Cycling UK Forum

Hi All,

I have a dynamo light set up on my own bike using a Shimano Dynamo hub a B&M IQ-X dynamo headlight and from the headlight runs a cable to power the rear B&M dynamo light.

Because I find this set up so convenient what with the fact that there's no charging and it's always ready to go I want to put a similar dynamo hub set up on y sons bike for the winter for riding to school and everything else. Now I wasn't planning on spending as much as a B&M IQ-X and so I was looking at some of the cheaper B&M dynamo lights. But there range is really big with lots of very similar sounding lights going by the product name and so I'm finding the range a bit baffling.

https://www.bumm.de/en/products/dynamo-scheinwerfer.html

But I've got a fairly straightforward question which is can you run a rear dynamo light from any of the B&M dynamo headlights or is it only specific ones? And if it's only from specific ones how am I meant to tell which ones from the B&M website because I'm reading the descriptions but I can't find any reference to this.

Many thanks to anyone who can help.

Best regards,

John

As far as I'm aware, pretty much yes.
Peter White's website is IMO the go to for all things dynamo, it says:

6 volt B&M taillights are compatible with almost all 6 volt B&M headlights, all Schmidt headlights, and Spanninga headlights. One Busch & Müller headlight is not compatible with a few older taillights. This is the Luxos U headlight.

https://www.peterwhitecycles.com/taillights.php

You've got to look at the instructions (PDF download links on B+M pages) to see the rear light connection details.

Only the models with on/off switches have a rear dynamo light connection (so the switch works the rear light too). Either Senso or "N" in the model name. Using a sidewall dynamo, on/off is controlled by the dynamo position, and both lights are wired to it in parallel.

Some have male 2.8 mm spade terminals on the back of the light, usually 2 pairs with one pair for the dynamo cable and one pair for the rear light. These are mostly the older models.

Other models have a long built in cable for the dynamo, and a short (10-15 cm) lead ending in two male spade connectors for the rear light. The leads seem to be photoshopped out of the B+M supplied images that all the shop sites use.

As far as I know all B&M hub dynamo headlights are provided with a "voltage protected" output intended to be directly wired to the rear light.
In such a case:

What wire is good for taillights is 20awg ok?

alexnharvey wrote:... for taillights is 20awg ok?

20awg is 0.5mm2 approx ... too good!!

No. 20 AWG has a diameter of 0.812 mm and an area of 0.518 mm^2.
https://en.wikipedia.org/wiki/American_wire_gauge

SJS standard dynamo twin core wire has an area of 0.75 mm^2 (per way).
https://www.sjscycles.co.uk/dynamos/dynamo-wire-twin-core-black-per-metre/

Jonathan

SON suggests to use a Coaxial Cable 2 x 0.5 mm².

alexnharvey wrote:What wire is good for taillights is 20awg ok?

The coaxial cable is IMO far better than the twin core stuff, it's not so much about capacity, it's just far less prone to breaking.
https://www.sjscycles.co.uk/dynamos/sch ... per-metre/

EDIT - ConRAD got in there first, while I was grabbing the link

I find it easier to run and lay twin core on the frame.

Jonathan

Jdsk wrote:No. 20 AWG has a diameter of 0.812 mm and an area of 0.518 mm^2.
https://en.wikipedia.org/wiki/American_wire_gauge

SJS standard dynamo twin core wire has an area of 0.75 mm^2 (per way).
https://www.sjscycles.co.uk/dynamos/dynamo-wire-twin-core-black-per-metre/

Jonathan

Are you sure? 20awg can carry 5amps, more than enough for a rear light?

I could well be wrong. All checking welcome.

Jonathan

It makes me wonder if a much thinner wire would do, led rear lights only use 100mA?

You can work back from current capacity to size (or AWG) from the table in that linked Wikipedia article.

But I wonder if fragility might become the size-limiting constraint.

Jonathan

alexnharvey wrote:Are you sure? 20awg can carry 5amps, more than enough for a rear light?

In many applications the current capacity is irrelevant, it is voltage dropped due to the resistance of the wire that is a killer. 20AWG is roughly 10 milli ohms per foot. My background includes electrical installation requirements in aircraft where I might be obsessed with this. Absolutely irrelevant with the low current requirements of a rear light.

20 years ago I used 20AWG to a 20W headlight on my bike. The wire ran a bit warm due to voltage drop, but it was fine since that light (my other was 5W) was used only for a sharp potholed drop in a nasty lane.

I have just moved my dynamo to another machine and considered using the SON coax, but instead used the B&M twin core (I had both available) because it lies flatter along the frame for securing. Fragility of the smaller wire should not be a problem if you install it well. I simple moved the B&M wire across from the previous cycle where it had been in use for several years.