Car Slipstream Explained
A slipstream is a region behind a moving object in which a wake of fluid (typically air or water) is moving at velocities comparable to the moving object (in comparison to the ambient fluid through which the object is moving). The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "drafting" works because of the relative motion of the fluid in the slipstream.
A slipstream created by turbulent flow has a slightly lower pressure than the ambient fluid around the object. When the flow is laminar, the pressure behind the object is higher than the surrounding fluid.
The shape of an object determines how strong the effect is. In general, the more aerodynamic an object is, the smaller and weaker its slipstream will be. For example, a box-like front (relative to the object's motion) will collide with the medium's particles at a high rate, transferring more momentum from the object to the fluid than a more aerodynamic object. A bullet-like profile will cause less turbulence and create a more laminar flow.
A tapered rear will permit the particles of the medium to rejoin more easily and quickly than a truncated rear. This reduces lower-pressure effect in the slipstream, but also increases skin friction (in engineering designs, these effects must be balanced).
The term "slipstreaming" describes an object traveling inside the slipstream of another object (most often objects moving through the air though not necessarily flying). If an object is inside the slipstream behind another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently. In addition, the leading object will be able to move faster than it could independently, because the rear object reduces the effect of the low-pressure region on the leading object.
Slipstreaming, also known as drafting is important in a number of contexts, including:
- In fast bicycle races, competitors attempt to use one another's slipstream, or "draft", breaking out to overtake the leader only at the last possible moment. In recreational cycling, on the other hand, members of a group can take turns at the leading position, enabling one another to rest a little. In a group of cooperative cyclists with sufficient group-riding skill, stronger members can spend more time leading to give weaker riders more rest, enabling riders of different strengths to ride together, at least on relatively flat routes. On inclines, however, the benefit of drafting is relatively less, as speeds are slower and the cyclist's primary effort is working against gravity. The flat or hilly nature of a route has consequences for both racing and recreational cycling, with different types of routes favouring different types of cyclists.
- Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved fuel efficiency due to reduced atmospheric drag. Truck convoys are a common example, travelling highways in a single-file queue several vehicles long. Auto racing drivers also draft in order to conserve fuel, the better to gain competitive advantage by reducing the frequency of fuel stops or, more often, to reach a higher speed before pulling out to attempt to overtake another driver.
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
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.
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.
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.
Drafting was discovered by stock car racers in the 1960 Daytona 500, when Junior Johnson found that he could use drafting as a strategy that helped him overcome the fact that his Chevrolet could not keep up with other cars, allowing him to win the race. Drivers, like Johnson, 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 and/or use less gas (resulting in fewer pit-stops) than a single car running by itself.
Drafting is most important at NASCAR's restrictor plate tracks, Talladega Superspeedway and Daytona International Speedway, where the plates mean that much less power is 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.
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. 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'. 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 Penske teammate Brad Keselowski, they ran 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.This strategy had also been very prominent at Talladega. In 2011, two-car tandem drafting was used for the extent of the Aaron's 499, with many drivers drafting their own teammates (e.g., Jimmie Johnson and Dale Earnhardt, Jr. drafted together, as did Jeff Gordon and Mark Martin).
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 (a 10% increase in efficiency of certain hybrid vehicles). 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. 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 blind spot is in that area, and if the truck stops quickly there is much less time to react. Truckers are not fond of the extra stress this puts on them, worrying about cars on their tail. Additionally, on the same episode, Mythbusters demonstrated that 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