Wednesday, June 16

Carbon Bike Proof Testing – Part 2

Cervelo Carbon Fork, After Failing Impact Test - from Wikimedia Commons, here
The Impact in this Case Was With a Garage While Being Carried on the Roof of a Car

REAL WORLD EXAMPLE –CARBON NON SUSPENSION FORK

Picking a component at random, the carbon fork seems like a good subject as an example case study. It is expensive enough to be worth a little effort to save if its integrity is in question. It is critical enough that you could be hurt badly if it unexpectedly failed at an awkward moment (inconveniently, that is the time it is most likely TO fail). It is comparatively simple to test, since the only places loads get applied are at the dropouts and at any bosses where brakes or racks are attached. The dropouts are the critical load application point unless those bosses are badly damaged to the point that a proof test is not needed anyway.

At this point, I’ll refer to EN 14781 because it seems more cyclist oriented to me than its CPSC counterparts. I would not infer from that statement anything at all, either in favor or against either standard. Anyway, EN 14781 defines the following conditions for bike forks:

Impact Test – Paragraph 4.8.2 – This paragraph is intended for a complete bike/fork assembly, so you wouldn’t normally use it just to test a fork. It is also a difficult test to perform at home without collateral damage.

Fork Clearance Test – Paragraph 4.9.3.2 – This paragraph is more applicable to suspension forks, but it will assure that your carbon fork won’t collapse as you ride along without heavy braking being involved. The standard requires an ultimate load of 2,800N (630lb). An appropriate proof load for this would be 420lb. If you were to apply the load to the single leg of the fork that has the damage, this would be 210lb. Be careful to align the load and stabilize the fork so it doesn’t go flying across your shop.

Fork Tension Test – Paragraph 4.9.3 – This paragraph is one that I’d dispense with. In the real world, you are NOT going to be pulling on your fork unless some gremlin has nailed your front wheel to the ground!

Fork Bending Test Schematic from EN Standard
Fork Bending – Paragraph 4.9.4 – This paragraph is one that is VERY relevant to you. Your fork must be able to withstand the bending imposed by braking and potholes, not to mention when those are combined with stuff like potholes. Here, the ultimate load recommended by the standard is 1200N (270 lb). 2/3 of this load would give you a proof load of 180lb. I imagine most of you know someone of about this weight. To actually apply the load, the thing I would be most concerned with would be to avoid damaging the steerer or the head bearings. Carefully support the upper end and then apply the load. I would recommend having the load tester bounce several times on the dropouts to verify that the fork is sound (don’t have them actually jump up and down). When you are done, make sure that the fork is not bent due to the load. If there IS any set, it is probably due to something in the upper end and you must take care of that, but your carbon is OK. For those that are engineers (or architects), please note that the “bending” test in the standard is not a pure bending test. It is actually a combination of shear (the 1200N) and bending, which is dependent on the length of the fork. In this case, the actual bending is the shear times the length measured from the dropouts. If you REALLY wanted to do a fancy schmancy (is that spelled right?) test, you could change the angle of the load to get a combined compression and braking with bending, but the standards people didn’t think of that!

Fork Impact Test from EN Standard

Fork Impact – Paragraph 4.9.5 – Another one that is VERY relevant to you. I haven’t rigorously compared this to the fork bending test, but basically it involves dropping a 50lb weight from a height of 28 inches. Paradoxically, if the fork were metal, the drop height would be only 16 inches. I have a strong suspicion that this test is more severe than the static bending test since the standard allows a greater permanent set than for that test. If I were doing this test, I’d brace the fork upper end and “known to be good leg” firmly against movement and twisting, and then drop 25lb from a height of 16 inches onto the dropout of the fork leg with suspected damage. Actually, I’d do it a bit differently. I’d tie weight to the dropout with wire of the right length, and drop it from the fork. That would avoid the possibility of damaging the dropout.

Fork Bending Fatigue – Paragraph 4.9.6 – This is basically the same test as the static bending except the load is only 620N (130 lb) which is basically half of the load noted above. I’d suggest that a 130 lb load would also be a sensible and standards-based alternate to the 180 lb load derived from the static bending test. Actually, I’d go simpler yet, and suggest that you just get someone that weighs somewhere between 130 and 180. Have them bounce several times just to make sure. If you feel more daring, get someone a bit heavier. DO NOT PERFORM THIS TEST WITH THE FRONT FORK HANGING OVER A PIT OF BOILING OIL! If it breaks, the tester getting injured would show “bad form.”

CASE STUDY WRAPUP
Basically, all this translates into two fairly simple tests. Have someone bounce on the dropouts with the upper end securely supported, and drop a weight with things securely supported. If you want to be extra cautious, take the wheels off and find the heaviest person you know to weight the front fork in compression. IMHO, this is something entirely practical for a cyclist to do in order to confirm his/her front fork is not about to do something really ugly.

3 comments:

Ham said...

Interesting - I'd say that my forks got that most days with me on top combined with the London Roads. Except of course, that the impact/stress is all applied to the lower part. But, the attenuation of the force as a result of being at the bottom end of the arc is then compounded by speed. I must work out the moments involved hitting a 4" pothole at 15mph on a 26" wheel.

Steve A said...

Ham, keep in mind that 180lb aft load represents a "1g" deceleration for a typical "non lightweight" cyclist. You will not be able to apply that much load to the fork with the brakes without probably flying over the handlebars, in which case you won't be worried about the fork.

The impact load is intended to simulate a moderate pothole. I'm certain that either of us could find potholes that'd break any fork if we hit it at speed.

The basic compression load is that 420lb load. I have not seen forks fail in that mode. I think the more common mysterious failure is the cyclist experiences a fork failure while braking, often after an impact that he may not have even noticed.

Rantwick said...

Hey, thanks again for more carbon related stuff. The first part of the post made me despair that I wasn't smart enough to grasp load testing, but then the second half sorted me out! Well done.

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