“Martens Motard XSM750” Yamaha XS650 Supermoto

Yamaha XS650 Supermoto

A truly “Handmade Motard” built for the 2018 Dirtbag Challenge…

The Dirtbag Challenge, born in San Francisco, is a self-proclaimed “low rent chopper build off,” where participants have one month and a $1000 budget to build a bike capable of riding at least 100 miles on all kinds of roads. The rules have evolved over the years, but the philosophy of building the bike you want, your way, without blowing a ton of money — has remained constant. As the founders say:

“There is a belief that custom bikes are the exclusive right of rich pretty boys and media personalities, rolling around on cookie cutter motorcycles that cost the same as a small family home. Well that’s bullshit.”

Enter Justin Martens (@Jmartens2004), a native Minnesotan who now restores and builds motorcycles out of his two-car garage in San Rafael, California. The idea for this build started innocently enough:  Justin had an XS650 engine and a set of SV650 wheels lying around and decided to put them together. What he didn’t have was a frame, so he decided to build his own.

Yamaha XS650 Supermoto

The result is the “Martens Motard XSM 750″ (as the DMV paperwork states) — a 750cc, trellis-frame Yamaha XS650 supermoto built for slaying Husky 701’s through the twisties of West Marin. Justin the frame from 1″ X .083” mild steel tubing, utilizing a manual pipe bender, MIG welder, and a jig he built himself. He rephased the XS650 crank and installed a 750cc big bore along with a slew of other goodies.

Yamaha XS650 Supermoto

Rarely do we see builds full of such heart and ambition. What’s more, Justin built the entire bike in less then 60 days for under $1500, then rode it 300 miles on a camping trip, winning the  COOLEST award at the 2018 Dirtbag Challenge. Well earned, brother!

Below, we get the full story on this canyon-carving 701-slayer.

“Handmade Motard”: Builder Interview

Yamaha XS650 Supermoto

• Please tell us a bit about yourself, your history with motorcycles, and your workshop.

Well, my name is Justin Martens. I was born and raised in the metro area of Minnesota but now call California my home. I’ve always been a gearhead but got into restoring and building custom motorcycles around 2013 after moving to the Bay Area. One of the biggest influences into my passion of building and riding motorcycles has to be the Dirtbag Challenge, a San Francisco local bike build-off that is just, real…truly honest, if you don’t know what I mean, you gotta check it out. I do everything out of my 2-car garage at my home in San Rafael, California. I have several motorcycles and am always getting new ones.

Yamaha XS650 Supermoto

• What’s the make, model, and year of the bike?

The bike is a blue tag special construction “Martens Motard XSM 750” — that’s what I put on the DMV paperwork, lol.

Yamaha XS650 Supermoto

• Why was this bike built?

I thought up the idea while sitting on the toilet texting back and forth with my buddies about motorcycles. I had an extra XS650 motor and a set of SV wheels so I decided that i was gonna build my own frame and make a custom supermoto. After making a plan, the Dirtbag Challenge was announced and I figured that I would use this 2-month time frame to get things going.

Yamaha XS650 Supermoto

• What was the design concept and what influenced the build?

I wanted to build a canyon-carver supermoto to ride the twisties of West Marin, so main influences were supermoto bikes in general but I knew I also wanted to a trellis frame. Function and comfort needed to be number one so I took several measurements from my KX 250 to determine seat position, height, and peg location.

Yamaha XS650 Supermoto


• What custom work was done to the bike?

The bike is a trellis frame supermoto hand-made using 1″ X .083″ mild steel tubing. I made a frame jig with the help of my buddy and set the steering stem angle on the neck to 27 degrees, same as my KX. Once I pinned the swing arm in, I made the top tubes that would hold the seat and connect to the upper shock mount.

Yamaha XS650 Supermoto

Once that was fitted, I placed the motor in the frame jig getting the sprocket alignment correct and built the rest of the frame around that using a simple manual tubing bender and mig welder. Once the frame was finished it weighed 1/2 the amount of a stock XS frame at just over 26lbs. I primed then the painted the frame with Little Daddy Roth “Bad AZZ blue pearl” and finished it with some 2k clear.

Yamaha XS650 Supermoto

The engine is a 1981 XS650. I rephrased the crank to 277 myself in my garage. I then installed a stage 2 cam from SmedSpeed in the UK along with a 750 kit provided by mikes XS.

Yamaha XS650 Supermoto

I am running a 277 Pamco with Emgo Green Monster coils. The charging system has been converted to a permanent magnet setup also from mikes XS topped off with clear stator cover I hand-carved with my company logo into.

Yamaha XS650 Supermoto

The carbs are stock Mikuni CV’s jetted and tuned. The exhaust is partially custom using the stock headers with dual D & D mufflers off a Honda 919.

Yamaha XS650 Supermoto

The wheels are Suzuki SV with Pirelli superbike slicks.

Yamaha XS650 Supermoto

The rear has a swingarm and shock is from a yz250, bandit hub, SV wheel, sprocket specialist custom sprocket, GSX-R1000 caliper/master cylinder with hand made brake stay, and hand made dropouts to fit a SV axle.

Yamaha XS650 Supermoto

Front forks are 55mm Conventional WP from a KTM LC4 with full floating rotor on an SV wheel, handmade brake stay with Nissin caliper and Brembo master cylinder.

Yamaha XS650 Supermoto

The seatpan is a 2014 YZ upholstered in ostrich skin. Pro taper 1 1/8 evo bars with Emig clamp on grips.

Yamaha XS650 Supermoto

The fuel tank is handmade 16 gauge steel located under the seat with about 1.8 gallon capacity which allows for about a 70 mile range.

Yamaha XS650 Supermoto

• Does the bike have a nickname?

“Handmade Motard.”

Yamaha XS650 Supermoto

• How would you classify this bike?

The bike is a supermoto, 701 slayer hahah.

Yamaha XS650 Supermoto

• Was there anything done during this build that you are particularly proud of?

I am pretty proud of the fact that the bike actually works, this thing is badass to say the least, corners like it’s on rails, accelerates like a dragster, comfortable to ride, it just feels right and I was pretty out of my depth during the whole project. Oh and did I mention I built the entire bike and then rode it 300 miles on a camping trip in less then 60 days for under 1500$ — Dirtbag Challenge 2018 COOLEST award winner.

Yamaha XS650 SupermotoYamaha XS650 Supermoto

Follow the Builder

Instagram @Jmartens2004
Email Martenscustommoto@gmail.com


  1. Jagdesh Ramjag

    I think the rims doesn’t match the bike design

  2. cool bike, I’ve been thinking of building one of these out of titanium for awhile (expensive!).
    I was wondering why you went with mild instead of chromoly, was it too expensive?

  3. Chuck Lantz

    Nice work! And I think that the wheels are perfect for this bike.

  4. Very Cool,Looks Great!! That’s A hell of a lot bang for your buck!!!! Well done.

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