Mystery of the Self-Driving Car
December 2, 2014 1:24 PM Subscribe
Last night my friend parked her car and went inside briefly. When she came back out to the car 5-10 minutes later, the car was not where she parked it. It was across the street against a rockery. How did the car get there? More details inside.
• Volkswagen Golf, manual transmission.
• Quiet residential street, car parked on flat ground facing north, with very very slight slope downward in front of car.
• About 7:40pm, with streetlights on.
• The car ended up facing south, against a rockery across the street nearly even north-south to the place it was originally parked.
• Width of the street is about 30 feet, which is the diameter of the arc the car may have traveled.
• Friend said she locked the door. Car was found with door locked, purse inside the car, nothing missing.
• Friend said she left the engine off, and this is also how the car was found.
• Let's assume we don't know whether the parking brake was engaged, or what gear it was in.
• Car was verified by another party to be parked where my friend said she parked it. (They both went inside the house together)
• Car was not damaged in any way, even though it rolled into a rockery.
• No other cars were parked nearby.
• Air temperature was about 32°F. There was a minor residual dusting of snow from a few days ago, but no ice.
• The street that the car drove across was bare and no tire tracks were visible.
• Event occurred within a 5-10 minute period when my friend went into the house.
• We tried to recreate the arc the car might have taken by parking the car as originally parked, then starting the engine, turning the wheel sharply left, releasing it, then letting the car roll in neutral. The car naturally straightened out quickly and the trajectory was not heading anywhere near the rockery, but into a neighbor's fence and a steep downhill driveway.
We are quite mystified. If some passers-by had seen the car rolling, they wouldn't have been able to redirect the car since the steering wheel wasn't accessible. And even so, wouldn't they have still been outside trying to identify the owner, when my friend returned to her car?
Also, it seems really unlikely that someone would have broken into the car, hot wired it, and taken a joy ride across the street, then locked the car and left, all within 5-10 minutes, and not even taking the purse.
My question is, how did this car take the wide arcing path that it took? And is there a better way for us to recreate what happened to achieve the actual path?
• Volkswagen Golf, manual transmission.
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• Quiet residential street, car parked on flat ground facing north, with very very slight slope downward in front of car.
• About 7:40pm, with streetlights on.
• The car ended up facing south, against a rockery across the street nearly even north-south to the place it was originally parked.
• Width of the street is about 30 feet, which is the diameter of the arc the car may have traveled.
• Friend said she locked the door. Car was found with door locked, purse inside the car, nothing missing.
• Friend said she left the engine off, and this is also how the car was found.
• Let's assume we don't know whether the parking brake was engaged, or what gear it was in.
• Car was verified by another party to be parked where my friend said she parked it. (They both went inside the house together)
• Car was not damaged in any way, even though it rolled into a rockery.
• No other cars were parked nearby.
• Air temperature was about 32°F. There was a minor residual dusting of snow from a few days ago, but no ice.
• The street that the car drove across was bare and no tire tracks were visible.
• Event occurred within a 5-10 minute period when my friend went into the house.
• We tried to recreate the arc the car might have taken by parking the car as originally parked, then starting the engine, turning the wheel sharply left, releasing it, then letting the car roll in neutral. The car naturally straightened out quickly and the trajectory was not heading anywhere near the rockery, but into a neighbor's fence and a steep downhill driveway.
We are quite mystified. If some passers-by had seen the car rolling, they wouldn't have been able to redirect the car since the steering wheel wasn't accessible. And even so, wouldn't they have still been outside trying to identify the owner, when my friend returned to her car?
Also, it seems really unlikely that someone would have broken into the car, hot wired it, and taken a joy ride across the street, then locked the car and left, all within 5-10 minutes, and not even taking the purse.
My question is, how did this car take the wide arcing path that it took? And is there a better way for us to recreate what happened to achieve the actual path?
This happened to me, way back in the dark ages when I drove an old VW Bug: yes, I had it in park, yes, I had the hand brake on, but also yes the hand brake failed and it rolled away.... fortunately I caught it before it ended up in traffic (!).
So basically, check the brakes, and until then park with the wheels butting up against a curb.
posted by easily confused at 1:28 PM on December 2, 2014 [4 favorites]
So basically, check the brakes, and until then park with the wheels butting up against a curb.
posted by easily confused at 1:28 PM on December 2, 2014 [4 favorites]
Both she and the friend forgot where she parked?
posted by daisystomper at 1:28 PM on December 2, 2014 [2 favorites]
posted by daisystomper at 1:28 PM on December 2, 2014 [2 favorites]
Best answer: "We tried to recreate the arc the car might have taken by parking the car as originally parked, then starting the engine, turning the wheel sharply left, releasing it, then letting the car roll in neutral. The car naturally straightened out quickly and the trajectory was not heading anywhere near the rockery, but into a neighbor's fence and a steep downhill driveway."
When the engine is off and the key removed the steering wheel on most cars is locked, so it wouldn't return to a neutral position. My guess is it just rolled.
posted by Gungho at 1:28 PM on December 2, 2014 [8 favorites]
When the engine is off and the key removed the steering wheel on most cars is locked, so it wouldn't return to a neutral position. My guess is it just rolled.
posted by Gungho at 1:28 PM on December 2, 2014 [8 favorites]
Best answer: The turning circle of a VW Golf is about 34 feet in diameter, so it should be possible for it to make that turn if the wheel is cranked all the way to one side.
posted by mbrubeck at 1:29 PM on December 2, 2014
We tried to recreate the arc the car might have taken by parking the car as originally parked, then starting the engine, turning the wheel sharply left, releasing it, then letting the car roll in neutral. The car naturally straightened out quicklyWith the engine off, the steering wheel would remain locked, so the car wouldn't straighten on its own as it did with the engine on.
posted by mbrubeck at 1:29 PM on December 2, 2014
Best answer: • Let's assume we don't know whether the parking brake was engaged, or what gear it was in.
My manual-transmission sedan will roll backward, not necessarily immediately, if I put it in neutral, forget the hand brake, and get out without noticing a slight grade. Similarly it will roll forward if I leave it in first and get out without engaging the hand brake. If the car moved, we can assume it was left in neutral or first, with the handbrake off.
Could the arc have been due to slight grades that you didn't notice?
posted by chesty_a_arthur at 1:30 PM on December 2, 2014 [1 favorite]
My manual-transmission sedan will roll backward, not necessarily immediately, if I put it in neutral, forget the hand brake, and get out without noticing a slight grade. Similarly it will roll forward if I leave it in first and get out without engaging the hand brake. If the car moved, we can assume it was left in neutral or first, with the handbrake off.
Could the arc have been due to slight grades that you didn't notice?
posted by chesty_a_arthur at 1:30 PM on December 2, 2014 [1 favorite]
To reiterate a previous answerer's note - this is why, on a grade, you face the wheels into the curb. That way, if the brake (or you) fails, the car won't really go anywhere.
posted by chesty_a_arthur at 1:31 PM on December 2, 2014 [5 favorites]
posted by chesty_a_arthur at 1:31 PM on December 2, 2014 [5 favorites]
Best answer: Why were you running the engine for the recreation? Running the engine could have engaged the power steering making the car straighten out. Heck, if the key wasn't in the car the steering column would be locked and couldn't straighten out. If it's a manual transmision, leave the shifter in a neutral position (with the engine off) with the engine off, no key in the ignition and the steering column turned west. Now you can recreate the original act.
posted by nobeagle at 1:33 PM on December 2, 2014 [1 favorite]
posted by nobeagle at 1:33 PM on December 2, 2014 [1 favorite]
It seems very much more likely that both of them misremembered where they parked the car.
posted by Kafkaesque at 1:41 PM on December 2, 2014 [3 favorites]
posted by Kafkaesque at 1:41 PM on December 2, 2014 [3 favorites]
Response by poster: Thanks for quickly solving our mystery!
posted by oxisos at 1:44 PM on December 2, 2014
posted by oxisos at 1:44 PM on December 2, 2014
When I was a teenager moving cars that were running was something that we did when the opportunity presented itself.
posted by COD at 2:35 PM on December 2, 2014 [1 favorite]
posted by COD at 2:35 PM on December 2, 2014 [1 favorite]
Similarly it will roll forward if I leave it in first and get out without engaging the hand brake.
A manual gearbox car will not roll if it is left in gear - first or reverse - whether the handbrake is on or not. Forwards or backwards. It must be in neutral to roll.
Running the engine could have engaged the power steering making the car straighten out.
Power steering does not straighten the wheel, caster effect straightens the wheel. The steering lock being engaged prevented the wheel straightening in the initial issue.
The car rolled in an arc. Luck and good fortune meant it happened so neatly to make a more bizarre explanation likelier than the obvious.
posted by Brockles at 2:46 PM on December 2, 2014 [1 favorite]
A manual gearbox car will not roll if it is left in gear - first or reverse - whether the handbrake is on or not. Forwards or backwards. It must be in neutral to roll.
Running the engine could have engaged the power steering making the car straighten out.
Power steering does not straighten the wheel, caster effect straightens the wheel. The steering lock being engaged prevented the wheel straightening in the initial issue.
The car rolled in an arc. Luck and good fortune meant it happened so neatly to make a more bizarre explanation likelier than the obvious.
posted by Brockles at 2:46 PM on December 2, 2014 [1 favorite]
This happened to me with a manual transmission in a parking lot a few years ago when I forgot to put on the parking brake and the car was left in neutral.
Your friend's car rolled.
posted by winna at 4:09 PM on December 2, 2014
Your friend's car rolled.
posted by winna at 4:09 PM on December 2, 2014
My friend just did this with his BMW. Sadly this did not turn out as nicely as your friend's case as his M3* rolled into a new Telsa*... A very costly mistake. He accidentally left the car in N when he jumped out the car to grab something he left inside.
* Why yes, I live in the Bay Area, what gave it away?
posted by saradarlin at 4:12 PM on December 2, 2014 [5 favorites]
* Why yes, I live in the Bay Area, what gave it away?
posted by saradarlin at 4:12 PM on December 2, 2014 [5 favorites]
A manual gearbox car will not roll if it is left in gear - first or reverse - whether the handbrake is on or not. Forwards or backwards. It must be in neutral to roll. You are right. Not sure what I was thinking here. That is WHY you (I) park it in gear. Duh.
posted by chesty_a_arthur at 12:52 PM on December 3, 2014
posted by chesty_a_arthur at 12:52 PM on December 3, 2014
Brockles: "A manual gearbox car will not roll if it is left in gear - first or reverse - whether the handbrake is on or not. Forwards or backwards. It must be in neutral to roll. "
This is pretty patently untrue. I'm no mechanic, but the explanation I've always heard is that if cylinder compression isn't up to snuff, the vehicle will move regardless (Car Talk backs me up here).
I had a manual-transmission compact pickup parked in first gear with the (admittedly barely functional) parking break on, on a hill, with the tires turned to the curb. After about 5 minutes, it proceeded to drift backwards (downhill), crossing the road backwards (thanks to the wheel being turned into the curb, no such luck) and then almost down over an embankment. Thankfully its progress was halted by a conveniently-placed telephone pole. (I was outside when this happened, with witnesses).
I also had a manual transmission 2-door small sedan that I could not park on a grade of any kind, anywhere, without concerns that it would move (most of the time before I could even get out of the car.) Being in gear mattered not. It even managed to roll down my father's pretty-much-flat driveway and into the ditch overnight. Of course, it was a piece of crap with a barely-working parking break too.
I think the moral here is, get your parking breaks fixed (that was certainly my lesson!)
posted by namewithoutwords at 7:48 PM on December 3, 2014 [1 favorite]
This is pretty patently untrue. I'm no mechanic, but the explanation I've always heard is that if cylinder compression isn't up to snuff, the vehicle will move regardless (Car Talk backs me up here).
I had a manual-transmission compact pickup parked in first gear with the (admittedly barely functional) parking break on, on a hill, with the tires turned to the curb. After about 5 minutes, it proceeded to drift backwards (downhill), crossing the road backwards (thanks to the wheel being turned into the curb, no such luck) and then almost down over an embankment. Thankfully its progress was halted by a conveniently-placed telephone pole. (I was outside when this happened, with witnesses).
I also had a manual transmission 2-door small sedan that I could not park on a grade of any kind, anywhere, without concerns that it would move (most of the time before I could even get out of the car.) Being in gear mattered not. It even managed to roll down my father's pretty-much-flat driveway and into the ditch overnight. Of course, it was a piece of crap with a barely-working parking break too.
I think the moral here is, get your parking breaks fixed (that was certainly my lesson!)
posted by namewithoutwords at 7:48 PM on December 3, 2014 [1 favorite]
This is not patently untrue based on two questionable anecdotal pieces of experience. Not even close. Push your car in neutral. Then put your car in gear and push it. Come back and tell me if it is a LOT harder if it is in gear or not (spoiler, it is). I'll bet you can't even do more than rock it in gear even if you could push it easily in neutral.
Being in gear means the natural drag of the engine (and the compression stroke) limits and/or prevents the car from rolling. This effect is magnified by the gears in the transmission (the gear ratio between wheel and crank). If you engine is near destroyed. and has no compression then this drag will be minimised, but the car being in gear should be enough to prevent the car from rolling unless on a severe incline, or popping out of gear or if ice is involved. The Car Talk thread you link to directly supports that.
It even managed to roll down my father's pretty-much-flat driveway and into the ditch overnight.
Your limited experience does not negate the physics involved. It was not in gear, or not in gear properly. It popped out of gear in some way. There is more than enough drag just running the gearbox backwards through the diff ratio and gearbox step up to provide enough drag that the car will not roll in almost all circumstances on any mild incline. There is no way that a 'pretty much flat' driveway is enough incline to turn the engine over enough times to produce the result you state with the car remaining in gear. I've parked a car without a handbrake for several years just by leaving it in gear.
posted by Brockles at 8:48 PM on December 3, 2014
Being in gear means the natural drag of the engine (and the compression stroke) limits and/or prevents the car from rolling. This effect is magnified by the gears in the transmission (the gear ratio between wheel and crank). If you engine is near destroyed. and has no compression then this drag will be minimised, but the car being in gear should be enough to prevent the car from rolling unless on a severe incline, or popping out of gear or if ice is involved. The Car Talk thread you link to directly supports that.
It even managed to roll down my father's pretty-much-flat driveway and into the ditch overnight.
Your limited experience does not negate the physics involved. It was not in gear, or not in gear properly. It popped out of gear in some way. There is more than enough drag just running the gearbox backwards through the diff ratio and gearbox step up to provide enough drag that the car will not roll in almost all circumstances on any mild incline. There is no way that a 'pretty much flat' driveway is enough incline to turn the engine over enough times to produce the result you state with the car remaining in gear. I've parked a car without a handbrake for several years just by leaving it in gear.
posted by Brockles at 8:48 PM on December 3, 2014
The comment on pranksters reminded me of a time when I worked at a bicycle shop, and at the end of the day, some twit had parked his car on the sidewalk, in a way that blocked the staff (bike) parking shed so we couldn't open the door wide enough to get our bikes out. We got it out of the way by bouncing the end of the car, and on the third bounce, lifting that end and shifting it over a bit. We typically got about 4-6" of sideways movement out of each shift.
Naturally, once we cleared the car away from the shed door, we expressed our appreciation for the courteous parking job by continuing to bounce the car over to a spot where it was pinned between a pair of immovable objects.
posted by kiwano at 11:10 PM on December 3, 2014
Naturally, once we cleared the car away from the shed door, we expressed our appreciation for the courteous parking job by continuing to bounce the car over to a spot where it was pinned between a pair of immovable objects.
posted by kiwano at 11:10 PM on December 3, 2014
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