Drafting

Why would the lead rider's drag increase as a result of having another rider behind him? Having another body following closely behind would theoretically decrease the net drag on the leader, by causing the airflow to move away from his trailing edges.

At biking speeds, 'blocking the wind' is a pretty decent description of what's going on. Changes in drag and suction forces pulling the trailing rider are so miniscule they're not even worth mentioning.

frenchyge is right. Thoeretically a single rider has wind resistance in front slowing him and a vacuum behind also causing drag. A rider behind him fills the vacuum drag and that drag is affecting him. The longer the "train" of riders the faster they can go. This is what works at the large speedways in NASCAR racing. But as frenchyge said, in a bicycle draft, the drag felt by the trailing riders is very small. They just get a lot of benefit by the leader taking on the headwind. That's why running second is usually better in a race... it lets you conserve power for the end....And the sprint to the finish.

OscarC said:

I know the benefits the rider behind has in a draft, but does the lead rider's drag increase thereby actually pulling the rider behind?

Very good question, actually. I don't know if anybody has investigated this for road cyclists, but for the generic configuration of two cyclinders, one behind the other, with axes normal to the flow, the answer is: It depends --- on the distance between the cylinders.

There is a distance for which indeed the drag of the front cylinder increases a little above the single-cylinder value, so this could intuitively be described as the first cylinder "pulling" the second one (although this is not an accurate way to look at things; the drag of the second cylinder decreases by more than the amount that the drag of the first increases). I don't have the corresponding paper handy right now, but as I recall the distance for which this situation occurs is quite small, and may be such that the analogous effect can never occur for cyclists (at least not with non-overlapping wheels, which is a good idea from a practical point of view...).

It is also interesting to note that, given the right distance, both cylinders will see a decrease in drag, even though the effect will be larger for the cylinder in the back, corresponding to intuition.

As far as I can tell as a cyclist, however, any effects on the front rider seem to be very small, and are essentially negligible. So, in practice, I don't think the front rider will be affected significantly by the person drafting behind him.

I'm still trying to find a definitive explantion on this in a physics site but haven't found one yet, so I'll give this a shot and try to explain my thoughts.

If rider "A" in front has a drag factor of 2, he creates a slip stream along his edges. Then rider "B" slips into rider "A's" draft his drag factor 2 has now been added to rider "A's" drag to some extent. Rider "B"'s drag just doesn't dissappear. Because rider "A" is pusing the wind out of the way, that creates an almost "sucking" motion in the air behind rider "A". One can argue that rider "B" can pass easily because he is rested from riding in the draft of rider "A", right? Not because he is being pulled along. To use the NASCAR example, a car when drafting behind another can sling shot his way past the lead car as was so beautifully done by the late Dale Sr. at Daytona or Talledega. One cannot say that the car behind passed easily becuase the car behind was tired. Like the NASCAR example, so too can a rider sling shot himself past the lead rider. I think the speed is gained from the sucking air motion behind rider "A" plus the energy generated on his own. Is this wrong??? In short I still argue that rider "A" is to some extend, pulling rider "B".

Dietmar said:

There is a distance for which indeed the drag of the front cylinder increases a little above the single-cylinder value, so this could intuitively be described as the first cylinder "pulling" the second one (although this is not an accurate way to look at things; the drag of the second cylinder decreases by more than the amount that the drag of the first increases).

OscarC said:

Is this wrong??? In short I still argue that rider "A" is to some extend, pulling rider "B".

That depends on whether we're talking about Nascar, or cycling. At Nascar speed there's a huge turbulent area behind the car which creates a negative pressure area. At cycling speed the air is able to remain fairly laminar, with the exception of some small eddys off the rough trailing edges, which means that there is not a large negative pressure area behind the rider. The speed at which this transition occurs depends on the shape of the bodies and the viscosity of the fluid moving over them.

As far as being able to 'slingshot' past another car/rider, that effect is caused by being able to accelerate in the wake of the leader, and then pull out into the wind with more speed than the leader has, and carry that additional momentum for a brief period past the leader. The passing car/rider will quickly slow back down to the original speed, but by then they've already made the pass or won the race.

While staying in the wake of another car, less horsepower is needed to overcome the wind resistance and the driver can let up a little on the gas. There's still enough drag and friction that the driver has to use *some* gas to hold his speed -- if he let off completely the car would not continue to be sucked along. While passing, the following car has much more horsepower available than what is needed, and is able to accelerate. Whether he's getting sucked forward, or is moving forward because he pushed the pedal the rest of the way down, is splitting hairs.

Edit: in any case, a bike rider is certainly not being pulled forward towards the rider in front.

I was not successful in finding anything to support my beliefs, so I guess I don't have to worry about someone slowing me down when they draft me.

OscarC said:

I was not successful in finding anything to support my beliefs, so I guess I don't have to worry about someone slowing me down when they draft me.

On the contrary, I'd say having a rider close on your wheel more likely speeds you up, since he is helping to fill in the low-pressure turbulent zone that occurs when the airflow closes around you. After all, a good portion of the drag that you normally experience is due to the low-pressure zone at your back; that's why TT helmets and HPV streamliners have a long, carefully tapered rear fairings. Believe it has something to do with keeping the laminar flow attached. Maybe someone who understands Reynold's number can chime in here....I never did.

No saying this is a big effect at all; certainly nothing like the 25% benefit the guy sucking your wheel is getting...you'll still want him to trade pulls 🙂

OscarC said:

I was not successful in finding anything to support my beliefs, so I guess I don't have to worry about someone slowing me down when they draft me.

Unless they're grabbing your seatpost...😉