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Bicycle Stem Caps
This project came about after stripping the threads of a stem cap on one of my bicycles.
What is a stem cap? (Also known as a top cap)
A stem cap is effectively a washer that is used to preload the two thrust bearings in the headset assembly of a bicycle - the area that allows your handle bars to pivot. I found a nice cross section from Akkana here.
It is worth mentioning that after pre-loading thrust bearings, the stem cap is purely cosmetic as the handlebar clamping mechanism carries the bearing preload and torque transfer through stem.
Objective
The objective of this project is to make a stem cap that can be customized easily, made cheaply in low quantities, specifically geared to sell to local bicycle shops to expand their marketing presence.
The concept I am shooting for here is similar to car dealerships putting their logo on the back of vehicles sold through them. If local bicycle shops put custom branded stem caps on all of their new bikes, the customer retention / customer loyalty could build for that shop.
Rev B: 3DP Proof of Concept
Below are some photos of RevB stem cap, which uses a countersunk flat head bolt for a clean, flat, visible surface. I did not like the return chamfer near the OD of this revision. The chamfer was too gradual, taking up too much flat top surface that could be used for etch detail.
The benefit of aligning the stem cap with one bottom surface and a countersunk hole is that the bolt will largely align the stem cap to be centered in the stem assembly. A common alternative is a counterbored socket head cap bolt, but this replies on user concentricity alignment to guarantee good cosmetic appearance on your bicycle.
The flat head bolt design allows for a thinner overall profile of the stem cap.
Profile Strength Validation
The following analysis was completed using Ansys Mechanical to validate the overall strength of the stem cap. The benchmark load case is a M6x1 socket head cap bolt torqued to industry standard 4-6Nm, producing 1149N of axial compressive force assuming dry mild steel on aluminum.
A worse case scenario is checked, considering greased steel on steel contact. This situation creates
. Bearing preload force not checked.
The actual load case used is 3000N chosen to model an extreme case of over-torquing the stem cap bolt to preload the bearings.
Equivalent Stress Plots:
Stress is approaching yield limit at the bottom fillet in this load case. This risk is accepted as SOP does not indicate a greased bolt/stem cap interface.
Total Deformation Plots:
Etch Detail
The following etch details are made in combination with ChatGPT image generation for complex shapes, and Inkscape for fine tuning and simple changes, lettering, etc.
Build Details
The stem cap will be Black colour, type II anodized, targeting 20μm thickness all over. According to the manufacturer, Aluminum 6063 has the best finish for primarily cosmetic anodized parts.
Aluminum 6063 T4 or higher is chosen to ensure sufficient strength to withstand the load case described above.
Supplier Quote
I got numerous quotes from different suppliers, while also working with them to determine the most cost effective anodizing approach. The best prices I got are:
QTY1: $65 USD
QTY100: $5.60 USD / pp
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