Test Rode Cervelo Rouvida Road vs Trek Domane+ AL 5 Today. My Analysis

[Stefan Mikes]

Next: How does Mahle cheat about the x20 torque? Explaining again.

Mahle has never said the x20 hub drive motor had 55 Nm. The brand said:

They said: "Comparable to a 55 Nm Mid Drive". Source.

Let see what it actually means. The hub-drive motor propels the rear wheel directly. The mid-drive motor acts on the rear wheel via the drivetrain, so the torque at the rear wheel can be either amplified (MTB low gears) or reduced (high gears). Mahle has assumed the rider would ride at the 55/23 = 2.39 : 1 gearing. The large chainring on Scott Addict RC eRide Ultimate is 52T. Therefore, the selected cassette gear would be 52/2.39 = 22 teeth. It is a middle gear on a 11-34T cassette.

So far so good. Mahle has explained its reasoning. Now, let us assume you are climbing. Let us compare a mid-drive 50 Nm motor (TQ HPR 50) on a TREK Domane+ 6 SLR Pro. The chainring is 46T and let us assume the 22T cassette cog selected for the comparison. The gearing would be 46/22 = 2.09. The TQ motor offers max 47 Nm torque (300 W/6.28). The effective torque at the rear wheel would be 47 / 2.09 = 22.5 Nm.

A very interesting result! It looks Mahle has nailed it for a typical road riding scenario? Let us analyze a mountain climbing case then.

A TREK carbon Domane+ 6 rider may drop the gearing down to 46/36T = 1.277 gearing ratio. Now, the effective torque at the rear wheel is 47 Nm/1.277 = 36.8 Nm. Mahle x20 would provide 23 Nm, not more.
A TREK carbon Domane+ 6 rider may install a 2x 52/36T chainring and a front derailleur (it is in the specs). Now, the rider would downshift to 36T chainring and 36T cassette cog giving the gearing ratio of 1:1. The TQ HPR motor would give 47 Nm at the rear wheel while the x20 would stay at 23 Nm in this climbing scenario.

Moreover, hub-drive motors are poor climbers. As the e-bike speed drops, the hub-drive motor becomes inefficient. It delivers a little power/torque at low speed and overheats.

The Bottom Line
It is very easy to juggle with the numbers, Mahle! You have almost convinced me because your scenario was about riding on the flat, or on a soft incline. When it comes to serious climbs, you can shove your data where the sun don't shine It is a good mid-drive motor for road e-bikes that shines.

 

[Recycled Cycler]

Next: How does Mahle cheat about the x20 torque? Explaining again.

Mahle has never said the x20 hub drive motor had 55 Nm. The brand said:


They said: "Comparable to a 55 Nm Mid Drive". Source.

Stefan: on the Scott website they say this about the Mahle X20:

Motor Hub drive 55Nm max Torque / EU: 25kmh / US: 20mph
Mahle Motor Hub drive 55Nm max Torque, up to 407Wh

Doesn't say comparable at all.

I interpret max torque to mean the motor is capable of 55nM. Period. Nothing is said about comparing Nm to a mid motor drive.

 

[RidgeRunner500]

Jeremy, let me explain it in more detail.

the manufacturers are unwilling to share the actual motor power to avoid investigation from the European Union as you know the nominal motor power limit is 250 W there. The figure of torque is used for the marketing purposes instead.

If the manufacturers are making up the published power figures, how are they more reliable metrics than the torque figures?

 

[Stefan Mikes]

If the manufacturers are making up the published power figures, how are they more reliable metrics than the torque figures?

Manufacturers do not lie about power figures (mechanical peak power).

You will certainly agree with me the 50, 70, 85 and 90 Nm figures look too nice to be real.

 

[RidgeRunner500]

Manufacturers do not lie about power figures (mechanical peak power).

You said that manufacturers don’t share the actual power figures. In any case, why would they be honest about power but lie about torque?

You will certainly agree with me the 50, 70, 85 and 90 Nm figures look too nice to be real.

These are nominal specifications. They don’t seem much rounder than that power figures.

 

[jabberwocky]

I think its safe to assume that most of what you read on torque and power specs was probably written by a marketer, not an engineer. And theres a lot more to a motor system than peak torque/power/whatever. I would be wary of saying "I rode a bike I liked that said it had 50nm of torque so that means I'll like any other motor that says it has 50nm of torque." Test ride, test ride, test ride. If at all possible.

 

[Stefan Mikes]

You said that manufacturers don’t share the actual power figures.

Specialized does.
Yamaha does.

Only recently DJI Avinox, Specialized/S-Works 3.1 and Bosch Gen 5 have decided to advertise the torque together with the peak power in their marketing (not technical) material. Which brought a controversy in Europe, as something such as 750 W peak power is a way above the declared 250 W nominal.

You can measure the max battery current draw and the voltage on older Specialized e-bikes. For instance, the measured max battery current on a Vado 6.0, which is specified for 520 W max mechanical power is 18.5 A at 36 V, and it is 666 W. If you divide 520/666 you will get efficiency of 0.78 (78%), which is highly probable. On the other hand, you have no way to measure the motor torque yourself.

Similarly, Specialized gives the peak power of 240 W for the SL 1.1 motor. The measured power draw is 303 W. Divide 240/303 and you will get the motor efficiency of 79%.

 

[dodgeman]

From a physics point of view torque is a twisting motion with no movement. Imagine twisting on a screw with a screwdriver but there is no movement, that is torque. Power on the other had has movement. Do the same amount of work twice as fast and you have more power. You can fudge power numbers by having unrealistic cadence numbers. My cadence is probably 60 on average and maybe 70 at times. If you take the cadence at 140 and have the same torque that you do at 70 you have a bunch more power, but most people won’t pedal that fast.

My Trek Verve 3 has 4 steps of 35, 40, 45 and 50nm with the power settings. 5 nm is a noticeable difference in power.

 

[dodgeman]

To The OP I looked up the bike you bought and it looks like a nice one. Keep in mind I don’t offer this service to everybody but I’ll break it in for you. That first 500 to 600 miles can be difficult so I’ll do that for you, just drop it off at my house.

 

[Jeremy McCreary]

From a physics point of view torque is a twisting motion with no movement. Imagine twisting on a screw with a screwdriver but there is no movement, that is torque. Power on the other had has movement. Do the same amount of work twice as fast and you have more power.

Well put with one quibble: You can put power into any resistance, including a pure twisting motion against, say, frictional resistance. Hence the power loss at wheel bearings.

In a beautiful symmetry of nature, the classical equations for translation and pure rotation have exactly the same form. In both cases, the power P lost to a resistance is just the resistance times the speed against it.

In the purely rotational case, the resistance isn't a force. It's a torque T in Nm. And the speed isn't a linear speed like ground speed. It's an angular speed W in radians/second, with W being proportional to rotational frequency F in rpm: W = F (pi / 30) ~ F / 10 .

Then the power P lost to T at F rpm is

P = T W = T F (pi / 30) ~ T F / 10.