Racing Dan

Was looking into buying an ebike, and I have noticed everyone being obsessed with the torque spec of the motor. Why is that, when the spec needed to compare speed and acceleration is ave. power and peak power?

Wattsup

In the realm of automobiles, marketing blurbs often refer to torque or "low-end torque." Of course, all the torque in the world couldn't pull a toothpick out of a cupcake...it takes power to do that. So when you see automobile manufacturers bragging about torque, they're really bragging about the horsepower curve...the engine makes a lot of torque, (and hence, more horsepower) at low-ish RPM's.

Electric motors are generally very "torquey" ....they make some very good horsepower at lower RPM's....they pull away from a stop rapidly, and they can pull through gears without having to downshift the transmission as much, say like up a hill. They can get a heavy load moving rapidly from a stop.

I don't know anything about the ebike market, but to borrow from my car experience, I'd say torquey (good horsepower at low RPM..low cadence) ebikes might be desirable in that they can get heavy loads rapidly moving from a stop, and can go up hills without the rider having to downshift so much.

Racing Dan

Yes, I know all that, thats why Im asking. "Acceleration" seem to be what manufacturers associate with torque.

From Bosch

"Maximum torque of up to 50 Nm ensures harmonious, agile acceleration."

However, from my understanding, more torque does not equal more acceleration. More power does. Say two motors both have a peak power of 500W but one can only manage 40NM and the other 60NM. Then, to my knowledge, both motors would accelerate the same load equally fast, you just need to put a smaller sprocket on the lower torque motor or simply change into a bigger sprocket in the back.

Right?

Wattsup

I have to admit, I need more education. Try:

https://electricbikereview.com/forum...-gimmick.2467/

https://www.google.com/search?client...71.p-Z0hlBUcoQ

nfmisso

power = torque x rpm.

Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488
Power (HP) = Torque (lb.in) x Speed (RPM) / 63,025
Power (HP) = Torque (lb.ft) x Speed (RPM) / 5252

Torque (translated to force at the road contact area) determines how steep a hill can be climbed, Power determines how fast you can climb the hill.

Doc_Wui

If all of this is a concern, and after reading this guy's woes with his class II bike, you might consider the merits of building a bike, especially if you want to go faster than what is off the shelf.

Poor gear ratio

Racing Dan

Was looking into this issue some more

What you write is not entirely true. Given the right gearing between motor and wheel (think Bosh) an inherently low torque motor will perform just as well on a steep hill as a high torque motor, if they put out the same power. At the same power the low torque motor just spins faster and needs a smaller drive sprocket.

Now why then do they advertise torque rather than power? - Beats me. In Europe motors are limited to nominal 250W and that is what is advertised along with the torque spec. However, as I suspected the high torque motors are in reality higher peak power, in accordance to relevant theory. ("torque=power/rpm"). For instance the Bosch Performance CX, 20 mph is 600W max compared to 295W max in the Active line, 20 mph, even if they both are 250W nominal.

https://www.bosch-ebike.com/us/products/drive-units/

2old

The torque curve for a given mass of copper can be "adjusted" by the gauge of the wire. The power can be regulated further by controlling the back EMF. There are a myriad of variables which influence the power of an electric motor, but as DW says, you need to go the DIY route (or get a de-restrict device if you want an OEM product). Several years ago it was possible to get more speed from a Bosch system by moving the magnet and sensor from the wheel area to the crank (which moves more slowly), but they've made changes in the controls to prevent this. There are individuals on endless sphere that have "bikes" which exceed 60 mph (one does 100+mph in the quarter mile). If we knew your needs, we might be able to help. Note that Bosch uses a small front chainring because they employ a Schlumph (sp?) drive which magnifies by 250%.

tyrion

Gearing can increase torque (at the expense of speed), just as using a longer handled wrench can. A long handled wrench increases the torque you can apply, but doesn't increase your power.

You can have tons of power and not get up the hill unless you use gearing to increase the torque applied to the wheel.

@nfmisso is an engineer, he knows what he' talking about.

Racing Dan

I didnt claim "a long handled wrench increases the torque you can apply, but doesn't increase your power." I doesnt make any sense.

I also didnt claim gearing is not important. I said "Given the right gearing between motor and wheel (think Bosh) an inherently low torque motor will perform just as well on a steep hill as a high torque motor, if they put out the same power", meaning gearing is all important.

Maybe nfmisso should answer the post? I believe I said not wrong in #7

tyrion

Correct. I claimed that.

Racing Dan

You framed it as if you refuted something I said. You didnt. I didnt say or imply anything resembling what you wrote.

tyrion

I didn't meant to imply the torque wrench thing was something you said. Sorry.

unterhausen

both torque and power are important. Changing gear ratios is effectively changing torque. I have seen tiny electric motors that delivered a lot of torque at high speed through a geartrain. But that's not really what you want in an ebike.

Maybe Bosch is advertising their torque numbers because they are power-limited?

Racing Dan

"I have seen tiny electric motors that delivered a lot of torque at high speed through a geartrain. But that's not really what you want in an ebike. "

Why is that? I appears that is what Bosch is doing.

All the manufacturers advertise torque.