I've just ordered one of the DiGrappa X braces. It got me thinking
about the whole chassis brace problem.

Why not brace all four suspension points against one another with a
large sheet of carbon fiber, designed to act as both a monocoque and
an ground-effects undertray? You could shoot a number of birds with
one stone:

• Increase torsional rigidity of the whole platform
  
• Lower underbody drag
  
• Better downforce
  
• Use the heat of the catalytic converter to force air from under the car
  
• Increased corrosion resistance

The problem with composites is that they're strong and stiff per
pound, but they don't handle point stresses well. Also, a sheet isn't
going to be naturally strong under compression. The compression
problem gets solved by having two sheets, bonded together in places,
creating tube-like structures running laterally across the bottom of
the car. I need to look to see if there is room for these where
needed.

So how do you attach this sheet to the bottom of the car, so that it
can take tens of thousands of pounds of horizontal stress, with a low
profile, no stress points, and easily detachable for service?

Velcro. Wait, I'm not on drugs! I'm thinking of a mating surface
with hundreds of zigzag ridges for transmitting shear forces. If it's
low profile enough it can be light enough to have the sheet attach to
every point on the bottom of the car where frame members are exposed
-- maybe more than 1000 square inches. Any dirt in the mating surface
would clearly screw up the system, as would misalignment. But with
just a few 10s of PSI of shear strength, you could develop insane
rigidity. Actually, you'd have to be careful that the undertray didn't
make the chassis so stiff that sharp vertical loads on the suspension
points could crush the steel structure above.

I know that some airplanes use ducting around their radiators which
not only has low drag, it actually converts the heat dumped into a
small amount of forward thrust. I'm wondering if suitable ducting
around the catalytic converter could be used to pump air out from
under the car faster than it might otherwise leave.

 

well I believe the more bracing the better, but sheets do bend. Even if it is carbon fiber. If it is too thin, it will bend. Just like 1/4 flat bar steel, which is long and thin, only has ridigity perpendicular to its length. you would need thicker or hollow bars (tubular or box style) to reinforce the chassis, then sheets to improve aerodynamics. I've been doing a lot of research with material sizes while developing my rear x-brace, now called the rear-N brace. With this, I'm using hollow 3/4" box steel, which is lighter weight than solid, and eliminates most of the area where max stress is (the center). And because of its square shape, it takes a lot of force to bend

 

Sure, sheets bend. The sheet is like the stay wires on
the mast of a sailboat. The wires themselves are worthless
for resisting lateral force up in the air, but when added
to the mast, they make the mast much stiffer. The X-brace
that you're working on sounds like using another mast
from the side of the boat to the tip of the main mast.
That's just fine actually -- big sailboats use stainless
steel rod rather than wires. But it's not light enough
to extend the solution to bracing the car front to back.

The trouble with my sheet idea is this super velcro. I
looked at 3M's site, and they talk about dual-lock, which
is I think what I remember working with many years ago.
You use a fist or a hammer, and the stuff makes an audible
pop when it engages. But dual lock is not going to work
in this application -- 1.1 pounds of shear per square
inch, when I need 10 PSI.

I'd also have to figure out how to make a sheet guaranteed
to rip apart before the frame crumples. I'm sure the frame
is plenty strong, but I'd still need to have good
consistency in how I built that sheet. Ugh.

Naah. Much easier to buy stuff from you or DiGrappa!

 

Man I wouldnt want to do a clutch on your car! is servicability a factor in your design? If its one peice and it runs the length of the car and its super velcro'd on, i think it would be almost impossible to remove. Would a three peice design work? All three peices would be joined by a row of fasteners. Each section would be removable which would not only aid in servicability but in cost of replacement. If you downed the front of the car one time nad cracked this plate you would need an entire new plate instead of just a front one. Just my 2 cents.

 

Servicability? Think VELCRO. That's the point.

The dual-lock stuff used to come off by putting a
chisel in it from the side and giving it a sharp
whack. You'd hear a BANG and it would let go.

This plate would come off basically the same
way, except to avoid damage I'd put detents in the
side of the thing so you could get a chisel in there
easily. Servicing the thing would involve ten
whacks with a chisel down each side of the car,
followed by a blow to your head as the plate fell
down on top of you!

I agree a 3 piece design would be more convenient
but it would then put a lot of constraints on those
intermediate fasteners. You'd now have front-to-
back strain on the plate going through four
fasteners, and the compliance of all four will add
up, so that the plate won't even start doing anything
until the chassis has already deflected a fair bit.
Leaving it as a single piece has the strain going
through two fasteners, which is bad enough.

 

At least it wouldnt weigh much. My dad uses this velcro style in his lab and it is very strong. It does not let go very easy.
I think on a stiff surface, it would be even less comliant to release.

 

Sounds like an idea with lots of potential. I'm more concerned about cost and rust issues?
Best of luck with your brainstorming :=)

 

Fullthrottle: I think on a stiff surface, it would be even less comliant to release.

I think you're right. I'd want to make the material behind the
velcro more easily bendable without making it stretchy. That
sounds like a single think fiber mat in a somehow rubberized
epoxy matrix. Hmm. Never seen anything like that before,
but I'll bet it exists. I'll have to look around.

Babydragon: The sheet would tend to keep muddy, salty water
near the frame members it attaches to, which is of course bad.
The actual attachment point is probably okay, since you'd
sand off the existing corrosion protection but then immediately
apply a layer of epoxy with which one side of the velcro gets
bonded. Unless the epoxy develops cracks, that joint will be
watertight.

As for cost, the cost is going to be in the labor to epoxy all
these velcro pads on the first time. For DIYers it probably
wouldn't be all that bad, but getting a shop to do it would
probably be pricey. I haven't yet figured out how to actually
install the thing, since you would kinda prefer to have the
sheet lock-in in a slightly prestressed condition.

 

I'll leave the design up to you, but I'm sure I can get the assistance of a bodyshop or two for installation. Would it be possible if you can design a single sheet with small cutouts for maintenance, etc? A nice lift can make all the difference in the world. Maybe we can make a simple rig to prestress the areas that we want . I have a 96 Civic, or my wife's 92 Accord that I would want to test this design on before using it on my S baby :=)

 

[QUOTE]Originally posted by babydragon
[B]Sounds like an idea with lots of potential.