Lotus Elise - Safe?
10-15-2005, 08:45 PM
#11
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Originally Posted by rai,Oct 15 2005, 10:21 PM
^ crash tests, the way they are done in the US, do not take into account the weight of the vehicle. If they did, people would be scared of small cars.
Here's the way a crash test works: a car is crashed into a fixed barrier at a certain speed I think it's 45 mph. This simulates a crash between 2 cars both going the same speed both with the same weight crashing into each other. A small car doesn't pack as much of a punch so for example in this case it's like an Elise crashing into another Elise. Or a 6000 lb Ford F150 crashing into another F150.
Now if say instead of a fixed barrier, the car was crashed into a sled weighing the same weight no matter what car was being tested. We'd get more "real world" data. However small cars would do poorly and large cars would do better.
For example... Take a 2000 lb Elise can crash it into a 4000 sled. The mass/energy would be transferred such that the Elise would take far more damage. Now if a 6000 lb truck crashed into the same 4000 lb sled the truck would come out with far less damage.
I think the the industry keeps fixed barrier crash test partly b/c if it didn't then cars would shake out mostly on their weight.
For this reason. I don't care how good an Elise does on a government crash test. Because the crash test score is a farce. It's not fair to subject a 6000 lb truck to a 6000 lb crash force while subjecting a 1800 lb car to 1800 lb crash force and try to say the car with the better score it's safer. That's just nonsense. Crash an Elise into a F150 and tell me which one is safer.
Here's the way a crash test works: a car is crashed into a fixed barrier at a certain speed I think it's 45 mph. This simulates a crash between 2 cars both going the same speed both with the same weight crashing into each other. A small car doesn't pack as much of a punch so for example in this case it's like an Elise crashing into another Elise. Or a 6000 lb Ford F150 crashing into another F150.
Now if say instead of a fixed barrier, the car was crashed into a sled weighing the same weight no matter what car was being tested. We'd get more "real world" data. However small cars would do poorly and large cars would do better.
For example... Take a 2000 lb Elise can crash it into a 4000 sled. The mass/energy would be transferred such that the Elise would take far more damage. Now if a 6000 lb truck crashed into the same 4000 lb sled the truck would come out with far less damage.
I think the the industry keeps fixed barrier crash test partly b/c if it didn't then cars would shake out mostly on their weight.
For this reason. I don't care how good an Elise does on a government crash test. Because the crash test score is a farce. It's not fair to subject a 6000 lb truck to a 6000 lb crash force while subjecting a 1800 lb car to 1800 lb crash force and try to say the car with the better score it's safer. That's just nonsense. Crash an Elise into a F150 and tell me which one is safer.
Like hitting a tree.
It is not like hitting another vehicle of the same weight.
10-15-2005, 08:54 PM
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Ztec, do you have pics of your ex-gf's Elise after the accident? I would like as much information as possible to make sure my gf knows what exactly she's getting into.
Thanks in advance.
Thanks in advance.
10-15-2005, 08:59 PM
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Well, she is my ex 
we speak but not often, I will do my best to get some pics if any were taken - I assume there would have been!
She is on her 3rd Elise right now I am no crash expert but I feel much safer in my S and even safer in my 4runner.
we speak but not often, I will do my best to get some pics if any were taken - I assume there would have been!She is on her 3rd Elise right now
I am no crash expert but I feel much safer in my S and even safer in my 4runner.
10-15-2005, 10:23 PM
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I was very close to ordering an Elise. I shyed away due to my own safety concerns. Now that my gf is seriously considering it, I am a bit concerned for her safety. She's an excellent driver, but my concern relates to those uncontrollable situations.
Anyway, great info thus far. Please keep it coming.
Anyway, great info thus far. Please keep it coming.
10-16-2005, 06:19 AM
#17
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Originally Posted by Ckcrigger,Oct 15 2005, 08:45 PM
It is not like hitting another vehicle of the same weight.
Because you're a small car (for example) 2000 lbs going 45 mph. When you hit a fixed barrier you are carring only so much force. Because of your mass. Your force is ONLY the force of a 2000 lb car going 45 mph. That's where I got f=ma (force = mass x acceleration).
Now if you're a 6000 lb truck going 45 mph and you hit a fixed barrier you'll be carring 3x as much force. So it's exactly like hitting another 6000 lb truck head on.
That's not real world. b/c 6000 lb truck might not always hit another 6000 lb truck. If it did it's more likely do poorly b/c it's hitting another car that has a lot of force. So a big car has a harder job to do great on the crash test.
A real world crash may involve a F150 hitting a Civic. In that case the F150 has 5400 lbs of mass vs 2600 lbs of mass for the civic. The Civic will come out looking 2x as bad.
10-16-2005, 06:26 AM
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Separation of body panels and crumpling of compartments is not necessarily a sign of an unsafe car. Energy dissipation is a primary safety design goal and crumple zones are one way to acheive it.
10-16-2005, 07:19 AM
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The way I see it colliding with a wall would be like colliding with a car of infinite mass.
For example in a head on collision between two Elises each going 22.5 mph for a total of 45 each car would have to receive half of the total energy resulting in relatively equal damage. If the Elise hits the stationary wall at 45 mph (Which by the way is equivalent to just that a car hitting a stationary object not one coming back at the same speed.) The wall merely sits there and can only adsorb energy in the damage it takes, which is very little, thus causing the car to take up nearly all the damage.
Now should we be back to a 2,000 lb. Elise and a 6,000 lb. F-150 in a head on collision at 22.5 mph each for a total of 45 mph. This collision is already less violent because of the extra crumple zone of the truck adsorbs that much more of the impact over the wall. The vehicles are going to adsorb the rest of the energy proportionally to their weight however so the elise essentially adsorbs the trucks inertia minus its own so that the truck only adsorbs 2,000 lb. X 22.5mph (whatever energy would be measured in) as opposed to the Elise adsorbing 4,000 lb. X 22.5mph.
For example in a head on collision between two Elises each going 22.5 mph for a total of 45 each car would have to receive half of the total energy resulting in relatively equal damage. If the Elise hits the stationary wall at 45 mph (Which by the way is equivalent to just that a car hitting a stationary object not one coming back at the same speed.) The wall merely sits there and can only adsorb energy in the damage it takes, which is very little, thus causing the car to take up nearly all the damage.
Now should we be back to a 2,000 lb. Elise and a 6,000 lb. F-150 in a head on collision at 22.5 mph each for a total of 45 mph. This collision is already less violent because of the extra crumple zone of the truck adsorbs that much more of the impact over the wall. The vehicles are going to adsorb the rest of the energy proportionally to their weight however so the elise essentially adsorbs the trucks inertia minus its own so that the truck only adsorbs 2,000 lb. X 22.5mph (whatever energy would be measured in) as opposed to the Elise adsorbing 4,000 lb. X 22.5mph.
10-16-2005, 08:46 AM
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Force is not exactly what you want to look at. It is actually force multiplied by the change in time of the collision that matters. Conservation of momentum is what you're looking for.
p = F*delta_t = mv
Meaning if a 2000kg (4400lb) truck is travelling at 20m/s (45mph), it carries a momentum of 40000 kg-m/s. If T-bones an Elise that is travelling with a lateral speed of 0 m/s (meaning it's effectively travelling orthogonal to the path of the truck.. ie T-bone), in an ideal physics problem, all that momentum would be transferred to the Elise. If we suppose that the truck has a very small crumple zone, or none at all, we can consider this an elastic collision, meaning all momentum is transferred to the Elise. So here we go:
p = mv = (2000kg)(20m/s) = (900kg)(v_elise)
v_elise = 44.4 m/s ~ 99 mph
In layman's terms, the Elise will go from 0 mph laterally, to 99 mph laterally. Of course in reality, there are crumple zones, friction, impact beams, and much more to factor into our little analysis. However, the key here isn't just the 99mph, it's the force that you "feel" due to the impact. This force isn't dependent upon the mass and speed alone, it is dependent also upon time of impact or exchange of momentum from one body to the next.
In our case, suppose the collision takes 1 second to transfer momentum from the truck to the Elise. Keep in mind, 1 second is fairly unrealistic. At 40000 kg-m/s, a 1 second collision would result in 40kN of force hitting you from the side. At 0.5 seconds, that's 80kN. At a realistic 0.1 seconds, thats 400kN.
Just to emphasize this even further, a 3500kg truck hitting a Buick at the same speed generates an impact force of about 180kN.
p = F*delta_t = mv
Meaning if a 2000kg (4400lb) truck is travelling at 20m/s (45mph), it carries a momentum of 40000 kg-m/s. If T-bones an Elise that is travelling with a lateral speed of 0 m/s (meaning it's effectively travelling orthogonal to the path of the truck.. ie T-bone), in an ideal physics problem, all that momentum would be transferred to the Elise. If we suppose that the truck has a very small crumple zone, or none at all, we can consider this an elastic collision, meaning all momentum is transferred to the Elise. So here we go:
p = mv = (2000kg)(20m/s) = (900kg)(v_elise)
v_elise = 44.4 m/s ~ 99 mph
In layman's terms, the Elise will go from 0 mph laterally, to 99 mph laterally. Of course in reality, there are crumple zones, friction, impact beams, and much more to factor into our little analysis. However, the key here isn't just the 99mph, it's the force that you "feel" due to the impact. This force isn't dependent upon the mass and speed alone, it is dependent also upon time of impact or exchange of momentum from one body to the next.
In our case, suppose the collision takes 1 second to transfer momentum from the truck to the Elise. Keep in mind, 1 second is fairly unrealistic. At 40000 kg-m/s, a 1 second collision would result in 40kN of force hitting you from the side. At 0.5 seconds, that's 80kN. At a realistic 0.1 seconds, thats 400kN.
Just to emphasize this even further, a 3500kg truck hitting a Buick at the same speed generates an impact force of about 180kN.