'73 Suzuki GT250 - Part 19 - Carbon fibre rear fender

Here's a quick overview how I made a plug and one piece fibreglass mold for replicating '85-89 Honda CR250R rear fender out of carbon fibre. I used same method with front fender I posted earlier so I won't go through steps here with same detail. 

I'm pretty pleased with the result. I only used two layers of carbon fibre cloth which ended up with a very light part. I might do another one later with three layers of cloth to achieve a bit stronger and less flexible piece.

Next steps would be to make a lower piece for the rear fender and a seat pan out of carbon fibre. I have not fully figured out yet how to make plugs and molds for those pieces. I've also been planning modifications to my '01 Honda CBR600F4i PC35. Hopefully I can figure those out and share some of them within this year. Happy new year!

Plug made out of pieces of wood and
sheets of polypropylene

Plug waxed and taped

Close up of waxing and taping

Two layers of gelcoat

Fibreglass and polyester resin
applied on top of gelcoat

Excess fibreglass cut off with scissors and
trim lines marked with ballpoint pen

Edges trimmed with an angle grinder

Closer look of imperfections
in the mold caused by wax surface

Look after first round of wet sanding

Masking with vinyl tape

Surface after two coats of gelcoat

Surface after wet sanding up to 600 grid

Mold waxed and ready for hand laminating

Vacuum bagging after hand laminating

Vacuum bag, peel ply, release film and
breather cloth removed after vacuum bagging

Third attempt gave best result (left)

Part after separating it from mold
and before trimming edges of it

Edges vinyl taped and ready for trimming

Edges trimmed and part ready for clear coat

'73 Suzuki GT250 - Part 18 - Carbon fibre front fender

Creating carbon fibre parts using vacuum bagging has been one thing I have been wanting to try for a long time and one of the goals to get done with this project bike. I decided to start by replicating a front fender I had purchased a long time ago for a moped of mine. The simple shape of it made it the perfect candidate to get start with.

Front fender by UFOPLAST
for Suzuki RM 85 00-07 (#SU03967102)

I made a plug out of pieces plywood, planks and a sheet of 3 mm polypropylene. I traced side profile of the front fender to the piece of plywood with a square and cut them to shape with a band saw. Blanks were screwed horizontally between the two pieces of plywood. Hardest part was to bend the sheet of polypropylene on top of the plug without warping it. This was done with a help of a heat gun. I would suggest to use a thinner sheet of polypropylene than 3 mm or add third piece of plywood in the middle of the plug as support to avoid warping. Mounting holes of the front fender were used to attach it to the plug by a piece of wood and screws. Wax was used to seal edges between front fender and sheet of polypropylene. I also used small pieces of clear tape to block off mounting holes of the front fender.

Plug from front...

...and back

Wax used in the plug

Once the plug was ready it was time to use it to make a mold using gelcoat, fibreglass and polyester resin. I waxed the plug before applying two coats of gelcoat. I let each coat to harden for one to two hours before applying fibreglass and resin. I used woven fibreglass on the bottom and fibreglass chopped strand mat on the top layers. I suggest to use smaller pieces over bigger pieces since they are easier to manipulate to difficult shapes. I started with a bit too big piece of woven fibreglass which ended up lifting and creating air pockets between gelcoat and fibreglass. I filled those air pockets with a quick cure epoxy. Surface of the mold where strips of wax were used were quite rough so I sanded those parts of the mold smoother and added another coat of gelcoat. Once those new coats had harden I wet sanded them with 400, 600 and 1000 grit sand paper, hand polished surface of the mold with polishing paste and applied few light coats of spray wax to it. I was afraid that outer surface would have sharp edges and pierce vacuum bag under vacuum so I glued a layer of breather cloth to it. It also made the mold bit easier to handle during vacuum bagging since I did not have to use so big piece of breather cloth to cover whole mold.

Norslipp 9860
mould release agent
and Norpol WAX W-70

Polyester resin,
gelcoat and hardener

Woven fibreglass and fibreglass chopped strand mat

Two layers of gelcoat...

...and a lot of fibreglass on top

Edges trimmed with an angle grinder
and outer surface sanded

Close up of mold edges

Surface before and after sanding 

Small air pocket between fibreglass and gelcoat

Big air pocket between fibreglass and gelcoat

Quick cure epoxy for filling air pockets

Big air pocket filled with quick cure epoxy

Small air pocket filled with quick cure epoxy

Mold masked and painted with gelcoat

Sand papers for wet sanding (400, 600 and 1000)

Half of mold sanded

Comparison of sanded and non sanded surfaces

Polishing paste

Mold buffed with polishing paste and waxed

Piece of breather cloth glued
to outer surface of the mold

Next step was to prepare materials for hand laminating and vacuum bagging:

• carbon fibre cloth 200 g/m2 2/2 twill 1250 (1st layer)
• peel ply (2nd layer)
• release film (3th layer)
• breather cloth (4th layer)
• vacuum bag

I laminated total of 3 layers of carbon fibre cloth. Standard epoxy resin I used gave me around an hour of working time before it started to gel up and overheat. I left the part under vacuum for 6 hours and let it harden for 24 hours before separating the part from the mold. Quick release coupling allowed to detach the vacuum pump and vacuum bag and kept the bag air tight during hardening.

Last step after separating part from the mold was to trim the excess carbon fibre cloth from the edges of the part. Loose strands I cut with scissors and then I marked final edge with vinyl tape. I suggest to use a brighter color tape than mine. Dark blue got lost a few times under the carbon dust. Remember to wear a good respirator while cutting carbon since it is very bad for your health. Angle grinder was very good for cutting most of the excess material and belt sander and sand paper helped finalizing the edges.

Carbon fibre cloth, peel ply, release film,
breather cloth and vacuum bag

Epoxy resin

Vacuum pump

Under vacuum

Piece separated from mold
and roughly trimmed with scissors

Close up

Other side

Close up

Bottom surface

Edges outlined with 6 mm vinyl tape

Angle grinder, belt sander and sand paper
were used to trim the edges

I'm fairly happy with the end result for this first try. You can see some distortion in the cloth on the sides and also bit of resin buildup in the edges on the top. I think I will buy a vacuum regulator and try hand laminating this part again without release film. That could work bit better. I will also hand laminate only 2 layers of carbon fibre cloth next time. I think the fender will remain sturdy enough with just 2 layers of carbon. I will update this blog post after testing that. In the next post I will show how I replicated the rear fender. Stay tuned for that 😎