EXHAUST HYDROFORMING
Pip Higham makes his own SDR200 exhaust!
When I bought my SDR200 it was a bit of a mess. In fact it looked like it had been upside down in a pond in Higashikagura for a few years, which was a bit of a shame. The SDR was only sold in Japan for a short time, about two years max, but it has quite a cult following. It is lean and funky but as I took it apart I was a bit taken aback by the weight of the exhaust system. The bike weighed about 115kg, but the exhaust made up 6.5kg of that! Something had to be done! I’d seen hydroformed systems a few years ago at my pal Dave Lord’s place in Farnworth; he had made a few for two-stroke grass-track bikes back then so where better than Dave’s to give me a leg up. Slight problem, he had shut up shop when the foot and mouth epidemic pretty much killed his business stone dead in 2001, after that he had been making titanium wheelchairs for disabled sports men and women. After a bit of searching I found Dave again in Bolton where he had once again set up as Trak Plus building beautifully fabricated components, mainly again for grass-track and speedway racers. Dave gave me a few pointers and then it was up to me and my greatly inferior ability to try to create the rather tortuous shape as seen in the pictures. The concept is quite simple, take two identical slender sheets of mild steel, weld them around the edges and then, by pressurising them with water the void will deform to an extent that stretches the material into the desired shape. The trick is to take the original three dimensional component and convert it into a two dimensional pattern. This takes a bit of imagination, the concept might not be immediately easy to deal with but bear with me. You must imagine several points at which the pipe changes direction; these are where cuts will occur, these cuts will
be exactly perpendicular to the centre-line of the pattern. Taking each section in isolation, mark these with a chalk line, the diameters at the start and finish will give us the width of our pattern with a small dose of simple maths: Diameter at point A = 60mm, therefore the circumference is pi x 60, i.e. 188mm. As we are using two pieces of material to form the circumference we need half of this dimension, (i.e. 94mm). For a straight section of pipe with different diameters at each end (i.e. a cone) the pattern will be straight with the calculated dimensions (as above) at each end of the cone. Where the pipe takes on
a sweeping bend this is best created by using a piece of coat hanger wire to mimic the centre-line of this bend. Cut and form this wire into the desired shape and then lay it on the pattern picking up the centre-line of the previous section. The centre-line is always at the centre of the component and the start of one section is dimensionally identical to its neighbour so it stands to reason that each section follows on smoothly (we hope) from the adjoining section. The quest is to create a shape made up of a series of individual sections, which, when pressurised (I’m refraining from saying ‘blown up’ here for obvious reasons) makes
the three-dimensional shape we want. By using water to pressurise the shape the element of any danger of explosion is negated; it simply can’t happen. If a weld fails there will simply be a leak akin to a water pistol and if this were to happen the pressure would immediately drop. In the case of a boiler or a vessel pressurised with gas, air or steam the story is rather different and very dangerous, we don’t use air or gas for this very reason: water safe, gas not safe! When it’s been pressurised it will create a shape. To effectively pressurise our pattern the two components need to be welded together. This takes time and care, the
best way to achieve a good joint is to tweak the edges of both patterns with a tool as shown above. This is just a slender slot sawn into a fairly hefty piece of mild steel. With this, it’s quite easy to form a regular flange round the edge of the patterns. Oh and don’t bend them both in the same direction! We’re keen to have a top and bottom, forming a space between, by doing this the welded edge has a head start and is much less likely to split when pressure is applied. The patterns need to be meticulously seam welded, leaving enough space at one end to weld on a small steel threaded fitting, this will require a screwed thread compatible with the outlet from the pump used to pressurise the system. I borrowed a pump used to test pressure vessels from a friend of mine. On the page opposite you can see the pressure gauge reading around 50 Bar or 700 psi! I pumped my creation up to this pressure to illustrate the inherent strength of a couple of pieces of 1mm sheet steel. In practice there’s no need to use anything like this pressure, 100 psi will generally suffice. It’s a good move to pressurise up to 30 or 40 psi a couple of times and then relieve the pressure, preferably with the pump above the big squiggly metal worm. This will help to bleed out any air bubbles as air is bad! As the pressure is gradually increased it will help to get a couple of hitting sticks to help the process along, a panel beating hammer is great for external curves but these will generally push out quite freely. The troublesome areas are the internal curves and great care will be needed here to eliminate kinks and creases, I used a couple of steel bars of different diameters to tap and persuade. It’s around this time that the quality of the welding will become apparent! There will be leaks: remove the fill pipe, drain off thoroughly and weld up any cracks or splits, and repeat. If you’re serious about giving it a go it might be preferable to pick one section of the pipe in question and make it in isolation. As you’ll be welding the pieces together it matters not if it is made as one piece, or a series of smaller parts, the principle is identical. With the shape of the parts resembling the pattern, albeit in two dimensions, it’s time to cut it into pieces! Consider the implications of each cut before firing up the hacksaw. Corrections made by grinding either side have consequences! Grinding the end which has an expanding cone will increase the size of the aperture, conversely the contracting side will decrease in size; it’s possible to correct such discrepancy (so long as it’s minimal) by carefully tapping the smaller opening around a dolly. This will have the effect of stretching the material slightly which may well correct any discrepancy in the diameters, normal rules of engagement apply: measure twice, cut once! Assembly is best done on the bike. Remove the fuel tank, disconnect the battery and wherever possible disconnect any potentially tender electronic components. Now make up the first short section of pipe with its fitting to the cylinder and bolt it into place. You may
decide to use an original flange and header pipe, make sure it’s rot free and in good shape, from here you may need to think in a few directions at the same time, position the component parts on the bike and, starting at the front end, tack each subsequent section into place. I know I’ve made this bit sound easy, and it really isn’t, but given patience and a bit of lateral thought the jigsaw will come together. It’s advisable to be 100% happy with each section in turn as any ‘retro tweaking’, even by tiny amounts, will have a considerable effect on the bits further back. Be warned. There are several different welding techniques, any of which will work, but my preference would be for either gas welding (old school but lovely) or TIG (new school, tricky to weld thin sections but rather pretty when done properly). In any event make certain there are plenty of tacks in place before the main course as otherwise distortion may well occur. After I’d completed the fabrication work and added the required flexible mountings, I thoroughly cleaned the pipe and took it to my local friendly nickel plater, Dave. I think it looks quite nice. When I finally get round to spinning it up I’ll dyno it and see how it compares to the OE system, that should be fun!