This post reveals the properties of carbon dropouts and the issues that they create.
What are carbon dropouts?
The dropouts are the parts of the frame and fork to which the axles attach.
Since the wheels “drop out” from there during removal, the term “dropouts” is used to reference those locations of the frame and fork.
As expected, a carbon dropout is made out of carbon, but in some cases, it can also have an aluminum face added for protection.
Lack Of Toughness Is The Main Problem
Carbon is a very strong material when put in the right context, but it certainly isn’t the toughest, especially when there’s a direct impact.
The dropouts are a contact point of the frame as they’re constantly squeezed by quick-release skewers or thru-axles. When the wheel is clamped properly the architecture is stable and the dropouts are protected.
However, quick-release skewers and axle nuts have serrated ends designed to increase the friction against the dropouts. Those serrations dig into the dropout and over time the stress adds up, especially when the wheel isn’t fully secured and moves ever so slightly in the dropout.
Damage can also occur when transporting or working on the bike. For example, if the wheel is removed and the dropouts accidentally hit the pavement, they can be damaged severely.
It’s also possible to impair the dropouts when they’re clamped in a car rack and the bike is pushed sideways. Carbon doesn’t like being twisted and the dropouts might snap.
That brittleness is why even “weight weenies” stay away from carbon dropouts.
Carbon Dropout Repair Is Possible But Costly
Currently, the most reliable method to repair a severely broken/damaged carbon dropout is to replace it with one made via injection molding. As the name suggests, injection molding is a manufacturing process for producing parts by injecting molten material into a mold.
The repair requires a specialized shop with a fair amount of equipment and before all experience working with carbon frames. If the frame is cheap, then the repair could result in a less-than-ideal cost-to-benefit ratio. If the frame is super expensive, then the repair would be worth it.
If the wear is more superficial and affects only the thin upper layers of the dropout, less aggressive solutions can analyzed.
One of the most common problems is a partially “eaten” inner side of the dropout. In that scenario, the wheel goes off-center due to the missing material on one side. In more extreme cases, the wheel may even rub against the chainstays.
Usually, this issue is caused by insufficient clamping power allowing the wheel to move ever so slightly into the dropouts under load. As a result, the hub eats a little bit of the dropout over time.
People sometimes use DIY methods to remedy this problem. Those protocols could work only if the wear is small and thus the strength of the dropout is fundamentally the same as when the frame was new.
If the wear is severe, none of the fixes below should be used as they’re just space fillers rather than strengtheners.
Option 1: Take a steel washer with an inner diameter matching the opening of the dropout. Cut one side to mimic the standard dropout opening. File the rough edges and the washer so that it sits flush with the frame. Glue the washer to the frame with a strong epoxy.
Option 2: Use two-way epoxy to fill up the space and shape it with a small file. Be gentle.
The first option requires more work, but it’s more resistant to abrasion because metal is harder than epoxy.
Note: Before doing any of this ask the manufacturer of the frame if similar damages are covered under the warranty agreement. They may give you a brand-new frame. But if you choose to fix it yourself, you’re technically altering the product and thus the warranty may not be respected.
Not all damage to a carbon dropout needs attention. It’s very common for the quick-release skewer serrations to leave indentations in the frame paint as shown in the image below.
Paint chips/ridges in the frame are expected when combining carbon dropouts with quick-release skewers.
To minimize the damage some people put super glue on the dropout face to create another soft layer of protection.
Thru-Axles Are More Carbon-Friendly
One of the main issues with quick-release skewers is that they don’t have a consistent position. There’s always a small amount of variation that takes place every time a wheel is re-installed. As a result, the clamping force isn’t applied evenly to the same locations of the dropout.
Thru-axles, on the other hand, have the same position. In addition, one end threads into the frame/fork. Thus, the thru-axle is pulling the dropouts together rather than squeezing them.
The consistent position and the tightening process greatly reduce the impact/external stress on the dropouts.
There’s one issue, however, that could be seen as a weak point – the threads on the frame and fork. In some cases, they can be stripped. When that happens, the axle will fail to secure the wheel.
The solution is a Helicoil-type thread repair which works as follows:
- The remaining threads are drilled out.
- New threads are made.
- A sleeve with inner and outer threads is threaded into the dropout.
The sleeve is necessary because it reduces the diameter to the original dimension. Otherwise, the axle will be loose.
Another benefit of thru-axles in this case is that they allow the user to apply the same torque every time via a torque wrench. (Quick-release skewers do not provide that opportunity – another inconsistency that comes with them).
Thru-axles have a recommended torque written on them, usually 8-10Nm. The consistently applied torque reduces the chance of damaging the dropouts.
Carbon Is Not a Good Material For Dropouts
At the end of the day, it’s hard to deny the obvious – carbon fiber isn’t a good material choice for bicycle dropouts.
Carbon is strong but lacks toughness and thus has poor resistance to impact. For that reason, many manufacturers add an aluminum cover to the drop-out faces to act as an additional layer protecting the carbon frame.
In the best-case scenario, that aluminum cover is replaceable because aluminum wears down too. This method prolongs the life of the frame without adding an appreciable amount of additional weight in comparison to carbon-only dropouts.
If Carbon Dropouts Are So Fragile, Why Are They Popular?
There are two main reasons: 1) carbon is trendy 2) carbon is lighter.
By relying on carbon dropouts, manufacturers can streamline the production process and label the frame as 100% carbon. As a bonus, the frame is a few grams lighter.
Those properties are inconsequential when we talk about recreational cyclists, but people obsessed with having a bike with stunning specifications could consider them important.