# Slipstreaming - is there a penalty?

When a following vehicle slipstreams close to a vehicle in front, by placing itself in a region of air moving at a similar velocity to the front vehicle, it is afforded a benefit, e.g. improved fuel efficiency.

Do the effects from this action introduce any type of inefficiency/penalty to the front vehicle?

• This is a complex question but my experience from biking in a pelaton (which is perhaps a different situation) is that being in the front is indistinguishable from biking solo. Dec 13, 2021 at 14:50
• The penalty is what the police issues for following too closely ;) Dec 13, 2021 at 21:48
• @JimmyJames in a (small) peloton you might still save 10% effort - see the answer below, and the paper it links to. But this is within the margin most people vary ion performance from day to day, so it feels quite normal Dec 14, 2021 at 11:13
• A good example of close following with no penalty might be lines of box-cars in a train. In theory they should be close as possible, with enough space to follow a corner but no more. I routinely see empty flatcars in the middle of a train. Dec 14, 2021 at 22:18
• assuming the object leave a low pressure area, 2nd object reside in that area, very simply it has to move less mass around itself to continue it's displacement. It get a benefit and that's it. An additional assumption can sometimes be made. The low pressure zone requires air to fill it in. That requires mass to accelerate and basically work or energy. The 2nd object reduces that volume of low pressure and therefore reduces energy used by the 1st objects motion in that sense. Dec 15, 2021 at 5:24

The opposite seems to be true.

There was, or maybe still is, a company that was looking into designing a system that allowed two semi trucks to "link" to each other (basically the truck in back had its braking and acceleration controlled by the truck in the lead -- this allows for a much closer follow distance and for the trucker in the back to have autonomous driving). In testing both trucks reportedly saw a benefit - the lead truck got about a 5% boost to fuel efficiency and the slipstreaming truck got about a 10-15% boost.

Edit:

I would assume the physics for the lead truck are similar to adding the rear flaps that are becoming more and more common:

https://slate.com/culture/2013/04/truck-panels-what-do-they-do-explained-photos.html

• Why do I have the feeling that a single truck of double the size would be the most efficient (and cheapest)? Dec 14, 2021 at 8:12
• This also seems to be true for flocks of geese. The leading bird also gains an advantage. Though I guess it’s still “altruistic” behavior which only arose because geese fly in closely related family groups (from the point of view of your genes it can make sense to sacrifice for your children or siblings). Dec 14, 2021 at 9:49
• @Vilx- it would, apart from the difficulty of manoeuvring around existing infrastructure and the hazard to other road users. That's how you get to road trains, which have to form up at an appropriate point to join a suitable road Dec 14, 2021 at 11:09
• "we have a convoy" Dec 14, 2021 at 14:27
• Reminds me of the anecdote of the German Kaiser who had tasked a committee of mathematicians with finding the optimal height/diameter ratio of a food cans, in order to minimize the tin needed to can military provisions. After the scholars presented their finding the Kaiser laughed and said "Now every child can see that this is not the optimum -- see here!", took two cans, stacked them and pointed out that his configuration needed even less tin than the committee's because you could omit the two facing sides. Dec 14, 2021 at 14:54

For cycling, there doesn't seem to be a penalty, but even a small advantage to the rider in front of a group. Computational fluid dynamics simulations and wind tunnel testing have shown air resistance values of riders in a group compared to an isolated rider. This image shows interesting results:

I don't understand the details well enough to explain these results properly, but the paper is pretty good in my opinion.

All from the paper "Aerodynamic drag in cycling pelotons: New insights by CFD simulation and wind tunnel testing" from Blocken et al. (Open access article)

• The percentages on the image tell you how much drag that cyclist experiences relative to a solo rider at the same speed. So the lead rider gets a whopping 14% reduction by having the peloton behind! Riders near the back are practically coasting, with only 5-7% of the wind drag of a solo rider. Dec 13, 2021 at 21:18
• having done bike racing myself, personal perception is absolutely in agreement with this. In the front you don't feel much difference - but it has to be there. The reduction in the second row feels a little optimistic, but yeah in row 10 or 15 you can basically follow with very little effort. That said - this depends a lot on the speed and the inclination of the road. In an uphill, gains these reduce a lot. On a shallow decent, where speeds are around 50-60 km/h, these gains are probably most pronounced. Dec 14, 2021 at 8:17
• @CarlBerger The same author just published a paper on uphill aerodynamics: link.springer.com/article/10.1007/s12283-021-00345-2 Those conclusions come with a lot of caveats, but there is still a significant advantage, even while climbing gradients over 7%. Dec 14, 2021 at 8:52
• @CarlBerger The "penalty" is then crashing your teeth on the bike in front of you if anything goes wrong, or getting ran over by your fellow riders if you're in the front. Id be terrified to ride at that speed so close to others. Dec 14, 2021 at 9:43
• @EricDuminil in big groups it gets really scary, but 2 or 3 riders, close but not pro-peloton close, can be really fun. 70+km/h down hill on a bike is always exciting and can be done without much effort on the right hill but 50+km/h on the flat, taking turns on the front, feels like flying Dec 14, 2021 at 11:05

To a certain extent, this depends on the aerodynamic design of the vehicles. But for a simple "boxy" shape, both the lead and trail vehicles could gain an advantage.

The rearmost vehicle would create significant turbulence behind. This will form a low-pressure region that contributes to drag. If another vehicle occupied that space, the turbulence and the low-pressure region are reduced.

• I'm struggling to follow the logic here. First you claim that the lead vehicle gains an advantage but offer no explanation. I'm also pretty sure that there can't be a vehicle behind the rearmost vehicle, by definition. Dec 13, 2021 at 14:46
• My translation: if vehicle A has vehicle B behind it, turbulence behind vehicle A is reduced and A gets a benefit. If vehicle B has vehicle A in front of it, drag from air hitting vehicle B is reduced and B gets a benefit. Therefore both benefit from being in a convoy. Dec 13, 2021 at 15:01
• Don't both vehicles basically become a single vehicle with the low-pressure region moving back, behind the rearmost vehicle? The drag would be the same, just acting on the rearmost vehicle instead of the foremost one. Dec 13, 2021 at 20:45
• @Ruslan If they were far apart you have drag from air hitting the front of A, drag from the turbulence behind A, drag from air hitting the front of B, and drag from turbulence behind B. Put them in convoy and the two middle sources of drag (turbulence behind A and air hitting the front of B) are reduced, so each is better off. The convoy has at least as much drag as a single vehicle but also more mass, so the same absolute magnitude of drag slows it down less.
– Ben
Dec 13, 2021 at 22:42

The answers by Jeroen and eps show evidence that there is no penalty, but instead a benefit to the lead vehicle. Let me explain intuitively what is happening.

The drag on a moving vehicle has two components - a pushing force from high-pressure air in front of the vehicle, and a pulling force from the low-pressure wake (slipstream) behind it. What happens during a drafting maneuver is that the trailing car enters the wake in front of it, reducing the pressure on its bumper. This uses the low pressure slipstream area behind the leading car, but does not create it - the wake dragging on the leading vehicle (and possibly pulling the trailing one) exists regardless.
On the contrary, the trailing car also pushes air in front of it, to slightly increase the pressure in between the vehicles and benefit the leading car. The high-pressure area in front of a vehicle does not extend nearly as far as its low-pressure slipstream, so this "pushing" effect is much smaller. Effectively, the aerodynamic losses due to turbulence between the two cars are greatly reduced when they are close together, and the conserved energy is distributed between them.

Some additional information on the maneuver: Drafting (aerodynamics)

Interesting side-note: The wake of the leading car is not always beneficial, if it contains too much turbulence and the trailing car needs laminar flow for downforce, see this Quora answer