Maarten Janssens is Building the MJ Works Racing Supermono

Maarten Janssens is building a KTM powered  Supermono

Maarten Janssens is building a KTM powered Supermono - this rendering is what the finished racer will look like in a few months

Recently, I got a note from Maarten Janssens, you remember him, he's the student who designed and built his own 250cc racer from scratch. Some of you commented the work he did on that project was a strong indication we might be hearing more from him and, sure enough, his first racer was just a warm up.

Maarten just turned 22, he's still studying, but after the first bike was completed he still had an itch to build his dream bike, a bike he could race in a competitive class where prototypes were allowed, so he chose the supermono class.

There's a lot to see in what he did here, look closely at the rear suspension, and notice the dummy tank and seat buildup, using wire mesh like John Britten. It's a neat build.

Here's Maarten in his own words:

The Ducati Supermono was my dream bike, but one thing that bothered me on the Ducati was that the engine was some sort of compromise with the normal v-twin crank using one of the rods as a balancer. Knowing a thing or two about KTM engines I saved up 2000euro (2500 U.S. dollars) working weekends and -sadly- selling parts from the 250RR project.

For that money I got a KTM LC4 Duke 690 engine from 2009 with only a 1000 miles on it + the injection system, wiring loom, exhaust, airbox, throttle system, clutch, radiator and fuel tank. I bought the engine with all these things because I knew I could use them or parts from them. Eventually I used a part of the inlet funnel that was attached to the airbox, I used the fuel pump from the tank, I modified the radiator and used the front pipe piece that fits into the engine. Money well spent I'd say!

KTM engine mates to MJ Works frame

KTM engine mates to MJ Works frame

Likewise for the Ohlins shock and the Showa race forks from a Ducati 998S by the way! I bought those for $350! The wheels come from the 250RR project but I'm looking for a magnesium rear wheel as I'm writing this.

Designing the frame:

Rear suspension closeup - pushrod activated horizontal shock

Rear suspension closeup - pushrod activated horizontal shock - click to enlarge

The first thing you'll notice when you look at the frame is probably the rear suspension. The entire point of the bike is mass centralization for accurate and fast control. Although a lot of people are skeptical about it I believe it has a lot of advantages:

1) Mass centralization: because of the position of the spring there is more room in front of the rear wheel than with a conventional system so you can put the fuel there and the spring closer to the engine so an overall gain in mass centralization is achieved. I believe mass centralization on a bike is more important than a low centre of gravity. because the fuel sits underneath the seat the total bike will be more sensitive to rider position.

2) Mass distribution: there is little difference in mass distribution between the front and rear wheel when the tank is full or empty, this makes the bike easier to ride during a race.

3) Room for the airbox: because the fuel could be moved (read above) there is more room for an airbox and electronic parts above the engine.

4) Continuously variable setting of the angle of the swing arm with the push rods: this angle has a great effect on the anti-squat effect and the weight distribution.

5) Forces: the spring forces generated by the swing arm are put onto the spring aligned with the springs axle and working from both ways. they react as negatives against each other! they don't disturb the frame and aren't going towards the headstock!

6) Fuel temperature: Because of the position of the fuel it will keep cooler (not closely behind the radiator or above the hot engine) and thus makes it easier to generate more power.

7) Spring adjustments: the spring is easy to reach and (dis)mount and in a couple of minutes I can change the system from progressive to linear and back.

The idea for placing the spring there formed the rest of the bike because of my belief in the advantages above.

MJ Supermono racer on wheels

MJ Supermono racer on wheels

The rest of the frame is a basic trellis frame made from 25CrMo4 seamless cold-drawn tubes. The main tubes are 25mm diameter with a wall thickness of 1.5mm. other tubes are 20x1.5mm and 14x0.8mm. More about the frame in the next point.

I had a lot of help designing the pivoting triangles the "push-rod" pushes upon. I now have the option of switching from linear suspension travel to progressive by putting the push-rods in other mounting holes. A local CNC firm made them for free for me from a solidworks file another friend helped me to make. All I did was pay for the blocks of aluminum!

Making the frame(s)

Yes, you've read it right! I'm not ashamed to admit I make mistakes but this was a big one! I made a new jig for this frame so I could build around the engine to ensure tight packaging. So far so good.

I started machining all the tubes and started welding them. Now that's where it all went wrong. I never welded CrMo steel before so I thought it was just the material being a pain in the ass (sorry) during welding and I can't say that I was content about the result but I carried on. Lucky for me I met the right friend at the right time. I didn't make all that much parts to fit the frame yet (well I did but I don't want to think about the old subframe, airbox an air-inlet I had to redo πŸ˜‰ He told me I used the wrong filler rod. So I chose to make a second frame.

The second frame turned out better because I had a real life example now and I could change little things that made a big difference. I also made it in a manner I thought would be better to avoid distortion due to welding.

I first built the top two "triangles" on a flat plate and connected them with the tubes that run across the frame; again on the machined steel plate. Then I connected this part with other triangles I made to fit the swingarm pivot. Last, but not least, I welded on the headstock, free from any tension of the other welds on the frame!

(Maarten wanted to be sure you knew some of the photos of his build on Facebook are of the old "bad" frame with the incorrect welds so you wouldn't be too hard on him, but somehow, I don't think he needed to be concerned about that. - Paul)

Making the swingarm

Swingarm begins to take shape

Swingarm begins to take shape

The swingarm eventually had to be home made so I chose to make an aluminum one that would also work as a part of the fairing. It was made from billet blocks and sheet plate I bent on my working bench. It was an unbelievable amount of work but it paid of! the pictures tell it all.


Forming the shape of the bodywork like John Britten using wire mesh

Forming the shape of the bodywork like John Britten using wire mesh

Since I was on a roll making everything myself again I decided to make the dummy tank/seat assembly myself too. Inspired by the John Britten documentary I bought me some $10 of wire and wallpaper glue and started on making a shape!

Dummy tank and seat mounted on the MJ supermono

Dummy tank and seat mounted on the MJ supermono

I had a lot of help from the guys at Polycress when making the mold. I also received a lot of support from the man behind Tyga-Europe who sponsored me with the fairing + footpegs and exhausts for the project!!

Fuel tank, airbox and subframe

They are made from alloy plate, tube, blood, sweat and yes...tears!

(current) SPECS

wheelbase: 1350mm
swingarm length (ctc): 600mm
headstock angle: 23.3Β°
fork offset: 35mm
current weight without fuel: 110kg (242 pounds) (I'm hoping to stay underneath 120kg and I think that that's reasonable given the heavy wheels.
current horsepower: 72HP (will be tuned!)
current torque: 65Nm

MJ Works Supermono with fairing in place and exhaust mounted

MJ Works Supermono with fairing in place and exhaust mounted


Thanks, Maarten.

(This project has been very well documented, Maarten took LOTS of photos, so choosing a few to highlight what he did was difficult. Follow the link below to see more of them.)

Maarten Janssens

Maarten Janssens

I think it's important for everyone to see all of the work involved in a project like this, including the mistakes along the way. Look around the Internet and you'll usually see shiny bikes with perfect paint, ready to ride. They look great and make for a gorgeous photo, but it's deceptive and all too often, as we've seen in many articles, comments begin by critiquing one part or another, how the builder should have done this or that, how the commenter's Ducati or Honda or whatever is better, cheaper, faster and prettier.

A photo of a finished bike draws the mind away from how it came to be there in the first place. A story like this emphasizes the process, the motivation, the skill, the determination to see it through; making mistakes and learning from them, and then, continuing on.

Many want to say they've built a motorcycle, to be able to point to it and look at it, however, the learning and doing isn't always fun, it can be really hard work, so they never quite get around to it, and then we come across some builders, like Maarten, who know there's a lot more to having a bike than wishing and dreaming, so he rolls up his sleeves and gets to work. In a few more months, when the Supermono rolls out of the shop and onto the track, if someone asks, he will be able to proudly say, "Yes, I did build that." Very nice work, Maarten!

Link: MJ Works on Facebook


  1. HigherRPM says

    Having raced singles/supermono back in the day, this project really intrigued me. The engine choice, shock mount position and all the β€œin house” fabrication were highlights in my opinion. This young man has a great future. I’m hoping for some video and commentary on its race track debut. Keep us posted Maarten. Ride on…..

  2. BICHO says

    I seat here….,watching the(real superduke)bike, dont know what to say…………….amazing,really exceptional concept and work!BRAVO Maarten,sei tutti noi!

  3. Kenny says

    *Long low whistle*
    Wow! That looks stunning and I’m loving the attention to detail.
    Just curious but why’d you go for a dual pipe exhaust?

    • says

      you need volume in the exhaust pipe with this big single engine and making one big pipe would be making a pipe that makes to much noise. That’s why there are two megaphones –> volume!

      kind regards,

      Maarten Janssens

  4. Taichi says

    I thought a high cog was preferred over Low one? Wasn’t that why it was so counter-intuitive when Britten was doing it?

    • Taichi says

      Sorry, that was an incomplete thought. Disregard the second sentence. Isn’t high cog better for handling? Quicker turning and less bike lean per cornering G.

      I’ve don’t the math my self, calculating rotational moment as well as the angle of the center of gravity of the total system from the ground. Total system being rider and Bike, with the rider hanging off.

  5. Steve says

    Well done.
    Looking forward to updates! More people should be doing this sort of thing (me included). If you look up Locost and see what can be done with donor cars, there is no reason why bikes are different in that respect.

  6. says

    Excellent build. The perforated swing arm construction looks strong. Clever, too.

    I like the rear shock setup. The Vyrus 600 cc Moto 2 racer had something similar but I never heard how it performed in an actual race. Congratulations so far & best of luck going forward with the racing!

    • says

      Nice project! A similar shock arrangement was used by Yamaha on one of their older OW racebike (can’t remember which one exactly, it is in Tony Foales book) Also Ian Drysdale used a similar set-up it for his V8 bike…
      A shame the shock is placed at where the leg of the rider is…making less room for the legs to tuck in. Good luck with finalising the bike…that is the most difficult part (the last 5%)

      • says

        I found the Tony Foale page online (you must love the internet for that) it was the Yamaha OW61 racebike from 1982(!) which had that set-up

      • says

        what you say about the shock being in the way of the leg is false. the shock is actually behind the knee cavity, so not in the way of the rider what so ever!

        thanks for the link below by the way, was looking for that!

  7. Drums says

    Great to see someone having a go, learning along the way, and employing some original thinking. One misconception I must correct – the Ducati Supermono engine wasn’t a compromise – a V twin with one cylinder taken off as a half assed attempt at making a single. It was actually a clever adaptation of the perfect primary balance of a 90 deg V twin to a single cylinder format. The result is one of the smoothest single cylinder engines (without balance shaft) ever produced.
    Anyway, great work so far, and I look forward to seeing the finished product on the racetrack.

    • says

      It does have a balancing weight instead of a balancing shaft, that is driven by the con rod that would normally move the second piston. So saying it doesn’t have a balance shaft is a bit deceiving. Don’t get me wrong, I love the ducati supermono but the engine is a combination of purpose build cases with parts from other ducati engines. Sounds like I’m negative about it but I’m not. πŸ˜‰ thanks for the kind comments on your behalf!

  8. Kurt says

    I’ve been watching the progress on this since the day he Maarten posted that he had received his engine for the project. I look forward to each time he posts new pix of his progress. We’ve actually discussed frame design personally, and I’m impressed by how nice of a guy he is. I cant wait to see Youtube vids of his test-n-tune sessions on the bike. It’s going to be awesome. Best of luck to him, and his fantastic looking bike. I’m sure he’ll do well.

  9. rohorn says


    This is far more encouraging to see than a 1000 more Man Barbies.

    Love the Locost comment above – sadly, too many perfectly good bikes are led down the aisle to the alter of style, where performance is sacrificed for fashion. Eew.

    Not that Real Men disgrace themselves with that sort of poofery…

    PS: reCAPTHA says “racemed” – oh yeah……

  10. says

    Very nice job Maarten, I particularly like the “cross the frame” shock position. The frame is very “business-like” as well, it looks light and strong.

    I also used a “cross frame” shock position on my 750-V8 ( see website ), maybe we should form some sort of club ?

    Cheers IAN

  11. says

    Very awesome project, great work!!

    A question about the aim for a low center of gravity… I had never thought this would be a big concern? I don’t know much about building bikes, geometry or that kind of thing, but I do know that all other things being equal, a bike with a lower CoG will require MORE lean angle for a given speed.

    Or is there a point where having the CoG too high can cause problems?

    • David says

      I remember reading somewhere as well that too low of a CoG (I think below the axle plane) cause adverse handling characteristics.

    • says

      Too high or too low is never a good thing. The TU Delft university in Delft, Netherlands had build a supermono with the petrol tank underneath the engine and they said the handling wasn’t good any more.
      My main point/aim in this bike is mass centralisation, not a lower centre of gravity! I thought I’d made that clear but it will be my English that doesn’t suffice.

      The more the mass is centralised the easier it is to handle it. Think about a halter with weights on the ends or the weights in the middle. The latter will be easier to turn! That is what I’m trying to do with this bike. read poin 1) under “Designing the frame” again.

      kind regards!


  12. Bart says

    Generally, minimizing the polar moment around the roll axis helps a lot. Having the bike CG too far from the loaded CG slows down roll, which means the bike doesn’t tip in as well, takes more force at the bars, alll other things being equal (which they never are!)

  13. rohorn says

    If you find Supermonos interesting (And who doesn’t?), MCNSPORT 2012 TT magazine has an excellent section on 6 British Supermonos, including a KTM 690 and a homebuilt frame/swingarm/bodywork/etc… by Martin Jarvis.

    I get the impression that singles racing never really took off in the US, thrived then died recently in the UK, but is still going strong elsewhere in Europe. If someone has some observations they can share, I’d love to hear them – I hope that class provides inspiration for racing projects like the MJ-Works for a long time.

    It is sad that the Britten single racer never got finished…

  14. Chris Walker says

    Excellent work. I’d only question a few things from what I can see.

    1. Why use adjustable length dogbones, when fixed length units would be lighter & using the shocks ride height adjuster would give a single point of adjustment without the differential length issues of the 2 dogbones !?

    1a. Why are the shock lever pivots unbraced on their top side ? The swingarm will generate hi leverage forces through the dogbones to those pivot points, the angle is forward & up, it may work fine, but not if excessive force is applied like when landing from jumping a crest etc.

    2. I hope the bottom of the petrol tank is NOT the bump stop for the swingarm, it looks close ! May crack the tank if it is !

    3. The throttle body/airbox appear to be lacking bellmouths ? This will hurt max torque & thus power !

    Looks brilliant, hope it goes as well as it looks.

    • says

      hey some answers to your questions:

      1) a shock with a ride hight adjuster would become to long and thus too wide for this kind of placement on my slim frame.

      1a) If you look carefully you can see they are braced with a bar that also holds the subframe and fuel tank.

      2) There is room enough πŸ˜‰

      3) specially for you: (not easy to take a good picture from it) I’m not that stupid that I don’t know you need a bellmouth πŸ˜›

      thanks for wishing me good luck!

  15. B50 Jim says

    Fabulous! Extremely well thought out; good mental engineering translated into metal; great build quality. Maarten, like one of his predecessors Soichiro Honda, learns by doing and isn’t afraid to get technical help when he runs into a problem he can’t solve. My Bell’s off to him!

  16. Carolynne says

    Just turned 22 and already producing something this, very impressive. You have a bright future ahead

  17. Carolynne says

    MJ I just checked out your facebook page. I am even more impressed!!! Awesome work. How did you end up following this path so early?

    • B50 Jim says

      WOW — just putting the Facebook page together takes a lot of work; I wonder where he finds time to build the bike? Maybe he has also discovered a way to get by without sleep!

      • says

        hey Carolynne: I just love motorcycles I guess. In the article about the 250RR on the kneeslider there is a bit of my personal history I think.

        B50 Jim: If you don’t make time for these things you’ll never find it. I work in the weekends, I go to school and study during the week and I have a nice girlfriend for over 6 years now that I’ll do anything for! I don’t have time I make it πŸ˜‰

        • B50 Jim says

          MJ — the joys of being 22! I remember it…. working during the day, rebuilding engines at night, going out on Saturday nights, taking night courses, having a love life, and not being tired at all! Enjoy it! You’ve earned it! Perhaps you could come to my house and finish remodeling the bathroom!

  18. lmfg says

    Did you FEA those bell crank mounts? They look a little thin for single sheer and off axis force input into the bell crank.

  19. Kiernan says

    This is awesome. I’m 30 and just dreaming of building this stuff…
    I raced an NSR250 proddie bike in NZ for a few seasons in F3. Just clubracing. But out there doing laps. Got children now, so I just work…
    A challenge for you: build a hossack type front end like the Britten used with your across the frame shock mount…