Condensed Answer: Star nuts scratch the insides of fork steerers during the installation process. When the steerer is made out of carbon, this is detrimental and can lead to cracks.
For that reason, star nuts are used only with steerers made of metal.
For carbon steerers, one has to use a compression plug because it doesn’t scratch the steerer and reinforces it.
Full Carbon Forks
A star nut is installed into the steerer via forceful taps. Its sharp edges slide against the inside walls of the steerer until the nut is wedged at the desired spot.
The friction between the star nut’s sides and the steerer results in scratches. When the steerer is made of carbon this is problematic because the damaged upper layer could eventually grow into a crack.
Therefore, to be on the safe side, it’s recommended to avoid using a star nut with carbon steerers unless the manufacturer of the fork has specified otherwise.
To my knowledge, the only full carbon forks that can accept star nuts are old models produced by Cannondale.
To overcome, the carbon issue created by star nuts, Cannondale used long cylindrical top caps inside the steerers. The caps served a dual purpose:
- To protect the inside of the steerer from the star nut
- To reinforce the inner walls of the steerer and prevent the stem from crushing the carbon steerer. (Carbon is notably weak when squeezing force is applied onto it.)
The official statement is as follows:
For Cannondale forks with carbon fiber composite steering tubes, we have designed special long cylindrical top caps. These work as part of the Star nut system. But they have another very important role. These metal tubes fit closely within the carbon steering tube inside diameter, supporting it against the clamping force of the stem. The long cylindrical shape supports the clamping force of the stem.source
This applies only to older models. To be sure, contact Cannondale and ask if your particular fork is compatible with a star nut.
Carbon Blade Forks
Some carbon forks come with an alloy steerer. Those models are compatible with star nuts because the steerer is made of metal and thus the scratches caused by the star nut are not detrimental to its structure.
What To Use Instead of Star Nuts?
If the fork has a carbon steerer, it is recommended to use a compression plug. The compression plug is not taped into the steerer. Instead, it’s press-fitted into the fork.
The compression plug has two purposes:
- Eliminate the need to use a star nut
- Reinforce the steerer and reduce the chances of damage to the carbon walls due to compression
The video below shows how a compression plug is installed:
FAQ: I already have a star nut in my carbon fork. What should I do?
If the carbon fork has an alloy steerer, the star nut is not a problem.
If the fork has a carbon steerer, however, the fork is already compromised. The standard routine is to replace the fork as it’s not safe to ride.
The steerer of a bike is an integral part. If it fails, a crash is guaranteed. Thus, there’s no place for compromises.
If you’ve bought a bike with a carbon steerer and a star nut in it from a bike shop, then the people at it are responsible for the outcome and should reimburse you. However, a professional mechanic is unlikely to make such a mistake. Thus, this occurrence is somewhat rare but not unheard of.
Another reason why damage to the steerer is so dangerous is the inability to monitor the element. The steerer is not visible and thus when it fails, it’s always sudden and largely unexpected.
Note: First, it has to be clarified whether the fork that you have isn’t part of the models with a sleeve in the steerer allowing the use of a star nut. To know that, contact the manufacturer and/or search for model information online.
FAQ: What is the purpose of a star nut?
The star nut is an anchor point which allows the mechanic to compress the entire headset (steering bearings) of the bike by tightening the top cap bolt going into the star nut.
Тhe bolt pulls the fork upward into the headtube. When the procedure is successful, there’s no play in the head set.
The star nut is not securing the fork to the bike even though it may appear this way. The actual support comes from the stem which is tightened around the upper part of the fork’s steerer. If the stem is removed, and you lift the bike, the fork will slide out.
FAQ: What is the purpose of a compression plug?
The compression plug has the same role as a star nut from a functional perspective. It’s just a different engineering solution compatible with carbon steerers.
Summary: What You Need To Know
- Star nuts are not compatible with carbon steerers.
- Installing a star nut into a carbon steerer may cause a crack and a catastrophic failure of the steerer.
- If a fork has a carbon steerer, it’s recommended to use a compression plug instead.
- The compression plug doesn’t scratch the inner walls of the fork and reinforces the steerer. This is beneficial because carbon is weak against compression forces.