I need some help from more experienced wrenchers/fabricators. I'll try to give the short version of the story:
I have a Lotus Elise, which is street driven on weekends and otherwise a track/autocross toy. I purchased a very thin single-skin CF roof for it (NASA will not let me run the OEM soft top on track, which is heavier anyway, and topless it's like having an air brake), but the original method of mounting it (essentially straps front and rear) failed on VIR's back straight at around 110mph in spectacular fashion (this is another story I can share later). The top is still solid, though not as pretty. I need a better way to attach it.
I can probably fabricate, or have someone local fabricate, some brackets that will bolt to the existing vehicle structure so I have more solid mounting points, and that should be pretty simple. My question is, what is the best way to firmly attach the thin carbon fiber to the brackets below it? Options as I see them:
1) aerocatches - bulky when mounting 4 on a small top, and would require much more complicated brackets underneath
2) Rivets - will 1-2 rivets at each corner really hold the roof on at speed, considering I now know the airflow would like to take it for a ride? What type of rivet would hold through this very thin CF and a metal or CF-angle bracket underneath?
3) rivet on a 'doubler plate' and bolt through that to the bracket underneath. This is likely a much less clean looking install and more complicated/more holes to drill in the roof.
What else?
I'll post some pics in a minute to help out...
roof:
here's the curvature I'm dealing with at the outer edges, but its much flatter inboard a few inches:
Rear mounting point:
front mounting point:
No matter what you do, you will need to spread the load over a fairly large area to keep from pulling thru the carbon fiber skin. A doubler that is bonded to the inside surface and has a few rivets to locate/pull-up would be the easiest. 1-2 rivets per attachment will not work for long. Since the material is thin, you will have to use domed head rivets, not countersunk. The aero loads on the roof panel have a very high lifting force near the front, which then lets the air rip it it up and over more easily. I would put the majority of the strength in the front of the roof panel.
stafford1500 speaks truth
I'd probably recommend avoiding drilling through the panel... since layup and fiber direction and composition is unknown. CF is really bad in handling shear. Better to bond something to the underside that increases the thickness of the panel there and helps spread the load out... or add another stiffening rib behind the front one that allows you to put a small plywood spacer to bolt through the webs.
two other things...
is there a gasket in the front of the panel that keeps any air from getting under the panel? If not, even after you find a better way to affix this... I'd recommend taping the whole from seam so there's no chance of air getting to the underside of the front panel.
can you give us rough dimensions of the panel? Any guesses for Cl stafford?
Thanks for the thoughtful replies.
Sleepyhead, there is a stock gasket of sorts under the CF panel (the panel sits on top of it, basically), so it would keep water out, but there is a gap up in the airstream that likely caused the original failure. I'll share a photo in a minute. Part of re-using this panel would entail finding an appropriate seal for the front (likely re-engineer an OEM hard top seal).
The roof is roughly 44" width by 30" length. It's 0.15" thick at the edges.
Forgive me for being a bit of a composites novice, but...
Is it feasible to reinforce the mounting areas with additional CF? I have some twill fabric and epoxy. My only experience is wet-laying some flat pieces (speaker blanks and switch mounting plates basically). That is, could I sand down an area underneath and wet-lay up a few more layers to add some strength and thickness for fixing it? Someone else also pointed me toward pre-made CF angles - is it possible to securely bond those underneath without using additional hardware
Both of you mention bonding something to the existing CF - what product is best for that?
Where could I find appropriate doubler or backing plates?
The transverse ribs that are visible on the underside of the panel are very thin, and cracked in several places from the prior attachment failure; hardware/straps ripped out of the rib after air got under the front (I figure thats what happened- it happened in a split second). So similar to above, I was considering adding 1-2 layers of CF wet-laid over those ribs to repair the cracks and strengthen them, at least in the front. The front rib holds two forward-facing tabs that fit into slots in the windshield frame; they're not really a secure attachment point but are useful for properly locating the roof in place and keep the middle somewhat pulled down.
Some more pics -
leading edge:
front tabs I referred to - see cracks in rib next to them:
Another shot of the rib with crack:
How about pop riveting a piece of aluminum right angle stock all the way across the rib on the inside and a couple additional flat pieces of aluminum on the windshield side also sandwiching the CF in the bracket area to spread the load. Use wide head rivets on the CF side where you're only riveting the CF to the aluminum. You could always sandwich the whole rib.
honestly, I'd guess the straps stretched which allowed the panel to lift up enough to increase its angle of attack, which created enough lift to exceed the tensile strength of the straps. but, I'll be interested to hear other theories.
before I suggest fixes... can I ask if this is a product of a company? or was this built up by someone on the lotus forums, or the like? I ask because it looks like this is built of 1 layer of carbon, and two layers of fiberglass.
Myself, I would run a piece of aluminum on the outside front to back on each side, maybe something like a 1" round rod cut down the center and rounded nicely on the ends. Inside I would run a similar piece, but probably more like 1/2x1.5" or so. If you ran the ribs from where the front mount is to where the rear is, all your attachments could be to the aluminum, and if you are careful you can drill and tap up from the bottom rib into the top rib but not quite all the way through, so you keep a clean line on the outside. Paint the outer rib either black to minimize it or body color to feature it.
In reply to sleepyhead :
It was sold originally by Blackwatch racing, but made by someone else who they wont disclose - "their CF guy". It was advertised as "close-weave carbon fiber with core mat for lightweight strength. Vacuum bagged construction for lightest weight and smooth surface." There are a handful of lotus track guys using them who have not had the experience I had. The top is no longer available, but google can still find the page:
http://www.blackwatchracing.com/product-p/1135.htm
In reply to NOT A TA :
I could do that, but if I just strengthen those ribs with aluminum, then what? bolt a new custom bracket to those transverse ridges? or go back to using the strap system?
