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# Comparing Regen to a Freewheel

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# Comparing Regen to a Freewheel

02-09-09, 09:49 AM
#26
safe
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It was made clear from the beginning that this was situation dependent.

The rules of the race will really be the thing that would determine if regen could compete or not. You basically need a situation where the battery pack is very small (and limited) and the motor size is unlimited to make regen win.

If the battery size is unlimited and the power is limited for the motor then the freewheel seems to have the upper hand.

Math can prove either side as the winner... it all depends on the rules...

.
02-09-09, 10:03 AM
#27
safe
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Let's review what happens with regen:

First the regen bike slows relative to the freewheel on a downhill.

Second the regen bike uses it's stored energy to attempt to catch the freewheel.

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

Whether or not the regen bike can catch up depends on:

Motor power limits (if any)

Battery size limits (if any)

...because the freewheel doesn't have to slow down if it doesn't need to.

.
02-09-09, 10:47 AM
#28
unime

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Originally Posted by safe
Our Math Agrees...
OK. Now I'm guilty of not reading your new scenario carefully and misunderstanding it.

My problem with you is that each time I debunk one of your scenarios, you come back with a different scenario, which also needs to be debunked, rather than understanding the simple case. Are you willing to look at my work and admit it has been corret? We can move on to arbitrarily complex race conditions, but only after we agree on the simple ones.

Regarding the scenario I was criticizing, you added a ridiculous and unreasonable handicap to Regen in order to get it to work out the way you believe it should. Dropping motor efficiency by 10% due to "heating" is nonsensical. First, temperature is not going to have such a dramatic impact on efficiency. Second, Freewheel will actually have worse efficiency due to lower speed (assuming we are still talking about hub motors). Third, we have been talking about running these motors at less than 300 watts - a very modest amount of power, not likely to induce high operating temperatures. The fact the Regen could finish anywhere close to Regen with 10% less efficiency should help convince you of the value of regeneration for improving race times.

The bottom line is that when regeneration is readily available (as on a direct drive hub motor), there will always be times when it is useful: For capturing energy while braking; and for trading a few mph at high speed (with high drag penalties) for gains at low speed (where drag is far lower and the speed gained has a much greater effect on race time). There isn't really a need to work through examples to understand this - the examples will merely show how significant the effect will be in differing circumstances.

Again, I do not ride with regeneration. The reason I don't use it is because I ride a very efficient, geared, chain driven ebike. My extra efficiency more than makes up for the energy recovery available on a direct drive hub motor, especially in the hills (where recovery would be most useful). That is, of course, until my battery runs out.
02-09-09, 10:57 AM
#29
safe
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Okay... so we are in agreement on the math...

The first scenario we were using set the uphill climb rate at a very low level for the freewheel at only 7 mph.

This original 7 mph value was too "lazy" on the part of the freewheel bike.

This gave the regen system plenty of time to catch up. In a real racing situation there would be some kind of limitation on the motor either by controlling the input or restricting what motors could be run. This just has to be the case or we are not talking about ebikes anymore and instead are talking about emotorcycles. So in this scenario we use a power limit of 1000 watts input. On the freewheel side we "reverse calculate" to figure how fast up the hill the freewheel can go and still hold the lead.

I also modified the pack size, but it seems to not matter much...

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

What if we vary the battery size?

Freewheel pack is set at 30 lbs (so change the online calc to read 150 total lbs)

Regen pack is set at 15 lbs (so change the online calc to read 135 total lbs)

The freewheel pack has 800 Wh.

The regen pack has 400 Wh.

Now we do the calculations again:

Downhill (5% negative slope for 10 miles)

Freewheel - 10 miles / 26.8 mph = 0.37 hours (net power gain/loss is zero)

Regen - 10 miles / 15.00 mph = 0.67 hours (behind by 0.30 hours)

Recaptured Energy = 0.156 hp * 746 watt/hp = 116.4 watt

116.4 watt * 0.67 hours = 78 Wh * 0.7 (motor losses) = 54.6 Wh

54.6 Wh * 0.9 (battery losses) = 49 Wh (this is the energy recovered)

Uphill (5% positive slope for 10 miles)

Freewheel - 10 miles / 13.5 mph = 0.74 hours

Power Needed - 0.476 hp * 746 watt/hp = 355 watt / 0.7 (motor losses) = 507 watt

507 watt * 0.74 hours = 376 Wh

Regen - Must do 10 miles in 0.74 hours - 0.30 hours = 0.44 hours (or less)

10 miles / 22.94 mph = 0.44 hours

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

1000 watt * 0.44 hours = 440 Wh

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

440 Wh - 49 Wh = 391 Wh

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

Just use your eyeballs and logic to verify these results are correct. (I've gone over them a couple times, but human error is always possible)

.

Last edited by safe; 02-09-09 at 11:44 AM.
02-09-09, 11:07 AM
#30
safe
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Starving ("dropping") the Regen bike

What should become obvious about this last example is that when power input is limited the freewheel bike can in effect "starve" the regen bike by using enough power to force the regen bike to fall behind. (in effect you are forcing the regen bike to become "dropped" since it can't catch up)

The regen bike uses it's maximum power (1000 watts input) up the hill.

The freewheel bike uses just 507 watts input to make it up the hill. (480 watts in the example below)

...in this scenario the freewheel "wins" because it never gets lazy.

The "Hare" in the story only loses because it gets so lazy as to allow the "Tortoise" to catch up and pass while it was sleeping...

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

Back To The Original

This goes back to the default values. The freewheel is allowed to go just fast enough to "starve" the regen bike that is limited to 1000 watts. The default weight is used on this. (I thought this might make it easier because we've done these numbers already)

Downhill (5% negative slope for 10 miles)

Freewheel - 10 miles / 24.35 mph = 0.41 hours (net power gain/loss is zero)

Regen - 10 miles / 15.00 mph = 0.67 hours (behind by 0.26 hours)

Recaptured Energy = 0.133 hp * 746 watt = 99.22 watt

99.22 watt * 0.67 hours = 66.48 Wh * 0.7 (motor losses) = 46.5 Wh

46.5 Wh * 0.9 (battery losses) = 42 Wh (this is the energy recovered)

Uphill (5% positive slope for 10 miles)

Freewheel - 10 miles / 14.7 mph = 0.68 hours

Power Needed - 0.451 hp * 746 watt/hp = 336 watt / 0.7 (motor losses) = 480 watt

480 watt * 0.68 hours = 326 Wh

Regen - Must do 10 miles in 0.68 hours - 0.26 hours = 0.42 hours (or less)

10 miles / 23.89 mph = 0.42 hours

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

1000 watt * 0.42 hours = 420 Wh

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

420 Wh - 42 Wh = 378 Wh

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

The "smart" freewheel rider goes fast enough to never let the regen bike back into the race...

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

As long as the (Hare) freewheel bike doesn't get "lazy" the (Tortoise) regen never gets a chance to catch up.

.

Last edited by safe; 02-09-09 at 11:51 AM.
02-10-09, 12:00 AM
#31
unime

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Originally Posted by safe
In a real racing situation there would be some kind of limitation on the motor either by controlling the input or restricting what motors could be run. This just has to be the case or we are not talking about ebikes anymore and instead are talking about emotorcycles.
There you go, making assumptions again - motor limitations are not necessary. If I were running a race, I'd limit battery capacity, and let competitors run whatever motor and controller they care to.

Do you realize that the only reason you are able to show Freewheel winning is that you are choosing bad strategies for Regen? If you want to see Regen's advantage, then set some race conditions and tell me how Freewheel rides the race. Don't pick Regen's strategy, because you set him up for failure. Let me show you his strategy. If Regen can accumulate energy on a downhill and use it on a climb (even flat) regen will win. But let Regen use the same battery pack, you really don't need another variable at this time.
02-10-09, 07:33 AM
#32
safe
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It seems you have recognized my point, whether you fully agree with it or not.

The rules of the race will determine if regen helps or hurts. (or does no good)

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

Just some historical information on race rules... in the electric car drag racing they set the rules based on voltage. So you would race in a class based on what voltage you are running at and you can use whatever battery you want as long as the voltage was appropriate. In some of the electric bike races they have tried battery restrictions, but that's been difficult because it's hard to get similiar (fair) packs for all the racers who might be using differing chemistries.

The lastest idea (the ePower Challenge at Portland International Raceway) is to use a wattage restriction circuit and then allow whatever battery people want to use. I know this from blogs where the person running it was searching for people to design the circuit for him. (I have no idea on the progress of this)

Since we are talking about "ebikes" the future really needs to involve some kind of power limit because this will reflect the laws about ebikes. The Federal Law for ebikes "recommends" that all American bikes are 750 watts of output. The states can override this if they so choose and in some states (like my own) they allow up to 3hp.

In the end, if the laws go and favor the power restriction and drop the battery restriction then regen is essentially of no value because people will load up with as many batteries as they need and just "race" to the finish.

I like the idea of a "race" being flat out anyway... there's just something really lame about people slowing down on purpose just to refill their battery. (it definitely makes such racing boring to ride and boring to watch) We like the danger and the speed of the downhills and the freewheel makes that more exciting.

So it all comes down to the rules...
02-10-09, 12:25 PM
#33
cerewa

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It seems that y'all have realized where the disagreement comes from and are mostly agreed on the following points:

In situations where motor wattage is limited and battery watt-hour capacity is not limited, battery regen is likely to be of no use; changing your design to accomodate regenerative braking is likely to hamper your other design goals.

In situations where battery watt-hour capacity is limited and motor wattage is not limited, a poorly designed regenerative-braking system is probably not useful but a well-designed regenerative-braking system (for example, one where the regen motor is not connected except during braking) is probably useful.

Factors that might have a bearing on the usefulness of regen are:
-whether you're emphasizing maximum speed, or maximum range
-whether you're in a race (and what kind), or using the bike for transportation (and what kind), or neither
-to what extent low cost and simplicity/reliability are important
02-10-09, 05:12 PM
#34
safe
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I think you pretty well summed it up.

The act of using regen means that it slows you down for part of your ride so that you can then make that time back and (with good efficiency) then gain a lead.

Regen makes the most sense when the battery is the limiting factor.

If power is limited then the freewheel rider can conserve just enough energy to maintain his lead while the regen bike tries to catch up (at full power) but without success.

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

In the long run I'm "guessing" the rules are going to favor power limiting because it's an easier definition of "What is an electric bicycle?"

Answer: Roughly 750 watts of power output in the US.

However, I know for a fact that races are being held now that use strict battery specs, so in those contests the regen makes sense. (there are no laws or standards for ebikes that apply to batteries that I'm aware of)

Any contest that allows only a very weak battery compared to the race distance will favor the regen over the freewheel.
02-11-09, 07:16 AM
#35
Mabman
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Where are these races? And when?
02-11-09, 09:47 AM
#36
safe
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Originally Posted by Mabman
Where are these races? And when?
https://flickr.com/photos/humanpowerc...7605508079377/

https://ohpv.org/HPC/RacingNews.html

https://ohpv.org/HPC/RacingSchedule.html

https://ohpv.org/HPC/ePowerEvents.html

"I have followed the forums regarding the topic of e-assist racing and have observed the variety of suggestions put forth by the community, and have settled on a “starting point” on which to build from. The ePower Challenge (tentative name), will be a venue built upon the premise that these types of vehicles are “power assist” bicycles, not electric motorcycles. I understand and appreciate the folks that want to develop the ultimate power bikes (that’s cool, it’s all good), but at this venue we are going to be advocates of showing off to the public the viability and forward thinking aspects of how personally and socially empowering these types of vehicles are, and thus will be setting limits on how much “assist” you can derive from your electrical beastie. Of course, the surest way of keeping everyone on an even playing field would be to hook up a watt/hour meter on every bike and limit the usage to a certain amount for a certain distance of race. You would then get to figure out your own “consumption” formula (volts, amps drawn, etc), but until the race gets lots’o extra \$’s to outfit each racer with such a device, we’ll need to have some rough justice applied."
02-11-09, 09:54 AM
#37
safe
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Bigger Motor Does NOT Help...

It's good to go back sometimes and make sure you didn't draw a conclusion that was actually false. As it turns out the larger the motor allowed (more power) the WORSE things get for regen because it means that if the freewheel bike also has an equally powerful motor then it can zip up the hill with equal ease.

In this scenario I'm using a whopping FIVE HORSEPOWER (5000 watt maximum input) motor on both bikes, otherwise it's the default settings for the online calculator:

Downhill (5% negative slope for 10 miles)

Freewheel - 10 miles / 24.35 mph = 0.41 hours (net power gain/loss is zero)

Regen - 10 miles / 15.00 mph = 0.67 hours (behind by 0.26 hours)

Recaptured Energy = 0.133 hp * 746 watt = 99.22 watt

99.22 watt * 0.67 hours = 66.48 Wh * 0.7 (motor losses) = 46.5 Wh

46.5 Wh * 0.9 (battery losses) = 42 Wh (this is the energy recovered)

Uphill (5% positive slope for 10 miles)

Freewheel - 10 miles / 22.2 mph = 0.45 hours

Power Needed - 0.830 hp * 746 watt/hp = 620 watt / 0.7 (motor losses) = 885 watt

885 watt * 0.45 hours = 398 Wh

Regen - Must do 10 miles in 0.45 hours - 0.26 hours = 0.19 hours (or less)

10 miles / 51.87 mph = 0.19 hours

Maximum input power - 4.692 hp * 746 watt/hp = 3500 watt / 0.7 (motor losses) = 5000 watt

5000 watt * 0.19 hours = 964 Wh

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

964 Wh - 42 Wh = 922 Wh

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

So what happens is that the more power there is to draw upon the more you have to deal with wind resistance on the UPHILL side. So the more power you have the less value there is in there being hills because the uphills are ridden like it was flat.

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

The case where regen seems to work is when the motor is of very low power and the battery size is very small. Then the mountains become larger from the ebikes perspective. The bigger the motor and bigger the battery the more you flatten out the mountains and then regen makes less sense.

It's somewhat counter intuitive... but that's what the math points to...

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

Also, it's good to remind ourselves that for tourists (who are not in a big hurry) that regen does in fact make their tour easier. It's going to slow down the tour, but if the ride is on a beautiful road then you can just look off into the horizon and enjoy the view. For racing regen doesn't help much, but for the tourist it's still a great choice.

.

Last edited by safe; 02-11-09 at 11:44 AM.
02-11-09, 10:12 AM
#38
cerewa

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Wow. Batteries that can run 5hp for any useful length of time = \$\$\$.

Safe, one of the assumtions you seem to hold is that we are all thinking of e-bike racing in our comparisons.

From what I have seen, almost everybody in this e-bike forum is interested in non-racing ebikes. They're also interested in e-bike usage patterns that give a good combination of:

-low battery price / low overall price
-good range
-low battery weight

trying to meet all three of those conditions makes regen sound like a good idea to a lot of people, though today's commercially-available systems with regen still leave a lot to be desired.
02-11-09, 10:17 AM
#39
safe
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For the casual tourist you are absolutely right that regen can make life easier and extend the usefulness of the battery.

In the end that thesis/equation I presented earlier seems to be correct:

Regen (gained) = Time (lost)

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

If you are not being "goal oriented" and having a desire to win any kind of race or push yourself to any kind of "personal best" in your ride then regen is good for you.

But for racing situations and sport riders it's just not of much value.

.
02-11-09, 03:23 PM
#40
cerewa

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For the casual tourist you are absolutely right that regen can make life easier and extend the usefulness of the battery.

In the end that thesis/equation I presented earlier seems to be correct:

Regen (gained) = Time (lost)
Well, if it extends your range, you lose time at one point and gain time (when you continue riding with electric assist because your battery hasn't run out) at another point. In that situation, Regen (gained) => Time (gained).

Since the amount of energy lost to air resistance goes up with the square of speed (if you double your speed, you use up 4 times as many Watt-hours overcoming air resistance in covering the same distance), using an electric motor with regen to hold speed nearly constant on uphills and downhills could increase your average speed if batteries are your limiting factor (battery cost, battery weight, battery watthour capacity).
02-11-09, 08:00 PM
#41
safe
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I guess it's a two part equation really:

Regen (gained) = Time (lost) : So you lose time in the act of saving energy.

Regen (spent) = Time (recaptured) : You then make back time with the saved energy.

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

When you combine the Time (lost) and the Time (recaptured) the result is usually about the same as if you used a freewheel. Energy lost to wind resistance is about the same as energy lost in coverting electrical energy to a battery and back... both systems come about nearly equal.

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

With regen you just need to forget about time... the goal of regen is not to win any races, but to get yourself somewhere with the least effort. The slower you can go the less effort it takes to get somewhere. This is true with or without regen.

Regen used sparingly (like just in "authentic" cases when you need to use the brakes) makes the most sense and taking to the next level like with KERS (F1) would be the way to improve on the system for racing purposes.

Regen for touring is fine...

.

Last edited by safe; 02-11-09 at 08:05 PM.
02-12-09, 06:46 AM
#42
safe
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600 Watt Break Even

What I've done is just taken the exact same scenario (repeated again and again above) and inserted those values into a spreadsheet and plotted the results. What we see is that in the scenario described before (15 mph regen down a 5% hill) that motors of a size of 600 watts or less will allow the regen bike to consume less energy to achieve the same overall time. Motors of 600 watts or more (on both freewheel and regen ebikes) will tend to allow the freewheel bike to consume less energy.

So we can see that with the 750 watt limit on motors that regen and racing makes no sense.

Attached Images

Last edited by safe; 02-12-09 at 06:53 AM.
02-12-09, 09:45 AM
#43
Ypedal
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Still and spreading useless confusion i see....
02-12-09, 07:53 PM
#44
safe
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We've gone through the math pretty carefully.

The facts seem to show that there is a very limited area in which regen can show any benefits. In an actual race situation where the motor is of "ebike proportions" (typically around 750 watts) regen does not work so well.

I know you want to make Justin your hero over at Endless-Sphere but the facts just don't seem to favor the idea of regen for anything but touring.

Regen is of uncertain value for people in any kind of competitive contest.

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

There was a guy posting earlier that knew his math and he eventually seems to have come to accept the numbers as they are.

I welcome anyone to face the facts, crunch the numbers, and find ways for regen to win a race.

(we've identified the case of "limited battery + limited motor" as the only case so far)
02-12-09, 11:33 PM
#45
JeanCoutu
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Originally Posted by safe
There was a guy posting earlier that knew his math and he eventually seems to have come to accept the numbers as they are.

Safe, I wish I didn't have to be so blunt, but does the word “clueless” mean anything to you? I'm asking because it appears you're reading, but not comprehending. The way you keep insistingly posting in spite of this is aggravating to people who would wish to discuss or read about the topic, because in doing so you're ruining the thread. I believe this is the principal reason the guy posting earlier has apparently lost interest in this discussion.

Last edited by JeanCoutu; 02-12-09 at 11:41 PM.
02-13-09, 10:10 AM
#46
Mabman
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Thanks for the links Safe. I had searched awhile back for any events like this and came up empty. Maybe we'll see you there?
02-13-09, 11:28 AM
#47
safe
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I'd love to go to that race at Portland International Raceway. California is my home state (third generation) but I'm living in Missouri these days. (got out before the housing bubble burst) The guy who is managing those races seems to have a firm grasp of where ebike racing needs to go. I'm very much in support of their activities.

The most logical way to limit ebike racing is with a "wattage limiting circuit" which would allow ebikes of any voltage to compete against each other. The circuit would multiply voltage by current to arrive at the limiting value. However, such a circuit is (to my knowledge) not in existence in such a way as to ensure that no one cheats.

Assuming that wattage limiting is the future of ebike racing then the use of regen becomes a real problem because you are always in this situation of falling behind while doing regen and then needing to supply extra power to catch up. If the freewheel bike is configured with an adequate battery at the start of the race and the motor wattage limit is 1000 watts then it essentially phases out regen as a potential option. (at least if winning is your goal)

However... there are cases where regen applies, such as touring, where best time does not matter. That's more or less the point of this thread, that regen is a time dependent activity... the more you use the regen the more you slow down.
02-13-09, 01:09 PM
#48
Mabman
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I personally am more interested in energy consumption. Over a given course, say 40 miles, you get there in an hour and use xx+ energy, but another competitor gets there in two hours and is compliant with federal regs and uses -x energy, that is the one I would call the winner.

I know it is not going that way with this series but real world use of motor assist bicycles in as many ways as possible is my goal. Getting them to be the best that they can be within the current regs at as low a cost as possible is what needs to happen, and yesterday.
02-13-09, 04:11 PM
#49
safe
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When speeds go up, efficiency goes down.

Regen seems to occupy a space where the overall time to complete a distance is very large and the speeds are very low.

At the root of everything is wind resistance... rolling resistance is a constant, but wind resistance increases rapidly with increasing speed. If wind resistance was zero then regen would not show any benefits.

The core idea of regen is to be "more efficient" than wind resistance.

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

Sometimes you just can't coast at full speed anyway... sometimes you actually need to use the brakes... so the "ideal" regen system would be based on the idea of recapturing the "true waste" of braking.

One only truly wastes energy when one uses the brakes.

(that's kind of the point of the comparison of regen to freewheel... when wind resistance is the only criteria the difference between wind resistance losses verses electrical losses is not that much)
02-13-09, 07:26 PM
#50
unime

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Originally Posted by safe
So we can see that with the 750 watt limit on motors that regen and racing makes no sense.
WRONG!

What you have shown is that Regen loses when he chooses a poor strategy. If I were riding against Freewheel under the proposed conditions, I would win. Maybe you can figure out why.

There are certainly conditions in which Freewheel wins, but your approach is haphazard and wrongheaded, providing you with little understanding of the big picture.