How and why do spark plug pieces break car windows so easily?

November 30, 2004 6:56 AM

How and why do spark plug pieces break car windows so easily?

I found this article pretty easily, but I can't find a copy even with offsite access to my university's library system. I just want to know why, dammit.

posted by Optimus Chyme to Science & Nature (31 answers total)

Because the little metal gizmo on the end of the spark plug is concentrating all the force you apply to the window into one little teeny spot on the window. The glass, which maybe could handle that kind of pressure across its entire surface, shatters when the same amount of pressure is condensed into one spot.

An analogy: Squeeze an egg, barehanded. (Over the sink, preferably.) Try to apply equal pressure all over the egg's surface. You have to squeeze surprisingly hard to get it to break. Now try the same thing, while wearing a ring. The ring will concentrate the force you're applying, and you wind up with goop in your hands.

Another analogy: You can safely lie on a bed of nails -- heck, they have 'em out at Ripley's museums and the like -- because they evenly distribute your weight across lots of nails, and the force applied to any one nail isn't enough for it to break or even bruise your skin. But you certainly wouldn't want your entire weight to rest on one nail.

As the article you linked to mentions, spark plugs are made out of fairly hard material, and if broken spark plug insulators (the ceramic part) are used to break into a car, their jagged, sharp edges may be a factor as well.
posted by Vidiot at 7:03 AM on November 30, 2004

Slightly off-topic: Don't thieves sometimes use a spark plug and a hammer to punch in the trunk/boot lock of a car, then gain entry through the back seat? There's something else popular for that, too, a more common item, but I can't think what.
posted by Shane at 7:09 AM on November 30, 2004

Response by poster: Hmm. Then wouldn't, say, a small sharp rock work just as well? I was under the impression that you could actually just throw the spark plug piece to break the window, and not have to wield it with your hand. If that's true, then somehow it requires far less force with one type of material than with another, similarly hard, object.

Part of me wants to go test this out, but I don't have a junkyard with liberal glassbreaking policies nearby.
posted by Optimus Chyme at 7:11 AM on November 30, 2004

I suppose even a rock doesn't have the overall density of a spark plug, unless it happened to be a lead rock, or iridium.
posted by brownpau at 7:23 AM on November 30, 2004

I've heard that some car thieves use a chunk of porcelain for this task as well. True?
posted by Succa at 7:30 AM on November 30, 2004

You hold the sparkplug in your hand with the metal tip sticking out. Swing your hand at the window and it'll break. A small sharp rock would be hard to wrap your hand aroud though a smallish oblong one would work, the sparkplug just happens to have good characteristics for concealment and a nice shape for your hand.

I've seen little tools with a metal or ceramic tip for breaking car windows from the inside in the event of an emergency. It's a similar idea. You smash it into your window, the sharp point concentrates the forces and then there's bits of safety glass all over the place.

I've full on punched the rear window of a car in highschool and it didn't break and I'm a big guy, it just sort of warped in the frame like plastic momentarily.
posted by substrate at 7:32 AM on November 30, 2004

I suppose even a rock doesn't have the overall density of a spark plug, unless it happened to be a lead rock, or iridium.

Exactly. Ceramics can be HARD. Knifemakers have experimented with ceramic blades that go off the metal hardness scale. Boker sold one for a while, maybe still does. From what I hear, when the blade finally gets dull, you certainly can't sharpen it with a wetstone.

Just about anything metal in an automobile engine could be made of ceramic, and auto manufacturers have experimented with this. Ceramic tiles have also been used for the heat shields of spaceship re-entry modules.

Someone more technical can expound on these fascinating tidbits and correct my loose terminology, if interested...
posted by Shane at 7:33 AM on November 30, 2004

I always thought that a spring loaded center punch was the preferred method of entry due to its convenience and silence. That was the method used to break into my car. Too bad for the thief that he was observed, reported to the police and arrested.
posted by caddis at 7:36 AM on November 30, 2004

Spark plug pieces have hard sharp points, structurally quite similar to the window punches used by emergency rescure personnel to shatter tempered glass windows. Read a bit more about emergency window removal here.
posted by RichardP at 7:36 AM on November 30, 2004

I've full on punched the rear window of a car in highschool and it didn't break and I'm a big guy, it just sort of warped in the frame like plastic momentarily.

It's much easier, of course, to punch out the side or front windshields, which are made of safety glass and are designed to break into neat, almost square little pieces. For whatever reason, the side seems to be more easily broken than the front, which often dents and spiderwebs before actually busting in.
posted by Shane at 7:36 AM on November 30, 2004

Hrm, come to think of it I accidently broke a fishtank when I was a kid with a marble. I was squeezing the marble between my thumb and index finger to shoot it, it flung off in a random direction into our fishtank and shattered it. Glass or ceramic doesn't have much give to it, they're not malleable so none of the kinetic energy gets wasted in deformation. You could probably throw a piece of sparkplug at a window and shatter it.
posted by substrate at 7:37 AM on November 30, 2004

It's much easier, of course, to punch out the side or front windshields, which are made of safety glass and are designed to break into neat, almost square little pieces. For whatever reason, the side seems to be more easily broken than the front, which often dents and spiderwebs before actually busting in.

Yeah, I knew that but the car had stopped at a stop sign and I had to run to catch it.
posted by substrate at 7:39 AM on November 30, 2004

Yeah, I knew that but the car had stopped at a stop sign and I had to run to catch it.

LOL.
posted by Shane at 7:40 AM on November 30, 2004

Plus, a sparkplug is both hard and easily concealed, as others have said above, and it's much more plausible to be walking around with one of those than a spring-loaded punch.
posted by Vidiot at 7:53 AM on November 30, 2004

Yeah, the front and possibly rear windows of cars are made of laminated glass to keep you from flying out in an accident. I assume the side windows are still breakable so you can break out of a car if you need to escape.
posted by zsazsa at 7:54 AM on November 30, 2004

My truck was once broken into with a spark plug fragment. The cop said they are referred to as "Ninja Rocks." They are popular with thieves because they don't make a sound when they hit the window and the widow shatters fairly quietly. At least, that's what the cop told me.
posted by bunktone at 8:07 AM on November 30, 2004

Here is s slight bit of info.

Here is a court case on the matter.
posted by bunktone at 8:20 AM on November 30, 2004

Indeed. Sadly, I speak from experience -- four breakins, two definitely done with sparkplugs, the other two probably done that way as well. I'd never heard of this as a break-in method until the cop happened to mention it.
posted by Vidiot at 8:22 AM on November 30, 2004

I assume the side windows are still breakable so you can break out of a car if you need to escape.

I think they actually want the front and sides breakable so you can fly out, and they make them from safety glass so there are no large, jagged pieces to kill you. In a high-speed crash without airbags (or if airbags don't employ), if you can't go through the window, your head's going to splatter like a pumpkin on the glass.
posted by Shane at 8:34 AM on November 30, 2004

Response by poster: Bunktone, thanks for that article, but I linked to it in the very first comment.

Also thanks to everyone for their comments. I had thought that there was something in addition to the hardness and sharpness of the sparkplug that facilitated window-breakage; apparently it's just basic physics.
posted by Optimus Chyme at 8:49 AM on November 30, 2004

Auto Glass FAQ
posted by caddis at 8:50 AM on November 30, 2004

Thanks, caddis. I guess I'm behind the times. Before airbags, the last thing anyone wanted was a break-proof front windshield. I've known of people whose lives were saved because they went through the glass. Of course, they weren't wearing safety belts, but some people claim that is occasionally an advantage too.
posted by Shane at 8:54 AM on November 30, 2004

Laminated glass now seems to appear in side windows to inhibit break-ins, but it makes rescues more difficult.
posted by caddis at 8:57 AM on November 30, 2004

Metals are generally hard and tough while ceramics are hard but brittle. Thats one of the the reasons they are used for different things.
Also i think the trick for breaking large toughened windows in the event of fire is to hit them with a sharp object a couple of inches in from the corner rather than pounding the center of the glass. The energy transfer is better as the stiffness of the held edge prevents absorption.
posted by stuartmm at 9:25 AM on November 30, 2004

Getting more off topic... Shane, the nice thing about the laminated glass is that the when the glass layers break, the pane has give due to the thin plastic layer. As a rule, in a crash you want to stay in the car, even without airbags (which only reduce fatality rates by about 10%). From the NHTSA: The relative risk of fatality of ejected to non-ejected individuals in all ejection crashes is 3.55 for drivers and 3.15 for front seat passengers, which translates into 72 percent and 68 percent reductions in fatalities when ejection is eliminated, respectively. Yes, in a few incidents getting ejected out of the vehicle will save you, but you're more than 3 times more likely to live if you stay in the car.
posted by zsazsa at 9:58 AM on November 30, 2004

caddis, it does make rescues more difficult. Side windows, which are tempered glass, are easy (as noted) to break using a spring-loaded centerpunch or other point impact. Getting through windshields generally involves cutting with a saw or some other tool. The center plastic sheet that holds the two pieces of glass is very tough and most of the effort is in cutting it. It also leaves a sharp edge, which doesn't happen with tempered glass.

In a lot of cases, side access is fastest and usually where the rescuers go first (unless that's where the impact was) so laminated glass will slow them down and complicate things slightly.
posted by tommasz at 10:01 AM on November 30, 2004

Re windshield glass: Someone recently vandalized my Mom's car and it pretty much illustrates all of this. From the patterns of damage, it was easy to surmise what happened:

First, probably with the tip of a baseball bat, they banged the front windshield, leaving a round, spiderwebbed dent. Then they tried harder, hit the window with the side of the bat, and left a long cracked crease, still not completely shattering the glass.

Then they gave up on the front windshield and hit the driver-side glass, which shattered readily.

zsazsa, I see what you're saying, but I still think it mostly applies to cases in which a seatbelt was worn. I worked with a woman whose husband was hit head-on in an old car doing at least 50 MPH with no safety belt. He went through the front windshield, either bounced off the other car or sailed clear over it, and skidded down the road a bit. The windshield broke easily, but the impact trauma from hitting the glass still caused his head to swell up literally to the size of a pumpkin. Pumpkin Head. Pretty disturbing to see, I guess. If the windshield hadn't shattered, his head would have shattered.

But I guess it all depends on the situations and the variables involved.
posted by Shane at 10:22 AM on November 30, 2004

Who needs a spark plug? [.mov, nsfw]
posted by ArcAm at 11:18 AM on November 30, 2004

Do you use the sparky end? Or the, uh, pluggy end?
posted by spilon at 12:04 PM on November 30, 2004

Shane - the tool commonly and effectively used to remove lock cylinders from trunks and steering columns is a dent puller.
posted by roboto at 1:35 PM on November 30, 2004

Thanks, I never thought of that, roboto. I was thinking of something about the same diameter as a trunk lock, a little less wide, used to bash the trunk lock right into the trunk rather than pull it out. I think "B Monkey" had a cigarette lighter or something she used.
posted by Shane at 2:00 PM on November 30, 2004