Technical FAQ: Physics and the bike throw

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Dear Lennard, Per classic Newtonian physics the centre of mass of the bike-rider system will continue moving at the same velocity unless an external force is applied. You’re entirely correct that throwing the rider’s weight rearwards relative to the bike does not apply an external force — only pedaling can do that — but it’s not correct to think that this makes no difference in a sprint finish: Moving a 72kg rider backwards X distance must result in moving an 8kg bike forwards 9x distance; so the centre of the rider will cross the line later but the front of the bike will cross the line earlier.

If we say that Sagan is maybe 2cm behind where he would normally be on the bike he’s gained 18cm on the front wheel. Kristoff looks to me to be 10cm forward to a seated position — if he’d just sat back down he’d have gained 9cm and won the stage; a Sagan-distance throw from where he is would gain him 27cm and make the photo redundant.

The heavier you are compared to your bike the more you can gain with a throw.— Phil

Dear Phil, You were one who caught my post early last Tuesday. Thanks for your analysis, which, unlike what I had posted briefly on the subject, is spot on.

Last week, after staring cross-eyed at photo-finish images of riders throwing their bikes at the line into the wee hours of the morning, I started thinking about how the bike throw works. I hadn’t begun digging through my veloqna@comcast.net email box for questions to answer for my column until late at night on Monday, and my column goes up on Tuesday morning. I was happy to find the wonderful question about why spokes are curved on photos from photo-finish cameras, and I set about answering it.

In a sleep-deprived state after answering that question, I thought I had come up with a revelation that the bike throw is a myth. I then went on to write out a wild theory regarding last Monday’s defeat by Peter Sagan of Alexander Kristoff by a hair in a photo finish that was attributed to the Slovak throwing his bike forward better than the Norwegian. Ignoring a lesson that I had learned many times before, namely not sending in an article that I work on late at night until I’ve slept on it, I sent it in right then. I awoke late the following morning with a jolt, wondering, “What was I thinking?”

I hoped that when I looked at VeloNews.com that it would not have yet been posted, but our web team is very efficient, and it was already up. You were among the many dedicated readers (thank you for your dedication!) who saw it before we chopped the bike-throw part off of the curved-spokes-in-photo-finish explanation. Anyway, the theory I put forward there was totally wrong, but there is still a lot going on with the bike throw that I think is fun to think about.

The reason for doing a “bike throw” at the line is obvious; the rider is trying to push the bicycle out in front of him in order to have the leading edge of the front tire cross the finish line earlier than it otherwise would have, had he not “thrown” it. It is the cycling equivalent of the runner who learns forward in order to break the tape with his or her chest a bit earlier.

Unlike the runner, a bicycle is rolling, and here is where my sleep-deprived mind started veering off into the weeds. I was thinking that as long as its wheels roll on the ground without slipping, the only thing that can get it across the line sooner is by pushing harder on the pedals; the rider cannot generate any other force to cause it to move forward faster. I got excited and wrote on and on in this vein and submitted it for posting while bathed in the soft light of a beautiful full moon. I went to bed with a smile on my face.

My analysis was, of …

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