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?



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?
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.):


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:


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.