Exactly why I opted to use constant current drive for my front light described above. The "voltage regulator" in my circuit is actually used as a constant current source in this configuration (set for 20mA, by the 1.25V reference drop across the resistor).
The rear light is slightly different, as the flashing LED has an integral IC that automatically limits the current, so this string of LEDs can be driven by any suitable voltage. The flashing LED spec gives an operating voltage of 3.5V to 14V. This is increased by the forward voltage drop of each of the series connected LEDs, in this case to about 16V to 26V. The 28V zener diode increases the operating voltage range again to about 44V to 54V, making the unit suitable for my 48V battery.
I believe that the reason the white LEDs showed signs of early failure in the original posters design was because they were not adequately current limited.
Jeremy
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