Last week Cannondale debuted an 8th generation Lefty Ocho XC fork, paired to their new F-Si hardtail. From the outside, its move to an industry standard steerer & single-crown were highlights, but inside the fork (strut!) got a total overhaul promising more suppleness & handling precision. So what makes Cannondale’s unique Lefty design both supple & precise compared to regular two-legged lightweight XC forks?

Inside the 100mm travel Cannondale Lefty Ocho XC fork

Without a doubt the one-legged mountain bike concept has been a decisive issue since Cannondale’s first Lefty fork was introduced back in 2000. (Yes, since this is a suspension technology article I should be clear that the Lefty is technically a strut since it only gets a single leg. With that out of way, I’ll go back to calling it a fork since that’s what we are comparing it to.) But while haters will joke about the missing leg, even those who adopt the Lefty have to adjust their brain to looking down and seeing just one leg.

Those who have ridden the Lefty know it is often touted as the stiffest XC race fork on  the market, so much so that this latest Lefty Ocho iteration actually detunes some of that signature stiffness for more predictable handling.

We sat down with Cannondale’s suspension designers & engineers to explain what makes the new Lefty Ocho tick, then we hopped on the new bike with them to get a feel for it.

Mountain Bike Fork Stiffness & Impacts on Bike Handling

Mountain bike fork stiffness can be broken down into three components, effectively the X, Y & Z axes about the headtube – Torsional flex, Fore/Aft flex & Lateral flex. Each aspect lends its own character to the ride, but generally more stiffness is better for predictable handling and smooth suspension operation. Specifically a fork that flexes front-to-back or side-to-side makes the bike harder to control, and especially with a traditional two-legged design, flex in any direction can increase friction in the telescoping bushings & seals.

Torsional Stiffness

Maybe counterintuitively (or because Cannondale was compensating for rider uncertainty) the Lefty has always been a benchmark of XC race level stiffness. In fact, the Lefty has sometimes felt as being too stiff, so Cannondale dialed back the torsional stiffness of the new Lefty Ocho by 14%. The engineering team says this lets the new fork track more predictably through rough, choppy trail & rock gardens – allowing the front wheel to deflect around big impacts instead of just bouncing off. Looking at the actual test figures Cannondale showed us, the new design places the Lefty Ocho on par with SID WC for torsional stiffness (actually 2% under the SID), both more than 1/3 stiffer than the 32 SC Factory 29er.

Fore/Aft & Lateral Stiffness

Front to back the new Lefty claims to be almost 18% stiffer than the SID and 8% stiffer than the 32. Side to side the Lefty is again the stiffest – 3% more than the SID & 7% more than the 32.

Cumulative or Overall Total Stiffness

Cannondale touts total stiffness as being the key factor in XC fork performance, simply adding up the torsional, fore/aft & lateral stiffness numbers. That might be an oversimplification, especially since the torsion figure where their fork is most flexible relatively, seems to actually improve ride quality. In any case cumulatively the Lefty Ocho looks to still be overall 9% stiffer than the slightly heavier SID World Cup, and 11% stiffer than the slightly lighter 32 Step Cast.

So how does that happen? How is the Lefty stiffer with just one leg?

A couple of things come into play here. First, since the Lefty is an upside down fork, the smallest diameter of the fork (the same 32mm sliding stanchions for Lefty, SID & 32) is closer to the axle at the end of the cantilevered arm. As opposed to having the narrowest element far away from the impacts coming through the wheel like a regular fork. Next, since there is only one leg, one stanchion, the Lefty can build in more material to resist flex. But that doesn’t answer it all, because a fork needs to be stiff, yet still slide smoothly.

 

Stiffness through Smooth Telescoping Action

 

Most telescoping forks rely on bushings that maintain a tight fit, but smooth sliding of the stanchion. That generally works well, but creates some friction that a bump has to overcome to move the fork. Also bushings tend to distribute the load over a relatively short area (around 2cm for example).

The Lefty does use a bushing at the lower contact between the stanchion and the upper/outer tube where the forces are lower, but uses a widely spaced ~7cm long set of needle bearings higher up in the fork to more evenly distribute forces.

The Lefty Ocho also moves to a new 3-sided design vs. the previous 4-sided. Basically it comes down to the fact that when you engineer a 4-sided system to take loads, it kind of needed to be overbuilt to provide the desired stiffness, resisting forces that came in at all angles to the bearings. With the new 3-sided design the ‘quartering’ forces that come in between two sets of bearings end up transferring the load across to the third bearing,  then more perfectly aligned to resist movement.

Of course more bearing surfaces would generally be thought to provide more support. But then you would have to reduce the size of the bearings & bearing surfaces to fit within the tube, and likely end up with more weight.

Smoothness through fewer Bushings & Seals

One of the biggest issues a fork needs to overcome in order to have supple suspension is bushing & seal stiction. Smoothness is then a balance of the tight fit of the bushings & seals needed to maintain the stiffness of the fork. Cannondale says that just by dropping one leg they cut the number of bushings & seals in half (bushings & bearings in green above, compressed & uncompressed).

The Lefty with one leg cuts the number of those friction-creating elements again by replacing the second bushing (typically in each leg) and replacing it with needle bearings which roll more smoothly under load. The result is a single stanchion seal, one Glide Bearing bushing pressed into the machined end of the fork upper/outer tube, and one rack of needle bearings – and the lowest friction under load of the three World Cup level XC forks (Lefty Ocho in green above), at least 140% less friction when static & at least 75% less when the fork is moving.

To manage the three-sided telescoping slider, Cannondale now put those needle bearings into a single Delta Cage hinged plastic carrier that makes them easier to service. The design also let them use fewer of needle bearings overall, for both lower weight & less friction.The bearing cage and 3-sided design also allowed for lower bearing preload, again reducing friction in the fork.

Suspension Internals

The Lefty forks have always had to combine air springs and hydraulic damping into a single leg, while the majority of dual legged forks spilt the two up.

Cannondale’s suspension engineers told us that got even more difficult when they decided to chop about 20% off the overall length of the fork for the new single-crown Lefty Ocho. So the internals got completely redesigned.

Chamber Damper

The new chamber hydraulic damper includes a simple 23-click dial rebound damping adjustment on the bottom of the fork leg and a 6 click compression adjuster on the top of the fork that then pairs with a cable remote lockout.

Inside is an Independent Floating Piston (IFP) that Cannondale says is designed to compensate for heat buildup and is self-bleeding.

OppO Air Spring System

The new OppO air spring design functions around the outside of the hydraulic damping cartridge, and uses a dimple machined into the internal of the upper/outer tube that automatically equalizes positive and negative air chamber pressure as you move the fork through its travel with a single Sidecar air valve (moved outside of the brake so any contaminant sprayed from the valve won’t end up on your rotor.

A stack of white plastic Ramp Clamp spacer at the bottom of the OppO air setup (around the plunger of the Chamber damper) then give you the chance to adjust air volume in the fork to tune the ride.

We did spend some time riding the new Lefty Ocho Carbon fork. Check out our first ride impressions here to see if the tech is backed up by trail riding & XC race course proficiency.

The fun never ends. Stay tuned for a new post each week that explores one small suspension tech, tuning or product topic. Check out past posts here. Got a question you want answered? Email us. Want your brand or product featured? We can do that too.

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20 COMMENTS

  1. Whilst it would obviously kill the biggest gimmick, I still hope Cannondale make a ‘LeftyRighty’ version of this fork using the needle bearing stanchion on two sides with a normal axle interface. I loved my Lefty Max TPC way back when, but beyond 140mm-ish travel packaging and damping become issues. A two sided fork with the resistance to twisting and that unique Lefty ground tracking feel would potentially make a Fox 36 look very silly indeed.

    • Agreed, that would be very cool! An added benefit would be separation of spring and damper each in a dedicated leg.
      In fact, there was such a fork around 1999, it was called the Moto FR. It had a hyper-stiff tubular arch and two (I think square) legs rolling on needle bearings. It also won some World Cup DH races if Google is to be trusted. 26″, rim brakes only as I recall, 100mm QR axle, not sure about weight. Double crown, though, which probably is not waht you’d like. They stopped making it when they switched to the Lefty, so it was made for only a year or two.

      • Sorry Tim, you’re mistaken. It was the “Moto 120 fork” that won World Cup DH races with Missy Giove (and others?) Search google images for “Moto 120 fork” and “Moto FR fork” and you’ll see there were massive differences. For one, the Moto 120 had the stancions on the bottom, while the Moto FR was opposite.

        • You are right on with a lot of those points CMC, however you should take another look at the Moto FR as it did not have stancions on the top, as you mention in your last sentence. The shock boots kind of disguise it, but look how skinny the lower legs are…there is no room for upper tubes to slide into them. It was upside down, but was rare in that it also managed to use rim brakes, which is difficult to package. They achieved that mix by using super short upper/outer tubes, which would have resulted in terrible stiffness on a normal fork, but with their square tube/roller bearing design they could get away with it. The only other upsidedown fork with rim brakes I can recall was the Halson Inversion, which had slots in the upper tubes, through which brake posts were affixed to the lower tubes, and they could slide up and down with the fork’s movement.

          • Wow, Kaiser. You do go way back. I had a Halson Inversion. The funny thing was, that fork worked pretty well for the time, but not for the reason it was inverted. They claimed that the inverted design meant lower unsprung weight, but in reality those lower tubes were on the heavy side, I am pretty sure heavier than RS magnesium lower legs. And the brace on it was both a tank and very flexy. The reason the inversion worked so well is that it had a very long (7″) spring stack with color-coded elastomers you could easily swap out; you could really tune the spring rate, and the rebound was manageable.
            The second generation Halson fork, the PDS, had an incredibly stiff and much lighter brace, but the internals on mine broke after I had ridden the fork for not so long.
            I also chatted with one of the founders of Halson Designs, he said that as forks got longer travel, they had to move away from inverted designs. The third gen of Halson forks, which never happened, were going to have a traditional design, but teardrop legs to resist twisting.
            There also was some European company that made an inverted fork with rim brakes, I saw one while in Finland and took a picture. If you drop me some kind of contact, I can send you the pic.

  2. lol
    everything what was the best regarding stiffness & smoothness is the opposite with the ocho now?

    Cannondale is for sure a genius of marketing.

  3. Until I see a manual showing how to do a complete re-build posted on Cannondale’s website this is all jibber jabber.

    I’ve owned a Lefty and it was a great fork…….until it was time to do a seal re-fresh and I had to take it to a local shop…..who then had to send it to Pennsylvania…..and got it back “in about 4-6 weeks maybe”.

    Total fricking joke as even in Boulder, Colorado there is not one shop that can even just do what equates to a lower oil bath refresh. For crying out loud PUSH is in Loveland and Dirt Labs is in Longmont who specialize in bicycle suspension and even they don’t service Lefty’s. Your only option is to hope you never have have an unplanned issue with the fork OR buy two of them so you’re not without a bike for months waiting for your fork to come back.

    It’s a cool design but any benefit is outweighed by the complete lack of ability for anyone to service the fork, buy the seal kits, see a manual, etc.

    • Project321 in Bend, OR is authorized to service the newer inverted Lefty’s. My latest Lefty is a 150mm Supermax on my M3+

    • Agree with brass nipple.
      None of this makes a difference if you don’t have a certified lefty tech within a reasonable distance.

  4. The single crown means that the largest frame size of their Slate can now have a taller stack. Right now the double crown limits it to the same as the stack on the next largest size. So, any word on when this show up on their other lefty bikes?

  5. 3% stiffer than SID = not stiff. I understand the motivation to decrease weight, but all mumbo-jumbo about “the-former-Leftry-was-too-stiff-so-we-‘tuned’-the-stiffness-of-the-new-Lefty” sounds more like a cover-up to hide the fact that due to this design, Cannondale couldn’t MAKE it as stiff as the former Lefty.

    Despite all the marketing bluffs, i don’t really see any advantage in a flexy fork. Anyone who has ridden former Lefties know that their biggest advantage is the fact that they don’t flex, by this creating a very predictable handling over rough terrain.

    Obviously, the lower weight is an advantage, and it’s OK if someone prefers to trade some performance for a lighter fork. But don’t tell us it is “better”

    • I agree. “this latest Lefty Ocho iteration actually detunes some of that signature stiffness for more predictable handling” Gee, I bet we will start seeing single crowns in DH for the same reason, more predictable handling = faster = “better”!

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