The results have been named the XB12S Lightning and XB12R Firebolt and it was the XB12S streetfighter that motoring.co.za was given for evaluation.

The throttle bodies on the dynamic digital electronic fuel-injection system have been enlarged from 45 to 49mm diameter to provide the extra input; peak power is up from 69kW at 7200rpm to 76.8kW at 6600. While this bit of inspired tinkering can hardly be said to make the motor rev harder, it's undeniably more muscular with peak torque up from 92Nm to a much more righteous 109.7 at 6000rpm.

That's enough to pull the naked XB12S Lightning up to a genuine 221km/h, right on the power peak; on one flat-out run it just kissed the rev-limiter in top gear at 6700rpm. That makes it the fastest Harley-engined machine I've ridden; the half-faired XB12R with its narrower bars, smaller frontal area and better aerodynamics could be even quicker.

It is, however, understandably thirsty when pushed hard; over the review period, which included some very hard riding as well as a week's commuting, the Lightning averaged 6.95 litres/100km.

The rest of the drive train is unchanged from the XB9, including the unburstable, large-diameter clutch with its slightly over-centre action and the slick, quick but vocal five-speed gearbox. The Kevlar-reinforced Gates final drive toothed belt is tensioned by a huge adjustable guide-wheel under its lower run.

Heavy vibes

The long-stroke motor does, however, bring with it one problem that afflicted the previous X1 Lightning models – excessive vibration. Despite the Uniplanar power train vibration isolation system, an arrangement of rubber bushings, rose joints and stay rods that lets the motor flap around in the frame like a landed fish, the XB12 vibrates badly enough at idle to unfocus my eyes – and I'm used to vibratious motorcycles; I grew up on a three-cylinder.

The shakes smooth out between two and four-five, becoming merely annoying, but above 5000rpm the feedback becomes harsh and the motor begins to feel a little stressed. 4800 revs, however, corresponds to a rather naughty 140km/h cruise, so it's never more than a short-term problem.

I can't help feeling that the Lightning vibrates worse than the Harley-Davidson 1200 Sportster, in which what is essentially the same motor is solidly mounted. If this bike were mine I would try replacing the rubber bushes with solid high-impact nylon; I've done that before on a big V-twin with surprisingly pleasant results.

The XB9R Firebolt I rode in November 2002 had abbreviated clip-on handlebars and handled like a GP bike; the Lightning variant has high, wide streetfighter bars, ideal for the short wheelbase, and monstrous torque to pull superbly controllable wheelies.

They do, however, make the bike very sensitive to rider input; the seating position is short-coupled and very upright and even the rider's weight affects the handling.

Buell acknowledges this by including in the owner's manual a two-page tabulation of median suspension settings against rider mass; the bike is fitted with superb, fully-adjustable Showa upside-downies in front with a similarly tuneable monoshock from the same maker to control the back wheel.

Personalised suspension

I spent a few minutes adjusting the settings at both ends for my 106kg. The bike immediately became more stable and – more importantly - more predictable with far less desire to shake its head on long fast sweeps and quick upshifts. Incidentally, there's noticeably less wiggle on changing up if you do so without using the clutch, a clear indication of too much handlebar leverage.

The streetfighter layout also causes an odd handling quirk; although the bike turns in like a terrier after a rabbit – anything with a 21° steering head angle is going to change direction as fast as a politician – it takes surprisingly high effort on the inside bar to initiate a turn.

This is because, although the main masses of rider and motor are concentrated in the middle of the bike, the centre of effort is high off the ground. The big lumps have to move through a surprisingly long arc to attain a given angle of lean; if you don't believe me, get a mate to hold the bike and measure it for yourself; the steering on a cruiserfeels a lot lighter because all the hefty bits are that much closer to the road.

On the top end runs I draped myself over the tank to take whatever advantage I could from the minuscule fly screen and held on to the bars with just my fingertips – an old trick to reduce rider input on naked bikes. In that configuration the bike was rock steady right up to terminal velocity, except for a slight waver going through 210km/h.

The radical chassis remains essentially unchanged from the smaller-engined version, with no painted body components; all the red stuff you see in the photos is actually colour-impregnated polycarbonate and thus practically scratchproof.

Also retained from the XB9 is the amazing 375mm rim-mounted floating disc front brake with a six-pot, opposed piston calliper; what it lacks in instant bite it makes up for in controllable, easily modulated power. Put that together with the short wheelbase and you've got a recipe for a wild stoppie machine.

If all this sounds like I'm describing a custom-built stunt bike you're not far wrong, given the Lightning's cobby chassis and accurate controls.

One point Buell has picked up on, from the earlier X1 Lightning, is the instrument mountings. The clocks on that bike, and the little fly screen in front of them, thrashed around more than 25mm at idle and were vulnerable to damaging themselves as a result.

The very neat instrument pod on the XB12, which places the rev-counter in the centre with the speedo under the same glass on its left and all the warning lights on the right, is firmly mounted, as is the shield; neither could be seen to move at any time and the dials remained clearly legible at all speeds.

Set for my weight, the suspension was firm but not harsh, reacting well to small bumps although some of our test route's potholes rattled my fillings; the back end felt firmer than the front (although that's subjective – it's difficult to quantify) but it kept both wheels under control under all but the worst conditions.

I managed to induce some rear-wheel patter at lowish speeds over the bumpiest stretches.

Hooligan tooling

Despite the effort involved at the bars the bike is superbly flickable and, once the turn has been initiated, goes down on its ear like a motocross bike. The crankcases' bulk is well off the ground and the pegs tucked in – only the very loony will touch down something on this machine.

It repays being ridden deep into a corner and turned in hard, flat-tracker style, rather than the classic "turn in early and keep it smooth" wisdom of the 1950's GP riders – giving you the opportunity to out-brake the riders of some supposedly sportier machinery and embarrass them on your favourite twisties – even though they will flash past you halfway down the next straight.

The tighter the corners the bigger your advantage on this, Buell's answer to the Ducati Monster. The accurate fuel injection, while a little snatchy at small throttle openings, makes it a surprisingly good commuter. Around town, which is this bike's intended habitat, its thundering mid-range acceleration and hair-trigger steering will get you ahead of the pack without slipping the clutch or lighting up the rear tyre.

This chassis was designed as a sports bike and in some ways the Firebolt is a better package than the Lightning; what they share, however, is the radical engineering that makes it possible to put a 1957-vintage V-twin motor into a 1320mm wheelbase and make it work, as well as the impressive build quality and attention to detail that lifts Buells for ever out of the kit-bike category.

• Test bike from Cape Town Harley-Davidson.
  
  Specifications:
  
  Motor: Air-cooled four-stroke 45-degree V-twin.
  Capacity: 1203cc.
  Bore x stroke: 88.9 x 96.8mm
  Valvegear: Pushrods with two overhead valves per cylinder.
  Compression ratio: 10:1.
  Power: 76.8kW at 6600rpm.
  Torque: 109.7Nm at 6000rpm.
  Induction: Dynamic digital electronic fuel-injection with 49mm throttle bodies.
  Ignition: Eelectronic.
  Starting: Electric.
  Clutch: Multiplate wet clutch.
  Transmission: Five-speed, constant-mesh gearbox with belt final drive.
  Suspension: Showa inverted forks adjustable for preload, compression and rebound damping at front, Showa monoshock adjustable for preload, compression and rebound damping at rear.
  Brakes: 375mm rim-mounted floating disc with six-pot opposed piston calliper at front, single 230mm disc with single piston floating calliper at rear.
  Tyres: Front: 120/70-ZR17 tubeless. Rear: 180/55-ZR17 tubeless.
  Wheelbase: 1320mm.
  Seat height: 775mm.
  Dry weight: 175kg.
  Fuel capacity: 14 litres.