DM3 Carbon-Steel Single Legged Fork Update, Answers

DM3 F69 single leg carbon steel rigid fork for 26 and 29er mountain bikes

Our original post on the DM3 F69 single leg carbon fiber and steel fork left a few questions, so we followed up.

To recap, it’s a steel single legged fork that works with both 26″ and 29er wheels via different axle slots with a sliding carbon fiber and alloy shaft inside the leg to adjust the height. Shown here is a much more polished version, but creator Dimitri Robin is quick to point out it’s still a test unit. After they finish their own testing on both 26″ and 29er mountain bikes on the trails and roads, they’ll go after CEN certification.

In the meantime, he’s given us a bit more of a breakdown on its construction…

DM3 F69 single leg carbon steel rigid fork for 26 and 29er mountain bikes

Inside the steel tube is a carbon fiber tube to add rigidity. Attached to that at the bottom is a 7075 T6 aluminum tube where the axle mounts.

DM3 F69 single leg carbon steel rigid fork for 26 and 29er mountain bikes

The axle is the same alloy and is 20mm in diameter, using an M8 bolt to attach and hold it in place in the fork. The axle is held by a supported hole in the steel outer tube and counter-bored in the internal alloy tube.

Comments

dgaddis - 11/15/11 - 10:28am

I still don’t see the market for this.

Steve M - 11/15/11 - 10:30am

This thing has not gone away yet?

Bmark - 11/15/11 - 10:42am

The real question lingers – what’s the advantage? Is this lighter? Stiffer? Stronger? Dumber? One of those, certainly.

Ollie - 11/15/11 - 11:12am

FEA says those cutouts are just DUMB.

mmmmfish - 11/15/11 - 11:15am

It makes changing a flat super easy!

BJ - 11/15/11 - 11:22am

The Left from C-Dale makes sense, and they post all their benefits with stiffness and weight

This just looks stupid and isn’t going to last very long

Steve - 11/15/11 - 11:24am

Oh man…I want one of these for when I go back to riding a slower, more hurtful rigid bike. NOT.

Collin S - 11/15/11 - 11:33am

I do see a small market for this. For people in areas where some days rigid is a better tool then a suspension fork, but they are not a devoted sadist, and do run suspension on a regular basis, and have already committed to Lefties (Ie have good lefty wheels) then this make since. I am one of those. On my 26er, which was my first mountain bike and a cannondale, so all of my wheelsets have a lefty front wheel. I recently tore the thing apart to sell the pricey items, but had the idea of keeping the frame to run rigid and keep the bike as a beater winter bike. This issue came up that if I would do that, not only would I have to buy a rigid fork, but a new front wheel and headset, so this cheap beater project, now will cost a couple hundred bucks. If there was a cheap rigid lefty (which I don’t think this one would be, but hey, cell phones were $1,000 when they first came out) I would throw that fork onto my existing wheels and have a rigid lefty.

Luke - 11/15/11 - 11:36am

As a rider from an area where rigid, single-speed 29er’s are the weapon of choice, this fork looks awesome! Hopefully we’ll see some production models in the near future!

Dantrell Eisenstien - 11/15/11 - 12:32pm

Colin S –
Cannondale already has a rigid Lefty fork for its Badboy series of bikes that you may be able to source from your local dealer. I’m not sure how it would affect your geometry but I would look there first for your rigid lefty needs. The use of steel and carbon could allow this fork to be more compliant for a rigid fork but that seems contradictory to the advantages of the lefty design – super stiff so that the fork doesn’t flex and instead goes right into its’ travel. I’m not really sure if I would want a flexy single sided fork… I am glad to see other companies experiment with the concept but I’m not so sure about this one.

Gillis - 11/15/11 - 2:28pm

@mmmmfish: have you ever tried fixing a flat on a lefty? In my experience if its anything but a soft/loose beaded tire its a pain in the ass and you’re better off just removing the wheel.

peter - 11/15/11 - 3:55pm

While I think the idea is novel, I do have at least one reservation: Galvanic corrosion. Steel and carbon are not friends. Has this somehow been addressed via coatings, or isolation of the materials? If not, then I couldnt endorse the design.

Topmounter - 11/15/11 - 6:49pm

Riding rigid is like having unprotected sex… yeah, it’s risky, but damn does it feel good.

I think I’ll stick with my Niner CF Fork, but it’s good to know that there may be a rigid option available for the Lefty crowd.

greg - 11/15/11 - 9:58pm

this is NOT an rigid option for the Lefty crowd. it uses a 20mm thru axle, not Cannondale’s tapered variety (25mm to 12 or so).
this fork is really only for those that like the aesthetics.
rigid lefty = black cat bone. look it up, it’s pretty cool.

rjv - 11/15/11 - 10:41pm

Seems like an innovative solution and might offer some great benefits as far as cost and weight reduction is concerned. Some cycles are better off with single stanchion forks,
Reading some of the other brainier comments from the product gurus above, I am interested to find out what other innovations are DUMB or look STUPID just so I don’t get tricked into trying to reason thru the logic of the product and waste time considering that in some situations this might be a very useful and innovative. After all, there is only one way to ride and one type of terrain to ride in. Some reader might be surprised to know there are folding off road cycles like Montague bikes and others that this might work very well for.

professor - 11/16/11 - 11:28am

Too bad it looks cheap and homemade. Hire a designer and try again.

LanceAndre - 11/17/11 - 1:37pm

But like most bikers i’m right handed… wonder if they have a “Righty” in the works?

rideTall - 11/17/11 - 4:04pm

yeah, and welder!!! my dead g’ma can lay down a better bead than that one!

Marshall Hance - 11/17/11 - 7:43pm

I wonder what they’ve done to keep the carbon from galvanically corroding the steel (worse than salt water!). It’s also curious to put the stiffer material inside the softer where it can do nothing to support and only to damage.

Alvis - 03/26/12 - 9:30am

Putting the stiffer material on the inside is the smart way to go– As the geometrical section gets smaller the material stiffness is less effective, unless of course you put a stiffer material there to compensate the reduced geometrical section. This is an engineering solution not like a nasty carbon wrap stem. At some point in most composite structures you’re going to end up with a metal to composite joint, its quite possible to do this and protect agaisnt galvanic corrosion. Regardless of what you think of the looks and functionality of this thing as a fork engineering wise it’s smart.

Tyler Pendley - 02/01/13 - 2:18pm

Hey man I really like your idea, I’d like to try to fab u something similar. How did you make the axle? Are there plans out there for one? Thanks

Mazza - 08/26/13 - 5:23pm

Hmm, i personally like this for a very simple reason.. rigid, long fork with a 20mm thru axle! (the latter being the most important!)

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