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The head tube angle is entirely dependent on the frame, and the stem has no impact on its absolute value.

Nevertheless, the stem can contribute to or take away from the head tube angle’s effect on a bike’s handling.

Hence the importance of selecting a stem that compliments the geometry and function of a particular bicycle.

Head tube angles are divided into two groups – steep and slack.

Steep head tube angles make the bike more responsive, especially at slow speeds, and facilitate climbing whereas slack head tube angles improve the bike’s performance when descending on off-road terrain and make it easier to lift the front wheel (read more).

Steep Head Tube Angle + Long Stem

The stem’s length is the first important property that one should analyze.

The general rule is as follows:

• Long stem = slow response
• Short stem = fast response

A long stem increases the delay between the rider’s input (steering command) and the fork/front wheel response.

This property of long stems makes them suitable for road bikes and other models designed for pedaling efficiency on flat roads, hills, and descents.

A longer stem results in more stable performance at high downhill speeds on paved roads because the minuscule movements that the rider might make are “softened” before reaching the fork.

As a result, high-speed descents feel safer and the user has an easier time reaching greater speed.

The delay that a long stem introduces is also complimentary to a steep head tube angle.

A steep head tube angle increases responsiveness whereas a long stem decreases it. Consequently, the combination becomes necessary for some bicycles.

For example, road bikes require a steep head tube angle for optimal climbing, but they also need stability at high speed. Thus, it makes perfect sense to use a long stem on a road machine.

Short Stem + Slack Head Tube Angle

A short stem does the exact opposite – it increases the responsiveness of the fork.

The more aggressive the MTB, the shorter the stem. Downhill bikes (arguably the most aggressive MTBs with the slackest HTA) have the shortest stems mounted directly onto the fork (hence the name – direct mount stems).

Why?

The short stem provides snappy steering making it easier to navigate the bike around off-road obstacles.

Unlike road bikes, MTBs require sudden and yet precise maneuvering because the terrain isn’t as predictable. The only way to achieve this type of effective input speed is to make the stem shorter.

The short stem is combined with a slack head tube angle because this geometry makes descending on off-road terrain easier and safer.

The only way for this combination (slack HTA + short stem) to work is to use wide handlebars.

The shorter the stem gets, the wider the bars on an MTB become to get extra leverage and reduce the effort needed to control the front end.

If you put short (e.g., 40cm) handlebars on an MTB, the steering will be very poor due to the shorter lever arm making it difficult to control the twitchiness coming from the short stem.

FAQ: Can I Combine a Short Stem with a Steep Head Tube Angle?

Freestyle BMX bikes have steep head tube angles and short stems. After all, the purpose of BMX bikes is to perform technical stunts requiring precision and control. If you were to put a long stem on a BMX, the bike would perform poorly due to the delayed input.

And if a BMX has a slack HTA, its performance at slow speeds will suffer. The only option left is to use a steep HTA and a short stem.

However, for other styles, the short answer is – You can, but why?

If you plan on putting a very short stem on a road bike, the handling of the bike will be affected negatively.

Here’s what will happen:

The short handlebars common for road bikes combined with the short stem will make the front end too responsive.

The effect of the road on the fork will also be more difficult to control. (In other words, the rider will have a harder time “neutralizing” the forces coming from the ground.)

That said, if you install much wider handlebars on the bicycle, the twitchiness will be greatly reduced. But if you do that to a road bike, you will lose aero points and change the entire bike fit.

Additionally, the shorter stem will reduce the “effective reach” and will position more of the rider’s weight on the rear wheel. The result will be a less aerodynamic position, smaller glute involvement, and reduced front wheel traction making climbing harder.

FAQ: Can I combine a Long Stem With a Slack Head Tube Angle?

The short answer is – You can, but why?

A longer stem will negate some of the benefits that come from having a slacker head tube angle.

For example, the effective reach will increase and more of the rider’s weight will shift onto the front wheel.

The weight shift will make it harder to overcome road irregularities and increase the chances of going over the handlebars.

As a bonus, the longer stem will hurt the bike’s handling at slow speeds and produce a ton of “lag”.

What About Stem Rise and Drop?

Modern threadless stems come with an angle that can either rise or drop the handlebars by a certain degree.

The stem angle has no impact on the head tube angle, but it does affect the effective stack and reach of the bicycle (learn more).

If the stem has a positive rise and is replaced with a longer one, both the effective stack height and the reach will increase.

As a result, the rider will be stretched forward more, but the back angle will become slightly more upright too. Consequently, this modification is not completely unreasonable even if the bike has a fairly slack head tube angle.

Of course, the additional length of the stem shouldn’t be excessive or else the effects of the slack HTA will be hurt as explained above.

In practice, this move could be fine for an XC bike that has a slightly smaller frame than needed.

If the stem has a negative rise and is replaced with a longer one, the effective stack height will decrease, but the effective reach will increase. In other words, the rider will be stretched forward and the back angle will become more horizontal.

This change is more beneficial to road and track bikes because the position is more aerodynamic and increases front-wheel traction.

That said, some XC riders rely on a negative stem too to reduce drag. However, you will never see this practice on an aggressive MTB designed for descents as it brings a lot more negatives than positives.

Comparing Stem Lengths

The next few tables present the head tube angles of various bikes along with the stock stem length. The tables focus on medium size frames.

Road Bikes

XC Bikes

Trail Bikes

Conclusion: As expected, the slacker the angle gets, the shorter the stem becomes.

Note: BMX bikes are an exception to this rule as they have a 75º head tube angle on average a 50mm stem.

Summary: What You Need To Know

• Long stems reduce the steering responsiveness and make the bike less twitchy even when the handlebars are fairly short. For that reason, bikes designed for efficient pedaling (road, gravel, XC…etc.) have longer stems.

• Longer stems compliment steeper head tube angles by positioning the rider closer to the front wheel.

• Short stems make for very responsive steering and are rarely successfully combined with narrow handlebars.

• The more aggressive an MTB is, the shorter its stem gets while the HTA gets slacker. In this case, the responsiveness helps the rider maneuver around technical terrain whereas the long handlebars negate the twitchiness that would otherwise arise.

• BMX bikes are an exception as they combine a steep HTA and a short stem. The purpose of this combination is to keep the bike ultra agile – a requirement for the performance of street stunts.

• The average stem of a road bike is about 100mm whereas that of a trail bike is about 50mm. The stems of road bikes get longer with larger frame sizes whereas that of trail bikes stay the same length for all sizes.

• XC bikes have fairly long stems averaging about 70mm