MJ-Works 250RR – Designing and Building Your Own 250cc Racer

MJ Works 250RR by Maarten Janssens

MJ Works 250RR by Maarten Janssens

Maarten Janssens, a 20 year old industrial engineering student from Belgium, got the motorcycle bug very early, courtesy of his enthusiast dad, who had Maarten riding a little scooter at the tender age of 4. By the time he was 18 he was riding in the Belgian national enduro championship and he got the idea he might like to build a racer from his KTM 250cc 2 stroke enduro bike. Of course, thinking and doing are two different things, but Maarten is a "doer" so, over the Christmas holiday in 2009, he began drawing his plans, by Easter break 2010, he started building, just finishing in March of this year.

MJ Works 250RR by Maarten Janssens - design work

MJ Works 250RR by Maarten Janssens - design work

He has the advantage of having a pretty well equipped shop at home, lathe, milling machine and several welding posts, but tools don't build on their own, someone has to learn how to use them and then put in the time. It looks like these were well used.

MJ Works 250RR by Maarten Janssens - early stages of the build

MJ Works 250RR by Maarten Janssens - early stages of the build

Beginning by creating his design on the computer, he TIG welded the entire frame and swingarm from steel. The gas tank plus front and rear subframes are aluminum. The front subframe holds the radiators and battery. Front forks and caliper are Suzuki 750 SRAD, seat comes from a Honda RSW 125 and the fairing is modified Aprilia RS125. Rear shock is Yamaha R1, wheels are Aprilia on both ends.

MJ Works 250RR by Maarten Janssens - front subframe closeup

MJ Works 250RR by Maarten Janssens - front subframe closeup

The KTM 250cc engine work began by lightening the flywheel by 540 grams and removing 300 grams from the outer clutch hub. He polished the exhaust ports and custom made the exhaust. Maarten says the power kicks in at 9000rpm and runs to about 12500, he's getting about 70 hp at the crank.

MJ Works 250RR by Maarten Janssens

MJ Works 250RR by Maarten Janssens

Wheelbase is 53.5 inches, fuel capacity is 4.2 gallons and dry weight is a mere 209 pounds!

He still has a little engine sorting to do but the build looks great. Not bad for a 20 year old student, in fact, it's not bad for anyone, period. Going from idea to racer, from computer design to finished bike, it's a big leap from the idea (the easy part) and the person who gets to work, gets his hands dirty and makes it happen (the hard part). I'd be willing to bet Maarten has a great future ahead. Nice work!

Link: MJ-Works on Facebook

Maarten Janssens with his MJ-Works 250RR

Maarten Janssens with his MJ-Works 250RR


  1. akaaccount says

    Oh one day when I don’t work and study constantly. Thanks for this kind of stuff, Kneeslider.

  2. B50 Jim says

    When I was 20 I was trying to figure the easiest way to change spark plugs on my Road Runner’s 383. This is the equivalent of BUILDING that Road Runner. Thanks, Maarten, for making me feel totally inadequate, 38 years later.

    Great work, by the way. This kid will go very far.

    • Russell B! says

      Hey, you had a Road Runner w/a 383 @ 20-that’s not bad.

      Mr. Janssens, however, is in a rare league. He deserves mad props from all of us.

      I would still love a stinky old RD 350 or 400.

  3. maarten MJ-Works says

    Thank you for the article kneeslider!! I’m very happy seeing my bike here 😉 If there are any questions, please do ask them! And don’t hesitate liking the facebook page, that might help me raising some money for the next built.



    • gildas says

      In what region of Belgium are you from? I live in Gent and if you have good infor on how to buy a lathe for cheap…

  4. todd says

    Very nice. I’d be inclined to keep the fairing OFF.

    Oh, this would be the perfect bike for my ride to work through all the hills. It’s just difficult (if not impossible) to get a modern two stroke licensed for the street. I’d trade my M900 for it, lickedy-split.


  5. Kevin says

    Awesome bike. I wish I have the skills and tools to do this. That must be one fun bike.

  6. JustThunkin says

    A 20 y/o engineering student…hmmm, who would ever guess that a love of motorcycles AND an education could make a difference?? There’s probably a large dose of family DNA that contributes to initiative and desire in the mix, too. Probably not the last time we see or hear about this enthusiast, I’ll bet.

  7. leston says

    i dont know why he is in industrial…we could use him in mechanical much more than those stop watch users can. hahah

    mad props to the guy.

  8. B50 Jim says

    Russel B! —
    I was lucky to have survived the Road Runner — it was brutal. Traded it on an MGB and had years of much safer fun.

    Most of us dream about building things like Maarten’s bike, but very few have the stuff to go ahead and do it. He gives me new hope for the young generation.

  9. rohorn says

    VERY neat project!

    Anyone who has a copy of “The Ultimate Racers” by Alan Cathcart will recognize that frame. Except the one in the book has a perforated stainless steel backbone.

    • Paulinator says

      This thing is stunning. I was wondering about that frame, though. Is there an internal shear-web where the rear down-tubes converge at the back-bone? And 70 hp from a 250? Damn!!!

      • maarten MJ-Works says

        there is a special type of cylinder with internal “webbing” inserted from the back of the main frame tube. This is also the cilinder that holds the upper mount of the rear spring. Some good “wondering” you’ve been doing 😉

  10. Nick52222 says

    Industrial engineer representing and showing we are jacks of all trades! Good job Maarten.

  11. R6Power says

    Extremely simple, light and must be fun to drive. Whats amazing is that the engine is as powerful as the Aprilia 250 twin engine.

    My mechanical skills are limited to changing the oil, but one day I would love to do something as cool as this.

  12. Aaburouss says

    Great work man. Really inspiring. Did you use a finite element software for the design of the frame? And how did you come up with the dimensions in the picture above?
    I’m an engineering student myself and I’m hooked on motorcycles ( I own a Rotax 500 powered bike) and I wish I had the ability and tools to build such a bike. Great job. Looking forward to your next project.

  13. maarten MJ-Works says

    the frame design has been put into autodesk inventor just to check if it would hold and how stiff it would be, I’ve only added a little strut at the bottom of the steering head after seeing the results in the program. Tha basic dimensions are based upon a 250 aprilia gp bike and off course lots and lots off research about steering head angle’s and center of gravity and so on. You need to read a few books before you start 😉 Maybe start by rebuilding your own bike into a café racer you cab really learn a lot from that. I did with a scrap honda cb125 (see the facebook page) it’s awesome because with little money you can make a big difference!

  14. jp says

    Love it!! Although my inner lunatic thinks a KX500 lump would be a slightly better fit :)

  15. Jeram says


    Im currently halfway through building a KTM380 in a Mito Chassis…

    That is a very nice job you’ve done.

    lets have a race oneday :)

  16. Jeram says

    quick question that I forgot to post in my original message above

    How does the ‘backbone’ style frame cope with torsion along the y-axis?
    (and by that I mean twisting of the frame rail along its axis)

    Im guessing that you got the 70hp purely with pipe design?
    you designed it for a narrow band of condensed power?

  17. maarten MJ-Works says

    There have been multiple things done for that HP. Fully programmable battery ignition, totally reworking the cylinder/crankcase fitments; altering the crankcase volume; adjusted exhaust ports; and so on…BUT you were right about the pipe design for a narrow band of condensed power; this engine used to belong to an off-road bike so it was made for a relatively smooth power delivery, among others; the adjustments mentioned above were mainly to transform the engine to a higher rpm engine. The weight I’ve managed to get off the flywheel and clutch helps the engine in picking up it’s rpm’s easier.

    Where do you live at? I’d be happy racing you :-). I believe that with a motorcycle in the right package can beat an other motorcycle that may have more horsepower 😉



  18. rafe03 says

    Using another frame as inspiration for yours doesn’t detract from the drive, skill, tenacity, perseverance, & application required to get such a tok sick magic machine from idea onto the road. Truly magnificent machinery1 Go Maartin!

    • says

      If I can find a honda rs125 I’ll be able to tell you! meanwhile I’ve been doing allot of modifications to the bike and the frame and it’s just been a few weeks that I’m truly satisfied with the handling and vibrations. I’ve also started conceiving my next bike and bought the engine; suspension and wheels for that one! Wish me luck and follow it on the facebook page 😉



  19. Clive says

    I would have bet money that those forks were from a ductai. They look exactly like the forks on my s2r.