Pros and cons of single-sided forks (Cannondale Lefty)?

  • Looks like they save some weight, but how do they have a better strength/weight ratio than a traditional two legged fork?

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    With the single-legged fork the fork and axle must resist huge bending all the time, producing even more of a problem than with the 2-legged fork under uneven compression.

    So how is this fork constructed and what advantages and disadvantages does this bring?

    Add "marketing" to both the pros and cons lists. So it stands out in the shop / catalog / photos, but its also attractive to the light-fingered. I'm curious why the rear triangle hasn't been offered in some single-sided format, like some sport motorbikes.

    My guess for the lack of single-sided rear triangles is that there isn't room for both cassette and disk brake on one side. Also, the chain is oily and I wouldn't have it near the disk.

    Can you turn further right than you can to the left? I'd expect the crown to foul on the frame when turning full left at low speed.

    @Criggie Yes, you can turn further right, but you don't usually turn past 90deg because that switches turn sides.

    Shame Cannondale don't make a righty, for countries that ride on the left side of the road.

    Pro: It looks really cool. Con: It looks a bit odd Beauty in the eye of the beholder, etc

    Cannondale have done a single sided rear, which did come with a "righty" fork. It was just a one-off special though. http://www.velovision.com/ftp-admin/VVArchive/www.velovisionmag.co.uk/showStory6004.html?storynum=877 Other single sided rears have been done: http://road.cc/content/tech-news/211246-rob-english%E2%80%99s-head-turning-single-sided-project-right-bikes The only production single sided rears I know of are various Mike Burrows designs though.

    There is folding bike that uses one sided front and rear hubs/forks. Search for "Strida Folding bike". Sure they use a belt instead of a chain, thus grease is not a problem for the disc brakes.

  • Jerryno

    Jerryno Correct answer

    6 years ago

    The single-legged fork must truly withstand heavier bending forces than conventional forks, simply due to physics and asymmetricity. But because of its different construction, the fork is actually stiffer than most 2-legged.

    Pros

    • The top is attached like a dual crown downhill fork, which is much stiffer than a single-crown.

    • The wheel axle is one-piece with the bottom part, which is stiffer than a quick-release axle (which is not solid at all) and on same level as a 15 thru-axle or 20 thru-axle. This design is used also on cars where the wheel doesn't have support from the other side.

    • The biggest difference is inside. While normal forks have round tubes that slide on oil film, the Lefty uses needle bearings on a square profile (same as the front leg of Boeing airliners.):

      lefty fork, pulled out to show bearings

    • Such solution works with almost same friction under heavy side-loads, whereas sliding tubes, when side-loaded (uneven compression of legs), do get friction losses (rolling resistance wins).

    • Having conventional fork with one leg wouldn't work, such a fork would get stuck. Having bearing-fitted 2-legged fork is an overkill and unnecessary weight.

    • The heaviest Lefty is still lighter than lightest carbon 2-legged.

    • You can replace the tire without removing the wheel.

    Cons

    • If you need to remove the front wheel, you need to remove the brake caliper also.

    • The axle works only with Cannondale hubs.

    • The front of bike will look unbalanced until you get used to it.

    • Pre-2013 models of Lefty are not sealed, having the bearings track just under a rubber sleeve. This required almost constant maintenance. Post-2013 models have a rubber-sealed leg with bearings further inside and with one plain bearing at the bottom which allows for service intervals as normal forks:

      image of pre- and post-2013 Lefties

    • Previous models, as some bikers on forums complained, had more linear action: they were easier to bottom than normal forks. So now the newer ones should have more progressive compression. (This is opinion based, but quite a lot of it online, there are no measurements.)

    • A Lefty is much harder to service than a normal fork. You need a couple of special tools, and the assembly is complicated.

    Regarding punctures, with 1-legged fork you can replace the tire without removing wheel.

    @ojs Good point, I'll add it to the pros section.

    Spot-on answer. I'll just add one or two things: removing the caliper to remove the wheel is not a disadvantage because the most likely reason to do so is to fix a flat tire, which you can do with the wheel in. You need special hubs for the lefty, but a lot of brands have them, you don't have to buy them from cannondale. And I don't think maintenance on a lefty is harder than other high-end suspensions.

    Another con is that you're entirely at the mercy of Cannondale when it comes to spare parts, there's no way to swap the fork for one that's still maintainable. Which is fine if you buy a new bike every couple of years, but if you ride the bike until it wears out, it's likely to be "can't maintain the fork" that means you repleace the bike.

    You also forgot to mention that most of the weight savings on the fork are spent on the crown and the overbuilt hub. Lefty forks are often a wash weight wise when you consider the system as a whole.

    @BlamBlamBlam Pretty much every brand of fork or shock uses proprietary replacement parts. Thats not specific to Cannondale or the Lefty forks.

    @zenbike I didn't know that, I thought headsets were fairly standard and any fork with roughly the right travel could be put into a bike to replace a dead one. But my experience with suspension is limited to the worn-out Cannondale Headshok that I can't replace because it uses a custom size headset. But yes, if the lefty uses a standard headset then replacing it is as easy as buying a new fork, dropping it into the bike and slotting the front wheel in.

    @BlamBlamBlam "Standard" is not quite accurate. The Headshok uses a 1.5" headset. Every fork uses a particular size of headset. There are about 40 different "standards" for headsets. The headset is not really a "fork part", but I get that it is affected by the fork sizing.

    The front wheel being harder to remove is a disadvantage is you need to transport the bike in a case, in a small car trunk or in a car rack that requires the front axle mount to hold the bike. I.E.you'll need to consider this if you frequently do any of this with your current bike.

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