Fork steerer tubes are generally made from aluminum and use a star nut, which is part of the headset installation assembly. The star nut is pressed into the steerer tube and is used to allow the headset to be compressed or stacked. It’s been used for the last 30 years and to this day, the same system and standard persists, which is kind of amazing in itself.
Sometime in the last couple of decades, when carbon fibre emerged as the raw material of choice for those looking for a better strength-to-weight ratio, some high-end fork manufacturers started making carbon forks as well, but with aluminum steerer tubes and then full-carbon models, which were mainly used on road bikes. Eventually, the technology migrated to mountain bikes, too, in hopes of shaving weight from already-robust builds. Fat-bike fork manufacturers jumped on board, which helped save weight and bolster the rigidity of the massive tire/wheel-build combos on those burly rigs.
While carbon steerer tubes are often lighter, and arguably stronger than aluminum steerer tubes, there is another important distinction between forks with one or the other type of steer tube:
when it comes to mounting and installation, all-carbon forks do not use a star nut, unlike forks with aluminum steer tubes, but rather they use what’s called a compression nut. The compression nut is also referred to as an expander plug.
How does it work?
The compression nut is composed of 2 parts: the top cap and the compression insert.
The compression insert is inserted into the steerer tube and, when tightened with a hex key, expands the wall of the compression insert against the inner wall of the steerer tube. Once it is properly tightened and secured, it can be used in the exact same way as a star nut, to compress/stack the headset down and also allow the stem to be properly tightened down.
Proper installation is crucial. Just like any other part on your bike, the compression nut needs to be installed correctly to allow you to ride your bike safely. If the compression nut is not installed properly, you could be at risk of causing yourself serious injury.
What follows below are step-by-step instructions for installing the compression nut into a carbon steerer tube, using the RSD Mayor fat bike carbon fork for demonstration.
You will need the following tools:
- torque wrench with a 6mm hex key
- 5mm hex key
- crown race installer
- carbon paste/carbon assembly compound
- Inspect your carbon steerer tube for any imperfections or irregularities. If something looks odd or abnormal, contact the fork manufacturer and ask them to confirm what the irregularity might be, and don’t proceed until the matter has been resolved to the manufacturer’s satisfaction.
- Cut your steerer tube to the desired length using a proper cutting tool.
- Using the 5mm hex key, separate the top cap from the compression insert. Make sure that the compression insert is still fully assembled before proceeding to the next step.
- Apply a tiny amount of carbon paste onto the compression insert outer walls and then insert it into the steerer tube.
- Ensure that the compression insert is 100% fully-inserted and flush with the steerer tubing wall.
- Using the torque wrench with a 6mm hex key, tighten the compression insert, which is now loose inside the steerer tube, to 10-12 N/m.
It’s now time to install your crown race:
- Apply grease to the steerer tube at the crown junction.
- Position your crown race through the steerer tube and make sure it’s somewhat straight or parallel to the fork crown.
- Using a hammer and the crown race installer, hammer down the crown race.
- Once that’s done inspect the crown race all around – it must be 100% flush and distributed across the crown evenly onto the steerer tube, with a zero value gap tolerance.
Now to stack/compress your headset:
- Insert 52mm headset bearing chamfer up onto the steerer tube.
- Insert fork steerer tube inside the frame’s headtube.
- Insert 41mm bearing chamfer
- Insert top headset assembly, then spacers and stem.
- The last spacer or top spacer must be 3mm to 5mm higher than the top of the steerer tube to allow the top cap to compress the entire headset down.
- Straighten your stem and tighten using the manufacturer’s recommended instructions.
Note: The headset top cap is always tightened before the stem (not the other way around).
Potential installation error signs to look for:
If your fork is not assembled or installed properly, you may notice the following signs and you should stop riding your bike immediately and have it inspected.
Your headset feels like it’s not tight enough
- Inspect the compression insert and make sure it’s still fully tightened and fully flush with the steerer tube. If not, re-stack headset. If the problem persists, have your fork inspected by a professional.
- Inspect the fork for possible crack(s) in the steerer tube or fork legs.
- Inspect the front wheel to make sure it’s fully tightened.
Your front wheel feels like it has some play in it when you apply the front brake
- First, make sure the fork axle is tighten all the way. 18N/m to 20N/m
- See above.
At a standstill, you apply front brake and you can feel movement when rocking the front wheel back and forth
- See above.
- The spacers above or beneath your stem are turning freely
- See above.
You notice some carbon fibre layers peeling off, or you hear a cracking sound when applying power/pressure
- Get your fork inspected right away. Do not ride with the fork.
- You have crashed and the front end of your bike feels ‘off’
- Get your fork inspected right away. Do not ride with the fork
Fork installation is a crucial element to a properly functioning bike. It should be treated as such and installed with the utmost care. A poorly-installed fork, star nut, compression nut, headset, crown race, etc., could result in a mechanical failure that could lead to serious injury.
If you do not have the proper tools or skills to follow these steps, or if you have never done this before, take your bike to your local bike shop, and let professionals do what they do best. You will enjoy your new bike or fork a lot more once it’s all installed properly.