# Why do plastic soda bottles have those weird five-nubbed bottoms? **URL:** \ **Category:** Factual Questions\ **Created:** [March 30, 2007, 5:42am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087 "2007-03-30T05:42:49Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Johnny\_Hildo](https://avatars.discourse-cdn.com/v4/letter/j/bbce88/32.png) [@Johnny\_Hildo](https://boards.straightdope.com/u/Johnny_Hildo)\ **Post date:** [March 30, 2007, 5:42am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/1 "2007-03-30T05:42:49Z")
Glass ones don’t. Is it just a weird design quirk or does it actually serve some purpose? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 30, 2007, 5:53am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/2 "2007-03-30T05:53:43Z")
I think the folding gives the plastic rigidity where required in that area. Glass is strong enough without. ---
**Author:** ![Doctor\_Who](https://avatars.discourse-cdn.com/v4/letter/d/ed8c4c/32.png) [@Doctor\_Who](https://boards.straightdope.com/u/Doctor_Who)\ **Post date:** [March 30, 2007, 6:11am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/3 "2007-03-30T06:11:48Z")
[QUOTE=Princhester] I think the folding gives the plastic rigidity where required in that area. Glass is strong enough without. [/QUOTE] That’s an excellent assumption. I don’t know the answer - but my first instinct was PROFITS! PROFITS! PROFITS! They make them like that so they can sell you less soda for the same price! It’s a conspiracy! But then I realized that it’s, what, 1/2 an ounce of sugar and water. Hardly enough to cut into their markup. Oh well, back to making my tin foil hat. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 30, 2007, 6:19am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/4 "2007-03-30T06:19:45Z")
WAG here: I’m guessing that you’re young enough not to remember (or maybe you just never noticed) that 2-L soda bottles used to be made of two pieces: the clear plastic bottle, which had a perfectly hemispherical base, and a separate opaque plastic piece that was glued to the bottom of the bottle. I believe I heard at some point that the change between the two was occasioned by improved blow-molding techniques, but I might be misremembering — for all I know, it was just something that nobody had ever thought of before. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 30, 2007, 6:24am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/5 "2007-03-30T06:24:16Z")
What gives me some confidence is that I remember that when plastic bottles first came out (at least here in Australia) the bottom of the bottle itself was a simple rounded end, of much the same thickness and strength as the sides. Then there was a separate thicker plastic part, like a cup, that provided the necessary toughness and stability and rigidity, bonded to the bottom of the bottle itself. It was (at least here) usually black. Then (I’m assuming) they figured out how to make the bottom rigid and strong enough to survive on its own so that they could use a one piece design, thus reducing production costs. Princhester (who’s childish hobbies include making water rockets out of bottles and thus knows far, far more about plastic bottle shapes than any sensible adult should). ---
**Author:** ![mamboman](https://avatars.discourse-cdn.com/v4/letter/m/ecb155/32.png) [@mamboman](https://boards.straightdope.com/u/mamboman)\ **Post date:** [March 30, 2007, 8:04am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/6 "2007-03-30T08:04:34Z")
… ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [March 30, 2007, 11:53am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/7 "2007-03-30T11:53:35Z")
I’m certain that it is to give the bottle rigidity and the ability to stand up, as has been stated. I don’t know if it’s improved blow-mold technique (I would’ve thought they’d had that worked out long before) or them just getting cleverer about it. There is a desire for profit – but it’s not the need to save the small amount of soda – the amount hasn;t changed, it’s marked on the bottle. But, as someone has pointed out, the older version (here in the States as well) had a two-piece construction of a round-ended plastic bottle stuck into a base. It costs money to mold a separate base (out of different plastic, too – the bottle’s PET, and I think the base was Polyethylene) and go to the expense and trouble of gluing them together. Making it all one piece of the same plastic saves a lot of time, effort, and materials. Possibly the reason they didn’t do it earlier was the need to keep the contents under pressure. The old round-ended bottles used the classic rounded design that gives great strength against internal pressure (if they don’t keep pressure, your soda goes flat). If you just molded squared-off end on the bottle, as with a glass bottle, the stresses woulf concentrate near those bends and the bottle would crack when it was being handled. The current five-point design uses less -tigthly turned corners and distributes the stresses so that the botles won’t break. I’ll bet it took some time to figure that out. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 30, 2007, 12:21pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/8 "2007-03-30T12:21:39Z")
[QUOTE=CalMeacham] The old round-ended bottles used the classic rounded design that gives great strength against internal pressure… [/QUOTE] And would have been lighter and more aerodynamic. Bummer that I started making water rockets after they stopped making these bottles. ---
**Author:** ![SmackFu](https://avatars.discourse-cdn.com/v4/letter/s/53a042/32.png) [@SmackFu](https://boards.straightdope.com/u/SmackFu)\ **Post date:** [March 30, 2007, 12:56pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/9 "2007-03-30T12:56:53Z")
The old one was harder to recycle too. ---
**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\ **Post date:** [March 30, 2007, 2:18pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/10 "2007-03-30T14:18:42Z")
FYI and of no real help, current bottles are made from injection molded blanks that look like a big thick fat test tube with a threaded end for the cap. These are then blow molded into the finished bottle shape. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [March 30, 2007, 2:56pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/11 "2007-03-30T14:56:28Z")
[QUOTE=MikeS] WAG here: I’m guessing that you’re young enough not to remember (or maybe you just never noticed) that 2-L soda bottles used to be made of two pieces: the clear plastic bottle, which had a perfectly hemispherical base, and a separate opaque plastic piece that was glued to the bottom of the bottle. I believe I heard at some point that the change between the two was occasioned by improved blow-molding techniques, but I might be misremembering — for all I know, it was just something that nobody had ever thought of before. [/QUOTE] These two-part bottles are still in use in some places. Eastern Europe, for example. [QUOTE=SmackFu] The old one was harder to recycle too. [/QUOTE] Actually, in many places they are refilled rather than recycled. When I lived in Budapest in 2003, you paid a deposit on 2- and 3-litre Coke bottles, which you got back when you returned them to be refilled. I can’t say I was impressed with this arrangement, since the cola always had an off flavour; I presume this had something to do with either the bottle material or the cleaning process. The non-refillable plastic bottles (1 litre and less) did not have this problem. ---
**Author:** ![jharvey963](https://avatars.discourse-cdn.com/v4/letter/j/54ee81/32.png) [@jharvey963](https://boards.straightdope.com/u/jharvey963)\ **Post date:** [March 30, 2007, 4:18pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/12 "2007-03-30T16:18:02Z")
Yeah, but why “5”? Why not 3, 4, 6, or 7? J. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [March 30, 2007, 5:30pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/13 "2007-03-30T17:30:14Z")
I actually wrote to the container-design company Ball about this (you know, the folks who used to make canning jars and now do lots of plastic containers). I’ll let you know if they write back! (Forgive me for slightly misrepresenting fellow Dopers as my “students”, but I thought that that would make it more likely to come across as a serious, if kind of peculiar, inquiry rather than a silly prank.) > [@](#): > > Date: Fri, 30 Mar 2007 > From: [Kimstu] > Subject: Design query about Ball’s blow-molded CSD bottles > To: [smccarty@ball.com](mailto:smccarty@ball.com) > > Dear Mr. McCarty, > > As a long-time user of Ball products and as a teacher > of very active-minded and curious students, I have a > question about the design of plastic soda bottles that > I hope somebody at Ball will be able to answer. > > Namely: Why do standard blow-molded soft drink > bottles, such as Ball’s CSD containers, have five > “bumps” on the bottom? Why is the bottom of the > bottle not flat or slightly concave, as in the case of > most glass bottles, and why five bumps instead of > three, six, etc.? > > Many thanks for any information you or any of your > colleagues at Ball may be able to provide. > > Yours sincerely, > [Kimstu] ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [March 30, 2007, 6:35pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/14 "2007-03-30T18:35:35Z")
> [@](#): > > Yeah, but why “5”? Why not 3, 4, 6, or 7? My guess: the same reason office chairs anymore have five caster legs – it gives them a lot of stability and harder to tip over. A three-legged chair is very stable in that three points determine a plane, but you don’t have to tip it up very far (especially if the legs ar on wheels) to tip it over. Four legged things tend to get aligned with the pairs of legs parallel to the way you’re sitting. But with five legs, if you lean backwards, you’re going to be pushing toward one unpaired leg, and it’ll be harder to tip. So with bottles, I think three or four nubs don’t give you enough resistance against easy tipping. You could go higher, but the more nubs, the tighter the curvature becomes, and the more likely the bottle is to split. Five is probably a good compromise. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [March 30, 2007, 6:46pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/15 "2007-03-30T18:46:28Z")
The answer I got from the folks at Ball, although not very detailed, seems to bear out **Cal** ’s reasoning: > [@](#): > > Subject: Re: Design query about Ball’s blow-molded CSD bottles > Date: Fri, 30 Mar 2007 > From: “McCarty, Scott” \ > To: [Kimstu] > > Kim, you might be surprised to find out that the bumps - which together > are called the “foot” of a bottle - have a number of patents and a lot > of engineering behind them. The short anser is that the foot is > designed that way for stability both during manufacture and filling, as well > as to help in shelf life and the way the bottles can be stacked for > transport. Boy, that raises more questions than it answers, doesn’t it? “To help in shelf life”? How would a five-footed bumpy bottle bottom prolong the shelf life of the bottle contents, I wonder? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [March 30, 2007, 7:00pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/16 "2007-03-30T19:00:56Z")
> [@](#): > > Boy, that raises more questions than it answers, doesn’t it? “To help in shelf life”? How would a five-footed bumpy bottle bottom prolong the shelf life of the bottle contents, I wonder? My guess (these are all guesses on my part) is that he’s referring to the total design “retaining freshness”, not just the number of legs. As i said earlier, the bottle not only has to stand upright, it has to retain pressure as well, or else the soda will go flat. That requires relatively gentle curves, since tight curves tend to concentrate strain, and if it’s too tight the bottle will break. Five feet is a good enough compromise for stability (the bottle doesn’t fall over easily, which is good if it’s open) , yet will keep its integreity when the contents are under pressure. ---
**Author:** ![Lamar\_Mundane](https://avatars.discourse-cdn.com/v4/letter/l/85f322/32.png) [@Lamar\_Mundane](https://boards.straightdope.com/u/Lamar_Mundane)\ **Post date:** [March 30, 2007, 7:51pm UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/17 "2007-03-30T19:51:28Z")
[QUOTE=Johnny Hildo] Glass ones don’t. Is it just a weird design quirk or does it actually serve some purpose? [/QUOTE] Those nubs are where the Mentos fit. If you look down the bottle after you drop one in you’ll see. ---
**Author:** ![Labtrash](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/labtrash/32/528_2.png) [@Labtrash](https://boards.straightdope.com/u/Labtrash)\ **Post date:** [March 31, 2007, 2:27am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/18 "2007-03-31T02:27:13Z")
[QUOTE=Lamar Mundane] Those nubs are where the Mentos fit. If you look down the bottle after you drop one in you’ll see. [/QUOTE] You can see it better if you look directly down into the bottle after dropping the Mento(or better yet, a few of them) in it. 🙂 ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [March 31, 2007, 2:32am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/19 "2007-03-31T02:32:42Z")
[QUOTE=Princhester] Bummer that I started making water rockets after they stopped making these bottles. [/QUOTE] You have a link on making those? 🙂 ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [March 31, 2007, 5:12am UTC](https://boards.straightdope.com/t/why-do-plastic-soda-bottles-have-those-weird-five-nubbed-bottoms/398087/20 "2007-03-31T05:12:39Z")
There are zillions. Just google water bottle rocket and you’ll find more than I could ever give you.