DM3 F69 single leg carbon steel rigid mountain bike fork that adjusts for both 26 inch and 29er wheels

DM3 is a small brand run by Dimitri Robin that makes carbon chainguides and such. It’s his latest project that grabbed my eye, though.

Called F69, it’s a steel, single-legged fork with an internal carbon fiber reinforcement tube. That’s just the beginning. The carbon tube has an alloy sleeve mounted to the bottom that the brake posts bolt to, and the whole thing slides between two keyed slots to adjust the axle and brake mounts for either 26″ or 29er wheels.

Pics and more after the break…

DM3 F69 single leg carbon steel rigid mountain bike fork that adjusts for both 26 inch and 29er wheels

Robin said he’s working on a tapered version, too, but the first goal is just getting it to market. The whole F69 moniker also refers to a Frame concept to that could be ridden as either a 26″ bike, 29er or 69er hybrid.

DM3 F69 single leg carbon steel rigid mountain bike fork that adjusts for both 26 inch and 29er wheels

The fork was shown set up for 26″ wheels. Unbolt the post mounts and the allen bolt on the side, slide it down and (voila!) a 29er rigid fork.

DM3 carbon fiber ISCG and bottom bracket mount chain guides for mountain bikes

His original products include a wide variety of chainguides and bash guards. They’re available in BB and ISCG mount varieties.

DM3 carbon fiber ISCG and bottom bracket mount chain guides for mountain bikes

DM3 carbon fiber ISCG and bottom bracket mount chain guides for mountain bikes

DM3 carbon fiber ISCG and bottom bracket mount chain guides for mountain bikes

This last one felt lighter than air.


  1. That “fork” is the scariest thing I have seen in a long time. I can’t even contemplate what that axle screw is doing……

  2. @Steve M – That was my first thought as well – I’m sure an M6 screw can handle that kind of load…. The “F69” relief in the half-crown may create a few stress risers as well, I’m not sure that’s the best place for a company logo.

  3. @Steve M

    You do realize many aircraft – including those which have to endure carrier landings – use a single leg design and have done so for decades. Mono arms have also been used on motorcycles. It’s only a new design for bicycles (and even then it’s not – look at Cannondale)

  4. Realistically, a monoleg design should be lighter and/or stiffer than a fork, as long as it (and it’s accompanying hub) are designed properly. Bending stiffness is proportional to the square of the tube’s diameter, so a single tube that uses the same amount of material as two smaller ones will be stiffer…

    This seems to be a little bit more on the bling side of things, though. Not quite sure on the engineering here.

  5. @Chris
    Yeah I have owned Honda Interceptors and currently own a BMW GS1200 so I am pretty familiar with single sided swingarms. I am also cognizant of what it takes to put structure in shear, usually lots of diameter and mass. Also cutting logos with sharp corners into the fork crown and hanging a wheel off of a 6 or 8mm screw is never going to be a good idea.

    Not for me thanks

  6. That fork is just scary looking. The welds are absolutely terrible, the crown logo is a really bad idea and why would anyone want a fork that is adjustable for 26″ and 29″ wheels?
    The whole thing reeks of a high school shop project gone wrong.

  7. @Steve M

    Agree with you (and others) about the logo. Vanity ahead of engineering is never a good idea though it seems to be the norm in cycling.

  8. Should call it a strut instead of a fork, monoarm, or monoleg.

    I think it’s overbuilt and the 26/29 thing is just an extra feature they kept from the prototype stage (probably intended to make testing easier and to try a 69er or different geo) because it worked. I think they’d like to invest in a carbon version, knowing the market is pretty open, but want to help clear the fear of struts before they go into developing the carbon version and get it so it has no weak points, like a bonded axle or steerer. The lightweight SS rigid crowd would probably love a carbon version and when the demand comes in…

  9. The axle is part of the alloy sleeve. That screw just helps secure the position, along with the disc brake caliper tabs.

  10. I’m more worried about the logo cut out in the leg than the crown….

    The big question here is: why? Why would someone want a fork that is either 26″ or 29″?

  11. Simple mechanics shows a stub axle can never be lighter than a similiarly engineered traditional fork – the stub axle and fork union to the crown are under a turning moment from the point of contact on the ground – the forces through the traditional fork are (largely) compression/tension.

    Image this fork from the front view leaning over to the right with all your weight going vertically down, the stub axle is trying to bend upwards around its contection to the fork blade and the fork leg is trying to bend to the left around its connection to the crown.

    The traditional fork is triangulated – as we all now from riding bikes, traingles are the strongest structure – one fork leg balance out the other.

  12. @Turbofrog…

    The bending stiffness of a beam with a circular cross section in proportional to the diameter raised to the FOURTH power. It’s in the second area moment of intertia term…. I=(pi/64)*(d^4) ….(for a solid cross section). It’s a big reason why my lefty kicks ass.

  13. This company looks to be 1 accident/lawsuit away from devastating many people…

    I used to use a Kastan Strut (on a BMX bike) when they came out and loved it, but they were recalled due to failure and they looked to be much better built/designed than this.

    I’m hoping this was just a (bad) idea that made it to the trade show to get people talking about the company. If it’s not in fact a marketing ploy, then I hope they rethink it before moving forward with production…..

  14. To the armchair engineers with your beam bending formulas… you’re neglecting to consider the location of the neutral axis.
    Stiffness of a circular beam does indeed scale with the 4th power of radius, because the neutral axis is the centre of the circle.
    If you have two circular beams (like a conventional fork), the neutral axis is at the centreline, and you have to consider the displacement of the beams from the neutral axis. Second moment of area. That’s when you stop quoting simple “D^4” formulas and start doing calculus. Lateral stiffness is higher than longitudinal, because the beams are laterally spaced from the axis.
    For torsion as well… two slender beams spaced a finite distance apart can be as stiff or stiffer in torsion than a large single beam. That’s demonstrated by the Lefty – it needs to be extremely fat to match the torsional stiffness of a conventional fork with smaller stanchions. (The Lefty also demonstrates that by clever application of precision engineering, it’s possible to make telescopic suspension work on a single strut, despite it being a fundamentally stupid idea to even try).

What do you think?