Question: We all know that drafting reduces the effort for the person in back. Given the laws of energy conservation, is there an opposite effect for the person in front? Does having someone draft off you make you work harder? Or does it actually give you push? I found this article about auto racing that discusses drafting and makes the point: In car drafting, the lead car is also getting a benefit. Trailing cars fill in the lead car’s low-pressure wake, thereby cutting down pressure drag. — Bill Rosenfeld
Bob Howland Replies: Briefly, at the back of a race car there is turbulent air, in a low-pressure system created from the Bernoulli Effect as air sweeps up over the top of the car and down its rear area — think, aircraft wing. But another air mass flows under the car, passing between chassis and road surface. The different air masses rejoin at the back of the car and swirl, which essentially creates trailing drag.
If a second car pushes its pressure cone into this swirling air, it smoothes out the swirl, thus reducing trailing drag. So the “push” of the second car is a result of the second car forcing that swirling air to continue on under the second car, and over the second car. Two cars working in conjunction act as a single, longer entity. There is only swirling air, or trailing drag, behind the second car. They both supposedly can go faster than a single car. But the second car has it easier, drafting in the low pressure of the first car.
So, what about bicycling?
Well, I have watched many lead-out trains with 5-6 riders. There is NO question to me that an organized lead-out train can go much faster than a single rider trying to get by them. One lead-out train really is never challenged — unless by another lead-out train acting as a single entity. So, at the approximately 35-40 mph elite racers can do, I am led to believe the drafting-pull effect is there.
Yes, they are going slower than a 200-mph race car, but the cyclists have a much smaller frontal surface area, so my guess would be the effect is there, just proportionally scaled down.
We all know you can feel the draft when you’re behind another rider. But, realistically, is there a “push” that can be felt by the lead car or the lead cyclist? For cars, maybe. I am speculating. For cyclists, I’d say the effect is there, but small enough not to be felt — unless maybe by those elite racers riding at 35 mph in team time trials or lead-out trains. (If the lead rider is getting a push, it means that the drag coefficient of two riders combined is smaller than that of the lead rider alone.)
Five old guys cooking at 25 mph is fun, and I bet the effect is there, but so negligible as not to be felt.
I’d have to say, I know it when someone comes up silently behind me if I am cruising at around 20 mph because somehow the airflow feels different than riding alone. But I would NOT describe it as a push. I would also say, when you ride deep within a peloton, the air flow feels different than when you are at the very back of the group.
RBR Premium Member Bob Howland is a physics teacher in Florida. He’s also written on the physics of riding in the wind and the physics of aero wheels for RBR.
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