TECH: IN THE WIND TUN­NEL

Held aloft as the ul­ti­mate go-faster tool for cy­clists, wind tun­nel use has ex­ploded re­cently as gains be­come harder to ind. Pro­cy­cling talks to in­dus­try brains about the lat­est test meth­ods and what the fu­ture holds

Procycling - - Contents -

How pro­fes­sional teams and equip­ment com­pa­nies are har­ness­ing aero tech

The foe that grows stronger with your ev­ery strike, wind is a cy­clist’s re­al­life night­mare. If you want to go twice as fast, your en­emy the wind pushes back four times as hard and you will need eight times the power to match it. Drag in­creases to ve­loc­ity squared, power to ve­loc­ity cubed. Be­com­ing a stronger rider is the most di­min­ish­ing of re­turns. But the wind tun­nel is how we fight back. We can’t change the strength of the wind but we can give it less to grab hold of, and wind tun­nels are how teams and the cy­cling in­dus­try test, ex­per­i­ment, de­velop and mea­sure their ef­forts.

From rudi­men­tary be­gin­nings in the 1980s, wind tun­nel use in cy­cling has come a long way. World and Olympic cham­pion Chris Board­man, more re­cently one of the brains be­hind Bri­tish Cy­cling’s suc­cess be­fore found­ing his own bike brand, re­calls his first ex­pe­ri­ence in late 1991: “I went to the tun­nel at MIRA [Mo­tor In­dus­try Re­search As­so­ci­a­tion, War­wick­shire, UK] with Mike Bur­rows. It was made for test­ing out trucks, but it was the first time we’d been able to test po­si­tions to get real num­bers. We came out with more ques­tions than an­swers, but it was a start.”

Cy­cling is one of the rel­a­tively more triv­ial uses of a wind tun­nel. As with so much tech­nol­ogy, this early 20th cen­tury in­ven­tion went through ac­cel­er­ated de­vel­op­ment dur­ing the two world wars and through the ad­vent of the jet era, lead­ing to tun­nels ca­pa­ble

of su­per­sonic speeds and able to ac­com­mo­date full­size air­craft. NASA has the world’s big­gest wind tun­nel at the Ames Re­search Cen­tre in Cal­i­for­nia, USA, with a vast test area cross-sec­tion of 80x120ft.

Motorsport – es­pe­cially For­mula One, with its high bud­gets and fierce pace of de­vel­op­ment – was the con­duit for this tech­nol­ogy moving from mil­i­tary us­age to cy­cling. It has also given cy­cling some of its best brains, in­clud­ing Si­mon Smart of Drag2Zero, a con­sult­ing aero­dy­nam­i­cist for Enve wheels and Scott Bikes, among oth­ers. He’s ar­guably the fore­most TT po­si­tion­ing ex­pert in the world – six of the top 10 on GC at the last Tour de France had been in his tun­nel to hone their po­si­tions.

“In F1, the goal was for a 10 per cent im­prove­ment per year. We had three or four groups of en­gi­neers work­ing 24 hours, six and even seven days per week. That’s the dirty way of find­ing per­for­mance, mak­ing ev­ery shape you can think of and test­ing and test­ing,” Smart remembers “When I started in cy­cling, at first I thought, ‘This is easy, it’s only a cy­clist’ but you have a much lower drag force, around 3kg at 30mph com­pared to 100kg at 100mph for an F1 car, and such chaotic flow be­cause a cy­clist is a bluff body and al­ways moving. It’s cer­tainly a chal­leng­ing busi­ness, but we’re now test­ing re­peat­ably to within 1 Watt, which I never thought would be pos­si­ble with a rider. At the same time, all of the low-hang­ing fruit – the frame shapes, wheels – has gone, so now we’re into sec­ond or­der stuff such as skin­suit fab­rics and per­for­mance in os­cil­lat­ing winds. When you look at all of it, those sec­ond-or­der el­e­ments can add up to first-or­der gains.”

KNOWN UN­KNOWNS

For all of the progress since then, it can some­times seem as if the main thing that’s been learned is just how lit­tle is known. “We’re still in the in­fancy of our un­der­stand­ing of aero in wind tun­nels,” says Kevin Quan, de­signer of Knight wheels and the rad­i­cal new Di­a­mond­back An­dean tri bike. “Most peo­ple in the in­dus­try are not PhDs, with the ex­cep­tion of the guys at Spe­cial­ized. We’re en­gi­neers; we’re ap­ply­ing the tech, not cre­at­ing it. The tun­nel gen­er­ates num­bers; it doesn’t ex­plain why some­thing is faster or slower.”

Get­ting be­yond trial and er­ror and into some real un­der­stand­ing is the goal of ev­ery aero­dy­nam­i­cist, but that goal is also in­cred­i­bly time con­sum­ing, as Board­man re­calls: “In one Olympic cy­cle we tested 10,000 ma­te­ri­als on a tube in a small tun­nel, test­ing fab­ric ten­sion, grain di­rec­tion… We tested se­quins, fur, all sorts of weird and won­der­ful things, just to see. Some­times you do one test and it gives you

“In one Olympic cy­cle we tested 10,000 ma­te­ri­als on a tube in a small tun­nel. We tested se­quins, fur, all sorts”

a whole other di­rec­tion to look at. We found some cloth­ing in the sec­ond Olympic cy­cle that worked ex­tremely well, but we didn’t know why. So we spent the next four years work­ing to un­der­stand it.”

UK Sport’s bud­get dwarfs that of most cy­cling brands, so that level of re­search was out of reach to them for a long time. How­ever, now the need to find sec­ond-or­der gains is push­ing brands into mak­ing a big­ger com­mit­ment.

“The com­mer­cial pres­sures of get­ting out a prod­uct ne­ces­si­tated a trial-and-er­ror ap­proach,” says Quan. “Now some com­pa­nies are moving to­wards re­search – such as Spe­cial­ized and the Eurore­gional Bike Val­ley hub in Bel­gium [shared by Flem­ish com­pa­nies Ri­d­ley, Lazer Sport and Bio­racer] – but that’s a big in­vest­ment and then you have to make it pay.”

The man re­spon­si­ble for the Spe­cial­ized fa­cil­ity, dubbed the Win Tun­nel, is Dr Chris Yu, di­rec­tor of in­te­grated tech­nolo­gies. Yu told Pro­cy­cling how their tun­nel is de­signed to be cy­cling-spe­cific: “We had three main ob­jec­tives: speed, size and sen­si­tiv­ity. We wanted to be able to cover speeds from 15-110kph [ie up and down moun­tains] so we could test flap­ping cloth­ing, cool­ing etc. The size [16x10x30ft] al­lows us to test TTTs and lead-out trains.

“Fi­nally, aside from the qual­ity of air flow, the most important el­e­ment is the force bal­ance. It turns out that cy­cling is unique in that the res­o­lu­tion re­quired is very high; we need to ac­cu­rately mea­sure changes of one gram. At the same time, the peak forces are quite high, such as when we have a sprinter rid­ing at max­i­mum power. It’s chal­leng­ing to de­sign a scale that is both very sen­si­tive and very ro­bust.”

For all of the science, wind tun­nels have their idiosyncrasies, too. Each one is a dif­fer­ent size and shape. The smaller ‘closed jet’ test ar­eas limit the size of the sub­jects that can be tested in them. “You don’t want the ob­ject to be big­ger than five per cent of the to­tal area. The walls can af­fect aero per­for­mance and can dis­tort re­sults,” says Smart. For this rea­son, ‘open jet’ tun­nels are more suited to test­ing rid­ers on bikes at high yaw an­gles.

More techy still is the met­ric of ‘ini­tial tur­bu­lence’ – how smooth the air flow is as it reaches the sub­ject, hav­ing ear­lier been chopped and swirled by the fans. “This is div­ing quite deep into how tun­nels work and how dif­fer­ent aero phe­nom­ena can be af­fected,” says Yu. “We tar­geted a free stream tur­bu­lence in­ten­sity of just un­der one per cent, which from our stud­ies is ap­pro­pri­ate for out­door cy­cling. This is con­trolled by tur­bu­lence screens, as well as the con­trac­tion at the in­let of the tun­nel. Val­ues that are too low or too high can re­sult in mis­lead­ing con­clu­sions about the per­for­mance of dif­fer­ent aero shapes, es­pe­cially when it comes to flow sep­a­ra­tion and at­tach­ment.”

Smart adds a less sci­en­tific eval­u­a­tion of it. “It’s these things that give tun­nels their own char­ac­ter,” he says. “But we’ve also solved a couple of those known un­knowns, and we’re re­ally ex­cited about the next few years.”

TEST­ING VERSUS DE­VEL­OP­MENT

Many prod­ucts and bikes are claimed to be wind tun­nel ‘tested’ but that’s a world away from be­ing wind tun­nel ‘de­vel­oped’. Only the lat­ter uses data to make im­prove­ments and new it­er­a­tions. If you’re test­ing a signed-off pro­duc­tion model, all you can do is hope that it’s fast. It’s even been known for a brand to book an hour in a wind tun­nel just for a cat­a­logue pho­to­shoot. Those mar­ket­ing peo­ple will prob­a­bly ask for smoke to be blown over the prod­uct, too, but the en­gi­neers will be rolling their eyes. Quan ex­plains that smoke “works on much big­ger ob­jects, but not in cy­cling, with lower flow speed and smaller air­foils. Cot­ton strings like­wise, and oil map­ping just makes a mess. The lat­est method of flow vi­su­al­i­sa­tion is Par­ti­cle Im­age Ve­locime­try (PIV). You flood the tun­nel with smoke, then fire a sheet of laser through it to see vor­tices. If we can get PIV here that would def­i­nitely progress our un­der­stand­ing.”

Dr Dave Marshall, who runs the wind tun­nel at the Univer­sity of Southamp­ton (where Pro­cy­cling tested the wheels in this is­sue and where Bri­tish Cy­cling and Team Sky have done much work) be­lieves finer data res­o­lu­tion is the goal: “We take 20 sec­ond av­er­ages for each con­fig­u­ra­tion, and it would be good to know how things are chang­ing within that. For an inan­i­mate ob­ject such as a race car, not much is hap­pen­ing, so you can take shorter av­er­ages and get the same sort

of data qual­ity. But a cy­clist is moving, so see­ing a time his­tory of where the drag is com­ing from through a pedal ro­ta­tion would be good, per­haps in a live read­out over­laid with slow-mo­tion video.”

Data mea­sure­ment is also where the great­est progress has been made over the last cen­tury. “This tun­nel dates back to the 1930s,” Marshall says. “Quite early on they had very good flow qual­ity and a good un­der­stand­ing of what the air was do­ing. The dif­fer­ence is in the in­stru­men­ta­tion and res­o­lu­tion – we can mea­sure thou­sands of times per sec­ond to very fine ac­cu­racy. The bal­ance used to be just that: an ana­logue bal­ance with weights.”

SIM­U­LA­TION VERSUS CON­TROL

Ac­cu­rate test­ing is all about con­trol and re­peata­bil­ity, but that cre­ates a para­dox – the more con­trolled the test, the less like the real world it be­comes. For an ex­am­ple of this, look at TT down­tubes from 10 years ago. With the front wheel clamped in place, en­gi­neers were spoilt by con­sis­tent flow, so they de­signed thin, wheel-hug­ging down­tubes, but on the road the front wheel is rarely aligned so neatly. Mod­ern TT and aero road bikes have more ver­sa­tile in­ter­ac­tions be­tween the two. So which is more important, sim­u­lat­ing the real world or cre­at­ing a per­fectly con­trolled test?

“That’s delv­ing into a de­bate that has been go­ing for eight years,” says Board­man. “You can choose. If you go with a man­nequin you gain con­trol and get very re­peat­able data, or you can go to the track and get very close to re­al­ity, but the data is very noisy.” Smart agrees to an ex­tent, but he also sees an­other way: “Tun­nels have been driven to be as pure as pos­si­ble for the ul­ti­mate fidelity, but then you move away from the real world. Our job as en­gi­neers is to find the sweet spot. That’s what the fu­ture holds. Out­door test­ing is good for val­i­da­tion. Just 10 years ago an anemome­ter needed a back­pack full of data-log­ging gear. Now you can run it from an iPhone.”

Board­man, Quan, Yu and Smart agree that a wind tun­nel may be the best sin­gle tool, but it’s at its best when it’s com­bined and cross-ref­er­enced with CFD stud­ies and real-world mea­sure­ment.

At worst, ex­ces­sively con­trolled test­ing risks skew­ing R&D ef­forts, as Marshall ex­plains: “A man­nequin test gives you ul­ti­mate re­peata­bil­ity, but rider shapes vary, so you cre­ate the risk of de­sign­ing some­thing that only works for cer­tain shape rid­ers.”

A case in point: at the last Win­ter Olympics the US speed skat­ing team blamed their poor per­for­mance on their new Un­der Ar­mour ‘Mach 39’ skin­suits, which had been her­alded as the fastest ever af­ter de­vel­op­ment but pro­duced poor times and no medals. Like­wise, in cy­cling, POC pro­duced the Tem­por TT hel­met, which is also in­cred­i­bly fast but only in the ul­tra-low po­si­tion of the pro who helped de­velop it, and while it’s good at 30mph, above 45mph it starts gen­er­at­ing lift like a wing and wob­bling around. Such mis­ad­ven­tures are re­told now as cau­tion­ary tales.

WINDS OF CHANGE

While For­mula One teams con­tinue to push the tech – their lat­est in­no­va­tion is the ‘adap­tive wall tun­nel’, bet­ter sim­u­lat­ing out­door con­di­tions and cost­ing £50m to build – this most elite of per­for­mance tools is about to be­come democra­tised, at least in the UK.

In Novem­ber, the Board­man Per­for­mance Cen­tre will open its doors. Along­side a con­cept store and café it’ll house a cut­ting-edge, cy­cling-spe­cific tun­nel that Board­man prom­ises will be far more af­ford­able than we’re used to. “The con­cept started over a curry with Rob Lewis, an aero­dy­nam­i­cist I worked with at Bri­tish Cy­cling,” he ex­plains. “He said, ‘Wouldn’t it be great if we could run tests for the price of a curry?’ So we started to ex­plore the con­cept and took away the 150mph fans, the rolling road, the heavy bal­ance for cars, and by the end of the meal we had a plan.

“It’ll be a cus­tom open jet and open re­turn tun­nel, housed in­side a tem­per­a­ture con­trolled build­ing. It’s de­signed to be fast to change a bike, with a bal­ance built to clear up data noise from ped­alling. Hun­dreds of hours of CFD have al­ready gone into this. I’m pretty con­fi­dent we’ll have the best tun­nel in the world.

“My big­gest chal­lenge is to get peo­ple to see how ex­cit­ing it is to go to find out stuff and have that eureka mo­ment.”

“A man­nequin test gives you ul­ti­mate re­peata­bil­ity, but rider shapes vary in the real world”

In­side the Spe­cial­ized ‘ Win Tun­nel’ test­ing fa­cil­ity, which was de­signed speci ically for test­ing bikes

Alex Dowsett has spent hours hav­ing his po­si­tion honed in wind tun­nels

Chris Board­man was one of the irst rid­ers to make ex­ten­sive use of wind tun­nel test­ing

Team Spe­cial­ized, with per­for­mance man­ager Mark Cote (left) and Dr Yu (right)

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