Why this year’s World Cup ball may not fly as far
Why this year’s World Cup ball may not fly as far
为什么今年世界杯的比赛用球可能飞得没那么远
Much is new about this month’s upcoming FIFA World Cup tournament, which will be held in the US, Canada, and Mexico. It hosts more teams than ever before. It’s the first to occur in three different host countries. And, like predecessor cups for over half a century, it will employ a soccer ball with a brand-new design. 本月即将举行的国际足联（FIFA）世界杯在许多方面都焕然一新，比赛将在美国、加拿大和墨西哥举行。本届赛事参赛球队数量创历史新高，也是首次由三个国家共同主办。此外，与过去半个多世纪的历届世界杯一样，本届比赛将采用一款全新设计的足球。
One group of researchers that has been testing the physics of World Cup balls for the past 20 years recently studied this new entry, called the Trionda. Made by Adidas, the Trionda features four red, green, and blue panels textured with deep grooves and maple leaf, green eagle, and star emblems to represent the three host countries. 过去20年来一直致力于测试世界杯足球物理特性的研究团队，最近对这款名为“Trionda”的新球进行了研究。Trionda由阿迪达斯制造，由四块红、绿、蓝三色的嵌板组成，表面带有深槽纹理，并印有代表三个主办国的枫叶、绿鹰和星星图案。
Through wind-tunnel experiments, the research team found that this ball improves over previous versions in some ways, but long-distance kicks might not go as far as they did in the past. “The simple picture is that Trionda may very slightly punish extreme distance, but it should reward clean technique and predictable flight,” says team member John Eric Goff, who researches sports physics and is an incoming professor of engineering practice at Purdue University. “Goalkeepers, defenders hitting long passes, and long-range shooters are where I would look first for visible differences.” 通过风洞实验，研究团队发现这款球在某些方面比以往版本有所改进，但长距离射门可能无法像过去那样飞得那么远。研究小组成员、运动物理学研究者、普渡大学即将上任的工程实践教授约翰·埃里克·戈夫（John Eric Goff）表示：“简单来说，Trionda可能会在超远距离射门上略微吃亏，但它能奖励精准的脚法和可预测的飞行轨迹。守门员、长传的后卫以及远射球员，将是首先感受到明显差异的人群。”
Adidas has been designing new balls for each World Cup since the 1970s. Some of the design changes in the first few decades were aesthetic: The 1986 ball featured graphics inspired by Aztec temples for the Mexico tournament, and 1994’s had space graphics in honor of the moon landing’s 25th anniversary. There were some structural differences too, such as upgraded foam cores and improved water resistance. But by and large, the balls used the same design of 32 pentagonal panels stitched together. 自20世纪70年代以来，阿迪达斯一直为每届世界杯设计新球。最初几十年的部分设计变化主要体现在美学上：1986年墨西哥世界杯的比赛用球采用了受阿兹特克神庙启发的图案，而1994年的用球则印有太空图案，以纪念登月25周年。结构上也存在一些差异，例如升级了泡沫内芯和增强了防水性能。但总的来说，这些球大多沿用了32块五边形嵌板缝合在一起的相同设计。
That changed in the 2006 World Cup in Germany, when Adidas introduced the +Teamgeist ball. It featured just 14 curved panels, which were thermally bonded together rather than stitched. The design helped keep moisture out so the ball wouldn’t grow heavier throughout the game, Goff says. It was around this time that he started studying soccer balls. 这种情况在2006年德国世界杯上发生了改变，当时阿迪达斯推出了“+Teamgeist”足球。它仅由14块弧形嵌板组成，采用热粘合而非缝合工艺。戈夫说，这种设计有助于阻隔水分，使球在比赛过程中不会变重。也就是在那个时候，他开始研究足球。
In the years since then, he and his colleagues have followed the transformations as Adidas has released balls with different surface textures and even fewer panels—design changes significant enough to affect game play. 此后的几年里，他和同事们一直关注着阿迪达斯推出的各种足球，这些球采用了不同的表面纹理，嵌板数量甚至更少——这些设计上的变化足以影响比赛表现。
In-flight motion
飞行运动
Goff discovered early on that by analyzing a ball’s trajectory data, he could derive its drag coefficient—a number that determines the air resistance it experiences midflight at a given speed. Shortly after, he began working with a team in Japan to analyze how the World Cup ball’s in-flight behavior changes with each new design. 戈夫很早就发现，通过分析足球的轨迹数据，他可以推导出其阻力系数——这是一个决定球在特定速度下飞行时所受空气阻力的数值。不久之后，他开始与日本的一个团队合作，分析世界杯用球的飞行表现如何随着每一代新设计而改变。
The experiments, carried out at the University of Tsukuba in Japan, have been purposely consistent over the years because “maintaining continuity is important for comparing new data with historical data sets,” says Takeshi Asai, a professor there who works on the experiments. They entail attaching the ball to a metal rod connected to an instrument called a force balance, which measures aerodynamic forces such as drag and lift as the ball is exposed to the same wind speeds it would experience in a real soccer game—seven to 35 meters per second. 这些实验在日本筑波大学进行，多年来一直保持一致，因为“保持连续性对于将新数据与历史数据集进行比较非常重要，”参与实验的筑波大学教授浅井武（Takeshi Asai）说。实验过程是将足球固定在连接着“测力天平”仪器的金属杆上，当球暴露在与真实足球比赛中相同的风速（每秒7到35米）下时，仪器会测量阻力和升力等空气动力。
The team tests the ball in different orientations, “but you can only do a few because the Trionda ball is $170,” Goff says, and each new test effectively destroys it. The experiments show the team how the drag coefficient changes with speed, and Goff then writes code to simulate the ball’s overall trajectory as it flies through the air. 团队会在不同的方向上测试足球，“但你只能做几次，因为Trionda球要170美元，”戈夫说，而且每一次新的测试实际上都会损坏球体。实验向团队展示了阻力系数如何随速度变化，随后戈夫会编写代码来模拟球在空中飞行时的整体轨迹。
The team’s analysis has shown how recent World Cup balls evolved since the eight-panel Jabulani ball for the 2010 event. The Jabulani faced much criticism from players—particularly goalkeepers, who said it had a deceptive trajectory that “dipped wickedly,” as one player told the Guardian. 该团队的分析展示了自2010年世界杯采用8块嵌板的“Jabulani”以来，世界杯用球是如何演变的。Jabulani曾遭到球员们的强烈批评，尤其是守门员，正如一位球员告诉《卫报》的那样，他们认为该球的轨迹具有欺骗性，会“诡异地下坠”。
The ball had one key flaw: It was too smooth. Even though its drag coefficient was relatively low at high speeds, once the ball slowed to a certain point the coefficient would ratchet up, causing it to lose speed quite fast and behave as the 2010 players complained. This sudden transition—called the drag crisis—occurs at higher speeds for smoother balls, but with added texture like seams and grooves, the transition can be avoided until a ball reaches lower speeds. 这款球有一个关键缺陷：它太光滑了。尽管它在高速下的阻力系数相对较低，但一旦球速减慢到一定程度，阻力系数就会激增，导致球速迅速下降，从而出现2010年球员们所抱怨的那种表现。这种突然的转变被称为“阻力危机”（drag crisis），对于更光滑的球，这种转变发生在更高的速度下；但通过增加接缝和凹槽等纹理，可以推迟这种转变，直到球速降得更低。
This allows the ball to travel farther and generally behave in a more predictable way during typical play. “It’s the same reason why golf balls have dimples and baseballs have those nice 108 double stitches. If those rough features of those balls were not there, you would not get anywhere near the kind of distance when those balls are thrown or hit that you see now,” Goff says. “There has to be some kind of a roughness on the ball to move this transition to a smaller speed.” 这使得球能飞得更远，并且在常规比赛中表现得更具可预测性。“这和高尔夫球有凹坑、棒球有那108道双缝线的原因是一样的。如果这些球上没有那些粗糙的特征，当它们被投掷或击打时，你根本无法获得现在所看到的飞行距离，”戈夫说。“球面上必须有某种粗糙度，才能将这种转变推迟到更低的速度。”
New grooves
新的凹槽
Subsequent designs have been able to push the drag crisis to lower speeds, according to the analysis by Goff and his colleagues. The Brazuca ball used in 2014, for instance, has only six panels, but their total seam length is much longer, adding to the surface’s roughness. And this year’s Trionda ball contains just four panels, but each panel also has three deep grooves for more texture. 根据戈夫及其同事的分析，后续的设计已经能够将“阻力危机”推迟到更低的速度。例如，2014年使用的“Brazuca”足球只有6块嵌板，但其接缝总长度要长得多，增加了表面的粗糙度。而今年的Trionda球仅包含4块嵌板，但每块嵌板都有三条深槽，以增加纹理。
There’s a trade-off to this roughness, though. While Goff and his colleagues found that the Trionda ball experiences the drag crisis at the slowest speed since 2010, its drag coefficient is also higher than that of the other balls at high speeds. That means that even though the most dramatic change doesn’t happen until the ball is moving quite slowly, the ball will still slow down faster. 然而，这种粗糙度是有代价的。虽然戈夫和他的同事发现Trionda球在2010年以来的所有用球中，发生“阻力危机”的速度最慢，但它在高速下的阻力系数也比其他球更高。这意味着，尽管最剧烈的变化要等到球速变得很慢时才会发生，但球在飞行过程中减速的速度依然会更快。