Should You Throw Your Water Bottle When Racing a Road Bike?

The other day, with 1.5 laps left in a cat 3 crit, the guy in front of me reached down into his bottle cage, grabbed his bottle, and threw it wildly onto the sidewalk, almost hitting a spectator.

This process caused him to open up a two or three bike length gap in front of him which he then had to pedal hard to close... almost certainly negating any gains he made by losing the weight of the bottle for the final sprint.

So I'm pretty confident that this particular bottle-throwing instance was a poor decision, but it motivated me to look into the
actual savings of the bottle-throw, which is really just an excuse to blog about PHYSICS!!  And who doesn't love MATH?!

IMPORTANT NOTE:  Throwing your bottle creates trash.  Trash creates unhappy residents on race courses.  Unhappy residents create problems for race promoters.  If you're throwing your bottle onto someone's lawn -- you are the kind of racer that all promoters hate.  Don't do it.

Let's set some ground rules:

1) A full bottle weighs one pound (0.45kg)
2) The aerodynamics of your bike are basically the same with or without the bottle.
3) The placebo effect of throwing the bottle doesn't make you go faster.

We're going to use energy equations to solve this.  The equation for kinetic energy is 0.5*mass*velocity squared.

The equation for potential energy is mass*height*gravity (9.8m/s/s).

At any given moment, the energy of a water bottle is based on how fast it's moving (kinetic energy) and how high above the lowest point of reference it is (potential energy).  If the energy of the bottle changes, that energy came from YOUR LEGS.

If the sprint is uphill, what benefit does throwing the bottle give me?

The GMSR crit (scene of the original bottle throw) has one of the more uphill sprint finishes out there.  On the last lap, I came through the final turn at 38mph (17m/s).  I crossed the line 12 seconds later at 32 mph (14.3 m/s) and about 20 feet (7m) higher than the final turn.  There was a (full) water bottle on my bike -- let's look at the energy involved.

At the turn, the bottle's kinetic energy was 65.025 joules  (0.5*0.45*17*17).  It's potential energy was zero, since it's at the lowest point in its path.

My body and my bike also have significant kinetic energy here, but they have the same energy whether there's a bottle attached or not, so it doesn't matter.

At the finish, the bottle's kinetic energy was down to 46.025 joules  (0.5*0.45*14.3*14.3) because I can't sprint at 38 miles per hour up hill.  But, the bottle's potential energy was now 30.87 joules, because it had moved 7 meters higher up the hill.  Adding these together, the energy the bottle had at the end of the sprint was 77 joules, a gain of 12 joules!  

These joules had to come from somewhere (my legs), so dragging that stupid bottle all the way through the 12-second sprint cost me 1 watt on average!  

(A watt is a joule per second, so 12 seconds at 1 watt = 12 joules)

(Probably don't need to throw your bottle in this sprint since it saves you literally one watt of sprinting power)

Even if I was making so much power that I didn't slow down at all in the uphill sprint (costing me kinetic energy), the savings would have only been 2.6w. 

If the sprint is flat, what benefit does throwing the bottle give me?

Most sprints aren't uphill.  What if you're sprinting in a normal, flat crit?  You're gonna accelerate during the sprint and if you didn't have to accelerate that big fat bottle you'd, like, totally win.

Let's assume you go through the last corner at 35mph (let's face it, you're a cat 3, if you get a 35 mph leadout it's a miracle) and hit the line at 40mph (because you're awesome).  The course is flat, so the potential energy of the bottle on your bike is unchanged.  The only question is how much kinetic energy has to be added to the system to accelerate a bottle from 35mph to 40mph?

35mph energy:  55.08 J

40 mph energy: 71.2 J

Difference:  16.2 J, or 1.3 watts over 12 seconds.

(Probably don't need to throw your bottle here, either)

(Oh and there's probably no way you can accelerate from 35mph to 40 in 12 seconds, because you're a scrub, I was just using some really optimistic numbers to illustrate that throwing you bottle is basically meaningless here, too)

What if I'm at the bottom the steep part of Middlebury Gap with a full bottle on my back in the Cat 3 race?

OH LOOK ANOTHER REAL LIFE EXAMPLE!

I started GMSR Stage 3 with an extra bottle because I was worried about survival.  This bottle ended up going over Middlebury Gap.  I got dropped on Middlebury Gap.  Was it the bottle's fault?

The steep part of Middlebury Gap climbs 263 meters.  It took me 12:15 (shut up).    The potential energy of a water bottle at the top of Middlebury Gap is 1160 joules (relative to the base of the climb), so dragging this stupid bottle up the mountain cost me 1.625w for twelve minutes.

While this number sounds small (just like the sprint numbers), it's worth five seconds on the climb.  Was five seconds the difference between me maintaining contact and not getting dropped?  No.  But it's a non-trivial portion of the time savings I needed to stay in touch.  I could have taken the GoPro off my bike, and not ridden with a tube and pump, and used my lighter wheels, and taken about 3 pounds off my race setup.  Now we're up to 15 seconds faster...

If I could lose five pounds of old-man-metabolism fat between now and GMSR 2017... now I'm down eight pounds... at the same power I'd go over Middlebury gap 33 seconds faster... oh god this is how cyclists get eating disorders, isn't it?

...

So, uh, don't throw your bottle, ever.  But if you're gonna throw your bottle, throw it along with your spare tube and CO2 and GoPro and 5 pounds of fat, so you don't get shelled by the freaking Cat 3 field on Middlebury Gap.




Comments

Hill Junkie said…
Haha, great post. You wouldn't believe how many times I've run through numbers like these for the Mt Washington hillclimb race. I knew every component on my bike not in terms of grams, but in terms of seconds it costs me in a pure uphill event.
Kevin said…
Wait. What about the effect of momentum. If you were able to find a direction to throw opposite the direction of travel--I am sure the rest of the bike racing field will understand--AND throw the with sufficient velocity it might be impactful... #armwatts
Colin R said…
If you can throw your water bottle backwards at a few hundred miles per hour during the sprint, you absolutely should do it. Those gains would be LEGIT.
Dieter said…
Great analysis. The cycling community is ripe with false assumptions, misdirection, and downright myths when it comes to weight savings, like buying $10k worth of weight savings when you could simply stop drinking beer after your ride if it's that important to you...
matt said…
Has the original water bottle thrower been ID'd?
Colin R said…
Of course, but I only support witch hunts for guys who cut the course in 24 hour races.
Aki Sato said…
Heh 24 hour cut guys. When I saw you at the race I thought of that and wondered if that guy is still racing.

I love this. I usually toss a bottle if it's full, and in one race where they displayed 2 to go way too early, I tossed a full tall one, a full short one. In my defense I do toss them to save them since the nice ones are Podium Ice bottles, unavailable for a few years. The not-as-nice ones are Podium Chills which are sort of useless for cold water in an hour crit so I carry them to dump water on me and then I toss the bottle to the Missus.

I didn't toss a bottle this year as I only did Tues night races but I stopped tossing the Podium Ices because unavailability. I did spend about an hour looking for an Ice that I tossed at Keith Berger one year, finally found it.

On a Tues night this year, in 20 seconds of effort I went from 22->37>22->35 mph, FYI, starting slow due to one turn, slowing a bit much for another turn. Once I got back up to speed the second time I had a big enough gap that I could soft pedal for 20 seconds to the line. I don't know the math on that but I didn't toss my bottle before the effort. I will say that I was carrying around a solid 15 lbs more than when I was placing in Cat 3 races in 2010, at least 10 lbs more than my other good recent season 2015.
Colin R said…
"20 seconds of effort I went from 22->37>22->35 mph,"

I would LOVE to see the power profile from this. It seems insane to me, but I realize you are a LOT more aerodynamic than me. The time it takes me to go from 33 to 35 in the wind at 1000w is... LONG.
Aki Sato said…
I don't have power numbers, just speed and HR. It was June 14, 2016, B race at the Rent. My post, and in the comments I describe what I think happened in terms of power/speed:
http://sprinterdellacasa.blogspot.com/2016/06/ccap-tuesday-night-race-june-14-2016-b.html

I added a screenshot of the HR/speed from Golden Cheetah.

Stava data:
https://www.strava.com/activities/609526190/analysis/3079/3144

Now that I look at it I'm not sure that I slowed to 22 mph, although I'd trust the SRM before GPS data. I do know I slowed really hard because I thought was going off the road in the last turn. We started slow because no one was going, therefore I jumped first. Also, because I was doing the Bs, I wasn't really looking to win the sprint as much as to do a full on effort. A break had won the race so I felt okay sprinting. Two days previously I'd done 1000w for 17s, over 900w for 23s.
Colin R said…
Thanks Aki! Wish you had power data for that, it would be fun to try to compute your CdA just from looking at wattage and speed change!
Aki Sato said…
I should dig up some files. Unfortunately I think I cooked my SRM my third battery soldering so nothing good from this year. Last good file was from 2015. I'll be sending my SRM to CO when I can to get it looked at.

I haven't done a proper max effort, like the GCN video from over the summer where they went in a low gear/speed, high gear - low speed, high gear - high speed, since about 2011. I want to go and do that once again, and GCN sort of confirmed my thoughts that I get max power doing a real sprint, not a jump from a light, so I'll want a high speed "leadout" aka a downhill going into something where I can go 20 seconds without worrying, even at 40 mph. I wouldn't expect to go much faster than that, maybe 43-44 with a tailwind. My race sprints were 39 mph into light wind, more.

Gotta go will continue my thoughts if there's any more.
JKLM said…
I have power data for 22->36mph over 14 seconds. 1380w peak, 1245w average (73kg at the time). I wish I could do that twice in 5 seconds longer. Not quite slowing back down to low 20s and back up again though. Aki, is your peak power something like 25w/kg or what?
Anonymous said…
throwing a bottle is also known as littering (same for gels, bars) and it is not permitted by state law. It should be done only in the feedzone as allowed by the rules.

Unfortunately, the officials are not enforcing this rule very often (I think it is DQ the penalty), so it continue to happen and it is still littering in the eyes of the general population creating problems later for the promoters.

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