Rocky Mountain Element Conversion

[jackhandy]

Morning d8veh,

Not having my thinking head on yet - Does pulling 8.5 amps from 2 x 36 volt motors mean 306 watts, or is the voltage effectively 72 volts, making 612 watts?

 

[Deleted member 4366]

8.5 amps is what's being drawn from the battery at about 39v = 331w from the battery. That current then splits and goes to each motor, so something like 4.5 amps to the rear and 4 amps to the front.

When a motor is spinning in its sweet zone, it has an efficiency of about 80%. The sweet zone moves with the amount of throttle so that with less throttle, the sweet zone will be at less rpm. When you have a single low-powered motor going up a hill, it's at full throttle but doesn't have enough power to rev into the sweet zone, so it's efficiency drops to perhaps 30% or 40% depending on speed. In this case most of the amps drawn from the battery are used to make heat rather than motion, so you see a lot of power on your wattmeter, but you're not actually making a lot of motion power.

You can see this effect using the simulator here: ebikes.ca Hub Motor and Ebike Simulator
Use an Ezee motor in 700c wheel and the lowest amp controller and set the hill to 10%. Then look at the green curve, which shows the amount of battery being converted to motion, then set the throttle to 50% and re-plot and you'll see how the peak efficiency moves to a lower speed.

 

[Deleted member 4366]

I got some data from my Speedict today after I did my normal commute, but without stopping (just turned round and went straight back for testing)
Outward leg: steep downhill for rhe first two miles, then slight downhill for the rest. Slight heasdwind. Pedalling steadily at 15mph, but without much effort.

Distance: 14.66miles
Time: 57m 35s
AH: 3.547
Ave watts: 155w
Max Watts: 781

Return leg same speed with slight tailwind
Dist: 14.87miles
Time: 63m 53s
aH: 5.336
Ave watts: 203.4
Max watts: 960

Overall average is 15mph which is bang on my normal speed with BPM motor. Consumption was 11.5 wH/mile, which is less than my 15wH/mile average on my BPM bikes. I used about the same (as BPM) on the outward leg because the PAs was causing the rear motor to try to go 18mph all the time. I was more efficient on the return because I used the throttle all the way to cancel the PAS so bike was trying to go only 15mph.

Conclusion: This bike is more efficient than a single BPM motor even with 2.1" Knobbly tyres instead of semi-slicks. It's also quieter even with both motors running. Range is about 40 miles at 15mph with my light pedal effort.

 

[cwah]

That is a damn insightful stat D8veh!!

For hill climbing, a dual motor is way better than a single one! And not just a few percent, it's as big as 25% difference.

Big enough difference to buy a new bike if you do a lot of mountain cycling.


Still wonder what would be the efficiency on flat road and on city commuting. (and also about max speed difference )

City commuting is full of start and stops! Maybe a dual motor would be more appropriate in this type of environment too!

 

[NRG]

Really nice build D8, great choice of donor bike and implementation.

Fascinating to read about the gain in efficiency, I'll really have to try a twin motor build at some point.....when works stops getting in the way that is!

 

[Deleted member 4366]

I'm going to swap the BPM motor in my Giant NRS to twin 328rpm Cuties now. This is my main commuter/touring bike. With a large motor and 20AH battery on the back, I have to be careful. Hopefully the new setup will be more manageable. Never mind anything else, 2WD is brilliant when the surface is a bit dodgy. I just ordered the parts from Greenbikekit.com to see how they compare with BMSBattery.

 

[Deleted member 4366]

A little more data in case anyone's interested. I went on a 40 mile ride today, but a bit slower using throttle only. A bit of a pain having to hold the thumb-throttle all the time, but that's the only way I can go slow because as soon as I let go the throttle, the PAS kicks in with full power. I'm going to have to make a cruise control for the rear motor so that I can tame it a bit. I wish I had connected the electronic cruise wires now.

Anyway, here's the data, which shows that this bike can be as economical with power as most - even with two motors. The battery was only 3/4 depleted - still showing 39.2v at the end of the journey, so overall range would be about 53 miles - not bad for two motors! The terrain was mixed with flat and hilly bits with some steep hills, one of which is long.

Dist: 40.15 miles
aH from battery: 8.256aH
Ave Watts: 100.5W
wH/mile: 8.8wH/mile
Ave spd: 11.4mph

 

[cwah]

That is incredibly low D8veh. Even on the simulator, under the same condition I have something like 9.x wh/mile. It's like you have a 85% efficiency motor at this speed.

I'm now thinking about a dual BPM, I hope I'll have the same results!

 

[Deleted member 4366]

Cwah, I take my hat off to you. I remember when you said you wanted a 30mph Brompton with 72v, everybody said it wouldn't work. I cringed when you said you'd fit a MAC to a Bromptom, but you got them both working. A twin BPM Brompton will go like a tank on steroids. Go for it, and we'll all learn something from your project. Personally, I don't think you'll gain so much for the effort, but it'll be fun finding out and we might be pleasantly surprised.

I got my low consumption by going slower, where leg powr takes over from motor power. The main point is that it can be riden slowly with the motors going. Also, less power is used on the hills with two motors.

 

[cwah]

Thanks for your kind words D8veh.

I'm not sure yet about which to get, the Cute100 doesn't look powerful enough and double BPM a bit on the heavy side... I was thinking about dual Ezee V2 but they may overspin and Cyclezee doesn't want to sell them to me..

Now I just had my Mac that stopped working, I think somehow it's the hall sensors that snapped (or melted), have to check that. Hope it's not the gears or I give up the geared motors for my brompton.

 

[Deleted member 4366]

In a few weeks, I'll have coonverted my Giant to twin 328 rppm Q100s. If you can wait 'til then I'll have some answers for you. Twin Bafang SWX is another option. They can handle 900w, so that would give you 1800W total.

Don't worry about this over-spinning thing. I don't believe it has any effect on efficiency. I regularly pedalled my old Sunlova in that zone, and I got huge range out of it.

Another thing. Will you be able to get the big motors in your forks. It's a bit of a stretch to get the small ones in. I know they're supposed to be the same width, but they're wide for a larger distance (diameter).

 

[Cyclezee]

Thanks for your kind words D8veh.

I was thinking about dual Ezee V2 but they may overspin and Cyclezee doesn't want to sell them to me..

I would just explain why we felt it unwise to supply cwah with V2 eZee motors.

After discussion with Wai Won Ching, it was felt that it would be irresponsible to supply motors with the prior knowledge that they would be used to create a bike that is both illegal and a potential hazard to the rider and third parties.

 

[Deleted member 4366]

I would just explain why we felt it unwise to supply cwah with V2 eZee motors.

After discussion with Wai Won Ching, it was felt that it would be irresponsible to supply motors with the prior knowledge that they would be used to create a bike that is both illegal and a potential hazard to the rider and third parties.

Spoil-sport!

 

[Cyclezee]

Spoil-sport!

Hi D8veh,

I did expect those sort of comments, but I hope that you can appreciate my position.

 

[flecc]

Spoil-sport!

This is what Wai Won Ching posted on the first of February 2008:

"In a conflict of interest between what customer or the market likes and what is legal. eZee takes the legal way, this is absolutely essential for long term consideration."

Link to that post

His comments show that he clearly doesn't approve of his agent not complying.

 

[Scimitar]

I would just explain why we felt it unwise to supply cwah with V2 eZee motors.

After discussion with Wai Won Ching, it was felt that it would be irresponsible to supply motors with the prior knowledge that they would be used to create a bike that is both illegal and a potential hazard to the rider and third parties.

Well, aren't you and he self-important?
Congratulations on smugness.

 

[Deleted member 4366]

Hi D8veh,

I did expect those sort of comments, but I hope that you can appreciate my position.

Of course I can, but if I was Cwah and wanted those motors, I'd get a couple of my mates to order them, so the result would be the same.

 

[cwah]

In a few weeks, I'll have coonverted my Giant to twin 328 rppm Q100s. If you can wait 'til then I'll have some answers for you. Twin Bafang SWX is another option. They can handle 900w, so that would give you 1800W total.

Don't worry about this over-spinning thing. I don't believe it has any effect on efficiency. I regularly pedalled my old Sunlova in that zone, and I got huge range out of it.

Another thing. Will you be able to get the big motors in your forks. It's a bit of a stretch to get the small ones in. I know they're supposed to be the same width, but they're wide for a larger distance (diameter).

Yeah, maybe the Bafang SWX would be an option. I wouldn't be cruising at much more than 900W anyway, but acceleration wise I'm doing now 4000W with my Mac now

My only worry is that these 2 SWX together may not be as torquy as 1 Mac 8T... So I'm still considering...

And you're right, I have to check the front fork width. The rear one is already done thanks to John (the other one )

 

[cwah]

Of course I can, but if I was Cwah and wanted those motors, I'd get a couple of my mates to order them, so the result would be the same.

Hehe, thanks for the insight

But for now I'm still considering different options.

Ezee V2 is still very appealing because according to the simulator it's more torquy than the BPM and BMC. And that's what I'm looking for more than the top speed (25 mph is enough for me) But apparently it may be quite inefficient on 16" wheel...

And dual BPM would still be very torquy, much more efficient on 16" wheel and much cheaper.. So still considering...

 

[wurly]

D8veh's two wheel drive thing for ebikes could be catching.
I just checked my ebay folder today and the front forks are 100mm.
It just so happens i have a 260rpm tongxin hub looking for bike to power, It should be an easy thing to do, i will have to get some shorter spokes and relace the tongxin into the 20"rim and it also makes my decision to up the volts to 36 and have dual controllers.

I have a question about dual thottle's though.
Would it not be possible to join the 2 signals wires and have one thottle and one sensor? the 0V would be commoned by the 0V from the battery. Just wondering if the sensor would sink current from 2 controllers ok? i think they are usually working in micro amps?