Using RC motors on E-bikes [Archive]

[Miles]

changing voltage isn't at all comparable to changing gears, changing voltage only changes the power output, the rpm/torque ratio is the same, gears keeps the power output the same but changes the rpm/torque ratio.

Yes, Kv and Kt are interdependent.

Using back-emf constant Ke [ V(supply)-IR/speed(rad/sec) ]

Then, in coherent units, Ke = Kt

 

[safe]

High powered e-bikes only need a few gears.

The solutions that would avoid "gear-down to gear up" could be...

If we're talking about 750 watts then you need a lot of gears to span the 0-40 mph range that the power can cover. You need a minimum of three (based on past calculations) and more like 5-8 is better. If you want to use something like Armature Current Limiting to narrow the powerband and reduce the heat you will need even more gears and lower ones to cover the lower speeds. The more efficient you want to be with 750 watts (in every sense) the more important the gears are. Just because you "can" get a motor to work in some situations with fewer gears doesn't mean it's doing the job well.

The more "illegal" you increase your motor power the less it matters that you use gears. But that's been my point... why waste time building these illegal machines if 750 watts is the law? I'm "okay" with parallel development, one path going towards legal "production" bikes and another going towards "formula" racing machines, but there needs to be a distinction.

As for "gear-down to gear up"...

The way I've suggested is to go:

From the Pedals gear UP to the motor. (chain one)

From the Motor gear DOWN to the rear wheel. (chain two)

___ So figure something like 44 pedal sprocket to 11 motor side for step one. (increasing pedal speed from 76 rpm to 304 rpm)

___ The motor does a geardown from 3000 rpm down to 300 rpm. (you would need a secondary if you went with the high rpm RC motor... the point of the thread)

___ Then the motor goes to the rear wheel at whatever gearing option you want to do with a single derailler or internally geared hub.

The speed of 300 rpm is about right because 500 rpm is right around 40 mph or so with a typical bike. This is not very hard to do, but you have to abandon the idea of making the pedal bottom bracket the "linkage spot" for the two power systems. The motor should be the "center" and the pedals should be feeding into the motor...

 

[Miles]

.... but you have to abandon the idea of making the pedal bottom bracket the "linkage spot" for the two power systems. The motor should be the "center" and the pedals should be feeding into the motor...[/color]

So, Optibike isn't the "bee's knees" after all, then?

 

[safe]

So, Optibike isn't the "bee's knees" after all, then?

There are many ways to do this... Optibike has it's own way... but I think the RC motor with it's high rpms and light weight is a great way to go too. Double planetary gears in the Optibike is excellent, but it's also complex and expensive.

Optibike sets a high standard that's for sure...

 

[Miles]

But, in the Optibike, the motor feeds the pedals......

 

[Mark_A_W]

Well the motor drives the final output stage which is coupled to the Ring Gear, with a freewheel I guess.

And the pedals are coupled to the Ring Gear with a freewheel. The chain guard is important if you want to keep your pants in one piece!

The trick is they geared the motor down to human cadence, then back up again at the back wheel. That's not perfect for the motor, but I think it's the best overall when packaging is included in the picture. Otherwise you have two completely different drive mechs, which is not perfect either - harder to lay out.

I watched their production flash. It's made in a job shop, basically by hand. That's why it cost so much. They could half the cost if they ramped up the volume, got some economies of scale and less labour intensive production methods. They build Optibikes exactly the same way my work builds medical instruments.

The next step for Optibike is full blown production, like a Taiwanese bike factory, without the luxury of cheap labour.

If they get that right I may have a chance to own one one day.

The Elation kit is the closest bolt on equivalent, but it's clunky in comparison, with the exposed motor and chain. Or the Cyclone, which is clunkier again...

 

[Miles]

Well the motor drives the final output stage which is coupled to the Ring Gear, with a freewheel I guess.

And the pedals are coupled to the Ring Gear with a freewheel. The chain guard is important if you want to keep your pants in one piece!

As it's a pedelec system, I don't think there is a freewheel for the pedals...

I hope there is one for the motor.....

 

[safe]

As it's a pedelec system, I don't think there is a freewheel for the pedals...

No way...

There have to be freewheels between the pedals and the motor going in both directions. There's no way they would produce something with this much precise machine work and omit such an important necessity.

That would be insane.

There's nothing wrong with gearing the motor all the way down to the pedal rpm, but it would be better to increase the pedal rpm up half way to the motor and then send that back to the rear wheel. That way the two rpms (3000 and 100) meet somewhere in the middle right where the rear wheel rpm is:

Motor Rpm - 3000 max (more for RC motor)
Pedal Rpm - 100 max
Rear Wheel Rpm - 500 max

Using a #35 chain you could go 114/11 from pedals to motor and that means that your 100 rpm translates up to 1000 rpm. Now the motor only needs to come down by three and then back down by two more... some very easy gears to handle.

 

[Miles]

Not insane at all. It's a bicycle. Bicycles are designed to be pedalled.

It's not a "sheep in wolf's clothing" motorbike.

 

[safe]

Not insane at all. It's a bicycle. Bicycles are designed to be pedalled.

Yeah, but they need to be independent systems... 850 watts of power is too much to tie directly to the crank. Let me check the pdf file... will return...

Okay, I'm right, they are independent.

Basically the drive spindle "floats" on top of both the crank freewheel and the motor freewheel, so the one that is going the fastest drives the spindle and the chain. It's a "double freewheel" in the forward direction which is perfect.

You do not need to pedal if the motor is running.

You do not need the motor running to pedal.

 

[TylerDurden]

Pedalec via torque-sensor seems a good method IMO: esp. for folks with drop-bars and/or carpal tunnel syndrome. Throttles are a PITH.

 

[Miles]

Pedalec via torque-sensor seems a good method IMO: esp. for folks with drop-bars and/or carpal tunnel syndrome. Throttles are a PITH.

I agree, but it does seem to have a straightforward throttle system on the Opti. I mis-remembered:
http://www.optibike.com/images/stories/manuals/2008%20Manual.pdf

 

[safe]

Not insane at all. It's a bicycle. Bicycles are designed to be pedalled.

Okay, I'm right, they are independent.

Basically the drive spindle "floats" on top of both the crank freewheel and the motor freewheel, so the one that is going the fastest drives the spindle and the chain. It's a "double freewheel" in the forward direction which is perfect.

You do not need to pedal if the motor is running.

You do not need the motor running to pedal.

 

[Miles]

Yes, you're right. I miss-remembered.

Certainly wouldn't make it insane if it was a true pedelec system, though..... well maybe it would be insane in the US.....

Come back and preach to us when you've actually made a bike with pedals, safe.

 

[safe]

Come back and preach to us when you've actually made a bike with pedals, safe.

I'm working on a pedal version... Project #003.

I'll get there... people these days seem to want everything all of a sudden (like a tv show episode where everything gets "solved" in an hour) but it takes a long time to do this stuff. The next task will be fiberglassing the molds for the front and rear fairings. I'll probably retrofit my existing bike with them too. There's always more stuff to do than time or energy to do it.

Eventually I do plan to get to an RC motor, but at the rate I'm going it will be at least a year and more likely two to three years... (by then I might be able to buy a real product and save myself the hassle)

The most valuable thing about these discussions is to spot and anticipate the future trends. The RC motor idea (high rpm small motor) seems to fit in two categories:

The "Big Iron" Killer: Building an electric bike with a small high rpm motor could deliver the power that a much larger and heavier hub motor can do. So "pure power" is one angle. These machines are completely illegal however and would be reserved for racetracks or people willing to tempt the police.

Multiple Speed Legal Ebikes: Being able to do what the Optibike has done and integrate the power and the pedals and the gears into a harmonious whole is likely what will actually be sold at the store. You would be able to buy and ride these legally.

 

[eP]

My question is: how often average e-biker do want change the gears ?

The next question is: how dense gears should be for e-biker (not a motorbiker) needs ?

Lets look for two cases. Common data for each one: Rm=40mohm, Io(@10V) = 2A, initial speed v=15mph (@ I=22A U=10.88V) P_in=239.36 W, P_out=200 W, eff. n=83.6%, no human input at the moment , P_in max = 1080 W (battery voltage 36V, current limit 30A)

case A) moderate hill begin - twice much torque needed to keep the same speed

case B) high hill begin - triple much torque needed for the same reason

How much dense (motor)gears e-biker need for both cases ?
What the gear ratios e-biker need for case A and for case B to keep decent efficiency (or any other actions - own (human) input for example) ?

What you want to do for both cases ?

Best regards

 

[Miles]

.... people these days seem to want everything all of a sudden (like a tv show episode where everything gets "solved" in an hour).....

I wouldn't know about that - I don't have a TV.... :wink:

 

[eP]

My point is: if we have a really efficient motor we don't need dense (motor)gears.
But if we don't need many dense gears - do we need gears at all ?
If we would be happy enough with two gears - why we should use manual motor gears instead two motors which would be swiched by onboard computer ?

 

[recumpence]

.... people these days seem to want everything all of a sudden (like a tv show episode where everything gets "solved" in an hour).....

I wouldn't know about that - I don't have a TV.... :wink:

Neither do I. :wink:

Matt

 

[Miles]

I wouldn't know about that - I don't have a TV.... :wink:

Neither do I. :wink:

Too few hours in the day as it is......

 

[Drunkskunk]

I think the optimum number of gears is... One. A good motor has plenty of torque for bicycle. for 99% of the people in the world, a one speed motor is plenty of torque for propelling A bicycle. over roads ment for bicycles.

There's always going to be a need for some specalized builds, but the extra weight and complexity of gears can be off set by having a bigger battery able to give enough extra preformance to cope with the lower efficancy of a single speed system.

 

[Miles]

My point is: if we have a really efficient motor we don't need dense (motor)gears.
But if we don't need many dense gears - do we need gears at all ?
If we would be happy enough with two gears - why we should use manual motor gears instead two motors which would be swiched by onboard computer ?

I'm happy with my 3 gears.

You could use 2 motors and there are lots of different ways that you could use them, I guess.

If you want an automatic 2 speed, you could also achieve it with retro-direct gearing, by getting the computer to reverse the motor.

 

[Miles]

I think the optimum number of gears is... One. A good motor has plenty of torque for bicycle. for 99% of the people in the world, a one speed motor is plenty of torque for propelling A bicycle. over roads ment for bicycles.

That depends on what you need in the way of speed:gradient ratio - unless you're talking about quite powerful motors.

 

[eP]

I think the optimum number of gears is... One.

So lets calculate how efficiency will change for case A and B if we have only one gear.

For case B: I=62 A ( (22-2)*3 +2 ) U=10+62*.04= 12.48 V
P_in=62*12.48=773.76 W
efficiency n=77.5%

so we see a few percent drop in efficiency

If we could switch the gear ratio to 1:2 and double the voltage to 20V then we could save some watts.
Lets assume Io=3A (@20V) then I=20*3*0.5+3A=33A, U=20+33*.04= 21.32 V
P_in= 703.56 W

so we could save 70 W for case B this way.

 

[eP]

That depends on what you need in the way of speed:gradient ratio - unless you're talking about quite powerful motors.

Quite powerful motors needs high gears ratio for the sake of high rpm.
But for the same reason they are low efficient at light load at the same gear.
But two gears are quite enough IMO.
Third and next gears could give us very little savings.