I know I'm an idiot, but does my car know?

November 22, 2012 8:29 AM

What did I do to my tire? Is my car smart enough to know I'm an idiot?

I may have accidentally nudged a pointy curb corner coming out of a parking garage. Not overly hard, but hard enough to look behind me to make sure I still had my (cheap, plastic, easily knocked off) hubcap. I did.

However, a block or two later it's apparent something is wrong - there's an obviously bad sound coming from the back end of my car, and my car seems to be losing power. The gas is depressed, but it's like the car has chosen to ignore me. I don't remember any warning lights on the dash.

Luckily I was able to pull over, and sure enough my tire is flatter than flat. It's a pancake.

Once the very nice roadside assistance people came out and changed my tire for me, all was good with the car again.

A couple questions - how does a curb nudge flatten a tire so quickly? And two, is my 2011 VW Golf, featureless except for heated seats, smart enough to know I'm driving on my rim and to cut power? That was the part that really confused me... a flat I can almost understand (but really, I didn't hit the curb *that* hard!), but it was the lack of power that freaked me out more.

Thanks!

posted by cgg to travel & transportation (16 answers total)

Did the pointy curb push it hard enough on the tire to create a gap between the tire and the rim? That would let the air out hella fast.

As for the losing power, I got nothin'.
posted by PuppetMcSockerson at 8:32 AM on November 22, 2012 [2 favorites]

There are tire pressure detection systems, but they won't slow your car, i don't think. They just give you a warning.
posted by empath at 8:37 AM on November 22, 2012

The gas is depressed, but it's like the car has chosen to ignore me.

There may be some confusion about how you're 'losing power'. The electrical features of your car (lights on the dash, gauges, radio, cabin lights, etc.) are all working properly, only the thing that worried you was sluggish acceleration and steering control, right?

Because sluggish acceleration and poor steering control are just what happens when you've got a flat and are running your vehicle on a rim.
posted by carsonb at 8:39 AM on November 22, 2012 [3 favorites]

You don't have to hit a curb very hard, just scraping it in the wrong (or right) way can cut a little slash in the tire and flatten it instantly. The guy who changed mine when that happened to me called it something like "kissing the curb."
posted by DestinationUnknown at 8:40 AM on November 22, 2012

There are tire pressure detection systems, but they won't slow your car, i don't think.

A traction-control system might. (Though I also don't know for sure.)
posted by mhoye at 8:45 AM on November 22, 2012 [1 favorite]

And two, is my 2011 VW Golf, featureless except for heated seats, smart enough to know I'm driving on my rim and to cut power?

The car didn't lose power. It's just that a flat tyre is way, way more drag inducing than you realise. If there was no warning lights, then there was most likely nothing wrong with the car otherwise, just the drag from the tyre being flat.

how does a curb nudge flatten a tire so quickly?

Either it pushes it off the bead (sealing face) and air rushes out (you usually hear that) or it will fold the tyre a little and pinch it and you will have a relatively fast puncture over a minute or so because the wall integrity of the tyre is compromised and it no longer holds air.
posted by Brockles at 8:46 AM on November 22, 2012

Maybe the car has traction control, ya know for cornering. It distributes the power accordingly so you have less of a chance to lose control. With a flat it was all, WTF? Or what mhoye said.
posted by Max Power at 8:47 AM on November 22, 2012

Because sluggish acceleration and poor steering control are just what happens

Sorry, this is kind of opaque. If the tire came un-seated from the rim, there wouldn't be anything affixing the tire to the spinning rim. So you press the gas and the rim spins without pulling the tire along with it. The horrible sound you heard was probably the rim spinning in the tire.
posted by carsonb at 8:47 AM on November 22, 2012

Yeah, sorry for the wrong terminology, I realized it was a little unclear. The car had power, it just decelerated astonishingly quickly, even with the gas pressed. In this case the steering was fine, I actually didn't even notice anything wrong with that, which also confused me. I've driven on what was essentially just a rim before, it wasn't like this.

The tire getting un-seated from the rim actually makes sense; I didn't see any obvious slashes in the tire itself, but then again it was dark and I didn't look that closely.
posted by cgg at 9:00 AM on November 22, 2012

If there was no warning lights, then there was most likely nothing wrong with the car otherwise, just the drag from the tyre being flat.

To clarify, there is a slight chance of the ESP/stability control coming in if the rear wheel was rotating at a very different speed to the fronts (although it'd have to be a weird puncture for that to happen) but you'd see a flashing yellow light on the dash while that happened. This is the only thing that could have made the car feel like it was losing power as a result of the puncture.
posted by Brockles at 9:04 AM on November 22, 2012

Could be a pinch flat, could be a tear in the sidewall, could have unseated the bead- all are surprisingly easy to do. There's really not much going on in the sidewall.
Imagine rolling a tire town the street. Now imagine laying that same tire on its side and dragging it down the street. That's essentially what your car's trying to do when you have a flat, and its more noticeable at slow speeds.
On a side note, you should learn to change your own tires. It saves you some money and a lot of time, and it's way easier then you think.
posted by gally99 at 10:00 AM on November 22, 2012

Sorry - because it hasn't been covered yet: you're sure the *only* damage is to the tire/rim and that you've haven't damaged something else or dislodged something that has put your car in limp home mode?
posted by MuffinMan at 10:06 AM on November 22, 2012

You likely tore the sidewall on the curb. My wife has achieved this several times.

Tire inflation sensors would tell that pressure is low with a light, if your car had them, but not stop the car.

I'm guessing you've never driven on a flat before this. The feeling of a flat tire is unique and you really don't know it until you feel it for yourself.
posted by Argyle at 10:37 AM on November 22, 2012

So, two things.

First, you said no warning light. Your 2011 Golf has a tire pressure monitor system (assuming you're in the US, those have been federally mandated since 2008 or thereabouts) and so if it didn't go off, you should mention that to the dealer on your next visit. Incidentally, accidentally ripping off the tire's valve stem on the curb is a great way to deflate your tire quickly and completely, and it doesn't take much of an impact.

Second, traction control systems work to reduce engine power if wheel slippage is detected, and in the case of a completely-flat tire, wheel slippage is definitely going to be detected. So the traction control system isn't designed to identify a flat tire specifically, but since that flat tire is going to reduce traction significantly (detectable even in a straight line when the tire's totally flat), it will step in to protect you. Of course, some cars flash their traction control light when this is happening, but I don't know if VWs do.

On the rear tire drag being enough to slow the car down: from personal experience driving a car for several miles with a completely flat rear tire, I can attest that as long as you're going roughly in a straight line in a front wheel drive car on reasonably smooth roads and not driving recklessly, it is entirely possible not to notice a tire is flat -- especially on low-profile tires -- so I personally wouldn't think that was a contributor here (or at least not nearly as large a contributor as the traction control system.)
posted by davejay at 12:33 PM on November 22, 2012 [1 favorite]

Second, traction control systems work to reduce engine power if wheel slippage is detected, and in the case of a completely-flat tire, wheel slippage is definitely going to be detected.

Not necessarily definitely, as it should have just rolled along (being a rear wheel). But I guess if the wheel started rotating slower than the front wheels as it got really REALLY flat (at the point the tyre is deforming from not being round and is being dragged rather than rolled) then the car may assume the front wheels are spinning and cut power. That'd give the symptoms of loss of power.

VW's definitely have a flashing light for traction control, though, which was why I had discounted it. Possibly relying on noticing a light flashing is not all that good a hard parameter, though...
posted by Brockles at 1:47 PM on November 22, 2012

The traction control system on our Prius freaked way out when I had to put the (fully inflated, but much smaller) donut spare on a few months back. There was a considerable power decrease as the computer tried to match the wheel speeds at all four corners but couldn't because the fourth corner was much smaller than the others. It was the strangest feeling. But there was a flashing light.

I'm going to go with the other commenters here and say the drag induced from the flat is what you were feeling. Cars just aren't designed to roll on flats.
posted by OHSnap at 4:34 PM on November 22, 2012