Relationship between leg watts and e-watts?

Discuss light weight issues concerning road bikes & parts.
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Shrike
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by Shrike

I see e-bikes starting to pop up on Strava. Seems they have 250 watt motors after taking a look.

How close is 250 watts purely from the e-bike's motor compared to you putting out a sustained 250 watts in terms of speed - is a watt a watt? Can't be the same thing surely, those motors look tiny, though I've read a blog post on them now that says the motors are actually underrated to skirt regulations.

by Weenie


2lo8
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by 2lo8

A watt is a watt. There might be a minor discrepancy between watts in and watts out due to efficiency, but yes those little motors can actually produce hundreds of watts of output. They're often paired with heavy bikes, slow tires, and very unaero riders and direct drive motors without transmissions might not be operating at peak power RPM. Also I know from first hand experience being passed by e-bikes going over 20mph uphill. They actually put out that many watts. People simply don't put out that much power, that's why we don't drive pedalcars on freeways. For reference, 1hp is 746w if you want to think about how much more powerful your lawn mower is than you.
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Shrike
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by Shrike

Thanks 2lo8, had no idea they were that powerful :shock:

mattr
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by mattr

2lo8 wrote:A watt is a watt.
Depends on more than just that. There will be a curve for the motor (and your legs) and you'll develop power differently at various speeds (motor shaft/leg) part of the motor equation is making sure it develops headline power at a point within normal leg speed range. I don't know what the curves look like, what the gear ratios are, but i'd guess(?) that the headline power is not available throughout the range.

Also quite dependent on motor tech and management as well. And what the original design prerequistes were!

FWIW The little motor in my RC car develops well over 2kW. Unfortunately it needs to be running at over 20000 rpm to do this........


sungod
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by sungod

for watts delivered, a watt is a watt. but if you look at the input power needed to get that output power delivered, there's a big difference

normal electric motors maximum efficiency is generally quoted around 75% (the rest goes to heating things up)

for a 250w e-bike, that makes around 190w at best, but as above, the operating conditions matter, efficiency could be much lower in some situations

a cyclist who can ride at 250w will easily out perform that

but this is power delivered, the efficiency of the cyclist in terms of turning fuel (food) into power to the pedals is probably 18-25%, the rest is generating a lot of body heat, which is why you can get pretty hot cycling

taking input power (assuming the human has an excellent 25% efficiency, and the motor 75%)

4 human input watts delivers 1 watt power
1.33 motor input watts delivers 1 watt power

bremerradkurier
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by bremerradkurier

2lo8 wrote:A watt is a watt. There might be a minor discrepancy between watts in and watts out due to efficiency, but yes those little motors can actually produce hundreds of watts of output. They're often paired with heavy bikes, slow tires, and very unaero riders and direct drive motors without transmissions might not be operating at peak power RPM. Also I know from first hand experience being passed by e-bikes going over 20mph uphill. They actually put out that many watts. People simply don't put out that much power, that's why we don't drive pedalcars on freeways. For reference, 1hp is 746w if you want to think about how much more powerful your lawn mower is than you.


Watch the video of quadzilla Robert Forstemann powering a toaster at 700 watts.

https://www.youtube.com/watch?v=S4O5voOCqAQ

sungod
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by sungod

i can do that too - for a few seconds :)

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Rick
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by Rick

I really wouldn't trust the accuracy of any ebike's power rating; and of course it depends on whether that is "watts required" or "watts delivered to the wheel".

I was passed by a guy on a mountain bike once, so I started trying to catch up. I noticed it seemed really hard, until finally I was on the edge of absolutely killing myself to try to catch him.
But I finally did, and then we both had to stop for a light, and I noticed he had an e-motor in his rear hub. So I started laughing and asking him about it. He said it was rated at 500 watts.
I can very rarely produce 500 Watts for about a minute!

When we started up from the light, he immediately just zoomed ahead by about 50 yards. Since I now knew he had an emotor, I was in an all out sprint to get on his wheel. I finally got on and the rest of the ride was like a motor-pacing workout! I could just barely stay on his wheel through careful drafting and suffering the tortures of the damned! . He was just looking around and laughing at me!

2lo8
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by 2lo8

mattr wrote:Depends on more than just that. There will be a curve for the motor (and your legs) and you'll develop power differently at various speeds (motor shaft/leg) part of the motor equation is making sure it develops headline power at a point within normal leg speed range. I don't know what the curves look like, what the gear ratios are, but i'd guess(?) that the headline power is not available throughout the range.

Also quite dependent on motor tech and management as well. And what the original design prerequistes were!

FWIW The little motor in my RC car develops well over 2kW. Unfortunately it needs to be running at over 20000 rpm to do this........

I believe I mentioned all of that. Including the discrepancy of input vs output, and whether it operates at peak power RPM. If you have RC motors, you should you you can force an even higher wattage than their rating through them, but they'll eventually overheat and can't sustain it. As noted by the OP, electric motors are often underrated to skirt import laws, those ratings are hardly precise.
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2lo8
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by 2lo8

Rick wrote:I really wouldn't trust the accuracy of any ebike's power rating; and of course it depends on whether that is "watts required" or "watts delivered to the wheel".

I was passed by a guy on a mountain bike once, so I started trying to catch up. I noticed it seemed really hard, until finally I was on the edge of absolutely killing myself to try to catch him.
But I finally did, and then we both had to stop for a light, and I noticed he had an e-motor in his rear hub. So I started laughing and asking him about it. He said it was rated at 500 watts.
I can very rarely produce 500 Watts for about a minute!

There's also the issue of whether he was maxing out the throttle (or PAS). You don't slam the gas pedal to max hp revs when driving your car all the time do you?

It's also worth noting that every thing else has to be rated to deliver power including the batteries and motor controller. And if you want to be technical, bicycles also have a drive train efficiency loss, and tires losses, so what you really want is dynamo power, or power at the wheel (not hub). So even then leg watts aren't quite the same as speed watts.
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kulivontot
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by kulivontot

This is like wheel horsepower vs engine horsepower argument. There's a drivetrain in between.
Even watts at the pedal vs watts at the wheel will differ.
Short of an experiment, you'd have to take an armchair guess at the actual relationship. One of those hidden crank motors with a powertap would be an interesting study.

by Weenie


Marin
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Location: Vienna Austria

by Marin

My father is 81 years, he has a 250w full suspension MTB. I don't know how many watts he can pedal for a longer time - 125?

I can barely hang with him on a steeper climb, and I can do 325 for an hour...

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