Using RC motors on E-bikes [Archive]

[Miles]

Piotrek,

What is the maximum gradient you have in mind?

 

[safe]

But two gears are quite enough IMO.
Third and next gears could give us very little savings.

A Thought Experiment

Let's say you have two bikes:

One bike needs to climb a 10% slope at 10 mph and reach a top speed of 40 mph using the minimum amount of power.

The second bike needs to be able to climb a 10% slope (at any speed) and only needs to reach a top speed of 20 mph. (pretty much the legal definition of an American ebike)

We can go through the formal math on it, but let's just observe the obvious first... the bike that only needs to go 20 mph for a top speed can fairly easily be able to climb the hill with a moderate amount of power. The bike that needs to hit the faster top speed needs a wider range of gearing so it needs more gears to operate efficiently. Try to climb the hill on the second bike and it's no problem because the natural power peak is so low (since it's geared for 20 mph max) that you can just have a one speed.

But the other question becomes how much power you are allowed.

The less power you are allowed for the motor the more gears are needed because you need to gear down for the hills. One might think of a sixteen wheeler truck transporting several tons of lumber (let's say) up a steep incline, such a machine has a very small motor compared to the weight it's trying to lift. In order to do that you need many forward gears so that you can climb slowly enough to allow the motor to spin many revolutions. Lowering the gear ratio is the same as raising the torque.

http://en.wikipedia.org/wiki/Torque

Now let's say you want to have a top speed that equals the human pedal powered record on flat land which is 80 mph. It's pretty obvious that if you want to be able to climb hills and also reach a top speed of 80 mph that you will need enough gears to cover all those conditions. If your aerodynamics are good enough you could get to 80 mph with not much more than 750 watts. (I think they measured the guy doing more like 1000 watts, but you get the idea)

 

[Mark_A_W]

I don't know about you guys, but I pedal my ebike as hard as a normal bike (clipless pedals and all).

I use all of the rear cluster and the two larger ring gears at the front. If I have an electrical problem I use the smallest ring gear at the front....

Personally I would not like to give up any gears for pedalling.

However I think it would be nice if someone could come up with a 2 or 3 stage gearbox for the motor, that can shift under full load. A CV would be nice too.

I think the motor needs about 3 gears, maybe two (climb and speed). The person however, needs lots of gears - it's a bike, not a motorbike. Under US law the 20mph limit means the person has to help if you want speed. I wish we had a 750w 32kmh law like you guys.

 

[eP]

Piotrek,

What is the maximum gradient you have in mind?

I don't have any paricular maximum gradient in mind.
Maximum gradient is power and total weight related.

Two gears are flexible enough to cover any gradient you want, for any weight and at limited power budget - to keep efficiency at decent level all the time.

We should keep in mind the smaller power budget we have ( we are more limited) the less important is efficiency as our absolute energy savings would be smaller too.
So until we are not creazy about each one Wh we dont need more than two motor gears (right way selected).

@ safe

Now let's say you want to have a top speed that equals the human pedal powered record on flat land which is 80 mph. It's pretty obvious that if you want to be able to climb hills and also reach a top speed of 80 mph that you will need enough gears to cover all those conditions. If your aerodynamics are good enough you could get to 80 mph with not much more than 750 watts. (I think they measured the guy doing more like 1000 watts, but you get the idea)

My point is we need only two gears for motor.
If you sure we want more than two - so lets go to the math.
Make a an example you want - than we will see what we will lose if we would be limited to two motor gears only.

If we are going to use really efficient motor than we should stay very happy at two motor gears i'm sure.

If i'm wrong than lets go to the math to see how much i'm wrong.

@Mark_A_W

I think the motor needs about 3 gears, maybe two (climb and speed). The person however, needs lots of gears - it's a bike, not a motorbike. Under US law the 20mph limit means the person has to help if you want speed. I wish we had a 750w 32kmh law like you guys.

The two - climb and speed - are enough, that is my point ( for any limited power )

 

[Miles]

I'm with Mark on this,

My bike has 3 gears for the motor and 8 for pedals. The 3 motor gears are already there as part of the pedal drive, so.... More gears for the motor would just be a nuisance - fewer gears for the pedals would be insufficient, for me, when used as a normal bike.

Of course the number of gears you need for the motor depends on load, speed, gradient and efficiency. eP, if you want to select so that two gears is enough, fine.... but isn't that because you already favour a 2 motor solution?

safe, I think we understand the basics, already......

 

[eP]

Of course the number of gears you need for the motor depends on load, speed, gradient and efficiency. eP, if you want to select so that two gears is enough, fine.... but isn't that because you already favour a 2 motor

No it is not !! Wrong sequence Miles
I favour 2 motor because i see 2 gears are more than enough.

The key element is max motor efficiency. If that efficiency is at 90% level or very close to that, than motors efficiency characteristic is very flat.
As a result we are able to use only two edge gears (for speed and climb) at moderate drop in efficiency for intermediate conditions (loads).

75-80% max efficient motors needs much dense gears to keep efficiency at moderate level (very close to the max )

~90% efficient motor is quite different case for two reasons:
- first is obvious: higher top efficiency allow for higher drop in efficiency and still efficiency remain decent
- second reason is less obvious but is even much more important : ultra efficient motors have very flat efficiency characteristic

Load and gradient are much less important (until you not going to climb on vertical walls ).

If you still can't belive me than give some data you want : load, weight, max gradient, max speed, max power.
Than i will select efficient motor and two gears only.
Than we will see what we could lose without intermediate gears.

OK ?

 

[Miles]

Piotrek,



Ok

Total load: 100kg

Max. speed: 40 kph

Max. gradient: 20%

 

[eP]

Motor params: Rm =40 mohm , Io(@20V) = 2.4 A, Io(@32V) = 3A

1) climb gear:
100kg*20% =200N
climb speed ~ 2.5m/s (9km/h)
power =500W
working point : 18V , 30A => power out 540W ( 40 W excess for transmission loses )

so: I= 30+2.4= 32.4A
U=18+0.04*32.4=19.3V
P_in = 625 W => efficiency n= 500/625 = 80% (decent)

max speed at this gear @36V ~5m/s (18km/h) at light load

2) speed gear => 2.5 lower gear ratio than climb gear (taller gear)
45km/h @ Uworking =36V
40km/h @ Uworking =32V

lets assume 2V sag @ 30A load =>Uworking = 36-2-30.004=32.8V => max speed 41 km/h

Pout = (30-3)*32.8=885 W lets assume transmission lose =65 W so we still have 820W at wheel what should be enough for 40km/h

P_in =30*34=1020 W so still efficiency is at 80% level (decent)

============================================

Would you like Miles give 3 intermediate working points ( speed and wattage at wheel) than we could check efficiency at each one for climb gear (within 0-18km/h speed range) and for speed gear ?

I'm sure efficiency will stay very close to 80% for any point you want.

 

[Miles]

Now let's assume that for 90% of the time you will be travelling at 25kph, with gradients up to 5% and lots of stopping and starting....

 

[eP]

Now let's assume that for 90% of the time you will be travelling at 25kph.

At what gradient ? Give me the wattage at wheel. Or you can give me three different wattages for that speed (three different working points).

I'm going to make your 3 wishes like golden fish

5% grade at 15mph need 462 W at wheel
http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=5182&p=77894#p77894

since 25km/h is more than 18km/h so we can calculate efficiency for the speed gear only

Uworking = 25/40*32V= 20V
so we need I=25A for P out =500W
than I=25+2.4=27.4A
U= 20+27.4*.04=21.1V P_in=27*21.08=578 W

efficiency 462/578= 79.9% almost decent 80%

If we have more dense gears than we could achieve maybe 84% but who care about 4% ?

,so give me the next two working points Miles

 

[Miles]

Sorry, I edited my post after you read it.

Cruising speed 25kph

Let's say for gradients of 0%, 5% and 10%

Lots of stopping at traffic lights etc.

I'm giving you a usage profile....

 

[Miles]

If we have more dense gears than we could achieve maybe 84% but who care about 4% ?

Why would you not want the best efficiency at the point where you spend most of the time?

 

[eP]

OK so lets check the next two points

A) 0% grade need ~ 100W

Uw=20V Iw=6A I=6+2.4= 8.4A
U=20+8.4*.04=20.34V
P_in =170.8W efficiency = 100/170.8 = 58.5% efficiency is really low i have to admit but who care cruise only 25km/h at flat when 1000W of power is ready to use ?

If we want cruise at low speed than we are able to keep 20km/h at flat by legs only.


B) 10% grade

 

[Miles]

A) 0% grade need ~ 100W
efficiency = 100/170.8 = 58.5% efficiency is really low i have to admit but who care cruise only 25km/h at flat when 1000W of power is ready to use ?

Now you're dictating my usage profile

QED

 

[eP]

If we have more dense gears than we could achieve maybe 84% but who care about 4% ?

Why would you not want the best efficiency at the point where you spend most of the time?

For the sake of convenience.
Who care all the time about speed, power, efficiency ?
Why 4% have to keep my all attention ?
4% is acceptable drop for me as a cost of freedom ( free of motor gears selection).

To telling the truth we still don't know we talking about 4 or 2% only. And the speed gear i chosed maybe was not the optimal choice.

If we have better describd profile than we are able to make better choice i'm sure.

 

[Miles]

4% is acceptable drop for me as a cost of freedom ( free of motor gears selection).

I don't have to think much about changing gears, either. My middle gear copes with most conditions and I usually start in it, unless I'm on a steepish hill.

4% efficiency drop is for the 5% gradient. The average gradient is 0%.

I think you could build a great system with 2 motors (after all, I set the conditions so that I would win... ) but I still think 3 gears are better for my needs - especially as I can use them for the pedals, as well.

 

[eP]

A) 0% grade need ~ 100W
efficiency = 100/170.8 = 58.5% efficiency is really low i have to admit but who care cruise only 25km/h at flat when 1000W of power is ready to use ?

Now you're dictating my usage profile

QED

OK so lets change the speed gear. Lets make them 4 times taller than climb one instead 2.5 times difference.
so for 40 km/h you will need Uw=20 V, Iw=40 A ,
so I=40+2.4=42.4 A
U=20+42.4*.04= 21.7 V
P_in = 920 W
P_out 20*40= 800 W lets assume 730W at wheel

efficiency n=730/920= 79.3%


So lets again calculate them for 25km/h at 0,5,10% grades.

Uw=12.5 V, Io(@12.5V)= 2A

A) 0% grade P=~100W, P_out = 112.5 W => Iw =9A
I=9+2=11A
U=12.5+11*.04=12.94V
P_in=142.3W
efficiency n=100/142.3 = 70.2% not bad for such light load !


B)grade 10% P=~850W, Pout=950W => Iw = 76 A
I=76+2 =78A
U=12.5+78*.04=15.62V
P_in=1218 W
n =850/1218= 69.8% is low again but as an option you still can slow down to 16km/h and reduce to climb gear if such efficiency is too low for you. You can also add 250W your own input to lower current by 20A to ~58A this way efficiency could go up to 75.6%


C) grade 5% P=~462W P_out 500W => Iw= 40A
I=40+2=42A
U=12.5+42*.04=14.18V
P_in= 596 W
n= 462/596= 77.5% not bad at all

Maybe it is not optimal choice again but IMO is quite acceptable.

If we add few cells to 48V and change the climb gear sligtly taller to cover 25km/h speed at climb gear too, than at two motor sloution case we could use both motors at 5% and 10% grades at 25km/h to reach better efficiency.

So i'm wonder what you can't accept - what is the primary reason for extra motor gears ?

 

[Miles]

Maybe it is not optimal choice again but IMO is quite acceptable.
So i'm wonder what you can't accept - what is the primary reason for extra motor gears ?

As I said, I think you could make a perfectly acceptable system with 2 motors.

3 gears is the optimum, for my conditions, as you proved.

I have the 3 gears anyway - they are there for the pedals, so....

Please build your 2 motor design, it has a lot going for it. Fully automatic change is great.

 

[Miles]

How would you take the drive to the wheel with 2 motors?

 

[eP]

I think you could build a great system with 2 motors (after all, I set the conditions so that I would win... ) but I still think 3 gears are better for my needs - especially as I can use them for the pedals, as well.

Are you sure you have better than 70% efficiency (battery to wheel) at ~100W power level at wheel and at your own input ?

What is the real reason for assistance at flat ?

25km/h is realy weird condition - i have to admit. It is too high for padals only, but is yet too low for powerful motor.
But for me it is not a problem. If i don't care views than i can go faster than 25km/h - 32-35km/h is still safe at flat.
Or when i'm not hurry up than i can biking 16-18 km/h without assistance at flat quite comfortable.
If i wasn't made a few thousands km (3500-4000 km) last year by pedals only than maybe i would agree the 25km/h at flat assistance is needed. But now i'm sure it is not a case for me at least.
Not for the sake i'm biased towards two motors or two gears, but for the sake of my real needs - assistance at +3% grades especially at +10% grades.

I would like to see your number Miles. What is the difference in power between your second and third gear at 25km/h at flat ?
I would like to see also your trip profile - how many percentage of time you spend at 100W power out level, and how much percent your battery energy you use at that condition.
I'm not going to argue my idea is better than yours - but i'm not aware how much important is that for you and why.

In my opinion 15-30 minutes assistance for most critical parts of trip is essential for average e-biker, assistance at flat is much less important. I said more - flat parts are too delicious to be killed by assistance until you have only flat around you.

How would you take the drive to the wheel with 2 motors?

Additional stage at second motor shaft ( for climb gear).
And two pulleys at first motor shaft (additional pulley (with freewhell) 2.5 -4 times larger in diameter than first one) . The same size small pulley at the second motor shaft.

 

[Miles]

Are you sure you have better than 70% efficiency (battery to wheel) at ~100W power level at wheel and at your own input ?

I'm hoping to do some proper testing when I get my new battery - I'll let you know exact efficiency, then. I have a logging device. I'm thinking of getting a power output meter, as well, so that I can measure realtime efficiency.

To be honest, I agree with you, I prefer to keep assistance for hills.

My gear ratios are: 0.734.....1.0......1.362

 

[Miles]

If i don't care views than i can go faster than 25km/h - 32-35km/h is still safe at flat.

Not in London.....

 

[Miles]

How would you take the drive to the wheel with 2 motors?

Additional stage at second motor shaft ( for climb gear).
And two pulleys at first motor shaft (additional pulley (with freewhell) 2.5 -4 times larger in diameter than first one) . The same size small pulley at the second motor shaft.

Sounds reasonable.

What motors? What speeds will you run them at?

 

[recumpence]

On my recumbent, I average an easy 18mph on flats without any assist. If I feel strong that day, 22mph is not hard to achieve consistantly without any assist on flats. My reason for assist is multifacted;

#1 The occasional 40mph blast on high speed roads to get from one town to another.

#2 Lots of power to get up the steep grades around my area without downshifting to a uselessly low gear.

#3 Pulling a trailer.

Lastly, I just love the technical side of this. I am used to powerful motorcycles. So, a low power assist has no real attraction for me. At that point, I would rather just eliminate the assist altogeter and pedal only.

By the way, I am a double motor kind of guy. I am planning on setting up a second motor soon.

Just some thoughts. :wink:

Matt

 

[eP]

If i don't care views than i can go faster than 25km/h - 32-35km/h is still safe at flat.

Not in London.....

OK you convinced me for 25km/h at least.

But i see:

My gear ratios are: 0.734.....1.0......1.362

so you have very narrow gears span.

So maybe you don't need 20% climb ability ?

If you ocassionally need 20% climbing (not regulary) than you don't need true climb gear.
If this is the case so we can chnge the gears once again.

Lets make previous climb gear twice mauch taller to cover 0-36 km/h range at 36V battery voltage.
That way the same gear will be able to cover 3 cases: 20% gradient, 10% gradient and 5% gradient.
For flat road - low load - would be second speed/idle (low load) gear. Lets make it 3.125 taller than first one.

So lets recalculate all once again.

20% gradient 9km/h 9V 60A P_out= 540 W Io(@9V) =1.8A

I=60+1.8= 61.8A
U= 9+61.8*0.04=11.47V
P_in= 709W

efficiency n=500/709= 70.5% not bad if this is occasionally case
if you could give 180 W of your own input then efficiency would rise to 73.5%
-------------------------

10% gradient 25km/h 25V Io(@25V)=2.7A
P=~850W, Pout=950W => Iw = 38 A
I=38+2.7 =40.7A
U=25+40.7*.04=26.63V
P_in=1084 W

850/1084 = 78.4 % quite nice

5% gradient
P=~462W P_out 500W => Iw= 20A
I=20+2.7= 22.7 A
U=25+22.7*.04=25.9V
P_in= 588 W
n= 462/588= 78.6% not bad at all
==============================

The last case: v=25km/h, Uw=8V
0% grade P=~100W, P_out = 112 W => Iw =13.5A
I=13.5+1.8=15.3A
U=8+15.3*.04=8.61V
P_in=131.8W
efficiency n=100/131.8 = 75.9% not bad even for regular trips

as a bonus you should reach 56km/h (35mph) at this gear, easy way if you want.