Power to load budget and battery and controller choice

[misterjones]

Math not being one of my natural talents, I’m struggling a bit with estimating some operating parameters of a build (a lightweight cargo bike with front hub motor, no pas - throttle only with rider effort), in this case the battery voltage, controller amps, and how to set things like cruise control. My plan is to use a MAC10T front hub on a 20” wheel but I’m not sure whether to get a 48V or 52V battery and whether to use a 20A, 25A, or 30A controller.

My thinking about this project is, loosely speaking, to have the hub motor to provide the power to push itself, the bike, and load, and me to provide the rest. As I have already been providing all the wattage when riding around on my non-motorized bikes to date I figure I can still be a significant contributor to the wattage needed to push around heavier loads, but with a desire to hand over more of it to a powered front wheel, maybe 50-50 me-motor, but maybe more 40-60 me-motor. I'm new to this so I've no real idea.

Using the online bike calculator (bikecalculator.com) I entered what I believe are some good approximations of working conditions. My weight (162lbs) plus bike weight (bike, motor, battery and nominal cargo load of 60lbs) together to get an approximate 290lbs of gross weight. Setting two scenarios, one a 12% grade at 6mph, and another of 0% grade at 20mph. I don’t really know what the hell I’m doing with those figures really, but they seemed to bracket a “general zone of satisfaction”. The 12% grade tells me I would need about 460W to push that load up it at 6mph. The 0% tells me I need only about 260W to achieve 20mph. It’s a little vague in some figures since the bike will have a 20” front wheel and 700c rear.

I’m still not sure how these figures mesh with, say, the motor power/load/battery power figures I get when I use the (amazing) motor simulator over at bikes.ca. and hence my uncertainty in regard to battery voltage and controller amps. And furthermore, how to set a cruise control in a way that provides a budgeted power flow. Said another way; how could I set the cruise control so that the battery power is a kind of “fixed income” to my more flexible addition? or do I just have to learn how to feather the throttle?

 

[harryS]

Cruise control holds a certain speed so it's not a power budgeted process. I suggest you hook up the pedal assist so you can free up your throttle hand. Having that hand fixed on the bars is no way to ride a bike, in my opinion. Set the assist level lower and you can pedal beyond what the motor wants to give and do most of the work yourself on flat land. You can always turn off PAS and feather the throttle too,

It's a geared motor isn't it? Should pull fine in a 20" wheel. Just make sure the motor doesn't rip out the front fork dropouts. I've never done the math or analysis on them simulators. More volts and current is good. You can always not use it, but if you don't have enough ....I look at it this way. If you can move 260 pounds of cargo bike up the hill now in a low gear, I think you'll be fine with your proposal.





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[misterjones]

Cruise control holds a certain speed so it's not a power budgeted process. I suggest you hook up the pedal assist so you can free up your throttle hand. Having that hand fixed on the bars is no way to ride a bike, in my opinion. Set the assist level lower and you can pedal beyond what the motor wants to give and do most of the work yourself on flat land. You can always turn off PAS and feather the throttle too,

It's a geared motor isn't it? Should pull fine in a 20" wheel. Just make sure the motor doesn't rip out the front fork dropouts. I've never done the math or analysis on them simulators. More volts and current is good. You can always not use it, but if you don't have enough ....I look at it this way. If you can move 260 pounds of cargo bike up the hill now in a low gear, I think you'll be fine with your proposal.

Hey harryS, thanks for that. I don't think I'm as clear in my writing as I think I am! I think what I'm trying to say is that I was going to set the CC as a fixed input, a set amount of wattage when activated, and then if I hit a hill, or want to go faster I just pedal harder. I was using another members set up, JRA, as the idea of riding over the top of the motor power made sense to me. Like riding a tandem where your stoker is a little motor instead of a big person. I still have a BBHD at the back of my mind as an option, but I wasn't keen on the way they mount onto the frame - potentially permanently damaging the BB. Switching out a motorized front wheel for a regular one (repairs, failure, touring) is pretty darn simple.

I really only push about 180-200lbs of me and my big steel road bike right now. This cargo thing is a new project to get me out of my truck more often.