mschwett
Well-Known Member
- Region
- USA
there was an interesting discussion here https://forums.electricbikereview.com/threads/another-new-tq-motor-hpr40.57935/page-13 about climbing hills on lightweight hub drive and mid-drive motors, especially the new TQ HPR40 which looks very, very promising, and @Yako is currently riding.
i haven't been riding much lately, but thought i'd revisit the question on my way home from work. i have a couple bikes - a very lightweight road bike (±14lb s-works aethos), a lightweight road e-bike (±24lb scott addict rc eride) and a 500w front-hub commuter. both the road bikes have 4iiii dual sided power meters on them, and i've ridden maybe 15k miles on them over the years, with basically every ride in strava with power, heart rate, cadence, speed, etc.
i live at the top of this hill, and have accurate-enough map and contour data. the average grade of the fat yellow segment (no stoplights) is 10.57 percent, and the distance is (obviously) also precisely known.
so, yesterday i made sure both the mahle smartBike app and my regular cycling app (which records the rider power at the cranks, heart rate, cadence, etc) were running and rode home up the hill slowly. i have health issues which require keeping my heart rate quite low, so i didn't push it, just an easy commute but under controlled conditions.
average speed was 5.99 mph. average rider power was 147 watts, and average power output from the motor was 145 watts. you can see in the following chart a couple things : the small hub drive of the x20 is unable to produce full power at very low speeds, with the maximum power limited to around 125 watts at 5mph. by 10mph, the power is very closely approaching the maximum 200 mechanical/output watts. the output at lower speeds comes very close to the theoretical maximum of the x20, which can be shortened to w = 28.8 x mph based on the relationship between torque, speed, and power. 5mph should yield as much as 144w, but 125 average is close enough. the spiky green rider power line (sampled every second) stays fairly close to the average regardless of grade. the slightly less spiky red motor power line (sampled twice every second) drops when speed drops and increases when speed increases. the little blips in speed are the flat spots at intersections. i'll try this again soon with a longer climb without intersections that i have similarly accurate data for, but i don't expect the result to be meaningfully different.
but, do we know if these power meter and mahle app values are correct!?!? let's plug the grade, total weight (i had on a backpack with a laptop, clothes, etc, so a little heavier than i'd normally be riding), estimates for drag and friction into the great cycling calculator that @Jeremy McCreary pointed me to. in this case we'll take the drivetrain loss (estimated) for the rider power out, since we shouldn't apply that to the hub motor which doesn't go through the chain and we'll reduce the rider power as input, for a total of 284w. guess what? near perfect match. i'm always a skeptic of things like this but when the main factor is gravity and weight and power, it's really super predictable. it all falls apart when you start going really fast, since aerodynamic drag is a much less forgiving mistress.
any meaningful conclusions from this? at slow speeds, you can very easily predict how fast you can go with a given combination of rider power and motor power if you know grade and weight. the science is really simple. if you know how fast you're going, you can very easily figure power, probably up to 12 or maybe 15 mph. if you know RIDER power and speed, you can easily figure out how much your motor is really putting out.... which brings me to the big question here, and one that is still a question for me. how efficienct is a small hub motor like this on steep, slow climbs? we've seen figures published in the 70-80% range. i'm not ready to make a claim here because the ride is too short to really trust the battery voltage and percentage indicators, and the battery was pretty low. i'm going to repeat this on some longer climbs at different charge levels and see if the data is meaningful. i'll say for now that based on this one, efficiency is lower than it should be.
i haven't been riding much lately, but thought i'd revisit the question on my way home from work. i have a couple bikes - a very lightweight road bike (±14lb s-works aethos), a lightweight road e-bike (±24lb scott addict rc eride) and a 500w front-hub commuter. both the road bikes have 4iiii dual sided power meters on them, and i've ridden maybe 15k miles on them over the years, with basically every ride in strava with power, heart rate, cadence, speed, etc.
i live at the top of this hill, and have accurate-enough map and contour data. the average grade of the fat yellow segment (no stoplights) is 10.57 percent, and the distance is (obviously) also precisely known.
so, yesterday i made sure both the mahle smartBike app and my regular cycling app (which records the rider power at the cranks, heart rate, cadence, etc) were running and rode home up the hill slowly. i have health issues which require keeping my heart rate quite low, so i didn't push it, just an easy commute but under controlled conditions.
average speed was 5.99 mph. average rider power was 147 watts, and average power output from the motor was 145 watts. you can see in the following chart a couple things : the small hub drive of the x20 is unable to produce full power at very low speeds, with the maximum power limited to around 125 watts at 5mph. by 10mph, the power is very closely approaching the maximum 200 mechanical/output watts. the output at lower speeds comes very close to the theoretical maximum of the x20, which can be shortened to w = 28.8 x mph based on the relationship between torque, speed, and power. 5mph should yield as much as 144w, but 125 average is close enough. the spiky green rider power line (sampled every second) stays fairly close to the average regardless of grade. the slightly less spiky red motor power line (sampled twice every second) drops when speed drops and increases when speed increases. the little blips in speed are the flat spots at intersections. i'll try this again soon with a longer climb without intersections that i have similarly accurate data for, but i don't expect the result to be meaningfully different.
but, do we know if these power meter and mahle app values are correct!?!? let's plug the grade, total weight (i had on a backpack with a laptop, clothes, etc, so a little heavier than i'd normally be riding), estimates for drag and friction into the great cycling calculator that @Jeremy McCreary pointed me to. in this case we'll take the drivetrain loss (estimated) for the rider power out, since we shouldn't apply that to the hub motor which doesn't go through the chain and we'll reduce the rider power as input, for a total of 284w. guess what? near perfect match. i'm always a skeptic of things like this but when the main factor is gravity and weight and power, it's really super predictable. it all falls apart when you start going really fast, since aerodynamic drag is a much less forgiving mistress.
any meaningful conclusions from this? at slow speeds, you can very easily predict how fast you can go with a given combination of rider power and motor power if you know grade and weight. the science is really simple. if you know how fast you're going, you can very easily figure power, probably up to 12 or maybe 15 mph. if you know RIDER power and speed, you can easily figure out how much your motor is really putting out.... which brings me to the big question here, and one that is still a question for me. how efficienct is a small hub motor like this on steep, slow climbs? we've seen figures published in the 70-80% range. i'm not ready to make a claim here because the ride is too short to really trust the battery voltage and percentage indicators, and the battery was pretty low. i'm going to repeat this on some longer climbs at different charge levels and see if the data is meaningful. i'll say for now that based on this one, efficiency is lower than it should be.
