Condensed Answer: It’s not recommended to put carbon bikes on racks that hold the bike by the frame. The vibrations of the vehicle translate to the rack and respectively the top tube.
If this practice is done long enough without any frame protection, it will lead to scratches that may eventually result in cracks and render a carbon frame obsolete.
Thus, it’s recommended to avoid using racks holding the bike by the frame when transporting carbon models.
Carbon Is a Spoiled Material
Carbon fiber allows manufacturers to produce light futuristic-looking bikes. However, the material is not particularly resilient and needs special treatment.
Carbon fiber components require frequent examination because a scratch can lead to a crack and a subsequent tube failure. Users need to be vigilant when exploiting, maintaining and transporting the bike.
Racks holding and supporting the bike by the frame create friction which increases on rough terrain. The contact between the rack and the top tube or downtube can result not only in damaged paintwork but in scratches too.
A scratch on a steel frame doesn’t mean much because the material is tough and highly resistant to external abuse. Carbon operates differently. Once its upper layer is compromised the affected part is no longer as strong as before and can potentially fail.
To prevent the bike from moving and sliding against the rack, one can be tempted to clamp it very firmly, but carbon doesn’t like being squeezed because it has poor resistance to compression and can be crushed. This is especially true for super light racing bikes. (Hence why it’s not recommended to clamp a carbon bike by the frame when using a repair stand.)
The safest solution is to rely on a rack that doesn’t hold the bike by the frame.
However, some people who already have carbon bikes and trunk racks often use foam isolation (the type used for pipes) as a layer of protection between the frame and the bike rack.
In most cases, the foam isolation is enough to prevent scratches. The downside of this approach is the extra piece of equipment that one has to manipulate.
The other option is to simply avoid basic trunk racks.
The most common solutions are:
1. Trailer hitch bike racks
As the name suggests, those bike racks attach to the car hitch. Their main advantages are:
Stability. Hitch bike racks don’t sway as much as trunk racks.
No stress on the frame. Most hitch racks hold the bike by the front and rear tire. This makes them safer for carbon frames.
Access to the trunk. The user can lower the rack and access the contents of the trunk without having to remove the bike(s).
Easy to Install and Remove. Hitch racks are the easiest to install and remove from the car.
No clearance issues. Unlike roof racks, hitch racks do not increase the height of the vehicle. Hence you don’t have to remove the rack and the bikes to get into a garage (as long as it’s long enough.)
Easy Loading and Unloading: Hitch bikes make it easier to mount and dismount bicycles.
Mirror Feedback: Hitch and trunk racks allow you to see the state of the bikes in the rearview and side mirrors.
2. Roof Racks
Roof racks that don’t hold the bike by the frame are another carbon-friendly option.
Their other advantages are:
Trunk Access. A roof rack gives the user full access to the trunk.
Multiple bikes. A roof rack can support the greatest number of bikes – one of the reasons why Tour De France team cars use roof racks.
Protection. If the vehicle gets rear-ended, the bikes are unlikely to get damaged.
That said roof racks have a long list of serious downsides too:
Compromised Vertical Clearance. If the driver forgets that there are bikes on the roof, it’s quite realistic to smash them into something like an advertising panel or the ceiling of a garage. When that happens, the bikes, the car, and the rack will be damaged.
No Feedback. Since the bikes are on the roof, neither the driver nor the passengers can monitor them.
Difficult bike installation: Roof racks are not short people friendly because one has to lift the bikes overhead to install them.
Drag. Roof racks create drag when they’re loaded with bikes. This lowers the gas mileage.
FAQ: Are carbon frames that delicate?
When it comes to carbon, there are two main groups of people:
Group A: Thinks that carbon is as delicate as cardboard and deconstructs upon touching it from the wrong angle.
Group B: Thinks that carbon is a very strong material that can take a brutal amount of stress.
As always, the truth is somewhere in the middle. Carbon is obviously strong enough or else no one would be using carbon frames. That said, the material requires a lot of extra attention that an average cyclist would consider an annoyance.
For that reason, people who aren’t thrilled to consistently check their bikes for cracks don’t like carbon.
Summary: What You Need To Know
Any bike rack that operates by clamping the frame or coming in contact with it causes rubbing and/or inserts compression force which may damage a carbon product.
One can reduce the risk by wrapping pipe insulation around the points of contact. However, the safest way to avoid this problem is to simply stay away from racks that grab or touch the frame.
This requirement leaves the user with two options – hitch or roof racks.