Archive for category i. Composite gas tank fabrication

Composite gas tank fabrication

The factory gas tank for the GS500 uses a small vacuum operated petcock to provide fuel to the carburetors. There is nowhere on the gas tank that a fuel pump could easily be adapted and welding in a flange to accept a fuel pump is extremely unsafe. An empty used gas tank, even with thorough effort towards cleaning, contains fuel vapors that are easily ignited during welding. Brand new gas tanks are prohibitively expensive to purchase.  Fabricating a new gas tank offered a unique opportunity to shed significant weight from the motorcycle. This weight loss, especially in a position significantly higher than the center of gravity, leads to increased performance, efficiency, and handling. To bring this weight down, the same path was taken as the charge pipes. Carbon fiber was used to fabricate a new tank with the means to accept a fuel pump. This tank was based off of the factory tank with major work done to rework the bottom surfaces.

Figure GT1: Fiberglass mold of the bottom of the factory tank

Figure GT1 shows the beginning of the molding process from the factory tank. Several layers of fiberglass were used to replicate the bottom of the tank.

Figure GT2: Bottom gas tank mold

The fiberglass mold was removed, as seen in figure GT2.

Figure GT3: Half top gas tank mold

Due to required draft angles for mold removal, the top surface of the tank was molded in two pieces, bisecting laterally.

Figure GT4: Top fiberglass tank molds

Once removed from the tank, the top two molds connect together, making a shell of the outside surface of the gas tank. With the bottom mold from above put in place, the inside cavity of these molds is an exact replica of the space occupied by the stock gas tank and all its exterior surfaces.

Figure GT5: Foam gas tank plug

The exterior molds from the tank were assembled and the cavity left filled with a two part expanding polyurethane foam. This foam expands to fill the molds, making a foam replica of the stock gas tank. From here, the surfaces along the bottom of the tank are easily reworked to address the needs of mounting a fuel pump.

Figure GT6: Reworked bottom tank surface

Figure GT6 shows a comparison between the mold of the bottom of the stock tank and the large flat surfaces created to allow for the mounting of a fuel pump. The shape of the tank was also changed along the bottom of the tank to create a flat interface surface for later bonding of the top and bottom tank pieces.

Figure GT7: Surfacing work on the tank

Efforts were also taken to smooth and perfect the top surface of the tank. The original tank that molds were made from suffered from large dents and less than ideal surface finish. After the dents were filled, the foam tank was primered and brought to high gloss surface finish to prepare for final mold making.

Figure GT8: Bottom surface masked off for mold making

Figure GT9: Bottom covered in gel coat

The bottom surface of the mold, after primer and surfacing, was masked off, then covered in an epoxy gel coat. This provides a better surface finish than directly applying fiberglass. Several layers of fiberglass were then added to make a strong final mold.

Figure GT10: First half of top molds

The same process was followed for the top surface of the tank, again in two pieces.

Figure GT11: The final tank molds next to the tank plug

Figure GT12: Final mold surface finish

The surface finish of the final molds directly corresponds to the surface finish of the carbon parts as they leave the mold. The better the finish now, the less finishing needed to finalize the parts. With the molds finalized, the tank can be made, starting in 2 pieces.

Figure GT13: Materials needed to vacuum bag tank bottom

Figure GT14: Tank bottom mold under vacuum

Figure GT15: Tank top under vacuum

When fabricating parts out of carbon fiber, steps are taken to minimize excess resin in the layup and evacuate all air bubbles. This is especially important in cases of structural components or something that must not leak or seep. To achieve this, the tank components are vacuum bagged. The carbon fiber is saturated in resin, then applied to the mold surface. This whole assembly is then placed in an air tight bag, sealed, and held under vacuum during curing. Air pressure outside the bag compresses the layers of carbon fiber together, squeezing out excess resin and any trapped air.

In addition to concerns about leaking, there are safety issues with using a purely carbon fiber tank. Carbon fiber, while incredibly strong, tends to shatter when catastrophically damaged. In the event of a collision of significant impact, this would cause the gas tank to leak fuel onto the road. To prevent this, kevlar is added to the lay up. Kevlar, while less strong that carbon fiber, will soften when catastrophically damaged, but will not shatter. This behavior is very similar to safety glass used in automotive windshields.

For the tank, 4 layers of 2×2 twill 5.7 oz carbon fiber is used along with 2 layers of kevlar along the inside of the tank as seen in figure GT16.

Figure GT16: Kevlar tank lining

When removed from the mold, the tank has a high gloss surface finish and is extremeley strong.

Figure GT17: Top half fresh out of the mold

At this point, the tank is two separate pieces; a top and bottom; and does not have any cut outs for the fuel pump, mounts, or filler cap. Also, some minor surface finishing is needed where the mold seams fell.

Figure GT18: Gas cap bonded in place

Figure GT19: Gas cap close up

After drilling a hole, the gas cap assembly is bonded into place. The fuel pump is mounted to the bottom of the tank in a similar fashion, although with further carbon fiber reinforcement.

Figure GT20: Fuel pump with carbon fiber mounting ring

The fuel pump being used here is a stock piece from a 2003 Suzuki GSXR600 motorcycle. It has an internal pressure regulator, limiting fuel pressure to 43.5 psi.

It is mounted to the bottom of the tank with the mounting ring show above. Bolts squeeze the tank bottom between the fuel pump and the mounting ring, with the same resin used for fabricating the tank used, thickened with glass microballoons.

Figure GT21: Fuel pump in place, tank sealant applied

The epoxy resin used to make the tank is listed by the manufacture to be non reactive with gasoline. To be safe, however, the tank is coated in several layers of POR15 tank sealant. This provides extra insurance against any leaking. This is painted on with the tank halves separated, then applied again through the gas filler hole after the tank halves are bonded together.

Figure GT22: Top tank sealant

With the two halves sealed and prepped, they are bonded together, mounts are added, and a final gas tank is ready for use on the bike. Final weight for the gas tank, with the fuel pump in place, is 4.4 lbs. Nearly 25% of the stock tank’s 16 lbs weight and an approximately 5% reduction of the overall vehicle weight.

Figure GT23: Finished gas tank

Figure GT24: Finished tank on bike