When does carbon break on a road bike? - Handlebar breakage, fork breakage, frame breakage - carbon road bike risk?
Robert Kühnen
· 09.06.2021
The crack happened under the eyes of the public: during the spring race Le Samyn, the handlebar arch on Mathieu van der Poel's Canyon Aeroad broke. Miraculously, the cyclo-cross world champion did not go down. But the missing handlebar section and the loosely dangling shift brake lever were a reminder that carbon harbours certain risks. Even companies with a lot of technical expertise like Canyon are not immune to damage to their products. However, the handlebar breakage, presumably due to the interaction of the shift brake lever fixation with the carbon handlebars, is an atypical incident. Forks break much more frequently.
For example, this year at the Omloop Het Nieuwsblad race. The fork on EF pro Tom Van Asbroeck's Factor Ostro VAM buckled above the head tube - fortunately without a serious fall. Bike manufacturer Factor analysed the damage and blamed it on a faulty steerer expander and incorrect assembly. In 2019, Swiss pro Simon Pellaud went down at the Tour du Doubs when the fork on his Bianchi Specialissima broke off during a sprint. Pellaud crashed, but was lucky and did not suffer any serious injuries. He wrote on Twitter: "I'll never forget the moment when I suddenly had the handlebars in my hands on the start. But I made it across the finish line before I flew off and I managed to avoid the barriers." He also tweeted the picture of his broken racer, but soon deleted it. Copies of it can still be found online today. Is this a coincidental accumulation of problems with carbon components? Probably not. However, the number one problem zone on racing bikes is not the handlebars, but the steerer tube, says graduate engineer Dirk Zedler, who, as a tester and expert, has an overview of breakages on bicycles (see interview below). Google also finds far more hits on broken forks than on broken handlebars. Zedler says: "Although products have become safer overall in recent years, clamping the stem to the carbon steerer tube still harbours risks."
Problem area: carbon fibre steerer tubes
The well-known fork breakages among professionals are just the tip of the iceberg; recreational riders also have their forks break. "A broken fork is the most horrific accident of all," says engineer Peter Denk, who designs racing bikes for Specialized and looks back on a long career as a developer. "Every now and then I have the thought on a descent: What if the stem breaks off now?" Professionals are tough in this respect and see it as an occupational hazard, he says. However, he wants to avoid any risk: "I never want the stem of my bike to break under any circumstances. I do a lot of lightweight construction, but not on the steerer tube, I refuse to do that. I want this part to be over-engineered, which is why it has extra layers - even though all the tests show that this is actually superfluous. I accept the extra weight of 30 grams, we'll have to make up for it elsewhere."
Fork steerer breakage - the typical damage pattern of broken carbon fibre
Most major manufacturers have had to recall forks over the past 15 years, the list reads like a who's who of the bike industry. This does little to inspire confidence, especially as all major production sites are presumably affected. There are apparently several reasons for this: "The way in which the stem is connected to the steerer tube is not fibre-compatible," says Arne Burkhardt, development engineer at Merida. They have simply adopted the design of metal parts. Even overtightened stem bolts can damage the steerer tube. High loads, such as when riding through potholes, also stress the steerer tube. Sharp-edged stems in particular can then break the outer fibre layer with every additional hard impact. The weak point: fibre-reinforced plastics have little strength across the direction of the fibres. Over time, the damage looks as if the stem has been sawn off. The harder you ride, the greater the risk. Cobblestone races, where athletic riders rumble over thick stones, are the worst case scenario.
The whole thing becomes particularly explosive because so many factors that are difficult to calculate have an influence, especially when parts are somehow put together. Zedler therefore sees integrated designs, where the manufacturer has more control, as a possible way to alleviate the problem (see interview).
Regular inspection of the carbon fork
What can you do to reduce the risk of breakage? "Regularly inspect the steerer tube," advise Denk and Zedler. After all, breakages work their way through slowly. Expanders that support the steerer tube from the inside help to prevent it from being compressed. If they are long enough, they also protect against permanent loads. "However, they must be inserted into the steerer tube over the lower edge of the stem," says Peter Denk. You should also keep the tightening torques of the stem bolts low and apply carbon assembly paste to the steerer tube to reduce the clamping force required to hold the stem securely. This also applies to all other carbon clamp connections such as handlebar/stem or frame/seatpost, and the clamping collar of the stem should surround the steerer tube as fully as possible and with rounded edges instead of narrow, sharp-edged webs.
Problems with carbon: manual labour and quality fluctuations
Arne Burkhardt systematically researched steerer tubes for his master's thesis at Zedler-Institut and discovered that good expanders and aluminium sleeves between the stem and steerer tube can significantly reduce the risk of breakage. The manufacturing quality must also be right, which is subject to fluctuations with carbon, because a lot of manual labour is involved that is difficult to control. Air pockets in the material are enough to cause fibres to break when the stem is tightened. Peter Denk's trick against these imponderables: a lot helps a lot. In his experience, more wall thickness for the steerer tube is the best insurance against damage.
Carbon fracture tests are (still) uncharted scientific territory
Technical failure of carbon fork steerer tubes on racing bikes is the most common reason for recalls. Paradoxically, the ISO:4210 test standard for road bikes does not cover the extremely safety-relevant connection between steerer tube and stem. In a joint project, Zedler-Institut and the bicycle manufacturer Merida have researched the basics for a safe connection between carbon steerer tube and stem. Zedler has provided six newly constructed fork test stands and expertise. Merida's development engineer Arne Burkhardt used the resources as part of his master's thesis to delve deep into the subject matter: Burkhardt destroyed 220 full carbon forks and 120 stems lost their lives at the interface.
A good expander allows the fork to withstand 80 times more load changes!
The result of Burkhardt's experiments: The quality of the steerer tube is crucial. A high proportion of glass fibres, as used in cheaper racing bikes, weakens the fork. The stem should have as few narrow webs as possible in the clamping area and no sharp edges. The expander, which serves as an abutment for the steering head bearing, additionally stabilises the steerer tube, and does so considerably. However, not all expanders are the same: it depends on their shape. A sufficiently thick sleeve with a closed surface between the steerer tube and the contact surface of the stem also considerably reduces the loads on the steerer tube, but requires stems with a larger bore.
Dirk Zedler on the breakage risk of carbon shafts: "Integrated solutions are the right way forward"
TOUR What is the most vulnerable carbon component on a road bike?
ZEDLER Still the steerer tube. I see steerer tube breakages regularly, although not as often as before.
Why do steerer tubes break?
Unfortunately, the issue is complicated, we have five influencing factors: the quality of the stem, clamping position and number of spacers, expander, stem quality and assembly. If the manufacturer and fitter do everything right, it's safe.
This seems to be difficult in practice, as almost every manufacturer has already recalled forks. Wouldn't it be better to design the connection differently?
We have seen a significant leap in quality with integrated solutions where everything comes from a single source. I think manufacturers are on the right track with this.
Why do the test standards not cover this topic?
Unfortunately, the standards are developing very slowly and the manufacturers have been testing better for a long time anyway. In connection with fork recalls, we tested steerer tubes with the standardised forces of the handlebar test and were unable to reproduce the fractures in the field. That's why we test harder and can thus reproduce the damage patterns in practice. Heavy loads in particular damage carbon fibre.
How do I make sure that the steerer tube on my bike is in good condition?
It is a good idea to check the shaft regularly. Carbon very rarely breaks abruptly. I also recommend using a good expander.
How can you recognise a good expander?
It should be long enough and extend beyond the lower edge of the stem, fit snugly and be able to be tightened with little force. A rough surface, grease inside the clamping mechanism and carbon assembly paste on the surface also help. Carbon assembly paste always helps; without it we would not be able to test many carbon parts.
Is the steerer tube inevitably damaged if the handlebars are twisted in a fall?
When we prepare expert reports, we replace forks after such accidents. But crashing is part and parcel of road cycling, and replacing them after every crash is unrealistic. We see no connection between scraped handlebars and broken fork stems.
Is my bike unsafe if a solid expander cannot be used because, for example, the shaft is not cylindrical?
It is up to the manufacturer to design the connection securely and test it correctly. In addition to the stem, the stem also has a major influence on safety.
Dirk Zedler is an engineer and bicycle expert. He founded and runs the "Zedler-Institut" testing institute for bicycle technology of the same name.
Rules for the safe handling of carbon components
Note the torque!
Carbon tubes do not like a lot of pressure from the outside - this applies in particular to the clamping points of the stem (steerer tube and handlebars) and the seatpost. It is therefore always important to find a compromise between secure but gentle clamping. Gentle means: the fit is precise so that the parts rest snugly, there are no sharp edges and the bolts are tightened gradually in a controlled manner using a torque spanner. Use carbon assembly paste, which reduces the required tightening torques of the bolts by 30 per cent. The torques specified on the stems are maximum values. With assembly paste, you can stay 30 per cent below this and check whether the connection is tight.
Regular checks!
Once a year, a fork with a carbon steerer tube should be completely removed and inspected very carefully. This is the only way to detect damage that could potentially become life-threatening. It is particularly important to take a close look at the clamping area of the stem. Are there any deformations or incipient cracks?
