Fork Column Length and Sizing
Cutting and Sizing Forks
Typical Tools and Supplies:
- Saw Guides SG-1, SG-2, SG-3, SG-6
- Hex wrenches (commonly 5mm and 6mm)
- Hack saw, good quality blade of 24 TPI (32 TPI for carbon)
- Files, flat and round
- Tape measure and caliper
- Bench vise (optional)
This article will discuss cutting and sizing of bicycle forks. See also related articles:
The steering column on new forks typically come longer than required. The column is then cut for the size according the particular bike it is to be installed in. There are two basic methods for determining steering column length. The fastest method is to take appropriate measurements and determine the length mathematically. Another option is to installing the fork without cutting it and then measuring the amount necessary to cut. The fork is then removed, cut, and re-installed. In either case, use care as cutting a steering column too short can be an expensive mistake.
Threadless Type Forks
Threadless type forks have no threads on the outside of the steering column. Headset bearing adjustment is done by pressure from an adjusting cap on top of the fork. The sizing procedure using measurements is described first.
Begin by measuring the length for the head tube and write this number down. Do not include any pressed cups or races.
Next, determine the stack height of the headset to be used. Stack height is basically the amount of steering column length the headset will occupy. A new headset in the box will commonly have the stack height listed on the box. The stack height listed on new headsets does NOT include any extra washers and the stem height. YOU MUST ADD STEM HEIGHT AND ANY EXTRA WASHER TO STACK HEIGHT BEFORE CUTTING.
It is always a good idea to double check the stack height listed by actual measurements. Assemble the lower part of the headset with bearings in place. Measure the from the fork crown race to where the lower pressed race would enter the head tube.
Next, measure the upper stack from where the upper pressed race would enter the head tube to the top of the adjusting race. Add any spacers and add the height of the stem.
Add all these numbers together. You must now deduct a bit to allow a gap between the top of the stem and the top of the steering column. Typically, 3mm is adequate. See image below for adequately sized steering column.
Threadless forks with carbon fiber steering columns have special concerns. There is a limit to the amount of spacers between the stem and the headset. Too many spacers may stress the carbon fiber and lead to failure. Contact the fork manufacturer for limits in regards to your fork. Generally, manufacturers recommend no more than 20mm additional stack height between stem and upper race.
Threadless Sizing Example
- Our bicycle has a head tube of 103mm.
- The lower stack measures 12.5mm.
- The upper stack, including a wide washer, measures 20.5mm.
- The stem height is 41mm.
- The total of lower stack, upper stack, washers, and stem is 74mm. That would place the fork exactly at the end of the stem, so we deduct 3mm for a working total of 71mm
- Add our 71mm to the head tube length of 103mm for a steering column length of 174mm. If you end up with a decimal answer, round up when cutting.
Assembly / Disassembly Technique
If the headset is already installed, an alternate but slower technique to determine steering column length is to assemble the steering column into the head tube with bearings in place, and assemble stem and spacers on top. Scribe fork at top of stem. Remove fork and cut 3mm below scribe line.
Cutting Steering Column- Threadless
It is recommend to use saw guide SG-6 to cut threadless steering columns. This tool holds the fork square to the saw blade. Use of a good quality and sharp hack blade is needed for a properly cut column. Generally, 24 teeth per inch are recommended for steering columns.
NOTE FOR CARBON FIBER: For carbon fiber steering column, a finer 32 TPI blade is recommended. To minimize dust from the carbon, keep the blade wet. If you prefer to use a "diamond rod blade", it is possible to install washers to widen the guide slot of the SG-6. Another option is to use the outer blade as a guide. Hold the diamond blade against the outside steel face of the SG-6. Align the cut mark to this outside face, and press gently against the tool as you cut. Note that this will visually mar the tool. Use a fine emery cloth to finish the end.
- Determine correct length of steering column. Mark column using marker or scribe.
- Place fork inside SG-6. Loosely secure handle.
- Move SG-6 saw guide opening over mark on column.
- Secure SG-6 handle and place SG-6 in vise.
Cut through column. (NOTE: Cut with pressure only in forward direction. Do not apply excessive pressure on blade.)
- Loosen handle and move column to slightly protrude past cover plate.
- Use flat file to finish end of column. Use round file or de-burring tool to remove sharp inside edge of column.
- Loose handle and move column further through cover plate. Leave handle loose to allow column to rotate.
- Rotate fork and use flat file to bevel outer sharp edge of column.
- Remove fork from SG-6.
Threaded Type Forks
Threaded forks require similar measurements as threadless forks. Begin by measuring head tube length. Next, determine stack height. The stem is not included in figuring stack height at all. If you are measuring stack height, assemble the lower part of the headset with bearings in place. Measure the from the fork crown race to where the lower pressed race would enter the head tube. Next, measure the upper stack from where the upper pressed race would enter the head tube to the top of the adjusting race. Add any spacers, including any brake housing stops or reflector brackets. Measure the height of the locknut, but deduct the amount of any "lip" at the top of the nut. The steering column should not contact this inner lip when the locknut is secured. See cross section below.
Threaded Fork Example
- Our bike has a head tube of 205mm.
- The stack height including washers, and accounting for locknut lip, is 42mm.
- The steering column should be 247mm long.
Cutting Steering Column - Threaded
It is recommend to use saw guide SG-1 (1" column), SG-2 (1-1/8" column), or SG-3 (1-1/4" column) to cut threaded steering columns. This tool holds the fork square to the saw blade. Use of a good quality and sharp hack blade is needed for a properly cut column. Generally, 24 teeth per inch are recommended for steering columns.
- Determine correct length of steering column.
- Clamp SG securely into vise.
- Thread column into Saw Guide SG. Measure from base of fork crown to gap in guide plates. Thread SG as necessary to achieve correct measurement.
- Cut through column. (NOTE: Cut with pressure only in forward direction. Do not apply excessive pressure on blade.)
- Thread column until it slightly protrudes past cover plate.
- Use flat file to finish end of column. Use round file or deburring tool to finish inside edge of column.
- Thread column further through cover plate.
Use flat file to bevel outer thread at about a 45 degree angle. This allow easy start of threaded races and locknut.
- Remove fork from SG