Love ’em or hate ’em, BMW’s GS behemoths have dominated the ADV market for almost four decades now. Five years ago, the 500,000th GS rolled off the Berlin production line, and we’re betting it won’t be long before the go-anywhere boxer hits the million mark.

We don’t see many GS customs, though. These machines are a triumph of function over form, and most owners like it that way.

Custom BMW R100GS adventure bike
So this R100GS is something of a rarity—and its backstory is rather strange too.

It belongs to Hasselblad Master photographer and bike builder Gregor Halenda, who is best known for an amazing KTM 2-wheel-drive motorcycle he collaborated on for the apparel brand REV’IT!

Custom BMW R100GS adventure bike
In some respects this is a cautionary tale about buying a project bike started by someone else. When the R100 GS arrived in Gregor’s workshop in Portland, Oregon, it didn’t take long for problems to surface.

“While the fabrication was great on the tanks, the overall mechanical condition was horrible,” says Gregor.

Custom BMW R100GS adventure bike
“The bike burned a quart of oil every 100 miles. It was down on power, and it wouldn’t run in the mornings because the petcocks were plumbing parts that let water and debris into the carbs.”

“I don’t know why the engine was in such bad shape, and maybe the previous owner didn’t either.”

Custom BMW R100GS adventure bike
Gregor didn’t complain because he considered this bike to be a test bed for his next build. He’s replaced everything except the three custom tanks—the rear is in a monocoque subframe—the frame, and the exhaust.

When Gregor took the engine apart he realized that a full rebuild was in order. “The heads were shot, the cylinders were shot, the pistons were toast, the rods were fried, the crank was shot and the crank bearings and block were ruined!”

Custom BMW R100GS adventure bike
So Gregor dumped that engine and installed a solid motor from an early 80s BMW R100RS, complete with a big valve conversion and head work by the Portland, Oregon shop Baisley Hi-Performance.

It fits just nicely into the braced frame, raised up and tipped back a little for extra clearance. The GS now has some serious grunt.

Custom BMW R100GS adventure bike
The 304 stainless steel exhaust came with the GS, but Gregor had to modify it by changing brackets and the backpressure. “It’s actually the wrong diameter, so I’ll remake it in the correct diameter.”

The suspension is another big upgrade: Gregor’s installed beefy 48mm WP upside-down forks from a KTM 690R, using KTM 450 SX triples and a custom fabricated steering stem.

Custom BMW R100GS adventure bike
The single disc brakes are Brembo, with a twin-piston floating caliper at the front.

The swingarm is from an R1100 GS—but it’s been cut and sectioned, with the shock mount repositioned, to allow fitment of the biggest possible 18-inch tire.

Custom BMW R100GS adventure bike
The driveshaft is a hybrid of parts from the R1100GS and R100GS, with the final drive coming from an R850R. It’s got 37/11 gearing, the lowest possible.

New rims and billet hubs from Denver-based Woody’s Wheel Works have made a huge difference.

Custom BMW R100GS adventure bike
They are very narrow: 21×1.6 and 18×2.15. “These are true off-road rims,” says Gregor. “BMWs are made for 17-inch rims, and the rear wheel squeezes the huge 140/80-18 Goldentyre GT723R into a very tall, round shape.”

“It rolls over things much better, and the round profile makes it much quicker to turn now.”

Custom BMW R100GS adventure bike
An extreme enduro Goldentyre ‘Fatty’ front complements the giant Rally Raid-style rear. “The traction is mind-bending for a big bike,” says Gregor. “It also has over 11 inches of suspension travel; previously it was nine, so it sits much taller now but with greater stroke.”

Cutting 20 pounds off the rotating weight apparently feels like more than 100 pounds. “The original owner got the GS down from 500 to 428 pounds, and I’ve taken it down to 400—with all that being in the rotating and unsprung weight,” says Gregor.

Custom BMW R100GS adventure bike
Adding to the rider enjoyment are new ProTaper bars in a ‘CR High’ bend, Renthal half waffle grips, and modified Fastway pegs from Pro Moto Billet. Gregor also made up new shifter and brake assemblies using stainless steel and precision needle bearings

The wiring loom on the BMW was neatly done, but it’s now upgraded with a Euro MotoElectric (EME) charging system and ignition.

Custom BMW R100GS adventure bike
There’s also a Trail Tech Voyager Pro instrument—a cutting-edge off-road GPS system with built-in Bluetooth for phone connections, and a 4-inch color touchscreen.

“I feel like the bike is ‘mine’ now,” says Gregor. “I’ve spent more time sorting it than the original owner did building it! I consider it a mule—a test bed for my next bike.”

Custom BMW R100GS adventure bike
“To get the weight I really want to be at—350 pounds—will require a new frame and much lighter bodywork. But the bike works great now, and it’s a blast to ride.

Gregor’s now going to design a new frame from scratch and test out some more motor mods. “I’ve become friends with Walt Siegl and he’s convincing me to do composites for the body and tank.”

Custom BMW R100GS adventure bike
“My long-term goal is to build the next bike into the ultimate custom adventure bike. A couple of people have expressed interest in having me build a series of bikes, like Walt does, but on the BMW platform.”

“Custom adventure bikes are the next big thing. I’ve raced, rallied and adventured, and I know what works. My bikes are function and form together: never at the expense of one or the other.”

Amen to that. And we can’t wait to see what this formidable GS turns into next.

Gregor Halenda Instagram

Custom BMW R100GS adventure bike

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