Attaching Cafe seat pan to tail section

When i got the seat pan and the tail piece(see previous post), I immediately began to wonder how to mount the two together. BMW’s R100S seat and cowl is a great design and works well with the hinges. But this is a Short Wheel Base model and we are going with the fiberglass tail piece. The tail piece mounts to the subframe through the saddle mount bag loop at the rear of the subframe and a small ‘tang’ that goes into the main frame tube.
The suggestions I got for mounting the seat pan to the tail piece were:
Velcro
Industrial Velcro
Upholstery with snaps (no thanks)
And then finally… i found out about Well Nuts. BINGO!
Well nuts seem perfect because they act as a thin rubber buffer between the to parts. Also when you tighten them they pull the pan snug onto the tail piece.

To do the job, you are going to need about
– 7 Well Nuts
– 6/32 screws to match the Well Nuts – about .75 inches long
– Fender washers for the Screws
– and a 5/32 and 5/16 drill bit.

Well Nuts purchased at the local hardware store (who has lots of my money):

Screws (I went with Stainless Steel):
Fender Washers:

The first step was to mark 3 holes to mount the rear of the seat pan to the tail piece.

The holes are drilled with the 5/32 bit. Just big enough to get my screws through.
The Well Nuts in the photo will eventually live in the seat pan.

Then snug up the upholstered seat onto the tail piece as best you can, and mark through the holes.

I started by drilling holes into the seat pan with the 5/32 bit. BE CAREFUL. DRILL SLOW. If you ram the drill bit through the seat pan, it will go through the other side and ruin the upholstery.

After i drilled the holes in the seat pan, i mounted it to the seat and ran screws through the tail and into the pan to make sure they lined up.

i then widened the holes with the 5/16 bit. AGAIN. DRILL SLOW AND CAREFULLY.

Time to insert the Well Nuts into the rear of the seat pan:

Now that the seat pan is attached to the tail piece, I marked 4 additional holes for some extra reinforcement.
NOTE: Make sure these holes do not conflict with the subframe cross member that contains the fender mounting bracket.

With the seat pan attached, I drilled (5/32 bit) carefully and slowly though the tail piece straight through the seat pan. i did this for all 4 holes so they would match perfectly.

I then widened the holes on the seat pan ONLY with the 5/16 bit:

All done. 7 screws all lined up. Seat pan is securely attached to the tail section.

BMW Cafe tail and seat

A while back, we purchased some fiberglass parts from Craig at Boxer Cafe. Craig has been great to work with and talk to. He even made a custom front fender specially for us.

As with building any custom bike, the parts you buy may not be perfect. They usually require some modification. But they are a great starting point.

Here is a rolling mock up of the bike build before i completely tore everything apart for rebuild and powder coat:

Craig didn’t have seat pans ready when we purchased the bodywork so a month or so later, the seat pan arrived:

It was good to test fit and take notice of the specific clearances/gaps/and spaces we had to work with:

The lower front corners of the seat pan had a sharp corner to it. I thought it would end up tearing fabric over time so i rounded it out.

The seat went to Autos International in San Diego for upholstery.

Thankfully, somebody in the upholster shop has a keen eye and noticed that the seat pan is slightly uneven. The tail sections (seat Cowl) and seat pan are not symmetrical parts but should fit the bike without uneven gaps and without looking crooked.

So the seat came back to my garage with the foam attached and thankfully it did. I re-test fit everything and then shaved down some areas to even the gap between the tank and the seat.

Again, I’m glad the seat came back into my hands because I also realize that the foam was a bit too high. It made the seat just a bit too tall. I’m only 5’9″ and the bike’s owner is a bit shorter then I am. So some foam has to go so we can touch the ground with both feet.

More notes for the upholstery shop:

The seat was returned about a month later and it looks stunning. Smells good too! And great craftsmanship.


Seat and Tail together. I can’t wait to have this on a running bike.

I am probably going to do one final modification. There is a small piece of the tail piece that sticks out under the seat. I am going to shave it down before the tail piece goes to get painted.

Tail Light bracket

A few final parts needed to be fitted before I can send everything off for blasting and power coat.
Those include a tail light hanger/bracket and small hangers for the rear turn signals.
I found these great old British turn signals. I have no idea what they are for, but for $35, i had to pick it up. The box is dated 1968 and i thought it would make a great tail light. If anybody has any information on it, please contact me? I’d love to know what it was intended for.

WIPAC British turn signal.



Our new tail light

We also bought another rear tail light that comes with a great hinged license plate bracket just in case the British light didn’t fit or work.
Seeing that the British light would work, i decided to incorporate it with the new license plate bracket (it is the black bracket seen in the next few photos).

I made a prototype bracket to hang the tail light from out of a sheet of aluminum i had laying around. Then sent it to a family friend who is a machinist for Coke. He took my template and made a bracket out of thick stainless steel. He made it a bit larger so i had to trim it down.

Bracket development. The stainless steel is a thick gauge and shouldn't 'unbend'.




Prototype tail light bracket.

I found these nice and small bullet shaped turn signals. I plan to have them blasted and coated to match the rest of the parts. Unfortunately, the small holes on the subframe that I intended to hang them from conflict with the lower edge of the seat cowl. I thought about trimming the seat/fiberglass but it would disturb the lines that the seat makes as it covers subframe.
So i made some small brackets to drop the turn signal down away from the seat.

Bullet rear turn signal



I didn't like how long the stems were so I trimmed them down.

Booty shot.

tail light and rear turn signal

All done. I love a few hours on a Sunday afternoon grinding and drilling metal.