Cellphones, pedal power stroke and Power Assist Levels

Mike leroy

Active Member
50% of your pedal stroke(i.e., one RPM) has no effect or loses power! The power fades from the 5'oclock position. Power gain does not recover until the 2 o'clock position. A significant power loss occurs in each pedal upstroke, according to the following videos.

The peanut-shaped watt curve shows power gain and loss in each pedal cycle. Maximum power is generated along the diagonal line connecting 2 and 5 o'clock positions. The diagonal line represents the downstroke on each pedal.

(Link Removed - No Longer Exists)
WattBike Pedal Power Stoke in Watts

To maximize battery energy, only the complementary amount of motor power should be drawn to overcome the upstroke pedal loss. Complementary motor power maintains speed without straining the battery. Graph pedal power in relation to cadence in real-time with your cell phone accelerometer. Systems with only a few battery "Eco-modes" appear to lack battery energy sensitivity.

You might also consider using a cellphone metronome to develop a smoother cadence. For a 60 RPM cadence, set the metronome to 120 beats per minute. Each beat corresponds to an upstroke. I use a metronome to train for uphill sprinting. I run up a 10% grade with the metronome set to 80-100 beats per second (~90 RPM). I know of no better way to improve uphill running speed.

I am rapidly coming to the conclusion that torque sensors unnecessairly drive eBike cost up by $500-$1000 -- over-engineering is great business for component manufacturers. An accelerometer and gear sensor make more sense to me. Polaris eBikes feature gear sensors. Chris at lectricCycles.com told me an 8Fun gear sensor and controller will be available this year.

Pedal power fluctuates; most power is generated from the 2 to 5 o'clock position(25% of your cadence). Your legs generate and lose power in cycles or RPMs. The analogy is jumping rope. You exert most energy when the rope is below your waist, as you push off the ground. When you land, you lose energy because you must absorb the impact.

Pedal stroke power loss is seriously increased on hills. The beauty of eBikes is compensating for uphill stroke power loss.

The difference between jumping rope and the pedal upstroke is the loss of power due to the pedal pushing your foot upwards. A common misconception is the foot pulls the pedal upwards during the upstroke.

My guess is the optimal Power Assist Level (PAL) is 25% to 75% of your leg power. The idea is to use the motor to make up for the gaps in the power stroke to provide a consistent level, rather than fluctuating power. Ideally, the motor only adds power in the valleys of your leg power stroke, but not so much as to increase peak power from the 2 to 5 o'clock position.

You should be able to feel the optimal, complementary power boost by paying attention to the power applied to your foot by the motor during the upstroke. By the same token, you should feel like the motor is neutral on the power downstroke.

The power fluctuations may also be audible. Attach something to the tire or spokes. Record the sound of your motor your cell phone and graph it. On a steep hill, the light level from a generator may visibly fluctuate.

You can physically demonstrate balanced power with the eBike on an indoor trainer. Simply have another person record the wattage output fom the eBike display. Computer access is also available. CycleAnalyst should provide adequate information. Plot your own polar graph, as in the WattBike video, with cell phone acceleramometer.

Tracking battery consumption will pinpoint the optimal tradeoffs. In this way, the eBike system functions more like a car transmission by providing appropriate power to a given situation. The goal is keep the motor between "lugging" and over-revving, like in a manually shifted car. The WattBike Polar Graph will resemble a sausage more than a peanut, as described in the above video.

I believe my theory was confirmed in this blog. Power levels were set to 10% increments and maximum assist is 63% of motor wattage.

The controller is monitoring 1000 times per second. Take full advantage of the cadence sensor. The motor provides most power at low cadence (i.e., 55 RPM) and shuts off around 80 RPM. For a 750W motor, each RPM is approximately a 25W drop in motor power, at 100% PAL. Assuming 10W per Nm of torque, one RPM drop in cadence is roughly 2Nm of torque.

Calculating battery distance is one way to apply this information.

Slowing down to speed up may sound ironic, but twin powers seem balanced in this fashion. Gradual, smooth cadence changes, rather than abrupt, standing-up sprints extract the greatest torque from your motor.

How to tune the firmware parameters is unclear. Most people pedal around 60 RPM, which generates max motor power. Setting PALs in gradual 10% increments aligns optimal RPMs to cadences.

The car analogy is adding more cylinders. More power is transmitted to the crankshaft during each RPM by adding a cylinder or complementary bike motor power.

My intuitive guess is most PAL levels, i.e., over 100% of leg power, waste battery power. Configuring the nine PAL levels in the controller as smooth, subtle 10% increments, from 10% to 100%, can prove or disprove the theory. The evidence is most conspicuous on 10%, or higher grades.

The 8Fun BBS02 motor is the only system that I know of, that permits PAL customization.

When more than 100% of leg power is needed, use the throttle as a turbo charger. After finding the most appropriate PALs, the need for a throttle should disappear.

Battery powered car drivers have efficiency contests. The principle is coasting greatly improves gas mileage. I can improve my gas mileage by 30% by monitoring the car TPS (Throttle Position) from the computer diagnostics.

Can you provide some evidence that correlates PAL levels with pedal stroke?
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Very interesting post would add to the mix crank length in improving your pedaling smoothness and efficency I use 165mm cranks the standard cranks with BBS 01/2 kit are too long for me.
My natural cadence is about 75 rpm, I use Nuvinchi N360 with Harmony gear shift I set my cadence to my desired rpm so that is pretty much a constant.
My BBS 750w is set for 9 levels of assist I use level 2 or 3 most of the time and moderate inputs of throttle to maintain speed and leg effort for given road conditions.
The smoother my ride style no suprise the better my range gets so the initial thrill of blasting around at speed on high levels of assist has given way to riding smart using just enough assist/throttle but still achive respectable pace.
I ride at between 15 to 20mph as for me that feels like good speed on a bike, once you push much faster your using too much energy pushing thru the air.
my range on an 8Ah batt can be as little as 8 miles at level 6/7 with lots of full throttle use giving a very fast ride to a much better 16 miles in my eco smooth style of riding.


Hi Mike,

this is close to rocket science!

Am afraid that I'm no intellectual match here... did you post this over @ Endless-sphere yet?

You might get more qualified feedback there...

My (mechanical) viewpoint: Cyclists try for some time to both stretch and/or "fatten their peanuts" with cleats (e.g. SPD pedals) utilizing the upstroke as well. Shimano and others tried to fatten the peanut with elliptical BioPace chainwheels.

Practically however I guess most cyclists don't bother... or simply add a few or more watts using a motor, no matter what's the drain on the batteries, which will include me for the near future I'm afraid.

Hope you are or get involved into the design of really smart controllers... you are way ahead of the crowd here...
and ahead of the cougar in your valley it seems ;)

I'll get back to you once I'm ready to tweak my controller settings, OK?

Hey Spotty, haven't you hooked your controller to your computer yet? What about your Speedict mercury wired into the system providing information about speed, battery state, voltage drawn etc all displayed via your smartphone? Any logs for Mike to analyze?

Time to go to bed here and recuperate,

so long,

Really have not got to grips with the Speedict full fuctionality yet and only use it for accurate mph and batt voltage readings will be fitting the temp probe soon after reading a useful link to Electric-FatBike.com in Go-Coo new thread.
It will prove interesting to see how hard/hot the motor is working at different levels of assist or using throttle in relation to amps drawn.