Q&A: Star Nuts and Full Carbon Forks? NO!

Jim’s Tech Talk

By Jim Langley

The interesting tech questions keep arriving, thank you! We aim to answer as many as possible, especially when they have to do with safety. And, Stephen Turk’s does!

Stephen asks about his Cannondale Six13

“I have a 2005 Cannondale Six13 that has a lot of good miles on it. The instructions for its Slice full carbon fork specify a star nut and go on to say.. 

“ “We have done extensive testing that has shown that star nuts do not adversely [EDITOR’S NOTE: word missing here] the strength of our forks. In fact, our Cannondale system, which includes a long cylindrical top cap, properly supports the clamping force from the stem and dramatically improves the strength and longevity of the steerer tube. Many of the headset compression devices on the market do not extend far enough down into the steerer tube and thus do not support the clamping force from the bottom stem bolt.” “

Conclusion – when in doubt, follow the manufacturer’s instructions.”

The Bike and Fork in Question

Thanks to the Vintage Cannondale website I was able to find Stephen’s bike. Here’s a link in case you ever need specs on older C’dales: http://vintagecannondale.com/year/2005/2005.pdf.

And, here’s a photo of Stephen’s Six13.

I looked up the 2005 Six13 to learn more about the fork. I wondered if maybe there was some confusion and it actually was equipped with an aluminum steerer. If so, then the star nut comment would make sense.

Star Nuts Are Used in Steel and Aluminum Forks

Here’s a photo of a star nut in a fork so you can see what Stephen is asking about. This photo is courtesy of Park Tool from their how-to on installing star nuts here: https://www.parktool.com/blog/repair-help/star-fangled-nut-and-expansion-plug-installation.

What I want to point out is the scoring/scratching visible on the inside of the aluminum steerer tube made by the sharp edges of the star nut. Star nuts are made of spring steel so they’re much harder than the aluminum steerer and they cut into it a bit as they’re forced down. 

Even pushed into a steel steerer the star nut will leave marks because it’s a little oversize which creates outward pressure on the steerer.

The star nut is made to be driven down into the fork to a certain depth. The directional design of the star nut only lets it move down, not up. Which means that it will stay in place and allow adjusting the headset once the fork is installed in the frame. This works fine in steel and aluminum forks.

Star Nuts Should Never Be Used in Full Carbon Forks

I think Stephen’s final sentence, which includes “follow the manufacturer’s instructions,” was an attempt at humor – if that was actually the full instructions (keep reading).

But, there’s nothing humorous about star nuts in full carbon forks. I had one break a carbon fork I was testing for review and I’ll never forget it. I did not know they had installed the fork with a star nut.

Luckily the fork did not break at 40mph on the big descent that day or I wouldn’t be writing this. It waited until I was on a flat stretch crossing railroad tracks. I usually jump bikes over tracks and that did it. The fork made a loud “crack!” and then broke and came out of the frame. This happened instantly.

Instead of going over the bars or falling to the side, I went straight down. My crank hit the pavement and one foot got stuck in the pedal causing me to roll over on my ankle. I almost passed out from the pain. I was sure I’d broken my ankle, but it turned out it was only sprained.

Star Nuts Cut Carbon

Later, when I could walk again and got to inspect the broken fork, I saw that the failure occurred right at the star nut. Carbon is a composite material, basically a fabric that’s turned into a solid with a hardening agent (resin). Carbon is super strong when cured but it can’t handle things that cut into it.

The way star nuts work they are like little saw blades with teeth all around exerting constant pressure on the walls of the steerer. Add the pulling force of the headset bolt and the flexing of the carbon fork when riding and you can see how my fork broke and why you should never use a star nut in a carbon fork.

Bit of a Mystery

I can’t explain why Cannondale would make such a mistake. So, I wonder if that was a typo. I noticed one word was missing in the instructions Stephen quoted. So maybe there was another paragraph explaining that for aluminum and steel forks, star nuts are okay. That seems possible to me since a lot of Cannondale’s statement was about their compression nut, which is the safe way to install carbon forks.

I think Stephen already figured this out, but just in case, I’ll end by telling him and anyone else installing a carbon fork, do NOT use a star nut. When you buy a fork (or a bike kit), you should get the compression fitting designed to be used with the fork with it. And, instructions to go along with it. That’s what you should use. If you didn’t get installation parts with the fork, reach out to the company to determine what they recommend and buy and use the right thing, which will NOT be a star nut.

10,200 Daily Rides in a Row

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Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.