OV9

Anyone know much about bike dynamos?

The question doesn't relate to my personal use - just looking for info.

My only experience of bike dynamos is as a kid - which is quite a few years ago now - where they used a heavily sprung knurled drivewheel squeezed onto (into) the side of your tyre. They were horrible draggy bastards, and you had to physically stop to pull the unit off the tyre, as trying to do so on the move either removed the skin from your fingertips, or you found your fingers in the (moving) spokes. This was a right PITA when you really wanted to retain any momentum you had built up.

I see modern versions instead run off the wheel rim (using a rubber o-ring for grip). I understand they are far more efficient (thank fook), but assume
1. You still need to physically lever them off the rim to fully remove the drag (ie no handlebar mounted actuation of unit in contact / in free air).
2. They can slip if the rim is dirty/wet

I see you also now get wheel-hub dynos - which look like a cool concept. My understanding is (compared to current rim-dynos)
1. More expensive
2. Unaffected by dirty/wet conditions.
3. Less power output than rim dynamos (?)
4. Drag is proportional to electrical load. So you could have a handlebar mounted switch to cut off the electrical load completely for climbing gradients - reducing drag to "almost zero"? (Possibly also true for modern rim-mounted?)

Any help much appreciated!

mik wrote: Wed May 17, 2023 9:38 am I see you also now get wheel-hub dynos....

They're almost as old as the hills ! I remember my Dad had a Sturmey Archer Dynohub on the bike he used for getting to work. Google seems to say the first one was 1936:

http://www.sturmey-archerheritage.com/i ... tail&id=51

And you're correct. No frictional loss from the dynamo part. The load only kicks in when the load is engaged. I remember being fascinated by this as a child - with the light off you could spin the wheel, and it would spin freely, then when you turned the light on it would suddenly slow down. Same principle as regenerative braking on EVs.

I'm sure modern versions will be far more efficient. I have a Shimano one in the garage on a bike I never use. I really ought to.

Also - stating the obvious - it only generates power when you're moving This was not really ideal IRL when you need to stop at traffic lights etc. Modern light systems will have a small battery to cover that, and the hub will both power the lights & keep the battery topped up. IIRC you can even get USB dongles to (slowly) charge your phone or whatever while you're riding.

Sundayjumper wrote: Wed May 17, 2023 9:52 am Also - stating the obvious - it only generates power when you're moving This was not really ideal IRL when you need to stop at traffic lights etc. Modern light systems will have a small battery to cover that, and the hub will both power the lights & keep the battery topped up. IIRC you can even get USB dongles to (slowly) charge your phone or whatever while you're riding.

Yes - and thanks for responding.

Would be used in conjunction with a battery - mainly to top the battery up and extend use time.

And on the bike I rode to school, I had a bottom bracket mounted dynamo the same as this:








Neater than a bottle dynamo, but being right down there it got covered in crap all the time.

What would it be powering? Batteries these days are small and light and could easily be selected to power any device longer than the user would be able to be on the bike.

If it is for lights look at Reelight's products. They use magnetic induction, so there's no drag. I had them on all my bikes in Denmark and they did a sterling job. Didn't matter how dirty or wet they got (in normal use).

Rich B wrote: Wed May 17, 2023 10:26 am What would it be powering? Batteries these days are small and light and could easily be selected to power any device longer than the user would be able to be on the bike.

Rich B is correct- unless you've got some very specific use case ?

Rich B wrote: Wed May 17, 2023 10:26 am What would it be powering? Batteries these days are small and light and could easily be selected to power any device longer than the user would be able to be on the bike.

I hear he is riding to Mexico.

Dave!

V8Granite wrote: Wed May 17, 2023 12:21 pm

Rich B wrote: Wed May 17, 2023 10:26 am What would it be powering? Batteries these days are small and light and could easily be selected to power any device longer than the user would be able to be on the bike.

I hear he is riding to Mexico.

Dave!

Sorry to be cryptic - it's planned to power something with an estimated load of 50W for several hours. Battery alone might do it - this would be to provide top-up to battery and extend it's use time (or allow a smaller battery to be used).

Example of the wheel-rim dynamo I was looking at.
https://www.velogical-engineering.com/d ... n/?lang=en

mik wrote: Wed May 17, 2023 12:30 pm ...it's planned to power something with an estimated load of 50W for several hours.

50 Watts !

Website:

...6V 1.5 watts at 15kph (9,3mph)...

I think you might be pissing in the wind

50w would be like cycling on level ground at pretty high speed, so to pull that much you’d have to be producing 100w yourself which wouldn’t be very enjoyable. You’d be better off with a power bank but it would also be pretty hefty if it’s containing 150wh.

Bear in mind that there are losses in the generation of power; 50W of your energy does not produce 50W of electricity (is that what Mito is trying to say above?) It's a ridiculous idea; you don't get anything for nothing and you'd be better off sticking a powerbank in your pocket.

Jobbo wrote: Thu May 18, 2023 8:41 am It's a ridiculous idea; you don't get anything for nothing and you'd be better off sticking a powerbank in your pocket.

See comment above. There is no intent to power a 50w device from a dynamo - this is intended to extend the use time of / reduce the size of the battery that will be needed. 50w estimation is hopefully higher than actual too.

I still think you're passing water into a strong breeze

Throwing some round numbers around, with losses, running a 1.5W dynamo for an hour will probably run your load for a minute. It's hardly worth it. A solar panel on your back would be way better. It'll keep going while you stop for lunch, for example.

@Sundayjumper quite possibly. Also if you manage to extend the use time of the battery by 5 mins, but the extra drag causes you to take 5 mins longer to get there - it’s useless.

Solar panel(s) also under consideration for power output/cost/size etc

My question is linked to a concept that needs to be tested. It’s not a finished solution.

It's a mobile disco, isn't it ?

You're not trying make a pedal powered Hovercraft to cross the bearing sea, are you ??