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Nov.2026 03
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Carbon Rim Drain Holes: Why They Matter and When Are They Necessary?
Introduction
Learn why carbon bicycle rims have drain holes, how they help release water and air pressure, and why road and MTB rims have different requirements.
Details
When developing carbon bicycle rims, small details can make a difference in product reliability. Drain holes (2.5mm) are one example.
You may have noticed small holes drilled into the sidewall of some carbon rims. These are often called drain holes or drainage holes. At NTP, we consider two main factors when deciding whether to add drain holes: water drainage and air pressure release during tire inflation. The requirements can vary between road and MTB rims.
Functions of Drain Holes on Carbon Rims
Water drainage
Water can enter the rim chamber through spoke holes, valve holes, or other openings, depending on the rim construction and setup. This is more likely when wheels are used in wet conditions.
If water remains trapped inside the rim, it can add unnecessary weight, create noise, and potentially cause problems over time. Drain holes provide an outlet for accumulated water, helping it escape from the rim chamber.
Air pressure release during tire inflation
The second function is less obvious but equally important.
With an improper tubeless tire setup, air can sometimes leak through the rim tape and enter the rim chamber during inflation. As a result, the tire beads may fail to snap into place, even as the rider continues pumping air into the tire.
If a large volume of air enters a sealed rim chamber, pressure can build up inside. Under certain conditions, this may cause rim damage. Small drain holes provide an additional path for air to escape, helping reduce this risk.
Road/Gravel & MTB Rims: When Are Drain Holes Necessary?
Road and gravel tires are typically inflated to higher pressures than MTB tires. During inflation, an improper tire setup can allow air to enter the rim chamber, making pressure management an important consideration.
For this reason, we generally recommend 2.5mm drain holes for our road rims.
For MTB rims, our approach is different. MTB tires generally operate at lower inflation pressures, so the risks associated with high-pressure inflation are less of a concern.
At the same time, MTB rims need to withstand demanding riding conditions, including impacts and repeated loads. Maintaining the strength of the rim structure is therefore a key priority.
For this reason, we generally do not recommend drain holes for MTB rims unless there is a specific product requirement.
Can Drain Holes Be Customized?
Yes. Drain hole design can be discussed during product development. At NTP, we typically use 2.5mm drain holes, but their number and placement can be customized upon request.
The right solution depends on the rim design, tire compatibility, inflation pressure, intended use, and weather conditions in the target market. It's best to consider these details before finalizing the rim tooling and drilling specifications.
If you're developing a new carbon rim or customizing an existing open-mold model, feel free to contact our team to discuss the right drain hole design for your application!
