Saturday, May 29

Carbon Bike Proof Testing – Part 1

Cycler has a VERY Apropos Sense of Humor!

Presuming that you have decided that the damage to a carbon bike component is enough that you don’t want to just take the chance that everything will be OK (Choice #1 or Doohickie’s Choice #5), but it doesn’t warrant spending $300 for a repair, you now face “Choice #3.” At this point, my own past aircraft carbon experience is irrelevant. Fortunately, the path to validation by test is very similar whether one is talking bikes, planes, trains, or automobiles. The following information is not especially complex, but there is a LOT of information and so you may want to print this off and absorb things slowly.

This Article Came out Around February, Raving Over a New Carbon Bike, Using a Truss
That article didn't mention the race experience. For that, go HERE
No need to proof test THAT bike! PS: The rider wasn't seriously hurt

STRENGTH
You are not entirely on your own. While bike manufacturers do not make strength specifications readily available for their products, basic information can be obtained, and used as a basis for validating that the component is reasonably safe. This information is publicly available. Two major sources are the US Consumer Products Safety Commission and the European Committee for Standardization. Any bike sold in the US must meet the first set of requirements, and it’s likely that anything sold by a major manufacturer will meet both sets.

CPSC Bike Regulations (These are Code of Federal Regulations, Title 16, Part 1512)
ASTM Bike Standards (these cost money to get copies)
EN 14781 (for Racing bikes. EN 14764 is the equivalent for city and trekking bikes)

COMPONENT TEST APPROACHES
Frame – Normally, unless it is a “low end” carbon frame, I suggest repair. Properly done, a carbon repair can completely restore the strength of the frame, and you may have difficulty even being able to tell the difference from an undamaged frame. If you are doing a home repair, I suggest a proof load to validate the strength. The specifications give methods of accomplishing this proof load.

Front Fork – If damage has actually thrown your front fork out of alignment, perhaps by bending of a metal steerer, that should be corrected before testing. If you can’t get the fork straight, get a new one. I also suggest that this test is best conducted with the fork removed from the bike, and the loads reacted by blocking up the steerer, so that you do not damage bearings, the steerer, or the frame during the test. Because carbon front forks are ubiquitous nowadays, I have picked the front fork as a proof test case study  and it will be part 2 of this post.

Seat Post – I suggest this test be performed using a seat tube that you do not require for riding afterwards. It need not be a high quality seat tube, just the right diameter.

Handlebars – I suggest this test be performed using a stem you do not require for riding afterwards. It should be noted that stem choice can affect handlebar test results.

COMPONENT TEST GENERAL PROCEDURE
Obtain a helper of sufficient weight to obtain the desired proof load. You may achieve this artificially by clever addition of free weight such as provided by barbells, pulleys, or other means of load amplification. In almost all cases, the “human scale” of a bike means the loads are also “human scale.”

Apply proof load. Note any “popping sound,” or any sharp “crack.” The first is an indication that you have loaded the part more than it has been loaded during riding, or during manufacturer actions. This is not necessarily bad. That is part of the point of the test. The “pop” likely is due to resin cracking that is not structural. A “crack” may or may not be serious. Regardless of any noise, or of no noise, the next step is identical.

Remove proof load. Check to ensure that the part is back at the starting position and that it is visibly unchanged.

Reapply proof load. If all is well, you will not hear noises the second time around. If the sounds are worse than the first time, that is not a good sign.

Repeat the cycle for a total of at least a half dozen load applications. Carefully reinspect, and if it passed, reinstall and know that your carbon component meets reasonable strength levels. Just to be safe, I’d also get some wet layup resin, or even a bit of clear fingernail polish, if the damage is small, to touch up and stick down any exposed fibers. While this isn’t really structural, it does help stabilize everything and makes it look a little prettier.

DESIGN VERUS FATIGUE VERSUS ULTIMATE LOAD AND YOUR PROOF LOAD

ULTIMATE LOAD – BANG!
The CPSC and European standards are loads below which stuff is not allowed to actually break or get bent beyond limits they pick somewhat arbitrarily. The tested component need not be fully usable afterwards. This should be considered as “ultimate” load for the standard in question, though the standard doesn’t really match up with the traditional definition of “ultimate” as the breaking load. Clearly, there is no point testing your bike component if you cannot use it afterwards. This quandary is part of what differentiates “design” loads from “ultimate” loads. The first is the maximum load the component should see in normal service and normal service shouldn’t permanently bend things. The second is the minimum acceptable failure load.

DESIGN LOAD
While the standards do not directly provide information on the design loads, aircraft use a 1.5 factor between “design” and “ultimate.” This means the “design” load is 2/3 the “ultimate” load. Conveniently enough, for metal structure, this is enough of a factor to ensure that permanent set is minimal for “design,” while avoiding excessive weight to achieve the “ultimate” strength.

FATIGUE LOAD
The bike standards also provide fatigue loads for some components. In the standards, they’ll typically specify that the fatigue load will be applied several thousand times. Fortunately, since we’re talking about carbon, the failure load will be pretty much the same whether you apply the load once or a few hundred thousand times.

YOUR PROOF LOAD
You definitely do NOT want to load your bike up to the “ultimate” load since you’d run the risk of just breaking the thing even if nothing was wrong. I suggest you consider using a convenient load in between the specified fatigue load (if available), and 2/3 of the specified failure loads as a proof load. This minimizes the chances of destroying a “good” part while providing confidence in the part’s capability. It also ensures that the proof load is at or above the expectation for normal use of the part. This difference also points out one of the fundamental differences between metal and carbon. The metal gets permanently bent before it breaks. The fork gets bent and you don’t use it. The carbon does NOT usually get permanently bent, unless associated metal bits are what got bent. The GOOD news is that a proof test gives a high level of assurance that the carbon has at least a reasonable capability. One cannot know if a metal component is about to crack. A proof load should be applied at least a half dozen times to provide confidence that the loading was not on the verge of failure. The BAD news is that if your carbon gets hit five minutes after the proof test, it might no longer be good.

ONE OTHER CONSIDERATION
Cycler inquired about static versus dynamic loading. She also wondered about combined loads. Fortunately (or not), carbon behaves quite differently under dynamic conditions than metal. Metal is very sensitive to the rate at which load is applied. Carbon is not. If a hunk of carbon is going to break under a thousand pound load, it’ll break at a thousand pounds no matter how quickly or slowly the load is applied. A metal hunk may withstand a rapidly applied load that would break it if the load were applied slowly. This phenomenon makes things simpler for the cyclist determined to check if a damaged carbon component remains safe. Combined loads are a little stickier, but are mostly ignored by the testing standards. This makes things simpler for the cyclist, but raises a possibly awkward question about the true safety of that brand new, undamaged carbon bike you just bought. That question, however, is the subject of another post.

2 comments:

Rantwick said...

Wow. Quality information, man. While it unlikely that I'll be doing this kind of thing, it is great to have my curiosity satisfied so well. I'm looking forward to part two.

dave said...

Great information
One of my favorite C&H's is when Dad explains B&W
photography.

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No Need for Non-Robot proof here!