Cycling UK Forum

I've always wired front and rear lamps independently directly to the dynamo. However, if the headlamp comes fitted with rear lamp feed wires must I use them? Can I not tape them off and feed the rear lamp directly from the dynamo ?

depends if there is over-voltage protection in the front/rear lights I think, but the B&M lights are designed so that the rear is fed from the front lamp, so that's probably the safest option?

james01 wrote:I've always wired front and rear lamps independently directly to the dynamo. However, if the headlamp comes fitted with rear lamp feed wires must I use them? Can I not tape them off and feed the rear lamp directly from the dynamo ?

Do you have a switch on the rear lamp ? Or a dynamo that disengages ? A lot of current designs assume a hub dynamo which lacks any switch, so the front lamp provides that switch.

I have my B&M LED lamp with a switch, and that then feeds the rear lamp (a much older B&M LED rear). Powered by a front-wheel hub dynamo. Both lamps have capacitors to keep the light on when stationary. If I switch the front light off at the switch, it goes off instantly, but the rear will stay running until the capacitor decays away - anything up to about 4 minutes.
That same rear lamp was in use on another bike for many years, wired in parallel with the front lamp and powered from a sidewall/bottle dynamo: it worked fine in that configuration. So, I don't see any reason why you can't wire as you propose provided the dynamo has another means to be switched off.

Nigel wrote: So, I don't see any reason why you can't wire as you propose provided the dynamo has another means to be switched off.

I should have said - it's a sidewall dynamo, so a switch is irrelevant. The only reason for wanting to avoid wiring via the headlamp is that the dynamo is at the rear and it seems a bit unnecessary to have the main wires running forward along the top tube, and yet another pair of wires running back along it to feed the rear lamp. I accept that a sudden headlamp failure could result in a cooked tail lamp, which presumably wouldn't happen using B & M's wiring as I imagine the rear supply would be automatically cut if this were to happen.

you can run the two lights in parallel which (if the bottle dynamo is mounted on the rear wheel) may avoid having to run more wires forwards to the front light.

These days the front light normally protects the rear light because the former has a protection circuit built into it. However it is not clear if this is proof against every eventuality, because that circuit is often in parallel with the rear light output too, meaning that the front light and/or the protection circuit can fail and the rear light can see the full generator output after all.

However if you wire through the front light the system is proof against one type of failure, in which the generator is temporarily disconnected from the front light. If you have the front light in parallel with the rear light as you propose then again the rear light sees the full output under these conditions.

Rear lights should be assumed to be incapable of withstanding the full generator output unless it explicitly states otherwise in the product manual. Even then there can be some nasty surprises ahead of you; it doesn't always work out as planned if you choose to (or accidentally) run the rear light only.

Some B&M rear lights state that their protection circuit is of a 'temporary' nature (for brief use should the front light fail) and others say it is 'permanent', meaning that you can run with just a rear light connected to a generator and it won't kill it.

FWIW you can minimise the wiring by using a ground return arrangement (which in fact is how most bottle generators are designed to be used). However this arrangement is less reliable than twin-wiring because the ground return connections tend to corrode over time. Also (with a typical crown-mounted headlight) the ground return current has to go through the headset bearings (or along one of the brake/gear cables) if there is no bridging wire between the forks and the main part of the frame.

cheers