Make your own … Air Filter
The air filter housing of the venerable Yamaha TY250 twin-shock was never the best in the world for airflow. We wanted to improve the flow characteristics and use a modern air filter. One way is to construct a trick-looking new housing from fibreglass.
Everyone has their own preferred way of building parts; some like to carve a former in wood and others to bend and weld steel or aluminium, which is fantastic if you have the skills. Yvan likes to get messy with epoxy and fibreglass sheets. There are certainly many different ways to fabricate parts for your trials machine. The ‘Lost Foam’ technique is just one of them. Since the mould is destroyed after the part is built this method is not a production process. The advantage is that any shape that can be carved or moulded out of Styrofoam can be turned into a fibreglass/epoxy composite. Yvan wanted to improve on the TY250 twin-shock airbox in which he already used a Montesa 315 filter and had removed the restriction into the connecting tube. However the flow was still restricted by the narrow sides of the standard airbox. Here is how he made his custom solution.
Thereare five basic steps in the process. 1. Cut a foam pattern. 2. Wrap with packing tape and apply a mould release wax. 3. Cover the pattern with fibreglass cloth and ‘wet out’ with resin. 4. Melt out the foam. 5. Final finishing of the part Seems pretty simple doesn’t it, so are you ready to try?
Remember you are working with materials hazardous to your health and safety. Please take all appropriate precautions following the manufacturers’ safety data sheets for the products you are using. Classic Trial Magazine declines all responsibility for those inspired by this article in the event of any harm or accident.
STEP 1: Mould-Making
Take a big chunk of dense foam — pink, grey or blue extruded foam is much easier to carve and sand. Depending on the size of the part you may need to glue several thicknesses together. Then cut the outline, using a carpet knife or a kitchen knife with small teeth, a band saw or a scroll saw. Complete the shaping and rounding by rough cutting with the knife and then use course sandpaper to create the final shape. You can also use a dry-wall sanding pad; this is a mesh of abrasive material with plenty of holes where the foam crumbs can lodge themselves instead of rolling and making dents in your foam or some rough sandpaper. All this can be done in around an hour.
STEP 2: Tape and Wax
Once satisfied with the accuracy and level of detail, wrap the pattern with one layer of packing tape to prevent the epoxy from getting into the foam. One thing to pay attention to: epoxy does not adhere to the shiny side of the tape but it will lock itself into wrinkles and gaps in the tape. So do a neat job of applying to avoid difficulty removing the tape from the cured part. Apply some mould-release wax to help remove the tape later on. Top tip: you can mount it on a fixture with a nail at each end to hold the foam core offthe working table. You can even use a kebab skewer. Thiswill allow you to work on all sides.
STEP 3: Cover with Fibreglass and Resin
Apply one layer of thin fibreglass plain-weave cloth (2oz or 3oz) using a multipurpose spray adhesive (e.g. 3M #77) to stick the fibreglass matting to the mould before the epoxy is applied. Apply the cloth in 5 to 8cm (2” to 3”) overlapping strips. To ease placement make the strips as wide as will conform to the shape of the core without producing wrinkles. In some areas they may be quite narrow and others quite wide. Mix your epoxy as per the manufacturer’s recommendations. Then brush into the fibreglass thoroughly, saturating it. Use paper towels to absorb as much of the excess epoxy as possible. When pulling offthe paper be careful not to liftthe glass cloth. Continue this process until the paper can absorb no more epoxy. Apply the number of layers needed to obtain the rigidity you want by the addition of additional layers of medium grade cloth, with more layers in the most vulnerable areas. If more glass is needed apply it in the first 24 hours. Let the epoxy harden for at least 24 hours if no more fibreglass is to be applied. You may want to rough up the surface with sandpaper first.
Finishing (only needed for perfectionists!)
After the epoxy is fully cured sand offthe high spots and rough up the surface. Mix up a blend of epoxy and microspheres that is just liquid enough to be brushed on but not so fluid so as to let it run. Brush the filler over the outer surface using sandpaper of decreasing roughness. Sand all the mixture off except what is left inthe hollows. Repeat until you get a smooth surface.
STEP 4: Foam Removal
It is better to get rid of the foam after all the surface preparation is done as it provides a solid base to apply pressure. Now get the solvent out — we use acetone. Remember it is a highly flammable liquid so use with caution. Work over a large bucket to receive the dripping solvent, in an area with excellent ventilation. Pour a small quantity of solvent at a time and let it dissolve the foam. You will be left with a bit of thick paste at the end; don’t worry. For the airbox, we used about a cup of thinner. Next remove the packing tape; if you waxed it, pulling a corner from inside will extract most of the tape and leave a nice clean interior, bringing with it any remaining foam paste.
STEP 5: Final Finishing
Spray with the finish colour you want. Firstly use an automotive primer or something similar. Wet-sand, and repeat this until you are happy with the surface finish. If you make a sports model you can spray with one or two coats of matt black. Silver wheel paint makes for a very nice aluminium look.
• Measure the approximate size of the item to be produced. Buy foam sheets which are just oversized when glued together to minimise waste. • To be more accurate make cardboard templates. Check the mould can actually be fitted to the machine. • Apply your reinforcement (carbon fibres, Kevlar cloth, etc.) in the first layers as you don’t want to sand these off if they protrude through the surface. • Remember that there will be traces of wax inside which will need to be removed before reinforcement or painting. • Sanding may weaken a couple of spots on your final product. You can reinforce those up from the inside by applying patches of cloth soaked with epoxy. • Fibreglass is weaker on sharp corners. Round the foam plug and add more microspheres/epoxy paste to be able to carve/sand the sharp corner in the end.