Suspension - Chassis Stiffening
Reducing and eliminating chassis flex is important to vehicle handling. Any flex or movement in the body and
structure of the vehicle will reduce the accuracy and effectiveness of the suspension and negatively affect the
Strut Tower Brace (Front and Rear)
The strut tower brace is the most common and visible chassis brace, spanning across the top of thee engine
compartment (and trunk) to create a structurally rigid connection between the upper suspension mounting points
on the left and right side of the car. The weight of the vehicle is supported on the strut towers, where the
spring rests against the chassis, and the stress on these points is significant. The logic behind this brace
is solid. The engine compartment (and trunk) is open on top, and lacks a structure to keep it from flexing
back and forth with the forces of the suspension moving up and down. Bridging between the two upper suspension
mounting points with a rigid brace will prevent the unibody chassis (stamped sheet metal held together with
spot welds), from flexing. This will prevent stress on those spot welds, and will also enhance handling and
steering response, by restricting all of the movement to the suspension, where it is acted upon by the
suspension, instead of being absorbed in chassis flex.
Look for strut braces which have two or more bolt positions on each end. Ideally, three or more. This will
prevent the strut brace mounts from becoming their own source of flex. Mounting a brace via a single bolt
(such as the common hammered-end and welded-L-bracket type from
Robie the Robot and others) does not provide
enough rigidity and structure for the brace to effectively reduce movement and flexing. A single end mounting
point simply redirects the chassis flex from the horizontal or crossways, to fore-aft, diagonal, and twisting
directions. As will be discussed in the following paragraph about Gacchiri or cowl braces, there are nine
(9) sheet metal body panels which come together between the strut tower and the A pillar, and there are plenty
of directions and opportunities for flex.
The Isuzuperformance offered strut braces for both front and rear of the Geo Storm. The front strut brace end
plates mount to the strut towers have four bolt holes on each end and mount to all
three studs from the upper strut mounts, or all four studs when used with four bolt camber plates. This forms
a rigid connection to the suspension, and sandwiches the strut tower between the two brackets to further enhance
the structure. The Isuzperformance front strut brace is also the only professional quality front strut brace
with a low profile design to fit beneath the lower hood line of the Geo Storm and Isuzu sister car Coupes
(Impulse, Gemini Coupe, Piazza, and Asuna Sunfire). The Japanese companies offered front braces that were
taller, and fit only the Gemini Sedan. And the Japanese companies' rear strut brace fit the Sedan only, and they
did not offer a rear strut brace for the Coupe.
Lower Cross Brace / Tie Brace
The mounting points for the lower control arms are another point that can benefit from bracing. The bottom of
the engine compartment (and rear suspension) lacks direct structure between the left and right sides of the car,
and this provides an opportunity for flex. Linking the mounting points at the bottom of the suspension will
provide more of the same benefits as a strut tower brace does at the top: better steering response and
At the front of the vehicle, a tie brace that connects the fore and aft mounting points of the front lower
control arms, reduces flex and improves steering response.
The rear suspension is mounted to the car via a cross brace or sub frame assembly. But the lateral control arms
are attached to this cross brace by cantilevered attachment bolt. The mounting bolt threads through one control
arm, then through the cross member box beam, then through the other control arm, and a nut secures the assembly
together. There is no rigid support point on the outside attachment points of the control arm bushing, and this
provides the opportunity for flex. Linking across the front of the forward facing side, and back of the rear
side of the control arms, with tie braces, significantly strengthens the rigidity of the rear suspension.
Isuzuperformance made lower tie braces for the Geo Storm and Isuzu cars.
Gacchiri – Cowl Brace
Between the strut tower and the A pillar of the Geo Storm, nine (9) sheet metal body panels come together.
These panels are secured in place with spot welds, and make up nine sources of flex in the front unibody
structure. The previous generation R-Body car, the Isuzu I-Mark Lotus Suspension and Gemini JT0 Lotus and
Irmscher cars, had a cowl brace linking the structure between the A pillar and the frame channel beside the
strut tower. Cowl braces are not uncommon as original equipment on many cars, especially sport models. And
the same brace, under a new name, “Gacchiri Brace”, is a popular upgrade for drifting, rally, and circuit
track use. Aftermarket Gacchiri braces typically mount over the top of the door hinges, providing six secure
bolting points, and require the addition of mounting points to the side of the frame channel beside the strut
brace. The brace forms a triangle between the three mounting points, preventing movement of the strut tower
and flexing between the strut tower and the A pillar.
Chassis Stiffening Foam – Unproven Benefit, Serious Draw Backs
Around 2003 or 2004, chassis stiffening foam became the new fad in the compact car magazines, most likely
due to the large influx of advertising dollars devoted to selling chassis stiffening foam. The product
descriptions make this foam sound like it cures everything without any negative aspects. Spray this
expanding foam into the C channel and box frame sections of the car (A, B, and C Pillars and Rockers),
and it increases the stiffness of the car body without intruding into the passenger area of the vehicle.
But the reality in the details is a little different, and quite a bit less enticing:
As can be seen, once chassis stiffening foam is examined with a critical eye, it is revealed to have
no proven benefit, and numerous serious drawbacks.
- No one has ever published any data on the exact amount of stiffness improvement that chassis stiffening
foam provides. This is probably because the type of equipment needed to twist an automobile frame is
available only to someone with a budget of a large multi-national corporation, like an automobile manufacturer.
All of the marketing claims made by chassis foam vendors are open-floored, meaning they all claim "Up to..."
or "As much as...". This type of wording allows the vendor to cite over inflated numbers that they know the
product can not possibly deliver, because the statement means "This product will provide between ZERO
improvement and the number provided in the text of this statement". And we all know that ZERO is most often
the actual number whenever someone phrases their claim this way.
- None of the companies making and selling chassis stiffening foam have easily recognizable names of known
auto body suppliers. Notice that none of these products are made by 3M, SEM, Dupont, Sherwin Williams, PPG,
or any of the other big names in paint and auto body. Certainly, if there were any benefit for chassis
foam, all of these companies would have their own products on the market to compete in the market.
- The tension and compression forces on a frame section are highest at the outer edge of the skin,
and negligible in the middle. Look at somethign simple in building structure like an I-beam and notice that
the material is used most at the top edge (compression) and the bottom edge (tension)(It needs only to resist
up and down forces). The web joining the two together is just to keep them attached. Take that example to
structures that are designed to resist not only up and down forces, but sideways forces, such as in an
automobile. Look at a frame section and notice it is not solid, it is a tube. This is because all of the
forces it must resist are at the edges, not in the middle. Unibody chassis design is based on folding pieces
of sheet metal to form tubes around the perimeter of the passenger and mechanical compartments. Cylindrical
shapes and folded edges enhance the strength of the tubes. And the larger the tube is made, the thinner the
material required to provide a given stiffness and rigidity. The open area in the middle of the tube is not
important to the stiffness of the tube. If it were, automakers would be making vehicles with solid 2 inch
by 2 inch square bar in the A, B, and C Pillars and Rockers.
- You may or may not be aware that there are several rather important electrical wires run through the
Pillars and various frame channels of the vehicle. These wires include the rear window defroster, high
mounted third brake light, radio antenna, dome light, door pin switches, and door speakers. Most of these
wires transit from the body to the doors and hatch, or are mounted to the body, and have the electrical
connector plugs located within the Pillars of the vehicle. Removing the doors or rear hatch, or servicing
any of these electrical devices requires pulling the wiring harness through an access hole so that the
wire can be unplugged for disassembly, or stuffing the connected wire back through the access hole into
the Pillar for reassembly. If the pillar (frame channel) has been filled with foam, foam that is rigid
enough to supposedly stiffen the chassis of the vehicle, then there is no way to access these electrical
connectors in order to service these items.
In addition to this, should it be needed to either run an additional wire within the Pillars, or replace
an original wire within the Pillar, this is impossible to do if the Pillar has been filled with foam.
- The Rocker channel running along the right and left sides of the vehicle has drip holes. This is
because the automaker knows that there will be water leakage into the chassis of the vehicle, and the
automaker has provided a passage for that water to be evacuated from the vehicle, by running down the
Pillars and out the drip holes in the Rockers. If the Pillars and Rockers are full of foam, there is
no passage for the evacuation of water out of the vehicle chassis. The foam itself MAY OR MAY NOT absorb
water like a giant sponge. If it does, then it will hold the water within the unibody chassis where
it will cause extreme rust damage. If it does not hold water, it will cause the water to collect and
puddle within the Pillars and Rockers. It will hold the water against the sides metal and prevent
evaporation by the air within the frame channels, because that air has been displaced by foam. And
the foam will impede the flow of water and block the drip holes where the water is meant to evacuate the
- In the event that the vehicle body must be repaired via welding, such as cutting out a rust
hole and welding in a patch, the foam becomes a significant impediment to this. The foam may be described
as "non-flammable", but expose just about anything to a fresh bead of molten steel, and it will burn.
In order to make a repair weld, the foam will have to be removed from the immediate and adjacent area where
it might be exposed to excessive heat.
- The flexing in an automobile unibody frame occurs at the seams and connections between the sheets of
stamped steel, not at the formed frame channels of the Pillars and the Rockers. These seams are along the
edges of the floor pan, the fire wall, outer sides of the strut towers and front suspension structure,
and wherever the one sheets of stamped steel is attached to another sheet of stamped steel. Foam inside
of the frame channels does nothing to increase the rigidity of these seams. Only supplemental welding will
accomplish this goal.
Spot welding joins two pieces of metal by clamping electrodes on opposing sides of an overlap joint, running
an electrical current between the two electrodes, and melting the facing metal surfaces together in a little
round spot. When spot welds are run along a seam in a continuous line with 1 ½ - 2 inches of space between
each spot weld, they form a reasonably strong joint.
Spot welding seems to be a lost art. Though ever car made in the last 30 years was assembled almost exclusively
with spot welds, no body shop uses spot welding to replace body panels. Instead, after removing a damaged panel,
they loosely line up the new panel, and plug weld (lance a Mig or Tig welder through one panel and into the
panel behind it), typically with five inches of spacing between each plug weld. A plug weld is weak, because the
panels are not squeezed together, and because the plug welds are typically spaced too far apart. "How weak?" you
might ask. One owner paid to have the rocker panels replaced by a body shop who installed the new rocker panels
with plug welds spaced at five inch offsets. He raised the front of the car to change the wheels and tires, by
placing a floor jack under the front jacking point. The frame of the car flexed so much that the doors could
no longer be opened or closed.
Oddly enough, spot welders are offered for sale by Miller and other manufacturers, in 220 and 110 volt power
In the event that the vehicle has been repaired using a plug welding technique, it is a very good idea to go over
the seams with a spot welder with 2 inch spacing between the welds, to return the structural strength to the
For improved chassis rigidity, adding more spot weld points between the original spot welds, will significantly
increase chassis strength. ½ inch offset spot welds along the seams around the door, hood, hatch, window openings,
windshield opening, and along the base of the rocker panel where it attaches to the floor pan. This will
transform the handling and steering response of the vehicle.
Seam Welding / Stitch Welding
Seam welding provides for the same improved joint rigidity as spot welding discussed above. Seam welding is a
little less subtle and easier to see. But seam welding can be done in areas that a spot welder will not reach.
A good quality Mig welder is recommended. This is one situation where the quality of the tool has a large
influence on the quality of the result, because a low quality welder will probably just blow holes through the
thin sheet metal of the vehicle body, while a good quality machine will run beautiful weld beads.
Welding and Race Rules
Rules for racing classes with limited allowable modifications typically ban chassis reinforcing by welds.
However, these same rules do not limit the thoroughness of body damage repairs. If it can be proven that the
vehicle was in an accident and required frame repair, or that panels had to be replaced or repaired due to rust
restoration, then just how zealously those repairs are performed is not outlined or restricted in the rules.
© 2010-2014 StormOwners.Com. All Rights Reserved.
Click here to return to the Geo Storm Owners Page Tuning Index.