02) Important fork, head tube and headset (bearing) dimensions

Maintenance and repairs

Maintenance and repairs

In a previous post headset construction was explained and important parts were noted. That post can be read here: Names of bicycle fork, head tube and headset parts. Part names defined in that post will be used here. This post will explain what needs to be measured when changing, or buying new bearings, or a fork (in terms of steering column, head tube and headset interface). First thing explained will be the fork measures, then the head tube ones and finally the important headset measures.

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Contents:
1. Important fork dimensions
…1.1. Fork crown diameter
…1.2. Steering column diameter
…1.3. Steering column length
…1.4. Stem column diameter (for threaded forks)
2. Important head tube dimensions
…2.1. Diameters of top and bottom head tube openings
……2.1.1. Bearing cup support dimensions (for integrated bearings)
…2.2. Head tube length
3. Important dimensions of headset bearings
…3.1. Fork crown race inner diameter
…3.2. Outer diameter of headset bearing cups
…3.3. Ball bearing, or cartridge bearing diameter

1. Important fork dimensions

1.1. Fork crown diameter

Fork crown diameter. Best done with Vernier calipers.

Fork crown diameter measurement Picture 1

Fork crown diameter measurement
Picture 1

This dimension is important for choosing appropriate fork crown race and matching head tube (inner) diameter.

1.2. Steering column diameter

For “tapered” forks (with steering column wider near the crown), diameter should be measured on the upper half. For “ordinary” forks it doesn’t matter.

Steerer tube diameter measurement Picture 2

Steering column diameter measurement
Picture 2

This dimension is important for choosing headset bearings and matching the head tube.

1.3. Steering column length

Nothing to it – just a tape measure and it’s done. 🙂

Steering column should be longer than the head tube – so that top and bottom bearing can fit with some room above for lockrings (for threaded forks), or stem attachment (for threadless forks).

1.4. Stem column diameter (for threaded forks)

For threaded forks, where stem column is fixed with a quill, inside the steering column, outer stem column diameter, as well as inner steering column diameter, must be compatible and their dimensions are thus important.

 

2. Important head tube dimensions

2.1. Diameters of top and bottom head tube openings

Head tubes for tapered forks have lower opening diameter visibly larger than the top one. Measuring diameters at head tube ends is important since that is where headset bearings should fit.

2.1.1. Bearing cup support dimensions (for integrated bearings)

With head tubes for integrated headset bearings, it is also important to check the depth of the bearing grooves, as well as the angle at which the angle is made. Obsolete standard is 36 degrees and the current is 45 degrees. More on this in a following post about headset standards.

Head tube bearing support groove for integrated bearings. Picture 6

Head tube bearing support groove for integrated bearings.
Picture 6

2.2. Head tube length

This dimension is necessary for choosing appropriate (minimal required) steering column lenght.

Measuring head tube length Picture 7

Measuring head tube length
Picture 7

3. Important dimensions of headset bearings

3.1. Fork crown race inner diameter

This dimension is important to see if the crown race fits the fork crown (onto which it is pressed).

Measuring fork crown race inner diameter Picture 8

Measuring fork crown race inner diameter
Picture 8

3.2. Outer diameter of headset bearing cups

Cup diameter is important since it needs to match the diameter of a head tube it is pressed into. Dimension of the cup part that is pressed into the head tube is the important one.

Measuring external bearing cup outer diameter Picture 9

Measuring external bearing cup outer diameter
Picture 9

In (an extremely rare) case that head tube for external bearings has different top and bottom opening diameters, that should be noted when choosing appropriate cups (and bearings). For such tapered head tubes (and forks), internal bearings are commonly used. Measuring internal bearing cup diameter principle is the same, just the part that remains outside the head tube is much smaller.

Headset cups: Left - internal Right - ahead Picture 14

Headset cups:
Left – internal
Right – external
Picture 10

3.3. Ball bearing, or cartridge bearing diameter

For bearings that use loose balls, bearing ball diameter should be measured when replacing them. For modern cartridge bearings, the whole bearing is measured.

Measuring cartridge bearing outer diameter Picture 11

Measuring cartridge bearing outer diameter
Picture 11

Other important dimension is the angle the cartridge bearing is finished with. This is primarily important for integrated bearings.

Angled bearing end. Standard is 45 degrees, 36 degree being considered obsolete standard Picture 12

Angled bearing end. Standard is 45 degrees, 36 degree being considered obsolete standard
Picture 12

This angle can be eyed, or the bearing can be leaned on a solid flat surface with the angled part down, then measured, or eyeballed – is the angle half way to vertical, or sharper.

This concludes the overview of important dimensions related to steerer (headset) bearings. Next post will explain the standards, so the appropriate type can easily be identified.

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