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
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
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
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.)
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.
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.
LED technology advances by leaps and bounds every year.
In the past, LED H4s weren't that great but over the last
year or two they've really improved. They're still
rather expensive but there's some good ones on the market
these days. Make sure you do your research before
investing in an LED H4.
© 2018 Drake Smith - Please do
not use or reproduce this elsewhere. Feel free to link
to it though.