LED Conversion for Dynamo Driven Bike Lights

It was at the end of December in 2016 that the short days, rapid sunsets and long nights finally prompted me into some kind of action. My Bertin C 37 had been converted to a randonneuse by the previous owner who had added vertical dropouts, brazed-on centerpull pivots, dual bottle cages, cable guides and shifter braze-ons on the downtube. Oddly, he had not added provision for generator driven lighting which was mandatory, along with fenders, under the randonneuring rules at that time.

The bike was purchased from Mike Barry at Bicycle Specialties in Toronto and it was there that a generator braze-on on the non-drive seatstay was added and the frame was drilled for internal routeing of the wiring for the taillight. Honjo fenders were added (to allow wire routeing through the rolled edge of the fender) and provision was made for a combined tail light/reflector on the rear fender. I assembled the bike with a Soubitez 6V 3W bottle generator and a matching front Soubitez headlight with 2.6 W screw base front halogen bulb and a 0.4 W rear halogen bulb. The front light was attached to the left side Mafac brake pivot bolt via a custom Mariposa tubular cromo bracket. The headlight wire went back to the lower head lug, through a hole drilled in the lug and frame and out the drilled bottom bracket, up inside the rear fender edge and into the generator. The fender stay attachment shown in the taillight photograph is a custom Mariposa style stay which uses cromo tubing and braze-on fittings to produce a rigid, stable and rattle free attachment for the plastic fender light/reflector combination. The single taillight wire goes from the generator to the inside of the rolled fender edge and down the fender to the taillight housing.

The result of all that plumbing was not impressive even if state of the art for the period. There was a warm, yellowish light puddled on the ground about 3 meters in front of the bike and a nice bright taillight. There was a reason all those old French randonneuse bikes had big, D cell battery powered flashlights on their front racks.

Visibility for this rider was poor at best and the lights and rear reflector really just acted as markers to let other road users know I was out there. It was obvious that a lighting upgrade was necessary regardless of the fact that I seldom ride at night anymore.

There has been a great deal of progress with LED based bicycle lighting in the last decade but much of the “modern” headlight and taillight designs are aesthetically incompatible(read: ugly) with older, classic bicycles. So, the obvious thing to do was to search out some kind of LED lighting which would be retroactively compatible with my current Soubitez lighting.

One of the great benefits of the older style of halogen/incandescent lighting was the single wire format with the circuit ground being through the frame. So, what I needed was a conversion to LEDs that was a simple bolt-on or screw in with LEDs being available on line.

A simple on-line search was immediately fruitful. Compass Bicycles in the USA offers a red LED rear light in 0.35 W with a built in stand light feature. As well, a front LED conversion light was also available through Bikeco in the UK.  The NL 432 LED was rated at 120 Lumens and was compatible with the 6V output of the Soubitez generator. One of the drawbacks of the NL 432 was the necessity of using a regulator to avoid overloading the front LED. It was possible to add a regulator to the circuit but to keep the conversion simple, I deliberately chose to use the Compass rear bulb. This contains a built in standlight function which avoids the need for a regulator and allows a simple plug in and ride approach to the bulb conversions. Bikeco offers both screw-in and push-in LED bases to facilitate the conversion.

On the Bertin, the conversion was quite straight forward. The front Soubitez headlight levers open and the reflector pulls out as it is a simple friction fit. The bulb/LED base pops out

of the reflector and the halogen bulb unscrews and the LED screws right in. Note that the Bikeco LED has a conical projection on the top of the LED. This is a collimnator which disperses the LED light from the same point relative to the reflector that an incandescent bulb filament would. The rear LED is not configured like this as there is no need to worry about focus as there is with the front light. The standlight  circuit is integrated into the rear bulb base and once the bulb is screwed into the base there is no other action to take. My installation was easy as the rear light lens unclips from the reflector housing, the old bulb unscrews and the new LED simply screws in as a replacement. The lens just clicks back into place when LED installation is complete. The result looks just like this:


Road Test:

Once the bulbs were installed, the bike was taken out and test ridden. Once the generator was activated, the two LEDs immediately came up to full brightness. There was no gradual increase of intensity as I experienced with halogen bulbs just immediate full function with no fall off in brightness even riding slowly at walking pace. The slight hum from the generator was the same as previously with the halogen bulbs.

The beam intensity was significantly brighter than the previous halogen headlight but I still would prefer more light. The stock Soubitez headlight cast a bright spot in roughly the same area as the previous halogen lamp and threw some light down at the base of the front wheel. The headlight colour is a bright white light and is easily visible from the front. The light intensity is sufficient around town and gives sufficient light to the front if one travels slowly. It would be readily possible to out ride the light. There was no diminution of the light after a half hour ride which indicates there was no over volting of the LED.

Caveat: Consider that my comments about light intensity and visibility are filtered through 70 year old eyes which have reduced night vision capacity.

The taillight was excellent. The red light was highly visible from the rear (easily from a city block away  under street lights) and the standlight was at full brightness for seven minutes with lesser intensity for another four or five minutes. Outstanding.


The conversion is worthwhile if you need a classic style lighting system for intermittent use. If you are regularly commuting in the dark, I do not believe that this would be sufficient for your needs. It would definitely not be adequate for fast riding or downhill use on a road bike.