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Relation formula between fork length change to headtube angle and bottom bracket height

6 min read

Asked by: Tony Barnett

How much does a longer fork change head angle?

As the fork gets longer, it raises handlebars up and slackens the head tube angle. As a rough estimate, each 20mm of travel added will correlate to a one-degree difference in the head tube angle.

How is headtube angle calculated?

Head tube angle – often shortened to ‘head angle’ – is measured from the horizontal to a line running down the centre of the fork steerer tube.

Does fork offset change headtube angle?

Both the headtube angle and fork offset can change the length of the trail number. By making the head tube angle slacker (decreasing the angle) you will make the trail longer. By reducing the fork offset, you will also make the trail longer.

What is headtube length?

The head tube length is measured from the bottom to the top of the head tube and – like the top tube – the angle of the head tube in relation to the ground is then the head tube angle. A longer head tube makes for a more upright position as it raises the front end somewhat.

How do you change a headtube angle?

Flip the bike upside down in the stand. Grease the offset lower cup and sit it on the head tube. The ‘-‘ should be at the front if you want a slacker head angle. Use the notches on either side of the cup like a gunsight to line it up centrally with the down tube.

How does fork length affect reach?

Any increase in fork travel will slacken the bike and shorten its reach. Slacker head angles boost confidence in steep descending terrain, but they make a mountain bike less agile at climbing technical singletrack as well as unbalancing the bike by moving the rider’s weight backward.

What is a good headtube angle?

The steeper the head tube angle, the better handling you’ll achieve, but with less stability at speed. Most trail bikes have settled on a minute range of somewhere between 65 and 67 degrees.

How do you measure a bottom bracket height?

Bottom bracket height is a vertical measurement from the ground to the center of a bike’s BB when the suspension is fully extended. Another measurement, BB drop, is a vertical measurement from a horizontal line through the center of the bike’s hubs to a parallel line at the center of the BB.

What is a slack headtube angle?

Slack Head Angles



A slack head angle means the front wheel will be further in front of your headset.

How is fork offset measured?

To start off, what is fork offset? Simply put, it’s the distance between the fork’s steering axis (i.e., the centerline of the steerer tube) and the center of the dropouts, measured perpendicular to the steering axis.

How do I choose a fork offset?

If you’re buying a new bike and cornering is your weakness, choosing a reduced offset fork can help. If you’re trying to race enduro or downhill competitively, a reduced offset fork can give you a slight edge. Ultimately, I think reduced offset forks work best on longer and slacker bikes with big travel.

Can I use a 29er fork on a 27.5 bike?

Condensed Answer: A 29″ fork can be installed on a 27.5″ bike, but it will have a notable effect on the bike’s geometry. The new fork will slacken the head tube and seat tube angle. Consequently, the handling of the bicycle will change in a manner that the rider may not like.

How does head tube angle affect reach?

In simple terms, head tube angle dictates how much effort is required to turn the front wheel. As head angle increases, the front wheel becomes easier to turn. It is also possible to make a sharper turn, and thus, the manoeuvrability of the bike generally increases with the angle of the head tube.

How does seat tube angle affect reach?

Seat tube angles range from about 70-75 degrees. A steeper angle moves the saddle forward while a shallower angle moves it back. Steep is fine for racers. It allows them to ride hard in an aero crouch, as the more open hip angle doesn’t constrain breathing or power transfer.

How can I increase my fork travel?

Quote from video: So you can change the dampers. And the air shafts around and it is super easy so if you've got a regular zeb or a regular lyric not the ultimate.

How do you slacken a head angle?

Quote from video: Get a either a spacer for your for your fork. This is a crown race spacer. Or you get the full on headset cup. And then you can also add some travel to your fork.

Why does seat tube angle matter?

Seat tube angles range from about 70-75 degrees. A steeper angle moves the saddle forward while a shallower angle moves it back. Steep is fine for racers. It allows them to ride hard in an aero crouch, as the more open hip angle doesn’t constrain breathing or power transfer.

How does head tube angle affect handling?

In simple terms, head tube angle dictates how much effort is required to turn the front wheel. As head angle increases, the front wheel becomes easier to turn. It is also possible to make a sharper turn, and thus, the manoeuvrability of the bike generally increases with the angle of the head tube.

Is a slack head angle better?

A slacker head angle makes the bike feel less nervous, more confident, and more stable through rough terrain, and more forgiving when the trail gets steep.

Is longer reach better MTB?

A longer reach and wheelbase means a more stable place to climb – well, as long as the seat angle is sufficiently steep to keep the front from lifting – and if the bike is stable then you’ll need less of the moving about as on the saddle as you attempt to find traction.

What is head angle MTB?

Head tube angle: Head tube angle is the measurement in degrees at which the head tube points towards the ground. A 90-degree head tube angle would be straight up and down. Think of a shopping cart with the wheels spinning in circles. Bikes use head tube angles ranging from 63 to 72 degrees.