Designing bikes is not as simple as just picking a few tube lengths, selecting some angles and calling it a day. A bike design should be focused on creating a specific experience that will enable the rider to get the maximum enjoyment out of their adventure.

While some numbers are specific to the fit of the bike, other numbers help dictate the handling characteristics of the bike itself. For example, chainstay length helps with tire clearance, but also help with the acceleration of the bike due to the way the bike is balanced. Bottom Bracket Drop influences side to side reaction time in both positive and negative ways. If the BB is too high, the bike will “sit” high and feel awkward when cornering. If the BB is too low, then it might feel too planted/ stable and not really want to be thrown around. These characteristics of the design dictate how the bike will handle in certain situations.

When it comes to the front end of the bike, Head Angle, Fork Offset, and Trail are the numbers to be looking at. These numbers tell how the bike is going to handle, but more importantly, they are part of the conversation as it relates to the fit of the bike itself for the rider. To understand these numbers, it is good to have a baseline understanding of what they mean and how they relate to each other (don’t worry, there are graphics below all of this text):

Head Angle:

• The head angle refers to the angle formed between the bicycle’s steering axis (an imaginary line running through the center of the head tube and the center of the fork) and the horizontal plane.
• A steeper head angle (smaller angle) positions the fork more vertically, resulting in quicker steering response and more agile handling.
• A slacker head angle (larger angle) positions the fork more horizontally, providing more stability at high speeds and better handling over rough terrain.

Fork Offset (also known as rake):

• Fork offset refers to the horizontal distance between the center of the steering axis (a line running through the center of the fork’s steerer tube) and a parallel line passing through the center of the front wheel’s axle.
• More fork offset results in less trail, making the bike feel more responsive to steering inputs but potentially less stable at high speeds.
• Less fork offset reduces trail, which stabilizes the bike and improves straight-line stability.

Trail:

• The term “Trail” refers to the distance between the point of contact of the front tire with the ground and the point at which the imaginary line extending from the steering axis intersects the ground.
• Trail is determined by a combination of factors including head angle, fork offset (rake), and wheel size.
• Larger trail values generally contribute to greater stability at higher speeds and smoother straight-line tracking (but a bit slower to initiate a turn).
• Smaller trail values typically result in more responsive steering, making the bike feel more nimble and agile, especially at low speeds and during cornering (but a bit less stable at high speed).
• Balancing head angle and fork offset to achieve the desired trail is the key to optimizing a bike’s stability, agility, and the appropriate handling characteristics for the type of terrain it will be ridden on.

Head angle and fork offset are interrelated and influence each other in determining a bike’s handling characteristics. Generally, when designers change the head angle, they often adjust the fork offset to maintain similar handling traits. For example, if a bicycle has a slacker head angle, designers might decrease the fork offset to maintain stability, or if they want to increase agility, they might increase the offset. Conversely, if the head angle is steepened, reducing the fork offset can help maintain stability while enhancing responsiveness.

The way to look at these numbers is that the Trail number is the number that tells you how a bike will handle in terms of stability, responsiveness, and overall ride quality. On one hand, the head angle gives a glimpse into one part of the equation, while the fork offset is on the other hand providing a similar view. Both numbers by themselves do not tell the complete story, so it is when they are combined that a complete picture can be had. The result of that picture is the Trail number.

When designing a new model or a custom bike for a specific person, I generally mockup the design to make sure the numbers are where they need to be. I then go about fitting the bike to the rider focusing on saddle placement, handlebar placement, and in some cases toe overlap. It is on this last point that the head angle, fork offset, and trail figures come into play.

If a frame design is running into toe overlap issues with the specific rider, then I focus on the front end of the bike and how best to eliminate it. Since I know what the desired trail number is for how I want the bike to handle, the focus turns to seeing if I can adjust the fork offset and/ or the head angle to help compensate for this problem. Running a shorter stem always helps to push the front end out a little bit further without sacrificing the handling characteristics of the design, but sometimes the head angle needs to be slackened a bit to help that along. As the head angle slackens further, the bars pull back towards the rider which decreases the reach. I can then push out the top tube to gain some much-needed space to make sure the fit is correct.

Now that the head angle has been slackened, a fork with a different offset is required to bring the trail number back to where it should be for the design. Sometimes the fork is pushed out further and other times it pulls it in closer. Forks from different manufacturers can use different offset numbers so it is possible that components might need to change depending on the spec of the build.

The end result is that the Trail number is where it should be, or at the very least close to it, and that the bike will handle the way it is intended. Head Angle and Fork Offset help define what the Trail number is going to be for a particular setup. It is up to the bike designer to determine what the optimum number will be for that particular design so that maximum enjoyment is to be had.