6/16/06
Before I can start fabricating the front sub-frame I need to make
several of the parts that will be bonded into to it.
I will be using a custom Phil Wood bottom bracket
The bottom bracket requires a
narrow shell to hold the bearings. I used a 1 ½” heavy walled tube and
machined the ends to accept the bearings leaving a 0.10” wide lip on the
middle to prevent the bearings from sliding one way or the other. The
total width of the shell is 1”.
I want to be able to
adjust the position the bottom bracket. I will make a sliding assembly
that will allow the position of the bottom bracket to change 5”. First I
made 2 sliders that the bottom bracket shell will be attached to. I used
1”x .058 tubes with 6mm binder bolts. Here is the setup before it was
brazed.
Here is the slider after it is
brazed and before a slot is cut between the centers of the binder bolts.
Next I tack brazed the bottom
bracket shell to the sliders making sure the sliders were parallel to
each other.
Then I slid 7/8” tubes through
the sliders and attached them to some pieces of 1” square tubes. It took
some time to get everything aligned so the bottom bracket shell would
slide easily. I also machined a 2” piece of steel to be a tight fit
inside the bottom bracket shell so it remained round when the final
brazing was done.
Here the bottom bracket
assembly is placed on the full size pattern for the front sub frame.
Next it was time to make a
carbon head tube. An aluminum mandrel was turned on my lathe and wrapped
with plastic wrap. A 30” x 7 ½” strip of 10 oz. carbon cloth was cut and
placed on a piece of plastic sheeting.
Once the carbon cloth was
thoroughly wetted with epoxy, it was carefully wound on the mandrel
starting at one end.
I tried to keep the cloth
centered on and wound tightly to the mandrel. The epoxy makes it hard to
keep a firm grip. I changed my gloves several time doing this.
Once all the cloth is wound on
the mandrel, I wound a shrink-wrap tape over the wetted carbon to
compress it and remove excess epoxy.
Tape was wrapped starting on
one end and overlapped 50% in a spiral pattern.
A piece of duct tap was used
to hold the end from unwrapping. A heat gun was used to shrink the tape
and squeeze any excess resin. The assembly was placed on sheet of
plastic to cure overnight.
The ends of the carbon head
tube were cut off exposing a section of the mandrel. This end was
chucked in the lathe. Several cuts with the lathe were done to true up
the ends and shave off some of the excess carbon to make the head tube
round. A shop vac is used to catch the carbon shavings as the piece is
cut.
Now it is time to make the sub
frame that will hold the bottom bracket and the head tube. A full sized
pattern is laid on a piece if 1” urethane foam. The foam will be cut to
the shape of the pattern and covered in several layers of carbon.
6/19/06
To make the front sub frame I cut some ½” and 1” thick urethane foam to the desired shape and then cover with several layers of carbon fiber. I cut the 1” foam to the shape of the full-size pattern and then cut several pieces of ½” foam that will be glued to the 1” foam to increase the thickness of the sub frame at the head tube area and along the bottom at the front. The ½” foam is shaped with some sandpaper to feather the sides.
The pieces of 1/2” foam are glued on one side of the 1” foam and the hole is cut for the bottom bracket assembly
The next step is to lightly spray glue several layers of 10oz carbon cloth to the foam. The foam is covered with carbon on 3 sides. The carbon cloth is placed at a 45 deg. orientation.
Once the carbon is glued to the foam,
peel ply and breather fabrics are cut to the correct size. These pieces are set aside and will be used after the epoxy is applied.
I am going to vacuum bad these parts so I need to make the bag. Garrie Hill lent me his heat sealer and it took some time to figure it out and get it to work, but eventually I was able to create a vacuum tight
bag.
The bag will be hooked up to a small vacuum pump that I have.
Once I had everything ready I brushed on a coat of epoxy over the carbon and foam. I then placed the
peel ply trying to get it as smooth as possible. Then covered everything with the breather. The pieces were placed on a plastic covered piece of MDF, placed in the bag, and left to cure overnight.
The next day I removed the pieces from the bag and removed the breather and
peel ply. The sides were sanded to remove excess carbon.
Now I glued the other pieces of ½” foam to the non-carboned side of the sub frame.
This side of the sub frame is
covered with carbon cloth on 3 sides.
The pieces were covered with epoxy,
peel ply, and breather and placed in the bag. This time the piece of MDF was not used. Care needs to be taken to remove as many wrinkles from the fabrics as possible. The pieces were left to cure overnight.
Here are the final pieces of the sub frame. The head tube needs to be bonded in and the steering support will be attached once the sub frame is attached to the tub.
This is a view looking at the front of the sub frame that shows the thickened sections. The center section needs to be 1” thick to provide clearance for the chain ring.
Next I will make the mounting
brackets for the rear wheel and then apply carbon to the inside of the
Tub on the Tub Fabrication page
