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I've been looking for a way to size an electric motor for a vehicle that could be assumed as a box-shaped load with a mass of 60 kg, up a 10° incline, at 1.6 m/s, using continuous tracks with 1 motor per side, but for some reason while searching for the answer I found different formulas for this and all of them gave me different answers.

What is the actual formula I should use to find the power required to move this load? As detailed as possible since it's for a practical application not only a theoretical question.

The last one I found is:

$$P = \frac{F_t\ v}{\eta}$$
Where $F_t = Rolling\;Resistance\; +\; Gradient\; Resistance\; + Air\; Resistance$
and $\eta = Transmission\; Efficiency$.

Is this the correct formula I should be using? Am I missing something? Should I consider Air Resistance since the vehicle is not moving too fast? I noticed both rolling resistance and gradient resistance have a friction coefficient, do I use the same one on both equations?

StainlessSteelRat
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Calpurnius Kazik
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2 Answers2

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At 1.6[m/s] the Air resistance is negligible so I would dismiss that term.

However, a word of caution: Rolling resistance Force is not the same as friction force if you are using wheels.

I would point you to the following page for a rough estimation of Rolling resistance.

The other factors that might come into play are negligible compared to what you have outlined (namely gradient and Rolling resistance).

If you use tires (which I expect with a load of 60 kg), the equation should have a form like:

$$ P = (c\cdot W\cos(\alpha)+ W\cdot sin \alpha)v$$

which can be approximated by the following (as you mentioned in your comment):

$$ P = (c\cdot W + W\cdot sin \alpha)v$$

NMech
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  • When calculating the gradient resistance, should I actually use a friction coefficient? I thought that maybe using tires that it wouldn't make that much of a difference, though tracks might. So my equation in this case becomes (withut air resistance): $$ P = (cW+ Wsin \alpha)v $$ – Calpurnius Kazik Oct 02 '20 at 10:29
  • Oh $c$ is the rolling resistance coefficient used for calculating the rolling resistance as it says on the link you provided before. The equation I posted would be the sum of the rolling resistance and the gradient resistance times velocity. I noticed your fixed equation looks a lot like the gradient resistance with the friction coefficient. – Calpurnius Kazik Oct 02 '20 at 11:55
  • I updated the post, basically incorporating your comments and my reply, while stressing that this is for pneumatic tires. However, I am curious as to where you got your initial equation with the Transmissability Ratio (I've encounter this term sometimes in Gear boxes or Vibration Isolation problems, so I am unsure of the context its being used). If you'd be so kind to provide a reference, I might get a better understanding of where you are coming from. – NMech Oct 02 '20 at 12:05
  • Yes, of course. I actually got the equation from the same site you [linked](https://www.engineeringtoolbox.com/cars-power-torque-d_1784.html), I didn't notice that the page for the rolling resistance was linked at it that's why I missed it until you linked it. Just so I can understand, if I use the rolling resistance coefficient, it replaces the friction coefficient from gradient resistance? – Calpurnius Kazik Oct 02 '20 at 12:07
  • I might be getting confused but it's best to ask than not. But would this be the correct equation? ($c$ is the rolling resistance coefficient and $\mu$ is the friction coefficient)$$P = (F_r + F_g)v = (cW + W(sin\alpha + \mu cos\alpha)))v$$ – Calpurnius Kazik Oct 02 '20 at 12:16
  • No. You will either use the rolling resistance (if you got tires) **or** the friction (if you got no tires). – NMech Oct 02 '20 at 12:19
  • Oh I see! That makes a lot more sense than what I was doing before. That probably explains why I was getting weird values. Thank you so much! – Calpurnius Kazik Oct 02 '20 at 12:22
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The friction coeficient is defined by the surfaces in contact, if they don’t change then it won’t.

So tires on tarmac has a friction coeficient but that changes with rain, snow or spilt diesel...

Solar Mike
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