DIAL IN YOUR MOUNTAIN BIKE’S HANDLING AND FIT WITH THE SCIENCE OF STEMS

Among the various components that make up a modern bike, the stem plays a pivotal role in determining how a bike handles and fits the rider, and, despite its relatively small size, significantly influences the rider’s experience. Connecting your handlebars to the fork’s steerer tube, your bike’s stem may seem fairly mundane on the surface, but it’s an incredibly important component that significantly affects your bike’s geometry, handling and your ride position.

Mountain biking is a sport that demands precision, control, and a keen understanding of one’s equipment, so an idea of how your cockpit influences your riding is key to maximizing your performance and confidence. We will delve into the science of mountain bike stems, examine how different lengths affect bike and rider fit, and what is best for various types of riding.

MORE THAN A CLAMP

The primary function of the stem is to transfer the rider’s input from the handlebars to the front wheel, but its length and height can significantly affect physical performance, comfort and front-end traction, making it one of the most critical yet often overlooked components of the bike.  “If riders are actively thinking about stem-length changes, they are on the right track,” says Specialized’s Leader of MTB, Sam Benedict. “If they are not thinking about stem-length changes, it could adversely impact their performance due to the inappropriate center of mass location that affects wheel-weight distribution and bike handling.”

Stems come in a range of lengths with different degrees of rise and drop with varying stack heights, which all affect the bike’s geometry and performance on the trail in different ways. Generally, longer stems are geared towards cross-country and marathon-style racing, while shorter stems cater more to trail and gravity-focused riders.

HANDLING CONCERNS
Stem length is measured from the center of the steerer tube to the center of the handlebars, typically ranging from 30mm to 60mm for most mountain bike riders. This seemingly minor measurement has a dramatic influence on the bike’s handling and the rider’s position, which can either give you a confidence-inspiring or terrifying experience out on the trail.

Modern trail, all-mountain, and enduro bikes are generally longer and slacker than those five-plus years ago, so stems have typically gotten shorter to compensate for this. However, that isn’t always the ideal setup for every rider. “I think it’s really easy to get caught up in trends,” says Jeff Lenosky (world-class trials rider, freerider and creator of the “Trail Boss” YouTube channel). “Reach has gotten longer and stems keep getting shorter. On steep downhill terrain, this works awesome, but on flatter or more technical terrain, it’s not always the best choice.” It all depends on your body’s dimensions, how you adapt to your bike’s geometry, and the types of trails that you ride the most often.

“The best place for your weight to be in a dynamic riding position is at or just behind the front axle,” says David Meredith of FSA Components. “This maximizes front-end traction while keeping your weight central on the bike and allows for the best possible braking performance. I’m not talking fit but actual riding position, which is dynamic across all terrain.” Bikes are designed to offer the best possible fit for a range of rider heights and sizes. Everyone is not the same, so adjusting the stem’s length allows the dynamic fit to be more tailored and can put the rider’s weight in that optimal position, oftentimes at or just behind the front axle.

LENGTH FOR FIT
Another big factor in choosing the proper stem length is driven by your biomechanical fit on the bike. A rider’s reach (not to be confused by the frame’s reach, although they are related) is directly tied to the stem length, along with the frame’s size and overall geometry. The standard-length stems that come spec’d on a bike are often good for most riders, but there are some who can benefit from something longer or shorter to dial in their fit.

Going too short or too long can compromise handling, though. In years past, it was not uncommon to sacrifice handling to get the ideal fit, but modern frame geometry has made that increasingly rare. That’s assuming you have the right-size frame in the first place, though.

If stem length is compromising handling on a modern mountain bike, you may want to consider getting a professional fit to make sure your frame is indeed the correct size. “Imagine a rider test riding at a shop and the fit feels too short; they may select the larger frame reach for fit reasons, but that may come with a penalty in handling related to the wheelbase and front center,” says Benedict. “What we’ve learned is that stem length and wheelbase interact and need to be considered together for a personalized experience on a rider’s home terrain.”

VARIABLES
Your other cockpit components (mainly the handlebars) also affect the effective stem length and your riding experience. Handlebars from different brands and manufacturers will have differing amounts of rise and backsweep, which can affect the rider’s reach. For example, running a bar with 10 degrees of backsweep will place your hand farther back than a bar with 6 degrees of backsweep. A stem-length change could compensate for this. It’s all about finding the bar and stem combination that will put your weight in the ideal spot and what is comfortable for you.

HEIGHT RITE
In addition to the length of the stem, the degree of rise or drop also plays a role in the bike’s handling and fit. The rise or drop refers to the vertical distance between the stem’s base and the handlebars and is typically measured in degrees. Most modern stems designed for trail, all-mountain and enduro riding have zero degrees of rise, and put the handlebars in a neutral position (since most of these riders will dial in the cockpit height with the handlebar rise), while longer stems for cross-country riding and racing typically have some degree of drop to further help with pedaling efficiency and put the rider further out front.

Bar height is often adjusted with the rise of the handlebars or spacers below the stem. When you run out of options on those fronts, it’s time to look at the stem. It’s not uncommon to see elite-level cross-country racers with radical negative-rise stems to get the bars as low as they need.

WHEN LESS IS MORE
Shorter mountain bike stems in the 35mm to 50mm range are popular with the majority of casual trail, all-mountain and enduro riders, as well as high-level racers. A shorter stem brings the handlebars closer to the rider, bringing your weight further behind the head tube and front axle, often resulting in better front-end traction and more confidence and precision on steeper descents. Particularly on steep downhills and chutes, where front wheel traction is paramount, having your weight centralized will give you more stability and let you take those sketchy lines with more confidence.

Along with a more centered weight distribution and increased traction, shorter stems allow for quicker handling and more responsive steering. On tight and technical trails where you’re constantly changing direction, more responsive steering can allow you to carry more speed and make better line choices. “Shorter stems handle better than longer stems because, as you turn the bars, shorter stems have less side-to-side sway,” says Lee McCormack of leelikesbikes.com. Another advantage to bringing the handlebars closer to the steerer tube is increased comfort and less strain, both while climbing and descending – especially if your stem is too long.

We can’t talk about short stems without mentioning Be More Bikes’ Raised Reverse stem. This unique take on the stem places the center of the bars 15mm behind the steerer tube with a reverse offset. It comes with either 115mm or 90mm of rise and is claimed to improve and calm the steering dynamics and make the bike corner much better. Stay tuned for a long-term review of this wild approach to the stem. Other ultra short stems have been tried in line with the steerer or just in front of it over the years with limited popularity.

That isn’t to say that shorter stems don’t have their drawbacks. As we mentioned above, being in a more upright riding position with more weight on your hips and legs can cause the front wheel to unweight or lift on steep climbs. Your bike’s seat tube angle and other geometry measurements also play into this significantly, but a 10 or 15-mm difference in stem length can be noticeable and lead to a change in handling while climbing. You’ll have to compensate by leaning farther forward or standing out of the saddle and putting more weight into your arms and hands.

Decreasing your reach too much can also have an effect on the stability of the bike through high-speed sections, rough rock gardens, or flatter and more gentle terrain; it can put too much weight on the rear end and not enough on the front wheel, leading to a loss of traction. “On steep terrain, you can still get weight on your front wheel with a shorter stem, but on flatter terrain, it’s not as easy, so a slightly longer stem can help get more weight on the front wheel and give you better traction,” says Jeff Lenosky.

A MIDDLE GROUND
Medium-length stems are slightly less popular than short stems, but they can be a versatile choice for riders who enjoy efficient climbing or spend multiple hours in the saddle. They strike a balance between the agility of shorter stems and the efficiency of longer stems. The 55mm–60mm stems are commonly found on shorter-travel trail and cross-country bikes where both climbing efficiency and descending control are important and a focus for the rider. These bikes typically have shorter top tubes and reach numbers compared to longer-travel bikes, so a slightly longer stem puts the rider in a better dynamic riding position and allows for more efficiency while climbing and pedaling.

There are times when running a longer stem can have advantages. Having a longer stem can also increase the straight-line and high-speed stability of the bike by increasing the reach and putting you in a more central position on the bike. “If a rider is looking for improved traction at the front tire, a small change in stem length can have a significant effect,” says Benedict. “Additionally, our studies have shown a more balanced suspension response when stem length is optimized.”

IT’S A STRETCH
Long stems above 60mm are typically reserved for elite cross-country racing and marathon-style riding where pedaling and climbing efficiency are paramount. A longer stem extends the rider’s reach, placing them in a more aerodynamic and efficient position for long hours in the saddle. For example, World Cup racer Lars Forester rides with an 80mm-long, -20-degree stem.

“Longer cockpits (within reason) tend to aid seated pedaling because they open up your upper body and encourage you to rotate your torso and hips forward,” says Lee McCormack. “These things tend to improve efficiency.” Cross-country race bikes typically have shorter and steeper geometry numbers compared to trail, all-mountain, and enduro bikes, so they are designed with longer stems in mind to place the rider’s weight in the optimal position to maximize pedaling and climbing performance while still giving optimal front-end traction.

THE SWEET SPOT
Finding your ideal handlebar position often takes experimentation on your part, or a professional fit. Think of it like an eye test; is this change better or worse than before? By making one change at a time and paying close attention to your fit on the bike and its handling, you can zero in on your ideal setup. And, it’s based on everyone’s individual needs, all of which are completely different when choosing a stem length. “The reality is, rider size and style may factor in, so it’s difficult to make a blanket statement,” says Benedict.