Originally Posted by SBinNYC

(snipped a lot of stuff)

The question of where to set the zener voltage depends on how one interprets the longevity specs for incandescent bulbs. I interpret them to be DC volts because that's what's on the spec sheet. That's also the instantaneous voltage. Therefore, I'd opt to set that threshold at 6 volts. I've used a 5.6 volt zener and assumed a nominal 0.7 volt forward voltage drop. That comes to a clipping voltage of 6.3 volts. I'm slightly over my desired threshold.

I used a 5 watt zener, like the 1N5339 because it comes in an axial package. I can solder the 2 zeners together and add leads to both ends. I can also put heat shrink around the diodes. The package looks like a wire. I attach it across my headlight because I use a bottom bracket generator and attachment there is not practical.