Why wouldn’t a Dyno hub work?

[Supratad]

Is there any inherent drag caused by magnets within a specially designed rear hub for an e-mtb?
I just had this idea while pootling along today, gently coasting down a slight slope.
Unlike the very old bike light dynamos that dragged on tyres to spin a dynamo, a specific design hub could have rotating magnets or wire coils, whichever works best round an axle housing, to generate a charge for extending a ride.
If there is no real drag created by a generator than it would seem a missed opportunity not to have this as an integral part of an emtb rear wheel.
Extra weight would be minimal , especially on an emtb, and close to centre of rotation which is ideal.

Any thoughts, good people? Have I missed a vital element?

 

[Gary]

dynamo hubs in a configuration as you suggest do already exist. to power low current bike lights
The current achieved would be next to pointless to charge a massive Emtb battery and involve a lot more electronics, weight and cost than you seem to think.

 

[Gary]

The most efficient power source on the bike is the rider!
Brake less, use lower assistance levels and pedal harder.
If we keep at it, who knows one day we might all even be able to ride bikes without a motor

 

[Planckus]

The most efficient power source on the bike is the rider!
Brake less, use lower assistance levels and pedal harder.
If we keep at it, who knows one day we might all even be able to ride bikes without a motor

Then we would need a new forum, and that would be a great shame … I just love the lovely people around here

 

[Supratad]

dynamo hubs in a configuration as you suggest do already exist. to power low current bike lights
The current achieved would be next to pointless to charge a massive Emtb battery and involve a lot more electronics, weight and cost than you seem to think.

Yes, but I'm wondering what sort of charge they currently give and how much bigger they might need to get to become useful as a range extending device. Extra weight and cost would be certain but on a 22kg +£6000 bike, would that really be that bad by comparison? A lot of weight and another £1500 to get 8% back on a ride would not be worthwhile IMO, but those are just figures plucked from the air to illustrate a point.

Websites selling dynamo hubs do mention there is a very slight effect on speed / drag etc.

For every ride the elevation gained is matched by the drop. This energy might be better used as "regenerative charging" .
It's a theoretical exercise that may not be borne out by market demand of course.



edit; in fact now I recall that when viewing ebikes in a shop back in 2019, one of my very first questions was about regenerative charging.

 

[Slapbassmunky]

Regenerative braking is a well known thing that had been going on for years with direct drive hub motors. Just have a look on the Endles*phere forums and read until you fall asleep. It's extremely easy to do with the right motor and controller setup. But the hub motor has to be sizable to cope with the heat and power generated, and it's rather inefficient. Loads of people use it on high powered setups as a drag brake to save their brakes when coming down (very) long descents.

Basically, it's a no go for anything with a mid drive motor, and on a proper MTB would be utterly pointless.

 

[steve_sordy]

The only way it would be net energy positive is if the regen was only powered when braking. If you were thinking of it to charge your battery whilst pedalling then you would have to put more energy in than the hub will generate for you.

 

[Supratad]

I used the term regenerative braking as an example, but I didn't mean actually under braking, just while going DH.
It doesn't need to be net energy positive as I'm not talking about having the same battery power at the end of a ride as at the start, but more modestly as a way of feeding some charge into a battery during a ride.

i.e. if you are going DH and the drag is almost nothing, then a hub could be feeding the battery a little bit. As said if it was just 10% but cheaply achieved, it would be worthwhile. If it were 10% at enormous extra cost then no one would buy it.

 

[Slapbassmunky]

I used the term regenerative braking as an example, but I didn't mean actually under braking, just while going DH.
It doesn't need to be net energy positive as I'm not talking about having the same battery power at the end of a ride as at the start, but more modestly as a way of feeding some charge into a battery during a ride.

i.e. if you are going DH and the drag is almost nothing, then a hub could be feeding the battery a little bit. As said if it was just 10% but cheaply achieved, it would be worthwhile. If it were 10% at enormous extra cost then no one would buy it.

But that's the thing, you notice a very small amount of drag on an MTB, and any drag is is going to kill the flow of riding a DH or trail section. Say you pull 50 Watts, you'd really notice that, and as a rough estimate you might get 2/3 or that back as electrical power. Just not worth it.

 

[Gyre]

Any weight you're adding on a hub is unsprung weight. Add enough and eventually it reaches the point where you'll notice the rear suspension tracking poorly.

Electrek has a good article on point: Regenerative braking: how it works and is it worth it in small EVs? - Electrek

For smaller EVs such as personal electric vehicles, the numbers aren’t quite as optimistic. On multiple electric bicycles with regenerative braking options, I’ve generally averaged around 4-5% regeneration, with a maximum of around 8% in hilly areas. Other personal electric vehicles including electric scooters and skateboards have similar results, usually in the lower single digits. Again, keep in mind this isn’t the raw efficiency of the system (as in how much braking energy is lost in the energy transfer), it’s the effectiveness (as in how much further your range increases due to the use of regenerative braking).

As I mentioned above, this is largely due to the lower weight of personal electric vehicles. They simply don’t carry much momentum and thus have less kinetic energy to convert back into the battery.

In the e-bike industry, regenerative braking can sometimes be used more as a marketing tool than as a feature. Because regenerative braking is generally only possible in electric bicycles with larger gearless motors, such e-bike manufacturers will tout the effectiveness of their models. At the same time, manufacturers of e-bikes with mid-drives and other geared motors that are incapable of regen braking will dismiss it as ineffective and simply not worth it.

 

[Supratad]

My last ride , a a small one on Sunday was 8.25 miles but 5.1 miles or 62% was with assistance. Therefore 38% of the ride drew no power from the bike, i.e. coasting and downhill.
47 mins, so about 18 mins of possible charging time when no power is also being drawn, from a small dyno hub. Sounds like it wouldn't be much.

 

[Supratad]

Good points made above. Thanks. I’d imagine battery tech and performance would advance much quicker than any ideas about energy scavenging.

 

[Zimmerframe]

If you think about charging your battery .. that takes say 4-6 hours - plugged into the mains.

If you want to charge your battery on the go with energy recovery, you'd need to pull a reasonable amount of current to make it worth while.

Think about the motor. Flat out you can flatten your battery in less than 1 hour. For those moments you want to recover some charge, you'd want to recover as much as possible to make it worth while. But if you pulled enough to charge the battery in an hour, the resistance would be massive - think of the same as riding in Turbo being pushed back against you. In order to recover a reasonable current, you'd need something the same size as the motor. The smaller your dynamo/generator, the less current it will recover. Make it too large, and it's heavy and contrary to what you want on the bike. Make it too small and the current recovered is so small that it's pointless having all the extra technology on board to give you 1% more range.

 

[Supratad]

The upshot then is that I’ve misunderstood the “drag” created by a generator. I was thinking it was only the bearings’ friction at play but in fact the more current drawn, the harder it is to turn a generator shaft.

 

[VWsurfbum]

I think about this a lot.
My solution would be to fix the rear hub (still have a cassette and derailleur etc) and rotate the chain which would on overrun regenerate the battery and have the freehub in the cranks as they do already. I think it would work to extend battery life with relatively very small amount of work?

 

[Zimmerframe]

I think about this a lot.
My solution would be to fix the rear hub (still have a cassette and derailleur etc) and rotate the chain which would on overrun regenerate the battery and have the freehub in the cranks as they do already. I think it would work to extend battery life with relatively very small amount of work?

This option always scares me.

At the moment, as you pedal and your feet come round, the chain is moving along with your feet. You stop pedalling and the chain stops.

Remove the rear freehub and the chain's always turning, which yes, means you can then theoretically use the motor as your generator.

It scares me though because it then means you have a chainsaw running right next to your feet/ankles on a mountain bike. Imagine how fast that chains going if you just finished a climb in first gear then shot down a steep descent. Twist your right heel inwards and brrrrrrrrrrrrrrrrrrp ... bone ....

Some gearbox bikes are setup like this though, so maybe I just overly worry.

 

[VWsurfbum]

I think that this is the next step forward on ebikes though. If I had the money I would patent the idea (after designing of course) but you saw it here first people!
Chains can be easily protected from sawing through calves.

 

[Gyre]

The upshot then is that I’ve misunderstood the “drag” created by a generator. I was thinking it was only the bearings’ friction at play but in fact the more current drawn, the harder it is to turn a generator shaft.

Definitely. No getting around thermodynamics.

 

[Zimmerframe]

If I had the money I would patent the idea (after designing of course) but you saw it here first people!