[xj-s] Carbon-Fiber Body Panel Replacement

Since it is winter and I have nothing better to do than consider
oddities that I shouldn’t, for the last couple of weeks I have been
casually researching carbon-fiber technology. My main motivation
for this is irrelevant, but brought me to the idea of
re-engineering the air intake on the '91. My idea is to put two
cold air scoops on the front of the hood, and blow it directly into
the intakes. Of course, the intake filter housings need to be
modified too. I have a few questions regarding what I am
contemplating:

1.) Has anyone here tried this? If so, were you sucessful? I
realize a hood is a pretty large part, and I have never worked with
carbon-fiber before. I do plan some smaller projects first, but
those don’t require a foam inner core.

2.) I am wanting to form an entirely new hood from carbon- fiber.
Has anyone here worked with it? I am considering both a female
mold and a male plug for my form. What would you recommend?

3.) Is a composite hood able to take the temperatures, and if so,
what epoxies would you recommend? I am currently looking at West
Systems, but if there is something better, I would like to know.

 I also intend for this custom hood to have vents that can cool 

the engine quicker. This is the one area where I have an issue
with the original design. I used to own a Nissan 280 ZX that had a
single vent above the turbo charger, and even though I ran it easy
for the last 15 minutes before I ever shut the engine down, there
was still an incredible amount of heat that rose through the vent.
Why didn’t Jaguar vent the hood on the XJS? They had to know heat
in these cars was a problem, didn’t they???

 BTW, if this project turns out well, I am considering an 

improved air-dam in the front, with brake ducts and a better design
which actually exerts some downforce on the nose at lower speeds.
This is a bit more complicated, but certainly not impossible. As a
pilot, I do know something of airflow, but cannot really think of
any way to do this but via a speed variable wing. This could be
easily accomplished by tying into the speed sensor and having a
stepper motor, controlled by a driver circuit with an inter-lock
system to allieviate power drain. Any ideas?

Matt
'95 VDP, '91 XJS V12–
Trilithon
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In reply to a message from Trilithon sent Sat 9 Feb 2008:

I’d be interested in an improved air dam for my track car, and
a carbon hood. I’m already running cold air intake, but vents
would be welcomed.

Keep me posted.

Charles–
Charles
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In reply to a message from Trilithon sent Sat 9 Feb 2008:

Go have a close look at a facelift 6.0L XJS (94-95) It has a pretty
good cold air induction that is easily modified to a great cold air
induction. There is a bulge in the center of the hood (I think it
was on the 6 cylinder cars before it was used on the V12). There
are brake cooling ducts in the front bumper and airdam that can be
hooked up to the back plates with a quick bit of flexible tubing. I
don’t have any data about downforce, but the later bumpers sure
look like they do a better job of it than the older cars. Any thing
you take off the car and replace with a lighter unit is a good
thing. And I do agree that venting the bonnet/hood is a good idea,
as long as you are sure that you aren’t disturbing the normal flow
of air out of the engine bay while moving.–
Penfold99
Atlanta, GA, United States
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In reply to a message from Penfold99 sent Sat 9 Feb 2008:

Hi Charles, Penfold99,

 I will most definitely keep all informed, and I plan on 

posting pictures as well, which I have never done on this site.
But as this project is still in it’s infancy, and I am still doing
alot of design considerations, I don’t expect a finished hood until
May at the earliest. If all goes well, then I design the airdam
with integral wing.

 And I agree... the face-lift XJS's is pretty good on both 

counts. But I want to incorporate my own designs here. Besides,
getting rid of the steel hood with give some weight reduction, and
I doubt the airdam from a face-lift model will be as light as my
design, unless those dams are carbon-fiber as well. This is the
main reason for incorporating the wing, and for making it speed
variable: To compensate for the weight reduction at higher
speeds. A secondary reason is to experiment with it throughout a
wide range of speeds. Then I can program a PIC microcontroller to
adjust according to a map. I know wind speed will also have to be
part of this mapping system. These inputs to the controller will
create an output to a driver circuit in order to turn a stepper
motor to the premapped position. Hopefully, this will improve
handling beyond stock of any production Jag, too.

 The problem will become more complicated if I decide to 

continue further with ground effects and a second, under-car wing
incorporated into a rear, under-bumper sweep. I think in this case
I would need to incorporate an accelerometer as an input to the
control unit, and alot more thought. :wink: But this is much further
down the road than I am willing to contemplate right now.

 Thanks for the responses, BTW.  I will keep the forum informed.

Matt
'95 VDP, '91 XSJ V12–
Trilithon
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In reply to a message from Trilithon sent Sun 10 Feb 2008:

If you build it, they will come…or at least I’ll come
and buy a hood from you!–
Rob Wade
Windsor Ontario, Canada
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In reply to a message from Trilithon sent Sat 9 Feb 2008:

On their web site V12sales.com lists a lightweight fiberglass hood
and they also make hood vents that can be used with that hood or a
standard metal hood, They look good too. Carbon fiber is great
stuff (I am a fairly hardcore triathlete/cyclist)and in the cycling
industry it is used for almost everything. The main advantage of
using carbon fiber in composites is to make for a very high strenth
to weight ratio and stiffness for hightly stressed parts and for
vibration dampening. I am not sure that the additional cost of
carbonfiber over fiberglass will be worth it for the hood. The hood
is not a highlt stressed body panel and although carbon will be
lighter than fiberglass how much lighter? One thing where it might
be better is I have seen lightweight fiberglass hoods and they
shake and flex in the center which I find quite unsightly when
driving a road car. Perhaps with carbon fiber it could be made
stiff enough not to flex but then it may not be that light weight.–
The original message included these comments:

 I also intend for this custom hood to have vents that can cool 

the engine quicker. This is the one area where I have an issue
with the original design. I used to own a Nissan 280 ZX that had a


Mike Blair 2x94 XJS(6.0 Coupe&4.0 Convert)+85 HE&96XJ12
powhatan/Va, United States
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In reply to a message from eyedoc sent Mon 11 Feb 2008:

Hi eyedoc,

 You bring up some very imprortant considerations that would 

bear close scrutiny, except that I have done so already. I do have
a few good reasons for choosing carbon-fibre over fiberglass, and I
shall try and enumerate them here.

1.) Fiberglass doesn’t have the modulus of CF. As such, it can
also use lighter core materials and still be stronger.

2.) Fiberglass requires more layers of heavier glass than CF
does. More layers means more matrix means more weight.

3.) Fiberglass, believe it or not, and in fact all glass, is a
liquid. Yes, you saw me correctly, glass is a liquid at normal
ambient temperatures. This means it is more prone to failure over
a long period of time. I know this probably won’t have much effect
in my lifetime, but could be an issue for those in the future.

I must qualify this by saying that I doubt that I higherly doubt
the surface tension of the individual fibers would ever allow much
cohesion, but it is a consideration that an amateur like myself
must consider, given the 200+ F temps that are involved here. I
want my hood/bonnet to last.

4.) Though heavier than Kevlar, CF is much easier to work with.
CF also has a higher specific modulus than Kevlar or fiberglass.
Higher than Kevlar by a factor of roughly 1.5, and higher than
fiberglass by a factor of roughly 4.75.

I think that with all things considered, the advantages of kevlar
are not worth the extra effort, nor are they more labor intensive,
but there are no real advantages to Kevlar over CF, unless you need
a highly impact resistant composite. I don’ know about you, but I
don’t have that many people shooting at my engine. Door panels
might be a different story. :wink:

But fiberglass is out for me… it is not as strong or flexible,
and it has other qualities that make me think it is inferior to
Kevlar or CF. Though I have used it with no complains, it just
isn’t as tough.

BTW, my design will allow very little flextion. This will be
accomplished by small spines of reinforcment material that will
prevent it. It may actually look like an H.R.Geiger painting under
the hood/bonnet, but except for the mounting points, their
extentions being necessary for stiffness at connecting points, and
a few other plates to captivate nuts and screws, it will be
entirely of carbon-fibre and the core of either honeycomb or foam.
Probably foam for the prototype.

Thanks for posting on this topic. I encourage disagreement, as it
leads to more thought. Your comments made me re-think the whole
thing, even if my decision remains the same.

Matt
'95 VDP, '91 XJS V12–
The original message included these comments:

On their web site V12sales.com lists a lightweight fiberglass hood
and they also make hood vents that can be used with that hood or a
standard metal hood, They look good too. Carbon fiber is great
stuff (I am a fairly hardcore triathlete/cyclist)and in the cycling
lighter than fiberglass how much lighter? One thing where it might
be better is I have seen lightweight fiberglass hoods and they
shake and flex in the center which I find quite unsightly when
driving a road car. Perhaps with carbon fiber it could be made
stiff enough not to flex but then it may not be that light weight.
Mike Blair 2x94 XJS(6.0 Coupe&4.0 Convert)+85 HE&96XJ12
powhatan/Va, United States


Trilithon
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In reply to a message from Trilithon sent Mon 11 Feb 2008:

I was thinking of using an extra bonnet as a male mold to make
a CF hood. I love the V12Sales guys, but I was wanting to try
this myself. I know the part would ‘grow’ a little, but there
should be enough room on the sides for that. I contacted Fiber
Glast which sells CF materials and they seem to think it’s
possible to do.

I would not suggest this for a street car, but my application
is less exact. I just would need to work out the reinforcing
of it…but it would not hinge, just lift off.–
Charles
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In reply to a message from Charles sent Mon 11 Feb 2008:

Hi Charles,

I considered this at first too Charles, and I have now doubt
whatsoever this would work. And if Fibre Glast said they thought
it would work, I am confident it would The way I would do it would
be to form the outer skin only over the original hood/bonnet,
unmold, and then position my core and do the rest of the lay-up.
But this would also require a flawless hood, and it couldn’t be
used to mold negetive alterations unless you cut holes in the extra
bonnet, effectively destroying it, and I don’t want to alter the
original hood in any permanent way.

However, a female mould would allow both negetive and positive
alterations, as I am leaning toward expandable foam as the mould
material. Protrusions could be modelled, and then stuch on the
original hood temporarily, and negetives could be modelled and
stuck onto the mold once the hood was parted from it. My design
requires both, so I must use a female mold, as opposed to a male
plug. Alternatively, I suppose I could use your method and then
just cut the holes where I needed them and make other CF pieces to
fit them, but I believe this could compromise strucural integrity,
which I am not willing to compromise on.

BTW, if you are seriously considering this and you are not in a
rush, I should have mine done within six months, and I intend on
reporting and showing pictures of it here. Once I get it made,
testing should only take a couple of days, and report whether my
bracing system worked as expected. That way, if mine worked, you
could use it on yours, and if it didn’t, you wouldn’t have wasted
the money, or come up with a superior solution. I would say
benefit from my mistakes if you can. My design will be based in
Jaguar’s original bracing structure, but with several structural
components added. That is why a core is important.

The core is integral to the whole structural integrity. If done
improperly, this can cause vibrations due to airflow over the hood,
expessed harmonically at certain speeds, much as the barring and
bracing functions in a stringed musical instrument. The barring in
that case is primarily for strength, but also for attenuation of
resonance of certain frequencies, and amplification of others.
This would probably not be a problem if I made the hood 1/2’’ thick,
but this would be way too expensive, and I would be ashamed of such
a sloppy design.

Glad to see someone else thinking about it. And remember, CF
doesn’t rust. Good luck with it.

Matt
'95 VDP, '91 XJS V12–
The original message included these comments:

I was thinking of using an extra bonnet as a male mold to make
a CF hood. I love the V12Sales guys, but I was wanting to try
this myself. I know the part would ‘grow’ a little, but there
should be enough room on the sides for that. I contacted Fiber
Glast which sells CF materials and they seem to think it’s
possible to do.
I would not suggest this for a street car, but my application
is less exact. I just would need to work out the reinforcing
of it…but it would not hinge, just lift off.
Charles


Trilithon
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In reply to a message from Trilithon sent Tue 12 Feb 2008:

Matt,

I’m in no rush, I can wait. And I have no superior solution!
In fact, I’m just dying to get the car back so I can drive it
on the track. It’s been thru a new engine in the past year or
more, so waiting to make it lighter would be icing on the cake
for me.

Cheers.–
The original message included these comments:

BTW, if you are seriously considering this and you are not in a
rush, I should have mine done within six months, and I intend on
reporting and showing pictures of it here. Once I get it made,
bracing system worked as expected. That way, if mine worked, you
could use it on yours, and if it didn’t, you wouldn’t have wasted
the money, or come up with a superior solution. I would say


Charles
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I’m contemplating making the entire front end clamshell, in fibreglass. Anyone know of anyone that has done this?

A noble, and intriguing project, … but why? It’s difficult enough to stop the damn thing bursting in flames if you look at it funny. I’d be spending my resources on other stuff.