By Rick Schultz
I am a bike fitter as well as a Shimano S-TEC certified mechanic, so, in addition to bike fits, I often get asked things like whether I can assemble a bike before the fit, put on a new stem, adjust bad shifting, etc. You get the idea — full time bike fitting makes you a full time mechanic.
I see so much damage to carbon forks that it is almost a requirement to remove the stem, drop the fork and inspect for damage and/or cracks. In my 10 year career as a pro level bike fitter, I see at least two forks per year that are ready to fail catastrophically. I mean specifically that these forks are cracked to the point that I am not sure how the stem is still attached. You can see that it has been covered elsewhere, like these two articles about catastrophic failures of the steerer tube.
What are the Cause(s) of These Failures
I see several main issues that can lead up to these failures.
- Professionals with very long (130mm-140mm-150mm) stems that can result in a too-long of lever-arm where the pro cyclist (i.e., sprinters) can basically twist the stem with such force that they can snap the carbon steerer tube.
- When bikes are assembled, a stem preload bolt (to preload the bearings) and an expander / compression plug (the other part of the bearing preload system as well as used to clamp the stem’s binder / pinch bolts against) are used as the interface between the handlebars and the bicycle frame. More often than not, an incorrect expander plug is used, which, when (over) tightening the stem’s pinch bolts can cause cracking of the carbon forks’ carbon steerer tube (more on this later – see photos below). I have also seen star nut expander plugs used on carbon steerers – Don’t do this.
- Along with the causes above, most stems are not properly prepped for assembly. Basically, when manufacturers machine the stems, some leave a small burr all around the inside of the stem which can result in an excessive concentrated pressure on the carbon steerer tube, i.e., stress rider. This can lead to potential stress cracking of the carbon steerer tube, not good. The fixes?
- Prepping the stem by sanding & polishing the inside removing all burrs & stress risers (see photos below).
- Use the correct expander plug. (In my opinion, the best is from Cervelo.)
- Grease the screws.
- Apply the correct torque to all screws.
- Every six months, remove the stem and fully inspect the entire fork.
Cervelo offers the best solution, with a steel sleeve that is epoxied into the steerer tube. The caveats are that the (a) inside of the steerer tube is lightly sanded and (b) all parts are thoroughly cleaned & degreased, so the epoxy bond does not fail. Now, a star nut can be used.
Make Sure to Use the Correct Compression Plug – The Longer the Better!
Most expander / compression plugs are too short for you to safely clamp against. Without backing support, the pinch bolts compress directly against the carbon steerer tube which can EASILY cause cracking of the steerer tube.
In this photo above, when this poorly designed compression plug is installed, there is support for the lower pinch bolt but nothing behind the upper pinch bolt to support the steerer tube which can easily cause a crack at the top.
When spacers are used, this is an even worse implementation since there is no support behind either pinch bolt. (See above image.)
This compression plug is even shorter than the one pictured above in image 2.
This is a worst case but common scenario for failure of the steerer tube since the top pinch bolt can be easily overtightened which can cause the top of the steerer tube to easily crack. (See above image.)
Adding a spacer is a common setup. Provides no support for the top pinch bolt and only minimal support for the bottom pinch bolt. (See above image.)
This is a far better designed compression plug. It is long enough to offer support behind both pinch bolts. (See above image.)
Even with a 1cm spacer, there is still support for both pinch bolts. (See above image.)
NEVER EVER use a Star Nut on a carbon steerer tube. The sharp edges of the star nut will cut into the carbon causing stress risers. This can cause a quick and catastrophic failure of the entire steerer. Star nuts are to be used with aluminum or steel steerers or only with carbon steerers that use a steel liner like Cervelo’s solution.
Summer will be here soon where everyone will be out riding. So, while you have some down time, it’s time to check your steerer and fork. While you have everything apart, polish up the inside of the stem. It could save your life!
Coach Rick Schultz is an avid cyclist who trains, races and coaches in Southern California. Rick is an engineer by trade, and in addition to being a coach, he’s a bike fitter and prolific product reviewer. He’s the author of Stretching & Core Strengthening for the Cyclist and Bike Fit 101: Your Toolset for a Great Bike Fit in the RBR eBookstore. Check his product reviews website, www.biketestreviews.com, and his coaching site, www.bikefitnesscoaching.com. Click to read Rick’s full bio.