Commentary on other pages: skjegg.blogspot.com

In March 2020 I exchanged some emails about a page with tests of dynamos and lights/USB converters regarding efficiency. I saw on cyclingabout ( https://www.cyclingabout.com/dynamo-hub-power-drag-testing-schmidt-son-shutter-precision-shimano/ ) a translation + commentary and as that shows some misunderstandings it was time to add this to my site:

My email from 2020-3-20 in reply to an email from 2020-3-17 from a bicycle dynamo manufacturer:

> Did you see this article? https://skjegg.blogspot.com/2020/02/dynamo-hubs-for-your-pleasure.html

No. Looks interesting for the measurements of power used by headlamps, i.e. efficiency, though it's not the real efficiency.

> It is a nice detailed write-up. Everyone who has shared the article has been boasting about power output from this new Shimano DH-UR700. I don't get the hype... it's only an extra 2-3W at normal speeds 🤔. I suppose that's a big increase relatively though.

2-3W extra would be a lot for a standard dynamo, but if you mean the table with a the MPPT, that seems wrong as the PD-8 only gets a bit more than 2W at 15 km/h in those figures whereas it should be at least 3W using the standard resistors as mandated by StVZO. I measured a PD-8 that way using an Agilent U1273A and it was exactly within spec. so with a MPPT it should give out more power (than 3W), defintely not less than with a fixed resistor value!

The Shimano dynamos in my testing with alternate ways to extract more power (than 3W at 15 km/h), are [ in my tests] of higher output than the SP and Schmidt dynamo hubs, so it's not really surprising but I see nothing that indicates the superiority of the UR700. Further, to know what power can be extracted you need to know whether the device is optimised for a particular dynamo... Using a device optimised for one dynamo on another dynamo with other characteristics will not give optimal results!

Further the drag is not the real drag, but the power used by the motor/chain, to drive the dynamo, so that is only of use in comparison with the other dynamos and lights and converters and the efficiency of the motor (and chain) is not linear, so this, though testing efficiency of the lights and USB converters is useful, without using the real efficiency (as we talked about long ago), these values are not comparable to anything else than that exact test rig so I think not that useful.

There are MPPTs for bicycle dynamos but I don't think commercially available (so only experimental). Forumslader is not really a MPPT but using resonance resistors [ edit: I meant capacitors ] unless that changed by now, but it works well enough, similar output to what is possible with a MPPT. They have different versions for dynamo hubs and the velogical rim dynamos due to the different characteristics.

On my site you can read about the physically noticeable impact of different characteristics of the electronics of headlamps in e.g. the frankenlamp where running it without smoothing capacitors gave me a lot of vibration in the front fork, with a large smoothing capacitor far less, and I mentioned that the Edelux produces stronger vibrations than other headlamps I tried with many dynamos. This is caused by the interaction of the electrical characteristics of the dynamo and that of the electronics. The same is of course the case for USB converters...

For my measurements of power output of various dynamos that show that the Shimano dynamos do provide a bit more power with a custom bike light driver (though that driver was developed on a low end Shimano dynamo which can provide even more power) and bare LEDs without driver with the Eco DS, see Comparison of power output of dynamos.

Measuring the real efficiency is not that hard. Olaf Schultz described the setup to do this on Fahradzukunft. I think that was for sidewall dynamos, perhaps also for dynamo hubs, but if not: for dynamo hubs you can make a similar setup with a torque arm to measure force at a certain distance with given rotation speed and thus speed and from that you can calculate the power going in to make it rotate at the given speed/rps (W=Fs, P=Fs/T = F v). There is no need for a flywheel as the author in that article mentioned and this type of setup gives instant results vs. a flywheel.

Last modified: 2020-6-30