Ebike Torque in 4W: What it is, Who needs it most When & Where

You probably happened to read a car or an ebike specifications, stumbling upon a word you hardly or vaguely know the meaning of: torque. Especially because power is clearly stated in watts or horsepower, what does torque exactly mean, and does it matter at all? In the end, power data should say it all, shouldn’t they?

Some physics about torque

Torque is the measurement of the force that causes something to rotate around a point, i.e. the measure of the force that can cause an object to rotate about an axis. Force is what causes an object to accelerate in linear kinematics. Similarly, torque is what causes an angular acceleration. Hence, torque can be defined as the rotational equivalent of linear force.

It’s measured in Newton meters, Isaac Newton having explained it. So, let’s imagine applying a force of 20 Newton meters to a lever rotating around a hub. If the lever is 0.5 meter long, the rotational force applied onto the hub will be 20 x 0.5, so 10 Nm. Now, should the lever be only 0.2 meters long, the rotational force applied onto the hub will be 20 x 0.2, so 4 Nm. Therefore, since a bike chain is farther from the axle on the larger sprockets, thus being a longer lever, these deliver more torque than the smaller ones. But moving a 60-tooth sprocket engages three times more links than moving a 22-tooth smaller sprocket. Hence, larger sprockets are better at starting, climbing and carrying heavy loads, whereas smaller ones are faster.

The same applies to motors: the higher the torque, the better they can carry loads, accelerate from a standstill, and climb. Consequently, diesel motors can deliver more torque, having a compression ratio (of the air in the cylinder) of 23 to 1,  which exerts more force on the rotating shaft, vs. the compression ratio of gasoline motors, about 10 to 1. The former are much better for trucks, the latter for faster cars. There are no diesel racing cars. Electric motors deliver the most torque of all. A gasoline Audi A5: 261 PS (HP) and 273 Nm, vs. an electric Volkswagen ID 7: 286 PS and an awesome 550 Nm. A gasoline Dacia Sandero delivers 95 Nm, weighing 1060 kg, versus 100 Nm for an average 25 kg ebike with the last Bosch Performance Line CX Gen5. Guess which one accelerates faster from zero to 25 Km/h, especially on a climb.

How ebike motors obtain torque

Electric motors operate through the interaction between the motor’s magnetic field and electric current in a wire winding to generate Laplace force in the form of torque applied on the motor’s shaft. Is both conceiving this part and leveraging internal gears that one can obtain more torque. This implies more sophisticated and complicated motors. Consequently, the higher the torque, the more expensive the ebike motor, although the power is always identical, respecting the 250 watts legal limit in europe, Whereas in the us ebike motors average 500 watts.

Torque to power ratio is lower with cheaper Chinese motors

Chinese cheaper motors deliver much less torque per Watt of power. Most 500 W chinese hub motors don’t exceed 50 Nm. Generally speaking hub motors are less torquey than mid motors. They are mostly used for city and trekking ebikes, whereas best European and American e-mountain bikes feature mid motors. Typical high quality 250 watts German and Japanese ebike motors sport 80-90 Nm.
Nevertheless, there are chinese motors sporting an excellent torque even with only 250 watts, like some Bafang and the Avinox.

Torque can greatly differ from only bike to another

Ebike motors offer a wider range of torque. It varies following the usage a specific ebike was conceived for, and also its price. Light 250 W motors aimed at light ebikes go from the 23 Nm of the Mahle in its new sporty rear hub drive X20, weighing only 1.375 kg, to the 55 Nm of the Bosch Performance Line SX. Cheap 250 W ebikes rarely offer more than 50 Nm, and you shouldn’t trust the ones that don’t even declare their torque.

Where torque is most needed in ebikes

Carrying heavy loads demands a hefty torque. Cargo ebikes shouldn’t have less than 80 Nm. Riding on hilly roads, even more on steep climbs, also requires a thorough amount of torque, exceeding 65 Nm and even more, depending on your muscles and your endurance.

The weaker you are, the more you need torque. In case your age is advanced, and your articulations can hurt, starting a bike from stand still and accelerating can be painful, unless you can benefit from a decent torque. Since torque is most effective for acceleration and riding at low speed, it’s extremely valuable up to the 25 kilometers per hour ebike legal speed limit in Europe.

Torque measures for riding or commuting on flat roads

For city, gravel or trekking ebike riding almost exclusively on flat roads, 40 Nm of torque is the least to avoid sweating. I would prefer at least 50 Nm to enjoy more comfort. To that, one should add, for a more enjoyable riding experience:

• One Newton meter for each year of age past 40 and another for each kilogram of overweight.
• 3 Nm for each percentage point of climbing inclination of your habitual paths.

That is tantamount to suggesting at least 69-70 Nm for any biker who is 50, 10 kilos overweight, often riding on 3% steep climbs. And so on.

Torque measures for e mountain biking, or eMTBs

I would start from 50 Nm, applying the same criteria to add some, as mentioned above.

Light ebikes for strong legs and athletic, young riders

Lately most ebikes high quality brands sell ebikes that are up to 10 kg lighter, weighing around 15 kg instead of about 25, but deliver much less torque, about half the Nm of a common 25 kg, 85 Nm ebike. Needless to say, the torque per kilogram ratio is still much better for heavier torquey ebikes, especially if we take into account the rider’s weight. When choosing such an ebike, is therefore essential to reckon the torque per kilogram ratio of the biker’s body, which can vary from one to 2-3 Nm per kilogram, depending on athletic prowess and age.

The more torque, the better? Not always

More torque implies more electricity consumption, which requires more expensive, cumbersome and heavier batteries. The bigger the torque, the heavier and more expensive the motor. High quality 250 W motors sold in Europe average roughly 2kg when delivering about 50 to 60 Nm, roughly 700 grams more as they sport 80 to 90 Nm. With 55 Nm of torque, the Bosch Performance Line SX weighs only 2 kg; the mightier Bosch Performance Line CX Gen5, with 100 Nm, 2.75 kg. The best torque for weight ratio is probably Dji Avinox’s : 105 Nm of torque for 2.5 kg.

Torque wears out transmission components

If we reckon an average 100 Nm of torque for our legs, to be summed up the motor torque, we can fathom the strain that ebike chain rings, chains, derailleurs and cassettes must endure.

Ebike torque is not inexpensive

With European 250 W motors one could approximately reckon 10 euros per each Newton meter of torque. Which results in a 40 Nm motor costing about €400, and an 85 Nm motor costing about €800, taxes excluded. More torque involves the necessity of more robust frame and transmission components, what makes the ebike more expensive and heavier.

Ebike torque performance sheet

Usage	40-45 Nm feels	50-60 Nm feels	65-70 Nm feels	75-85 Nm feels	90-120 Nm feels
Flat terrain	sufficient	easy	comfortable	like sailing	like flying
Shallow climb	a bit tiring	not too hard	quite well	easy	as it were flat
Steep climb	oh my!	barely bearable	sufficient	almost easy	comfortable
Emtb/cargo	need help	must push hard	sufficient	fun/easy	as if I were a squirrel/truck

Images: Pixabay, KTM, Tern