Porsche 918 Spyder Concept

[Giannis]

I see that the rear powertrain carrier appears to be a separate casting from the monocell tub. Curious to understand how it's bonded to the tub. Still impressive by any standards.

My guess would be that this was done in order to be able to replace parts of the chassis in case of a crash. You could keep your passenger shell and get a new rear part, if someone rear-ends you, without having to repair the CF locally.

I see that the rear part is bolted on the passenger shell. It's a discontinuity for the material, unwelcome in any case, but still a well designed one. Even if you replace the whole rear substructure, you will have the same rigidity as when new - locally repairing the CF is never done right.

Another idea would be for heat reasons, but I doubt that the CF, even in such an extreme vehicle, needs to have an expansion joint.

 

[paxah]

 

[klier]

My Lord....no words...

 

[tristatez28lt1]

Also interesting. Other applications only use the CFRP monocell for the passenger cabin, using metal subframes and substructures to support the engine and suspension components. Two typical examples are the Murcielago and the LaFerrari (as is the P1 of which I can't find a proper photo)

I found a partial picture of the P1's chassis

 

[martinbo]

^ The difference is that the fore and aft sections shown above are not structural, load bearing components - they are "merely" outer body work sections. Quite unlike what we see in that sexy rear powertrain carrier in the 918. It's definitely something new and hitherto unseen for me.

 

[Gianclaudio]

Quite unlike what we see in that sexy rear powertrain carrier in the 918. It's definitely something new and hitherto unseen for me.

Martin, thanks for constantly teaching us new words. Your english is fantastic, I love to read your posts.

 

[Soup]

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[Monster]

Also interesting to see how much carbon-fibre monocell extends as structure-bearing material for the engine bay side. You don't see that in anything else to my recollection. Very impressive.

The Carrera GT also used this carbon fiber engine cradle, and it was bolted onto the main monocoque.

From the looks of it the Veyron has a hybrid setup

 

[martinbo]

Fark, of course yes! Monster FTW. How could I ever have forgotten about the Carrera GT. Interesting about the Veyron too. Thanks mate.

 

[Monster]

Fark, of course yes! Monster FTW. How could I ever have forgotten about the Carrera GT. Interesting about the Veyron too. Thanks mate.

It is about time we start having discussions about automotive technology again. I wonder why Ferrari and the P1 chose not to follow Porsche's method. Also the suspensions are no longer in push rod configuration for the front and rear, Ferrari did the same thing from the Enzo to the LaFerrari.

 

[hoffmeister_fan]

It is about time we start having discussions about automotive technology again. I wonder why Ferrari and the P1 chose not to follow Porsche's method. Also the suspensions are no longer in push rod configuration for the front and rear, Ferrari did the same thing from the Enzo to the LaFerrari.

I know there's an "option" (which I clicked) but I figure this point needs to emphasized. That's why I enjoy coming to GCF!

 

[Levi68]

I know there's an "option" (which I clicked) but I figure this point needs to emphasized. That's why I enjoy coming to GCF!

Me too before all those ignorant badge fans came in comparing whose is bigger.

 

[Sunny]

My guess would be that this was done in order to be able to replace parts of the chassis in case of a crash. You could keep your passenger shell and get a new rear part, if someone rear-ends you, without having to repair the CF locally.

I see that the rear part is bolted on the passenger shell. It's a discontinuity for the material, unwelcome in any case, but still a well designed one. Even if you replace the whole rear substructure, you will have the same rigidity as when new - locally repairing the CF is never done right.

Another idea would be for heat reasons, but I doubt that the CF, even in such an extreme vehicle, needs to have an expansion joint.

I think the main reason is the passenger cell and the power-train carrier is made of different kinds of CF - the passenger cell is resin molded vs the power-train carrier being "pre-impregnated woven CF". Check the right bottom corner text below.

 

[Merc1]

Is it me or does it seem that this Porsche, as glorious as it seems, will be outdone by the P1 and the LaFerrari from day one?? Where is the comparison thread?

M

 

[Giannis]

It is about time we start having discussions about automotive technology again.

It's been so long time that it feels unnatural

\I wonder why Ferrari and the P1 chose not to follow Porsche's method.

I'd say cost, cost and development cost. CFRP is a tricky material. Macroscopically it's homogenous, but microscopically it's highly inhomogeneous. It also has an "unnatural" high modulus of elasticity (Young's modulus) and it's not easy to work together with steel. In structures, it's used only for retrofitting reasons, but mainly to increase confinement. The good thing is that it has an amazingly high deformability.

Moving away from structures (my field), into suspensions and substructures, the basic advantage is its weight per unit of strength. This enables the engineers to use very small cross-sections, but on the other hand, a more complex structure is needed in order to have a certain level of rigidity (be it bending, torsional, whatever). That's why you see this complex design in the rear of the 918 and the CGT, with those holes and "weird" shapes. Complexity is also necessary because you can't just take two CFRP pieces and glue them together. You need to have one huge piece, with no 90 degree angles, but soft curves, or the structure will break there. Internally (as in stress flux *) stress "flows" through the material and this has to be smooth.

Steel sub-structures, on the other hand, are simpler to design, require no special molds, no extreme engineering and you can just take a few parts and weld them together. During design stage, you only have to do some simple calculations to see if the welding is enough for the expected loads and then you are good to go.

Also the suspensions are no longer in push rod configuration for the front and rear, Ferrari did the same thing from the Enzo to the LaFerrari.

That's a very interesting question, actually, that's intriguing me, too!

____________________________
* Imagine the flux of internal stresses as if internal forces were water, flowing through a pipe. If the pipe was bent smoothly, the water would flow smoothly, too. If it had sharp bends, then the water would create unwelcome turbulence. As far as internal stresses are concerned, the building up of stress at a certain point must be avoided.

 

[Monster]

Is it me or does it seem that this Porsche, as glorious as it seems, will be outdone by the P1 and the LaFerrari from day one?? Where is the comparison thread?

M

Hyper Car Comparison

 

[martinbo]

Splendid post G. Thank you.

 

[Giannis]

Splendid post G. Thank you.

You are most welcome, Martin!

 

[Giannis]

I think the main reason is the passenger cell and the power-train carrier is made of different kinds of CF - the passenger cell is resin molded vs the power-train carrier being "pre-impregnated woven CF". Check the right bottom corner text below.

In my field, there are two types of CF used:

1. The first type is CF that is bought in rolls as if it was fabric and then it's applied to the structure by coating and smearing it with organic resin, usually epoxy resin. I think this is the same with "resin molded", as it's mentioned in the article. If you use a mold and apply the resin carefully, you will get a very nice visual outcome, because the "carbon fabric" is "perfect" with the carbon fibres being interwined in bunches vertical to each other, thus creating small rectangulars.

2. The second type is usually referred as TRM and it's essentially a pre-impregnated CF material that is applied on the structure, using some high strength, non-organic cement mortar. It's advantages is the fact that it's easier to apply, (most builders have decades of experience working with cement mortars, but not epoxy resin) and higher fire strength (it's not organic, it has no carbon that will be burnt up, thus altering the materials chemical composition), but on the downside it doesn't have the strength of the first type. Anyway, this isn't applicable in the automotive sector.

What I believe the second type of CF mentioned in that article is, is a CF material that's kind of like pre-fried potatoes. It has a certain level of initial treatment, before receiving its final shape. Since, it's pre-impregnated with the agglutinative substance, it won't have the visual outcome of the first type, when molded into the 918 sub-structure.

My guess is that they made the passenger cell using resin molded CF, because this will have a better visual outcome, despite being more expensive, but then they used the cheaper pre-impregnated material for the rear substructure, since the CF won't be exposed.

 

[donkeykong]

Micro site is up.
http://www.porsche.com/microsite/918-spyder/usa.aspx?ws=1