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# Comparing Regen to a Freewheel

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# Comparing Regen to a Freewheel

02-15-09, 06:01 AM
#51
safe
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Aerodynamic Race

How do the aerodynamic properties of the ebike effect the overall result?

There are many possible variables to consider, but in the online calculator we can place a different value in for wind resistance. Simply change:

Air Resistance Coefficient - From - "Straight Arms"

Air Resistance Coefficient - To - "Hill Descent"

...and the results will look like:

...I was actually expecting the lowered wind resistance to favor the freewheel since it could get down the hill faster, but it actually helps the regen bike catch up on the uphill and also allows it to recapture energy better on the downhill. So better aerodynamics actually favors the regen bike MORE than the freewheel bike.

Unime... you need to actually show some math... search for an answer if you can find one and then present it... otherwise all it sounds like is that you somehow want things to "magically" work out better with regen.

Just use the online calculator:

https://www.me.psu.edu/lamancusa/Prod...e/bikecalc.htm

...that way we can ALL AGREE on the results.

Since the math is already "done", please use the same format so that we aren't "reinventing the wheel" by having to go through the math again in a different way. I attached a zip file with the spreadsheet I used, so if you want to save time just use that and change the values.

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The "Math Format" for this problem: (just change the variables to test new ideas)

Downhill (5% negative slope for 10 miles)

Freewheel - 10 miles / 29.64 mph = 0.34 hours (net power gain/loss is zero)

Regen - 10 miles / 15.00 mph = 0.67 hours (behind by 0.33 hours)

Recaptured Energy = 0.160 hp * 746 watt = 119.36 watt

119.36 watt * 0.67 hours = 79.97 Wh * 0.7 (motor losses) = 56 Wh

56 Wh * 0.9 (battery losses) = 50 Wh (this is the energy recovered)

Uphill (5% positive slope for 10 miles)

Freewheel - 10 miles / 13.94 mph = 0.72 hours

Power Needed - 0.399 hp * 746 watt/hp = 298 watt / 0.7 (motor losses) = 425.7 watt

425.7 watt * 0.72 hours = 306 Wh

Regen - Must do 10 miles in 0.72 hours - 0.33 hours = 0.39 hours (or less)

10 miles / 25.81 mph = 0.39 hours

Maximum input power - 0.938 hp * 746 watt/hp = 700 watt / 0.7 (motor losses) = 1000 watt

1000 watt * 0.387 hours = 387 Wh

...but we get to subtract the "savings" so the actual value is:

387 Wh - 50 Wh = 337 Wh

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Last edited by safe; 02-15-09 at 06:46 AM.
02-15-09, 01:32 PM
#52
safe
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Motor Input (Watts) vs Regen Speed

With each addition to the spreadsheets I'm able to get closer to a multidimensional analysis of this issue of whether regen makes sense or not in a "race scenario" involving ebikes.

Let me repeat the rules of this comparison:

Both ebikes go downhill on a 5% slope for 10 miles.
Both ebikes go uphill on a 5% slope for 10 miles.
No braking is ever required to complete the course.
It is assumed that both bikes MUST complete the 20 miles in the same elapsed time.
The default settings of the online calculator are used.

https://www.me.psu.edu/lamancusa/Prod...e/bikecalc.htm

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Here are the results.

If two ebikes can achieve 1000 watts of input (max) then the freewheel bike can use LESS energy than the regen bike to achieve the same elapsed time.

If two ebikes can achieve 600 watts of input (max) then they will perform about the same.

If two ebikes can achieve 250 watts of input (max) then the regen bike can use LESS energy than the freewheel bike to achieve the same elapsed time.

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Last edited by safe; 02-15-09 at 01:49 PM.
02-15-09, 02:32 PM
#53
unime

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Originally Posted by safe
If two ebikes can achieve 1000 watts of input (max) then the freewheel bike can use LESS energy than the regen bike to achieve the same elapsed time.

If two ebikes can achieve 600 watts of input (max) then they will perform about the same.

If two ebikes can achieve 250 watts of input (max) then the regen bike can use LESS energy than the freewheel bike to achieve the same elapsed time.
Your presentation does not make sense. If the bikes have a limit on motor input power and unlimited battery capacity, riders will just run full throttle all the way and any attempt at regeneration will slow the finish time. You are failing to communicate what exactly it is you are analyzing, and I can only assume you are still getting mired in details while missing the big picture.

Speaking of missing the big picture: Real ebikes are fundamentally battery limited, and it is when you consider battery limitations, not race imposed motor power restrictions that regeneration shines. You are off track with your analyses. If you want to get back on track, state the scenario you want to analyze - clearly and plainly - and let's talk about the conditions before wasting all those bits with graphs and spreadsheets that miss the mark. Frankly, I have no idea what your graph means, and I highly doubt anyone else is paying close attention.

Do you comprehend that we can't hope to agree on results if we don't agree that the calculations are meaningful? Posting additional spreadsheets and graphs is not helpful in the least.
02-15-09, 02:38 PM
#54
safe
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Let me repeat the rules of this comparison:

Both ebikes go downhill on a 5% slope for 10 miles.
Both ebikes go uphill on a 5% slope for 10 miles.
No braking is ever required to complete the course.
It is assumed that both bikes MUST complete the 20 miles in the same elapsed time.
The default settings of the online calculator are used.

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I'm simply not assuming that battery is the limiting feature.

If you want to compare regen verses freewheel based on the battery that would be an entirely different discussion.

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The Daytona 500 is just starting... they have some gasoline limits I'm sure, but the limit is high enough that it's not what determines the power limitations of the race.

When the race is ENTIRELY about the battery then it's not a "race" but an efficiency contest.

But I see your point...

.
02-15-09, 02:45 PM
#55
Abneycat
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I don't get the need for 3 pages of muddling, opinionated technical discussion from both sides.

If you're talking about a race, a variable speed system (such as an in-line) beats both hub motors.

Everyday people don't pick their motor based on mathemancy.
I'll pick mine because its quiet, she'll pick hers because its a good ride when the power is off, he'll pick his because it has lotsa torque.
02-15-09, 03:45 PM
#56
unime

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Originally Posted by safe
Let me repeat the rules of this comparison:

Both ebikes go downhill on a 5% slope for 10 miles.
Both ebikes go uphill on a 5% slope for 10 miles.
No braking is ever required to complete the course.
It is assumed that both bikes MUST complete the 20 miles in the same elapsed time.
The default settings of the online calculator are used.
And...? Are there power or energy restrictions? Are they the same for both riders? Uphill and downhill? Regeneration doesn't help in a race if the participants can pack enough battery to run the motor at its limit the whole way (for reasonable distances, eventually battery weight becomes a handicap).

If you want to compare regen verses freewheel based on the battery that would be an entirely different discussion.
Indeed! It would be a discussion that had a point. When you eliminate battery capacity as a consideration and limit motor power, regeneration doesn't help. We could have agreed to that three pages ago.
02-15-09, 04:27 PM
#57
safe
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https://www.hobbycity.com/hobbycity/...0mAh_6S1P_30C_

22.2v * 5 = 111 Wh

111 Wh * 20 = 2220 Wh

770g * 20 = 34 lbs

So if we want to do our course we could go:

2220 Wh / 400 Wh = 5.55 times

20 miles * 5.55 = 111 miles

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Just how long of a race did you have in mind?

My point is that you can buy the amount of battery to cover the distance... but in order to have an actual "race" you need to strictly limit the power input with some kind of "wattage restriction circuit".

Otherwise the whole idea of a "race" becomes rather ridiculous.... doesn't it?

Range is just a matter of \$\$\$...

-----------------------------------

Ebikes are not limited by weight of batteries.... it's more a matter of people wanting to not have to buy expensive batteries... (it would cost \$2400 to buy the LiPoly)

.

Last edited by safe; 02-15-09 at 04:40 PM.
02-15-09, 04:53 PM
#58
Ypedal
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111 Mile long race **********?

How about a 20 mile run.. !?
02-15-09, 04:57 PM
#59
safe
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In Praise Of Regen

Don't people realize what this data means?

I'm saying that if the motor is restricted to below 600 watts (or somewhere in that range) that regen actually beats the efficiency of a freewheel.

Even in a race situation!

So I'm not knocking regen completely... I'm saying that is has a certain place in the larger picture of things...

.
02-15-09, 05:39 PM
#60
unime

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Originally Posted by safe
I'm saying that if the motor is restricted to below 600 watts (or somewhere in that range) that regen actually beats the efficiency of a freewheel.
You can say it, but how about explaining what you mean? Applying full power both down hill and up obviously wins (DUH!) when motor power is limited but batteries are not. Using your "rules" (unlimited battery capacity), Freewheel always wins. For my part, I don't know any riders who have more battery capacity than they can use (up to 1600Wh - enough to actually ride 100 miles), so I think your rules are evidence of tunnel vision designed to get the result you wanted.

In any case, what you may have discovered is that 700 watts gets you going fast enough uphill on a 5% slope that you speed is close to the speed you coast down the same hill. By "close", I mean near enough that the difference in drag is not enough to make up the losses you assumed in regeneration. Change the slop, and you'll probably come up with a different threshold (though I can't be sure because you have not explained your reasoning fully).
02-16-09, 06:19 AM
#61
safe
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Originally Posted by unime
In any case, what you may have discovered is that 700 watts gets you going fast enough uphill on a 5% slope that you speed is close to the speed you coast down the same hill. By "close", I mean near enough that the difference in drag is not enough to make up the losses you assumed in regeneration. Change the slope, and you'll probably come up with a different threshold.
Yeah that's pretty much correct. Since roads are mandated to be of a slope that is around 5% or less (otherwise it cannot be used by ordinary cars) it's an adequate assumption to limit slopes to 5% for the sake of the math. Slopes above 5% are rare.

https://en.wikipedia.org/wiki/Interst...hway_standards

"Maximum grade. Maximum grade is determined by a table, with up to 6% allowed in mountainous areas and hilly urban areas."

...the steepest climb I've ever ridden up (on a bicycle) in my life is 16% and it went on for only about 100 meters.

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The idea of "comparing equals" means that the regen bike and the freewheel bike need to be able to achieve the same time at the finish so that we can compare the underlying value of the two techniques as far as the ability to retain energy. It's true that in a real race regen ALWAYS loses because the guy that holds the throttle on the most is the winner when the battery supply is adequate.

But I'm just trying to be "fair" here... we know that regen loses in the "absolute sense" to someone who seeks speed alone, but we want to give some credibility to the idea of regen being able to extend the range while still more or less keeping up with the freewheel bike. If the power of the ebikes is kept very low (below 600 watts it appears) then the freewheel bike is forced to use up more energy to hold onto the lead. Above 600 watts and the freewheel wins in just about every category. (because with that much power wind resistance happens everywhere)

What is often forgotten is that in a race it's sometimes of value to force your competition into doing something they don't like to do. Maybe it's taking away their preferred line... or as in this case... to force the regen bike to do things that it doesn't like doing. The freewheel rider is the "leader" so he's the one that sets the tempo of the race. The freewheel rider wants to force the regen rider to have to do things that are not ideal for them. This would be normal in a race situation. (you have to assume that people would behave exactly like this)

So the idea is to narrow and isolate exactly when regen can have some advantage and when it cannot so that someone might decide under what conditions it might be pursued.

The best argument for regen is just to SLOW DOWN and become a tourist. For tourist type riding regen performs better than a freewheel... there is no doubt about that.

.

Last edited by safe; 02-16-09 at 06:44 AM.

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