The lightest 29er  frame

in production




Our goal was to build an extremely light, strong and well thought-out hardtail. The result is the lightest 29er hardtail in production, the first frame with internal cable routing that protects while being easy to live with, the first mountain bike with BBright, a frame with a geometry truly developed from scratch for 29er wheels and more. All features big and small are here in this section.

The hardtail comes in two versions; the sub-900g O-1.0 and the O-1.1 which is barely 100g more.

OPEN's  goal was to build an extremely light, strong and well thought-out hardtail. The result is the lightest 29er hardtail in production, the first frame with internal cable routing that protects while being easy to live with, the first mountain bike with BBright, a frame with a geometry truly developed from scratch for 29er wheels and more.


The hardtail comes in two versions; the sub-900g O-1.0 and the O-1.1 which is barely 100g more. Both frames are available with various build kits.


Frame £2150 or £1920

1:  TRCinTRS™ technology

“100% hi-modulus carbon”, “aero-space grade”, etc. Useless – and hopefully false (we’ll get to that) – claims meant to impress you.

It’s not about high- or low-modulus, it’s about the right carbon in the right spot. And because the bike industry loves techie-sounding abbreviations, we’ll humor them and call it TRCinTRS™.


The simple fact: stiffer carbon is more brittle. Strategically placed ultra-high-modulus carbon is a good idea. Putting the same in the headtube where you have big impact loads is not!

The best lay-up is not 100% of one modulus; it’s a blend. We use the highest modulus (stiffest) carbon of any bike manufacturer where possible, and tougher grades of carbon where we must. This makes the O-1.0 the lightest 29er hardtail frame in production AND helps it pass the toughest industry tests.

2:  Transition downtube

The downtube is the key for stiffness, connecting the steering center of your frame with the drivetrain. The downtube has:

Large dimensions (oversizing means more stiffness relative to the weight),

Flat sides far away from the center plane to put strategically placed strips of ultra-high modulus carbon (the flatness is important as this material doesn’t easily ply around complex shapes).

Continuously changing cross sections to best deal with the changing loads.

3:  Trapezium toptube

The toptube starts out wide, perfectly matching the width of the headtube to best get the loads from the headtube into the rest of the frame. Towards the other end it narrows down to our minimalist seattube and transitions smoothly into the seatstays.

That continuance of the shape is paramount to obtaining the best stiffness properties with the least amount of material, and therefore the lowest weight.

4:  Zero-setback seattube

Two characteristics of the seattube stand out:

Minimal diameter to allow for flex in seattube & seatpost. The seattube has twice as much flex as the next-lightest 29er frames in production. Especially on a hardtail, this is a big plus on rough terrain.

Seattube angle is adjusted to match a straight, zero-setback seatpost rather than a regular seatpost with setback (we’ve never understood those). Zero-setback posts are lighter, saving you another 10-30 grams (every little bit helps). As a bonus, people who like to sit further back (especially on smoother terrain) can do so with a setback post.

5:  Wire-stays

Seatstays are important to give the frame good torsional stiffness between the headtube and the rear dropout, but it doesn’t take a lot of material to achieve that. And material used to stiffen the frame in torsion also stiffens it vertically – thereby killing comfort.

That’s why our seatstays are so wire-thin, and flatted to give more stiffness sideways (when the frame is in torsion) and less in the vertical direction (think a ruler, stiff in one direction and flexible in the other).

6:  Semi-flex chainstays

If you want loads of vertical compliance, you obviously need a full suspension frame. But that also brings complexity and extra weight. Without full suspension, most vertical flex comes from your tyres (no matter what bike makers claim about their frames).

Although a hardtail’s frame design can add little to the comfort, that little bit can be maximized by keeping the vertical size of the chainstay cross section small.

The semi-flex chainstays are exactly that, small in the vertical dimension but large sideways to keep the power transfer from cranks to rear wheel optimal. And we call them “semi-flex” because we don’t want to promises the world when we know the effect is small.

7:  Tapered headtube

Now that lightweight suspension forks use carbon steerers, making the steerer bigger at the bottom makes a lot of sense. It allows for a better transfer of the loads from the (carbon) crown into the steerer and strengthens the whole area.

At the top of the steerer, the loads are much smaller, and hence a bigger cross section is not needed there (and would add weight to both the steerer and the upper bearing). So the bottom diameter grows to 1½ inch, the top diameter stays at 1⅛ inch, creating a tapered steerer and hence a tapered headtube.

8:  BBright™

This BBright bottom bracket shell fits cranks from SRAM, Shimano, Rotor, THM and many others. It's delivered standard with the Rotor BBright-specific crank, but your store can order a bottom bracket for the other brands as well. The BBright design is the result of simple physics:

A bigger axle means makes the axle and hence the drivetrain stiffer.

A wider bottom bracket makes the frame & axle and hence the drivetrain stiffer.

Extending the bottom bracket wider than the chainrings means the crank spider has to wrap around it, making the crank and hence the drivetrain less stiff.

What follows out of this is that the bottom bracket can be wider on the left side where the crank doesn’t have chainrings. That’s the BBright system that was developed at Cervélo. OPEN is the first mountain bike that uses it and our friends at Rotor make a fantastic crank for it.

9:  Fully-internal cables/hoses

External cables & hoses collect dirt, risk getting stuck behind objects (particularly expensive with electric shifting) and are frankly ugly. So the O-1.0 & O-1.1 runs them internally.

The easiest solution is to run full housing through the frame, as all you need is a hole in the frame. But housing is heavy and there’s no point having a sub-900g frame and adding 200g of housing! So we integrate cable stops inside the frame to keep the dirt out and let the cables run freely inside the frame.

The brake hoses also run internally, to protect them and complete the clean look. Of course they are full hoses, otherwise the oil runs out!

10:  MultiStop™

At the front, all cables and hoses enter the frame at the headtube through our unique MultiStop™. The MultiStop™ has several advantages:

It only interrupts the frame’s surface in one spot, which keeps the weight of the local reinforcement and the actual cable stops to a minimum.

The combined opening for 3 cables/hoses is large enough to easily guide electric connectors through.

11:  Electric-shifting-ready

The OPEN hardtail frame is the first frame in the world to be fully electric- and hydraulic-shifting-ready.

It’s unavoidable; the age-old cable shifting will disappear from bikes in the coming decade, to be replaced by electric and hydraulic shifting.

The O-1.0 is ready for Shimano Di2, with the head tube entry point easily large enough to fit the connectors through. There are several ways to guide the cables, so feel free to contact us or one of our retailers to discuss how to best do this for your O-1.0 or O-1.1.

12:  Hydraulic-shifting-ready

We’re big believers in hydraulic shifting due to its low weight and high precision. Hence we made the O-1.0 & O-1.1 compatible with the hydraulic shifting from Acros, for which you can order the custom cable stops and MultiStop.

At left, you see the four hydraulic hoses for the Acros shifting (two for the front derailleur and two for the rear) going into the MultiStop as well as the rear brake hose.

As more hydraulic shifting options become available, we will also provide – whenever practical – cable stops and MultiStops™ for those.

If hydraulics is not your thing, you can of course use the frame with regular cable shifting and even with electric shifting.


13:  S3 Front derailleur posts

There are a gazillion front derailleur mount standards. We feel the S3 standard is superior for several reasons:

Both Shimano and SRAM make derailleurs for it.

It mounts the derailleur directly on posts embedded in the frame, rather than using a band clamp around the seattube, which can damage the frame.

The posts are very close to the very stiff bottom bracket, so the derailleur will not move much relative to the rings (for the smarty-pants: unless you shift of course). Hence, front derailleur chain rub under heavy pedal loads is minimized.


14:  Dropout brake bridge

The mounting posts for the rear brake caliper are usually located on the seatstay. However, the seatstay is usually smaller and more flexible than the chainstay – especially in our case as we try to design that flex in. Therefore, it is better to mount the caliper posts on the chainstay.

To further reduce flex, we connect the two posts with a straight carbon bridge, resulting in a triangular left dropout that envelopes the caliper posts while still leaving sufficient room for the various brands of calipers.


15:  FrameSaver™ hanger

A replaceable rear derailleur hanger is meant to break in case of a high impact, thereby attempting to avoid bigger (and more expensive) damage to the rear derailleur or frame. But many replaceable derailleur hangers are threaded into the frame. But on impact, these threads inside the frame strip out, thus damaging the part it is trying to protect.

Our FrameSaver™ hanger reverses this design. The threads are in the hanger; the frame only has two simple holes through which the screws are inserted – from behind. This reduces the chance of any damage to the frame in the event of a crash.

No replaceable derailleur hanger can protect a frame from every possible derailleur impact, but the FrameSaver™ comes close.


16:  SafePost™ Pilot

Seatposts usually indicate a minimum insertion dimension. While it is important that there is enough seatpost inside the seatTUBE to support the former, it’s only half the story. The seatTUBE also needs to be supported properly, and therefore requires a minimum insertion.

In particular, you want the seatpost to be inserted well past the toptube to minimize the stresses on the frame. To make it easy for you to find the minimum insertion for the seatpost, we have made a small pilot hole in the seattube.

Simply make sure the seatpost is inserted past this point (you can either see the seatpost through the hole or check with a thin pin) AND past the minimum insertion stated on the post and you’re good to go.


17:  Tyre clearance

When designing a frame, there is a direct connection between frame weight, stiffness and tyre clearance:

There is only a fixed amount of space between the cranks (and chainrings).

That space needs to fit the rear tyre and the chainstays.

The bigger the tyre you want to fit, the narrower the space left for chainstays.

But the narrower the chainstays are, the more material you need to generate the same stiffness.

For a hardtail, especially a 29er, most people will not run a very wide tyre. 2.1 inch is already pretty wide for this application. Nonetheless, we designed the hardtail such that most 2.25-inch tyres will fit. Note that we say “most 2.25 inch tyres”, because there is another problem with tyre clearance.

Virtually no tyre manufacturer is honest about their tyre sizes. tyres can be up to 20% narrower than advertised, because riders like the sound of a wide tyre and the weight of a narrow tyre (so lying about the width “solves” both). You’re best off measuring a tyre yourself – both width & weight.


18:  Steering geometry

Our favorite feature on the hardtail may be the one you can’t really see: the steering geometry.

People often think they want a “stable” bike. Great if you want to go straight, but big forces are required to turn the wheel. And if the front wheel hits rocks and other debris at an angle, you need big forces to counteract those impacts.

Instead, we prefer light, agile but precise steering, which:

Ensures the minimum disruption from forces from the ground into your handlebars.

Let’s you point the bike exactly where you want to go.

Surprisingly (to others, not to us) it gives you very secure handling, also on steep descents.

It even allows for a different style of riding. Instead of grinding it out on short hills, you’ll find yourself standing up and powering over them more often.

View our geometry charts here.


19:  Seating geometry

Few topics generate as much confusion and bad advice as a frame’s geometry. There are a few reasons:

Sizing frames by seattube length (15”, 17”, etc.) is meaningless. It only determines how much seatpost sticks out, nothing else.

The toptube length you need is not fixed, it depends on the seattube angle (a slacker seattube requires a longer toptube to put the headtube in the same spot).

In the end, the frame has to connect your contact points: handlebars, saddle and cranks. Seattube, toptube and other dimensions only INDIRECTLY describe how a frame connects those three. Stack & reach on the other hand DIRECTLY describe the position of your headtube (and thus your handlebars) relative to your bottom bracket (cranks).

So first you put the saddle at your preferred height & setback (relative to your cranks). You can achieve this on almost ANY frame with the right seatpost & saddle rail position. Stack & reach now describe where the frame will put your stem/handlebar combo relative to your saddle & cranks. And that’s all there is to know.


20:  Testing

Our philosophy is that once we ship a frame out, we don’t want to see it again unless we meet you out on the trails. This may seem contradictory when designing very light frames, and of course mountain biking is a sport where damage can never be completely ruled out.

But we want to make the frames the absolute strongest they can be, which is why we subject them to the toughest internal lab tests we have. We then spend up to six months on the trails to judge the handling, stiffness, and general ride quality. This also allows us to solving any small annoyances like rattling or assembly difficulties.

Finally we ship it to EFBe, the German test experts for the bicycle industry. Their tests are so tough that most manufacturers don’t dare to send them their frames, knowing they won’t pass. But our philosophy is that we would rather know of a problem now than later. And we relished the challenge to become the lightest mountain bike frame ever to pass their test. The hardtail frame succeeded!



With regards to our geometry chart, please note the following:

We strongly believe in stack & reach as opposed to sizing by seattube length. Therefore our sizes are named after the frame’s reach.

We use a zero-setback seattube design, so the seattube angle will appear a little slacker than you will normally find (but a zero-setback seatpost will compensate that (and be lighter)).

You can use a suspension fork with 80, 90 or 100mm travel in our frame. Remember that the shorter the travel, the lower the front end and the more direct the handling. In general, we believe in relatively steep headtube angles for better performance in tight corners and less disruption from rocks and other debris you may find on your path.

It even allows for a different style of riding. Instead of grinding it out on short hills, you’ll find yourself standing up and powering over them more often.

Unit 9, Hatton Technology Park, Warwickshire. CV35 8XB                  Tel: (+44) 01926 842469


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