SuDiBe Tandemseiten  |
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| SuDiBe Tandemseiten |
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Agreed. Any fork that is designed for a single bike should not be used on a tandem. If it happens to be structurally suitable for a tandem, then it is over-built for a single.
> Santana ended its proposed collaboration with AME (Alpha-Q) 18 months ago.
> First, Mr. Lee admitted to me that he had never had any of his forks failure
> tested. Second, he declined to provide forks for independent lab testing.
> Santana recently purchased and tested a current sample of the fork that AME
> claims is "strong enough for tandems." The results were unimpressive, even by
> accepted single bike standards. The October 8 issue Velo News reported that
> the steerer of an Alpha-Q cyclocross fork pulled apart while being evaluated
> on a single bike.
We tested the Alpha Q Tandem fork to our satisfaction. We had to pay for the test forks. Mr. Lee generally does not give forks away. I don't know what fork Santana tested, whether it was a tandem specific fork or not. We distrust manufacturer's claims, ("strong enough for tandems") that's why we do our own testing. There are many ways to test a fork and there are many ways to interpret the test results. Bringing in a story of a cyclocross fork has nothing to do with Tandems. It wasn't a strength test, it was a ride test. Did the guy crash? Whatever the case, the lack of a full explanation of the situation makes it sound like "this is proof that all Alpha Q forks are bad".
It is not necessary to "failure test" composites. One should test it for the maximum loads it will experience and for delamination-causing microcracks within the first 1000 hours of use. Non-destructive testing and analysis are how most composites are tested these days. The difficulty lies in maintaining consistent standards in production.
> As for testing, Reynolds Composites... has never released a new fork until
> after they have destroyed 18 production samples... Every carbon fork built by
> Reynolds is considerably stronger than a premium steel fork.
I applaud Reynolds for testing to this level. We also offer the Reynolds forks and like them very much. While all this testing sounds good and helps to sell forks, it is easy to slightly overbuild a composite structure so it exceeds the commonly accepted performance standards. That way you can advertise that your fork is X times stronger than Brand Y, even if it doesn't make a bit of difference in the real world. The overbuilding also helps if your workers make a minor mistake that goes unnoticed, something easy to do in composite fabrication. The Mexican factory where the Reynolds forks are made is well run and produces impressively consistent quality. But mistakes do happen as we will see below.
> An earlier post surmised that the weight of a carbon fiber fork might indicate
> its strength. While this is generally true for steel forks, Reynolds...
> reports that the strength of equal-weight carbon fiber forks can vary by a
> factor of four!
Again, this is assuming that strength is the main issue. If it were, we would see a lot more new forks that suddenly broke because they were too weak. A poorly designed or flawed composite fork will fail in normal use by micro cracks starting and then growing into a full fledged delamination. Micro cracks start from stress risers, release agents left on a bond area, impact damage and faulty material. Testing to failure will reveal the stress risers that are in every fork. Fortunately, the test conditions are never seen in real life. If the fork is well designed for basic strength, as are most that stay on the market for more than a year, the main concern is whether it was made without the possibility of micro cracks to start. This has more to do with craftmanship, quality control and design of the manufacturing process (to eliminate the possibilty of errors).
> Instead of achieving doubled strength through additional plies of material,
> the Reynolds tandem fork instead uses larger diameters, especially the
> steerer tube (1-1/4 vs 1-1/8)--- a fork's all-important fulcrum point. To
> achieve this degree of strength without a tandem-sized steerer would require
> an extra 200-300 grams of material.
This is a veiled pitch for 1 1/4" steerers using flawed tech-speak. Again, strength is pointed to as the end-all quality measuring a fork's value. But even here the logic is flawed: I would expect the tandem fork to be 4X stronger than the single fork since it sees (as Bill says in the first paragraph) 4X the braking force.
The Santana/Reynolds Tandem fork uses an 1 1/4" steerer (and 5.5 cm rake) because that's what Bill asked them to build for him. I do not think it would take 200-300 grams (1/2 to 3/4 pound) more material to make a 1 1/8" steerer fork of the same strength. Also, a properly designed fork shifts the "fulcrum" away from the high stress riser of the crown race bearing on the steerer tube to a point lower into the crown area as both the Reynolds and the Alpha Q fork do well. The 1 1/4" steerer is most appropriate for an aluminum steerer where higher stiffness is needed to prevent fatigue.
> One of the various carbon forks Santana tested, a beefy-looking unbranded
> cyclocross fork with a 1-1/8 steerer, weighed 50% more than the new Reynolds
> fork, and yet was not strong enough to meet our standards. It is not
> surprising to me that the lightest tandem-specific carbon fork with a 1-1/8
> steerer (built by Wound-up) weighs 900 grams. If anyone tries to sell you a
> lighter 1-1/8 tandem fork you should be suspicious.
I wouldn't expect a cyclocross fork to hold up to tandem use. The Alpha Q 1 1/8" tandem fork weighs 470 grams. Out of the 60+ forks we have sold with our tandems in the past three years, none have failed under normal use and the two I inspected after minor crashes were also fine. I would be suspicious of over-generalizations.
> How much weight can tandem enthusiasts save with a carbon fork?
> Tandem-specific forks built from steel weigh about 1100 grams. The Reynolds
> tandem fork with its evolution-sized steerer weighs just over 500 grams.
> How comfortable is a carbon fiber fork? The answer may surprise readers who
> have been told... that the material itself is the primary reason carbon forks
> are more comfortable than steel forks. Actually, on an ounce-by-ounce basis
> the comfort difference between steel an carbon is nearly imperceptible. The
> primary reason for the comfort differences most of us have discovered is that
> the carbon forks we have ridden are lighter than the steel forks we've
> compared them to. While carbon fiber is modestly more comfortable than steel,
> a lighter fork (of any material) will transmit less shock than a heavier fork
> built with the same material.
The weight of a fork has little to do with the comfort, especially with a carbon fiber fork. The stiffness of a fork has the greatest influence. One can easily build a heavy, yet comfortable carbon fork (like the Aegis fork) or a lighter yet less comfortable fork (like the Alpha Q extra stiff version). In carbon, stiffness is mainly influenced by how the fiber is oriented. Assuming a minimum acceptable stiffness, switching to a carbon fiber fork will produce a noticeable increase in vibration damping. This is especially appreciated towards the end of a century ride.
> Compared to material or overall weight, however, the surest indicator of fork
> comfort is a pair of curved blades. Why do straight blades exist if curved
> blades are preferable? Because the mold tooling for a curved blade fork is
> hideously expensive, designers of carbon forks have tried to revive the old
> straight-blade fad. While straight forks look great, curved blades dissipate
> extra shock while maintaining better tire contact through uneven corners. By
> softening the impact force of bumps, curved blades attenuate fatiguing force
> before they reach the critical area of the crown and steerer (which in turn
> allows a curved-blade fork to be lighter or safer).
These are completely unsubstantiated claims designed to "steer" potential customers away from the Wound up and Alpha Q forks. I have not found curved blades any better at absorbing shock, damping vibration or maintaining better tire contact than an equivalent straight blade fork. We made a concerted effort to try and detect a difference between the Alpha Q curved and straight blade fork. We found no difference in ride quality. We did find that the curved Alpha Q forks occasionally had a wrinkle in the laminate right at the most curved part of the blade. This caused a stress riser in the fork which led to a crack starting. It takes more skill and quality consciousness to lay prepreg carbon into a curved mold without wrinkling it than it does a straight mold. Mr. Lee assured me that these few wrinkled laminate forks were made by an individual who had since been corrected. I didn't accept that fully, and decided to sell only his straight blade forks. I didn't want to have to depend on the mood of a worker. If you look at the curve of a Reynolds fork, you will notice that it is a very gradual curve - almost straight. I'm confident that this makes it much easier to lay prepreg in the mold without wrinkles. If Bill has any data to back up his claim about curved blade forks, I'd love to see it.
I applaud Bill for helping to lead the way in using carbon forks on tandems. I also thank him for introducing an excellent solution to stopping a tandem with the 8" disc (and a clever way to allow the use of a drum brake with the same frame fitting). I just wish the marketing language did not use loose technical reasons why other products are bad.