oldokie

Is there any difference between the spacing of the chainrings (triple) between a road bike and a MTB? I am trying to reuse a Sora 3300 triple crank with a 105 front deraileur driven by a MTB shifter. Will the MTB shifter position the FB correctly? It should if a road bike and MTB cranks have the same chainring spacing. If they are not the same, what is the spacing? I would measure it myself but I don't have a MTB crank to measure.

urbanknight

I think they do, but even if they don't, front derailleurs use micro-indexing or friction so it should work either way.

nitropowered

It should work

well biked

The potential problem is not with the chainring spacing, it's with the fact that the cable pull is different for road and mountain bike front derailleurs. For front derailleur shifting, road front shifters are designed to work with road FD's, and mtb front shifters are designed to work with mtb FD's. Since you're using a mountain bike front shifter with a road front derailleur, that's where the potential problem is. You might try it and see, although it's not ideal it might still work-

halfspeed

The chainring spacing won't be your problem. Unless you've got a friction shifter, it won't work properly. Shifters and =front= derailers aren't cross compatible between road and MTB in index mode. You'll need a MTB derailer to work with the shifter's indexing. You could still run into trouble, though. Shimano's MTB front derailers spec a minimum of 12 teeth difference in chainring size between the middle and large ring. If your triple uses a 30-42-52, the inner cage may rub the middle ring.

oldokie

Makes sense. I did not consider the different geometry between derailures of the road vs MTB. Even if chainrings have same spacing, the amount of cable pull might differ to deal with the slightly different geometry.
Looks like I will be forced to invest in a MTB crank & FD now. I intended to do that eventually anyway...just trying to delay that expense during my buildup.

well biked

I still say you could try it and see how it works, I've heard conflicting reports as to how much real-world difference it makes-

WillisB

There is a BIG difference in the cable pull for MT vs Road Derailleurs. I just finished switching from bar-end shifters to brifters on my commuter. Step 1 was to install a Tiagra FD. The reduced amount of lever throw = cable pull to shift was amazing. You might get away with it on a double. Not on a triple. You would never be able to properly hit the middle chainring. BTW the chainring spacing is the same.

Honestron

What is your road bike's center to center chainring spacing? This is of interest for a table that is in preparation. If you know the front cable pull also so much the better. Thanks. Honestron

HillRider

That isn't true as a general rule. Campy Ergo brifters (and not all of them) do use a ratcheting mechanism that is not true indexing and does allow many trim positions so they will work with a wide variety of front derailleurs and cranks. Shimano and SRAM shifters, both road and MTB, have a very limited number of "clicks" and trim positions and pretty much work only with compatible front derailleurs.

You are correct that friction shifters will work with nearly anything.

Chris_W

You can get a MTB shifter to work with a road FD acceptably but not perfectly, but it does take a bit of skill. Set the cable pull up so that the FD is lined up with the middle ring when the shifter is in position 2. Then set the position of the FD for the inner ring by using the inner limit screw (the shifter will release way more cable than needed, so the cable will go very slack when in this position, but that should not matter). Then set the outer limit screw of the FD to control the position for the big chainring. The shifter is going to try to pull too much cable to shift beyond this position, but hopefully you can find a place where the shifter clicks into the 3rd position and the FD is up against it's limit screw without requiring too much force in the shifter as you push against the limit screw - you may have to release a bit more cable to get this to work, which may put the FD a little too far inboard when in the 2nd position, but this is why I said it will probably not be perfect.

Again, this is not something that I'd recommend for someone who is not familiar with adjusting a front derailleur, it is something for the advanced class. However, I have had success once or twice and got the combination working reasonably well. Using a matching shifter and front derailleur is always preferable, though.