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View Poll Results: Compact or Crossover for Saftey?
Would stick to compact and drive safe 40 78.43%
Crossover for Saftey 0 0%
Would get a crossover if it had the same mpg 7 13.73%
Would pay up for saftey and get a larger car 4 7.84%
Voters: 51. You may not vote on this poll

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Old 02-14-2012, 12:14 PM   #31 (permalink)
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Quote:
Originally Posted by gascort View Post
You can double a car's mass and expect it to stop in the same distance if all else is held constant (especially center of gravity).
OK, I've got a little experiment you can try. Take a pickup (say my '88 Toyota), accelerate to a reasonable speed, and measure your stopping distance. Now put a good load in the bed (say a load of logs for firewood), accelerate to the same speed, and again measure your stopping distance. Bet you find that they aren't the same - 'cause I've tried it :-)

Quote:
A LARGER car is more safe in all accidents, especially one that's designed to crumple and absorb energy over a greater distance...
There you go again, with that thing you know for sure that just ain't so. See the actual real world data, as in e.g. the Wenzel & Ross paper I mention.

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Old 02-14-2012, 12:42 PM   #32 (permalink)
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Originally Posted by PaleMelanesian View Post
The Smart... held its structure
Which is the main issue with the Smart.
Its structure is very stiff as there's hardly any crumple zone.
As a result, the forces on its passengers are greater.
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Old 02-14-2012, 12:51 PM   #33 (permalink)
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Originally Posted by sheepdog 44 View Post
The biggest problem i see with those mini city cars in the video is that they just dont have enough of a crumple zone in the front! That is a design liability, and i do disagree with them being that short. You can have the same mass and add 3-4 feet to that car in length and it would be much much safer. Furthermore your mpg would get much higher. More length doesn't add to the area of drag and it'll improve aerodynamics so much like the Aero civic. Ecomodding Boat tails and bullet nose extension are good for gas and safety! Get with the picture car manufacturers!
Well put.

These things could be simple aerodynamic shapes over a well-designed crumple zone.

In addition, all (and all sorts of) vehicles should have their crumple zones in the same height range, so the crumple zones actually crash against each other for maximum effectiveness.
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Old 02-14-2012, 01:12 PM   #34 (permalink)
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[QUOTE=jamesqf;286788]OK, I've got a little experiment you can try. Take a pickup (say my '88 Toyota), accelerate to a reasonable speed, and measure your stopping distance. Now put a good load in the bed (say a load of logs for firewood), accelerate to the same speed, and again measure your stopping distance. Bet you find that they aren't the same - 'cause I've tried it :-)

QUOTE]

Not always true, I watched a show with 1 ton dually pickups, they stop faster with load, rear tire don't help without some weight back there.

Size advantage is more important when you hit someone smaller in head on collisions, the smaller car will go from 50 mpg east bound to 10 mph west bound (60 mph speed changed spread over the time of crumpling to give the G forces the driver needs to survive) the bigger car goes from 50 to 10, only 40 mph, the crumpling time would be the same, so G forces are way lower.

Turn that big car into semi, car goes 50 east to 45 west, 95 mph worth of G force impact (semi's don't have crumple zone so only one crumple zone, so time is basically half of first first example). Semi goes 50 to 45, on 5 mph impact to the driver as far as G forces go.

Running into a wall all undergo 50-0 decell G force, the longest crumple length reduces the G forces absorbed by the occupants. A longer car would make it more possible to have longer crumple zones, but construction is the key.
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Old 02-14-2012, 02:27 PM   #35 (permalink)
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Quote:
Originally Posted by IamIan View Post
Impact energy is the transfer of Kinetic energy ... they are not separate or different things.

You credited a larger mass itself for absorbing more impact energy:


Which is still incorrect.

It is the design of the structure that would allow it to absorb or not absorb the energy of the impact.

More mass itself ... will ALWAYS increases the energy of the impact ... mass itself does not absorb impact energy it adds more impact energy to the event.

Two vehicles of identical design expect Car A weighs more than Car B ... both vehicles experience the same collision event with a stationary object at the same speed ... Car A with it's greater mass will have more impact energy than car B ... This is just the way the world works ... KE=1/2MV^2 and F=MA ... A heavier , more mass itself always equals more energy and more force... what you wrote about heavier absorbing more impact energy ... is backwards.
The fact that a heavier car has more momentum means it will take longer to stop regardless of what it hits. This means that the impact forces on the occupants will be lower, smoother, and applied over a longer period of time. The impact forces on what it hits are irrelevant.

Weight isn't the only thing that matters in safety just as safety isn't the only thing that matters in a car, however weight is a big determinant factor in safety.

I might add that there are insurance statistics that show a definite trend in an increase in fatalities with a reduction of weight per number of cars registered. In other words being able to stop and turn quicker doesn't overcome the better protection provided by a heavier vehicle

link to follow as soon as I find it.
edit link: not the one I was talking about but the summary pretty much wraps it all up http://www-nrd.nhtsa.dot.gov/Pubs/808570.PDF
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Old 02-14-2012, 07:15 PM   #36 (permalink)
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...I'm not afraid of "moving" death-traps at all, it's the "suddenly-stopped" ones (with me inside) that I'm afraid of (being in).
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Old 02-14-2012, 07:39 PM   #37 (permalink)
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Quote:
Originally Posted by 5.4 View Post
What are your guys thought on driving small compacts vs small crossover/suv? Is it worth the extra saving in your wallet but to risk safety?
Obviously larger cars will fair better in a crash vs a small car but are they worth the extra $$$+Gas, Opinions?

After viewing the poll results I am very happy too see most people use common sense. You cannot buy a big and safe car to compensate for a lack of driving skill and awareness. When I am on the road its like I'm behind enemy lines. I don't trust anyone, I am always looking over my shoulder, and I quickly complete my objective and then return home. Any study will show you that the most common cause of accidents is a lack of attention. Now even if you are a great driver its possible to just be in the wrong place at the wrong time and that's just life. But you should not be turned away from compact cars based only on safety ratings. Driving skill and common sense is far superior to having 18 airbags.

I assume you are talking about buying a personal commuter car, not a family car. I would say protecting your loved ones while you are driving is much higher in priorities than mpg. And generally you would need a bigger car for families anyway so shopping for this kind of car sort of takes micros off the table. Buy what you enjoy and feel comfortable driving in. A car that you drive well and know the limits to will be much safer than something you don't like but is considered safe.
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Old 02-14-2012, 10:10 PM   #38 (permalink)
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Quote:
Originally Posted by gascort View Post
Your #3 argument seems to make sense until it contradicts itself. More energy in the car or in the passenger?
That gets back to the issue of design ... a better design will transfer as little energy to the passengers as possible... but the amount of energy the design as to deal with is increased with increased mass... designs have their limits and can only compensate so much for the negative of increased mass.

As I wrote:
Quote:
Originally Posted by IamIan
A good design can sometimes compensate for the mass penalties ... but that design benefit does not change the penalties of the greater mass ... the design benefits try to overcome or compete with the penalties of greater mass.
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Originally Posted by gascort View Post
I disagree with your #4. Greater inertia yes, more difficult to accelerate yes. What about proportionately more frictional force between the ground and tires? This cancels out any effect of greater inertia in the end.
It helps but I have my doubts it is a true 100% 1 to 1 relationship.

Tires have limits, brakes have limits, Anti-Lock brakes don't apply braking force 100% of the time, static friction is a function of the normal force not the mass ... on any surface that isn't 100% perfectly perpendicular to gravity the normal force is less , but the interia remains the same... going up hill it might help ... but doing down hill it will hurt... etc.

For example ... my Gen-1 light weight Honda Insight has been rate to stop from 60 to 0 MPH in as little as 120 feet ... which means if the accident is ~120 feet away from when I apply my brakes I have a chance to avoid it completely ... but despite the additional friction from a heavier vehicle from what I see those heavier vehicles always take longer to brake ...
Link

With my 120 feet braking I've avoided the accident that all of these heavier trucks run into.

Quote:
Originally Posted by gascort View Post
You can double a car's mass and expect it to stop in the same distance if all else is held constant (especially center of gravity).
Not always true.
Double the mass of a car going down a 5% grade ... it has 5% less static static friction for it's tires ... but now has 5% of it's (doubled) weight as an additional force acting against it if it tries to stop.

Quote:
Originally Posted by gascort View Post
A LARGER car is more safe in all accidents, especially one that's designed to crumple and absorb energy over a greater distance. Mass is only relevant in the "my car is bigger" argument, which yes, there is always someone bigger.
All accidents?
That is just not correct... in some accidents ... maybe... all ... nope.
The designed to crumple and absorb is a safety technology ... it is what is making such a vehicle safer ... the mass can very well work against you.

Quote:
Originally Posted by gascort View Post
Assuming both cars crush say, 0.50m, at the same rate, both drivers experience the exact same force during impact with said stationary object.
If we made that Assumption.

But that assumption does not agree with the example ... because both cars in the example are otherwise equal ... one does not have better crumple zones than the other ... the increased mass at the same speed means they will not both crumple the same ... the heavier vehicle will crumple more ... if it has any crumple zone left to crumple ... if the lighter vehicle had already reached it's crumple zone limit ... the heavier but otherwise equal other car will transfer all the remaining force / energy it still has into the reinforced passengers compartment and those passengers will experience an additional rapid acceleration ... which equals more force to the occupants.

With better crumple zones ... that better safety technology can compensate for the penalties of the additional mass... but even if they do , that doesn't means those mass penalties didn't exist.

Quote:
Originally Posted by gascort View Post
That said I voted for the compact and just drive safely. You can avoid accidents by being observant and alert, and not driving faster than necessary (as you mentioned, velocity is critical in Kinetic Energy)
Most wrecks are due to driver error or inattention so you can save a lot by being careful yourself and watching out for others.
Now that I 100% agree with.

- - - - - - - -

Quote:
Originally Posted by roosterk0031 View Post
A longer car would make it more possible to have longer crumple zones, but construction is the key.
Agree 100%

- - - - - - - -

Quote:
Originally Posted by ConnClark View Post
The fact that a heavier car has more momentum means it will take longer to stop regardless of what it hits.
Not always.
Once the crumple zone if crumpled the rest of the vehicle is usually designed as a ridged frame ... any remaining vehicle motion at that point is translated into abrupt deceleration.

For vehicles without crumple zones ... like some large trucks ... if it hits a truly stationary object ... it just stops abruptly... with little deceleration.

Also have your vehicle more crushed in an accident is not necessarily always a good thing either... repairable ... or totaled.

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Originally Posted by ConnClark View Post
The impact forces on what it hits are irrelevant.
Depending on what it hits ... some times it matters what it is that you hit and how hard you hit it.

Quote:
Originally Posted by ConnClark View Post
Weight isn't the only thing that matters in safety just as safety isn't the only thing that matters in a car, however weight is a big determinant factor in safety.
100% agree... it is not all about weight ... but that is kind of my point ... a lot of people have it in their heads that heavy = safer ... it isn't always true for every situation... does weight matter , yes ... but heavier does not always = safer.

Quote:
Originally Posted by ConnClark View Post
I might add that there are insurance statistics that show a definite trend in an increase in fatalities with a reduction of weight per number of cars registered. In other words being able to stop and turn quicker doesn't overcome the better protection provided by a heavier vehicle

link to follow as soon as I find it.
edit link: not the one I was talking about but the summary pretty much wraps it all up http://www-nrd.nhtsa.dot.gov/Pubs/808570.PDF
Unfortunately I don't see that link addressing those other issues ... it only looks at reported accidents ... not the non-reported non-accidents because the more agile vehicle was able to avoid the accident or stop sooner ... I've seen other studies make other similar types of comparisons ... but you can't compare against the data you don't have ... ie the accidents that were avoided , and thus never reported.
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Old 02-14-2012, 11:46 PM   #39 (permalink)
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POINT -- The "safest" accident there is, is the one that YOU are NOT involved in!
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Old 02-15-2012, 03:07 AM   #40 (permalink)
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Unfortunately I don't see that link addressing those other issues ... it only looks at reported accidents ... not the non-reported non-accidents because the more agile vehicle was able to avoid the accident or stop sooner ... I've seen other studies make other similar types of comparisons ... but you can't compare against the data you don't have ... ie the accidents that were avoided , and thus never reported.

well according to this, which is based on deaths per vehicle registered ( which would nullify any arguments that smaller cars have an advantage of avoiding an accident )

http://www.nhtsa.gov/staticfiles/rul...kshop-Lund.pdf

1) (page 13) Occupants of the smallest and/or lightest vehicles still have death rates about twice as high as occupants of the largest and/or heaviest vehicles

2) (page 16) Occupants of heavier vehicles typically will benefit from greater effective momentum

3) (page 17) Smaller vehicles are involved in more crashes (not fewer, as some have hypothesized)

4) (page 24) We will be ok if we let data on what works –not wishful thinking- guide our strategies.

5) graph on page 11 attached

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