Musings on best universal drive approach

[Solcar]

I got a chance to examine my classic ten speed layout yesterday. It is a Huffy Omni-10. My interpretation of the information I gathered is that the simplest system is left side drive of rear wheel, with the freewheel at the hub. Only need two freewheels this way, while driving the pedal chain requires three if turning an unpowered motor by pedaling is to be avoided. The two stage reduction gearing allows a smaller, faster turning motor. That is technically a much simpler motor to get manufactured. I use a Pittman servo motor which is two inches in diameter and 5 inches long. It uses brushes, which is easier and cheaper to control. My regret is that these are high-end surplus motors that I lucked upon; that is, I wish everyone could still get one for $15.

-The basic approach is freewheel on left side of hub with the chain from its attached big sprocket going up to the crank which it uses as an axis of rotation when driving a small sprocket of a two sprocket assembly.
-That two sprocket assembly is a small sprocket attached to a big sprocket.
-That big sprocket is driven by the motor sprocket.
-The motor is mounted to the lower main frame bar. It also might be mountable behind the seat.
-Controller: One current controlled PWM output. Doesn't require everything internal in triplicate to drive separate motor windings.

This approach keeps all the electric drive components out of the way so that maximum cargo space is available. I carry boxes of groceries on my bike. Your thoughts are welcome, and thanks in advance.

 

[liveforphysics]

Just a freewheel on the left side means when you pedal with-out the motor running, your pedaling drags the motor along.

For complete drive isolation, a pair of freewheels on a free-hub is the esoteric solution.

If you don't mind your legs dragging along the motor, or you never plan on a time where you would pedal without the motor, then your plan will work out nicely.

 

[Solcar]

What a wonderful polite response, liveforphysics. There would be the freewheel on the pedal side, under the 5 sprocket cassette and the other one added to the left side. I didn't mean to convey the notion that there is only one freewheel, but I write with an unconventional style!

 

[Miles]

That seems fine. I've always liked the idea of using the left side of the BB axle as a jackshaft. I suggested it, in a slightly different context, here: http://endless-sphere.com/forums/viewtopic.php?t=3984

I guess there's really no best universal drive system - you have to decide what your priorities are and then choose the system which is the best fit to those..... It seems like you've done that.....

 

[Solcar]

That seems fine. I've always liked the idea of using the left side of the BB axle as a jackshaft. I suggested it, in a slightly different context, here: http://endless-sphere.com/forums/viewtopic.php?t=3984

I guess there's really no best universal drive system - you have to decide what your priorities are and then choose the system which is the best fit to those..... It seems like you've done that.....

Alas, no universal drive system. I think a zero-in on one would help greatly in getting ebikes out there in the public. After all, Henry Ford got cars out there by making just one car, the model T, and the customer could have any color desired so long as it was black.

Like in the old days, the classic diamond framed bike was so common and if that were still true, it would facilitate closing in on that logically-best drive method, just as the diamond frame, itself, was the best combination of possible characteristics and trade-offs. But there are a hundred bike styles now! The closest I think we can get is some type of geared, brush hub motor.

 

[Miles]

But there is no consensus on what an ebike is....... For some, it has pedals so you can pedal home if the battery runs out, or it breaks down. For others, it has a motor for occasional help up hills etc.....

 

[Solcar]

But there is no consensus on what an ebike is....... For some, it has pedals so you can pedal home if the battery runs out, or it breaks down. For others, it has a motor for occasional help up hills etc.....

Indeed, but both are true since the same bike does both and the range is just shorter in the first case. I touch upon this point not because I am a fun-forbidding puritan, but because humanity is fast approaching the fabled lemming cliff.

In the meantime before the "Model T" bike is developed and always afterward, everyone keep enjoying your personal creativity on your own projects. I've seen some cool work on this forum!

 

[Miles]

Indeed, but both are true since the same bike does both and the range is just shorter in the first case.

At a simple level, sure... But you'd probably make different design decisions for one case or the other....

I can't see any particular system predominating anytime soon...

 

[Solcar]

Indeed, but both are true since the same bike does both and the range is just shorter in the first case.

At a simple level, sure... But you might make different design decisions for one case or the other....

It's like all cars use the same basic design with variation for different models, because that is the most practicle platform known. It is altered with variations of particulars like engine size and type or number of doors.

I can't see any particular system predominating anytime soon...

We have a long way to go before we get to a standard of ebikes that is basically as perfected as civilization can get to. The automobile has been there for 75 years now, but housing never did get there IMO, since it is much more complicated and multi-faceted.

 

[John in CR]

With the Chinese putting 20 million new 2 wheel EV's on the road each of the last 4 years, I think the world is way past the Model T stage. Peoples' ebike needs and preferences vary greatly, as do the local regulations, so there can't be a universal drive, at least unless someone comes up with a high power/high torque direct drive brushless hub motor that weighs well under 10lbs. Brushed motors aren't the answer because they don't last and electrical failures can result in a runaway full throttle condition. Geared hub motors have proven to be unreliable as well. Existing direct drive hubs motors suffer from either being under powered or they are too heavy, and only the heaviest have any meaningful hill climbing ability.

After riding near silent direct drive hub motors for almost 2 years, I could never go with something noisy, so for me that eliminates all high rpm motors and multiple reduction drives unless there's a way to almost totally absorb the sound inside some kind of housing with years of reliability.

John

 

[Solcar]

With the Chinese putting 20 million new 2 wheel EV's on the road each of the last 4 years, I think the world is way past the Model T stage. Peoples' ebike needs and preferences vary greatly, as do the local regulations, so there can't be a universal drive, at least unless someone comes up with a high power/high torque direct drive brushless hub motor that weighs well under 10lbs.

That hasn't been a problem as with the variations of the same basic car design.

Brushed motors aren't the answer because they don't last and electrical failures can result in a runaway full throttle condition.

That could be addressed simply with a panic button. A lot cheaper than lots of engineering work and resource use to manufacture a lot of gadgets to handle every evetuality.

Geared hub motors have proven to be unreliable as well. Existing direct drive hubs motors suffer from either being under powered or they are too heavy, and only the heaviest have any meaningful hill climbing ability.

After riding near silent direct drive hub motors for almost 2 years, I could never go with something noisy, so for me that eliminates all high rpm motors and multiple reduction drives unless there's a way to almost totally absorb the sound inside some kind of housing with years of reliability.

John

Hub motors like that need to be big so that the torque is decent enough not to get too inefficient at low speed and heavy load. That is resource intensive and expensive to make since a lot more magnet and copper is needed.

 

[ekline309]

Brushed motors aren't the answer because they don't last and electrical failures can result in a runaway

Could you expand on this John?? It seems like the simplicity of DC system could outweigh the drawbacks of maintenance and efficiency.

 

[Miles]

Is this the motor that you have, Solcar?

 

[liveforphysics]

Is this the motor that you have, Solcar?

If that's the servo motor, it's time to get a better suited motor. The winding resistance on that thing won't even allow it to be usefully powerful.

Unless you've got the version with the 12v wind, and your controller can take a 48-60v input voltage, you're not going to be able to trick at thing into making a useful amount of power with any reliability.

That motor is less powerful than a $20 RC motor the size of a golf-ball, and the RC motor has no brushes to wear.

 

[Solcar]

Is this the motor that you have, Solcar?

Neat find, Miles. Hmm. I remember the ratings given by the surplus house (Hosfelt Electronics) as rpm @12v = 2800 and stall torque @ 12v = 216 oz*in, though I seem a bit foggy on the rpm figure, wondering if it was really 1800 rpm. I might be able to find the paper copy of that catalogue page from 15 years ago. I measured the contact resistance as about 1 ohm. I also recall a light no load current rating, which I took as a sign of efficiency. I figure the bearings are ball type because the armature spins so freely. So freely that my present ebike is not unbearable to pedal with the motor unpowered, even though it has pedal crank drive input without freewheeling electric drive and the gear reduction is about 15:1. (To be fully rider friendly, it would need two more freewheels, 3 total for crank sprocket drive: one at the gearbox output, one at the chain ring sprocket, and one at the rear cassette.)

I would still like to insulate pedaling from the drive system by freewheeling, though, and also see how I would like being able to easily shift the pedal gears while running the motor. Those are some reasons why I am contemplating trying to build rear wheel drive on the left side. I don't really want the work or expense changing the pedal sprocket to freewheeling type. I wouldn't mind so much if I were using a Cyclone kit, but I prefer my more from-scratch approach.

I do have some classic Raleigh ten speeds with factory-installed front freewheels, but I'd rather not reconfigure things to utilize one of them for the foreseeable future. If I had had them in my possession 15 years ago, I would have used one of them to allow the bottom bracket drive without the pedals turning when running the motor.

I recall the Hosfelt catalogue listing my motors as ball type bearings, so I am wondering if the data sheet you provided is saying those have sleeve type, since it mentions "sintered bronze bearings".

Edit: I think the peak torque that came from the chart that Miles gave the link to is the correct figure. I said 216oz-in whereas the chart says 256oz-in.

 

[Solcar]

Is this the motor that you have, Solcar?

If that's the servo motor, it's time to get a better suited motor. The winding resistance on that thing won't even allow it to be usefully powerful.

Unless you've got the version with the 12v wind, and your controller can take a 48-60v input voltage, you're not going to be able to trick at thing into making a useful amount of power with any reliability.

That motor is less powerful than a $20 RC motor the size of a golf-ball, and the RC motor has no brushes to wear.

I know what you mean! I estimate that if I take the voltage to about 40, I can get about 200 watts of assist, even with the current limit that I use. On level ground, I was able to reach emergency speed of about 40mph. I wasn't ever able to go much over 30 without assist even in my youth.

My controller (with built-in charger) can convert 12v from anywhere between 0v and about 40v. It uses current mode control.

 

[Miles]

With such a small power output, even if it was 200 Watts continuous, you really need to keep running the motor through the pedal gearing, I think....

I still like the idea of left-side drive with a separate 2 speed gearbox, for more powerful motors, though. There are advantages to having a separate gearing system for the motor drive.

 

[liveforphysics]

The following is true if you connected your motor to an infinite current source (no current limiting).

If you've got 1ohm winding resistance, then your voltage will equal your stall current. At roughly half of the no-load peak RPM, current will also be roughly halved (from BEMF). Useful torque output reaches zero at peak RPM. These are important things to think about when you setup your gear ratio choices.

With brushed, the peak RPM is generally set by the limits of the rotor radial g-force loading on the windings (brushless has no copper in the rotor), so you're not as free to just pump up the voltage to maintain decent current draw unless you've got a way to ensure it never gets rev'd up with the tire off the ground, or goes down a steep hill or something.

 

[Solcar]

With such a small power output, even if it was 200 Watts continuous, you really need to keep running the motor through the pedal gearing, I think....

I still like the idea of left-side drive with a separate 2 speed gearbox, for more powerful motors, though. There are advantages to having a separate gearing system for the motor drive.

I think that is good general advice. Yet my way of riding at a leisurely pace, often with 50 pounds of groceries involves not really ever needing to go much over 10mph. Even slow like that, it is way way better than my alternatives of driving a car or walking.

 

[Solcar]

The following is true if you connected your motor to an infinite current source (no current limiting).

If you've got 1ohm winding resistance, then your voltage will equal your stall current. At roughly half of the no-load peak RPM, current will also be roughly halved (from BEMF). Useful torque output reaches zero at peak RPM. These are important things to think about when you setup your gear ratio choices.

With brushed, the peak RPM is generally set by the limits of the rotor radial g-force loading on the windings (brushless has no copper in the rotor), so you're not as free to just pump up the voltage to maintain decent current draw unless you've got a way to ensure it never gets rev'd up with the tire off the ground, or goes down a steep hill or something.

Those thoughts seem quite reasonable! Indeed, the output rectifiers on the voltage booster are 100v Schottky types. So I want to limit the output to about 45v.

 

[liveforphysics]

Those thoughts seem quite reasonable! Indeed, the output rectifiers on the voltage booster are 100v Schottky types. So I want to limit the output to about 45v.

Your controller boosts voltage rather than bucks voltage with PWM for current control???

Can you post a picture of the board? I'm pictureing a DC/DC up-switcher setup built into a current controlled DC motor driver, and it would be a great learning experience to get some pictures of the way they layed that all out.

Thanks!
-Luke

 

[Solcar]

Those thoughts seem quite reasonable! Indeed, the output rectifiers on the voltage booster are 100v Schottky types. So I want to limit the output to about 45v.

Your controller boosts voltage rather than bucks voltage with PWM for current control???

Can you post a picture of the board? I'm pictureing a DC/DC up-switcher setup built into a current controlled DC motor driver, and it would be a great learning experience to get some pictures of the way they layed that all out.

Thanks!
-Luke

I wouldn't mind at all! I designed the circuit using the free simulator LTspice provided by Linear Technology. However, people hate my style of drawing moderately complex analog circuits crammed on one page so the component symbols remain big but packed together. That way I can see the whole circuit without straining my eyes, but on others it seems to have the opposite effect. If you wouldn't mind joining my Yahoo 'switchmode' group, if I recall correctly, a partial version without the charger portion (that uses the same transformer as the booster/controller, but in the reverse direction) is in the files section. I would have uploaded the whole circuit including the charger to the files section, but I figured the motor/booster portion was hard enough for others to follow already.

http://tech.groups.yahoo.com/group/switchmode/

 

[Solcar]

Luke, I've dug up a simplified version of the voltage booster/motor controller. This version doesn't have the capacitive voltage doubler driven by the transformer, so it outputs a maximum of 24v as opposed 48v. I call using 74C14 hex Schmitt inverters that way in power supplies the USMPS (Universal Switchmode Power Supply) method, but the voltage booster/motor controller can also be done with the SG3525 PWM controller IC. The USMPS topologies are great for simulators because the 74C14 works well on them. I don't have a component model for the SG3525 and am not computer savvy enough to add models with good results.

The throttle control changes the bias point on the top PNP transistor. I left it out of the diagram.

Edit: I had forgotten to place two diodes in the diagram. The lower voltage side of the load should not have been connected to circuit ground. It should have been pulled down by isolation diodes connected to the MOSFET drains. I have replaced the diagram.

 

[Solcar]

I corrected an error in the voltage diagram in my last post.

I'd like to retouch upon the physical drive of this next project I am considering building, particularly, the left side rear sprocket that I would have to install onto the hub. The place where this freewheeling sprocket needs to be mounted has no threads and might be a bit short on width. So, the success of the project depends on being able to epoxy a freewheel sprocket, e.g. a 16 tooth one, over that bearing casing there.

Then a metal ring could be mounted to the teeth of that sprocket. Onto that ring, I am planning to mount a big sprocket. At this time, I am thinking to make it out of a lid from a black 5 gallon bucket (the kind like a restaurant pickle bucket). The black pigment should give some ultraviolet light protection. The teeth have to be quite accurate. I am trying to find the best trick for making this with rudimentary tools like a jigsaw and drill.

One possible trick might involve placing a hot chain over the roughed-in teeth on such a sprocket in order to try to get them to conform to the chain when the plastic melts a little. Presently, the chain I think I would use is #25.

The size of that rear sprocket being big should help the chain clear the kickstand where it attaches to the frame. The kickstand attachment point would be encompassed by the chain as it extends to something like a twenty tooth sprocket that rotates on the pedal crankshaft. The big rear plastic sprocket has about 140 teeth.

That 20 tooth sprocket is bonded or bolted to a 60 tooth sprocket, maybe a little smaller, though. Both spin on the pedal crankshaft acting as a jack shaft.

A chain goes from the possibly 60 tooth one to about a 14 tooth one on the motor. Better not to make the sprockets too small in order to keep noise down and efficiency up.

I calculate a total reduction ratio of about 28 to one. That gives enough mechanical advantage leeway for the motor to operate efficiently. Up to a point, I plan to raise the voltage to the motor to go faster. I rarely go over ten miles per hour these days, though.

 

[Solcar]

Here is bad artwork depicting the layout of the sprockets and chains.