nm vs watts

[Poider]

Hello, This is my first post, I have just ordered an earth T rex 650b Trekking.
The stats for this bike are 85nm limited to 250w. is there a website or source that shows what 85nm is compared to 250w, I have seen some bikes for sale that state 250w and 35nm, I am curious to know how to compare bikes listed in nm to bikes listed in watts.

[JPB]

They are measuring different things.
Nm is Newton Metres and measures Torque, how much twisting force is being applied.
Watts is measuring power in a period of time or how much work can be done.

My understanding is that the rate that the torque is being applied is the power. The impact probably occurs when going up hill, the motor can generate 85Nm of turning the pedals but can only do it at a speed limited by the power. It can probably climb steeper than the 35Nm bike.

[Mozziediver]

+1
N x m is force times distance, a measure of the energy delivered (joules). As I understand it (correction welcome) Watts measures the rate the energy is delivered. Nm x revs per second is the power in Watts.
Very roughly, at the higher torque rating and full power, it will produce more torque at lower rpm so better on hills.
Be aware that most ebikes have "ranges" that offer more than 100% at the expense of range.
*edit for clarity*

[uart]

. As I understand it (correction welcome) Watts measures the rate the energy is delivered. Nm x revs per second is the power in Watts.

Close, but not quite right. The power in Watts is equal to the torque in Nm times the rotational speed in radians per second (where one rev is equal to 2*pi radians).

So while 85 Nm is a lot of torque (it's equiv to about 50 kg at 170mm cranks), it will necessarily start to reduce at a fairly low cadence in order to stay under 250W.

To find where the torque starts to reduce, find where 85 * cadence (rpm) / 60 * 2*pi = 250. If my math is right, this comes out at only 28 rpm cadence. (If you've ever wondered why ebike riders pedal so slowly, well now you know LOL).

[find_bruce]

Close, but not quite right. The power in Watts is equal to the torque in Nm times the rotational speed in radians per second (where one rev is equal to 2*pi radians).

So while 85 Nm is a lot of torque (it's equiv to about 50 kg at 170mm cranks), it will necessarily start to reduce at a fairly low cadence in order to stay under 250W.

To find where the torque starts to reduce, find where 85 * cadence (rpm) / 60 * 2*pi = 250. If my math is right, this comes out at only 28 rpm cadence. (If you've ever wondered why ebike riders pedal so slowly, well now you know LOL).

Not much wrong with your maths.

But where I suspect the difference actually lies is in fuzzy definitions - the requirement for ebikes is not power, but rated continuous power - the motor can produce more power as long as it isn't rated for that continuously. As I understand it, that is controlled by software

[Poider]

Thank you all, That makes it a bit clearer.
So I assume that 85nnm is the torque the bike can produce, Whilst not going over 250w, seeing as the bike is limited to 250w does that mean that it is possible to change the limit for when riding on private property? and if so is there a limit that it can go to, I am sure that I can move the magnetic sensor to change the allowable speed, but I am not after more speed, just want more power for the hills.

[gsxrboy]

There's an old saying that goes.. Torque gets you up to speed, power keeps you there.

[Thoglette]

.... but I am not after more speed, just want more power for the hills.

Use (or change) the gearing. More teeth at the front or less at the back.

(I see a lot of Ebike riders in big/small combination wondering why they can't get up that hill )

[uart]

but I am not after more speed, just want more power for the hills.

You don't need more power, just lower gearing. The nice thing about mid drive systems is that they go through the full drive train, so their torque can be amplified by the bike's gearing.

Use (or change) the gearing. More teeth at the front or less at the back.

I think that you probably meant to say "less teeth at the front and more at the rear" for better hill climbing torque.

[Thoglette]

I think that you probably meant to say "less teeth at the front and more at the rear" for better hill climbing torque.

Oops