Ultimate touring/cargo bike question

[Warren]

Folks,

This is a question for Luke, and the other motor experts on this site. Running a motor through a gear reduction, means I can use a lower wind/higher rpm motor at the same battery voltage, just geared down more, for the same power/speed. This should allow lower motor current for the same output wattage, and greater electrical efficiency...correct?

I have had a new bike project present itself. My wife, after riding thousands of miles with me since 1995, and after several arthroscopic knee surgeries, and one knee replacement, now facing the other knee ultimately needing replacement, is giving up cycling. This leaves me with our RANS, dual 26" wheel, Screamer tandem. Here is a shot of me and our daughter from 16 years ago.

http://www.rowvelo.com/images/26-26Scr.jpg

I am thinking of pulling off the stoker seat, and chopping the crank arms off the stoker crankset. I would mount a 6T, "500 watt" MAC, from cell_man, Paul, in the frame, just below where my daughter's hands are, in the picture. I'd use his "1000 watt" controller. I would mount 12, 100Ah GBS cells, from Elite, 6 per side, down low on the frame back there. I'd run our "rough road" 2" slicks, instead of our usually 1.25" slicks. This rig would come in at about 145 pounds. I figure, since I have the extra bottom bracket back there, I will take advantage of it. That extra shaft allows me to up the motor rpms, without having to use custom chainrings to keep my cadence normal. I would go with the 100 Ah cells, because they match the weight/Ah of my big Ping pack. I figure if I run this bigger motor at 2.5 times the 350 watts, I run my current bike, or 875 watts, I should be able to cruise at 28 mph average, instead of my current 20 mph average. And with 3.3 times the energy available, and only 30% higher gross weight my range should be as good.

Warren, at 3000 miles on my bike

 

[deVries]

Running a motor through a gear reduction, means I can use a lower wind/higher rpm motor at the same battery voltage, just geared down more, for the same power/speed. This should allow lower motor current for the same output wattage, and greater electrical efficiency...correct?

Looking at your battery of 12 cells in series I think you're talking either 36v or 48v nominal. I think this is the voltage range the MAC is normally run in, so your "electrical efficiency" is not above normal at these voltages. But, yes, using this motor as a mid-drive should allow it to be more efficient, and, hopefully, match your ideal cadence rpm too when pedaling (assuming you want to). This is a heavier bike with all that battery, so I'm not sure about the performance level you are wanting... top speed & acceleration... geared right, you might be happy at 24-28mph with decent acceleration... otherwise, you might have to up the voltage or consider dual motors.

I'd make sure you order the MAC with new heavy-duty clutch & gears just in case that is still an upgrade option & not standard issue by now.

 

[Warren]

I know that both motors are designed to run on 36-48 volts. My question is, at the same input voltage, if they are both geared to produce the same bike speed/output wattage, will the faster turning motor have lower electrical loses?

 

[deVries]

I know that both motors are designed to run on 36-48 volts. My question is, at the same input voltage, if they are both geared to produce the same bike speed/output wattage, will the faster turning motor have lower electrical loses?

The answer will not be one way or the other. It's going to vary depending on each speed/load range or "target category" (how you define it or using motor specs) as you accelerate to your target speed. What speed target range do you want to be most efficient at? (My answer, personally, would be the cruising speed I will spend the most time traveling at... for me 25mph or 22mph-28mph. YMMV.)

You're not over-volting at a higher voltage, which is what most mods on ES are to get higher speed & lower "heat" by using lower amps. Less heat is more efficient. More volts is higher speed.

You are not over-volting (based on what you posted already), so you need to ask Cell_Man for the specs on the motor he sold you to know what is the ideal most efficient RPM to run your motor at. Then gear accordingly for the speed and cadence and torque you want. If you use no load values, then a good rule of thumb is to up the power you need by a margin of about 20%.

Go to Justin's website & use his motor simulation to help further explore your options. The MAC motor is similar to the BMC, but I don't know if Justin has either of these motors loaded into his simulator or not.

Maybe someone else can jump in with better or more accurate information & suggestions?

Talk to Cell_Man. I think he knows the most about the motor he sold you to tell you the most efficient (lowest) amp draw for X rpm at the speed "target range" you want the most efficient, imo.

Another way to get more accurate info is to use a Cycle Analyst. You can always change your gearing & test for various efficiencies doing "real world" experiments. The motor simulator & data is just an estimation anyway.

 

[Warren]

deVreis,

I run a CA on my current mid-drive bike. Great invention. Thank you Justin!

My hunch was, I could get much of the efficiency benefit of running higher voltage/lower gearing with a lower turn motor/lower gearing.

 

[Miles]

If you mean "run the motor at a higher speed and gear it down more", it will alter the range of greatest efficiency and increase the available power. Efficiency at the lower power levels will be a bit worse.

 

[Warren]

Miles,

Yes, "run the motor at a higher speed and gear it down more". I will still be spinning at my 80 rpm cadence, putting in my 100-150 watts, but this bigger motor will be spinning 40-50% faster than my current one, and hopefully putting out 2-2.5 times the power.

 

[Miles]

Yes, "run the motor at a higher speed and gear it down more". I will still be spinning at my 80 rpm cadence, putting in my 100-150 watts, but this bigger motor will be spinning 40-50% faster than my current one, and hopefully putting out 2-2.5 times the power.

If you run a motor faster, it will have the ability to output more power - you can get more power at the same torque level. Parasitic losses will increase, though, so the peak efficiency point will occur at a higher power level. For power levels below the new peak efficiency point, the efficiency will be worse than before. Direct drive hub motors are designed for low rpm, so not much attention is given to mitigating eddy current losses - hysteresis losses are the dominant parasitic loss. Eddy current losses go up as the square of the rpm however so, with the high pole count and thick laminations, there will be a limit to the speed that you can run a direct drive hub motor efficiently. For any motor, the eddy current losses depend, principally, on pole count and rpm (flux frequency), lamination thickness (losses increase as the square of lamination thickness) and core material.

 

[Warren]

Miles,

"For power levels below the new peak efficiency point, the efficiency will be worse than before."

Yes. That was my assumption. If I want to cruise at 20-21, my current bike is great. This bike would be my grocery getter, and Grand Tourismo machine. I figure the same or slightly better acceleration, more weight carrying potential, and a little higher cruise speed, same range.

Warren