I know that when riding close to another cyclist, I expend less energy to go the same speed. But does the Conservation of Energy "law" mean that the lead cyclist is going to expend more energy to allow me to expend less?
(i.e. Is there a physical reason to get angry at someone drafting behind you? Does riding in their slipstream cause drag for them?)
No, on the contrary the lead rider also gets a boost.
The reason to be unhappy about someone drafting you is that they're too close to be able to react if there's a problem - if you go down they will run over you.
The way I understand the boost is that a solo rider is effectively dragging around a volume of low pressure air - you push the air out of the way creating a high pressure volume in front of you, but that displaced air doesn't magically close in behind you, it flows out then flows back, creating a "missing air" volume behind you. If something fills in that space you don't get sucked back into it as much. There's a mention on wikipedia chain gang (riding) which links to the exploratorium article but not to a primary source that I can see. That explanation is common and makes sense, so I've never chased it further.
Here is a link to a wind tunnel video which shows that the lead rider receives a small benefit from having a drafter.
I recall this thread, and thought I'd add a link to a post describing an impromptu experiment I conducted this week, which tested the impact on the power demand of a test rider (172cm 60kg female on a track pursuit bike riding at a quasi-steady state velocity on an indoor wooden velodrome) of another rider (185cm 80kg male on mass start track bike riding in close proximity), and to compare this with the test rider's solo ride power demand.
The tests examined the following locations of the other rider relative to the test rider:
immediately in front of the test rider
riding next to the test rider (on their outside)
immediately behind the test rider
completely away from the test rider and not riding on the track (to provide data on the solo power demand for the test rider).
I use sophisticated technology to assess rider aerodynamics in real time and had the chance to perform this experiment at an indoor velodrome (Dunc Gray Velodrome, Sydney), so that we could at least conduct such an experiment in well controlled, no wind, low yaw angle conditions.
Test runs were repeated for validation and confirmation of results. The testing protocols and analysis of data provide values for the apparent CdA (coefficient of drag x frontal area, units: m^2) value for each of the test conditions. I then use the apparent CdA data to show the power demand for the test rider to maintain a 40km/h average speed.
This is the link to my write up, which includes links to other experiments and published science on the topic.
Here are the summary of the data in chart and table form, which show the power required for the test rider to maintain 40km/h while riding solo, and with the other rider in various relative positions:
In summary, compared with the power required (195W) for her to maintain 40km/h (lap average speed) on the velodrome:
Drafting immediately behind the other rider gives massive benefit (-76W, -39%). No surprises there.
Having a rider immediately behind (~1/2 wheel gap) provided ~ -7W (-3%) benefit for the lead rider.
Having a rider ride right next to her (~0.8m - 1.0m lateral gap between wheels) created an additional power demand of ~ +10W (+5%).
The result of a 7W (3%) benefit to the lead rider of having a rider immediately behind is in line with previous experimental results and published studies. So while the effect is small, and would be difficult to feel while riding, it is a real effect, at least in low wind conditions.
The side by side ride result showing an additional power demand of 10W (5%) in low yaw conditions is more novel, and has interesting implications for team formation events (e.g. team pursuit and team time trial) and rider changeovers.
Of course different rider morphologies, individual aerodynamic properties, riding alignment configurations and wind conditions will yield different results to this impromptu experiment, but I thought it interesting none the less.
The answer is ... it depends.
Normally, by reducing/filling the vacuum that exists behind the lead rider, the drafter would be expected to give the leader a slight boost (though nowhere near the boost the drafter gets). But fluid dynamics is a tricky thing, and there are probably configurations (based on a few millimeters movement one way or the other) where the leader can be slowed. I wouldn't expect the negative effect to occur very often, though.
The bigger effect on the leader is the demands placed on him to maintain a steady pace and better signal his intentions. Many are apt to find this responsibility stressful.
No.
The only reason to get angry at a drafter is if they are not safe or if they don't take their turn.
Conservation Law: oversimplification, but in this case if there are no drafters, the extra energy to separate the airflow is just wasted as the turbulence collapses.
If you are close enough to get into their Slipstream, you can essentially kill their drag. This might feel like a boost because the wind that was previously pulling them back is now transferred to YOUR rear and not sucking on them anymore.
Here is a photo of a bullet's slipstream (wake), where you can see the air sucking the bullet backwards.
There is no "physical" reason for someone to be mad that you are following so close behind them, in fact, the physics prove the reverse, unless they want that drag to help them train like how runners will use a parachute. Other than that, you might consider the stopping distance in an emergency, "personal space", and swapping the lead as courtesy reasons for following or not.
If there is an aerodynamic difference, it is so small it is utterly unnoticeable in practice.
In a paceline, the resistance experienced by the front rider is overwhelmingly dominated by slicing through the air in front of him.
Perhaps you're thinking of a velodrome race? What often happens is that the second rider will pass around the outside by quickly blowing-up all the energy he's saved by being behind the first guy. In the last lap of the race, the only way the front guy can respond is by speeding up and keeping the passer from getting in front before the last turn. This way the front guy can hang the passer "out to dry" on the turn forcing him to not only to keep up the acceleration but to travel a longer distance around the outside of turn.
That kind of race situation might lead someone to believe that the front rider gains some kind of an advantage when the second guy jumps. It is an illusion. The front guy has to "work for it" all the way.
