Regenerative braking

Electrically assisted bikes, trikes, etc.
Garry Booth
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Regenerative braking

Postby Garry Booth » 7 Aug 2020, 11:28am

Is there a simple reason that e-bikes don't have regenerative braking? Possibly cost, complexity or even weight?

thirdcrank
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Re: Regenerative braking

Postby thirdcrank » 7 Aug 2020, 11:30am

Completely off the top of my head, a system similar to that used in motor vehicles would need fixed wheel. Perhaps you could improvise with a very draggy dynamo?

Jdsk
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Re: Regenerative braking

Postby Jdsk » 7 Aug 2020, 12:42pm

Some do but most don't.

There is some cost and complexity, but both are much less than they were.

But the energetics of bikes and motor vehicles are completely different. Apparently it's something to with the relative power used to accelerate, to climb and to push air molecules around.

Jonathan

Vorpal
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Re: Regenerative braking

Postby Vorpal » 7 Aug 2020, 1:08pm

This has been discussed before viewtopic.php?f=55&t=129840

It is feasible, and a couple of companies have commercialised solutions, but IMO, it's marginal in engineering terms. As technology improves, it should become more effective.
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kwackers
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Re: Regenerative braking

Postby kwackers » 7 Aug 2020, 3:10pm

Bikes are simply too light for it to add any worthwhile amount of energy back.

All it really amounts to is a slightly more efficient way of braking, but like brakes you'd be better off looking ahead and not using it.
That's even true on electric cars. The best efficiency comes from simply slowing down using aerodynamics because you'll be looking that much further ahead.

richtea99
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Re: Regenerative braking

Postby richtea99 » 7 Aug 2020, 7:46pm

It's not just a light bicycle being braked - the real weight being decelerated is the bike + rider.
In my (weighty) case, that's quite a lot of kinetic energy bombing along.

Surely the problem is that the kinetic -> electric energy conversion is either inefficient, or the cost to make it efficient is just too great.

Cyril Haearn
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Re: Regenerative braking

Postby Cyril Haearn » 7 Aug 2020, 7:50pm

Flywheels are good for storing energy, but maybe impractical, too heavy, for cycle use
'Dangerous' too maybe
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Jdsk
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Re: Regenerative braking

Postby Jdsk » 7 Aug 2020, 8:03pm

richtea99 wrote:It's not just a light bicycle being braked - the real weight being decelerated is the bike + rider.
In my (weighty) case, that's quite a lot of kinetic energy bombing along.

Surely the problem is that the kinetic -> electric energy conversion is either inefficient, or the cost to make it efficient is just too great.

You can only save (some of) the energy that would otherwise be converted into heat when braking. And that isn't much for many modes of cycling.

Some calculations:
https://www.pandaebikes.com/regen-braking-ebikes-worth/

Shirley

kwackers
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Re: Regenerative braking

Postby kwackers » 8 Aug 2020, 6:20pm

richtea99 wrote:It's not just a light bicycle being braked - the real weight being decelerated is the bike + rider.
In my (weighty) case, that's quite a lot of kinetic energy bombing along.

Surely the problem is that the kinetic -> electric energy conversion is either inefficient, or the cost to make it efficient is just too great.

The problem is that if you're riding sensibly then you look ahead and don't use the brakes (or the regen).
That's the most efficient way to ride bar none - even if regen was 100% efficient which it isn't (not even close).

If you insist on braking like mad to get a few extra feet then even on a normal bike once you stop pedalling you soon lose all your speed. Air resistance steals it all from you long before you'd manage to get any of that kinetic energy back into the battery.

As I said it's also true of electric cars and they are considerably heavier.
Rolling to a stop saves far more energy than braking using regen mainly because you come off the power much earlier, thus your average speed drops and so does your wind resistance. Lower average speed equals less expended energy.
Where electric cars score on regen is coming back down hills although they've got significantly more potential energy at the top than a bicycle. (It's also handy because you don't need to faff around with 'low' gears or dabbing the brakes worrying about how hot they're getting).

Jdsk
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Re: Regenerative braking

Postby Jdsk » 8 Aug 2020, 6:55pm

kwackers wrote:Where electric cars score on regen is coming back down hills although they've got significantly more potential energy at the top than a bicycle.

But that potential energy is only in proportion to their mass. And *so is the energy to accelerate them or to go uphill. So that cancels out... the difference from bikes is in the aerodynamic energy.

Jonathan

* As long as inertial mass = gravitational mass... ; - )

kwackers
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Re: Regenerative braking

Postby kwackers » 8 Aug 2020, 8:20pm

Jdsk wrote:
kwackers wrote:Where electric cars score on regen is coming back down hills although they've got significantly more potential energy at the top than a bicycle.

But that potential energy is only in proportion to their mass. And *so is the energy to accelerate them or to go uphill. So that cancels out... the difference from bikes is in the aerodynamic energy.

Jonathan

I guess I should have qualified it by pointing out aerodynamic losses as a ratio of the potential energy are massively smaller for the car...

* As long as inertial mass = gravitational mass... ; - )

I reckon there's a Nobel prize up for grabs there... ;)

Jdsk
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Re: Regenerative braking

Postby Jdsk » 8 Aug 2020, 8:25pm

Sure is.

: - )

Jonathan

richtea99
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Re: Regenerative braking

Postby richtea99 » 8 Aug 2020, 10:57pm

Jdsk wrote:the difference from bikes is in the aerodynamic energy.


Ah, agreed, the aerodynamic loss on a bike is far greater proportionally than a smooth rounded car.
I stand corrected. :)

Lodge
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Location: Staffordshire Moorlands

Re: Regenerative braking

Postby Lodge » 9 Aug 2020, 7:45pm

I must admit I would like regenerative braking on our electric assist tandem. If as effective as those on our car they would act as a useful drag brake down the long and frequent descents we experience in the nearby Peak District where we ride most of the time.

We've already had to upgrade to ventilated discs & superior calipers to overcome brake failure with the original equipment (boiling brake fluid!).

nez
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Re: Regenerative braking

Postby nez » 11 Aug 2020, 10:13pm

Mrs Nez has a Koga with with an Ion motor in the rear wheel. It does recover some charge when you're not pedalling - going downhill mostly. Frankly it's not much and not really worthwhile unless you were cycling up and down repeated hills in Derbyshire or Yorkshire or something. I don't think regenerative braking would be worth the weight of the mechanical parts and inconvenience. So all round a bit negative!