Motors and bicycles

I’m practicing some semi-technical marketing talk for Stokemonkey here. Does it work for you?

A direct current electric motor operating at a given voltage will spin up to a certain speed, called the unloaded speed. At this speed the motor produces no useful mechanical power. Once you hook things up to the motor, giving it work to do, the motor slows down. Eventually, if you give it enough work to do, it will stall in an overloaded state. The motor produces no power while stalled, either. Between the zero-power overloaded and unloaded speeds, the power produced can be plotted as a curve. power

The most power is generally produced at 50% of the unloaded speed. If maximum power is your only goal, your task is to arrange for the motor to operate at this speed as much of the time as possible. However, you will likely also want to consider efficiency, the ratio between the electrical energy drawn by the motor and the mechanical power it produces.

Just as with power, efficiency at a given voltage is tied to motor speed, and is zero at the over and unloaded speeds. Unlike power, efficiency is highest at a speed well above 50% of the unloaded speed. In Stokemonkey’s motor, it is at about 80% of the unloaded speed. power and efficiency

The band of motor speeds between peak power and peak efficiency is thus only about 30% the total range of motor speeds. Operating the motor outside this sweet zone will always result in suboptimal performance.the sweet zone

Now let’s consider a typical mountain or touring bike. It may climb up a steep hill at only 5 MPH in low gear, but it will come down at closer to 30 MPH in high gear. That’s a 600% spread in wheel speeds, while the rider may pedal with similar speed and effort in both cases. About 30% similar, in fact (say, from 60 – 90 RPM): just like the sweet zone of an electric motor.

By hooking up such a motor to the pedaled cranks of a multi-speed bicycle, instead of to a wheel or subset of the rider’s gearing, the motor can provide good power efficiently over the widest range of speeds that bicycles attain in the real world. Furthermore, driving the cranks in synchronicity with the rider’s legs provides for the rider to constrain the motor’s speed to within the sweet zone constantly, through the normal bicycling skill of shifting to maintain a steady cadence. Stokemonkey is the only product that does this.

This is not rocket science, nor is it controversial; the benefits of multiple-speed transmissions as found on bicycles, power tools, and eighteen-wheeler trucks are widely understood and accepted. Why, then, do the large majority of electric bicycle products have the motor connected directly to a wheel? And why do the few that don’t use only a limited range of gears, excluding those based on triple front chainrings?

37 thoughts on “Motors and bicycles”

  • Bill Manewal

    <p>Works for me! Easy to understand. May want to see if you can edit it down somewhat for marketing purposes. Remember the TV/Computer game-trained mind has an attention span of 30 seconds.</p>

    <p><span class="caps">OK,</span> I can’t resist the former English teacher in me:</p>

    <p>Why Gears for the Monkey?</p>

    <p>Electric motors produce no power when overloaded to stall, or when free-wheeling (no load). Between the zero-power overloaded stall and the zero-power unloaded motor, the power produced can be plotted as a curve. [Graph 1]</p>

    <p>The most power is generally produced at 50% of the unloaded speed. Unfortunately a motor�s point of maximum efficiency isn�t the same as maximum power. Stokemonkey�s motor efficiency peak occurs at about 80% of the unloaded speed. [Graph 2]</p>

    <p>The band of motor speeds between peak power and peak efficiency is only about 30% the total range of motor speeds. Operating the motor outside this sweet zone will always sacrifice power or efficiency. [Graph 3]</p>

    <p>Now letââ?¬â?¢s consider a typical mountain or touring bike. It may climb up a steep hill at only 5 <span class="caps">MPH </span>in low gear, but it will come down at closer to 30 <span class="caps">MPH </span>in high gear. Thatââ?¬â?¢s a 600% spread in wheel speeds, while the rider may pedal with similar speed and effort in both cases. About 30% similar, in fact (say, from 60 ââ?¬â?? 90 <span class="caps">RPM</span>): just like the sweet zone of an electric motor.</p>

    <p>Hook a motor to the pedaled cranks of a multi-speed bicycle (instead of a wheel) and you can enjoy good power efficiently over the widest range of speeds that bicycles attain in the real world.</p>

    <p>Furthermore, driving the cranks in time with the rider�s legs allows the rider to intuitively tune the motor�s speed to the sweet zone through the normal bicycling skill of shifting to maintain a steady cadence. </p>

    <p>Stokemonkey is the only product that does this.</p>

    <p>This is not rocket science: the benefits of transmissions as found on bicycles, power tools, and eighteen-wheeler trucks are widely understood and accepted. Why, then, do the large majority of electric bicycle products have the motor connected directly to a wheel? We can�t figure it out, but we DO know how to haul 300 lbs. up a 30-degree incline <span class="caps">AND </span>go 30 mph on the level <span class="caps">AND </span>get 40 miles from one charge of the battery, all out of the same motored/geared/human-powered system. This is where engineering gets fun. This is Stokemonkey.</p>

  • Todd

    <p>I like your edits, Jim, except for the very end where it <strong>sounds</strong> like sales talk. I’m allergic to it. (Similarly, there will be no terminal nines in my prices — that’s always struck me as manipulative and thereby at least mildly insulting.) My best hope is that I won’t have to do much of this kind of writing at all, instead relying on word-of-mouth referrals from customers. As you know.</p>

  • Bill Manewal

    <p>Yeah, I have that allergy too. So just leave off the last two sentences?</p>

  • Jeff Osborne

    <p>Great blog Todd. </p>

    <p>You wrote:</p>

    <p>ââ?¬Å?Operating the motor outside this sweet zone will always result in suboptimal performance.ââ?¬Â?</p>

    <p>Maybe there�s a better way to phrase it:</p>

    <p>ââ?¬Å?Riders will get the best performance and longest battery life by operating the motor within this sweet zone.ââ?¬Â? </p>

    <p>Or similar in a way that emphasizes the positive.</p>

    <p>Maybe also consider expanding your use of bold font in an article like this one, to give readers more ââ?¬Å?points of entryââ?¬Â?. Right now the only bold I see is on a sentence that is overt marketing. </p>

    <p>Your allergy to marketing spiel should serve you well in building dialog with enthusiasts. If someone has read this far, they probably already believe what youââ?¬â?¢re saying, and they donââ?¬â?¢t need much pushing. Technical explanations and occasional marketing references related to seemingly objective quality and uniqueness are enough to pull the reader along. Do you get what I mean about ââ?¬Å?pushââ?¬Â? versus ââ?¬Å?pullââ?¬Â?? </p>

    <p>I like Bill�s use of references embedded in the text that point the reader to the appropriate diagram. And similarly you can embed a name into all diagrams. Although the link between text and diagram is usually obvious, I find that the references add valuable subconscious context to the paragraph. </p>

    <p>I�m having a hard time following the second clause of this sentence:<br />
    <br />
    ââ?¬Å?By hooking up such a motor to the pedaled cranks of a multi-speed bicycle, instead of to a wheel or subset of the riderââ?¬â?¢s gearing, the motor can provide good power efficiently over the widest range of speeds that bicycles attain in the real world.ââ?¬Â?</p>

    <p>Perhaps add an extra sentence of explanation of the point you wish to make re: the wheel or subset of the rider�s gearing. </p>

    <p>Taking the time to offer careful explanation of a technology or feature is a great way to build dialog with enthusiasts and early adopters. By pattern those types are going to represent your market in the beginning. Also, reading your later post describing the batteries was enjoyable, and it helped me imagine ways that I could use those batteries to power the xtracyle mounted stereo that I�ve been tinkering with. </p>

    <p>Bill also makes a good point about people�s attention span, and I�d encourage you to consider who the people are that you�re working to communicate with. Some people may only connect with your product if they�re given the opportunity to really learn the technology. Those may or may not be the people you want to communicate with.. and if they are, I�d focus on ways that you can make things clear, and concise, but not necessarily short. I enjoyed reading both the motor description and the battery description.</p>

  • Hbilt

    <p>You wrote: “Why, then, do the large majority of electric bicycle products have the <br />
    motor connected directly to a wheel?”.</p>

    <p>Thwe answer has to do with how electric bicycles are c lassified by the <span class="caps">DMV </span>(<br />
    Department of Motor vehicles). For eample, in California, <span class="caps">THE DMV</span> Code (section 406)<br />
    defines an an electric bicycle as:</p>

    <p>b) A “motorized bicycle” is also a device that has fully operative pedals for propulsion <br />
    by human power and has an electric motor that meets all of the following requirements:</p>

    <p>(1) Has a power output of not more than 1,000 watts.</p>

    <p>(2) Is incapable of propelling the device at a speed of more than 20 miles per hour on<br />
    ground level.</p>

    <p>(3) Is incapable of further increasing the speed of the device when human power is used <br />
    to propel the motorized bicycle faster than 20 miles per hour.</p>

    <p>Your electric motor configuration will clearly allow the bicycle to exceed speeds oof 20MPH <br />
    when human assistance is employed. Without upgrades to heavy duty brakes, and other related <br />
    safety componets, an operator of your vehicle may overtax the original (bicycle) equipment brakes when <br />
    descending hills or attempting to stop in inclement weather. It is for these reasons that<br />
    electrically driven two wheeled vehicles that can exceed 20MPH–but not 30MPH–are clasified <br />
    as mopeds instead of electric bicyles. These two wheeled motor driven vehicles are thus required to have the necessaary safety equipment and componets to ensure that they can operate safetly within the <span class="caps">INTENDED </span><br />
    design envelope of the <span class="caps">MANUFACTURED </span>vehicle. In otherwords, 20MPH is considered the maximum safe speed for a human powered bicycle, assisted, or unassisted.<br />
    or<br />
    components</p>

  • Todd

    <p>You’ve provided a partial straight answer to a rhetorical question, Hbilt. It’s true that Stokemonkey can be configured to fall foul of California law for street use, but equally so that it can be configured to conform. The laws are <a href="http://cleverchimp.com/products/stokemonkey/faq/#iisl" rel="nofollow">inconsistent across jurisdictions</a>. Because Stokemonkey is an aftermarket kit, instead of a manufactured vehicle, individual purchasers bear responsibility for configuration. </p>

    <p>I think Stokemonkey is among the first products to exploit these old mechanical principles because without the (fairly new) Xtracycle, the variability and physical clearances of typical bicycles aren’t adequate to assure broad compatibility, and complete manufactured vehicles might run into the legal issues you cite. Complementary advances in battery technology, heavy-duty bicycle componentry, and perhaps internet-enabled commerce further answer the “why not earlier?” question.</p>

    <p>The safety issues associated with speed are real, as they are on unassisted Xtracycles and indeed any vehicle — I <a href="http://cleverchimp.com/products/stokemonkey/warnings/" rel="nofollow">review these issues</a> personally with all customers. I disagree, though, that “20MPH is considered the maximum safe speed for a human powered bicycle”. Avid bicyclists commonly exceed 20MPH on the flat, many can exceed 30, and downhill is a different story. Tandem bicyclists have power, speed, and gross vehicular weight comparable to Stokemonkey installations in heavy service, and are considered safe, at least among those confident enough of their bicycling competence to be interested in the first place.</p>

    <p>These people are the kinds I’m trying to reach. I see the driven pedals as a kind of filter here: if you can’t handle ‘em, you probably can’t handle the rest of the package, either. Same deal with the cargo capacity: when people ask for the kit without the Xtracycle, I think either “should ride a plain bike” or “should get an honest scooter.”</p>

  • Customer (Mike)
    Customer (Mike) January 3, 2006 at 12:09 pm

    <p>Chiming in on the one-size-fits-all frequently taken in the creation of laws:</p>

    <p>“(1) Has a power output of not more than 1,000 watts.”</p>

    <p>There are a fair number of human beings that can generate greater than 1000 watts on a bike, so I guess they would be classified as mopeds also? ;-)</p>

  • Todd

    <p>Really, Mike? What’s your source? I thought humans could spike somewhat below 1hp (750W).</p>

  • Mike

    <p>On a grander scale, one of the Olympic gold medalists in 04 (Qually or something like that, from Britain put out between 1,600-2,000 peak watts, I don’t remember exactly, but I’m pretty sure it was 2,000. I’m sure he’s not unique as far as World class sprinters go. On a more local scale, I have a couple of friends that can put out 1,200-1,300 watts (measured on an <span class="caps">SRM</span>). I have put out 1,000, measured on a Computrainer (accuracy +/- 10%). I would be very surprised if every competitive cyclist couldn’t do at least the same, except for maybe the very light guys.</p>

  • Todd

    <p>Mike, according to http://www.kreuzotter.de/english/espeed.htm , 2000W is enough to take a streamlined recumbent up to 100MPH (well in excess of the 80-something record) and an ordinary road bike to 50MPH. I’m finding 2K hard to believe. what kinds of drugs are involved, and what kind of reconstructive surgery is required after these spikes? :^) I’m thinking of 5 Stokemonkeys operating in parallel at max output, and that’s a pretty scary thought! On the other hand, the same calculator tells me I could put out 1190W for a short time; I got a not-particularly aero touring bike up to 34MPH on a flat windless road a couple years back. Felt like I could have kept it up or exceeded that speed with taller gearing. Alas, that was then.</p>

    <p>Anyway, now I wonder how I had gotten stuck on the 750W figure being the human threshold, because I’ve been running these numbers for a couple years. I may need to revise some Stokemonkey specs, too!</p>

    <p>It appears that my usual calculator (http://www.analyticcycling.com/ForcesSpeed_Page.html) gives quite different results from the Kreuzotter one. Hm.</p>

  • Mike

    <p>Ineresting that you bring up the streamliner note. Jason Queally travelled over the pond with a streamliner called the Blue Yonder, built by an F1 racing team (Renault I think). Evidently Renault didn’t think they needed to look at current streamliner technology, as they brought over a boat. Jason ran 64mph or so, and was thoroughly beaten by the current world record holder Sam Whittingham, who ran 81.something. People did expect Jason to blow Sam away, as Sam does not put out that kind of mindblowing power. I think he’s in the 1100 or 1200 range.</p>

    <p>The current record holders are indeed shooting for 100mph. </p>

    <p>Be careful of the theoretical models. They are useful for gross calculations, but are not closely related to the real world, unless they have a lot of real world data. Exact cda, crr, weight, windspeed, etc. are very necessary to come up with accurate forecasts. The models are not good for a general streamliner category, as there is no such thing as a general streamliner. Their cda’s are all over the place.</p>

    <p>Hmmm, now let’s see a Stokemonkey in power mode tied to a streamliner with a cda of .45 or so, and a rider putting out 250 watts–should be able to haul some groceries at close to highway speeds. I’m getting verklempft just thinking about it.</p>

  • Hbilt

    <p>Todd,</p>

    <p> I hate to tell you this, but your illustrated power curve for <br />
    a series DC Motor, is incorrect, both as drawn, and as explained <br />
    by you. Remember that the most common formula for power (there are <br />
    several that can be used) is that P=IXV, where P=power is equal to<br />
    I (current) multiplied bythe applied voltage (V). In a DC series <br />
    motor, at stall, its DC resistance is minimum (infact its almost a <br />
    short circuit!) and its current, therefore, is maximum, for any <br />
    given input voltage. For example, if your motor draws 30 amps <br />
    at stall, at 24 volts, then its power is 750 watts. If it draws <br />
    15 amps at mid rpm range, again with a 24 volt input, its<br />
    power is only 450 watts, and if it only draws 5 amps at its <br />
    no-load (max rpm) speed, its power would be reflected as 120 <br />
    watts. In a nutshell, then, Mike, both power (and torque) are<br />
    always maximum at stall, in a DC series motor. Thus your graph<br />
    should be a downward sloping linear (line) graph, rather than a <br />
    bell curve graph.Additionally,it should show <span class="caps">MAX </span>power at stall, <br />
    with a downward slope toward minimum power at its no-load speed.<br />
    Your feedback is invited.</p>

    <p>it should start with</p>

  • Todd

    <p>Looks like your comment got clipped Hbilt. I remain completely confident that there are phenomena corresponding essentially to what I’ve drawn and explained here going on with Stokemonkey and in other electric vehicles both similar and different. You may be right that I’ve used some terms incorrectly, or that my analysis conflates some things that you’d think of separately. I am not an engineer, nor writing for an engineering audience, but I welcome your help in not annoying those with a more rigorous education in these areas. </p>

    <p>I am confused by your assertion that power is greatest at stall. Torque yes; power no. A stalled motor has no capacity to perform work, unless you count heating, which I don’t. You seem to be talking about input power when you look at voltage and amps apart from <span class="caps">RPM, </span>ignoring the fact that as <span class="caps">RPM</span>s decrease under load, and amp draw goes up, more and more of the power going in is lost to ohmic resistance in the coils, which heat increases the resistance further, starting a vicious cycle whose end result is a drive wheel turning much more slowly than it could if the motor were running much faster and being reduced through gearing.</p>

    <p>Do you suppose it’s been seven months since I posted this and gotten no substantive critical feedback because nobody wanted to embarrass me, because most people know what i’m talking about, or because few do?</p>

  • Hbilt

    <p>Hi,</p>

    <p> In answer to your question, posed in your final paragraph of<br />
    your response, I think that only a small percentage of your <br />
    audience is technically knowledgable about electrical terms, <br />
    electrical formulas, and how electrical graphs support the <br />
    fundamentals that the formulas and terms represent. Nevertheless,<br />
    when you are engaged in a business that must use this term (power)<br />
    to describe either how your product works, or which explains<br />
    its features and benefits, you have a legal obligation to <br />
    ensure that what you are saying is correct regardless of the<br />
    knowledge level of the consumer. Otherwise you are <br />
    misrepresenting the claims (selling points) upon which your <br />
    product is based.</p>

    <p> I am not sure if you are aware that there are two types of <br />
    forces (energy), in which the term ‘power’ can be used.<br />
    Those forces are electrical ‘power’, and mechanical ‘power’.<br />
    The mechanical work that an electrical motor performs is usually<br />
    measured in horsepower, which is directly derived from electrical <br />
    power, since 1 horsepower=746 watts. However both mechanical power,<br />
    <span class="caps">AND </span>electrical power, in a DC motor are <span class="caps">BOTH SIMULTANEOUSLY </span><br />
    greatest at stall. </p>

    <p>Here is where I think your stumbling block is: <br />
    the ‘stall’ that engineers refer to, in an electric motor,<br />
    is only momentary, in a properly coupled mechanical <br />
    transmission. That is, the ‘stall’ (zero rpm of <br />
    the motor’s shaft only exists for a fraction of a second while its <br />
    electrical fields are building in electromagnetic value, due to <br />
    the inrush of (stall) current. Once these fields are at maximum, <br />
    (again in a properly coupled mechanical transmission), maximum <br />
    torque is developed, and exerts itself to twist (turn) <br />
    the motor’s shaft, so that the motor can perform its work. It <br />
    is this very stall current, then, that allows the<br />
    electric motor to ‘overcome inertia’, and begin to rotate its shaft,<br />
    and any (if applicable) device that is mechanically coupled to the<br />
    motor’s output shaft.</p>

    <p>The electrical ‘power’, that the DC motor consumes, at stall is always<br />
    measured in watts, which then inturn, is transposed to a (mechanical)<br />
    horsepower rating. </p>

    <p>Hence, an electric motor will always produce its maximum <br />
    horsepower rating (peak horsepower), at stall, because its electrical power<br />
    power rating is also at its maximum (wattage) level at stall. Since manufacturers do not want their motors run under stall or near stall conditions in normal usage,<br />
    their motors use a continous horspower rating instead. This rating will <span class="caps">ALWAYS </span>be less<br />
    that the ‘stall’ (i.e., peak) horsepower rating.<br />
    <br />
    With all of this said, and now referring to your graph of power and <br />
    efficency, just what kind of ‘power’ were you referring to, that is at a minimum level at 0 rpm????rpm????It can’t be electrical power (since by virtue of<br />
    the power formula, electrical power is at a maximum wattage level, and it can’t be mechanical power since, mechanical power (hp)in an electrical motor, is derived from its<br />
    electrical power rating, and will also be at a maximum value at 0 rpm.. </p>

    <p>In closing, Tim, the graph ‘power’ solution is simple: display the usable rpm range of the<br />
    electric motor <span class="caps">BOTH </span>in power, and in efficency, on your graph, and <br />
    eliminate the reference of ‘power’ as being minimal, at zero rpm.</p>

    <p>Respectfully,<br />
    Hbilt</p>

  • Todd

    <p>Hbilt, concerning legal obligation to make accurate claims, the one place I cite hard power numbers is accompanied by the disclaimer “Power figures are extrapolated from data supplied by the motor manufacturer, field experience, and physics models; they are cited as approximates in good faith.” I don’t know of another business in the electric bike space who even tries to offer as much detail about performance as I do. Maybe you’re helping me understand why :^). Anyway, there are no numbers in the graphs given in this post, and the curves aren’t meant to represent actual dynamometer results of my or any other specific motors, but to illustrate general principles.</p>

    <p>When I refer to power, I usually mean (and often specify) output power, which you’d call mechanical power. I have noted the tendency of other electric vehicle vendors to cite input (electrical) power only, and I have regarded this as obscurantist at best and duplicitous at worst, because it doesn’t take efficiency into account. There are “1000 Watt” systems out there that can put out only a very minor fraction of that in real-world conditions, thanks to the efficiency penalty of an inadequately variable “properly coupled mechanical transmission.” When I was teaching English in Czechoslovakia in 1990, I was amused and appalled to learn that under the freshly collapsed communist system, the productivity of factories was measured not by how much stuff they made, but by how many resources they consumed in the process. So the factories stayed warm and lit all night, even though empty. If they made bike motors, I bet they’d all be “5,000W” :^)</p>

    <p>For the record, Stokemonkey’s present controller limits input to 20A, and as a freshly charged, nominally 36V NiMH battery pack has a surface charge of around 42V, electrical power consumption can exceed 800W. Draws above 700W @ 36V seem indefinitely sustainable as far as motor and controller go.</p>

    <p>You are right that I’ve been using “stall” in a very different sense than you do, and I’ll take your word for it that my usage is incorrect, at least without more qualification. By stall I mean the point at which the motor can no longer advance because the load is too great. Maximum amperage is inadequate to produce enough torque to keep the motor moving. At this point, work stops (no mechanical power) and the motor becomes primarily a heater that secondarily exerts some stress on the coupled mechanicals, torsional to the output shaft. This point is familiar to users of common hub motors climbing steep hills: it’s the point at which their vehicle stalls and they must pedal harder or get off and push. It doesn’t occur at 0 <span class="caps">MPH</span>/RPM, rather it collapses quickly to that from a low value. What’s the correct term for this?</p>

    <p>This is exactly what I was Googling for yesterday without success: http://www.innovatia.com/Design_Center/Maximum_Motor_Power.htm . If you find this page erroneous, then we disagree substantively, because I have a great deal of field experience supporting this explanation of the phenomena involved. If not, then my layman’s phrasing is to blame.</p>

    <p>By the way, I am neither Tim nor Mike, but Todd.</p>

  • Hbilt

    <p>Hi Tpdd,</p>

    <p>Thanks for responding to my e-mails concerning the subject of power: and i apologize for<br />
    referencing you as ‘Mike’, or “Tim’, by mistake., in my previous posts. </p>

    <p>In regards to posed your statement and question, “This point is familiar to users of<br />
    common hub motors climbing steep hills: it�s the point at which their vehicle stalls and <br />
    they must pedal harder or get off and push. It doesn�t occur at 0 <span class="caps">MPH</span>/RPM, rather it <br />
    collapses quickly to that from a low value. Whatââ?¬â?¢s the correct term for this?”<br />
    The answer and term for a motor that is subjected to these conditions is simple: <br />
    the motor is operating in an ‘OVERLOADED’ state/condition. The overloaded condition <br />
    is usually due to either insufficient horsepower for the intended load, incorrect <br />
    transmission gearing (for the intended load), or an actual load/incline condition that<br />
    physically exceeds the designed load parameters of the motor and its coupled transmission.<br />
    Under these conditions, the motor will draw its maximum current rating, in an attempt<br />
    to still move the load. If the conditon still continues, the motor will overheat and<br />
    may destroy itself, or related electrical components.</p>

    <p>I hope that this these posts have helped you, in some small way.</p>

    <p>Respectfully,</p>

    <p>Hbilt</p>

  • Todd

    <p>Hbilt, I have edited the original post above to say “overloaded” as the opposite of unloaded, and, I hope, disambiguated “stall” from the electrical engineering usage. This amendment may confuse readers who go on to read your constructive criticism, at least until they read to this point. Thanks for the clarifications.</p>

  • John Riley

    <p>Hello Todd<br />
    I admire your gift of language and your clear and correct description of some physical concepts. You are indeed correct when you say that at stall (zero rpm) an electric motor may exert maximum torque, but as power is the rate of doing work, and as work is the application of a force over a distance, and as there is no distance involved (no movement) then there can be no work done and thus no power exhibited.<br />
    Just one small correction that I noticed in your third paragraph, last sentence where you write: “However, you will likely also want to consider efficiency, the ratio between the electrical energy drawn by the motor and the mechanical power it produces.” Perhaps more correctly, efficiency should be expressed as (paid for) “power in”, divided by (useful) “power out”, or conversely, (paid for) “energy in” divided by (useful) “energy out”. Strictly, all power is measured with the same units. Horsepower is well outmoded and the watt is the best unit for <span class="caps">ALL </span>forms of power. Again, well done, and thanks for the great reading, John</p>

  • Bart

    <p>2 commonly used motors for electric bicycles are series DC<br />
    and brushless <span class="caps">DC. </span> Series (brushed) DC are what the stoke uses<br />
    and are cheap. Brushless DC are more expensive with costly <br />
    controllers, but their low end torque and efficiency are far <br />
    superior, making a transmission unnecessary above a certain power<br />
    to weight ratio.</p>

    <p>My 2c</p>

  • Todd

    <p>Bart, Stokemonkey uses a brushless DC motor, not a brushed as you state. I guess you’d say it’s within the power:weight band where a transmission pays. It certainly outperforms the same motors used as hubs by a large margin.</p>

  • oilnemisis

    This sounds just great. I would like to purchase one from you. I, like the majority of true cyclists, could not care less what "jurasdiction" our cycles fall into. Only that we can ride them. I would love a "good" electric bike platform, so that I can couple it with my next project "Lead-air Battery".....said to be as engergy dense as Ic engine technology with instant charge potential.

    I'm starting to mine for this "stokemonkey" now and if you actually have one I will be getting in touch!!

    mike

  • Richard

    Some brushless motors have a much wider powerband than others (brushed and brushless) and are in much less need of an array of gears.

    The controller of a brushless motor consigns the normally mechanical commutation of power within the motor to a microcontroller. This provides it with the ability to adapt to conditions and derive increased performance at a wide range of speeds and loads.


    Richard

  • Todd

    There's detailed performance modeling of the (Crystalyte) motors used in Stokemonkey at http://ebikes.ca/simulator/ . Stokemonkey's "high" mode is currently 407. There's a 2.75:1 RPM reduction to the cranks. You can figure out the rest from there.

  • Nicholas

    I think everyone has missed the main point and that is creating the most intuitive and thus safest system. That would be an amplifier that essentially measures the force exerted on the pedals and amplifies it by use of the motor, by say 10 or an amount that makes the user feel comfortable. In this way a bicycle laden with cargo and rider that weighs a total 400lbs will feel like it is 40lbs. The user uses the gears to keep his pedalling speed at a comfortable level i.e about 60 rpm. On that basis, whether the motor is mounted before or after the transmission is simply a technical issue unimportant to the user.

  • Todd (admin)

    I disagree that the "pedelec" system you propose, Nicholas, is safer or more intuitive than a manually controlled one. I do agree that it is simplest, in the sense that it presents the fewest control elements to the user. But a spork is seldom superior to a spoon and fork. Making one action produce two results compromises the ultimate degree of control, with negative consequences either for safety or efficacy.

  • John Garrish

    Can't comment on the accuracy of terminology, etc -- like you, not my training. But I thought the post was great marketing. Very informative, which to me, is the antidote to rampant salesmanship. In this post, and in others, I have learned a tremendous amount about your products but also about the topic and remain convinced that the Stokemonkey is really the best product in this category and certainly the best thought out. As marketing goes, I think one of the most compelling things you can discuss on your site is why and how you arrived at your design. As I'm finding out, there are TONS of motorized bicycle concepts out there. Mostly 2 and 4 stroke engines which have obvious disadvantages, but also a slew of electric models. Which, I might add, are VERY compelling to people who are not dyed-in-the-wool bike people. For instance, I work with a guy who bought his girlfriend one of the Bionx bikes to ride in San Francisco to work. Not because they were better bikes, but because they were likely better marketed... which is to say, more WIDELY marketed. And, I'm not critiquing your marketing plans or suggesting that you should emulate Bionx (Don't they have infomercials on TV??). My point is simply that marketing matters, and giving people reasons to buy your products matter, and this explanation you wrote was excellent in that respect.

  • Paul

    Sorry to post to a 2 year old thread but, in the event that anyone stumbles across this, I wanted to comment. First, the efficiency and power versus speed curves are qualitatively correct, zero at either end and a max in the middle. Anyone who doubts it can easily verify it with a simulator or by experiment. Finally, having just test-driven several ebike combos for the first time, I can say that for hills or a loaded bike, any solution that works through the bike's gears will be a much better solution than one that does not. Anyone who wants to talk about wide power bands on hub motors, etc. just needs to actually ride a Stokemonkey and a hub motor-equipped bike up a moderate hill. You will need some serious current to the the hub motor up that hill, if it will make it at all. The Stokemonkey equipped bike will walk right up; not necessarily fast, but consistently. I'd used all the simulators and calculators, but in this case, 10 minutes worth of riding was better than 10 hours of simulators and discussion groups. I defy anyone who has ridden a Stokemonkey-equipped bike and a similar-sized hub motor bike to say they are even close in performance on a hill of, say, 6-8% grade. On a flat road with an unloaded bike; that is a different story; a hub motor will do fine there. But put a hill in front of them and that is when the Stokemonkey comes into its own. The Stokemonkey/Xtracycle is more expensive than hub motor solutions, but it is truly a tool and not a toy. For me, the cost is prohibitive; if I spent over $2k on batteries, a Stokemonkey, and Xtracycle, I wouldn't even bother going home that night. The Stokemonkey might get to sleep in the garage, but I am pretty sure I would be in the yard. However in terms of performance, the Stokemonkey does what it is supposed to very well. It is also highly regarded for its longevity.

    To the actual post, I have a hard time doing better than 3 mph up a steep hill. 5 mph would actually be pretty good for me. And it takes a lot more effort on my part to climb the hill than to go on the flat, although the cadence might be the same.

  • fireofenergy

    I'm not an engineer by no meens, (either an "old post" avoider) But it's obvious that an electric motor attached to gears will find many sweet spots, that which are definately not at the stall point. Imagine though, all gear ratios will have their "stall" when starting uphill. Thus the stoke monkey approach (which I just "now" heard of, half a decade later, what's wrong with me?) offers the higher torque necessary to get it out of stall, sooner, thus more efficient (as long as you change gears appropiatly).
    To me, excess heat is the same as resistance which is waste.

    Thanks for the great read!

  • WillJL

    Why do cars and motorcycles use a transmission?

    It's because it allows the driver/rider to select the most efficient gear to take advantage of the engine's power in any given situation. Any motorist who has driven a stick shift must be familiar with this principle.

    That said, regardless of any semantic errors in the Stokemonkey's marketing above, the same logic applies to bicycles equipped with an electric motor. End of story.

    P.S.-- Big respect to Todd for dealing with Hbilt's vitriolic argumentativeness in such a classy way, even if it happened four years ago. Hbilt, if you ever read this thread again, I hope you realize what an ass you sounded like, and how genuinely unhelpful your misspelled attempt to sound smart really was.

  • Ted Howard

    I've been using a Stokemonkey on my Xtracycle for 3 years (Xtracycle for over 5 years), and have recently put on a Rolhoff 14 speed hub.

    This is now a complete 'truck', and is used 4-6 days a week for my business (kite stall and Permaculture gardener/designer/teacher. As such it's now very much in the very useful tool level of utility biking. I use it when teaching the alternative transport/appropriate technology module on a PDC (Permaculture Design Certificate course).


    As we slide out of an unsustainable industrial civilisation, this set up may or may not be sustainable in the long run. But as a transition from cars to intensely local living, it's ecologically light years ahead of cars regardless of what fuel they use. And unlike cars, I don't even really need a road, as a single dirt track will do if necessary.

    As a peak oil activist for over 13 years, this setup allows me to "ride my talk" in a way that's fun for me, and has inspired a bunch of my friends to get electric assist bikes for local transport needs.

    Regards
    Ted Howard
    Nelson, NZ

  • Leo Lichtveld
    Leo Lichtveld June 27, 2010 at 2:51 am

    WillJL Cars and motorcycles have combustion engines whose torque varies with RPM. This is why they need transmissions. Electric motors produce nearly the same torque no matter the RPM. The Tesla roadster has only 1 gear. Electric trains don't shift gears either. From Wikipedia:

    "Electric vehicles can also use a direct motor-to-wheel configuration which increases the amount of available power. ...
    A gearless or single gear design in some EVs eliminates the need for gear shifting, giving such vehicles both smoother acceleration and smoother braking. Because the torque of an electric motor is a function of current, not rotational speed, electric vehicles have a high torque over a larger range of speeds during acceleration, as compared to an internal combustion engine. As there is no delay in developing torque in an EV, EV drivers report generally high satisfaction with acceleration.

    For example, the Venturi Fetish delivers supercar acceleration despite a relatively modest 220 kW (295 hp), and top speed of around 160 km/h (100 mph). Some DC motor-equipped drag racer EVs, have simple two-speed transmissions to improve top speed."

    Stokemonkey works because it uses gears to multiply torque, not so much because of some "sweet spot".

  • Todd (admin)

    Hi Leo -

    Your statement "Electric motors produce nearly the same torque no matter the RPM" isn't true. Check the curves for any electric motor, e.g., http://ebikes.ca/simulator/ . They produce more torque at lower RPMs, but like other motors produce less _power_ at low RPMs. Electric cars/trains (and indeed some electric bikes resembling motorcycles) simply use such powerful electric motors and have such ample power supplies that variable gearing would add unnecessary complication and expense. It's a brute force approach. It's when you're constrained by the desirable minimum weight and displacement of battery packs on a bicycle that you can't afford to take a brute force approach, instead using variable gearing to make maximum power available at all road speeds.

  • Richard

    Very interesting discussion about motors vs. engines.

    Leo is mostly correct. And I'm glad to see someone say it, finally. Thanks, Leo. Electric motors have much broader power and torque curves than IC engines. The do still have a curve. So perfection only happens under certain operating conditions. But you're usually closer to perfection with electric power than internal combustion. Way closer. And many if not most electric vehicles are designed with just one gear because of it.

    The interesting thing about electric power is that the curve can be changed by adjusting the input power. Voltage and amperage. Even a small, slow vehicle can be made into a dragster by electrically moving the power curve. Momentarily increasing voltage or amps in the right way. Put more battery cells in parallel than in series, or vice versa. I've done this with my electric bike and gotten amazing performance under vastly differing conditions. High volts and lower amps will push my cheap bike over 40mph on a flat road -- well over 40mph if I was crazy enough. Higher amps will push me up my steep hill towards home at better than 25mph!

    If your system is way underpowered perfect matching of gear ratios is probably essential. Otherwise, one gear is usually plenty. And if you want to turn a golf cart into a rocket ship, just make the battery configuration adjustable. One gear is fine.

    Amazingly, with electric, when you push the limits this way things still tend to stay very efficient -- unlike internal combustion engines. A lead foot with an electric vehicle doesn't consume energy nearly as terribly as with it's counterpart.


    Richard

  • Todd (admin)

    Richard says "If your system is way underpowered perfect matching of gear ratios is probably essential."

    SM is less than 1 HP. Being "way underpowered" by motorcycle, electric dragster, train, Tesla Roadster, or even golf cart standards is what keeps electric-ASSIST bicycles light enough to carry passengers and goods, to lift over your head, to have 100+ mile range, to remain viable as 100% human-powered vehicles. There are no >1HP systems that meet these bicycle criteria. Battery technology is currently the limiting factor. Stokemonkey is a "low power" system by contemporary motor vehicle standards, but if anything, "overpowered" by bicycle standards. "Underpowered" implies comparison to a class of vehicles I don't mean to emulate.

    So, horses for courses. I have ridden a 2,500W electric "bicycle," with fixed ratio motor gearing. The instant you touched the throttle you knew that pedaling was entirely meaningless. It weighed maybe 175lbs and had only a 10 mile range. Its human gearing didn't work because the owner stopped using it or caring about pedaling. It felt quite dangerous to me, not having motorcycle-class brakes and other control components. It was fun, but I'd rather bike myself 10 miles than go by motor conveyance of any kind.

  • Paul

    "Amazingly, with electric, when you push the limits this way things still tend to stay very efficient"

    Not true. Efficiency for electric motors is zero at zero motor speed. You can increase power by increasing voltage or current but you have to remember that you have to take your energy source with you. If you increase voltage, for a fixed weight battery, you decrease capacity. You can buy your way out of the hole by stacking enough batteries on the bike but you don't end up with a very nice bike. I prefer the gears. Most single gear bikes that will do 40 mph on a flat aren't too good on a steep hill.

  • Richard

    Paul says:

    " Efficiency for electric motors is zero at zero motor speed. "

    Interesting claim, if a bit silly. I'll claim that electric motors are infinitely efficient at zero rpm. Just as silly, though a little more accurate.

    BTW: my bike weights about 50 lbs. with an inexpensive mountain bike at the core. I can lift it onto the bus bike-rack easily. It will go about 15 or 20 miles at 40mph. At slower speeds it will go a lot farther. Forty mph incurs a lot of wind resistance. It will go 25mph up my steep hill. Hill is 900 foot rise in about 1.5 miles. It's a direct drive, hub motor which is much more efficient than indirect drive systems, such as chain drives. Where direct drive fails is in allowing the bike to be used without the motor assist. The direct drive motor causes some drag, more than is acceptable for pedaling. I think that's fixable, but haven't had time to address it.


    Richard

  • Paul

    "I’ll claim that electric motors are infinitely efficient at zero rpm. Just as silly, though a little more accurate."

    Well, if you can go to: http://ebikes.ca/simulator/ you will see that motor efficiency is a strong function of motor speed. A hill that rises 900 ft in 1.5 miles is a 8.5% grade. I guess steep is relative; I was considering hills more on the scale of 15-20%.

    All of these systems have their good points and bad; none are magic. Gears keep the motor at higher efficiency. If you pile on enough battery voltage and capacity to drive a direct drive bike up a 15-20% grade at 40 mph, then it will be efficient. If you are doing 20 mph up the same hill on a bike that will do 40 mph, you are putting about half the energy of the battery into heat. You can't get around physics. The efficienty of indirect drive systems is not that dissimilar from direct drive systems; the motor does not turn when not powered. There is no cogging as found in direct drive.

Comments