New FD-5801 front mech

I've not seen much discussion about the revised 105 front mech so thought I'd share my observations.

TL:DR - easier shifting, easier setup, but you'll need to RTFM and invest in a good quality 2mm allen key.

My primary reason for upgrading was that the long arm of the 5800 mech was touching my mudguard, and being a sexy set of PDWs I really didn't want to have to notch them.

There are a couple of other reasons - less chain rub if you're running discs and cross-chain (but you'll also need to replace your chainset to get that - FC-RS510 in the case of 105), and lighter shifting. 

Its a relatively cheap upgrade (around £30) and if you read the manual will probably take around an hour to install. Depending on the condition of your shift cable you may need a new one as you won't be able to thread a frayed cable (or one cut very short) through the cable guides on the mech. You'll also definitely need to read the manual because the setup is completely different to recent Shimano front mechs. Make sure you've got a very good quality and ideally long-armed 2mm allen key as all the adjustment bolts require this. You need the long arm to get to the support bolt, and because this is thread-locked its very difficult to turn with a standard length key. The same key will adjust the limit screws and cable tension adjuster.

The first setup change is that there's no longer a cable position adjuster on the arm so you don't need to work out which side to route the cable before anchoring it. Cable anchor is on top of the mech so you just pull the cable reasonably tight and then adjust the tension with the on-mech adjuster. A big plus here is that there is now a scribed line on the mech so you can set it accurately first time - just line up the two lines by turning the cable tension adjustment bolt accordingly.

Then its simply a case of attaching the chain and doing the usual limit adjustments, which are now 2mm allen bolts rather than JIS screws. They're also on the side of the derailleur which for me made the adjustment slightly easier as the screwdriver didn't interfere with my view of the chain and chain guide. 

So, was it worth doing? Well, the mech is now nowhere near my mudguard, and there's no risk of the cable end tinging against either the wheel or the crank arm. The shifting is noticeably easier - lever throw is the same but it definitely requires less effort. Not much more force involved than going from big ring to little ring. I guess I could also remove the frame cable adjusters and just use the one on the mech but I'm not sure that's an improvement. On a new cable run you'll be able check visually whether the tension needs adjusting after everything has snugged up by referring to the scribe marks which do take the guesswork out of tension adjustment.

Compatability-wise this will work with your existing chainset and shifters, including Tiagra 4700 as the cable pull hasn't changed. Despite the fact that its designed for the latest DA, Ultegra, and 105 chainsets which have a larger gap between the rings I'm not seeing any issues with my FC-5800 double. Its only 0.4mm difference so on a ten-speed drivetrain (I'm running Tiagra 4700 shifters/rear mech) I guess there's enough tolerance in the system. Strictly speaking Shimano's compatability chart doesn't list this as a compatible combination but it's working fine for me.

Overall this was the easiest front mech I've installed and adjusted and it looks suitably neat and stealthy. The allen bolts are a definite improvement over the JIS screws for H/L adjustment and the shifting is much lighter.