Obeast

I ride a 26 wheel mtb frame with road components except for the crankset which I keep mtb. The crankset is for 8 speed and my entire drivetrain is 2016 tiagra 4700. I have trouble making the front derailleur move back and forth enough to shift from the 32 chainring down to the 22 chainring. Do I need cyclocross specific crankset to get the left shifter to shift properly? Is the bottom bracket axle of different length to road and mtb?

jsigone

Which size bottom bracket shell are you using, match the cranks spindle length to the BB shell.

Typically roadie cranks are same length as CX crank spindles. Which are narrower then MTBs aka closer to the frame.

You may want to try a MTB front derailleur to get the pull ratio correct. Maybe some one here know more or go down to the bike shop and ask, bring bike so they can see the problem.

Tim_Iowa

Aaaah, you've encountered the MTB/Road incompatibility problem. Welcome to the support group.
@jsigone has it correct, the issue is the different cable pull between the MTB front derailleur and the Road front shifter. They aren't the same.

You probably can't replace the front derialleur with an cyclocross/road one, because the cyclocross/road front derailleur won't reach out far enough for the MTB chainline.

And you probably can't easily replace the crankset with a cyclocross/road one with a shorter spindle, because: the small ring may hit the chainstay, and the chain line will be wrong.

Solutions others have used:
1) JTEK Shiftmate 7 (Shiftmate Compatibility Charts and Choices - Jtek Engineering)
2) experiment with old MTB/Touring front derailleurs from the 80s, they may have the proper "road" cable pull but with enough reach for your chain line.
3) experiment with a modern road front derailleur and a shorter spindle
4) change out the left shifter to friction (bar-end, Gevenalle, stem, downtube, etc)

vinuneuro

Sounds like he's already running a complete road groupset. It'd be simplest for him to just run a sub-compact road crankset.
@Obeast There is a lot of overlap between mtb and road bottom brackets. What frame do you have?

fietsbob

Cyclocross racing the crankset gear range is narrow, like a 39-46, if not just a single ring.. no low gears ..

because putting the bike on your shoulder, and running up a slope is faster , and in the race, faster lap times win.

it's probably not what you want.

Obeast

Itīs an old 1990s mtb frame with shimano 8sp components, no suspension but 1.125 modern steel fork. I have tried installing a 10sp road crankset but the smallest chainring rubs against the chain stay. I rather like the mtb crankset because it gives me the granny gear 22t but I prefer to ride the 32t most of the time. When I find a big hill, I hop over the bike and stick a tree twig into the crankset to move the chain back to the 22t. Would bar end friction shifter for the front derailleur solve this problem?

Tim_Iowa

Old-fashioned "manual" front shifting!

Yes, you could use the proper MTB front derailleur (still have the original?) with a friction left shifter. That shifter could be a bar-end, on the stem, on a clamp somewhere, or on a Gevenalle brake lever.

There are also some extinct, vintage ones that put a friction shifter near the brake lever, like Suntour Command or Kelly Take-offs.

Obeast

youīre absolutely correct. Sometimes old fashioned tech is the right choice especially when itīs designed for the frame. Yes, I still keep the old FD and front shifter.