Carbon Repair Testing
We have conducted tests to validate our carbon repairs. Long story short, they work pretty well. See below to see what we did.
Overview
If you are reading this, you are likely interested in carbon fiber bicycle repair, either because you broke your bike or you have curious interest.
You may be asking: Is carbon repair possible? Can a carbon bicycle be repaired? Are they any good? What do I get when I get my bike repaired? These are all great questions to ask!
Unfortunately, not much information exists that answers these questions. Our first take at answering these questions can be found in our tests below.
Experiment
The experiment we conducted was rather simple. In short, we dissected a Cannondale road bike into its individual tubes (top tube, down tube, chain stay, seat stay, seat tube), and we crushed them in a static frame breaking jig (shown to the right). This test was designed to replicate an accident, such as the garage door falling on your frame or a rock smashing into your down tube (both of which we have seen).
The frame breaking jig uses a lever and fulcrum with 4.5:1 lever ratio to apply a compressive force to the center of the tube in order to cause a failure. The force is applied with weight lifting weights with known mass so that we can easily record the force applied. In addition to recording applied force, we recorded the beam deflection, which will help us gauge the stiffness of the repair to the original.
Objective
The objective of this experiment is to validate the functionality of our repairs. There are three things that we wanted to accomplish. First, we needed to make a repair that was as strong as the original or greater. We also wanted to match the deflection properties of the original bicycle tube. Last, we wanted to make a repair that was nearly invisible for an elegant repair. With these qualities, we can obtain an idea of each tube’s strength and modulus. Then, we can determine the validity of the repair.
We understand that the design of a carbon bicycle is highly involved. More stresses and strains are applied to a bicycle frame other than what is demonstrated in this “accidental” situation. This test was specifically selected to demonstrate that a carbon bicycle can indeed be repaired, and to fit our budget (which is quite small right now because we are just starting up).
Below is a quick video showing the chain stay being tested. Just like you’d expect, the carbon part failed abruptly and catastrophically.
Results
Testing took us about a month to complete between the normal day job and Cleveland Carbon operations. Here is a quick breakdown of what is shown.
You will see two different plots for each tube tested. The top plot shows the raw results of the test with the amount of weight in pounds on the x-axis and the beam deflection in millimeters on the y-axis. The bottom plot shows the difference between the original tube deflection and the repaired tube deflection normalized about the smallest diameter of the tube.
The reason we normalized the deflection data about the smallest diameter of the tube is to basically put the numbers into perspective. Here is an example of what we are talking about. Say that you had to transport a load of 5000 lbs of gravel. You have a pickup truck and a dump truck. To the pickup truck, that 5000 lbs of gravel is going to look like far too much load to haul. To the dump truck, that same load is going to look like a perfectly normal load to haul. See where we are going here? In our case, the load is the beam deflection, and the tube size is the size of the truck. Hopefully that helps. On to the Results!
Chain Stay
The repair of the chain stay exhibited very good strength and similar deflection properties compared to the original chain stay. The top plot shows the original tube failing at a force of 112 lbs with deflection of 3 mm, and the repaired tube failing at a force of 225 lbs with deflection of 5 mm. This tells us that the repair is significantly stronger than the original. The bottom plot shows the repair deflection tracking the original deflection within 7% of the original.
Down Tube
The repair of the down tube exhibited very good strength and nearly identical deflection properties compared to the original down tube. The top plot shows the original tube failing at a force of 247 lbs with deflection of 6 mm, and the repaired tube failing at a force of 270 lbs with deflection of 11 mm. We see that the original and repair have similar strength. The bottom plot shows the repair deflection tracked the original deflection very well, within 4% of the original. Another observation we see is that the repair is slightly stronger than the original but with an increased deflection near the failure point.
Seat Stay
The repair of the seat stay exhibited good strength and fair deflection properties compared to the original seat stay. The top plot shows the original tube failing at a force of 90 lbs with deflection of 6 mm, and the repaired tube failing at a force of 112 lbs with deflection of 5 mm. We see that the original and repair have similar strength. The bottom plot shows the repair deflection deviating slightly from the original with maximum deviation of 31% from the original.
Seat Tube
The repair of the seat tube exhibited very good strength and nearly identical deflection properties compared to the original seat tube. The top plot shows the original tube failing at a force of 180 lbs with deflection of 3 mm, and the repaired tube failing at a force of 225 lbs with deflection of 4 mm. We see that the repair has greater strength. The bottom plot shows the repair deflection tracking the original tube very well, within 3% from the original.
Top Tube
Testing of the top tube made us scratch our heads a little bit. The top plot shows the original tube failing at a force of 180 lbs with deflection of 9 mm, and the repaired tube failing at a force of 315 lbs with deflection of 4 mm. We see that the repair has significanly greater strength. The bottom plot shows the repair deflection deviating from the original where the load reaches 100 lbs. The maximum deviation was 19% from the original.
Conclusion
In conclusion, our tests revealed that no matter what tube is broken, we can achieve similar or greater strength than the original tube, which is excellent. Of the five repairs that we tested, three demonstrated elastic properties within 10% of the original tube. This means that the repairs maintained very similar stiffness (modulus) to the original tube, at least with this type of applied force.
The other two repairs produced deflection characteristics with slight deviation from the original. The maximum deviation we measured was 31% on the top tube whose original tube displayed non-linear-elastic properties, something that wasn’t exactly observed in the original tubes. Why didn’t we observe this in the othe tubes? We are not sure, thus, futher testing is required.
We hope this gives you a good idea of what you get when we repair your bicycle. Like we mentioned earlier in this article, we conducted these repairs on a small budget. In the future when resources are more available, we plan to conduct some more advanced tests, including full-frame stress testing and fatigue testing. Keep an eye out on our site for more testing. If you have any questions or concerns, please feel free to reach us through our Contact Page.