Claimed Power: 57hp @ 7,700rpm (stock)
Top speed: 121mph (period test)
Engine: 864cc air-cooled OHC 90-degree L-twin with 900SS pistons and twin-spark cylinder heads, 86mm x 74.4mm bore and stroke, 9:1 compression ratio
Weight (w/half tank fuel): 436lb (198kg)
It took more than a second look to register the fact that the bright orange bike standing out from a gaggle of modern Italian motorcycles was actually a Ducati and not a Harley. But it only took a split second to appreciate that this bike was something very special.
Intrigued, I hung around and was lucky enough to meet Peter Koren, the bike’s owner. And when he started to describe how he built the bike himself, from scratch, I was left more than a little impressed. While the Harley-Davidson XR dirt tracker is a cult machine in its own right, favored by the likes of the late, great Evel Knievel and racer Cal Rayborn, Peter’s audacious and unexpected Ducati-based take on the XR works beautifully. You’d think it was a factory machine — but from Bologna or Milwaukee?
Transforming the Ducati 900GTS
“It was an abandoned project based on a 1979 900GTS I’d bought for spares for my 750GT and 900SS,” Peter explains. “But though covered in rust and missing parts, it was, on closer inspection, too good to break. I thought of building a Z-stripe Sport utilizing some parts I already had, but then it sat in the shed for two years while the idea of a flat tracker-style bike came into my head.”
Peter’s styling cue for the flat tracker might have come from the prettiest of them all, but he knew straight from the beginning that there was no point buying up loads of XR parts to make his bike. “A correct XR tank would never fit the frame,” Peter says. “I needed brakes, so I couldn’t run with a spool hub. I also had a fixed, low budget, as it was a bitsa. So as a starting point I bought a replica XR seat base and bars and began from there. I wanted it to look as if it had come out of the Ducati factory.”
Peter’s a little reticent about his many skills, but to build what was in his head, he just got on with it, he says. “I knew I’d have to make the tank myself, to enlarge the whole thing by 10 percent, so I made a wooden buck, got a sheet of aluminum, stole a croquet mallet from my kid and started bashing. I also realized I had to learn to gas weld aluminum to join up the bits — a two-piece base, two sides, one top and the tunnel — that was tricky!” Like the tank, Peter also made the side panels from 2mm sheet aluminum, in a similar manner. “The XR doesn’t have side panels as such, so I looked at the shape of the seat unit and the completed tank, and also took Ducati inspiration from my SS and GT side panels. I also added air scoops to break up what would have been large slabs of bodywork. I fabricated a steel seat pan, which was upholstered beautifully by a local fellow, Earl.”
The Ducati’s frame needed some serious adjustment, so Peter got to it with the grinder and oxyacetylene torch. “First of all, I constructed a jig to fix the frame down properly. I wanted to get rid of the ugly kinks that the rear part of the GTS frame has, because I predicted they would push out the high-level exhaust system I had in mind.” Peter says. “So I cut them out and added a gusset to mount the side panels to. The frame still didn’t look right on the mock-ups, and I realized the rake was just wrong, so I steepened it from 31 to 26 degrees. Twenty-six is fine, the bike steers really nicely. I cut through all the tubes, rewelded, fettled and filed, having set the angle on the jig, but in the end using the oxyacetylene was tough, and not wanting to risk distorting the frame, I had Exactweld in East Grinstead, England, complete the work and they did an excellent job.”
Undaunted, Peter sorted out the rest of the frame. “The swingarm is as original, but I had to add various brackets to the rest of the frame for the tank, coils, ignition, regulator and other parts. I fabricated lugs at the rear of the frame to accommodate the seat, and also made a rear sub-frame to support the seat base and rear light. I wanted to do the job properly — I hate stuff that when you scratch under the surface it all turns to crap,” Peter says.
For most of us lesser mortals, just the thought of cutting and shutting a Ducati frame would bring us out into a cold sweat, but Peter’s persistence continued throughout the rest of the bike build, and his eye for detail — and lines and proportions that just look “right” — extends to the cycle parts, too.
“I really enjoyed making the exhaust,” Peter says. “The tighter bend under the front cylinder came from a grab handle such as you find in a public bathroom — no, I didn’t steal one! — and the cheap Far-Eastern silencers have Imola-style cut-outs for the rear shock and sit nice and tight to the frame. The footrests are the original Ducati GTS900 items, adapted and rear-set to suit my height. I made the gear lever and rear brake lever, with brake cables that pass under the seat. I also fabricated, drilled and curved the silencer grilles, using an old Conti to get the form right. The forks are the original 38mm Marzocchis, reversed with calipers behind the fork leg, which also gives a cleaner look. The rear shocks are YSS, and the wheels are Akront 2.15 x 18s, with Bridgestone Trail Wing tires. The handlebars are from Wakula Racing. The clocks are from China, and look OK and do the job. The headlight is a Bates-type thing I would like to change at some point. I had originally planned for no lights, but an accident on my other Ducati changed my mind. I rebuilt the engine myself — luckily it already had a good crank and bearings, though it has longer 750GT rods, hence the plates beneath the barrels.”
Peter also did the painting (“My God, do I hate painting,” he says) in Harley Racing Orange with clear lacquer over clever decals designed with CAD, drawn on paper, then cut out on vinyl.
Future plans for the Ducati XR900? “Better rear shocks,” Peter says. “I’d like to change the headlamp because I don’t much like the lip on it. And I’d possibly like to carry out a Desmo conversion, too.”
Kicking the Duke over and with an easy start, Peter makes some passes for the camera. His creation is fast, skinny, essential and beautiful, and makes the kind of sound a proper bike should.
“I love the rawness of it, and the way it fits me,” Peter says. “It’s so slim and light compared to the other bikes I’ve owned, though at 436 pounds wet, it weighs a little more than I thought,” he says.
“But I’m very happy — I’ve made things all my life, always admired the Harley XR look, so I just had to scratch the itch with what I had at my disposal!” Lucky for us, that’s exactly what he did. MC