Claris front shifter with mtb derailleurs?

Question

My son's bike (Decathlon road bike with 24" wheels) has the following configuration: Shimano Claris 3x8 STI shifters: the crank set is a Suntour 48-38-28 teeth with 152mm crank length; the cogs are a vintage Sachs Maillard, made in France, 12-28 teeth freewheel (screw on type); both derailleurs are Shimano Claris.

Now, while the rear derailleur works like a charm, the front derailleur doesn't change very well (it needs several pushes and goes directly to the third chainring, doesn't come back easily etc.) Someone explained me that this might happen because the crankset is a MTB type, and there's a wider distance between chainrings than in a road crankset. My questions:

1. Is this explanation correct?
2. If this is the explanation, can I replace the Claris derailleur with a MTB type derailleur (i.e. Deore?) Will the Claris shifter change properly the gears on a MTB derailleur?
3. What other hacks could I try to make the bike shift gears easily? I don't want to give up the claris shifter, nor the crankset, so, it's only the derailleur that remains, apparently, available for hacks / tweaking :)

Answer 1

I assume you've got the triple version of the Claris FD.

There are no strict standards for chainring spacing so while if the crank you had was too far off what the shifter/derailer "expect" then yes, that could cause bad performance, that part isn't really a mountain vs road thing, and is more about what width chain the crank is intended for. If that was causing the problem, it would probably be pretty clear when trying to adjust the shifting.

To answer your second question, no, a Shimano MTB front derailer will not work well because their road and mountain front shifters pull different amounts of cable. It can be made to work badly and this is enough that some people will tell you it works.

One probable reason it's working badly is because you're going under the minimum mid/top tooth difference spec of your FD-2403 by 1 tooth, which forces the derailer to be mounted higher than it should be and can throw off the chain contact points. That will exacerbate its not quite optimal match with the profile of the rings you have. Both of these tend to cause larger problems than people often expect.

Just using an IRD Alpina may completely solve the problem. It is currently the only STI-compatible FD available optimized for this kind of application. (And the main one ever, the exception being one or two weird old Shimano ones, RSX I think). What I don't know offhand is how well it's going to play with an 8-speed chain. If that proves finicky you can use a 9.

Answer 2

A front derailleur has several things affecting its performance when changing the front chainrings: Double vs Single chainrings (total width traveled) Diameter of largest and smallest (road derailleur cage is larger than MTB's) Width of axle/spindle in bottom bracket (example: 68x107 is narrower than 68x110) You can make the changes but you have to have patience and pay attention to the travel movement while shifting/experimenting with the gear changes on the bike stand. Don't forget the total chain width will be affected and can in turn affect the rear derailleur.