Aero­dy­nam­ics is the sci­ence of ex­ploit­ing the air­flow around your race car for down­force, re­duc­ing drag, and cooling. It’s a pre­ci­sion task that can pay huge div­i­dends if you get it right, which is why the up­per ech­e­lons of mo­tor­sport pay so much at­ten­tion to it, with the likes of For­mula 1, tour­ing cars, and en­durance teams de­vot­ing huge bud­gets and re­sources to it. But even at a grass-roots level, get­ting this right (with­out break­ing the bank) can re­sult in last­ing and size­able im­prove­ments to your lap times.

We’ve all heard that a race car like Rod Millen’s Cel­ica should, the­o­ret­i­cally, be able to drive up­side down in a tun­nel at, say, 200kph, with the only thing hold­ing it to the roof of that the­o­ret­i­cal tun­nel the pass­ing air act­ing on the ve­hi­cle’s aero de­vices. While driv­ing up­side down would be trick and all, imag­ine the ef­fect that that level of down­force would have on your grip lev­els, thus, the pos­si­ble speed that you could take a cor­ner. This is where lit­eral sec­onds can be slashed from a PB if you’re able to get the balance of you aero­dy­namic ad­di­tions cor­rect. How­ever, this is where most come un­stuck — in be­ing able to test, or prove, the ef­fi­ciency of their de­sign. Un­like, say, en­gine up­grades, when a dyno will tell you the ex­act re­sult of any change, with­out a wind tun­nel or com­pu­ta­tional fluid dy­nam­ics (CFD) anal­y­sis soft­ware (and the knowl­edge to run it), we’re re­ally only guess­ing when it comes to aero, and must rely on the feed­back from the driver’s seat. Just look at the cur­rent F1 front wing and how it changes over a sea­son to get a pic­ture of how ex­act this sci­ence is.

De­spite the rise in pop­u­lar­ity of aero in lower level mo­tor­sports such as Time At­tack, in which wings, split­ters, ca­nards, dif­fusers, un­der­trays, and vents are now com­mon­place ad­di­tions, few teams have yet en­gaged the likes of Kiwi aero engi­neer David Hig­gins to en­sure that their cho­sen com­bi­na­tion is ef­fi­cient, ef­fec­tive, and mak­ing the most of their avail­able bud­get. The cheap­est way of en­sur­ing whether that big wing you pur­chased off Alibaba is ac­tu­ally a cred­i­ble de­sign is to grab a book on aero de­sign. David rec­om­mends two books as a start­ing point—Race Car Aero dy­nam­ics: De­sign­ing for Speed by Joseph Katz, and Com­pe­ti­tion Car Aero dy­nam­ics by Si­mon McBeath — each is avail­able on­line at around the $50 mark and will give you a solid un­der­stand­ing to make in­formed aero ad­di­tions.

One of the most com­mon mis­takes made, and the sin­gle big­gest piece of ad­vice that David likes to give to bud­ding de­sign­ers, is to de­sign your pack­age to suit not only your car but also your class rules. All too of­ten, peo­ple fall into the trap of us­ing the likes of Google to de­sign aero com­bi­na­tions or bor­row­ing de­signs from con­trol classes like FIA GT3. “Pro­fes­sion­ally built race cars are typ­i­cally built to a set of strict rules or balance of per­for­mance cri­te­ria. Many of the de­ci­sions made in their de­sign are due to rules, and, in New Zealand, the classes we have are var­ied and don’t carry the same rule re­stric­tions. You can cer­tainly pick up de­sign fun­da­men­tals and ideas from pro­fes­sion­ally built race cars, but, in terms of de­sign­ing cer­tain aero­dy­namic parts to mir­ror these cars, you may only be scratch­ing the sur­face. Hence, it’s best to un­der­stand the lo­cal classes, rules, and where it’s best to con­cen­trate for max­i­mum per­for­mance within your bud­get,” David says.

The in­cred­i­ble full-car­bon Evo of Mick Sigsworth is a cur­rent work-in-progress, hav­ing its new split­ter/ un­der­tray and rear dif­fuser de­signed by Dave. This was a large-bud­get project that saw a full 3D scan of the body made. The new parts are cur­rently un­der con­struc­tion, with a set for the WTAC Pro-Am Class and a nar­row set for door-to-door rac­ing

An­other area in which this work can come in handy is de­sign­ing your cooling pack­age. The en­durance rac­ing R35 GT-R we fea­tured back in Is­sue No. 235 saw huge gains af­ter David ran the num­bers: “For this car, we made a num­ber of changes to their cooling lay­out, pack­ag­ing, cooler sizes, and duct­ing to solve many cooling is­sues for en­gine temp, oil temp, gear­box temp, etc. We also made some mi­nor changes to their front split­ter, which net­ted them over a sec­ond in lap time and made the car re­li­able for tem­per­a­ture”

This is some­thing he is find­ing more and more of his cus­tomers are opt­ing for: a push in the right di­rec­tion with some men­tor­ing/coach­ing so that you can de­sign and con­struct your­self rather than a full de­sign ser­vice — in the same way that you might have an en­gine builder do all the ma­chine work then take over your­self to do the fi­nal assem­bly. Get­ting ad­vice on choos­ing the right wing el­e­ment or de­sign­ing your dif­fuser is a com­mon route to take. All too of­ten, these are poorly de­signed — in the most ex­treme cases, the lift gen­er­ated can make the car un­sta­ble and slow it down, or the parts added can make the car heav­ier; pro­duce in­ef­fi­cient down­force; or, in many cases, gen­er­ate more drag. A piece of al­loy or ACM pro­trud­ing out from the front of your bumper is an ex­tremely com­mon ad­di­tion, but it is not go­ing to be an ef­fec­tive aero de­vice, as it will cause air­flow sep­a­ra­tion on the lead­ing edge, leav­ing a large pocket of low pres­sure un­der­neath and block­ing air from flow­ing un­der the car.

One of David’s most suc­cess­ful au­to­mo­tive projects to date is Andy Duf­fin’s FD RX-7, which uti­lized CFD anal­y­sis. “We had four months (in­clud­ing build time) to trans­form the 3 Ro­tor Rac­ing FD RX-7 from what was es­sen­tially an SS2000 car to a WTAC [World Time At­tack Chal­lenge] Open Class car,” David told us. “We car­ried out a ba­sic CFD anal­y­sis us­ing an on­line-pur­chased CAD model and de­signed a new front un­der­tray/split­ter, bon­net, front guards, side skirts, [and] rear dif­fuser, and se­lected a rear wing top [for] which we de­signed our own end plates. In the first test, hav­ing never driven with aero, Andy was three sec­onds a lap faster, with more to come.”

CFD re­places the need for a wind tun­nel, which is good, as there are no full-size tun­nels in little old New Zealand any­way. To con­duct CFD anal­y­sis, you need a CAD model of your car, which can be pro­duced by scan­ning your ac­tual race car, or the cheaper op­tion, as they did with Duf­fin’s FD, is to pur­chase a pre-drawn CAD sur­face model, of which many are avail­able on­line. These CAD mod­els are not di­men­sion­ally the same as an ac­tual car, but, if they are used for the ex­ter­nal up­per sur­faces only, they can save a lot of time/money. The sur­face mod­els then have all the de­tail added, in­clud­ing in­side the guards, un­der the bon­net, the ra­di­a­tors, un­der the floor, the wheels, and the sus­pen­sion. Ba­si­cally, the more de­tail that is added in, the more ac­cu­rate the anal­y­sis will be. Once the CAD is com­pleted, it’s then over to the com­pli­cated and time-con­sum­ing work of CFD anal­y­sis, which tests each part that you’re think­ing of adding and how it will work as part of the en­tire pack­age.

Max­i­miz­ing your aero pack­age can start from as early as car se­lec­tion. It doesn’t take a ge­nius to fig­ure out that start­ing with a Sil­via rather than a Volvo is a huge step in the right di­rec­tion. If you are be­gin­ning a new project, re­gard­less of how long it will take to build and how many it­er­a­tions of de­sign it will end up go­ing through, choos­ing the right base car to suit the rules and aero­dy­namic op­por­tu­ni­ties is key. We can­not stress how im­por­tant this is. Just be­cause you have a car or a shell sit­ting in the garage doesn’t mean that you should use that as your base. Do your home­work at the start

Do­ing this al­lows you to eas­ily make changes to things such as the an­gle of your wing, the shape of your end plate, or even the ride height. If this were done in a wind tun­nel, you’d need to have pro­to­typed any parts that you want to tweak and be swap­ping and ad­just­ing them on the fly, which, when you’re pay­ing by the hour for a tun­nel, can soon mount up your costs. As with CFD, you’ll only pro­duce some­thing once a de­sign has been fi­nal­ized, and, at this stage, a CAD draw­ing can be sent out for the part to be CNC’d or con­structed from mea­sure­ments (taken by hand).

This type of de­sign work does come at a cost, but it’s a frac­tion of the ac­tual cost of pro­duc­ing the parts, as David states: “If you have ne­glected the cost of de­sign­ing prop­erly, then you have wasted a heap of money. It is al­ways go­ing to cost X-amount to con­struct a new body, split­ter, wing, etc., but if you don’t spend the time and [seek] ex­per­tise be­fore mak­ing it, then you have wasted a mas­sive op­por­tu­nity. In most cases, we work in the 80:20 space, where we try to get 80 per cent of the gain with 20 per cent of the nor­mal bud­get for an overseas, pro­fes­sional-style de­vel­op­ment. We are try­ing our best to ed­u­cate lo­cal grass-roots mo­tor­sport [about] ... the ben­e­fits of proper de­sign and make this style of de­vel­op­ment — whether your bud­get is CFD anal­y­sis or not — af­ford­able to lo­cal mo­tor­sport. For the cost of a few sets of tyres and some ba­sic fab­ri­ca­tion/com­pos­ites, you can get per­ma­nent lap-time gains. Throw­ing tyres at a car will only help you for a few laps.”

Over the next few is­sues, we will delve a little deeper into a few key ar­eas with David and lay down some ba­sic knowl­edge, in the hope that we all can get our cars run­ning a little faster at the track.

As you can see, a CFD model shows in great de­tail how the air in­ter­acts with your car’s body. You’ll be amazed at the dif­fer­ence even one de­gree of at­tack-an­gle change can make to the per­for­mance of the part or the next part that the air will come into con­tact with — It shows how easy it is to get it wrong

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