Should new convertibles have taller than roof roll bars?

[CBeck113]

Having been in an couple of accidents myself (and some in karts), I don't see much difference between your head hitting the car roof or hitting the road.

But there´s a difference. If you would have crashed with a convertible, the road would have been much closer to your head, to the point that probably you could have not survived. Instead of being hitted by the roof, you could have been crushed by the road

This is also not true - he would have hit the road. If this accident happened many years ago in an older roadster, then it would have been much more dangerous. In a modern convertable the safety standard is much higher than years ago. It is therefore your choice with which car you want to drive - i.e. cost of safety.
All vehicles undergo a simulated roll-over test, which is actually a static load test on the roof of the car. Convertables are not exempt to these tests, so it is difficult to say if they are that much more dangerous than normal sedans. I can say that I am against the proposed change - it would make most cars look horrible. Think of an Audi TT with the roll hoops sticking out a foot over the windshield. Nope.
Additionally, according to this study http://www-nrd.nhtsa.dot.gov/Pubs/809438.pdf, it would be smarter to get active in SUVs, since there are many more and the % on the road is growing massively vs. convertables. It's better to go after the low-hanging fruits first.

[Andres125sx]

Having been in an couple of accidents myself (and some in karts), I don't see much difference between your head hitting the car roof or hitting the road.

But there´s a difference. If you would have crashed with a convertible, the road would have been much closer to your head, to the point that probably you could have not survived. Instead of being hitted by the roof, you could have been crushed by the road

This is also not true - he would have hit the road. If this accident happened many years ago in an older roadster, then it would have been much more dangerous. In a modern convertable the safety standard is much higher than years ago.

Agree modern convertibles are much safer than years ago, but this does not imply they´re as safe as a car with a roof. Did you watch the video I posted? A car with a roof will not collapse as easy as that one

The windshield of a convertible is a tilted structure with no support, I think the word in english is a corbel, and they will never be as strong as another structure with a support at the end, like the roof. Even when the roof is not at the best angle to help with the load, it changes the whole thing. This is basic in structure calculus. Obviously it will depend on the section of each one, but the difference should be enormous to be comparable in resistance.

I wish I´d be able to explain it better, but my english is not good enough to explain this technical stuff. It´d be nice if some engineer reading this would explain it better. I´m construction engineer, but my english is much worse than my structure knowledge

[mrluke]

All cars have to pass the same test, if they have to increase the section of the 'A' pillar a little bit on the convertible then so be it.

We could come at this another way and say if that Mercedes was only 800kg rather than 1600kg the roof would not have collapsed so much therefore we should ban cars being >1000kg

[Andres125sx]

All cars have to pass the same test

Even if they have to pass the same test, results may differ

I´ve done a search and according to the results they are safe, but as expected, rear seats are much safer, it´s the windshield what collapse, so front passengers suffer more injuries

[youtube]http://www.youtube.com/watch?v=K3aoFbri25M[/youtube]

I personally don´t like this test. Depending on how the car rolls the windshield could receive a direct impact and collapse, or not. Mercedes test I posted in first page is much more reliable to test roof strenght, and as it shows, it will collapse

This could be solved, but...

[tommylommykins]

Does it collapse? It's obviously flexing a lot, but I assume this is just an artefact of absorbing the car's downwards-moving kinetic energy. I assume that as long as the cabin-height is not compromised so much that an occupant will get crushed, then some flexing (or a small collapse) of the pillar is acceptable?

Just to help clear my own thinking, I think there might be two different types of injury in this sort of crash:

• Crushing injuries caused by relative motion between the two different parts of the interior at the same time, where on part squashed an occupant against the other. In a convertible, this might also include when the car is crushing someone into the road. Looking at the mercedes video, this sort of injury might occur there the pillar buckled so much that the seat pushed an occupant into the road
• Ragdoll injuries caused by an occupant bouncing around inside the car, but not where two parts of the car are touching them at the same time. There is a situation where if an occupant of that mercedes was loosely buckled into the car, then they might move upwards in the cabin and hit their head on the road, but they would not be pushed into the road by the car. I assume that this would be just as bad as hitting your head on the roof of a car in a rollover -- but not as bad as getting crushed.

[Richard]

Pop up roll bars already exist. Here's the BMW example:

[youtube]http://youtube.com/watch?v=ceVbJ4RTjuA[/youtube]

Here's an Opel going for a roll, they even interview one of the dummies:

[youtube]http://youtube.com/watch?v=zdOd0fa3w0k[/youtube]

Yes we could have convertibles built like tanks, but where is the evidence get it is actually needed? The fact that something "could" be stronger is not the same as "needs" to be stronger.

Ps A crash test dummy is 188cm tall, so it's not going to nice if you are taller than that in a roll.

[Andres125sx]

Pop up roll bars already exist.

Off course, that´s the reason rear passengers are safer than front ones, pop up roll bars are much more rigid than the windshield, so they´re better protected

But since front passengers suffer more injuries, IMO they should:

A- Reinforce windshield pillars
B- Increase lenght of pop up roll bars
C- Start using front pop up roll bars


A is cheapest and less safe, while C is safest and most expensive

[Richard]

Firstly, where is the evidenceof roll injuries in convertibles? How does that compare to a solid roof cars?

Secondly, the slender pop up stubs on the Opel seem to be rather floppy and bend on first impact. A well designed windshield will be safer than a poorly designed roll bar, regardless of what they look like.

Ps - a windshield works as a cantilever. A corbel is short cantilever that works as a strut and tie. I'll find an image to show that later

[Andres125sx]

Firstly, where is the evidenceof roll injuries in convertibles? How does that compare to a solid roof cars?

Sincerely, I´m interested on the technical part of how to improve it, if it´s really needed or not does not bother me that much

Here they talk about how Citroen did improve safety on their C3 Pluriel, with roof arcs, reinforced seats... Maybe the reinforced arcs are a bit overkilled, but safety wise they´re unbeatable

Secondly, the slender pop up stubs on the Opel seem to be rather floppy and bend on first impact. A well designed windshield will be safer than a poorly designed roll bar, regardless of what they look like.

Of course, but if equally designed, roll bars will be safer by a huge margin

Ps - a windshield works as a cantilever. A corbel is short cantilever that works as a strut and tie. I'll find an image to show that later

Thank you Richard, now I got it. Diccionaries translate both as "voladizo" (the spanish word I was trying to translate) but searching as pictures it´s easy to see the difference

Corbel:


Cantilever:


A windshield works as a cantilever, with no struts it´s a very weak structure

[tommylommykins]

There's nothing wrong with the rollbar flexing is there? As long as it's been designed to do so?

Car manufacturers are very good at designing flexible parts for slowly absorbing energy in crashes. Although a rollbar doesn't have to spread out an impact over time like a crash structure, making it flexible could allow it to be lighter and smaller than a fully rigid rollbar?

[Richard]

Secondly, the slender pop up stubs on the Opel seem to be rather floppy and bend on first impact. A well designed windshield will be safer than a poorly designed roll bar, regardless of what they look like.

Of course, but if equally designed, roll bars will be safer by a huge margin

If well designed they'll have the same safety margin (ie defection and stress). I understand that a straight stub looks stronger than the windshield pillars, but I had a look a Merc convertible and the windshield pillars are very substantial. I think they look thin because they are integrated within the huge expanse of the window.

As for the front seats being more dangerous, are the higher number of injuries because many convertible only have two seats? You'll find there are zero injuries in the back of an MX5!

[Andres125sx]

Secondly, the slender pop up stubs on the Opel seem to be rather floppy and bend on first impact. A well designed windshield will be safer than a poorly designed roll bar, regardless of what they look like.

Of course, but if equally designed, roll bars will be safer by a huge margin

If well designed they'll have the same safety margin (ie defection and stress). I understand that a straight stub looks stronger than the ... ld pillars, but I had a look a Merc convertible and the windshield pillars are very substantial. I think they look thin because they are integrated within the huge expanse of the window.

It´s not a matter of how do they look, but how do they work. A tilted pillar that must take a vertical load applied at its end will never resist as much as a vertical pillar receiving the same load. Add to that the tilted pillar must be much longer to reach the same height (increasing the slenderness factor, if that´s its name in english ), and you have a very weak structure compared to the vertical counterpart

As for the front seats being more dangerous, are the higher number of injuries because many convertible only have two seats? You'll find there are zero injuries in the back of an MX5!

No, that´s the conclusion at one of those rollover crash test I´ve been watching. The idea is there must be two antiroll structures that once you draw a line between the upper part of each one, passenger´s head must be withing that line with some safe margin. On single seaters the front anti-roll structure is the cockpit itself (watch the graph at second post) so the rear antiroll structure must be much higher to keep driver´s head inside the safe area. On road cars the front structure is the windshield wich obviously is much higher than the cockpit of a single seater, so the rear structure doesn´t need to be that high. But once one of those structures fail, the line wich defines the safe area goes down. When it´s the windshield what fails the line goes down from the front end, so it´s front passengers head what goes outside the safe area first.

[RZS10]

Well it's the lack of the b pillar... basically... they could use an A pillar with a vertical support, but that would look sh** and no one would want a car with that

[neilbah]

I dont have any statistics but when i was involved in a car crash and ended up in hospital with a bad hand injury the surgeons said convertibles cause lots of hand injuries - peoples arms can be flung up without their control and touch the road as the car flips. In my own crash (not a convertible) the airbag deploying possibly contributed to my arm smashing the drivers side window and as the car rolled it touched the road, briefly, but enough to remove a lot of skin, damage tissue and fracture bones. I guess theres not alot that can be done to stop issues like that without racecar side netting- thats in a roofed car, without a roof it wouldnt help.. The only solution i can think of to protect passengers in a convertible during rollover is a rather big air bag that deploys from under the windscreen to a higher height than the roof line and over the front two seats. not very easy to achieve. or a device a little like the cycling air bag helmet.

[Richard]

It´s not a matter of how do they look, but how do they work. A tilted pillar that must take a vertical load applied at its end will never resist as much as a vertical pillar receiving the same load.

Never??? A well designed convertible could be stonger in a roll than a badly designed saloon.

The windshield on any car will be designed to resist the applied load within the deflection criteria. If a convertible has a more slender structure then it'll have a thicker section to give it enough strength. That's what engineers do.