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Regenerative brakes?

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Old 08-12-18, 06:47 PM
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3speeds2slow
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Regenerative brakes?

Has anyone ever heard of a regenerative braking system being used on bicycles? I was thinking along the lines of a fly wheel capturing the energy for storage and subsequent use.
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Old 08-12-18, 09:20 PM
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Probably only practical on an e-bike.

It is what the stand-on self balancing single wheel things do as their only means of braking. Which raises an interesting question of what happens if you start at the top of a hill with the battery already at the charge safety limit. Having taken the controllers apart, there are no braking resistors, only the battery to absorb energy.
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Old 08-12-18, 09:24 PM
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These keep coming up,..

Yes, they exist already. It's called a dynamo. Any time the hub is spinning, it is generating power.
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Old 08-12-18, 10:35 PM
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Interesting thread!

I like the idea of recouping energy lost from braking but I'm having trouble with the scale.

So help me out here:

A bike and rider weighs 200 pounds
A car and rider weighs 3170 pounds

Each are coasting at the same speed and must come to a complete stop.

It seems obvious that the heavier object would generate more energy stopping than the lighter object. Conversely the heavier object requires more energy to achieve the same speed than the lighter object... So is it comparable?

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Old 08-12-18, 10:42 PM
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I put a little bit of retinoic acid on my worn brake pads, and they grow the pads back.
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Old 08-12-18, 10:44 PM
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Regenerative brakes are already a thing on higher end E-bikes, using the motor(s) as a generator under moderate braking to recharge the onboard batteries.

The University of Michigan developed a hydraulic regenerative braking system for use on bicycles:

https://archive.epa.gov/otaq/technol...earch-hhb.html
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Old 08-12-18, 10:46 PM
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some of the nicer e-motor cycles to
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Old 08-13-18, 05:29 AM
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Originally Posted by 3speeds2slow View Post
Has anyone ever heard of a regenerative braking system being used on bicycles? I was thinking along the lines of a fly wheel capturing the energy for storage and subsequent use.
No way to do it without making a bike MUCH heavier. Slim version still will need efficient mechanical brakes.
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Old 08-13-18, 06:11 AM
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3speeds2slow
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Originally Posted by UniChris View Post
Probably only practical on an e-bike.

It is what the stand-on self balancing single wheel things do as their only means of braking. Which raises an interesting question of what happens if you start at the top of a hill with the battery already at the charge safety limit. Having taken the controllers apart, there are no braking resistors, only the battery to absorb energy.
I believe e bikes would use electronic devices and not flywheels.
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Old 08-13-18, 06:17 AM
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Originally Posted by DanBraden View Post
Interesting thread!

I like the idea of recouping energy lost from braking but I'm having trouble with the scale.

So help me out here:

A bike and rider weighs 200 pounds
A car and rider weighs 3170 pounds

Each are coasting at the same speed and must come to a complete stop.

It seems obvious that the heavier object would generate more energy stopping than the lighter object. Conversely the heavier object requires more energy to achieve the same speed than the lighter object... So is it comparable?

/no fighting, just illuminating!
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In some ways yes but in others no. Since the “engine” of a bicycle is the human physical exertion, it is even more important to not “waste” it. By recapturing the energy during braking that otherwise lost energy is reclaimed. Which means a bicyclist could ride farther for the same energy exertion.
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Old 08-13-18, 06:20 AM
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3speeds2slow
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Originally Posted by wgscott View Post
I put a little bit of retinoic acid on my worn brake pads, and they grow the pads back.
How’s that working out for you?
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Old 08-13-18, 06:23 AM
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Originally Posted by chelvel View Post
No way to do it without making a bike MUCH heavier. Slim version still will need efficient mechanical brakes.
But if the increased weigh is compensated for by the increased efficiency of energy usage, this would be a case where an increase in weigh is useful.
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Old 08-13-18, 06:37 AM
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Originally Posted by 3speeds2slow View Post

But if the increased weigh is compensated for by the increased efficiency of energy usage, this would be a case where an increase in weigh is useful.
TANSTAAFL.

There Ain't No Such Thing As A Free Lunch.

The weight costs you, and since humans are only 25% efficient, going uphill and then capturing the energy is only going to be 25% efficient.

It takes at least 4KCal, or "Food Calorie" to put about 1 watt on the road. It is that 1 watt that you might be able to recapture, but it represented 4KCals at the motor. (Normally 1KCal is about 1 Watt)
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Old 08-13-18, 06:47 AM
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Originally Posted by 3speeds2slow View Post
Since the “engine” of a bicycle is the human physical exertion, it is even more important to not “waste” it. By recapturing the energy during braking that otherwise lost energy is reclaimed. Which means a bicyclist could ride farther for the same energy exertion.
But there is no way to put significant power back into the human body from a regenerative brake system I suppose the thing could be wired to deliver electric shocks to the rider if s/he slowed down .....
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Old 08-13-18, 07:22 AM
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Originally Posted by Maelochs View Post
But there is no way to put significant power back into the human body from a regenerative brake system I suppose the thing could be wired to deliver electric shocks to the rider if s/he slowed down .....
Better use more lube.... that probe can't just go anywhere!
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Old 08-13-18, 07:22 AM
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If it made any sense, it would have been done long ago.

I seriously doubt that the weight penalty would outweigh the benefits, on anything but ebikes
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Old 08-13-18, 08:52 AM
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Originally Posted by rgconner View Post
The weight costs you, and since humans are only 25% efficient, going uphill and then capturing the energy is only going to be 25% efficient.
You would of course capture the energy on the downhills and at traffic lights, energy otherwise dissipated as heat in the brakes. So that part is free.

In terms of the weight of the system, while something like an ultra small 100 watt "hidden cheating motor" probably couldn't absorb all of the braking of a steep descent, it's also not going to weigh that much. Unfortunately, unless the bike is fixed gear, putting it anywhere upstream of the hub won't work for braking. So now you're back to talking about a hub motor, and those typically do weigh a bit.

Originally Posted by 3speeds2slow View Post
I believe e bikes would use electronic devices and not flywheels.
It's unlikely any sort of bike would use flywheels for this. They're mechanically complex, and if you want them to be light but still store useful re-acceleration energy across a traffic light duration, they are going to be exotic to the point where failure is explosive.
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Old 08-13-18, 09:18 AM
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Formula E race cars have the option of both regen and friction braking during the races..

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Old 08-13-18, 09:22 AM
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Originally Posted by jefnvk View Post
I seriously doubt that the weight penalty would outweigh the benefits, on anything but ebikes
Even on eBikes I wonder if it's just about marginal gains. Any idea on that? There's probably no weight penalty for recapturing the braking energy, but is it really enough to add very many minutes to the battery life?
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Old 08-13-18, 10:09 AM
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Originally Posted by JonathanGennick View Post
Even on eBikes I wonder if it's just about marginal gains. Any idea on that? There's probably no weight penalty for recapturing the braking energy, but is it really enough to add very many minutes to the battery life?
No clue on how useful they would be, I'm sure there is an electrical engineer around here that is better suited to answer that! My remark was more in regards to the fact that ebikes already have the major heavy components needed to do regen braking (in this case, electric motor), and any weight added if a circuit change was needed for charging would be minimal in the overall scheme of the ebike.
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Old 08-13-18, 10:47 AM
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Originally Posted by JonathanGennick View Post
Even on eBikes I wonder if it's just about marginal gains. Any idea on that? There's probably no weight penalty for recapturing the braking energy, but is it really enough to add very many minutes to the battery life?
It's directly proportional to how often you use your brakes. Similar to how a Toyota Prius gets better fuel economy in city driving than highway, because city driving involves lower speeds (less drag), and it can recoup most of the lost energy from stop/go traffic with its regenerative braking. Regular cars just dissipate the energy needed to slow the car down as heat.

Similarly, if you ride a completely flat route where you rarely or never use the brakes to slow down, you won't gain anything from regenerative braking. If you're in a very hilly area or you're in stop/go traffic, you will get a significant return from regen braking.
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Old 08-13-18, 11:06 AM
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Hub motor may be able to offer this, (motor-generator switching)

but for any significant hill climbing torque, a mid drive is where the action is..
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Old 08-13-18, 11:46 AM
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I briefly looked a bit into this when Porsche put KERS into the GT3R and quickly became disappointed.

As Geoff said above, look at how often you actually use the brakes on your bike, it probably isn't much. Even on moderate downhills, you usually just maintain speed for the inevitable uphill that follows probably 95% of the time. Sometimes you have to stop at the bottom of a hill and that's no fun, I admit.

If you figure that you want something like enough energy for 50' of climbing for something like 200 lbs of rider and bike, you need around 13.5kJ stored equivalent to that same rider going 38 mph. Figure that if you have to store that, and something goes wrong things are going to hurt a lot... Imagine being hit by a cyclist moving at near 40 miles per hour...

If you want to store that energy in a flywheel, it will need to really be moving if you want to keep the weight somewhat reasonable. Probably going to need to keep it a vacuum to prevent unnecessary losses, the bearings need to be very good look out for any little bumps to damage things... things get complicated very quickly.

Batteries would probably be more reasonable and tend to fail less spectacularly (though still quite spectacularly) but you run into the other issue of you probably don't actually use the brakes that much. Most of the work at any reasonable speed to to push air out of the way and other general losses. when you aren't pushing air out of the way, you aren't going very fast and don't have much energy to store anyway.

You are probably just better off with a lighter bike if you do a lot of stops and starts...
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Old 08-13-18, 01:58 PM
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Mista Sparkle:

If you had a bike equipped with regenerative braking, you could intentionally apply the brake moderately while going down a hill that you might otherwise coast down at top speed, in order to store a lot of the energy that is otherwise being wasted on overcoming drag at higher speed. Instead of going down a steep hill at, say, 40mph, if you use your regenerative brakes to maintain 20, you're storing a massive amount of energy for nothing lost except a bit of time, which is then made up for by using that energy to assist in the next hill climb.

It would make for a brilliant auto-brake feature on an e-bike, to be able to set your maximum speed at say, 20mph, and anything past that causes the regen brakes to kick in and keep you at 20 down hills, topping up the battery for the next climb automatically.
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Old 08-13-18, 08:08 PM
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Originally Posted by General Geoff View Post
Mista Sparkle:

If you had a bike equipped with regenerative braking, you could intentionally apply the brake moderately while going down a hill that you might otherwise coast down at top speed, in order to store a lot of the energy that is otherwise being wasted on overcoming drag at higher speed. Instead of going down a steep hill at, say, 40mph, if you use your regenerative brakes to maintain 20, you're storing a massive amount of energy for nothing lost except a bit of time, which is then made up for by using that energy to assist in the next hill climb.

It would make for a brilliant auto-brake feature on an e-bike, to be able to set your maximum speed at say, 20mph, and anything past that causes the regen brakes to kick in and keep you at 20 down hills, topping up the battery for the next climb automatically.
True, I can't see myself trying to go slow down a hill, but I don't have a lot of them around and they are all short and straight...

Trying to store any reasonable amount of energy in a flywheel though would be generally impractical on a bike, it either would have to be reasonably heavy(and therefore counterproductive) and/or go terrifyingly fast. Batteries are by far the better route as they should be able to handle the relatively low charge/discharge rate associated with moving a person on a bike at reasonable timescales without overheating.

Flywheels are "decent" in race cars because its a couple seconds of hard acceleration followed by hard braking etc etc and dumping that energy into a battery is difficult to do efficiently. They handle high peak power for short duration's well, but have generally poor energy density...
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