electric bicycle?

[quarky]

Greetings. I seek scrutiny from electrical engineering types and bike nuts.

Would it be possible, or feasible to build a bicycle that had a generator in the bottom bracket (where the cranks are) and a motor in the wheel hub?
If so, this might eliminate the chain and derailures, possibly adding little or no extra weight. A small battery might be needed, to even out the flow.
The idea would be to remain at a fairly constant cadence, turning electric energy into mechanical energy.

Would the losses be prohibitive, as compared to chains and multi-sprockets?

(if this has been done already, I could find nothing)

This idea is not the same as an electric assist on a bike, which is done all the time.

Thanks for any feedback.

 

[kookbreaker]

How do you plan to get the generated power to the rear wheel?

Keep in mind, a bicycle is amazingly efficient. By adding a generator and motor to the system you cut the efficiency at least in half (and I am probably being generous). In other words, you would need to pedal twice as hard for your system to be as fast as a conventional bike.

 

[GreyICE]

You have a kinetic energy - kinetic energy conversion (highly efficient) and you're proposing to replace it with a kinetic - electric electric - kinetic conversion?

Wow, yeah, pedal five times harder for the same result.

Prohibitive.

 

[Bikewer]

The pedals have been used to power such low-powered devices as tail-lights; usually not very efficiently. They tend to sort of fade and dim....

Cooking up enough wattage to power the bike itself does seem a little counter-productive.

There are a number of electric or electric-assisted bicycles on the market, but they all use conventional storage batteries.

 

[Ziggurat]

Would it be possible, or feasible to build a bicycle that had a generator in the bottom bracket (where the cranks are) and a motor in the wheel hub?

Possible? Yes. Desirable? No.

If so, this might eliminate the chain and derailures, possibly adding little or no extra weight.

Not likely, the chain/gear/derailures really don't weigh that much. A generator/motor combo will almost certainly weigh more, possibly quite a bit more.

Would the losses be prohibitive, as compared to chains and multi-sprockets?

Sprockets and chains are very low loss. Just flip a bike upside down and crank it by hand: it takes very little force to keep the chain spinning. Any generator/motor combo will most definitely be much more lossy than current chain systems. Electric assist bikes can afford the additional weight because they can lift themselves up hills, but with what you're suggesting, you get additional weight PLUS decreased efficiency and decreased performance, with no benefit anywhere.

 

[quarky]

Thanks. I was thinking that perhaps the losses could be minimal, and said bike would be easier to operate, with one putting out steady wattage, as on a workout machine; generator matched to one's output, a few hundred watts, at most. I see the wrongness, overall.

 

[Sunstealer]

The beauty of the (safety) bicycle is really in the machine's simplicity, which hasn't changed in 100 years or more. It's incredibly efficient; I recall figures like 98% efficient - I'm sure there are many studies around that address this issue . I'm also sure that such a set up that you are proposing would not be anywhere near as efficient and of course weigh more that what is available on the market today.

There have been huge developments in bicycle technology since the mid to late 80's mainly driven by the new sport of mountain biking, which required novel solutions to new problems coinciding with developments in new materials and processes. Throughout that time the good old chainset and dérailleurs have survived and been improved, alternatives never caught on.

I don't find it amazing that a chain driven system is very efficient, but I do find it amazing that gearing is still produced by derailing the chain from one sprocket/chain ring to the next and operated via wires which is quite crude i suppose. It's very slick, especially on a road bike, and unless internal freehubs/gearhubs are developed significantly then the traditional methods will be here for a very long time.

Incidentally I'm now reminded of the time I rode the Zike; a Clive Sinclair designed electric bike, at Alexandra Palace in the very early 90's. A friend of mine was exhibiting Trimble mountain bike frames at the time and I was part of that set up. Anyway, I had a go and dismounted at the end of the floor only to be confronted by Clive who asked, "so what do you think?". I was forthright and said it was crap. The concept was good, but the frame geometry and handling were so poor that it was difficult to ride. I showed him the specifics of why - too shorter stem and minimal fork rake and told him he'd be lucky to sell 100 and that the company would fold. Battery technology may have improved, but I doubt that it is a viable alternative to muscle power and getting fit.

 

[quarky]

One of the reasons I brought up this old idea, was because of another thread I started recently, in this forum.

It involved sucking heat, for heating, thru friction and a windwheel; minus electronics.
The idea was met with claims of how efficient electrical transfer can be; why try to go mechanical.

And so I remain confused, a little.

 

[GreyICE]

One of the reasons I brought up this old idea, was because of another thread I started recently, in this forum.

It involved sucking heat, for heating, thru friction and a windwheel; minus electronics.
The idea was met with claims of how efficient electrical transfer can be; why try to go mechanical.

And so I remain confused, a little.

Heat and mechanical energy are quite a bit different.

A good heat pump can easily get a 5:1 ratio on the Heat out:Energy in ratio. That's somewhat tough to beat.

Kinetic energy is a bit less efficient, obviously.

 

[PingOfPong]

One of the reasons I brought up this old idea, was because of another thread I started recently, in this forum.

It involved sucking heat, for heating, thru friction and a windwheel; minus electronics.
The idea was met with claims of how efficient electrical transfer can be; why try to go mechanical.

And so I remain confused, a little.

I was one of the contributors in your other thread and what I said there is still correct. Conversion for a pricey motor/generator could be high as 95% and above. FYI, energy efficiency in battery is not so great at around 80%.

Note that in your windmill discussion only a generator was needed. With your bike idea you've stumbled into a new realm of problems. Motors have something called a speed-torque curve. When an AC motor starts it tends to draw a massive current. This large current will continue until the rotor gets up to speed and produces a back emf to balance the source. For a windmill, we can expect a certain range of speeds and choose the proper generator that has the best characteristc curve for that speed range. For your bike, it has to work from a stanstill up to the maximum rider's speed. If we used a regular old fashioned induction motor (like I was thinking for your windmill idea) a rider would have to peddle like crazy (wasting energy) until the motor reaches something called the "breakover" speed. After that, the effort to speed conversion would feel normal for the rider.

Here are some alternatives.

DC/AC hybrids. Some AC motors can use a DC mode to "kickstart" itself up to the breakover speed after which it runs in the efficient AC mode. I haven't studied them so I can't say for certain they would do the job.

Exotic AC motors. Induction motors are not the only AC topology. There may be something on the market that addresses the problems of starting torque and current. I haven't studied these either.

DC motors. A good 'tried and true' choice. They're perfect for variable speed. Unfortunately, they're not all that efficient. They tend to be in the neighborhood of 85%.


P.S. I'm an EE but my focus is on embedded design. I did study this motor stuff but I'm far from an expert. The numbers I'm using are figures of merit that I was taught in school or picked up here and there. Look around if you like. There may be some hot new DC motor with astounding efficiency.

 

[Ziggurat]

Thanks. I was thinking that perhaps the losses could be minimal, and said bike would be easier to operate, with one putting out steady wattage, as on a workout machine;

If you're working too hard on an ordinary bike, either slow down your rate of peddling or drop the bike to a lower gear. With a 21-speed bike, for example, the gears are close enough that you can achieve close to constant-effort riding with just a bit of practice. If you want to spare the rider the skill requirement and effort of gear shifting, there's another alternative which is already in use: automatic shifters. The idea is that the bike shifts up or down for you, depending on the speed, in order to maintain a roughly constant peddling speed. These tend to have only a small number of gears, so they don't work as well as a 21-speed bike in keeping the gearing ratio optimized, but they do work.

 

[X]

Sorry Quarky.

You could do it, but it offers no benefits over the existing bicyckle drivetrain.


Have you ever ridden a bike with a generator-headlight?
Try it sometime to get a feel for the resistance of electrical generators.
Granted, the headlights ride on the rim, not the crank, and this amplifies the feeling of resistance somewhat. But it gives an idea.

If people want an easier time pedalling, go to higher-inflation pressures on the tires, or smoother tires.
I run my mountian bikes at 60psi (XC semi-slicktires), and can attain speeds of over 50 kph before my feet can't keep up to the pedals. I can cruise comfortably at 35kph. Road bikes run over 100psi, and with bigger gears, can cruise comfortable at 50kph.
The higher pressure/lower resistance tires roll easier, but if the going gets muddy of sketchy, they provide less traction.
It's a cheaper alternative to re-buiding your bike, if you are looking for ways to make pedalling easier.
It doesn't help with wind or hills, though.

 

[ben m]

It involved sucking heat, for heating, thru friction and a windwheel; minus electronics.
The idea was met with claims of how efficient electrical transfer can be; why try to go mechanical.

Moving mechanical energy around (chains, driveshafts) is hugely efficient, but tends to be inflexible and cumbersome.

Moving electrical energy around (wires, transformers) is hugely efficient and incredibly adaptable.

Moving heat around (insulated pipes) is lossy and inefficient, and also inflexible and cumbersome.

Changing energy from electrical to mechanical, or vice-versa, is inefficient; you lose a significant fraction to heat at every step.

Your original windmill idea was take mechanical (or possibly thermal) energy and move it around a lot---down a tower, across some land, into a residence, across the basement ceiling and into a water heater? Here, mechanical or thermal transmission sounds like an engineering nightmare, and if it's thermal it'll be very lossy.

This bike idea is to suffer two rounds of conversion loss in order to avoid ... lossless mechanical transmission over an 18-inch straight line.

 

[PingOfPong]

I just read a wiki article on bicycle performance. It claims that 10%-15% of power can be wasted by gearing mechanisms. That's more wiggle room than I expected.

Quarky, what do you think of this idea? The rider must pedal at an certain speed or at least a narrow range of speeds. This speed would be optimized for leg pumping comfort. At higher speeds the rider will have to pedal harder to supply the necessary torque. However, no matter how fast the bike moves the rider will still pedal at the same speed.

I'm thinking something along these lines: Pedals -> synchronous AC generator (not induction)-> rectifier -> DC motor -> wheel. The 'rectifier -> DC motor' section would be an efficiency problem of course. I think you're idea is kind of interesting as an engineering question so I might end up studying modern AC motor designs to see if there is an alternative.

I'll tell you what. If you'll do some digging to find out the range of power a bike rider outputs (Horse power or Watts will do), I'll do some internet window shopping and see what's available for your idea. Just so you know, there is no garuntee that any combination of parts exists that will get the job done cheaply, efficiently, or at all for that matter. I should know because I have a ton of ideas that are on the backburner until the parts I need become available or cheaper.

 

[PingOfPong]

Changing energy from electrical to mechanical, or vice-versa, is inefficient; you lose a significant fraction to heat at every step.

Not so much

 

[casebro]

Diesel electric locomotives have that market all sewed up. So rotational force>electrical energy>rotational does work - at some scales.

But I do think the world may be ready for the hybrid bicycle. They would be a real boon to the in-not-so-good-shape commuter. Just pedal as much as you feel good about, and still get up the hills.

 

[soylent]

A good, utility-scale generator can be 95% efficient; likewise for an electrical engine.

But that's not what you need at all. You need a constant-voltage, variable-speed generator to match your battery(or ultra-capacitor most likely, as they can deal with very high power densities and you didn't seem all that interested in energy density). And you need a variable speed DC motor.

There are electric bicycles. They use chain-drive and a motor; battery charged from the grid. They're quite popular in China I believe.

 

[Bob Blaylock]

It seems worth mentioning that on much larger-scale machines (such as railroad locomotives, ships, and submarines) it is very common to use an electrical generator-motor system rather than a mechanical system to get power from the engine to the wheels or propeller. On that scale, at least, it is apparently more efficient than the mechanical transmission systems that might otherwise be employed.

 

[quarky]

Much thanks, folks.

Actually, I'm fairly familiar with bikes; raced skinny 10 speeds in the 60's; ride a recumbent trike now. I have no problem with derailure systems, but I've noticed that most people can't shift properly, and often ignore the gears and don't try for a steady cadence.

Ping of Pong, I believe that the power range is somewhere between 500 watts (for a few seconds by athletes) and 100 watts, for more casual riding.
I have tried to generate electrical energy (to charge a battery) on a bike stand, and was frustrated with the available generators. (This was 30 years ago). Going ac hadn't occurred to me. Thanks for your willingness to consider the issue.

 

[Delvo]

I do think the world may be ready for the hybrid bicycle. They would be a real boon to the in-not-so-good-shape commuter. Just pedal as much as you feel good about, and still get up the hills.

That basic idea of mixing two power sources, human muscle power and something else, already exists and is called a "moped". The "mo" part refers to a fuel-burning ICE. Replacing that engine with an electrical power source and motor would be roughly the same thing as replacing the engine in a car with an electrical system: either you add a bunch of weight to get the same power and a shorter range, or you cut the weight down and get less power and range.

This bike idea is to suffer two rounds of conversion loss in order to avoid ... lossless mechanical transmission over an 18-inch straight line.

There are problems with the chain system. Aside from "exposure to the elements", there's the fact that the chain tends to slip off of the gear it's on when you want it to stay there, and to stay on it when you want it to shift to another. Since this happens mostly or only with the rear gear set, which has 5-7 gears of rather similar size, and rarely or never the front, which has 2 or 3 of more radically different sizes, this could be handled by just reducing the rear gear set to 2 or 3 gears (or even just 1 if the problem is instead related to the different system that's used for transfers between the rear gears). But no bikes like that seem to be built; there seem to always be lots of very similar-sized rear gears if the bike has any gear-shifting at all (and the ones without it are kids' bikes).

I have seen one that goes an entirely different route, using a purely mechanical system but no chains and nothing exposed. The pedals and the rear wheel are connected by a driveshaft as in cars, with gears at each end to convert the axis of rotation from x to z and back to x again. Those right-angle gears, along with whatever is used for a transmission, are encased, so the whole thing looks a bit like a limb bone: narrowish for most of its length (the driveshaft), and bulging on the ends (the gearboxes). But I didn't even test-ride it and haven't heard from anyone who has, and it hasn't caught on in the bike world yet (or I would have seen more than one).