I survived, but now some questions remain

[Johan Bornman]

Edman, Christie, GreatWhite, Bruce, Lefty and NickD - your phones are ringing.

 

First the survival story

 

Last week I survived the umpteenth attempt by truck drivers to kill me by release a truck wheel and aiming it at me in my car.

 

Approaching the Beyers Naude turnoff, I noticed a very large wheel bouncing over the trucks side of the highway, over the centre isle roadworks, over a barrier over two lanes of my side of the highway and up the embankment where I was now taking the turnoff.

 

I was on the phone at the time and said my goodbyes to the person on the other side with a very curt..."***, hier kom 'n wiel." The wheel was aimed exactly at me but then at the last moment hit something and bounced nicely over the car, causing a shadow as it passed my open sunroof, bounced off the concrete wall and then tried again to kill me.

 

I escaped once more. The last time this happened was in Robertson last year and then on the Alberton freeway before then. The Alberton one was  a joke compared to the other two since it was a mere taxi wheel.

 

A truck wheel weighs approximately 200 kgs and when coming for you at 120kph it will easily crush the car.

 

Here's the question:

 

I can swear that each time this happened, the wheel departed from its vehicle and passed it before losing direction. Is this an illusion? How can a wheel that's going at the same speed as a vehicle suddenly accellerate upon release?

 

As I told a bunch of friends of my experience (including the one who witnessed the episode over the phone), one cynical bastard, a bit like that Yellow Saddle guy, uttered "Bullsh*t, how can the wheel possibly go faster than the truck?"

 

Well, I don't have a video memory but my recollection, from an angle and not in a frame of mind to take careful note, is tha that the wheel was ahead of the truck, at least in the beginning of the episode before it started to go off course.

 

It's been giving me sleepless nights - not the prospect of 9 minus 3 lives, but the physics.

 

Is it possible that due to the truck's weight, the wheel, whilst loaded has more rolling resistance than when released. However, at the point of release it still has the same energy but less resistance and can therefore accellerate? Can I go back to my cynical friend and claim a six-pack or something. Or, does the mind play games at times of immiment death?

 

 

 

Johan Bornman2008-11-12 01:13:01

[TNT1]

Dunno the answer, but I've had a wheel come off the trailer I was towing, and it overtook me all on it's own as well...

[CAAD4]

Jeesh! I'd PM the guys at Brainiac.

[fandacious]

i'm no physicist, but i can imagine the rotational momentum could initially push it faster than the vehicle it was attached to?

[ngala24]

A friend from schools car was hit but a tyre on the highway near PPC a couple years back. He almost never made it and is now a liitle brain damaged

[gungi]

the wheel would have to be under a lot of strain to break off, maybe the built up energy propelled the wheel forward as it snapped off! hope this helps you sleep better:)

[bruce]

I would suggest that the balance of forces changes significantly.  The inertia of the wheel is far greater than the rolling and wind resistance it encounters when it releases - given a force imbalance, the wheel will accelerate initially, until the balance of forces is restored.

The truck either continues as it was, or actually decelerates if the wheel that has been released is one of the drive wheels.

[Johan Bornman]

Dunno the answer' date=' but I've had a wheel come off the trailer I was towing, and it overtook me all on it's own as well... [/quote']

 

That's good info. If you weren't drunk and you can swear by what you saw, I think it strengthens my case.

 

I've never had the opportunity to study the physics of a turning wheel from your perspective. Consider yourself blessed.

 

[Decado]

The wheel alone doesn't have the load bearing down on it and (up until the moment of release) has exactly the same torque driving it.

 

So F = ma (can't completey recall the angular version)

 

F is constant (although will decrease), mass is much less so acceleration increases.

 

[fandacious]

http://ask.metafilter.com/49668/Why-does-a-detached-tire-roll-faster-than-the-car

[niterider]

I'd expect the loose wheel to be subject to a lot less resistance initially, eg less (a lot less) rolling resistance from no carrying no load and no wheel bearing drag. 

[Domestiquehated]

I have never driven a vehicle that had a wheel come off and nor have I been attacked by a rogue wheel, but I guess that a possible reason would be that when drivers hear the sound of metal cracking they think to slow down (thus allowing the wheel to "accelarate" past the vehicle)?  Nah, silly me...

[loneranger]

I have seen this before as well.

 

Wheel of a bakkie came off, ran neatly alongside the vehicle then

passed infront of the vehicle and into the bushes. So yes, you're eyes still work perfectly! 

 

[Johan Bornman]

I have never driven a vehicle that had a wheel come off and nor have I been attacked by a rogue wheel' date=' but I guess that a possible reason would be that when drivers hear the sound of metal cracking they think to slow down (thus allowing the wheel to "accelarate" past the vehicle)?  Nah, silly me...

[/quote']

 

He he...funny. However, the drivers don't even know a wheel has broken off. If the truck is fully loaded, they only notice once the adjacent wheel bursts. In my first incident in Robertson, I chased the truck in my mighty Hyundai Atos (rental). He said the only thing he noticed was that the wheel lost pressure. Obviously he was fitted with one of those remote pressure sensors. However, the loss of pressure didn't bother him and he just drove on.

 

I must add that in the taxi incident I described, the driver did notice...it was his front wheel. If he didn't notice the slight lurch me definitely noticed the spray of sparks from the disk riding the R556.

 

 

[Bateleur1]

I think that load pressure is also a major source of energy that gets release thus allowing the wheel to accelerate.  If you would have a wheel on an axle travelling at 100 km/h with no load and you would just pull the axle out it would not accelerate. 

 

Similar than edging a little stone on the rode by driving over it with your bicycle.  That stone was not moving but the downward pressure squeezed the stone out of the way accelerating dramatically before it looses the energy again.

[slowpoke]

yo!!!! JB!!!! please confirmt hat you were using a hands free kit!!!!!

 

to put it very simply, the wheel, when attached to the vehicle, has a downwards weight on it - and as such it gets "slowed down" by this downward pressure. as soon as it's released the gyroscopic forces remain the saim but the downward pressure (causing friction etc) is gone so it accelerates. but then, because it is no longer being turned it will slow down