The Shift to Disc Brakes
As bicycle braking systems transition from rim brakes to disc brakes, the rim is one of the components that has undergone a major change in form.
The reason for this change is that the role assigned to the "rim," a key part of the wheel assembly, has shifted.
Back in the heyday of rim brakes, the rim’s role—as the name implies—was to form the wheel, withstand shocks and vibrations from the ground, hold the tire, and rotate. Additionally, it had the critical duty of accepting the brake pads on the rim sidewalls to control speed.
Today, however, the widespread adoption of disc brakes has led to the development of many disc-specific rims. Freed from the duty of handling braking forces, rims have shifted toward shapes that rotate more efficiently and dissipate impact more effectively.
Differences in Rim Shape
The photo above shows cross-sections of a typical disc brake rim and a rim brake rim.
The disc-specific rim on the left has a semi-cylindrical cross-section designed to efficiently dissipate impact and manage air resistance.
The rim brake version on the right has a more rectangular cross-section, where the parallel sidewalls serve as the surface for the brake pads to grip, controlling speed through friction. Because this area is subjected to significant forces, it is built sturdily to withstand that pressure.
As you can see, disc-specific rims no longer require a braking surface, eliminating concerns about wear.
The issues often seen during the rim brake era, where long-term use caused the rim to wear down from the brake pads—eventually leading to rim failure as shown in the photo below—have largely disappeared.
However, just because you no longer need to worry about wear doesn't mean you can let your guard down.
There are other parts that require careful observation.
Inspecting Your Rims is Crucial
The part in question is the connection point for the spoke nipples on the rim.
This is where the spoke tension, necessary to maintain the wheel's shape, is held along with the holes in the hub.
It is also a part of the wheel that is constantly under high stress.
Even metal fatigues; any area under constant tension will eventually exceed its limits and fail.
The photo below shows the stage just before that happens.
Notice the crack running cleanly in the longitudinal direction before and after the spoke hole.
This is clear evidence that the rim can no longer withstand the spoke tension due to metal fatigue.
Go ahead and inspect your rims before your next ride.
If you see lines like the ones in the photo, your rim is on the verge of failure.
Replace the rim before it leads to a major accident or damages other components of your bike.