[IamIan]

Originally Posted by gascort

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:

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.

Originally Posted by gascort

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.

Originally Posted by gascort

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.

Originally Posted by gascort

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.

Originally Posted by gascort

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.

Originally Posted by gascort

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.

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Originally Posted by roosterk0031

A longer car would make it more possible to have longer crumple zones, but construction is the key.

Agree 100%

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Originally Posted by ConnClark

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.

Originally Posted by ConnClark

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.

Originally Posted by ConnClark

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.

Originally Posted by ConnClark

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.