RATTLERS TECH TIPS

Building a Fibreglass Tank

Making a mould

Why build a tank from fibreglass I hear you ask? Well there are a few reasons, firstly none of us have much experience with sheet metal fabrication. Maybe enough to bash out a reasonable seat but that's about it. Secondly if we took the job to a sheet metal worker it would cost more than we would care to pay, anytime you say 'one off' or 'custom' the price of things tends to become unreasonable. Lastly we have experience working with fibreglass and have found it to be very versatile and fast to work with. The tank on Kitset Deathtrap is fibreglass and has been on the bike for two years with no issues.

This page is not going to be a step by step how to, but more of an outline of the stages involved. Ren wanted a sportster style tank for his xv400 Dragstar bobber project, originally we tried to fit a steel tank by removing the cumbersome airbox and fitting a couple of pod filters. We found that the bike wouldn't run properly and after having it tested on the Dyno, we discovered it was going to take a lot of work and cost approximately $1200 worth of mucking around to get it jetted properly. That's when we went for plan B - mould a tank that fitted over the airbox.

Below is a picture of the sportster tank we picked up for $50 from Trademe. We bogged up some small dents and gave it a coat of undercoat. Because of the shape of the thing, we were going to have to build a split mould. We ran a small cardboard divider along the curved edges, which makes it easier for sanding out later on. The cardboard divider is held in place by small cardboard wedges hot glued in place. The side of the divider without the wedges is then waxed up with a release agent and painted with gelcoat, the layers of glass are then applied over that half of the tank and up the side of the divider to make a flange. We went for a 5 mm thickness just to make it nice and strong. 

Once the glass has hardened you can tear away the cardboard divider and wedges, and repeat the wax, gelcoat, glassing process for the other side of the tank and create an opposing flange. When this side is hardened it's best to drill a series of holes through the dividing flanges, so it can be bolted together again to form the mould after you've separated it from the original tank. To release the mould from the steel tank you need to bash the fibreglass carefully with a rubber hammer.

Below is a picture of the finished split mould bolted together. You can see the flange to the left hand side. You can see the join line running through the mould. Surprising it is a lot easier to sand out any problem areas on your finished product if the mould is joined along curved areas of the tank, rather than flat areas, although it would seem easier and logical to make the mould split down the centre. If you're going to be fussy, you can sand out the inside of the mould with 1200 wet and dry and then finish off with a buffing compound on a rag before using release agent, gelcoat, and finally glassing in 5 layers for your finished product. It's generally best to lay in one layer of 300 weight chop strand mat as a tie in layer because sometimes you can fry your gelcoat by doing too many layers at once, which creates too much heat. Let that tie in layer harden, then give it a light sand and glass in 4 more layers of 450 chop strand. After it's hardened undo the bolts and bash the mould carefully with a rubber hammer to release the new tank shell. Depending on what you are going to use for a fuel inlet, now would be the time to use a hole saw and glass whatever you're using in from the inside before you attach the bottom of the tank. You can always make a hole afterwards and glass the fuel inlet from the outside, but it's a lot messier.

Nb: We used 2% catalyst to polyester resin and 2% catalyst to gelcoat without any dramas or fry ups. It pays to use one colour of gelcoat for the mould and a different colour for the tank shell so you can see when you are brushing one into the other and don't miss a spot. 

This photo shows how we made a bottom for the tank by glassing a template over the airbox area and top frame rails directly off the bike. The process we used was to hot glue some cardboard flanges around the airbox and on top of the frame rails. This gave us a flat boundary around the outside of the raised airbox area, which we would cut away later to the shape of the tank. The edge of the cardboard flanges had a raised lip made from masking tape to stop any run-off from the resin. The airbox and cardboard flanges were covered using a wide masking tape before release agent was applied and then we glassed in 5 layers all at once, as there was no gelcoat required. Make sure you use 2 layers of masking tape to avoid any leaks onto the bike. This process gave us a tank bottom that fitted like a glove.    

Once the bottom is hardened, take the tank shell you have made and place it on top and line it up straight and trace around it. Remove the shell again, pull the bottom off the bike and using a cut off blade and a hand held grinder carefully cut around the traceline. You want the bottom piece to sit flush within the shell, not on the bottom edge, so you will need to grind it smaller than your traceline until it fits snugly.

You will now need to attach the bottom to the shell with a polyester bogging technique. Turn the shell upside down and using masking tape at intervals, tape the bottom into position. You now need to make a bog by mixing talcum powder with the resin until you get a consistency that doesn't drip when held upside down. Don't forget to add the catalyst. Using a small putty knife bog in the gaps between the tape. The bog will harden in a couple of hours after which you remove the tape, mix up new bog and fill in the gaps left by the tape.

Fibreglass doesn't mould around 90 degree corners without creating airpockets, which you do not want when you are trying to seal a tank. This means you will need to round off the bottom edge of your tank before you glass it together over the bog.  

Using a grinding disc shave 2 mm of thickness out of the bottom 1 inch of the shell right around. This creates an inset area that the glass will take up when you glass the bottom to the shell, so everything remains flush. Next round off the bottom edge of the shell so the glass will form around it without any problems. Now for the glass: cut 1 inch strips of 300 weight chop strand mat and use two layers to glass the shell and bottom together. Only a small amount of automotive filler need be applied after to tidy things up and bring a straight edge back to your tank. 

For a fuel outlet we glassed in a suitable piece of tube after the tank was finished. We are running a filter and tap further down the fuel line. We glassed on some steel mounting fixtures.

We got some tank sealer from Red Baron Motorcycles here in Auckland. This makes sure the fuel doesn't eat into the composites. It takes 92 hours to cure and then you can test it with gas before you go to paint.  

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This is the finished tank all ready for final paint, 'T' has filled a few minor trouble spots and given it a dust of undercoat. Below is the filter we are going to run with this tank, hopefully it will all be up and running for this weekend 17/01/09 and we can get some shots of the bike completed and the ride to the Kumeu Hotrod and bike show.

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