Condensed Аnswer: Technically, aero bars can be installed on a mountain bike. However, they offer dubious benefits given the geometry and function of an MTB. In most cases, the practice isn’t worth it.
MTB Geometry “Rejects” Aero Bars
MTBs aren’t created with aero bars in mind. The geometry of the frames reflects that:
- A Slack Head Tube Angle
The head tube angle (HTA) is the angle formed by a line extending from the head tube and a line parallel to the ground. (image below)
A slacker head tube angle pushes the front wheel further in front of the rider. This increases stability during descents, especially on uneven terrain, and makes it harder to get over the handlebars. Also, the riding position is more upright.
- A Steeper Seat Post Angle
Another important angle is the Seat Tube Angle (STA) (image below).
A slack seat tube angle puts the rider closer to the rear wheel whereas a steep one positions the rider forward.
Modern MTBs are moving towards steeper seat tube angles which are the only way to compensate for the slack head tube geometry.
However, the seat tube angles of MTBs are not as steep as those found on TT bikes which are the prime candidate for aero bars.
- Small Saddle to Handlebars Drop
The saddle to bar drop describes the relation between the saddle height and the handlebars. TT and road bikes have a massive saddle to bars drop in comparison to MTBs.
A big saddle to bars drop allows the rider to get in a very aggressive aerodynamic position and get the most out of the aero bars.
However, in practice, most MTBs have a small or even a negative saddle to bar drop (the handlebars are taller than the saddle). As a consequence, the rider isn’t a lot more aero even when riding on aero bars.
One way to increase the saddle to bar drop would be to remove all the spacers under the stem (if there are any) and/or slam the stem.
Ultimately, the slack head tube angle, the insufficiently steep seat tube angle, and the minimal saddle to bar drop result in the following problems when using aero bars on an MTB:
- Limited room – unless the rider is very flexible chances are that he/she will hit his elbows and chest with his knees
- Uncomfortable position – an MTB frame isn’t designed for aggressive aero riding and will compress the rider unnaturally when riding in the hoods. The result is extra stress on the lower back.
- Inability to get as aero as possible – even if you remove all the spacers and slam the stem, the saddle to bar drop won’t be as large as what we see on TT bikes.
Additional Downsides of Aero Bars on an MTB
Aero bars are not ideal for an MTB due to the following issues:
Aero bars reduce the rider’s control. As a result, riding with aero bars on uneven terrain is a recipe for an accident. Ultimately, aero bars work best on TT bikes ridden on controlled paved roads without significant irregularities.
- No Brake and Shifter Access
TT bikes are designed for aero bars and give the rider access to the brakes and shifters from the aero position.
However, when you put aero bars on an MTB, the brakes and shifters will remain at their original location and won’t be accessible when riding in the aero bars. Thus, the rider’s ability to quickly slow down will be greatly diminished.
- Potential Hazards
The aero bars may catch on an external object or hurt the rider or another person during an accident. This is one of the reasons why aero bars are banned from professional cycling competitions outside of triathlons and time trials.
- Non-compatible with Handlebar Bags
Aero bars make the installation of handlebar bags a difficult if not impossible task.
- Aesthetic Destroyers
It’s subjective, but in most cases, aero bars look fine only on TT bikes. Putting them on hybrids, road bikes, and MTBs is often unsightly.
There are two main reasons to put aero bars on an MTB – to obtain an extra aero position and hand placements.
The same goals can be achieved via the following alternative routes:
Bar-ends are an old-school MTB accessory normally attached to the ends of the bars. They give you an extra hand position and are very helpful during climbing.
However, they don’t come with aero benefits, although some people install them between the brakes and lean on them as if they’re aero bars. It’s questionable to what extent this practice is “healthy”.
Drop-ends are bar-ends shaped like the drops on a road bar. They bolt on to the handlebars and give the rider an extra hand placement plus an aero position. Those work better on an MTB with older CX geometry than modern trail bikes. In fact, it would be funny to put drop-ends on an aggressive MTB machine.
3. Alternative Bars
One of the more aesthetic solutions is to switch to alternative handlebars that give you more hand positions. A cheap and fairly common choice, especially in Europe, would be butterfly bars. They give you multiple hand positions and some models come with a bend that can be used as semi-drops to gain an aero advantage.
4. Turn Your MTB Into a Hybrid
If you have a CX bike, you can easily turn it into a hybrid by putting slick tires on it.
For additional information on how to make a hybrid faster check out this article.
5. Buy a Road or Gravel Bike
If your goal is to be as fast as possible on paved roads, nothing beats a dedicated road bike. You certainly can’t match the speed by merely putting aero bars on an MTB.
The downside of road bikes is that they don’t operate well on uneven terrain. This is where gravel bikes come to save the day. They’re faster than MTBs on the road while also being able to cover off-road sections.
Summary: What You Need To Know
Technically, nothing stops you from installing aero bars on an MTB, but their function goes against the average MTB geometry.
The downsides are:
- The rider is less visible in traffic when pedaling in the aero position.
- Aero bars offer very little control and can be used only on paved roads without lots of traffic, intersections…etc.
- The rider cannot access the levers and the brakes from the aero position.
- The geometry of the bike can result in the rider’s knees hitting their elbows and chest.
- The rider may experience back pain.
If you’re looking for extra hand positions and speed on paved roads while still riding an MTB, it’s better to make the following changes:
- Mount slick tires
- Lock the suspension or switch to a rigid fork
- Install bar-ends or switch to alternative bars (e.g., butterfly).