Condensed answer: A negative stem positions the handlebars lower and allows the rider to shift more of their weight on the front wheel. The result is a more aerodynamic stance and easier climbing.
What is a negative stem?
A negative stem lowers the handlebars (hence why it’s “negative”) whereas a positive stem elevates the bars.
All threadless stems come with an angle that can be positive or negative depending on how you position the stem.
For example, if a stem has a 17-degree angle, and you position it at a slope towards the ground, you acquire a -17-degree negative angle. If you flip the stem, you will have a +17-degree positive angle.
For that reason, the angle of a stem has both a “+” and “-” in the description, and the logo is facing up and down.
The above applies only to threadless stems. (Threadless stems grab the fork’s steerer with a bolted mechanism.)
If you have an older quill stem, its angle cannot be switched from positive to negative or vice versa because the stem has a single orientation. (The body of a quill stem slides into the fork’s steerer.)
The Advantages of a Negative Stem In an XC Race
The main purpose of a negative stem is to lower the handlebars. This change has the following effects:
- The rider assumes a more horizontal back angle that reduces the drag created by the body (an aero stance). The more aero position makes it easier to maintain higher average speeds.
- The rider can position more of their body weight onto the front wheel. This move makes climbing easier by reducing the weight on the rear wheel.
- The rider can produce a more powerful pedal stroke. (We observe the same effect in the world of road cycling where drop bars are the norm.)
In short, a negative stem could make you faster overall in an XC race.
The Negatives Of a Negative Stem
- Stress on the Back
A negative stem forces the rider to bend over more. The more horizontal angle is more stressful on the back. In some cases, discomfort may occur.
- Front Wheel Lifting is More Difficult
A negative stem effectively steepens the head tube angle (HTA) and makes it more difficult to lift the front wheel of the bicycle. For that reason, stunt bikes such as dirt jumpers use positive stems and handlebars with a serious rise.
- Higher Chances of Falls
A negative stem shifts some of the rider’s weight on the front wheel and technically increases the chance of falling over the handlebars.
The cons of a negative stem aren’t a deal-breaker because XC riders do not have to perform massive bunny hops or extreme downhill descents. Instead, XC races include a lot of technical terrain with many uphill sections that are difficult to cover with tall handlebars.
How Much is The Average Angle of a Negative Stem?
It depends on the rider and the bike’s geometry. In some cases, the negative angle is subtle (e.g., 10 degrees). However, some riders prefer a more extreme angle (e.g., 27 or more degrees). The goal is to obtain a power pedaling position during difficult climbing sections.