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This post compares the advantages and disadvantages of slack and steep head tube angles.

Definitions

Head Tube Angle.

The head tube angle is formed by the head tube and the ground (or a horizontal line passing through the center of the bottom bracket. (image below)

What is a steep head tube angle?

The larger the head tube angle is, the steeper it is. For example, head tube angles around 71-73º or more are considered steep.

Steeper head tube angles are common for bicycles designed for efficient pedaling (e.g., road bikes, gravel bikes, urban commuters…etc.)

What is a slack head tube angle?

The smaller the head tube angle is, the slacker it is.

In general, angles under 70º are considered the beginning of the slack range. Slacker angles are common for mountain bikes designed to overcome massive obstacles during descents.

What Are The Advantages of Steeper Head Tube Angles

• Speed

A steeper head tube angle positions the rider’s mass closer to the front wheel. Consequently, it becomes easier to assume a more aerodynamic position. Reduced drag equals greater average speed and efficiency.

For that reason, road and TT bikes do their best to minimize the drag created by the torso by making the rider’s back angle as horizontal as possible to the ground.

• Climbing Power

Bicycles with steep head tube angles excel at climbing because the front wheel has better traction and there’s less mass over the rear end making it easier to spin the rear wheel.

As a result, the rider has an easier time maintaining a higher cadence. Cadence is a term indicating the rotations of the cranks per minute.

A higher cadence such as 90RPM or more produces a greater average speed and spares energy.

Also, the extra mass on the front wheel reduces the chances of accidental front wheel lifting when overcoming a section with a steep gradient.

If the front wheel goes up involuntarily, the rider loses control as well as the ability to maintain a steady pedaling output.

• Technical Control as Slow Speeds

A steeper head tube angle makes tight turns at slow speeds easier. This is beneficial when the rider is covering technical terrain. For that reason, XC bikes have steeper head tube angles than trail models.

• More Glute Involvement

The aerodynamic position resulting from the steeper HTA brings more of the rider’s hip extensors into the pedal stroke.

As a result, the rider can produce more power because the glutes are among the strongest muscles of the human body.

The Disadvantages of Steeper Head Tube Angles

• Obstacles = Problem

The more aggressive an MTB is, the slacker its head tube angle becomes. This geometry makes it easier to overcome road irregularities.

When the head tube angle is slack, the front wheel is further in front of the rider and there’s less weight on it.

Consequently, the front wheel has an easier time rolling over obstacles. And if the front wheel overcomes an obstruction the chances that the rear wheel will follow are very high.

Steeper head tube angles have the opposite effect. The extra weight on the front end and the position of the front wheel closer to the rider’s mass make it more difficult to get over irregularities.

• Falling Over The Handlebars

The steeper the head tube angle is, the easier it becomes to flip over the handlebars due to the extra mass on the front end.

• Horizontal Back Angle

The horizontal back angle is beneficial from an aero standpoint, but it does stress the rider’s back more. If the bike fit is poor, and the rider isn’t fit enough for the riding volume, chronic pain may arise.

• Wrist Stress

The shift towards the front end stresses the wrists, elbows, and shoulders because they have to support more of the rider’s weight.

The Advantages of Slack Head Tube Angles

Slack head tube angles weren’t a thing until MTBs acquired long travel suspension and became extremely aggressive downhill.

Until then, MTBs were essentially gravel bikes (although used on much harsher terrain) and had a very steep geometry.

The head tube angles of MTBs are getting slacker and slacker for the following reasons:

• Stability Downhill

By putting the front wheel further in front of the rider, it becomes easier to ride fast downhill on off-road terrain because the front wheel has an easier time overcoming obstacles. Hence the extremely slack HTAs on downhill bikes.

• Longer Travel Forks

A super slack head tube angle allows the user to run a fork with massive travel.

When the fork compresses, the head tube gets closer to the ground, and the head tube angle becomes steeper.

The slacker the starting head tube angle is, the more the fork can compress until it reaches its final position without putting the rider in danger.

Meanwhile, if you put such a fork on a regular bike with a steep head tube angle, the post-compression HTA will get dangerously sleep.

Also, the stress on the head tube itself will be massive, and sometimes the frame may crack (or completely disintegrate).

• Harder To Flip Over The Handlebars

The slacker the HTA, the harder it is to get over the handlebars because the rider’s mass is closer to the rear wheel.

• More Aggressive Front Braking

Thanks to the two points above, it’s possible to use the front brake slightly more aggressively. Of course, the extent of this technique depends on the rider’s skills, the terrain, the braking mechanism...etc.

• Tricks Are Easier

Slacker head tube angles facilitate the lifting of the front wheel – an essential move for the performance of many tricks such as the bunny hops and rear wheel manual.

In the case of MTBs, lifting the front wheel is also used to overcome obstacles such as tree roots, rocks, small logs…etc.

The Disadvantages of Slack Head Tube Angles

• Difficult Climbing

Slack head tube angles make climbing more difficult because the rear wheel is supporting more weight whereas the front wheel has less traction.

It’s also more difficult to exert force due to the body position of the rider. As a result, downhill bikes are rarely if ever used for climbing.

• Slow Speed Steering = Weird

The slack HTA makes steering at slow speeds a bit uncomfortable and “boat-like”.

As a result, going around tight corners is difficult, especially uphill.

• Turning Requires More Leaning In

Riders used to steeper head tube angles will find that steering via the bars only is not ideal when the HTA is extra slack.

The slacker the HTA is, the more the rider has to lean in the turns. Not everyone will like that.

• Upright Position

Slack HTAs put the rider in a more upright position creating more drag.

Steeper Seat Tube Angles Come to Save The Day

The slacker the HTA, the steeper the seat tube angle becomes. This engineering aims to negate some of the downsides caused by the slack head tube angles.

A steeper seat tube angle places more of the rider’s weight towards the front wheel and thus allows more efficient pedaling and improves front wheel traction.

The Average Head Tube Angles Of Road Bikes

The table below contains the head tube angles of some popular road models.

Note: The data in the tables focuses on medium size frames.

Conclusion: Road bikes’ average head tube angle is 72.475º and is therefore on the steeper side.

The Average Head Tube Angles Of Gravel Bikes

The next table shows the average head tube angle of gravel bikes.

Conclusion: The average head tube angle of gravel bikes is 71.79º or about 1% slacker than what we observe on road bikes.

The ever so slightly slacker head tube angle isn’t surprising given that gravel bikes are designed to conquer light off-road terrain.

The Average Head Tube Angles Of Touring Bikes

The next table focuses on the head tube angles of popular touring bikes.

Conclusion: Touring bikes have a much slacker head tube angle than road and gravel bikes averaging at 70.77°

Why? The slacker head tube angle puts the rider in a more upright position which relieves wrist and back stress. The relaxed stance is especially beneficial over long distances.

The Average Head Tube Angles Of XC Bikes

The next table focuses on the head tube angles of popular XC bikes.

Conclusion: The angles of modern XC bikes are quite slack at 67.59º on average. Not a surprise since modern XC bikes have become way more aggressive than their ancestors.

The Average Head Tube Angles Of Trail Bikes

The next table focuses on trail bikes.

Conclusion: The average head tube angle of trail bikes is quite slack at 65.1º. No surprises since trail bikes have to overcome hardcore off-road terrain.

The Average Head Tube Angles Of Downhill Bikes

The next table contains the head tube angles of popular downhill bikes.

Conclusion: Unsurprisingly, downhill bikes have the slackest head tube angle on average at 63.09º.

The Average Head Tube Angles Of Dirt Jump Bikes

The next table focuses on the head tube angle of dirt jumpers.

Conclusion: Dirt jumpers have a conservative head tube angle of around 69° that’s neither steep nor as slack as what we observe on aggressive MTBs.

The Average Head Tube Angles Of Race BMX Models

The next table analyzes the head tube angles of RACE BMXs.

Conclusion: Race BMX bikes have steep head tube angles averaging 73.2°

The Average Head Tube Angles Of Freestyle BMX Models

The next table presents the seat tube angles of freestyle BMX bikes.

Conclusion: Freestyle BMX bikes have very steep head tube angles averaging at about 75°. The goal in this case is to increase the responsiveness of the bike – a critical property when performing technical stunts.

Meanwhile, race BMX bikes do not have to be as technical and thus can get away with slightly slacker head tube angles (values above) making it easier to overcome obstacles in front of the rider.

The list below contains the average head tube angles in the tables going from the steepest to the slackest.