Weight

[popuptoaster]

Mclaren F1 is the only car ever to have driven away from the MIRA 40mph crash test into a concrete block< the passenger cell wasnt deformed at all so i suspect carbon fibre is much better to crash with than steel or aluminium.

The shocking video shows the famous supercar undergoing a simulated head-on collision at the Motor Industry Research Association (MIRA) crash testing facility in the UK. The clip clearly demonstrates the benefits of the car’s carbon fiber monocoque structure, which proved so secure that the Mac Daddy became the only car ever to be driven away from a MIRA test. Be warned: this video can induce uncontrollable sobbing.

http://uk.youtube.com/watch?v=mUPq760LC00

 

[wemorgan]

Mclaren F1 is the only car ever to have driven away from the MIRA 40mph crash test into a concrete block< the passenger cell wasnt deformed at all so i suspect carbon fibre is much better to crash with than steel or aluminium.

But near zero deformation is not the measure of a high performing crash safety vehicle. One day maybe I'll be involved in the design of a carbon fibre vehicle and I'll be able to answer such questions with authority, but for the moment I just have my opinion.

 

[Mudster]

Is all this weight to do with the high speed stability as many of these cars obtain what were previously only racing car top speeds.....

 

[popuptoaster]

But near zero deformation is not the measure of a high performing crash safety vehicle. One day maybe I'll be involved in the design of a carbon fibre vehicle and I'll be able to answer such questions with authority, but for the moment I just have my opinion.

Everytime i see anything about car safety a rigid passenger cell is top of the list, the rest of the car is sposed to crumple up and absorb the shock and leave the passenger area undamaged, which is exactly what happened with the Mclaren in the video i posted.

 

[popuptoaster]

Is all this weight to do with the high speed stability as many of these cars obtain what were previously only racing car top speeds.....

Thats aerodynamics not weight, if you take the lil black plastic things off the back window pillars of a granada scorpio hatchback (they only weigh a few ounces) it feels really horrible and squirmy at the back at high speed because the airflow round the back creates lift without them, i took em off mine when i was smoothing it off, soon put them back on! Grannies are pretty heavy but it was still trying to take off. :S

 

[wemorgan]

Everytime i see anything about car safety a rigid passenger cell is top of the list, the rest of the car is sposed to crumple up and absorb the shock and leave the passenger area undamaged, which is exactly what happened with the Mclaren in the video i posted.

I slightly misread your original post, sorry. What I meant to comment on was that a very strong body shell with minimal deformation is not the method for absorbing the energy in an occupant friendly manner. The lack of intrusion may stop your legs and chest being crushed, but on the other hand high deceleration will tear your internal organs apart.

I suspect that the SLR has very high decelerations managed by expensive occupant restraint systems.

 

[wemorgan]

Apologies for sounding like a cracked record, maybe it's only me interested in this thread

I'm interested to know whether 15kg is the total mass or just the seat frame? A friend tried to lift one up with the restraint systems, motors and rails and could barely lift it up. I guess he needs to eat more spinach.

Returning to the OP, it is interesting to look at the carbon fibre SLR and wonder where the 1800kg comes from. Putting aside some obvious heavy items such as, body shell, closures, powertrain, wheels, there must some other items overlooked that weigh more than expected?

I'd like to know the mass of the body structure and front sub-frame. It would be interesting to compare it to equivalent steel and aluminium shells. Both are very efficient energy absorbers that can be easily tuned to give a desired deceleration pulse, to enable occupants to survive a crash. I'm not sure that is so true for carbon fibre. Whilst it is indeed very light the failure mechanism is very complicated. As seen in F1, minimal intrusion is designed at the expense of a very high deceleration pulse. This is not such a problem in F1 where occupants are young, healthy and restrained in a harness. But in road vehicles needing to pass FMVSS legislation I suspect very advanced heavy restraint systems would be required to manage this deceleration pulse.

I spoke to a friend who's worked with carbon fibre vehicles and his assessment of them is:
There is little detailed understanding of the failure mechanisms of carbon fibre. As such carbon fibre vehicles tend to be over designed, which adds weight. With no deformation allowed in the occupant cell sacrificial carbon fibre cones are mounted on the front. These cones tend to be attached onto an aluminium cast front sub-frame. This sub frame is also designed to not deform under crash loads. Again this leads to a very thick walled heavy casting. For packaging reasons the carbon fibre cones tend to be short. So the vehicle has to decelerate over this short distance, which leads to a high declaration pulse. In general it's the lack of knowledge of carbon fibre as a deforming material that leads to very stiff/strong & ultimately heavy designs.

Returning to the Mclaren F1 car, I'm told that whilst the performance to a front crash is reasonable it would not pass a side pole impact, with the vehicle simply splitting in half.

Some food for thought I hope.

 

[Ade B]

So it would seem the weight loss benefits of CF are lost due to safety measures. The trade off would be a very stiff chassis which would help handling presumably..

Looking at other 'supercars' Murcielago 1665kg, R8 1565kg, 599GTB 1580kg, Veyron 1888kg..

None of them are lightweights..

Ade

 

[wemorgan]

Also, from Wikipedia:

Ferrari F40 V8 twin turbo: 1100kg
Ferrari F50 V12: 1349kg
Ferrari Enzo V12: 1365kg

 

[popuptoaster]

I know big alloy rims and tyres are bloody heavy, as are big brakes and the diff and drivetrain assemblies required to take the torque of big engines, even glass is pretty heavy if you need a big raked screen thats contributing to the strength of the shell.

Guess i wont be buying a Mclaren for its safety rating then!

 

[Mr E]

I was intrigued by the posting that said CF failure is not well understood, so i made couple of calls...

It appears that it is well understood by the right people. A lot of CF manufacturing would appear to be the replication of parts made with more traditional materials and an inadequate understanding of it's properties would make CF appear to fail in strange ways.

At this point the conversation started to go way over my head. But the upshot was that a CF structure can be designed to deform in the same way as a steel / metal structure. He quoted the SLR which has two cones of about 300mm length as the primary absorption component. When they do their job - which they have been correctly designed to do by the correct and precise alignment of the material - the car suffers no more deceleration force than the average car and can dissipate 4 times more energy that the equivalent steel structure.

 

[Dieselman]

the car suffers no more deceleration force than the average car and can dissipate 4 times more energy that the equivalent steel structure.

Very informative, thank you.

I'm confused by this bit. If it can dissipate 4 times the energy in the same distace doesn't that mean that the deceleration forces are also 4 times greater.?

 

[BTB 500]

If it can dissipate 4 times the energy in the same distace doesn't that mean that the deceleration forces are also 4 times greater.?

Deceleration forces are based on time though, rather than distance.

 

[popuptoaster]

Its more likely that it can be 4 times shorter (and lighter) than a steel structure to do the same job than that you'd expect it to save you from a 4 times bigger impact.

 

[wemorgan]

Given a constant impact velocity, as is always the case in legislative impact tests and a zero final velocity (ignoring rebound velocity for easy of explanation) then:

Deceleration = impact velocity^2 / distance x 2

So the deceleration is inversely proportional to the amount of distance the vehicle slows down over.

The time that impact event takes is then a consequence of the impact velocity and available distance. (t= v/a)

I would argue that a conventional mid size vehicle would have nearer to 500mm of crush space, roughly 50% more than the SLR.

Also it's interesting to note that the deceleration of the vehicle is not related to the vehicle's mass and therefore also not to the kinetic energy.
But where the mass of the vehicle does influence the design is the amount of energy the structure needs to absorb. This will influence the material gauge and grade but not it's overall length.

 

[Mactech]

This issue of weight really has grown in direct proportion to our lack of tolerance of danger or to put it another way, safety. That and our expectations of what luxuries we now take for granted.
It is the same in road cars and race cars. New safety regulations demand ever more design ingenuity if the car weight is not to spiral out of control.
I had a stark reminder of this a few years ago when the Le Mans Bentley chassis weighed 68kgs and the electronics weighed 70!
Carbon structures are kilo for kilo much more effective than steels or alloy at dissipating energy as a correctly designed carbon structure will break fibres and be reduced to dust as opposed to the relatively low energy absorption of bending metal. £ for £ it’s a different story!
There are now standards which didn’t exist in the past and designers do try to engineer cars to pass test rather than be ‘totally safe’. But that’s a big step up from the past. As we learn more, and if the economics allow we will get even safer cars in the future.
Road car seats can be really heavy. In a Bentley Conti GT I could save 60Kgs per side by fitting racing seats!
The McLaren F1 may well have had some issues with the side impact test, but as a race car with the driver in the middle I have seen some massive shunts with the driver escaping unscathed.
But they could be expensive! The repair bill for this one crept in at just under a quarter of a million!

 

[Mr E]

I'm confused by this bit. If it can dissipate 4 times the energy in the same distace doesn't that mean that the deceleration forces are also 4 times greater.?

I did say that it started going over my head at this point....

The SLR has far more than 300mm of deformation space - that's just the length of the cones (which I believe are ahead of the centre of the front wheel). The engine sits mainly behind the front wheels. I'm told that the "sacrificial" cones are mounted to an aluminium cross member that sits across two deformable structures that run back to the bulkhead.

 

[Robbie T]

I haven't got a great deal of time (or expertise) to add to this, and i certainly can't comment on the deformation of carbon fibre as that's well over my head!

I worked out the seat weight by adding up the key components :

Metal base-frame including motors 8.5kgs
Carbon tub 3kgs
Leather pads 3.5kg's total (roughly)

To be honest, it was just to say that they're not that heavy compared to a normal seat and certainly not 60kg's.

The crash structure is indeed, as Mr E stated, two carbon cones attached to a cross brace which has a rope running through it. In the case of a centre impact the rope shares the impact across the two cones. The engine is mounted quite far back in the bay also, the back end of the block is basically under the dash (hence why it's engine out for a spark plug change).

 

[wemorgan]

Thanks. I'm happy to be corrected over the weight of the SLR seat. Facts are always preferred over my guess work.

On the very pertinent issue of designing cars to pass legislation rather than be totally safe I'm in agreement. EuroNCAP has made a huge difference in people's perception to safety. But people need to understand the total risk to them on the open road. Bigger is not always best. An expensive car doesn't male a good driver. Airbags don't exempt you from risk. A nominal 50g deceleration from a frontal EuroNCAP test really really really hurts.