I had a small motorbike whose light got dim when rpm were lower.

in start I tried some big led but as voltage got lower they got exceptionally dim.

so I used lm338(similar to lm317) to regulate voltage(v input was 8-13 volts) lm338 is used to upgrade it to even higher outputs and hence more light.

Things needed

1.100 ohm resistor 1w=3

2.lm317

3.diodes=4(bridge rectifier)

4.wires

5.leds to your need

the zener diode,diode and resistor in last are needed if you are using some cheapo ic

Step 1: Diodes

I used bridge rectifier to ensure that no reverse polarity will occur as mechanics some times roll over the wires moreover easy installation con be done as these ic's don't like reverse polarity.i used doubled up diodes as these were of smaller size