2018 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.

K Bike Headlight Relays

2020 Update - LED Headlight: On this page I ran some tests to compare an LED H4 headlight bulb to some other bulbs. I'm now running LED headlights in my Ks because they work better than filament bulbs. Even though there's probably less need for headlight relays due to the reduced watts used by LEDs it's still not a bad idea to install headlight relays in order to get more voltage to the headlight.

K Bike Headlight Relays

Lots of people add headlight relays to their K bikes. Why?

1) The stock wiring for BMW headlights is rather wimpy. It suffices but given the small gauge of the wire, voltage is lost in the circuit. By upgrading to thicker wire (like 14 gauge or even 12) the amount of voltage lost is much less. More voltage to your headlight means more light.

2) Since, from the factory, the headlight does not use relays this means that all of the current travels through the contacts in the high beam/low beam switch on your left handlebar. Every time you switch between beams a little spark is created. Over time and many uses, these little sparks degrade the switch contacts and eventually cause the switch to fail. When you use relays, the current flowing through the switch is reduced to next to nothing and this greatly increases the longevity of your headlight switch.

3) Once you've upgraded the wiring and added relays you can switch to a higher power bulb. If you tried that with the stock wiring chances are you'd melt the insulation off of the wires and have some potentially serious problems. (I run a 100/90W Xenon H4 headlight bulb which is a massive improvement over the stock halogen H4 bulb.)

You have two options when adding headlight relays:

1) Buy a kit from Jim Davis at Eastern Beaver. If you're not into chasing electrons or just don't feel like making your own, Jim's kits are handy, fairly priced and used by lots of K bike owners. Link: Eastern Beaver relay kits.

2) "Roll your own"

You can find a couple of write-ups on this over at the IBMWR K Bike Tech Pages:


You can either use your existing headlight plug and cut the wires very close to the headlight socket or you can get a new H4 headlight socket and connect your higher gauge wiring to the socket. This is really the "right" way to do things.  To avoid possible heat issues, I use a ceramic H4 headlight socket like the one pictured below.

I recommend using Posi-Taps to tap into the headlight wires from your switch.  You'll need two of the PT2022 type. (I do not recommend using ScotchLocks or T-taps - both have a tendency to fail or cut through the wire you're tapping into.)

Here's a wiring diagram that shows how to wire headlight relays using standard Bosch automotive relays. The thicker lines indicate where heavier gauge wire should be used

And be sure to disconnect your battery before doing stuff like this. (Have your radio security code handy if you have an LT.)


Headlight Bulb:

K bikes use a standard, very common H4 headlight bulb. There's plenty of competition to put out the "best" bulb and they range in price from a few dollars to quite a bit.

Legally headlights are restricted to 60/55 watts for the high/low beams but once you've upgraded the headlight wiring and added relays you can run higher wattage bulbs.

What I find works really well and what I run in my K bikes is 100/90W "xenon" bulbs that cost less the ten dollars a pair on eBay.  Yes, they're cheapo Chinese stuff but they really do work well and put out a ton of light.  They don't last as long as name brand bulbs (the worst I've had lasted 20,000 miles) but at that price you can afford to carry a spare in the tail cowl.

Technically they're illegal but I've been running them in all of my bikes for over a decade and never had any issue with police.  I think the main reason is that K bikes have a very good cutoff at the top of the low beam so as long as you have your headlight low beam aimed properly you won't blind other road users.

HID Conversion Kits: Don't do it. Yes, HIDs put out a ton of light but they need to be installed in a headlight assembly with a reflector and lens that are designed for HID.  If you install HID in a headlight unit designed for regular bulbs then you'll get a lot of scattered very bright light that can blind other road users. You also might get a ticket.

2018 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.