# Wattage Calculator



## figofspee (Jul 19, 2018)

There is a wattage calculator online that has helped put into perspective the amount of wattage a typical rider experiences on a downhill. This calculator isn't eBike specific but it is something that can help people relate to how powerful an eBike is or can be. My attention span for physics is shorter then it needs to be to feel 100 percent confident in my interpretations, so feel free to correct me!

It appears that a descent on a 20 percent grade provides 2400 watts of assistance, and a descent of 30 percent provides 4500 watts of assistance.

https://www.gribble.org/cycling/power_v_speed.html


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## Moe Ped (Aug 24, 2009)

Looks like a very nice calculator, at a glance it looks like the math is correct. Not sure about some of the defaults; this is obviously geared (pardon the pun) towards road (racing) bikes. Both the aero and rolling drag are too low for MTB.

Of course for e-bikes there's another huge variable which is the efficiency of the drive system. Most manufacturers ain't telling!

Your example is too simplistic; at single points you might be getting those numbers---you're not mentioning at what speed nor do we know at what weight (very important).


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## figofspee (Jul 19, 2018)

Originally I used the stock parameters, which were in between dirt and pavement at .005. When I raised that number to .01, the weight of rider to 200, and the weight of bike to 50 (leaving all the other numbers to defaults) I get 3775 watts for minus 20 percent grades and 6530 watts for minus 30 percent grades.


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## figofspee (Jul 19, 2018)

Aero and drivetrain efficiency do not relate to this calculation as it is a measurement of gravity's assistance. Those fall into resistance forces which are relevant to your overall speed but not so relevant to the assistance measurement. It doesn't matter what speed you are going as the gravitational forces are the same regardless of speed, at least that is my assumption correct me if I am wrong.


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## Moe Ped (Aug 24, 2009)

figofspee said:


> Originally I used the stock parameters, which were in between dirt and pavement at .005. When I raised that number to .01, the weight of rider to 200, and the weight of bike to 50 (leaving all the other numbers to defaults) I get 3775 watts for minus 20 percent grades and 6530 watts for minus 30 percent grades.


At what speed?


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## Moe Ped (Aug 24, 2009)

figofspee said:


> Aero and drivetrain efficiency do not relate to this calculation as it is a measurement of gravity's assistance. Those fall into resistance forces which are relevant to your overall speed but not so relevant to the assistance measurement. It doesn't matter what speed you are going as the gravitational forces are the same regardless of speed, at least that is my assumption correct me if I am wrong.


Gravity is a constant, acceleration is a squared function. Until you start moving you are getting zero watts from gravity.


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## figofspee (Jul 19, 2018)

Moe Ped said:


> Gravity is a constant, acceleration is a squared function. Until you start moving you are getting zero watts from gravity.


Gravity is applying a force regardless of speed which is why you need to brake to keep from moving downward. The formula does not ask or tell a speed, so my assumption is that speed is not relevant.


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## hikerdave (Mar 8, 2006)

figofspee said:


> There is a wattage calculator online that has helped put into perspective the amount of wattage a typical rider experiences on a downhill. This calculator isn't eBike specific but it is something that can help people relate to how powerful an eBike is or can be. My attention span for physics is shorter then it needs to be to feel 100 percent confident in my interpretations, so feel free to correct me!
> 
> It appears that a descent on a 20 percent grade provides 2400 watts of assistance, and a descent of 30 percent provides 4500 watts of assistance.
> 
> https://www.gribble.org/cycling/power_v_speed.html


I use this one:

Bike Calculator


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## Bigwheel (Jan 12, 2004)

Does it really matter how many watts you are producing going down? Isn't it more about the flats and ups and the amount the assist actually produces









Swiped from a Lennard Zinn article.....


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## figofspee (Jul 19, 2018)

Bigwheel said:


> Does it really matter how many watts you are producing going down? Isn't it more about the flats and ups and the amount the assist actually produces
> 
> View attachment 1262517
> 
> ...


6530 divided by 575 is 11.3

The naturally occurring wattage that gravity produces is 11 times the maximum output of an average mid-drive motor. There needs to be caps on this out of control power accessible to all riders.


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## figofspee (Jul 19, 2018)

https://www.ebikes.ca/learn/power-ratings.html

"While an actual watt is an actual watt, There is NO SUCH THING as a "rated watt" or any standarized method for rating ebike motor power"

"When ebike companies talk about a motor power, there is no standard at all for whether this is a continuous power rating, a peak output power rating, or a peak input power rating, or something stamped on the product for legal compliance."

https://www.ebikes.ca/tools/trip-simulator.html


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