EBikes - is torque a good predictor of how easy it will feel to go uphill?

Question

Electric bikes sometimes quote the torque or max torque of the motor being used.

So for instance this bike has a Shimano Steps E8000 motor the torque of which is given as 70 Nm (confusingly enough the web page actually has another specification of torque for the same bike but let's ignore that for the moment).

Ignoring other variables for a moment (bike weight, battery charge, wind etc) would this bike go up a given hill at a given speed with less effort on the part of the rider than another bike with a quoted torque of, say, 90 Nm ?

If torque isn't a good measurement of "effort uphill to achieve given speed" what would a good metric be ?

Answer 1

To be frank, I would not put much stock in quoted ratings for motors.

Before I bought my electric bike, I test rode something like eight or nine different makes and models, equipped with a range of different motors (Bafang, Bosch, Brose, Yamaha, maybe some others), some with mid-drive, some with hub drive.

All of my test rides included some steep hill climbs.

The biggest difference I noted was that bikes that had a throttle were the easiest to go up the hill, and this was simply because it was possible to go up the hill with no pedaling at all.

Among the bikes that required pedaling for motor boost, there did not seem to be any direct relationship between quoted motor torque and perceived boost. I attribute this to there being too many variables involved.

For example, even within one particular line of bikes equipped with Bosch Performance CX motors, there were options for the gearing (Nuvinci, now Enviolo, vs Rohloff) as well as options for max speed (in the US, 20 mph vs 28 mph). The difference in gearing, as well as motor setting for the max speed, meant that even with the same nominal torque rating, different bikes climbed hills differently. For example, the 20 mph bikes were better climbers than the 28 mph bikes.

That said, all of the bikes made all but the steepest hills effortless to climb (up to the maximum speed rating for the bike), and even on the steepest hills I could easily double my speed as compared to riding a conventional bike (e.g. a hill I'd normally take at 4-5 mph, I could go up at 8-10 mph on an electric bike).

These tests were ridden entirely on paved surfaces, in a city where the steepest hills are about a 20% grade. Most of the test rides were in neighborhoods where the steepest hills available to me were in around 10-15% grade. Below 10% grade, most of the bikes provided enough power that I could reach the bike's maximum speed going uphill; the main exception being the 28 mph bike that I tried, which didn't have as much power going uphill as the other bikes.

Beyond the fact that the quoted torque for a motor won't tell you very much about how that bike compares to another, I will also suggest that you probably shouldn't care very much anyway. There is a vast difference between climbing a hill with an electric bike as compared to a conventional bike, and only very minor differences in effort climbing a hill comparing one electric bike to another.

In addition, there are a number of other aspects about an electric bike that IMHO should take much higher priority in terms of selection, including battery size and cost, drive configuration (mid- vs hub-drive), motor controller (I found the cadence-sensing motors very annoying, and the torque-sensing motors very natural to ride), and of course cost. And those aspects are overshadowed by the most important, which is simply how the bike feels to you when you ride it. How do you like the handling, how comfortable is the rider position between seat and handlerbars, etc.

Answer 2

(tl;dr, higher torque motor doesn't make "given hill" easier for you, it handles steeper hill before falling off in its ability to help. Going up a gentle hill on a high torque bike is not easier than going up a gentle hill on a low torque bike. Going up a moderate hill on a high torque bike is not easier than going up a moderate hill on a moderate torque bike. For a steep hill, technical terrain, you will appreciate the high torque bike. (All this assuming you maintain cadence, if you're a low cadence rider the high torque bike will feel better.) Best answer is always try out the bike you're interested in on the actual route you're planning to ride.)

Talking about mid-drive pedelecs since you asked about Shimano Steps, it's important to understand modern mid drive pedelecs do not all have '250W' motors. They have complicated systems of motor + gearing + controller that deliver a max of 250W of equivalent rider input assist at the crank. The claimed torque number is sort of a proxy for how broad a range of rider cadence+torque inputs over which the system can actually reach 250W of assist.

Mid-drive motor manufacturers like to reference the torque number so you have something to differentiate between units in a product line where everything is limited by regulation to a stated 250W of rider assist.

I know Bosch better than Shimano but the principles are the same. The assistance algorithm works on a power multiplier. Suppose you set it to 120% so when you put in about 208 watts the bike adds 250 watts of rider assist for an input at the crank of 458 watts which should get you up any grade recreational cyclists who aren't from San Francisco would consider riding. As long as you're riding in the sweet spot of cadence and torque input where your motor is able to generate 250W of assist, the higher torque motor won't require any less effort from you. You put in 208 watts, bike adds another 250. As long as your cadence+torque are within the motor's ability to assist it doesn't matter if it's a 40nm active-line motor burning at its absolute limit or a 85nm performance CX motor that is humming along effortlessly.

But as the hill gets steeper the beefier motor can still keep generating the 250W of assist in response to a broader range of cadence+torque input by you. The bigger motor can provide max assist at lower cadences and for fixed cadence will be able to generate max assist on a steeper hill.

So as far as your effort for 'given speed and given hill' it just depends on if the speed and hill you're talking about are within the maximum capability of the motor. If they are the higher torque motor won't require less effort from you. It handles steeper hills at the same effort.

Of course there are always confounding factors like e.g., with Bosch bikes if you set the assist to 'max' the power multiplier is not the same for all motors across the line, so the Performance CX actually will take less effort from you than a Performance to get it to hit max assist. (sometimes I think they do that just to make it feel 'more powerful' pedaling around a dealer's parking lot.) (but again once it's at max assist if you still need to put in more human effort to maintain the speed you want then it's still 250W plus whatever you're doing, the bike just hit 250W sooner.)

Answer 3

For every 1 kilogram of mass that you want to get 1 meter higher, you must supply 9.8 joules of energy just to overcome gravity.

Power is energy per unit time. So if you want to go up a given hill faster, you can either:

• reduce the mass of you and the bike, which reduces the energy requirements, or
• supply more power, which supplies the energy faster and thus climbs the hill in less time.

Ultimately the power must come from you or the bike. Climbing a hill feels easy when most of the power comes from the bike and little comes from your legs.

So is 70 newton-meters a lot of power?

It not power, it's torque. To get power for a motor, you multiply torque by angular velocity. So 70 newton-meters could be a lot of power or it could be very little. It depends on how fast it is spinning while producing that torque.

This is easy to observe in a bike without a motor. Ride up a hill. Now shift to a lower gear but keep riding at the same speed. You'll find you don't have to push down on the pedals so hard (less torque) but you have to spin them faster (more angular velocity).

So unfortunately, the torque doesn't tell you how hard it will be to go up a hill.

Power would be better, but this too doesn't really answer the question either, because less power doesn't mean a hill can't be climbed, it just means you'll climb slower because it takes more time to produce the required energy.

Your best bet is to find a bicycle hill climb calculator. You tell it how much you and the bike weigh, how steep the hill is, and the slowest you are willing to go, and it will tell you the power required to do it. Then look for an e-bike with at least that much power, which means the bike should be able to provide nearly all the power to get you up the hill, requiring your legs to do little work, making the climb feel "easy".