Make your own … Air Fil­ter

Classic Trial - - WORKSHOP AIR FILTER - Words: Matt Hep­ple­ston • Fab­ri­ca­tion: Yvan Marszalek Pic­tures: Yvan and Matt

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.

Five steps

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?

Safety Tips

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.

Top Tips

• 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.

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