Drafting (aerodynamics)

A paceline of drafting cyclists

For other uses, see Slipstream (disambiguation).

Drafting or slipstreaming is a technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream. Especially when high speeds are involved,as in motor racing and cycling, drafting can significantly reduce the paceline's average energy expenditure required to maintain a certain speed and can also slightly reduce the energy expenditure of the lead vehicle or object.

Drafting in sport

Drafting is used to reduce wind resistance and is seen most commonly in bicycle racing, car racing, and speedskating, though drafting is occasionally used even in cross-country skiing and running. Some forms of triathlon allow drafting. Drafting occurs in swimming as well: both in open-water races (occurring in natural bodies of water) and in traditional races in competition pools. In a competition pool a swimmer may hug the lane line that separating them from the swimmer they are abaft of thereby taking advantage of the liquid slipstream in the other swimmer's wake. Drafting also occurs in competitive longboarding.

It is believed, but not yet conclusively proven, that Thoroughbred racing horses draft each other, especially in longer races.^[1]

Bicycle racing

Belgian tourniquet (Belgischer Kreisel)

In cycling the main (largest) group of tightly packed cyclists in a race is called a peloton where cyclists ride in a long formation with each (but not the first rider) drafting behind the others before them.

When cyclists ride fast they form a paceline. Each cyclist (but not the first) is drafting behind another one. In order to ride very fast a team of some skilled cyclists may form the "Belgian tourniquet." Successively, each cyclist leads the group. Drafting can be cooperative: several competitors take turns in the lead position (which requires the most effort and energy consumption). It can also be competitive or tactical: one competitor will try to stay closely behind another leaving him or her more energy for a break-away push to the finish line.

Motorsport

Open-wheel racing

In single seater, open wheel racing series such as Formula One and the IndyCar Series a technique known as slipstreaming is used. Along a long straight a car following close behind another uses the slipstream created by the lead car to close the gap between them, hoping to be able to overtake the leader under braking for the next corner, or if he has a straightline speed advantage, to pass on the straight. However it is very difficult for cars to follow each other close together in fast corners as the "dirty" (turbulent) air that comes off the lead car unbalances the trailing car as its aerodynamic devices provide less grip. However, on the straight this effect is much less of a detriment due to the lower levels of grip required.

Stock car racing

NASCAR cars draft at Daytona International Speedway

On the faster speedways and superspeedways used by NASCAR, ARCA, and at one time the IROC series, two or more vehicles can race faster when lined up front-to-rear than a single car can race alone. The low-pressure wake behind a group's leading car reduces the aerodynamic resistance on the front of the trailing car allowing the second car to pull closer. As the second car nears the first it pushes high-pressure air forward so less fast-moving air hits the lead car's spoiler. The result is less drag for both cars, allowing faster speeds.^[2]

Handling in corners is affected by balance changes caused by the draft: the leading car has normal front downforce but less rear downforce. The trailing car has less front downforce but normal rear downforce. A car with drafting partners both ahead and behind will lose downforce at both ends.^[2]

Similar to the "Belgian tourniquet" in cycling, the "slingshot pass" is the most dramatic and widely noted maneuver associated with drafting. A trailing car (perhaps pushed by a line of drafting cars) uses the lead car's wake to pull up with maximum momentum at the end of a straightaway, enters a turn high, and turns down across the lead car's wake. The combination of running downhill and running across the zone of lowest aerodynamic drag allows the trailing car to carry extra speed and pass on the inside of the leader.^[2]

Drafting was discovered by stock car racers in the 1959 Daytona 500. Drivers found they picked up speed running closely behind other cars, and as they experimented they found that a line of cars could sustain higher speeds than a single car running by itself.^[2]

Bump drafting

Drafting is most important at Talladega Superspeedway and Daytona International Speedway where a restrictor plate is used, making much less power available to push the large bodies through the air. Race cars reach their highest speeds on these superspeedways, so the aerodynamic forces are highest, and the effects of drafting are strongest. Since restrictor plates were first used as a safety device, their effect has changed the nature of drafting. Vehicles no longer have sufficient horsepower or throttle response to maintain their drafting speeds upon exiting the draft; they can pull out and squeeze ahead but lack the response to clear the car being passed. This negates the slingshot maneuver. As a result, passing is often the result of cooperation between two or more drivers or is achieved by sucking air off the side of the car being passed, a technique called side-drafting.^[3]

Bump drafting is a tactic used at Talladega and Daytona. The technique was initially popularized by the Archer Brothers in the SCCA Sportruck series during the late 1980s.^[4] It begins as normal drafting, but the following car pulls up behind the lead car and bumps into the rear of it, pushing the lead car ahead, to maintain momentum.

If done roughly or in the wrong position (e.g. close to the entry of the turn), this tactic can destabilize the handling of the lead car sometimes causing a crash. Use of the tactic in this manner is known as slam drafting. Due to the danger, NASCAR has attempted to limit the bracing on bumpers on cars, disallowed bump drafting in turns, introduced "no bump zones" on certain portions of speedways where this practice is prevalent, and penalized drivers who are too rough in bump drafting. The 2010 NASCAR season allows drivers more freedom. Bump drafting is allowed anywhere, including turns.

Kyle Busch is largely responsible for the current popularity of bump drafting, which is now referred to as 'two-car drafting'^{[citation needed]}. At a 2007 test session in Talladega, he asked Ryan Newman to push him from behind, and was stunned to realize he was two seconds faster with Newman's help. At the newly paved Daytona International Speedway in 2011, Busch was the first to realize that the corners were smooth enough to allow a two-car draft for the complete length of the track. During test sessions on the track, when Busch was pushed by his brother's teammate Brad Keselowski, they were running 15 mph faster than single cars. Other drivers quickly picked up on Busch's strategy, and the two-car draft dominated the 2011 Daytona 500 and Budweiser Shootout.^[5]

Drafting in nature

Canada geese flying in V formation

Cooperative fluid dynamics techniques like drafting are found in nature as well. Flocks of geese and some other birds fly in a V formation because the wingtip vortices generated by the front bird will create up-wash circulations. The birds flying behind will receive lift force from these up-wash vortices. Thus other birds in the flock do not need to work as hard to achieve lift. Studies show that birds in a V formation place themselves roughly at the optimum distance predicted by simple aerodynamic theory.^[6] The theory behind this is the same as the one for the wings of an airplane in general.^{[citation needed]} However it does not work quite the same way as drafting in racing. Birds fly in a flock for better lifting force whereas the race cars stay close for less total aerodynamic drag force.

Other animals have been observed to use true drafting behavior reminiscent of auto racing or cycling. Caribbean spiny lobsters for example are known to migrate in close single-file formation "lobster trains".^[7][8]

Tailgating and hypermiling

Some drivers have been known to draft behind other vehicles, particularly tailgating larger vehicles, to save fuel. For example, hypermilers using this technique can achieve 75 mpg or more.^[9] Some sources say that the most common tailgating does not save gasoline even at freeway speeds because one is likely to accelerate and brake so frequently that any aerodynamic savings are lost through the brakes.^[10] On the show Mythbusters, drafting behind an 18-wheeler truck was tested and results showed that traveling 100 feet (30 m) behind the truck increased overall mpg efficiency by 11%. Traveling 10 feet (3.0 m) behind the truck produced a 39% gain in efficiency. Of course, they warn that this type of driving "is insane" because the truck's blindspot is in that area, and if the truck stops quickly there is much less time to react.^[11] Truckers are not fond of the extra stress this puts on them, worrying about cars on their tail. Additionally, it can be very dangerous for the following car if one of the truck's tires (or their recaps) delaminate, as the chunks of ejected rubber can be large enough to cause serious harm, even death, to a driver following too closely.

Computer simulation of drafting

Computer simulation (computational fluid dynamics or CFD) is increasingly being used to analyse drafting. It is important to understand the aerodynamic behaviour of a motor vehicle when drafting, for example if the rear car is too close to the front car, the air supply to its radiator will be reduced and there is a possibility of the engine overheating. Most motor sport aerodynamic analysis is performed using wind tunnel testing. This becomes difficult for drafting cases, if only because a very large wind tunnel is needed. CFD, a kind of virtual wind tunnel, is used by race teams to understand the car's performance while drafting.

References

1. ^ Andrew J. Spencea, Andrew Thurmanb, Michael Maherc, and Alan M. Wilson (2009). "Speed, strategy, drag and drafting in thoroughbred horse racing". Comparative Biochemistry and Physiology - Part A: Molecular & Integrative Physiology. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VNH-4WMFDH2-9C&_user=1090614&_coverDate=06%2F30%2F2009&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1239350059&_rerunOrigin=google&_acct=C000051509&_version=1&_urlVersion=0&_userid=1090614&md5=4b15de5c1c60da629281365a60e4418c. Retrieved 2010-03-08. 
2. ^ ^{a b c d} Lemasters Jr., Ron, "One Wild Wind", Stock Car Racing (ISSN 0734-7340), Volume 36, Number 1, January 2001.
3. ^ Jonathan Ingram (March 6, 2010). "CUP: Atlanta Is Side Drafting Heaven". Speed Channel, Inc.. http://nascar.speedtv.com/article/cup-atlanta-is-side-drafting-heaven. Retrieved 2010-05-27. 
4. ^ Glick, Shav, "Motor Racing", Los Angeles Times, p. 10, Dec 3, 1987.
5. ^ Anderson, Lars. "The Kid Wins a Wild One", Sports Illustrated, February 28, 2011.
6. ^ Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range; Defense Technical Information Center; April 2002; Retrieved February 27, 2008
7. ^ "Finding the Way". The Trials of Life. BBC. 1 November 1990.
8. ^ Kanciruk, Paul; Herrnkind, William (October 1978). "Mass Migration of Spiny Lobster, Panulirus Argus (Crustacea: Palinuridae): Behavior and Environmental Correlates". Bulletin of Marine Science 28 (4): 601–623. http://www.ingentaconnect.com/content/umrsmas/bullmar/1978/00000028/00000004/art00001. 
9. ^ ‘Hypermilers’ wring out every last bit of mpg, msnbc.com May 29, 2007
10. ^ Tailgate for Mother Earth! ecogeek.org, July 16, 2007
11. ^ Mythbusters study drafting on the road

External links

• Mark Martin's site with an extensive explanation of drafting in NASCAR