Join Date: Jun 2011
Location: Central NY
Bikes: Felt Brougham, Lotus Prestige, Cinelli Xperience,
Mentioned: 8 Post(s)
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Differences aren't necessarily huge:
MTB cranksets tend to favor triples, and seem to top out at 44, 46, or 48 tooth big ring. Roads have triples, but compact double is common, as is road standard, which means 50, 52, or 53 tooth big ring.
Front derailleurs have different cable pulls mtb vs road, and slightly different shapes to accomodate front ring size.
MTB cassettes have bigger range usually than road, so you might see a 12-36 mtb cassette where a road might top out at 12-28. Bigger range means bigger jump between gear shifts. These parts are interchangeable road vs mtn, the big thing is making sure that your rear derailleur can handle the range.
Road rear derailleurs tend to have shorter arms and are lighter weight than MTB. They are set up to cover a smaller cassette. MTB rear der's are designed for different things: mud clearance and getting smacked, so they seem to be beefier.
So with all of this, what you start with has some impact on your upgrade path, if you chose to upgrade. Stock bikes have the components that work together chosen so that they work. When you start upgrading, you have to stay with that scheme, or change enough to be compatible.
When I got my Sirrus, it had a MTB triple crank (48/38/28 teeth). I replaced that with a compact road (50/34 teeth) crank, and couldn't get it to shift well on the front, I think because the MTB front der had too steep of a curvature for the bigger crank, so it was hard to set the height and clearance correctly. But I couldn't just put on a road front der because road shifters vs mtb shifters have different cable pulls (they move a different amount of cable to actuate the der), so the curvature would be right but the movement wrong. I eventually, with the help of BF, found a road curvature mountain cable pull front der and it now shifts beautifully. Many bikes come stock with this set up, so there is no hassle figuring it out.
Hope this helps, and sorry I am wordy.