When high-mileage Dutch rider Willem Heijboer decided he wanted a sportier special version of the BMW K 1600 GT two-wheeled land yacht that he’d been criss-crossing Europe on for his work as CEO of his own safety signage company, there was only one person he could turn to – the Netherlands’ master baker of frame design, Nico Bakker.

Bakker is the couturier of chassis-builders, the welding-torch wizard who, when you want a high-class piece of hardware to wrap round a motor to create a unique bike that handles well and looks the business, is the bloke for the job.

Not many people can rival Nico’s experience when it comes to building a better BMW. Apart from marketing the first aluminium Deltabox chassis for the old-style airhead Boxer motor, Nico worked as a consultant for the German manufacturer in developing its R 1100 RS’s distinctive Telelever front end.

He has more first-hand knowledge than anyone outside BMW’s R&D department on how to make a modern flat-twin BMW handle properly, and it was under the Bakker badge that the first outright sportsbike using the then-new air/oil-cooled 8-valve Boxer motor was launched for street use, the Bakker BMW Bomber. But until recently, he’d never applied his trade around an in-line multi-cylinder BMW motor.

“I bought a K 1600 GT for my work, and I was very satisfied with the engine, which sounds lovely and is so satisfying to ride,” says Willem, 61.

But with his business expanding and his kids grown up, at the age of 48 Willem returned to riding a succession of R 1200 GS BMWs to visit his clients all over Europe, from London to Milan, Stockholm to Paris.

“I much prefer riding bikes for long distances than going in a car,” he says. “If I’m just going 20km to see someone, I always take the car because it’s more convenient, without having to pull out all my motorcycle clothing and put it on.

“But to ride from where I live near The Hague to see a client in Berlin 700km away, I always go by bike. It’s much quicker, especially with all the road works on the Autobahns nowadays, and anyway, it’s more enjoyable. So my BMW dealer suggested I buy a K-16, and I did. But when I took it back to him for the first service, he saw that I’d scraped the engine cases on both sides – so that’s when he told me to go and see Nico.”

Bakker remembers his initial reaction quite clearly. “‘Are you really sure that’s what you want?!’ But after talking with him, I realised he knew what he was doing, especially already owning an R 1200 GS as well – so I told him that it wouldn’t be an easy job, nor would it be cheap – but I’d give it a try!”

To create what he would eventually dub the Mammoth, Willem Heijboer purchased a brand new 2018-model K 1600 GT from his local BMW dealer, which meant that it was one of the slightly revamped versions of that model which the German manufacturer had introduced the previous year, with new catalytic converters and updated engine management. This allowed the 1649cc 24-valve DOHC six-cylinder engine to be Euro-4 compliant without any loss in claimed output, namely 118kW (160hp) at 7750 rpm, with 175 Nm of torque peaking at 5250rpm – 70 percent of which is available as low as 1500rpm.

It came with a ride-by-wire digital throttle offering three riding modes – Road, Dynamic and Rain – and also featured Dynamic ESA as standard, an active suspension system that automatically alters damping electronically in two modes, Road and Dynamic.

Once Nico got his hands on it, he stripped it down and retained the engine, shaft final-drive transmission, the trio of 320mm brake discs with Brembo calipers and Bosch ABS and the electric and electronic systems, including the ESA function and the shock to go with it.

Bakker then began designing the frame, which he opted to make in chrome-moly steel tubing as the best way to achieve a satisfactory stiffness to weight ratio.

“I wanted to make the bike lighter and easier to handle than the stock K 1600, as well as of course to fix Willem’s ground clearance problems with the engine,” he says. “To be fair though, the motor is very narrow for an in-line six – it’s just the way it was mounted in the standard bike which needed
to be changed.”

In fact, it’s the narrowest in-line six-cylinder engine yet made for motorcycle use. The 24-valve semi dry-sump unit with chain-driven DOHC which, despite its short-stroke dimensions of 72 x 67.5mm, is just 555mm wide, so only slightly wider than most in-line fours. It weighs just 102.6kg complete with 10-plate oil-bath clutch, six-speed gearbox, alternator and 52mm throttle body, all
of which are retained on the Mammoth, though with an altered airbox to make more space for a larger fuel tank.

In designing the Mammoth’s chassis, compared to the stock K-16, Bakker slightly lengthened the wheelbase to 1635mm (from 1618mm), but substantially sharpened the steering geometry. He’s ditched the stock BMW’s Telelever front end in favour of a more conventional but fully-adjustable 48mm WP upside down fork modified to the Mammoth’s specific needs by Dutch specialists HK Suspension, who also adapted the ESA rear end to suit its new application. Rake is now a snappier 25º with 100mm of trail, compared to the stock K-16’s 27.8º/106.4mm settings.

To adapt the R 1200 GS design ethos to the K-16, Bakker opted for a 19-inch front wheel matched to a 17-inch rear, and because Heijboer likes the look of wire wheels, obtained an Italian-made set from Kineo with forged aluminium rims and stainless steel spokes, which permit tubeless tyres to be fitted. These are a set of Pirelli Scorpion Trail IIs with the 120/70R19 front mounted on a three-inch rim, and a 190/55ZR17 on the six-inch rear, which had to be specially made so as to incorporate the housing for the shaft final-drive.

“It took them a while to do that, else it would have taken us just six months from start to finish to build the bike,” says Nico.

But the big difference is in weight, with the Bakker K 1600 GS scaling 310kg, split 48/52 rearwards with all fluids, and the massive specially-made 39-litre fuel tank fully fuelled.

“I don’t like to stop on a long journey until I have to,” says Willem – which presumably is more often for personal fluid exchanges than to refuel the bike? That’s versus 319kg for the stock K 1600 GT, with a 90 percent fuel load in its smaller 24-litre tank. Taking the commonly accepted stat of a litre of fuel weighing 0.77kg, that works out to a half-dry weight of 280kg with oil/water but no fuel for the Bakker GS, against 299kg for the K 1600 GT – both without luggage. That’s a lot!

In fact, luggage-wise the Mammoth carries two left-side panniers from the R1200GS Boxer twin (so, both are the full-size one, with no restriction caused by the right-mounted exhaust), and there’s also a rack for mounting a top box.

“When I’m travelling I always have my computer and all my work stuff like samples with me, and sometimes a suit and tie I must wear,” says Willem.

“I make sure I change somewhere just before arriving, so then I’m all official when I get there, and I have everything I need to take inside to the meeting in the top box. I can put my helmet in the pannier and park my bike in front of the office entrance.”

For convenience, both a centrestand and a side stand are provided.

The radiator was sourced from an Aprilia RSV1000 V-twin, with an overflow bottle on the right side of the bike, which looks like it belongs there. But the stainless-steel Akrapovic 6-2 exhaust with twin carbon-wrap silencers was specially made for the Mammoth, and satisfies Heijboer’s desire for a “little bit better a sound than the stock BMW – not noisy, but just a little more exciting with a bit more performance – it looks much better, but it’s the same weight.”

The original ECU has been modified to optimise engine mapping with the revised exhaust, and also to finesse the adaptation of the ESA to the Bakker-built bike. And he did all this without having BMW’s secret access codes to its various electronic systems.

“On this bike there’s a lot of electronic programmes, which all work together in unison,” says Nico.

“So you cannot remove one part to patch in a replacement, but must modify the entire system.”

It’s a sign of a contented customer that upon taking delivery of the Mammoth in April 2018, Willem Heijboer covered no less than 17,000km in the next 14 months – and that doesn’t include the four winter months from November to March when he took it off the road, opting for his R 1200 GS on salt and ice instead.

“I ride all year ’round, except when it snows, and I do about 25,000km each year,” says Willem matter-of-factly. “The Mammoth completely exceeded my best expectations – although it’s a heavy bike, the smoothness and the easy way of riding that it provides as stock have now been equalled by the handling. I took it to a track day at Assen last June, and I was amazed at how well it handles. And I didn’t touch the engine down any more – just the footrests!”

It’s quite common at European track days to see a well-ridden BMW R1200 GS putting the latest and greatest sportsbikes to shame – but still, I wonder how many riders Willem shared the track with realised they’d just been stuffed by a six-cylinder adventure tourer!

Willem generously insisted I took the bike away for a couple of hours to see for myself how well it lived up to his expectations – it would have been nice to make it longer, but he had a 600km ride ahead of him the next morning!

At rest it’s an undeniably imposing-looking bike with real presence to be genuinely comfy, and after climbing aboard and settling into the ultra-comfortable seat sourced from a R1200 GS, I found myself sitting quite upright, but well protected by the screen and its subframe mount, both also sourced from the GS, the same as the headlamps and the wide handlebar (complete with mittens). That subframe is a useful place for Willem to mount his GPS, but it’s also the source of his only criticism of the Mammoth.

“The screen flexes a little when I’m cruising at 180-200 km/h in Germany,” he says. “It gets a bit irritating, so I must try to fix it!”

While I didn’t run into any ground clearance problems on the Bakker GS beyond scraping the footrests either side, those are actually quite low, so the riding position is pretty spacious for a 180cm rider like myself, and would be great on long journeys – Willem measures 187cm tall, so it’s aimed at making him comfortable for the longer haul. But the 820mm seat height only allowed me to tiptoe both feet on the ground at rest, since the bike is inevitably quite wide between your knees, to add crucial space for fuel.

But despite the long wheelbase the Mammoth proved surprisingly adept in tight spaces – it felt very balanced riding it at low speeds in towns, thanks also to the light-action clutch and beautifully mapped throttle in Road and Rain modes (it didn’t, but I had to try it out!). Dynamic mode is what it says on the label, and to be honest I didn’t get the chance on Dutch country roads to put that to the test, but it certainly delivered added zest to the acceleration.

But the engine is so silky and flexible that you can let it run as low as 1500rpm in top gear, which means 40km/h on the TFT dash, then crack the throttle wide open, and it powers seamlessly towards the 8500rpm revlimiter without hesitation.

With its nominal off-road capability, which would make it at home on harder unsealed surfaces, this is a 250km/h-plus adventure tourer, so if BMW’s R 1200 GS is a two-wheeled Range Rover, the Bakker K1600 GS is the bike equivalent of a Lamborghini Urus!

The aria that issues from the twin Akrapovic carbon-wrap silencers is melodic and musical, with the Bakker GS’s exhaust note resembling a sotto voce version of the wondrous wail produced by BMW’s trademark six-cylinder Touring car racers. It starts singing as soon as you thumb the starter button, and peals into life then settles to a languid 950rpm idle.

But that aria’s not the most captivating thing about this motorcycle – it’s the effortless yet precise way in which it steers in low to medium speed turns. It’s rock solid on 120km/h sweepers, but in turns half that fast, it’s improbably nimble for such a big, long bike. It doesn’t exactly flick from side to side along a winding country road, but with minimal rider effort it changes direction in a very assured and confidence-inspiring way. The chosen geometry just tips the bike into turns controllably and relatively easily, without the rider raising a sweat, or with ground clearance ever becoming a problem.

At slow speeds the physically-massive motorcycle is cumbersome but well-balanced, with the clutch and mapping allowing U-turns or low-speed manoeuvres in town to be made with confidence, if some care.

And the triple 320mm disc brakes are undeniably effective, with the twin front four-piston calipers linked to the single twin-pot rear via the adjustable handlebar lever, with extra stopping power available from the rear foot pedal, and with BMW’s proven ABS fitted as standard.

You soon forget about the wide engine, with its oil tank incorporated in the rear of the crankcase casting to deliver a low centre of gravity for the whole bike. That’s partly because you can’t see it from the hot seat, but also because it seemingly has no effect on the K 1600 GS’s handling.

Looking at the bike from the side I suspect one reason for that is the way the Nico Bakker has carefully positioned the crankshaft of the slant-block engine with its cylinders canted forward at 55º in almost the exact epicentre of the motorcycle, halfway along the wheelbase. This means the EVO K-16’s centre of mass is close to its cee of gee, resulting in more neutral handling and together with Nico’s sharper steering geometry, the greater agility you immediately notice riding the bike at almost any sort of speed. Smooth, seductive, sophisticated and supremely swift, and with apologies to Kellogg’s, the Bakker BMW K1600 GS is special K on two wheels.

It’s a special motorcycle that you can’t help but think BMW should be making itself. Well – it wouldn’t be the first time that Nico Bakker has done some product development work for them.

But assuming they won’t do so, how much would Nico Bakker charge to construct a twin of Willem Heijboer’s transcontinental express?

“We can do it again, that’s not a problem,” says Nico. “Figure twice the price of a standard K 1600 GT to take a new one of those and transform it into a GS. But it takes time – we need six months to do it from the time the order is placed.”

Any takers?

Test Alan Cathcart     Photography Kel Edge