From Purpose Built Moto: The Ultimate Adventure Sportster Scrambler…
Back in 2019-2020, Tom Gilroy of Australia’s Purpose Built Moto faced the challenge of building a custom scrambler capable of handling one very strenuous cross-country test ride — a journey across Tasmania that would include a staggering array of weather conditions, road surfaces, and terrain.
What’s more, if the machine wasn’t up to the task, it would be obvious not just to several of Tom’s friends and fellow fabricators who’d hand-built their own bikes for the trip…but to audiences all over the world, as the ride was being filmed for Wide of the Mark.
“Enter our signature series Harley Sportster, built to do everything. Long range touring, mountain Twisties, off road logging tracks, tight single track trails, beach riding, and the occasional jump when the mood strikes. Sounds fun? Sounds like Wide of the Mark. Our latest film project…”
The idea for the film was that six of Australia’s top customizers would build bikes for a two-week trip across Australia’s largest island, Tasmania, which boasts an incredibly diverse geography, including temperate rainforest, mountainous highlands, moorlands, buttongrass plains, pine and cypress forests, and extensive beaches.
As if building a bike to survive such a journey wasn’t challenging enough, Tom decided to build up a 2006 Harley-Davidson Sportster 1200C, transforming the heavy factory cruiser into a two-wheeled all-terrain adventure vehicle.
To start, Tom focused on lighter weight and beefier suspension. He swapped the original front end with the forks and double-disc brakes from a 2015 Triumph Tiger adventure bike, matched with a 19-inch front wheel, two-stage fender, and custom fork guards with low-beam lights.
“The high fender has been hand built from sheet aluminium and the lower re-purposed and mounted from a spare fender I had at the shop. These worked a treat with minimal rocks or spray coming off the front wheel. They look pretty trick as well I think.”
The front headlight grille / number plate houses Stedi LED lighting, while ISR levers make it much easier to work the friction zone in tight off-road terrain.
Out back, the Sportster is running a lighter weight aluminum swingarm that adds 35mm of height, paired with 14.5″ K-Tech rear shocks. This adventure Sportster boasts 210 mm of ground clearance fully loaded — equivalent to BMW and Triumph adventure bikes, says Tom. A 17×5.5 alloy rim was laced up to the stock Sportster rear hub, with both wheels wrapped in Mitas E-10 rubber. Tom says he was impressed with the performance of these tires, both on- and off-road:
“Grippy in soft off road conditions and not noisy or lumpy on road, the only problem I had was in soft sand where the knobby tread pattern and weight of the sportster would simply dig the rear wheel a trench. That’s to be expected when you build a dirt bike out of a lump of American highway pig though.”
The original belt drive was converted to a standard chain, with Motoproducts.com.au supplying the sprockets: 22T front, 52T rear.
“That will get me a comfortable 100km/hr [62 mph], and a 1st gear speed of 25-40ks off road [15-25 mph], maybe a little to high still, but on an adventure tourer that’s about the best middle ground you could ask for.”
Tom opted for a tracker-style tail with a custom exo-skeleton for mounting a sissy bar, a Kreiga waterproof adventure side bag for tools / spares, and tie-down points for a larger backpack. The saddle, covered in black nubuck, detaches for access to a storage space sized for a rain cover or small tent.
Tom says the choice of tank was both aesthetic and functional, giving a range of about 330kms (200 miles).
The pegs were custom-mounted just forward of the swingarm pivot, allowing for stand-up riding. What’s more, mounting the pegs symmetrically left a little extra room on the chain side, which Tom used for one of the trickest, most functional parts of the build:
“Inside the peg mount basket I was able to fit up an onboard air compressor you can see in the small green canvas bag. Come off the beach with 18 PSI in the tyres, leave the bike running and hit the compressor and you can ride away on bitumen at 30PSI no worries. It came in handy more than a few times during our trip and is always readily accessible.”
The internals of the low-mileage engine were left alone, though Tom mounted a custom 2:1 exhaust and Lectron carb.
“Once the fabrication and finish welding was complete, the entire exhaust was bead blasted, cleaned, and heat treated to gain a consistent straw gold colour in a matte finish. This is a first on any bike that I know of and the risk definitely paid off. One of my favourite features on the bike.”
In stock trim, Sportster air cleaners hang off the right side of the bike, completely exposed to dirt, dust, and weather — a no-no for an adventure bike. Tom decided he needed an airbox, and found the perfect spot where the relocated ignition coil used to sit just behind the neck of the frame.
“Well protected from dust, mud, water crossings, and rain. I had a filter made by DNA Performance filters, built a protective airbox from sheet metal and clear Perspex, and linked it to the Lectron carb via a stainless steel manifold.”
Tom says the design does inhibit airflow slightly, but the bike still managed 64 hp on the dyno — and the airbox proved itself time and again on the trip:
“While everyone was suffering from clogged pod filters or water ingress, my Sporty only needed one clean the entire two-week trip and never let the bike choke in deep water crossings or heavy rain.”
Tom finished up the build with a custom aluminum bash plate and mounted his waterproof Kreiga side bag, ensuring it didn’t interfere while riding sitting down or standing up.
“The finished product is a blend of styles, perfectly suited to what I think a Sportster scrambler should look like. Tracker styling, but with some completely functional mods that make it a manageable, and enjoyable ride off road. Managing to strip 30 kg off the bike leaving it sitting at 225 kg wet, putting out 65 hp and with 180mm front wheel travel.”
Tom says the bike proved its mettle on the Wide of the Mark trip, handling all kinds of terrain with surprising grace. The only place it struggled was deep sand, though Tom admits that may come down to rider skill as much as machine performance.
Tom encourages you to watch the full build series of this bike on Youtube, and we look forward to showcasing its successor build in the next week or so.
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