Santana initiated our tandem-specific carbon fork project at the end of 1998. Since then I have worked with the design engineers of five vendors, and have had forks failure-tested at two different labs. I have ridden tandems hundreds of miles with five different carbon forks from three manufacturers. Here are some thoughts:
Decades of steel fork failures on tandems attest that a tandem fork needs to have greater fatigue resistance than a single bike fork. Not only is a tandem heavier, it often strikes bumps faster and harder (try to "lift" the front end of a tandem over a chuckhole). A tandem's fork will also need to resist up to 4x more front braking force than a single. Finally, the average tandem will see years of additional service.
Given all of the above, installing a single-bike carbon fork on a tandem is not a good idea. By definition, a well-designed single bike fork is simply too light. While many builders of carbon forks for single bikes may believe their forks are robust enough for tandem use, this would imply their designs are too heavy for their intended market. In any case, if a fork manufacturer claims their product is strong enough for an intended use, they should be able to back their assertion with test results.
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. A photo of the failed fork accompanies the review. Personally, I would not ride a tandem with an Alpha-Q fork. Hopefully, the new owners of AME (Mr. Lee sold his company at the end of September) will test and improve their product.
As for testing, Reynolds Composites (the California fork company, not the British tubing company), has never released a new fork until after they have destroyed 18 production samples. Their internal protocol uses 70% more force and five times as many cycles as the "international" voluntary test standard that some fork builders don't even bother to use. In addition to fatigue testing, Reynolds also employs impact tests (dropping a heavy weight on a dummy axle) and yield tests (applying increasing force through a hydraulic piston until the fork breaks). Every carbon fork built by Reynolds is considerably stronger than a premium steel fork. The new Reynolds carbon tandem fork is considerably stronger than a Santana steel fork.
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, who has tested the popular carbon forks from over a dozen various manufacturers, reports that the strength of equal-weight carbon fiber forks can vary by a factor of four!
Reynolds' new tandem fork is roughly twice as strong as their best single fork at a weight penalty of 100g (a quarter of a pound). How can a fork be twice as strong without being twice as heavy? 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.
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.
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. In that a pound is 454 grams, installing a tandem-specific carbon fork may save you as much as 21oz. (Reynolds) or as little as 7oz. (Wound-up).
How comfortable is a carbon fiber fork? The answer may surprise readers who have been told (by manufacturers of carbon forks) 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 and 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.
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 forces before they reach the critical area of the crown and steerer (which in turn allows a curved-blade fork to be lighter or safer).