New Cars Torn Apart: Why?



Hi John,

As a motoring expert, I would like to speak to you about a couple of things that have concerned me for long time now. 

Cars that are literally ripped  apart in an accident. Has any one ever tested any of these cars for metal fatigue? Some of the boom boxes in these cars you can hear them half way up the street and when they pull up beside you your car and ear drums vibrate. Surely this must be causing metal fatigue.

Poor seating posture, especially with our young drivers. I see so many of them, my son son included, drive with there seat laid back like a lounge seat. Surely they can't see or handle a car properly in the case of an emergency.

I have written to NRMA twice in recent years but they wrote back saying they don't think there is any interest in the story at this time.

What do you think?





Hi Craig,

Good questions - thanks.

Boom boxes and metal fatigue:

Boom boxes are annoying but they don't cause metal fatigue. Metals have two deformation ranges - elastic deformation and plastic deformation. In the elastic range (which varies from metal to metal based on composition and heat treatment) the metal can be deformed and released an unlimited number of times without fatigue. Example: a spring in the suspension - never wears out; experiences millions of deformation events in its life. Never fatigues. The so-called plastic range is where some of the deformation is permanent. The metal gets bent permanently. Metals that are deformed repeatedly in the plastic range get fatigue: crystalization occurs at the point of fatigue, causing brittleness. Example: bend a coathanger back and forward. It fatigues, crystalizes and breaks through fatigue.

It's inconceivable that a speaker, no matter how 'boomy' in a car, could cause metal fatigue.


The fact is, new cars are designed - for a very good reason - to deform tremendously in crashes. The reason they do this is to absorb impact energy before it gets to the driver. If the car were very rigid (think HQ Holden) the deceleration of the occupants in a collision would be very sudden. If the car had carefully engineered crumple zones (think: any car with a five-star ANCAP safety rating) the deceleration of the occupants is more gentle because it occurs over a greater period of time.

Example: Thomas Kelly. Killed not directly by a sucker punch thrown by a coward, but rather by the impact of his head on the pavement as a result of that punch. (I'm not making excuses for his killer, who is demonstrably a scumbag.) The impact with the rigid, unyielding pavement was so brief that the victim was unable to survive the deceleration of his brain inside his skull. Its physical limits were exceeded, blood vessels were damaged, an intra-cranial bleed was suffered and the rigid box of bone (the skull) left nowhere for the pressure to go. Death resulted.

If Mr Kelly's head had hit a few empty egg cartons on the pavement, he would undoubtedly be alive today. The egg cartons would be damaged, but the brain would have survived because the cartons absorbed the impact in a  controlled way, absorbing energy and extending the time to reduce the magnitude of the deceleration.

If you compare the two impacts, when it's head-versus-pavement, the head loses and the pavement wins. The pavement looks (and is) undamaged. When it's head-versus-egg-cartons-on-pavement the egg cartons lose and the head survives. 

The head V pavement scenario is crashing in a 1980s car. The head V egg carton scenario is crashing in a modern car. The structure is a shock absorber. This is one of the principle reasons why the road toll is way down. It's not policing, that's for sure.

I was an engineer. There's a huge body of technical research in support of this fact. Unfortunately, you need a degree in applied physics or engineering to understand it. If your head decelerates at 80G fo more than 80ms, you're dead. That's what the engineers aim to prevent - by using the car like the egg cartons as a shock absorber. (Similar limits apply for thoracic deceleration, and loads on lower limbs.)

Personally, I'd rather have the car be completely rooted, and me be okay after a crash - because the alternative is the other way around. (And you can't have both - something has got to give in this game.) Older cars weren't better built; they were death traps.

Seating position:

We agree on this. As upright as possible is a great idea. It boosts both vision and control as you say. It also maximises the capacity of the restraint system - seatbelts, pre-tensioners and airbags - to protect you in a crash. In a severe crash, if you're sitting too far reclined, you'll slide through the seatbelt, into the footwell. Ask paramedics and the fire brigade. Something should be done about this, but I'm not sure what. Probably a public education campaign.

Thanks for the questions. I understand how you think that about modern car construction. I've been asked that several times. It's intuitive. A high level of technical training is required to understand the reason for all that deformation. Thankfully though, they don't make 'em like they used to. Trust me on this; that's a good thing.


John Cadogan