Outbound Lighting Road Edition for Brompton?

[u_i]

It is below what body can realistically sense when pedalling.

 

[rogerzilla]

Generally, drag will be about 5-6W. It depends on speed, though.

 

[berlinonaut]

I couldn't find much info about drag value of SON XS under load. As far as I know without load it's drag value is 0,7W. Assuming both front and back lights will load full 3W of electrical load on the dynamo what would the drag be?

It is below what body can realistically sense when pedalling.

Generally, drag will be about 5-6W. It depends on speed, though.

As u_i said: You won't recognize it. When dealing with Watts:

• Do you know, how many Watts you need on your bike for a certain speed?
• Do you know how many Watts you are able do deliver continuously?
• Do you know which parts of your bike consume how many of those Watts (that thus cannot be used for speed)?
• Do you know how much of the necessary Watts at a given speed are caused/consumed by rolling resistance, how many by aerodynamics and how many by friction losses?

If not, a discussion about what amount of Watts a certain dynamo consumes is pretty pointless as you neither have an idea what effect it really has nor if there are other areas for optimization... It also helps to have a rough idea of the connection between electrical power, current and resistance in a bike lightening setup to better understand the amount of power needed from the rider on the input side.

Regarding the SON XS: There was a test of a prototype of the SON XS along with other dynamo hubs back in 2004 and these were the outcomes on a 20"/Etro 406 wheel:

Losses when idle (you should have in mind that a dynamoless hub would have losses, too):

Efficiency under load:

Current produced:

There's a newer test which includes other dynamo hubs as well (like the SP8) but does not include the SON XS (but other SON dynamo hubs) and it was performed on 28" wheels: https://fahrradzukunft.de/14/neue-nabendynamos-im-test

Here you see the losses when idle again:

Here is the electrical power produced at different speeds:

Here is the power consumed under load at different speeds to produce the results from above graphic:

If you want to get a ballpark idea of how much power you need to maintain a certain speed on your bike you should have a look at Kreuzotter: http://kreuzotter.de/english/espeed.htm
It helps to have in mind that roughly up to about 25 km/h rolling resistance is the dominating factor and above that speed wind resistance takes over more and more. Obviously other forces like friction in the hubs, the drivetrain with things like chain tensioner, hub gears or a filthy chain take their toll as well.

 

[CEBEP]

I don't understand why dynamo hub producers don't publish this data. It's all measurable after all and I'd be interested to know what efficiency the hub I'll want to buy will have and what drag.

 

[CEBEP]

@berlinonaut you will not be able to tell the difference being outside between - 18 or - 19 Co considering it will feel different if it's dry or humid, windy or still. Yet you are still checking exact temperature value before you go out. Since watts is the measure value for drag it absolutely makes sence to know this value to be able to compare with other hubs, even if I don't know how much W I input. Which I can calculate within 3 seconds though.

Charts you've posted are interesting. I've also looked through several reviews and hub tests but didn't find much info particularly about SON XS model. It seems like it's ideal drag at 10km/h is 0,7W and as far as I know the wheel hub will have 0,5W drag. So it's almost the same which is awesome since I don't do and don't plan to do night long rides. XS also seems to be among the most efficient hubs out there among wide range of speeds.

 

[rogerzilla]

Samples vary. I have a lot of dynamo hub wheels built over 20 years or so. With lights on, a spin test doesn't show much difference between SON, SP or Shimano 3N80 in the appropriate-sized wheel. Lights-off drag varies far more as bearing and seal friction are a larger percentage.

A Shimano 3N80 I have is better than the original "tin can" SON28. I have an SP SV-8 which is quite draggy but an SV-9 is much freer. Shimano hubs use cup and cone bearings and the more affordable ones, made outside Japan, tend to be very tight out of the box, so should be checked and adjusted before use.

My preference on a performance/cost basis is Shimano for 700c wheels but SP for smaller ones (I don't think the Shimano 74mm/8mm axle Brompton hub is still available - it was very heavy but otherwise excellent). SON hubs are very nicely made and would be my preference if I did huge mileages on a small-wheeled bike, since the bearings usually last better than SP's.