CNC Machined Bicycle Stems and Suspension Linkages in 7075-T6

The challenge

Stems and suspension linkages look simple, but they are highly loaded parts that the rider sees up close.

For the stem, the steerer clamp (Ø28.6 mm, the 1-1/8 in standard) and the handlebar clamp (Ø31.8 mm or 35 mm, depending on the bar) must be sized so the clamp closes evenly around the tube. A bore that is too tight can crush a carbon steerer; one that is too loose concentrates load at the bolt side. The slot edges and the inner bore edges must be smooth, with no sharp corners that start cracks.

For the linkage, two or three pivot bearing seats must be parallel and at the correct center distance, because the linkage defines the suspension leverage curve. Bearing seats for press-fit cartridge bearings need a fit that holds the bearing without distorting it.

Both parts also carry a cosmetic requirement: uniform bead-blasted texture and an even black anodize, with no visible tool steps on the curved surfaces.

Our approach

We split the work by part type but used the same principle for both: finish all related features in one setup.

Stem process route

1. Saw-cut 7075-T6 bar into blanks with stock on all sides.
2. 4-axis roughing of the body profile and lightening pockets.
3. Finish-boring of both clamp bores in a single setup on the rotary axis, so the angle between steerer and bar axes is fixed by the machine rather than by re-clamping.
4. Clamp slots cut with a slitting saw, then all slot and bore edges radiused with a form tool and finished by hand.
5. Bolt holes drilled and tapped; threads were roll-formed where the drawing allowed, as formed threads are stronger in aluminum.

Linkage process route

1. Roughing of the outline from plate, leaving a holding tab.
2. 5-axis finishing of the curved faces with constant-scallop toolpaths.
3. All pivot bores rough- and finish-bored in one clamping to hold parallelism and center distance.
4. Tab removal and edge blending.

Finishing for both

Parts were bead blasted for an even matte texture, then Type II black anodized. Bearing seats were sized with the anodize growth in mind and the bores were plugged during coating so their size stayed as machined. Logos and part numbers were laser marked after anodizing.

Quality plan

Inspection focused on the features that affect safety and fit.

• Clamp bores measured with bore gauges at several depths to check size and taper
• Pivot bearing seats measured for size and roundness; center distance and parallelism checked on CMM
• Edge quality inspected visually and by touch on every part, with special attention to slot ends and bore entry edges
• Threads checked with go/no-go thread gauges after anodizing
• Cosmetic check under consistent lighting for color uniformity and blast texture
• First-article inspection on CMM for each new size of stem and each linkage revision

All critical dimensions were 100% inspected before shipment, and material certificates for the 7075-T6 stock were included in each delivery.

What was delivered

We delivered prototype stems in several lengths and a set of prototype linkages, followed by a low-volume production order made on the same fixtures and programs. Each delivery included:

• First-article inspection reports for each stem length and the linkage
• Dimensional records for clamp bores, bearing seats and pivot center distances
• Material certificates for the 7075-T6 bar and plate
• Approved color and texture sample kept as a reference for later batches

Strength and fatigue testing of the stems and linkages was carried out by the customer according to their own test standards.

Questions about this project