p*********e
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weatherx (贫农--砖头党，金牌小密探) 于 (Wed Jul 15 13:36:00 2009, 美东) 提到:
我日。
http://www.autoblog.com/2009/07/15/video-tata-nano-gets-its-cra
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fatboyslim (fatboyslim) 于 (Wed Jul 15 13:41:11 2009, 美东) 提到:
我没根据，胡猜的。
nano是自己设计的，整体上考虑了安全性。
landwind的原型isuzu的那个能在美国卖，安全性应该也不差，但是江铃拿掉了很多东
西，或者材料上以次充好，结果车身强度大为降低。
是不是因为nano车本身轻crash test也很站便宜。
帝国主义亡我之心不死，crash test作弊！哈哈
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weatherx (贫农--砖头党，金牌小密探) 于 (Wed Jul 15 13:45:30 2009, 美东) 提到:
话说回来，tata原来就是个搞钢铁买卖的，估计这方面恐怕比国内好些厂家强不少。
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kushana (库夏娜) 于 (Wed Jul 15 22:18:38 2009, 美东) 提到:
但是nano也轻多了，同等速度要吸收的能量小很多啊。那个isuzu原型在美国卖的时候
已经有撞墙试验了？
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fatboyslim (fatboyslim) 于 (Thu Jul 16 13:05:53 2009, 美东) 提到:
有了，1997 isuzu rodeo
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 13:52:31 2009, 美东) 提到:
on the flip side, when the chassis becomes lighter and smaller it's also
more difficult for it to withstand the same impact.
moreover smaller cars don't have much crumple zone, so the safety cage
pretty much take all the impact.
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carbonfiber (碳纤维) 于 (Thu Jul 16 15:51:17 2009, 美东) 提到:
different classes don't have "the same impact" during the crash test
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 18:06:26 2009, 美东) 提到:
you didn't get the point. kushana only pointed out the reduced impact on
smaller cars. my point was there's also reduced capacity to withstand impact
on smaller cars.
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carbonfiber (碳纤维) 于 (Thu Jul 16 18:32:39 2009, 美东) 提到:
so, we can't compare cars directly if they're not in the same class for
crash testes,
right?
impact
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 18:53:44 2009, 美东) 提到:
stop repeating the obvious. i'm not comparing cars. i'm comparing the R&D of
two different companies. the capability of producing chassis that can
withstand the impact of its own mass moving at 35mph is a pretty good
indicator of the R&D level.
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 19:06:59 2009, 美东) 提到:
in this case, the smaller, the easier. size-wize.
There is a physics law behind this but i'm so sorry i can't recall the
term.
of
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 19:19:40 2009, 美东) 提到:
however one thing i can recall. Not 100% sure this can explain my previous
point
Let's compare two chassis with exactly the same material and design,
and only difference lies that the former one is one half of the other
in length.
The strength(ability to resisting stress) for a given material is
proportional
to 1-dimension, or length(think about the definition of Young's modulus),
and the mass is proportional to volume or length^3, and so is the kinetic
energy (1/2*mass*velocity^2).
Which means strength/impact energy ratio(inversely proportional to length^2)
will go down while size increases.
That's to say, it's easier to produce a smaller structure with same material
and better strength toward an impact with same initial/final speed.
Just my 2 cents, please correct it if anything's wrong.
of
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carbonfiber (碳纤维) 于 (Thu Jul 16 19:15:41 2009, 美东) 提到:
靠，印度造个几千美元的nano，crash test水平就比江铃什么的好
r u sure u r not comparing cars?
just kidding:-)
of
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DoomIII (HammerHeart) 于 (Thu Jul 16 19:43:25 2009, 美东) 提到:
that's natural law.
that's why there are more rats & roaches than elephants
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 20:12:16 2009, 美东) 提到:
i think it's a matter of the scale of the load. in the event of a crash, you
are no longer only dealing with reversible deformation characterized by the
young's modulus. a lot of the deformation is plastic. you are dealing with
more of a lattice of springs than a lattice of rigid tubes. the more
material you have to distribute the energy over, the easier for the
structure to retain its original shape (more reversible deformation, less
plastic). and it's the ability to retain shape that you are looking for in a
car chassis. imo simple rigidity is not the goal.
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 21:04:30 2009, 美东) 提到:
or we can treat the problem like this
overall crashing performance = reversible deformation plus non-reversible
ones.
With the same model mentioned before, even if the latter doesn't change
for both structures (suppose the ability of absorbing kinetic energy is
proportional to the total volume/mass of the structure in an non-reversible
deformation), the former part, which at least works for the bearly unchanged
saftty cage (if it does remain unchange), still gives the smaller structure
an edge, agreed?
And sure you can define an impact speed so high non-reversible deformation
plays the major role. However this means the safe cage has already crashed
and people in both structures are hard to survive.
you
the
with
a
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 21:17:56 2009, 美东) 提到:
no. crash performance == plastic deformation. all the crash tests only look
at the result of non-reversible deformation.
so a larger structure is actually advantageous. it can reversibly deform a
small amount to absorb a large amount of energy, before the remainder of the
energy force it into plastic deformation.
a smaller structure, on the other hand, might be more rigid, in the sense
that for a given load at one point that doesn't exceed elastic limit, it
deforms less. but at the same time it will not be able to absorb as much
energy before plastic deformation kicks in. (the fact that smaller cars have
smaller crumple zones doesn't help either.) this is the difference between
retaining shape vs simple rigidity.
plastic deformation doesn't need to be big. all that IIHS number you see,
the 1cm and 10cm of a pillar movement are all plastic deformations across
the chassis. there's no measurement for reversible deformation whatsoever.
reversible
unchanged
structure
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 21:24:35 2009, 美东) 提到:
remember that if you have a spring half as long and you stretch it the same amount, the force (or stress) is doubled. simplistically, a smaller structure therefore cannot deform as much before parts of it go into plastic deformation.
you can get yourself a copy of solidworks and the like and try the finite element package.
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 21:39:09 2009, 美东) 提到:
question:
What's the rough relationship between non-reversible deformation and size?
also, that's not a problem a smaller structure has a lower capability
in absorbing kinetic energy. What we are comparing is small vs large.
As long as the drop in crash performance is slower than that of the kinetic
energy with a smaller structure, we have a winner. That's why I am asking
the above question.
Plus, as i recall from the crash test video, the collision between the car
and the barrier is not completely non-elastic. The car falls back after
the contact, which means not all the kinetic energy are absorbed by the
frame.
In this scenario the frame rigidity definitely makes a difference, which
gives the smaller car an edge. Yes the IIHS only measure the plastic
deformation, however rigidity has a contribution to this number, too.
look
the
have
between
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 21:42:45 2009, 美东) 提到:
here's the problem. by doing this you suggests the smaller car will face
the same impact, however it does not. The kinetic energy is proportional to
the mass of the structure (given both cars are tested at the same collision
speed).
same amount, the force (or stress) is doubled. simplistically, a smaller
structure therefore cannot deform as much before parts of it go into plastic
deformation.
element package.
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 22:26:41 2009, 美东) 提到:
i didn't assume same impact. i compared on the ground of local displacement.
when you have a structure rather than a simple solid cylinder, half as much
KE doesn't translate into "half as much force at all places" or "half as
much displacement at all places".
if you really like to think of structures like solid cylinders, good for you
.
to
collision
plastic
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 22:38:55 2009, 美东) 提到:
if you don't, why you apply the same spring displacement(thus the same force)
to a same(materially speaking) spring with half the leength?
Yes it won't translate to "half as much ...",
but i'd say it's closer to half than to whole.
displacement.
much
you
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 23:06:41 2009, 美东) 提到:
force)
let me ask you two questions, college physics 101 style:
1) throughout the crush of crumple zone, is the force exerted by the crumple
zone on the safety cage determined by total vehicle weight?
2) at the point of total crush of the crumple zone, which of the two safety
cages have more displacement (assuming they behaves like springs and one is
twice the length of the other)? and which of the two safety cages have
absorbed more energy?
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 23:19:54 2009, 美东) 提到:
crumple
assuming both cars are traveling at the same speed, the kinetic energy
and the momentum are solely determined by the mass. And given the impact
time
are the same, the force generated by the impact, in the case of complete
non-elastic collision(final speed zero, means delta V is the same),
is solely determined by the mass.
is that enough?
safety
is
definitely the one with larger size. BUT, at the same time, it has more
energy to absorb.
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 23:35:19 2009, 美东) 提到:
1) fail. the force is not determined by the total vehicle weight. it's
determined by the crumple zone itself. in other words, how crumple zone
crumples determines how much force (at any time during the crush) it exerts
back onto the safety cage.
2) assume there's no plastic deformation of the safety cage, the amount of
energy stored in the larger car safety cage is about twice as much as the
smaller car.
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pandamalone (0000~史前圣灵猫熊) 于 (Thu Jul 16 23:47:26 2009, 美东) 提到:
sorry i messed up question 1. thought it's the force between
the front of the vehicle and the barrier.
But even considering the force from the crumple zone to the safety cage,
mass of the entire vehicle (through the kinetic energy and momentum)
still plays an important role in determining the aformentioned force.
It makes significant difference when the crumple zone received twice
as much impact(in terms of energy).
for 2)
if your definition for the larger car remains the same as my OP, i.e.,
twice larger in length and identically designed, then it has a mass
8 times of that of the smaller car. Which means with the same speed
before the impact, it has 8 times kinetic energy to be obsorbed.
Like you said it can store only about twice as much energy, so it makes
it HARDER for the larger car to be safe.
exerts
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weatherx (贫农--砖头党，金牌小密探) 于 (Thu Jul 16 23:57:47 2009, 美东) 提到:
1) no. it's entirely up to the crumple zone--it's designed to crumple in the
desired manner. think about it.
2) i assumed a car twice as long with other dimensions the same. width and
height really don't vary that much. do you know any car that's twice as wide
AND twice as high as the tata (or any other modern car)?
also weight of the car does not scale like that. look it up.
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pandamalone (0000~史前圣灵猫熊) 于 (Fri Jul 17 00:04:17 2009, 美东) 提到:
the
let's do it this way. will the "force" be the same when the car is traveling
at 1.414* speed (so the kinetic energy is twice, as in the twice-as-heavy
car)? How about at 2* speed when the momentum is comparable?
if not, why are you so sure "it's entirely up to the crumple zone"?
wide
i assumed you were following my scenario and thus obeying the definition,
sorry i misunderstood it. |
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