w******x
发帖数: 4396 | 1 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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p*********e
发帖数: 32207 | 2 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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p*********e
发帖数: 32207 | 3 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 |
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D*****I
发帖数: 8268 | 4 that's natural law.
that's why there are more rats & roaches than elephants |
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w******x
发帖数: 4396 | 5 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 l |
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p*********e
发帖数: 32207 | 6 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, |
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w******x
发帖数: 4396 | 7 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 |
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w******x
发帖数: 4396 | 8 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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p*********e
发帖数: 32207 | 9 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 complet |
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p*********e
发帖数: 32207 | 10 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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w******x
发帖数: 4396 | 11 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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p*********e
发帖数: 32207 | 12 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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w******x
发帖数: 4396 | 13
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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p*********e
发帖数: 32207 | 14
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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w******x
发帖数: 4396 | 15 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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p*********e
发帖数: 32207 | 16 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.,
tw |
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w******x
发帖数: 4396 | 17 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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p*********e
发帖数: 32207 | 18
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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y***g
发帖数: 10422 | 19 NHTSA 的这个速度低的可怜的100%正面碰撞还是要搞。
因为这个速度低的可怜的100%正面碰撞是法定的市场准入最低安全标准。也就是说，达
不到这个标准的车是禁止上市销售的。所以虽然这个标准太低，没什么技术上的指导意
义，但形式上还是必须的。
欧洲、日本、俄罗斯、中国大陆的市场准入最低安全标准也是这个速度低的可怜的100%
正面碰撞。中国产的江铃陆风、中华尊驰尽管在德国ADAC组织的 64km/h 40%正面偏置
碰撞（即新标准）中成绩惨不忍睹，但仍被允许在欧洲市场销售。就是因为它通过了法
定最低标准————这个速度低的可怜的100%正面碰撞测试。 |
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y***g
发帖数: 10422 | 20 江铃陆风和奇瑞旗云是几十年前的设计。但尊驰和QQ不是。
QQ 是仿 Daewoo Matiz。 Matiz 也就比 QQ 早两三年。Matiz 的碰撞成绩比 QQ 好得
多。
当时 Daewoo 财政困难，濒临倒闭。由于其他在中国合资建厂的汽车企业都赚翻了，
Daewoo 也想进入中国。当时正在和奇瑞谈判合资。奇瑞当时刚起步，只有 Seat
Tolado 一个车型，也想引进新车。于是一拍即合。Daewoo 当时已经提供了两个车型的
全套图纸和资料给奇瑞试生产。一个是 mid-size 的 Magnus，另一个是 微型车 Matiz。
没想到后来 Daewoo 被 GM 收购。由于 GM 在中国已经有合作伙伴，于是 Daewoo 和奇
瑞合资计划告吹。按理说奇瑞不能用 Daewoo 提供的车型资料。但奇瑞耍了个流氓，把
这两款车的外形改了一下换个名字就上市销售。Magnus 就是后来的东方之子，Matiz
就是 QQ。Magnus 也是后来 GM 的 Chevrolet Epica（景程）和 Suzuki Venora。
也就是说，QQ 并非简单的山寨 Matiz，而是（事实上）拿到 D... 阅读全帖 |
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d*******s
发帖数: 15155 | 21 这没啥奇怪的，都是流水线，加工都是车床，连MB都能国产了。Getrag的中方合作伙伴
是江铃，做大车也做不少年了。 |
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l**********l
发帖数: 4468 | 22 是的。这都快二十年了，国产车还在买三菱这个发动机。江铃新做的那个suv也是这个
发动机，不过变速箱很吊，国产八速的。
觉。 |
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l**********l
发帖数: 4468 | 23 BYD这么厉害。。。我暑假去看的江铃反正是很烂。
turbo |
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