it was like standing in a hot shower...

    I suppose the other thing which makes it bearable here is that we measure temperature in centigrade - 42C doesn't sound as bad as 108F :jester:

    Right now though it's winter, and not having a proper workshop, the rain is frustrating...
  18. pedalless

    pedalless Member

    The e-bicycle limit in all states of Australia is 200watt not 250watt


    The Motor Vehicles Act 1959, defines a power assisted pedal cycle to be a pedal cycle with an auxillary power drive attatched up to a maximum of 200 Watts.

    Part 2, Division 1, Section 9B, covers the exemption of registration for a power assisted pedal cycle.

    Part 3, Division 1, Section 25, states that a licence is not needed for a power assisted pedal cycle.

    Source: SA Motor Vehicles Act 1959.

    I hope to see this motor running soon best of luck
    with your construction.
  19. madact

    madact Member

    True - perhaps I was a little careless with my wording here (I was thinking in terms of motor design, not the system as a whole), I certainly don't intend to break the law. The speed controller will be tasked with keeping the power output at the rear wheel strictly within the legal limit.

    The reason I said "in the 250W region" is that 250W @ 60kph is what I'm using as my 'design spec' for the limits of safe operation of the motor, and it allows a little headroom for amateur build quality and hot weather. Because of the design of the motor, this will mean that technically, the motor itself could deliver 375W at 110kph, but that's down to the controller to put a 200W limit on it. At the end of the day, there is no such thing as a "200W" motor - I can buy a motor labeled "200W" at a bike store and hook it up to a battery and controller combo that would make it do 400W (for a while, anyway :rolleyes7:), and that would be illegal, just as I could go on ebay and buy a motor labeled "1000W", pair it with a controller which limited output to 200W and it would be legal - and likely somewhat more efficient (though I wouldn't like my chances of convincing a traffic cop of that, and going to court for no reason doesn't sound like fun - better than going to court for good reason though, I guess...).

    Before you point out that 110kph seems like something you'd require an illegal motor to achieve, I fully expect the vehicle in question to be capable of that on human power alone (and in real-world conditions, one might easily hit that on a slight decline - a speedo is a must)... if only for a few minutes, at my fitness level, which isn't spectacular. I know from experience that sustaining 70 to 80 for an hour or so in a similar vehicle is well within my capability. The motor is purely for extending cruise time, not increasing top speed... This is also the main reason I'm building instead of buying - the chances of a commercially available bicycle hub motor being efficient for velomobile speeds seems slim - these would likely be optimized for speeds of 30 to 60kph, not for speeds over 60.

    But thanks for your concern, and it's certainly good to see someone looking out for the interests of others.
    Last edited: Jun 19, 2010
  20. safe

    safe Active Member

    The laws alway deal with "output" power actually delivered and not "input" power introduced to the controller. So a 24 V 10 Amp controller might be allowing 240 watts of "input", but in most cases after losses in the motor you will have under 200 watts delivered to forward motion.

    This is why I've argued for off street racing classes to be defined in the reverse way to street laws. For racing you want equal power input levels and unequal output levels to give an advantage to people with higher efficiency. For the street the focus is on output without regard for how many losses occured to get there. Totally different mindset.