How do you get flat tires from nails?

October 11, 2007 10:56 PM

How do nails/screws get stuck in tires?

One would think that a nail or screw or other piece of metal would be lying flat on its side on the road, so when a vehicle runs over it, how does it raise up to puncture the tire?

Even if it were sitting upright, wouldn't the vibrations of the approaching vehicle cause it to fall over?

posted by atlman to travel & transportation (14 comments total)

I was hoping Googling would result in some high speed slow moltion video, but alas no luck.
posted by jaimev at 11:20 PM on October 11, 2007

Sometimes it's a dirty trick. Someone will deliberately prop a nail under a tire so that if a car starts to move, the nail will go in.
posted by Steven C. Den Beste at 11:27 PM on October 11, 2007

Very nice write up on this question. Gets a little technical, but stick in there.
posted by B(oYo)BIES at 11:32 PM on October 11, 2007

It always seems like they are stuck in a tread. My guess is that often they are picked up in a tread. On the next rotation, the side of the head hits and levers the tip a little, enough that it pierces the rubber slightly. After it clears the pavement, it is pulled into more of a radial configuration by centrifugal force, and the next rotation drives it in for the kill.
posted by Good Brain at 11:36 PM on October 11, 2007

I speculate that nails and sharp things don't get picked up right away but are blown around and tend to collect in depressions, holes, and cracks. Then they're in a better position to point upward.

Of course what I wonder is what nails are doing on roads to begin with. I squarely blame the bubbas who have pickups with no tailgate, and have the bed chock full of hoses, rope, beer cans, plastic bags, dogs, screws, and nails. Of course that stuff falls out eventually.
posted by rolypolyman at 11:58 PM on October 11, 2007

Great question! Especially since I got a flat tire this week and found a 3 cm long scalpel-blade stuck in the tire. Completely bizarre.

Thanks to the MacSci-post I won't loose any more sleep over this.
posted by Rabarberofficer at 12:34 AM on October 12, 2007

nail or screw or other piece of metal would be lying flat on its side on the road,

It's also not inconceivable that the tire picks up the nail and then runs over it again while it is tumbling. The head of a nail seems like it would even facilitate this fairly well by providing an easily dislodged and shallow initial puncture, thus whipping it up and in front of the tire for a second go round. Think of the messy craziness going on in your wheel wells when you go over a gravel road.
posted by IronLizard at 2:10 AM on October 12, 2007

The tire tread kind of twists the nail or screw as it drives over. This twisting makes it easier for the nail to assume puncturing position.
posted by JJ86 at 6:13 AM on October 12, 2007

Easy: you've got four tires. Two in front, two in back. Front two drive over the nail, kicking it up, back tires get the puncture.

Unless the nail is on top of other debris and is pointed slightly upward. All kinds of debris commonly finds it way towards the sides of the road (one of a myriad of of reasons why you should stay away from the sides of the road if at all possible), so it's not entirely unlikely. But my money would be on the first scenario.

Out of curiosity, was it the front or rear tires?
posted by Civil_Disobedient at 6:43 AM on October 12, 2007

Once the tip of the nail becomes imbedded, at even a shallow angle, the centrifugal force of the rotating tire would fling the head outward, causing a puncture on the next rotation.

A couple of years ago I got a flat from a small, random fragment of copper that looked like it had been run over hundreds of times. I remarked to the tire guy that it seemed pretty odd. He said that he fixed a lot of flats from possum teeth.
posted by Huplescat at 6:55 AM on October 12, 2007

I have a truck that attracts flats enough that I now just plug them wherever I am and reinflate the tire with a portable compressor rather than mess with the spare. Almost every time the offending debris has gotten wedged in the tread and worked its way in. I have even gotten flats from gravel in this way; this was a real problem since I have a long gravel driveway. Remember that tires flex considerably while they spin and this helps to work wayward objects through the tread
posted by TedW at 7:56 AM on October 12, 2007

I guess the screws and nails are oriented so as to be able to pierce your tires because they are sticking up out of a small piece of wood, a scrap or even a splinter, when you run over them.

The point pierces your tire, and the screw or nail is either pulled directly out of the already half-pulverized wood as the tire rolls over its length, or the crushing force of the tire tends to split or break the wood, which is spun off in the first or first few rotations, and then the screw or nail is driven farther into the tire with every turn of the wheel.
posted by jamjam at 8:23 AM on October 12, 2007

Civil_Disobedient has it. Most commonly the front tire kicks up nail and the rear tire gets punctured.

On bicycles, punctures are quite frequent so there are easily observable trends. It is usually the rear tire that flats. And the front tire will often puncture on group rides where we follow right behind fellow cyclists.
posted by randomstriker at 9:32 AM on October 12, 2007 [1 favorite has favorites]

It wasnt a particular tire this time, it has just happened to me and friends and family enough that I got to thinking how it could happen so frequently. You all answered my question though, fascinating!
posted by atlman at 5:22 PM on October 12, 2007