Your Power at 20mph?

In no wind conditions, with no pedaling, on a normal bicycle, and on flat ground, how many watts are you pulling from the battery at 20mph?

I know this question has been asked before in other formats, but not with these specifics. I'm working on adapting a power estimation calculator for Ebikes, and I want to get an average to compare my results to. I know other calculators exsist, but I wanted to make something simple, that will show people how much power they might need to go 35mph up a 5% grade on a mountian bike, all in a simple format without a lot of extra confusing inputs or graphs.

For the wind factor, an average of your power use on rides into the wind, and with the wind will work, or riding cross wind.


Other information I could use is:
Type of Motor:
Type of bike:
Tire type(Slick, semi-slick, knobby):
Rolling weight (bike+rider+normal carried equipment):
Your CoD and/or CoDA. (If you happen to know this for some reason):
Any other speed you want to add:

Type of Motor: SWXH rear hub
Type of bike: 20" wheel full suspension folder
Tire type(Slick, semi-slick, knobby): knobbly, wide, MTB type tyres
Rolling weight (bike+rider+normal carried equipment): ~103kg
Power is around 440W at 20mph (battery is 10S LiCoO2, motor is fast wind version intended for use at 24V)

Type of Motor: Q100 rear hub
Type of bike: 20" wheel Xootr Swift folder
Tire type(Slick, semi-slick, knobby): Schwalbe Marathon
Rolling weight (bike+rider+normal carried equipment): ~97kg
Power is around 360W at 20mph (battery is 15S LiCoO2, motor is 328rpm at 36V version)

Both these bikes are pretty upright (the Swift has slightly lower bars though), the main difference is that the Swift has noticeably less rolling resistance, probably down to the narrower tyres.

That's about my maximum speed at full throtle. So my motor is 500 watts.

Some data here: http://endless-sphere.com/forums/viewtopic.php?f=5&t=14786

I admire your efforts, but why not just use the proven calculators. ??
People have put a lot of effort into compiling data and checking these.
such as ..
http://www.kreuzotter.de/english/espeed.htm

Hillhater said:

I admire your efforts, but why not just use the proven calculators. ??
People have put a lot of effort into compiling data and checking these.
such as ..
http://www.kreuzotter.de/english/espeed.htm

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There are a lot of good ones out there. Few are setup for ebikes. And the results are based on what amount of human input at the pedals it would need to move the bike. They don't factor in things like the motor's efficiency. It might take 260 watts of human input to go 20mph, where as an Ebike sucks 300 watts out of the battery on the same bike.

I want to make something easier to understand. Something that gives a 1 field result for someone who may barely understand what these numbers mean, and who won't understand (Cd*A), (F= -1/2 * Cd * p * v^2), or (E = pJ)

I agree that drive train variations will make noticeable differences, but you must be aware that collecting empirical data in this uncontrolled way is meaningless.
the difference in simple things like tyre type or pressure can add 50% to power required.
A 2% grade can double the power needed, and a 5mph breeze can add (or subtract) another 50%
Road surface ?, Rider height,? clothing ? , etc etc
There are just too many major variables ...outside the motor / controller / drive line,.. to compare this data .

As a bare minimum,.. any data should be an average of a 2 way run, to eliminate wind and grade effects.

Hillhater said:

I agree that drive train variations will make noticeable differences, but you must be aware that collecting empirical data in this uncontrolled way is meaningless.

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On the contrary, it's very helpful. The data isn't about building the calculator, its about testing and checking the calculator when I'm done. This is an estimation calculator. I'm looking to make the best Estimate possible with the least data input by the user.
Physics are immutable, and the math is absolute, but there are as many variables to consider as there are ebikes. I will have to set parameters and make assumptions, as all the online power calculators do. Seeing other people's results will give me a pool of answers to check my questions against. If my questions and my assumptions don't lead to those answers, then I know I'm making the wrong assumptions, the wrong guesses, asking the wrong questions.

I only have 6 motors I can test personally. My range of variables are small. So using myself as a basis for checking my accuracy is meaningless.

For Example, Jeremy's info falls outside of the range of output from what I was working on. I'll have to start over on a few points, but thats a good thing, it will ultimately make this more accurate.

The intent of my running survey, was/is to allow users to find a contribution that closely resembles their own configuration. As time goes by, there may be a large enough number of contributions to cover a broad spectrum. It won't cover all, but maybe the mainstream.

Narrowing the band to 20mph has the benefits of less input data and a reasonable expectation that most riders will have a setup capable of 20mph.

Here's a log I kept for a while before I got bored. I was shooting for 20mph average speed to stretch my limited battery power all the way home. Might be some useful info in there for ya.



Gary

At 20mph I'm either pulling about 12kw, since the bike doesn't hit max power until about 30mph, OR negative 2-3kw in regen. I don't cruise at 20mph, so I'm either accelerating or regen braking at 20. I some conditions where low speed is warranted it's generally well below 20.

If it's not windy tomorrow I'll try to get a number more like what you're looking for by trying a steady cruise at 20 on a flat stretch of a residential street.

John

The problem with measuring this, is you never really have truly flat ground in NM. It looks realllly flat near my house, yet it's actually a slight grade on every street.

But by going both ways and noting the wattage, I tend to come up with a 400 w average to go 20 mph on most of my ebikes. Closer to 350 average if the bike has no panniers and I'm riding cleaner in the summer. By clean I mean no big flapping coat, just shorts and a t shirt. But most of my bikes have some kind of cargo bags, even if just a handlebar purse, or a battery bag that catches side winds, and riding near naked is not for all year at all. So 375-400w is about right for when there is no wind.

Not much wind or uphill grade can pump it to 500-600w.

It's a bit tricky to pin it down though, since 21 mph might take 450w and 19 mph as little as 300. You really have to pin it at exactly 20mph for quite a distance to establish that you aren't skewing the number with some stored momentum or a tiny variation in the grade.

I weigh 185 pounds btw, and tend to carry about 15 pounds of battery. Weight matters, I think I'd get different numbers if I weighed more. But I was suprised at how little it mattered to add 30 more pounds of battery and gear, bringing the total load to about 45 pounds not counting the motor itself. At 20 mph, I'd still get well under 500w.

Then if you have a frankenbike, it can get a lot more. I haven't the thousands of miles needed on my longtail to really confirm what my wattage is at 20 mph. It was slower than 20 with the 2810 on it, and now that I have a 5304 on it I tend to ride it 31 mph. But it's at least 600w for 20 mph, possibly more. That bike has a large box on the rear rack and 4 panniers. Any wind, and it's like riding with a sheet of plywood under one arm. Takes about 2000w to go 31 mph. Weight of the bike matters of course, this frankenbike is well over 100 pounds, but it's the bags all over it that really matter. Before I put the bags on it, it really rode more efficient.

I just came back from 1984. Power usage showed 1.21gigawatts.

go figure?

Very unscientific, but yesterday I was maintaining 20mph on a flat at about 500w. I know I was running into a head wind, I'll try to get more data and post again when I feel I have better conditions.

Type of Motor: Astro 3210; 5:1 primary reduction, 4:1 secondary reduction, 406mm wheel.
Type of bike: commuter bike - Dahon 20" folder
Tire type(Slick, semi-slick, knobby): semi-slick, 1.25" road tire
Rolling weight (bike+rider+normal carried equipment): ~240 lbs
Your CoD and/or CoDA. (If you happen to know this for some reason): Not sure what this is

414 Watts
Riding Position: Upright
Type of Motor: rear 9c 6x10
Type of bike: Trek 3700 MTB
Tire type(Slick, semi-slick, knobby): Knobby, 70psi
Rolling weight (bike+rider+normal carried equipment): 265 lbs

I actually have all my power data up to 38mph. This was done over a very flat section of smooth road for 5 miles.

The formula for my power usage is this:
Watts = 1.8152(Speed)^2 - 23.446(Speed) + 154.58
example:
Watts20 = 1.8152(20)^2 -23.336(20) + 154.59
Watts20 = 1.8152(400) - 466.72 + 154.59
Watts20 = 726.08 - 466.72 + 154.59
Watts20 = 414

Interestingly enough, the slope intercept (154.58watts) is about the no-load power required at full speed.

itchynackers said:

414 Watts
This was done over a very flat section of smooth road for 5 miles.

The formula for my power usage is this:
Watts = 1.8152(Speed)^2 - 23.446(Speed) + 154.58
.

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?? So the 414W is a calculated figure ? ..or an average figure over 5 miles (15 mins) ??

Where did you get the "constants" .. (1.8152, 23.446, 154.58 ).. from in your equation ?

I set the CA to max out at 2mph intervals and ran the bike over a 5 mile course. Recharge battery, repeat. I did this from 8mph to 38mph. Then enter the data into excel, and do a regression analysis. A binomial was the best fit.

itchynackers said:

I set the CA to max out at 2mph intervals and ran the bike over a 5 mile course. Recharge battery, repeat. I did this from 8mph to 38mph. .

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Wow .. i bet those 8, & 10 mph runs were fun !! :lol:

CrazyBike2 takes around 450W average to maintain 20MPH under no-wind conditons on flat roads.

It's semi-recumbent, but with big boxy cargo pods on either side of the rear wheel just behind the seat. According to the numbers I get from ebikes.ca simulator when modelling various setups on there, it actually has aero more like their definition of "mountain bike" than their "semi recumbent", which reports taking only about 3/5 of the power CB2 really takes on the road.

Presently it is a 9C 2807 20" rear DD hub, powered by a modified 18FET Methods controller with one shunt removed, so it's peak curent is only about 80-90A, IIRC. Battery is LiPo-like 20Ah, 16s, 66.3V hot off charger.

Tires are 26" Kenda Krossroads on front (slick center, slightly knobby edges), and I forget the brand of rear, but it is actually a jogger-babystroller tire, 1.8" or something like that max but a fairly flat crosssection with ~ -like tread, kinda like a really skinny car tire tread. 55PSI kept in each tire.

Typically weighs about 300lbs rider + bike + stuff I'm carrying at any one time.

Hillhater said:

itchynackers said:

I set the CA to max out at 2mph intervals and ran the bike over a 5 mile course. Recharge battery, repeat. I did this from 8mph to 38mph. .

Click to expand...

Wow .. i bet those 8, & 10 mph runs were fun !! :lol:

Click to expand...

Ya a little slow going in the early testing, but eventually got way more fun! This summer I plan on doing the 40-50mph test runs. 35s lipo!