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Tesla head on collision with a Honda

RDoc

S85D
Aug 24, 2012
2,723
1,572
Boston North Shore
I stand corrected. Two cars hitting each at 40 mph is not the same as hitting a wall at 80 for the reasons stated, there's half the energy compared to hitting the wall. Hitting a parked car at 80 is a bit more ambiguous since it depends on what happens after the collision. If the parked car is free to move it will carry away some of the energy in it's velocity, if it is prevented from moving the energy is all absorbed between the two cars.
 

yme

Member
Apr 22, 2013
9
0
NY
[...] my sense [is] that the 80/0 crash can be accurately modeled as occurring in two phases: the initial impact with very rapid accelerations where most of the damage is done, and then a second part, with much less rapid accelerations, where the center of mass comes to rest because of friction with the road.

I agree, for all practical purposes.

Basically, modelling it that way does pretend that there's no friction with the road during the actual collision, but this is ok because such friction is so much weaker than the other forces involved.

- - - Updated - - -

I was talking about head-on collisions.

Of course the time it takes for the energy to be expended is very important, but this time will be virtually the same for any head-on collision with two cars of specific types and a given difference in speed.

I can't figure out what your position is. You talk about energy being "released" or "expended", but you only calculate the initial kinetic energy. What if most of that energy remains after the collision? Then, it hasn't been expended after all.

Compare a 1-mph car hitting a stationary train with a 1-mph train hitting a stationary car. The moving train has thousands of times the kinetic energy of the moving car. But the damage to the car is the same in both cases, namely, very small.
 

vfx

Well-Known Member
Aug 18, 2006
14,790
40
CA CA
...
Crumple zones are also the reason why nascar racers can survive 150+mph crashes. These cars are designed with ridiculous crumple zones with parts flying off everywhere and twisted wreckage but yet the driver and climb right out of it. ....

Which has the added benefit of looking spec-tac-u-lar!
 

derekt75

Member
Jul 16, 2012
624
34
San Jose, CA
The reason why it fares batter is not just that it is heavier. If the extra weight was manifested in a block of lead behind the drivers seat, the driver of the heavier car would die first.

Well, yes, a heavier car needs a stiffer crumple zone if you want increased safety, but I don't think it's a detail that a driver needs to consider since heavier cars have stronger frames.

I think the key takeaway is that a heavier car fares better precisely because it is heavier. Yes, the optimal crumple zone on a heavy car is stiffer than the optimal crumple zone on a lighter car, but you can't just take a light car, stiffen the crumple zone and say that it's safer. You need the weight. Many school buses don't have seat belts, not because school bus drivers are super expert drivers, but rather because they weigh so much that bus occupants are unlikely to be seriously injured in a two vehicle accident.

The forces exerted on the two cars of unequal weight are equal, but the accelerations are different due to the different masses. That's why the fly gets squished with no consequences to the car.

As noted earlier in the thread, while heavy vehicles are safer for their occupants, they are much more dangerous to whatever is getting hit by the vehicle. A dust mite hitting a fly at 40 mph doesn't do much damage to the fly compared to a windshield.
 

Jeff Miller

Member
Jan 2, 2013
293
3
Chicago
The forces exerted on the two cars of unequal weight are equal, but the accelerations are different due to the different masses. That's why the fly gets squished with no consequences to the car.

That's right.

If a car of mass m_1 going speed v_1 (eg 40) collides with a car of mass m_2 going speed v_2 (eg -40), car 1 has
an acceleration a_1 of approximately

a_1 = -(m_2)/(m_1+m_2) [ (v_1-v_2)^2 ] / (2L)

where L is the distance over which the cars accelerate in the crash (making L as big as possible is the job of the crumple zone).
So car 1's acceleration is proportional to car 2's mass, divided by the sum of the masses of the two cars. Conversely for
car 2. All else being equal, a heavier car is good for the passengers of the heavy car and bad for the passengers of the lighter car.
 

neroden

Model S Owner and Frustrated Tesla Fan
Apr 25, 2011
14,676
62,627
Ithaca, NY, USA
Many school buses don't have seat belts, not because school bus drivers are super expert drivers, but rather because they weigh so much that bus occupants are unlikely to be seriously injured in a two vehicle accident.
This sloppy and inaccurate thinking has led to very dangerous school buses in the past, but thankfully some states (like NY) have finally required that school buses have lap belts. In any sort of high-speed accident, including a one-vehicle accident, the bus occupants are quite likely to be injured due to *their own inertia* -- when they continue moving while the bus does not, and strike parts of the bus interior at 65+ mph. Lap belts really, really matter, even in school buses, because school buses *do* take expressways.

Not so important in certain city buses which never exceed 30 mph.
 

walla2

Member
Jul 15, 2012
495
218
Lap belts really, really matter, even in school buses, because school buses *do* take

Children (and adults) in lap belts only frequently get burst fractures of the thoracolumbar junction when a vehicle wrecks at high speed. The large force just applied to the lap creates a tremendous amount of stress at the T11-L1 region as your rigid thoracic spine is whipped forward by the mobile segments of your lumbar region. This is one of the many reasons lap belts were replaced with shoulder/lap belts in combination. I would prefer getting bumped into a padded seat over getting a spinal cord injury. Imagine a bus load of spinal cord injury children. That will not be a pretty lawsuit when it hits in NY and the lawyers ask why they didn't have shoulder/lap belts.

I still see these types of injuries but mostly in kids/adults with the shoulder strap behind their shoulder (worn improperly) or in wrecks that occur in older cars with lap only belts. It's sad when I see that the only person injured in a car was the one wearing a lap belt and no one else was belted at all.
 

dsm363

Roadster + Sig Model S
May 17, 2009
18,278
151
Nevada
Children (and adults) in lap belts only frequently get burst fractures of the thoracolumbar junction when a vehicle wrecks at high speed. The large force just applied to the lap creates a tremendous amount of stress at the T11-L1 region as your rigid thoracic spine is whipped forward by the mobile segments of your lumbar region. This is one of the many reasons lap belts were replaced with shoulder/lap belts in combination. I would prefer getting bumped into a padded seat over getting a spinal cord injury. Imagine a bus load of spinal cord injury children. That will not be a pretty lawsuit when it hits in NY and the lawyers ask why they didn't have shoulder/lap belts.

I still see these types of injuries but mostly in kids/adults with the shoulder strap behind their shoulder (worn improperly) or in wrecks that occur in older cars with lap only belts. It's sad when I see that the only person injured in a car was the one wearing a lap belt and no one else was belted at all.

Very interesting. Can you point to a study showing you are safer in a high speed collision unbelted vs. lap only vs. lap plus shoulder belt? Are you a trauma or orthopedic surgeon of some kind (you said you still see these types of injuries)? I hadn't heard that before so was curious.

MSP - Safety Belt Myths and Facts

If you are traveling so fast that the seatbelt causes injury, you would almost certainly have been ejected from the car and not done well at all. Encouraging people to not wear seat belts isn't the best idea unless you have hard science to back up the claim. You are correct you are safer with the shoulder belt added and since every single car sold today has that, that's not an issue. If someone isn't using a safety system properly, that is their issue and being unbelted isn't the answer.
 
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K5ING

MegaMiler
Feb 6, 2013
293
65
Denton, Texas
The trouble with having no belts of any kind in buses is that due to the fact that buses are very top-heavy. Many bus accidents result in the buses going over on their sides, even in low speed collisions, and this is where most of the fatalities occur. Belts of any kind that would hold the occupants in their seats would reduce this (assuming that they are worn, of course).
 

NigelM

Recovering Member
Apr 3, 2011
13,386
555
Northern Virginia
The discussion on impact physics really isn't any less relevant to the thread topic than the various other tangent discussions on black boxes, tracking technology, lap belts vs shoulder belts, Russian (or was it Chinese)?) accidents on a Danish website or the weight of a Model S (all of which was discussed in the last several pages.

For all the people who don't like this thread for one reason or another....just stop reading it; it isn't mandatory.

Edit: The whole conversation on not reading this read went to random-chitchat
 
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deonb

Active Member
Mar 4, 2013
4,057
4,208
Redmond, WA
Many school buses don't have seat belts, not because school bus drivers are super expert drivers, but rather because they weigh so much that bus occupants are unlikely to be seriously injured in a two vehicle accident.

I know some school bus drivers tend to be a bit overweight, but to imply that they can be used as some sort of human airbag to prevent injuries during an accident is a bit much...
 

brianman

Burrito Founder
Nov 10, 2011
17,526
2,994
yea, old news, but wow - the comment on that one 'looks like the Tesla won!!' not cool when 2 ppl are dead.
Totally agree it's in bad taste but it might just be a overzealous car guy forgetting that humans were even part of the story.
 

Herbys

Member
Jul 3, 2012
73
19
The more than 2x greater weight means that .

Where does the "more than 2x" weight difference recurrently referenced comes from? An accord weights 2728 lb and a Model S 4600lb (assuming the Accord has an empty gas tank). That's a 70% difference, not a 2x difference. Still a big difference though, but most modern cars are heavier than the Accord.
 

Zythryn

Model Y custom Warming Stripes wrap.
Mar 18, 2009
2,169
1,189
Minnesota
Where does the "more than 2x" weight difference recurrently referenced comes from? An accord weights 2728 lb and a Model S 4600lb (assuming the Accord has an empty gas tank). That's a 70% difference, not a 2x difference. Still a big difference though, but most modern cars are heavier than the Accord.

I believe the weight of a 20 year old Accord was indeed half the weight.
 

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