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
Jim Langley is RBR’s Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his “cycling aficionado” website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim’s cycling streak 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.
STEPHEN TURK says
Thanks for the discussion. The original Cannondale document is indeed missing a word. I don’t have a current link for that document, but did find a related Cannondale document at http://wodocs.com/docum/541-cannondale_bicycle_corp-06_en_cannondale_carbon_road_fork_instructions_technote.html
The Slice Premium and other models referenced in the document are indeed full carbon forks with carbon steerers, and Cannondale installed star nuts in these forks at the factory. These headsets included special long cylindrical top caps. To quote Cannondale, “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.”
This old Cannondale system worked well – I have put over 40K miles on a couple of Slice Premium forks with Cannondale’s star nut system with no issues whatsoever. Compression nuts are not always as trouble free!
As I said originally – follow the manufacturer instructions. I wouldn’t use a star nut on a carbon steerer unless specifically instructed to do so.
Joe Maley says
Fascinating article since I too have the same 2005 Cannondale Six13 with over 40K trouble-free miles on it. Thanks for the great discussion.
Cannondale used a special star nut with their carbon fiber steerer tubes. Also a special installation tool and procedure. Here is one reference: https://wodocs.com/docum/541-cannondale_bicycle_corp-06_en_cannondale_carbon_road_fork_instructions_technote-2.html
Jim Langley says
Thanks, Chris, but that link is opening on my computer what appears to be a fishing/spam/scam website. Do you have a proper functioning link or can you copy and paste the text into a comment?
Try this link instead:
(eliminating the “s” on the http seems to work for me)
Jim Langley says
Thanks a lot, that did the trick!