Very high-powered drag braking to control speed while pedaling

For a while now I've had the idea of an exercise-focused EV that focuses on speed moderation as well as speed enhancement. This may seem somewhat strange, but I am going to attempt to explain below. (In a not-entirely-short but hopefully not-too-long exposition).

Speed and safety

All things being equal, going slower is safer. Impact forces will be less in a collision or fall, you will slide less far (and thus have to deal with fewer abrasion cycles), and you have more time to react to your environment (e.g., a child runs out in front of you).

Speed and situation

Matching traffic speeds is important when riding on roads that you share with cars. It is less dangerous to be going 10 mph slower than traffic than to be going 30 mph slower than traffic. Assuming you get to pick what roads you are on to some extent, going around 20-25 mph is probably sufficient for riding safely with car traffic (the trick is getting enough torque to do this up steep inclines). It is also important not to go very slow in bike lanes; forcing fast cyclists to pass you puts them at risk.

There are also some situations where going slowly is desirable. If you are on a paved nature trail or park trail that people take their kids and dogs on, going fast puts you and others at risk; the "best" speed for such trails is slow.

Speed generation

Electric vehicles have no problems generating speed. Particularly if you use something more powerful than a 250W DD hub motor.

Speed reduction

Here is where things get interesting. People using EVs for transportation typically want to preserve as much speed as they can, essentially only braking when necessary. However, when using EVs for exercise, it seems to me like lower speeds are superior in most cases that don't involve mixing with cars. Safety is one thing, but I find that I like to exercise most on paths, which often have pedestrian traffic on them, forcing lower speeds. Now, speaking from the perspective of someone who is moderately fit (rather than someone who is quite out of shape), it is difficult to get any sort of meaningful exercise while being forced to constantly check speed on such paths.

Another area that I find myself thinking about a lot is downhills. It is very hilly where I live, making any sort of exercise I do more akin to interval training than steady-state cardio. I huff and puff going up hills, but then zoom down them without being able to pedal since I'm already going quite fast. If I want to target my workout to be something like 30 minutes of 70% VO2 max, I can't, because I can never get consistent enough terrain to allow for steady output.

These two areas -- being able to exercise on paths that require low speeds, or just having having the option of exercising at lower speeds in general, and exercising while going down hills, are the main things I am interested in.

Options for speed reduction

Based on my research, the drag brakes of old, as used by tandems (among other tandem brake options) in the mountains, were typically drum brakes (in particular, the Arai drum brake, a part no longer in production, to my knowledge). The idea was to moderate speed using a big, heat-dissipating drum brake, so that rim brakes would not heat up on long descents and blow out tires.

Nowadays, a Tandem company called Santana has come out with a large-diameter disc brake that seems like it might be able to handle a lot of heat without fading, particularly if it only had to deal with the mass of a non-tandem bike and a single rider.

Additionally, DD hub motors can be used for regenerative braking, assuming motor controllers than can handle the amps and a BMS that keeps the battery cool enough. My understanding is that bigger hub motors are capable of more braking resistance (and winds and stator width can probably be optimized for maximum regeneration as well -- this stuff is beyond me at present). Quite a few people here on ES have mentioned that using regenerative braking saves brake pads a lot, particularly for cargo bikes.

Controlling speed reduction

It is my understanding that regenerative braking can be made progressive if you use a high-quality controller. I also know that the drum brakes of old typically used a form of "level selection" whereby a braking level was set so the captain wouldn't have to keep a brake lever compressed for miles on end.

What I want to be able to do, whether using friction brakes or regenerative braking, is to have a level selector in the manner of a gear shifter: level 1 for barely noticeable braking, and level 12 for basically locking the wheels. A completely analog selector would be even better - one that allowed for a full spectrum of braking levels rather than discrete options. It would probably be necessary to set different braking levels for the front and rear, since front brakes tend to dominate, and rear wheel(s) slip more as weight shifts forward

Questions

Alright, sorry for the length. Now that I've gotten through all the background, here are specific questions I have:

1. My understanding of traction is that braking eats up some amount of available traction. Does this mean that riding around with drag brakes always engaged is actually leaving you with less traction than you would otherwise have? (I can see this being a big problem with the front wheel).
2. Silly question -- can you even pedal while you are braking? This whole concept revolves around being able to continue to pedal with drag brakes engaged. (I would test this myself except I don't currently own any bikes to test with).
3. Would running two of the large tandem disc brakes be sufficient for limiting speed on pedal-power alone on flats?
4. How about the same setup when pedaling at full bore going down a 15% incline, say? The brakes would obviously heat up a lot, but given a single rider's mass, would they fade out if this was kept up for minutes on end? Would air-cooling keep up with the heat generation?
5. How about two big DD hub motors (like the Crystalyte H35) on the flats -- would they be able to sufficiently limit speed?
6. How about these same hub motors pedaling while going down a 15% incline?
7. How about a combination of the large DD hub motors and large disc brakes? I think the disc brakes would have more problems with cooling right up against the hub motors, but could the combination handle full speed reduction?

I think that's enough to start! Any other thoughts people have would also be appreciated.

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In case you want to read even more, here's some additional information:

• I am aware that many things can be done to slow things down without activating brakes. I am already planning on running fat tires at low-ish PSI on whatever EV I settle on, and am currently leaning towards modifying an Elliptigo for reasons outlined in this blog post of mine. The standing position on the Elliptigo is very much un-aerodynamic, which will help moderate speeds especially for downhills (air resistance being proportional to the square of velocity). You can always just put more watts through motors if you need the speed to mix with traffic, assuming you are running with high quality battery packs that have good energy density.
• The weight shifting when braking is an interesting subpoint that may be relevant after some of my initial questions get answered. Upright bikes have most of their weight on the back wheel, which is why it is usually best to put the motors there (either mid-drive through the drivetrain or via a rear hub motor): the back wheel doesn't slip nearly as easily. When braking heavily, however, weight shifts forward onto the front wheel, meaning that front brakes have much more of an impact than rear brakes.
• I have also thought about using a derailleur to disconnect the chain from the drivetrain when you don't want your pedaling to contribute to your speed. There would need to be some other form of resistance to pedal against ( a flywheel perhaps?), and this is also more fiddly than using progressive resistance since it is binary, and you would have to shift a lot to maintain a level speed (or just use hub motors for vehicle speed, and this other thing for exercise). I would be interested to hear if this would be possible though, since I've never heard of anything like it.

I'm not at all clear why you'd want brakes to drag all the time. Is this like some two-wheeled version of those CrossFit morons jogging around with a truck tire dragging behind them? If so, the truck tire is cheaper, easier, and more durable.

My experience with a big direct drive hub motor revealed a kind of passive speed limiting that was of some use, I guess, in hilly Seattle where I rode it. I had a front Crystalyte 5305 in a 700c wheel, running on 36 volts. Top speed on flat ground was about 22mph as I recall, and at only a little bit higher speed than that, the motor outran its free speed and began to apply a drag force. I think the fastest I ever saw on that bike, with a very steep downhill pushing me, was about 28mph.

I could afford to let the battery recharge in this uncontrolled way because it was a lead acid pack that was rated to charge at a multiple of its capacity. In either direction on my commute, there was a significant climb before I encountered any noteworthy downhills.

This same method could be employed to impose a practical top speed on any direct drive e-bike-- simply have a motor free speed that's a little less than your desired limit. But to make a consumer-ready version, it would have to be able to dump the braking current somewhere other than the battery when the battery is full.

I'm extremely confused by this as well. If you want to control speed, you can just use a cycle analyst to set a top speed..
If you want to do more pedaling, turn down the motor's power or top speed.

Anything other than wasting energy intentionally..

I can see where he's going with this, and while it isn't something desirable for most people, there are people that would want it.

Mostly those that otherwise would be riding stationary exercise machines that have an adjustable resistance--which is essentially what he's describing in the OP, except that he wants to go somewhere and enjoy the ride while being forced to exercise harder than normal, and being *able* to exercise on the parts of the ride that otherwise he would not be able to (downhills, etc).


For very slow parts of a path, one could simply change gears to some very very high gear that makes it difficult to pedal. But it would probably require higher gearing than most bikes have--may even require a third stage of gearing to allow it.




So using a DD hubmotor with a proportional EABS (active braking, not just regen, so it will still work at very very low speeds, essentially down to a stop to some degree) could allow this.

It would also allow safe e-braking even with a full battery, because EABS actively uses up power to cause the braking, not just regenerating it from the motor back to the battery (though some of them may do that too, whenever there's more power coming out of the motor than you need to cause the amount of braking requested).

Regular regen isn't going to be able to lockup the wheel, at least not in anything I've ever used; EABS is also unlikely to be able to for most controllers. Plug braking can, however, at least from higher speeds (perhaps not at lower ones). Plug braking essentially shorts the phase wires together (any pair for some braking, all three phases for full), but I don't know if any controllers do this on their own; you may have to build something to do it (first disconnecting the phase wires from the controller so you don't blow the FETs, then connecting the phase wires together).


Keep in mind that ebraking creates heat inside the motor, the phase wires to teh controller, and the controller itself, often much more heat than is created using the system as a motor. So you may have to actively cool the system to use it the way you would like; there are a number of threads about cooling hubmotors, if you do.


I"m not sure if the Cycle Analyst v3 has a way to adjust the amount of braking it could force a controller to do based on the Aux Input control vs speed limits, possibly in combination with presets, but you might check with Grin Tech to see if it could help you with that, to make it more automatic whenever you have it enabled.

One thing I know it can do is engage the ebrake output whenever you've exceeded the speed limit you've set in it, so you could use the Aux Input to control the speed limit (I think) and then set it to whatever you want for that particular path, trail, downhill, etc., so that it engages the brake once that is exceeded.

Or if there are only two or three speeds you'd ever need to limit to, you can just use presets for the speeds.

That at least makes it automatic for the slow riding, or maintaining a speed no matter how hard you pedal (whether or not you even use the motor as a motor).

I don't recall if the CA's ebrake output is analog or not. If it isn't, then your controller would need to support a braking mode where it has a digital input for activating the ebrake, and then uses a separate analog control for how much brake to provide. Then you could use a selector switch with different voltage divider points on it to feed that input, to choose how much braking you want.


You could also create your own system to do the controlling of when and by how much the ebrake is applied, either with analog electronics, or with an MCU (and potentially FETs or contactors, etc., depending on which braking forms you need to use).

Regarding disconnecting the pedals from the system at times: If you did this, youd' be better off having them be an indirect driver of the system, by pedalling a generator, which then helps to power the system, by recharging the batteries indirectly.

There are a number of threads about doing this, usually called a serial (or series) hybrid bike. (A regular ebike is a parallel hybrid)


For your case, the pedals power a generator which powers an inverter, which powers a charger which charges the battery. It would be very inefficient (certainly less than 50%), but since that's what you're after, essentially, that's fine in this case. The generator would be completley separate from the hubmotor system, other than the indirect charger connection.

IF you make a current-adjustment setting on the charger, or on the inverter, then the less current it can provide, the less resistance on the pedals. Disconnecting the inverter from the generator makes the least resistance. For really high resistance needs, if shifting gears to high gear doesn't do it, you could short the generator output wires (same thing as plug brakign except it only does this to your pedals, not the wheels).

If you still want the pedals to control the motion of the bike, there are a number of PAS controls (cadence, torque) that can be used to control the motor system with. The Cycle Analyst can use many of those to convert to a throttle signal a common ebike controller can handle.

The motor system can also still have the other ebrake systems in place for the downhill speed control if you like, but since you're pedalling against the generator resistance all the time anyway, it wouldn't matter if you ebraked on the downhills or not as far as pedal speed / resistance goes.

Thanks for the replies. I thought I had included enough background to outline what I am trying to do... amberwolf got it. From the OP:

These two areas -- being able to exercise on paths that require low speeds, or just having having the option of exercising at lower speeds in general, and exercising while going down hills, are the main things I am interested in.

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There are some places where speed is bad, as it puts yourself and others at risk (trails with pedestrians being a good example). The main idea is being able to keep the speed low in such situations -- and I mean arbitrarily low, as in right down to 0 mph, not "lower than it is with motors engaged" -- while also being able to get good exercise.

So it's not just having the motors cut out when you want to go slower or running less power through them -- the speeds I am thinking of are quite a bit below what bicycles are capable of on human power alone.

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Regarding "wasting power," one of the reasons I am asking about DD hub motors is that they can recapture some of the pedaling energy with regenerative braking rather than dissipating all of it as heat. However, I am not too concerned with energy efficiency here since I would never be running the drag brakes with motors engaged (if that is what you thought I was getting at). That would be eminently pointless. No, what I am interested in is limiting speed when it is undesirable but I still want to be able to pedal for exercise.

I dunno, maybe you think this is a terrible idea? It seems pretty logical to me.

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@amberwolf:

I had thought about shifting to higher gears, but then you are changing the resistance/cadence balance. It would be ideal to be able to pedal at variable resistance levels and RPMs just like normal, except with resulting slower speeds.

So in addition to disc brakes and regenerative braking from motors, there are also the options of EABS and plug braking? I'd come across plug braking, but I always sort of thought it was like regenerative braking without the battery connected. I didn't know it was stronger, in other words.

If I ask stupid questions, I apologize. I haven't ever owned an electric bike, and all of this is very abstract for me since I only know of things like motor controllers and the CycleAnalyst, not how they work or what precisely they do.

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Maybe we should focus on one thing at a time; that might be easier. So here's something to start:

Without getting into specifics (how to connect controllers, etc. and wiring things up), it sounds like it would be possible to keep very low speeds (even going downhill) while pedaling? Even going down a steep hill and pedaling hard?

Sure, it'd be just like sitting on a stationary exercise bike and adjusting it's resistance to whatever degree you want.

The only real limitation is how slow you can go and still balance the bike.


Technical limitations mean that if you intend to recover the pedalling energy even at very low speeds (5MPH and under, for example), you are better off disconnecting the pedals from the drivetrain, and using them with a generator, at all times. (would be easy to make a consistent amount of resistance at all times as well).

If you only really need to recover energy from higher speeds (10MPH and above) then regular regen braking on a DD hubmotor would be sufficient, with the proper controller.

To make it go really slow with high resistance you won't be able to use just a DD hubmtor with simple regen, but will probably need a combination of things.

StevenTammen said:

Thanks for the replies. I thought I had included enough background to outline what I am trying to do... amberwolf got it.

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Okay.

I'll repeat, then. Dragging a truck tire is cheaper, easier, and more durable.

Not quite sure I get your problem entirely (tldr), but if you've ever tried riding a DD ebike without power I think you'll find yourself at acceptably low speed on the flat. As for down hill, Regen even with pedalling will result in the same low speed on most hills. If not... Turn it up (the Regen that is)
Strong Regen on ( 1) motor will draw you down to ~15kph even on pretty steep hills.
Going slow is kind of a non issue... Use less throttle. Legs are pretty weak, even good legs compared to a motor so you won't be going too fast for too long with all that weight and resistance stopped to your bike.
2xregen motors and large discs? what are you trying to stop?....that would be adequate for 500kg easy.

Well, I have actually never ridden an electric bike, period. Which is why I may be asking some questions that would be pretty obvious if I had.

15kph is probably slow enough for comfort for the applications I am thinking of. I don't think I would need to go slower than that. Some more questions:

Understanding regenerative braking

I'm trying to get an idea of how much braking force a single DD hub motor can provide, assuming a quality controller/battery system that can handle pretty significant input. I know there is some internal resistance caused by the motor turning ("cogging"), and that this increases at higher RPMs. And my understanding is that the magnitude of braking force that can be applied is also dependent upon motor RPMs, meaning that regenerative braking works better the faster you are going (and doesn't work very well when you are already going slowly). Here is a quote from another ES thread that talks about this:

Spot on as usual SM. I'd like to add though, the one time I had regen working on a bike, it worked real strong when applied at 30 mph, but faded a lot as you came to a stop. Lower rpm, less regen force. So I would never advise relying on regen only. I had mine set up with a separate button for the regen, not the brake handles, so I could feel it change when braking only on regen.

Just so you know,, if you are setting up a bike with rear regen only, and front brakes, you end up with almost pure front brake as you come to a stop. This gets tricky if there is a bit of sand, gravel, or water on the road.

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I myself weigh around 190lbs (86kg), and am estimating that the frame, batteries, motors, etc. for whatever I end up going with will add a minimum of 60-70lbs (27-31kg). Being conservative, let's say the total weight is 130kg.

At higher RPMs, the motor could provide more braking force. I understand that. But I have no idea if a single big DD hub motor would effortlessly limit top speed for this 130kg total weight going down a 15% incline with me pedaling at 100W (as in, handle the load all day without struggling or overheating), or whether it wouldn't be able to handle this at all, or be able to provide some level of resistance but not all of it.

In a ballpark sense, assuming one keeps the speeds at least 15kph (~10mph), can big DD hub motors provide "lots" of braking force, "some", "a little", etc.? Is two like total overkill for braking?

amberwolf said:

So using a DD hubmotor with a proportional EABS (active braking, not just regen, so it will still work at very very low speeds, essentially down to a stop to some degree) could allow this.

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I think that may be overthinking it.

Let's say this guy wants to go 15mph. He gets a DD motor and a voltage mode controller (NOT a current mode) and sets the throttle to 15mph. If he pedals slower than that, the motor assists. If he pedals faster than that, the motor regens. That way his speed (and more importantly his cadence) stays basically the same, minus the "play" in the system.

You could get fancier and do a feedback look to keep the speed exactly at the setpoint but that's not really necessary in most cases.

billvon said:

Let's say this guy wants to go 15mph. He gets a DD motor and a voltage mode controller (NOT a current mode) and sets the throttle to 15mph. If he pedals slower than that, the motor assists. If he pedals faster than that, the motor regens.

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None of the controllers I've used do that, though they are all basic generic stuff, and older. (they just cease assisting)

If newer ones do, that's an interesting feature.

THe Cycle Analyst has a mode to allow this, though, for any controller that does regen or other electric braking modes, as long as it's a DD motor.

amberwolf said:

THe Cycle Analyst has a mode to allow this, though, for any controller that does regen or other electric braking modes, as long as it's a DD motor.

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Yes, that would definitely be the higher tech way to do that - and would be a lot more flexible.

StevenTammen said:

I'm trying to get an idea of how much braking force a single DD hub motor can provide, assuming a quality controller/battery system that can handle pretty significant input.

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That will depend on the wheel size, the motor winding, the motor magnet and stator width, the controller's design and regen or EABS / etc settings. (assuming the battery is not full, at which point it can't accept regen current)

If you go to http://ebikes.ca/simulator you can setup various controllers and motors to see how much torque they have at startup from a stop, at various currents. Assuming the same current limits for braking, that would be the maximum force applied as a brake, too, for the same setup. What actual force you'd get with a particular setup you'd probably have to test in the situation it's used in, to see if it would work.

At higher RPMs, the motor could provide more braking force. I understand that. But I have no idea if a single big DD hub motor would effortlessly limit top speed for this 130kg total weight going down a 15% incline with me pedaling at 100W (as in, handle the load all day without struggling or overheating), or whether it wouldn't be able to handle this at all, or be able to provide some level of resistance but not all of it.

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The system could easily handle the 100w of pedaling (assuming battery is not full); for the rest of it it would depend on what setup you used, and the battery's state of charge (assuming regen) for how much continuous current it could accept. YOu can calculate how much force would be needed to do the braking, and then determine how much braking force the motor used could create in the wheel size you're using it in, based on how much force it can create while being a motor.

Note that regen has quite a wide range of implementations in controllers, and none of them are documented, so you'd have to test with a specific controller to find out how much regen current it will actually provide in any particular situation, and thus how much braking force.

Same thing with EABS.

It's also difficult to know for sure whether a controller supports what I call EABS, or active braking, without testing it, because the term is used for regen as well, by various manufacturers, and sellers use whatever terms they feel like.

I don't know of any ebike controllers that do plug braking, so you'd have to build that (and in a way that fails safe, so it can't accidentally short the phases when you don't want it to, or accidentally short out the controller by failing to disconnect it before connecting the phases together).


But it would probably be simpler to find someone with a DD ebike in your area and test out what it is like.

I'm not a math guy, so I prefer experimentation.


My SWA guess is that the average DD motor setup could probably do it--whether it would be effortless or not, and how much heat buildup there'd be (and thus how long it could do it), is a question you'd have to answer the hard way, most likely.

Have a big red button that deploys chalos tyre! :lol: seriously you are getting way too in depth about a thing which is really not that difficult. Get a dd motor, a controller with variable Regen and your done-oh remember to turn it off when you want to go slow on the flat. Now go and build it, and start riding!!

Thanks again for all the responses. I feel like I have a better idea of things involved now, at least enough to see some of the considerations (and play with the simulator). I think, as suggested, that I'll probably have to do hands-on experimentation before really getting to the bottom of all my questions. Sort of a pricy experiment for just exercise feasibility though... just need to come up with other reasons to buy an electric bike .

Just one more main question: it would be more effective to put the regen braking (or EABS, etc.) on the front wheel, rather than the back wheel, if you are concerned with how much it can stop you, right? Since weight shifts forward when braking and you could apply more braking force up front without losing traction? Or am I thinking about this in the wrong way?

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If anyone has knowledge on the disc brake front, I'd still be interested in hearing whether or not it is realistic to have disc brakes that can handle pedaling while braking down hills without fading as well. It would seem to make sense that if rotors of a certain size can moderate speed on steep hills without fading, then just making them a little bigger (as with the tandem brakes) could probably deal with the relatively small output of human pedal-power without a problem.

I'm also still a bit fuzzy on how traction works with braking (does using drag brakes -- such that you are always braking to a degree --inherently reduce your overall traction?), if anyone wants to take a stab at explaining this to me.

StevenTammen said:

Sort of a pricy experiment for just exercise feasibility though... just need to come up with other reasons to buy an electric bike .

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Couple hundred bucks for the cheap DD kits on ebay; there's a bunch of threads about those and bikes built from them. Whether they do what you want you'd ahve to test though.

Better to first find someone in your area with a DD regen bike and try it out.

Just one more main question: it would be more effective to put the regen braking (or EABS, etc.) on the front wheel, rather than the back wheel, if you are concerned with how much it can stop you, right? Since weight shifts forward when braking and you could apply more braking force up front without losing traction? Or am I thinking about this in the wrong way?

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Depends on how good your brakes are to start with, but braking on the front , on a normal bike shape/size, will always have more braking power than the rear. If you're never braking hard enough to skid the rear wheel, it's basically irrelevant.

In regards to what has the best traction.. on a regular bicycle, it's the rear wheel because that's where most of your weight is..

If you are braking hard, it's better to lose traction on the rear wheel than the front.

If you want extra constant friction without electrics, buy a BMX bike and put the knobbiest and heaviest moped tires on it you can find. That's good for a workout :lol:

OP: your idea is super cool, you have a very positive idea flow going and I am stoked to see what your final build of this idea will look and function like.

Interesting topic. OP's idea reminds me of the "generate" assist level on the Bionx systems.

I feel the usefulness of braking with shorted windings and active braking is exaggerated a bit. Regular regen should give plenty of braking power with a suitable controller right down to like 5% of your system top speed. Typically they are used very briefly just to bring a vehicle to a complete stop.

A nice idea to use the CA to impose a speed limit, with regen occurring above this limit.

The solution to being able to get better exercise with an ebike is quite simple...choose more appropriate routes.

The non-simple solution is an ebike I've planned for some time. That is a high powered ebike capable of safely mixing with traffic, which I'm sorry isn't riding slower that traffic, but pacing traffic with sufficient acceleration and speed headroom to achieve safe spacing away from idiots in 2 ton motorized metal cages trying to kill you. Functional pedals in the drive system are pointless on a bike capable of pacing traffic on primary roads, but that doesn't mean they can't be used for a cardio workout.

Instead of driving the wheel, I envision what I term as an "electronic chain". The pedals will instead drive a generator that charges the battery. Due to the very low power involved only a tiny motor and controller wired to permanently regen are needed. I'd do this on very aerodynamic recumbent trike that sips electricity to keep up with traffic, so the energy recovered from pedaling is meaningful, as well as be able to continue the workout while stopped or even parked.

I like that idea CR, sure easy to find out how much power you can pedal back into a spare pack. Could even have a little 30cc weed wacker motor to help out. I was thinking that for long distance riding, I know its not a purist ebiker mentality of being green but to get that 125km or 250km to a town would sure help out, especially when there are no outlets anywhere for those distances. Dont have to spend tons of cash on a bigger battery, but spend wisely on a battery sufficient for your needs, but of course always add on a bit more so there is less strain on the battery. Been thinking of that idea for awhile. Thats my plans, though there are gas stations I can charge at. I build my batteries to the charger I have, for maximum efficiency of charge time. 1C is a nice healthy charge rate, or a bit lower all depends. Thats all I plan for, my long distance is no province to province, because BC is stricter and I cant mess around over there. I've seen an older 500W generator last night on craigslist, even the chinesium gens of similar wattage are a hundred and a bit. Can pedal along, with some tunes, charging a backup battery. Then wave at the asian tourists in Jasper/Banff.

John in CR said:

The pedals will instead drive a generator that charges the battery. Due to the very low power involved only a tiny motor and controller wired to permanently regen are needed. I'd do this on very aerodynamic recumbent trike that sips electricity to keep up with traffic, so the energy recovered from pedaling is meaningful, as well as be able to continue the workout while stopped or even parked.

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Why fool with a mechanical solution at all....or even a controller-based solution for that matter.
Increase KV (which reduces torque by same) of motor on-the-fly by simply switching between "delta" and "wye" configuration of the three armature windings.