We are minimalists, love reliability, and prefer to not follow convention.  When choosing our first ever bike build, we wanted to find a more aggressive platform than what’s found on normal cafes.  After some searching, a 1990 BMW K75S with 38k miles fell into our laps and after a quick ride we were smitten, even on the potholed streets of Chicago.  And so began our first build with exactly zero prior motorcycle wrenching experience.

The first order of business was to make a second key since the original is 24 years old and we always fear the worst.  Want a cheap key insurance policy?  keys4urride.com provided some blanks and a local locksmith worked their magic.  For $1 in labor, they were able to cut the key and provide some peace of mind for when we screw up.

We established a maintenance baseline including: engine oil, transmission oil, final drive oil, fork oil, coolant, spark plugs, brake fluid, fuel filter, air filter.  Splines had just been done, so we left them alone.

The best addition is subtraction.  All the tupperware had to go and we stripped the bike to its frame. 

With that we removed the handlebars, mirrors, headlight, and speedometer.  We decided to go with 41mm clip-ons and a 7 1/2″ British style headlight (found through our friends at DCC).  Without access to DIY space or proper tools, we used a rubber mallet and brute force when fitting the clip-ons (a most effective and knuckle bruising method).  With the upper fork mount was off, the handlebars were slid on and pounded into place.  The mounting hardware that was included easily secured the handlebars.  The controls also needed a little finessing (rubber mallet) when mounting onto the new clip-ons.  It took some playing around with the alignment and height, but eventually, we found a comfortable riding position that gave adequate tank clearance.

A $10 mounting bracket was found online (fits forks 38mm-42mm), but again with a little elbow grease the bracket mounted was widened and installed.  A set of LED turn signals  hooked up to the OEM wiring harness to round out the electronics.  The entire installation took about 1 hour and the all black front end makes the bike and its massive engine block look a little more sinister.

Since the dash pad and all electronics were ripped out during the handlebar project, we decided to relocate the ignition.  The wiring is a mile long and mounted neatly behind the airbox with some worm screws.

Going fast is fun, but stopping is crucial, so we decided to tackle the 25 year old brake lines.  The new handlebar position moved the lines out about 1″ so we needed some custom lines to fit the new setup.  Not knowing what exactly to ask for, we called Spiegler, who walked us through the entire process and were able to fabricate a new upper brake line with a special banjo bolt.  With new lines installed and brakes flushed, the brake lever was firmer and the bike stopped well and didn’t experience any fade.

Using the help of a friend with an eye for design we taped the front fenders, sketched out a more refined design and we went to work with a Dremel.  We finished off the handlebars with new grips and called the front end a day.

Not happy with any of the cafe tails online, we felt nothing off-the-shelf would mesh well with the awesome 80s angles of the K75.  We ran across a video that showed how to shape your own tail and lay it using fiberglass.  Without knowing what we were getting into, we set forth to build a custom tail.  Beware if attempting this, you will get glass splinters in your hands.

  1. Went to Home Depot and a craft store for: styrofoam board, florist foam, Loctite adhesive (buy the spray and not actual glue, we found out the hard way), Bondo fiberglass resin, Fiberglass cloth, Cheap paintbrushes, Latex gloves
  2. Lay the styrofoam on your frame and start gluing the florist block on top of the board to start to build up your tail.  Use a sharpie to trace out the frame on the board and use a file to carve out a channel for the tail section to rest in.
  3. Use a serrated bread knife to begin to shape your seat and tail section.  (It is always better to make more cuts and slowly shave down as you can’t add foam back.)  Once you’re happy with the general shape of your seat, STOP.  Do the rest by hand.
  4. The florist block is super fragile and it’s easy to achieve the lines you’re looking for this way.  By this time your workspace is going to be covered in a fine green dust like you just vaporized Oscar the Grouch.  So be sure to wear a mask and do this somewhere outside or DIY workspace!
  5. Give the seat a good brushing and shake off to get all the shaving off.  Take painters tape and cover the entire seat pan with it, making sure to cover below as well.
  6. Cover the tape in wax to act as a release agent for the fiberglass.
  7. Wait until your girlfriend goes to work.
  8. Prop up the seat pan, put down a tarp, and start to cut strips of fiberglass cloth (smaller the better).  Mix the fiberglass resin and pour it all over the seat, brushing it over all surfaces.  Start laying down fiberglass cloth and dab on more and more resin over the cloth until you have a giant, gelatinous mess.  You have about 10 minutes to finish this step so you need to move quickly.  Don’t think, just make sure you’ve got complete coverage.
  9. Step away, let the mold rest for 24 hours, tend to your wounds, and prepare to deal with an angry significant other when they come home.  Fiberglass is nasty stuff.  Not only was it all over the dining table, but all over the floors and for the next day we were finding glass splinters from our fingers.
  10. Repeat with more layers of just fiberglass resin for additional stability.
  11. After dry, use a Dremel to cut the bottom edge of mold away and pry the mold from the foam.
  12. Since the fiberglass itself isn’t strong enough to bear your weight, you’ll need a strong seatpan.  We had a few $5 Salvation Army chairs that seemed like they were good donors and cut the particle board from one of the chairs.
  13. Hit up our friends at the hardware store for 4 rubber stoppers, some bolts, washers and nuts.  Attached the stopper between the seat and frame, held down to the frame by the screws.  Nothing fancy, but got the job done.
  14. Used the chair fabric and foam to make a seat cushion.  Cut down the faux-leather and wrapped it around the foam, which laid on top of the fiberglass seat.  Super clunky and awkward, but at least it gave a little cushion and let us back on the street.

DIY Frame cutting time.  We had so many questions about the best way to do this, but at the end of the day just took a saw, marked off where we wanted our cuts, and went at it.  No overthinking here.  We also found some aluminum to fabricate side panels and hide all the internal electronics.

The K75 was rideable for the entire summer and fall, but an infamous Chicago pothole changed our fortune while riding home from work.  The impact of the pothole caused the entire seat to rise up and pivot on the rear screws, enough for the license plate to catch the rear wheel, which then ripped the license plate and taillight right through the seat.  Pothole 1.  DIY seat 0.

We guess a part of growing up is learning when to ask for help.  With the bike down for the winter, we decided to send it off to a custom fabrication shop for a seat that functions.  Fortunately the new seat is going to be metal so it’ll be a little more bulletproof and feature upholstry fit for a king.  We will work on our welding in the meantime.  After a long month away in the shop, the K75 is back and looks better than ever.

After some back and forth, we decided to do something about painting the tail section. A $3 can of Rustoleum truck bed liner was applied to the bottom to protect against water and debris getting thrown up into the tail.  We weren’t ready to commit to a full paint job on the bike and caved for a uniform look, courtesy of matte blue Plastidip. 

So, take it from rookie bike builders; don’t get discouraged by taking on any DIY task.  You can do a lot with a little DIY space, creativity, a good hammer, and some advice from those with more experience.  Will our bike win any beauty contests?  Nope.  But it is unique, presented us with a great learning experience, and makes us excited to do it all over again.



Leave a Reply