9 replies to this topic

[Duckman]

In the recent opinion thread about our pop-up headlights, Flash questioned the rigidity of our unibody cars if we switched the steel fenders for FB. Here's the link to the parts.

http://www.gt-racing...pdfs/GTR968.pdf

So, what about it? How would it affect the torsional strength of our cars? Handling? Would carbon fiber react differently than fiberglass?

I'm really just curious and not considering the change. However, if I ever had body damage in need of repair, I'd consider it in a heartbeat.

What do you think?

[flash]

i didn't mean to incinuate that the world would come to an end, but merely that some compensation needed to be done if one were to go that route - it is a frequent thing to replace sheet metal with fiberglass or carbon fiber in a race car, but then you have also added a bunch of tubes to reinforce the chassis

the concept of a unibody is an old one - rather than provide a frame for rigidity in the chassis, instead a structure of load transferring interconnected panels is used - the key to this working though is that all of the panels have to be there, intact, and reinforced in key areas

unibody construction is used to reduce mass, while still providing the necessary rigidity and crash protection - it also tends to provide a less "truckish" feel and ride - it is a very cost effective way to build a car

the downside of unibody cars is increased cost to repair, and the lack of ability to adequately repair them in a severe crash - the results are frequently beyond the ability to restore the car at a cost below the value of that car

as for fenders and stiffness, to some degree, every component of the body is a structural member - you will find in a unibody car, a lot more nuts and bolts holding things together along torsional axes than you would on a framed car - for example, a fender on a framed car might have 4 or 5 bolts along the longitudinal line, but a unibody car will often have as many as 10 and often larger in size - this is to hold that member in place, and allow the sheet metal to transfer and absorb load

as for replacing with other materials, while carbon fiber is actually stiffer than sheet metal, the problem is going to be the connection - if you reinforced the mounting edge with embedded metal, then a carbon fiber fender could probably add as much rigidity as a sheet metal fender - the cost of course would increase - without it though, it would rip under load at the connection points (been there done that)

over the years, i've had a chance to play around with some composite body panels on a number of cars, both framed and unibody - in fact, the entire front end of my last toy was fiberglass - the change is noticeable, but not unmanageable - reinforcing members are usually pretty easy to add to most cars

another thing to consider is the effect of mass - if you reduce a car's mass, you reduce the load on the chassis, and in turn the amount of work any given component is doing - so, in theory, you could mitigate a lot of the structural loss by eliminating a lot of weight from the car in non-structural areas (wheels, seats, etc)

the hard part though is getting fiberglass to look good - that takes a LOT of work - vacuum formed carbon fiber is easier in some respects, as the panels are generally closer to the oem size and shape, but it still has fitment issues - these kinds of panels are generally fine for race cars, but fail to come up to the aesthetic standards that most people want in a street car

did that help?

[xrad]

the front fenders are bolted on to a box type frame. I serioursly doubt the front fenders add any significant 'frame' rigidity in terms of handling(the majority of the mounting points are along the top edge and the bottom edge is nearly free). Although, they do add crash support. I have seen several 968 accidents where the front fenders were smashed. The frame underneath was prety much 'fine' and in two cases, the shock had punched through the front upper shock mount sheet metal lending creedence to the stiffness of the underlying frame(or the weakness of the shock mount!).

As Flash says: there will be fitment issues with fiberglass (such as 'warpage') but, after having worked with both, if you don't have the sheetmetal tools or experience or know how to lay-up fiberglass, either one will be difficult. However, there are many great bodyshops that have what you need.

Both sheetmetal and fiberglass have their +s and -s. Both can have beautiful results.

as for the back end, if you do any wheel well work or if you remove any of the body sheetmetal, you would need to add structural rienforcement with the change to fiberglass or the like.

If you need a body repair, I would suggest getting an original sheetmetal panel because it will definately fit better out of the box.

Edited by xrad, 06 August 2008 - 09:16 AM.

[flash]

actually, the shape of the fender has a lot to do with how much it adds - the fact that it curves in two directions essentially adds a longitudinal tube to the front end, making it harder to twist the two frame rails relative to each other - the fender will resist buckling due to those curvatures, and in turn stiffen the chassis - it is much the same as the little bends along the edges of heat shields

[Duckman]

Thanks, guys. Interesting input.

For now, I'm glad I don't NEED the replacements.

[flash]

lol - me too - that would push me right into having to decide how far to go - i foresee having a complete carbon fiber front end made and a front end structure to reinforce everything

i'm determined to be saving that for the race car, but you never know......................

[xrad]

Flash: you are right, no doubt the complex curves of the outer front panels add strength to the design (which of course would be stronger if they were epoxied or welded along the length of the contact surfaces..not good for ease of repairs, though) However, I was just stating that although the outer quarter panels do add strength, but it is not significant compared with the inner panels of the engine bay, mini subframe, or greenhouse components of our unibody design. I am curious to know exactly how much!

There is a four wall box frame on two sides of the engine and three crossmembers (firewall, engine mount, front edge) producing a 5 sided structure. Of course, there are some torsional stresses to this design which would be diminished with some upright triangular bracing through the firewall, tied to the shock mounts, and extending to front edge, but not necessary for street use. This is why I feel you can add fiberglass quarter panels(which will themselves add strength) to the front of the car without the need for 'additional' internal bracing. If you are on the track, that is another story.

Also of note, just tying the shock towers together with a simple brace adds a good amount horizontal rigidity, which shifts the stress to other points of the inner panels, and completes the vertical 'box' 6th side. Anybody notice a windshield crack 'grow' after adding a strut brace?

[flash]

absolutely - the outers don't do as much as the inners - sandwiching, like in the rear, does add, but i can't speak to how much

if you have an upper and lower brace at the control arms, the reinforcing that would need to be done is likely only up at the very front - there is a good shot of this in the red RS - basically it is something to prevent one frame arm from rotating on the central axis relative to the other, as well as bowing up and down - with the fenders in place this is minimized, due to the nature or the curvatures

i'me not talking about the car becoming flimsy though, so i don't mean to paint that picture - remember, even in stock form i find the car to be soft and in need of reinforcement - anything removed would only exacerbate that situation, and i'd want to put something back in there to make up for it

[xrad]

I agree .... and find that this is a common theme with many unibodies...especially after spring/strut upgrade

[flash]

lol - yup - long slippery slope for sure

when i finally get to the race car, i am basically removing the body and adding a space tube frame