What makes a stem a "mountain bike stem" and why might it not be suitable for (or clamp properly on) a road bike?

Question

I am about to purchase a stem labeled "Mountain Stem" with the two diameters:

• 1-1/8" steerer
• 31.8mm bar clamp

to install on a road bike. There is stock of "road bike stems", but of considerably longer lengths (generally 60mm to 110mm). I'm adapting an XS adult road bike for a child's reach. (I wish it had 650 wheels, not 700, but that's a separate issue.)

Is there any reason why an MTB stem would not work on a road bike? What makes a stem a "mountain stem"? Will it be ("just") heavier/more rugged than necessary?

From the bike's specs sheet, I know the steerer is 1-1/8", but I am not certain that the bar clamp is 31.8mm and not 30.9mm. My impression is that it is only with seatposts that one can end up with this nasty little surprise of confusing 31.8mm and 30.9mm, and that adult bike handlebars are 31.8mm, but are not made in the 30.9mm diameter; can you confirm?

Related:

• Is there a difference between mountain and road stems?

Answer 1

There used to be a time when road and MTB stems differed. MTB was 25.4mm, road was 26.0mm.

Today, the most common bar clamp is 31.8mm, common to road and MTB. There's no difference apart from the stem length. If you have a stem with length and angle that fits you, and also steerer and bar clamps are of the correct size, it's usable in any kind of bike, road or MTB. Today there start to be various aerodynamic tricks done in road stems but not in MTB stems, but you don't absolutely need those on road bikes.

However, a stem of a length not typical for the type of bike you have could obviously affect steering geometry, especially if you use drop bars, and could also be a sign that you have a bike with a poor fit.

I would be very surprised to find out if bar clamp is 30.9mm.

A short stem buyer's guide:

• Prefer aluminum
• Shiny aluminum has less tendency to develop fatigue cracks than black anodized
• Check steerer and bar clamp sizes
• Check length and angle
• Check that steerer clamp has 2 bolts and bar clamp 4 bolts
• Prefer gapless stems to avoid handlebar developing fatigue cracks in the most stressed point.

Answer 2

Modern MTBs are designed around long top tubes, slack head angles, and very short stems compared to road bikes. The slack head angle would normally slow the steering, and the short stems counteract that somewhat. The full rationale is more complicated. Some discussion is here in a review of newer gravel bike geometry, here in an article specific to stem length, and here in a longer article about bike geometry.

Basically, an MTB-length stem is probably 30-50mm. This would be very short for a road bike and is not ideal. A very small upper body movement will translate into a steering response that's not noticeable on a normal length stem.

Now, aside from that, the stem will physically work, and it is a temporary deal for a growing person. I would make the person in question aware of the issue with the shorter stem. We are all adaptable, and they may adapt successfully to that stem. And also, they're growing, so I can understand why you would want to just adapt an adult road bike.

As a historical note, the 2013 question you linked could be considered a duplicate. However, I think MTB geometry was still evolving then, so the stems were not quite as short (and the TTs not quite as long, and the head angles not quite as slack). Older steerer and handlebar standards may have been more persistent then (e.g. 1" steerers on road, 25.4mm bars on MTB, 26.0mm bars for road), whereas now almost everything is a 1-1/8" steerer and 31.8mm handlebar clamp diameter.