MATERIAL :- MILD STEEL

Mild steel is a type of carbon steel that's made from iron and carbon. It's
also known as low carbon steel because it contains a low amount of
carbon, typically between 0.05% and 0.25%.

Properties of mild steel

 Strength: Mild steel has high tensile and impact strength.
 Malleability: Mild steel is malleable and can be cold-formed.
 Weldability: Mild steel is easy to weld.
 Magnetic: Mild steel is magnetic because of its ferrite content.
 Cost: Mild steel is inexpensive compared to other materials.
 Reactivity: Mild steel is reactive and will rust when exposed to water, oxygen, and
ions.

Justification for using mild steel in chassis

Mild steel is an ideal material for rally car chassis and roll cages due to its combination of
strength, ductility, and weldability. It can withstand high stresses during crashes, absorbing
energy by deforming rather than breaking, thus protecting occupants. Its high weldability
ensures consistent joint quality, crucial for intricate roll cage designs. Mild steel is also cost-
effective, significantly cheaper than materials like chromoly or aluminum, making it a
budget-friendly choice for rally cars, with lower repair and replacement costs. The material's
availability and ease of workability allow for precise shaping to meet safety standards like
those set by the FIA. While heavier than aluminum, the extra weight enhances stability and
lowers the center of gravity, improving control on uneven rally terrains. Furthermore, mild
steel can meet safety standards when used with the correct thickness and dimensions. Though
prone to corrosion, treatments like powder coating, galvanization, or paint provide protection
against rust in harsh environments. Overall, mild steel strikes a balance between safety, cost,
and performance, making it a preferred choice for rally car construction.
 ROLL CAGE
 AISI 4130 (Chromoly Steel) –

AISI 4130, or chromoly steel, is an ideal material for rally car roll cages due to
its high strength-to-weight ratio, durability, and safety performance. Composed
of chromium and molybdenum, it provides exceptional strength while
remaining lightweight, ensuring effective crash energy absorption without
cracking. Its fatigue resistance makes it highly reliable under the extreme
stresses of rally racing, while its workability allows for precise fabrication of
complex, custom roll cage designs. Chromoly also offers moderate corrosion
resistance, enhancing its durability in harsh rally environments. While more
expensive than mild steel, chromoly delivers superior performance at a lower
cost than titanium, making it a cost-effective choice for high-performance
applications. For rally car roll cages, typical tubing diameters range from 1.25"
to 1.75", with wall thicknesses between 0.095" and 0.140". For larger tubes
(1.5" to 1.75" diameter), a wall thickness of 0.120" is commonly used to
balance strength and weight. Its compliance with FIA and SCCA safety
standards further solidifies its position as the preferred material for rally car roll
cages, balancing strength, weight, and safety in a way that meets the rigorous
demands of professional motorsport.

THICKNESS OF THE MATERIAL

1.Main Tubular Structure:

Thickness: 1.5 mm to 2.0 mm (14–16 gauge). keeping weight
manageable.
2.Roll Cage (Per FIA Standards):
Thickness: 2.0 mm (minimum)
3.Reinforcement Plates and Mounts:
Thickness: 3.0 mm to 5.0 mm.
4.Floor Pans and Non-Structural Panels:
Thickness: 1.2 mm to 1.5 mm
 Physical Properties Metric English
Density 7.85 g/cc 0.284 lb/in³

Mechanical Properties Metric English

Hardness, Brinell 197 197
Hardness, Knoop 219 219
Hardness, Rockwell B 92 92
Hardness, Rockwell C 13 13 Converted f
Hardness, Vickers 207 207
Tensile Strength, Ultimate 670 MPa 97200 psi
Tensile Strength, Yield 435 MPa 63100 psi
Elongation at Break 25.5 % 25.5 %
Reduction of Area 60 % 60 %
Modulus of Elasticity 205 GPa 29700 ksi
Bulk Modulus 160 GPa 23200 ksi
Poissons Ratio 0.29 0.29
Machinability 70 % 70 %
Shear Modulus 80.0 GPa 11600 ksi
Izod Impact 87.0 J 64.2 ft-lb

Thermal Properties Metric English

Component Elements Properties Metric English
Carbon, C 0.28 - 0.33 % 0.28 - 0.33 %
Chromium, Cr 0.80 - 1.1 % 0.80 - 1.1 %
Iron, Fe 97.03 - 98.22 % 97.03 - 98.22 %
Manganese, Mn 0.40 - 0.60 % 0.40 - 0.60 %
Molybdenum, Mo 0.15 - 0.25 % 0.15 - 0.25 %
Phosphorus, P <= 0.035 % <= 0.035 %
Silicon, Si 0.15 - 0.30 % 0.15 - 0.30 %
Sulfur, S <= 0.040 % <= 0.040 %
RALLY CAR CHASSIS
Monocoque Chassis

The monocoque chassis design, also known as the unibody design, fuses the
car frame and chassis into a single unit. It is the standard chassis type in most
modern vehicles, including sedans, hatchbacks, and SUVs. Therefore, the
monocoque chassis design goes beyond aesthetics; it provides exceptional
strength, rigidity, and crash resistance.

Pros

 It provides high torsional rigidity;

 The fused design helps to protect its components, making it more
durable;
 The monocoque design helps to absorb and effectively distribute crash
energy.

Cons

 Because of the fusing with the car frame, monocoque chassis is

expensive to manufacture;
 The fused design may make repairs difficult.
 Best Mild Steel Grades for Rally Car Chassis
AISI 1018 or 1020 (Low-Carbon Steel):
Properties:
 Yield Strength: ~350 MPa
 Tensile Strength: ~400 MPa
 Good weldability and ductility.

Using mild steel for a rally car chassis offers several advantages, particularly in terms of cost,
ease of fabrication, and specific performance factors. It is significantly more affordable than
alternatives like chromoly steel, aluminum, or carbon fiber, making it ideal for teams with
limited budgets or entry-level builds, with mild steel costing 50-70% less than chromoly steel
and being much cheaper than carbon fiber. Mild steel is also easy to weld, cut, and form,
making it highly suitable for custom chassis fabrication, as it doesn’t require special
equipment or advanced techniques like chromoly steel or aluminum. Its widespread
availability in various forms, such as tubes, sheets, and bars, ensures easy sourcing and
reduces downtime and costs for repairs or modifications. Mild steel’s ductility and toughness
allow it to absorb energy and deform in a controlled manner during a crash, enhancing safety,
whereas high-strength materials like chromoly or aluminum can be more brittle and may fail
catastrophically under impact. Additionally, damaged mild steel components are easier to
repair or replace on-site or in workshops, as standard welding tools and techniques are
sufficient, and its lower hardness makes reshaping or re-welding simpler. Its forgiving nature
and ease of fabrication also make mild steel an excellent choice for novice or grassroots
builders. While it is heavier than chromoly or aluminum, the added weight can contribute to a
lower center of gravity, improving stability, making mild steel a practical and versatile choice
for rally car chassis, especially in grassroots or regional competitions where cost and ease of
use are key considerations.

Density 7.85 g/cc 0.284 lb/in³

Mechanical Properties Metric English

Hardness, Brinell 293 293
Hardness, Knoop 319 319
Hardness, Rockwell B 99 99
Hardness, Rockwell C 31 31
Hardness, Vickers 309 309
Tensile Strength, Ultimate 965 MPa 140000 psi
Tensile Strength, Yield 585 MPa 84800 psi
Elongation at Break 12 % 12 %
Reduction of Area 17 % 17 %
Modulus of Elasticity 205 GPa 29700 ksi
Bulk Modulus 160 GPa 23200 ksi
Poissons Ratio 0.29 0.29
Shear Modulus 80.0 GPa 11600 ksi
Izod Impact 7.00 J 5.16 ft-lb
 Component Elements Properties Metric English
Carbon, C 0.75 - 0.88 % 0.75 - 0.88 %
Iron, Fe 98.13 - 98.65 % 98.13 - 98.65 %
Manganese, Mn 0.60 - 0.90 % 0.60 - 0.90 %
Phosphorus, P <= 0.040 % <= 0.040 %
Sulfur, S <= 0.050 % <= 0.050 %
Weight balance in a Car
A well-balanced rally car has a weight distribution that's close to 50/50, where the
center of gravity is midway between the front and rear axles. This distribution can
help the car be agile and calm, which can improve driver confidence.

Balancing the weight of a rally car is a crucial aspect of performance, as it affects the car's
handling, stability, and overall dynamics during high-speed driving on rough and varied
terrain. Proper weight distribution ensures that the car responds predictably, optimally
transfers weight during cornering, and maximizes traction.

Here are the key aspects of weight balance in a rally car:

1. Chassis Layout and Configuration

There are several chassis configurations used in rally cars, each with its own advantages for
handling and performance. A Front-Engine, Rear-Wheel Drive (FR) layout places the
engine at the front and drives the rear wheels, typically resulting in a front-heavy weight
distribution. To balance this, rally cars with this configuration often use a slightly rear-biased
weight distribution or specialized suspension tuning. Classic examples include the Ford
Escort Mk2 and Lancia Stratos. The Front-Engine, All-Wheel Drive (AWD) layout offers a
more balanced weight distribution (often 50:50 or slightly rear-biased), with power sent to
both the front and rear axles, enhancing stability. The engine at the front, along with
transmission and driveshaft components at the rear, helps to achieve this balance, and further
tuning ensures optimal handling, as seen in vehicles like the Subaru Impreza WRX and
Mitsubishi Lancer Evolution. The Mid-Engine, Rear-Wheel Drive (MR) layout, where the
engine is located between the front and rear axles, improves central weight distribution and
handling, making the car more responsive to driver inputs. This layout is seen in some rally
or specialized off-road cars, as well as RWD prototypes in endurance racing. Lastly, the
Rear-Engine, Rear-Wheel Drive (RR) configuration, though less common in rallying,
offers a rear-biased weight distribution that helps with traction. However, it requires precise
handling to prevent oversteering. An example of this setup is the classic Porsche 911,
typically more associated with circuit racing. Each of these configurations offers unique
handling characteristics that can be fine-tuned for the specific demands of rally racing.

2. Left-to-Right Weight Balance

In rally car design, achieving an ideal weight distribution is crucial for optimal performance.
Symmetry plays a key role, with the car ideally being symmetrical in terms of left-right weight
distribution. This balance helps ensure stability when cornering and prevents unpredictable handling
caused by uneven weight on one side of the car. Additionally, adjustments for the driver and co-
driver are often made to further fine-tune the car's balance. This can include adjusting seat
positioning and the placement of cockpit equipment, such as the co-driver's seat, to ensure that the
car remains well-balanced. In some cases, weight may also be added or redistributed using ballast to
optimize handling and maintain the car's overall balance during different stages of a rally. These
considerations are essential for achieving predictable, responsive handling in the challenging
conditions of rally racing.
3. Lowering the Center of Gravity (CoG)

A lower center of gravity (CoG) is essential in rally car design, as it helps prevent roll and enhances
overall stability, especially when navigating sharp corners and uneven terrain. By lowering the
weight of key components such as the engine, suspension, and transmission, and using lightweight
materials, the CoG can be reduced, improving the car's handling and control. Rally cars often
incorporate specialized components like coilover suspension systems, which allow for adjustable
ride height and better weight distribution, further lowering the CoG. Additionally, lightweight alloy
wheels are commonly used to reduce overall weight and contribute to the car's stability and agility.
These modifications collectively help ensure that the car remains responsive and stable under the
demanding conditions of rally racing.

4 . Roll Cage Design

The roll cage is a crucial component of a rally car chassis, playing a vital role in both safety
and performance. A well-designed roll cage ensures that the driver and co-driver are
protected in the event of a crash by providing structural integrity. However, the design and
placement of the roll cage also affect the car's weight balance. Strategic placement of the roll
cage is essential for optimizing handling and balance; for example, positioning parts of the
roll cage lower or more rearward can help counteract a front-heavy chassis, improving
stability. Additionally, the material used for the roll cage is important for weight and safety.
Lightweight materials such as chromoly steel or aluminum are commonly used in rally car
roll cages to reduce weight, which helps maintain the car’s agility without compromising the
protective qualities of the structure. These considerations ensure that the roll cage enhances
both the safety and handling characteristics of the car during the demanding conditions of
rally racing.

5. Weight Distribution Adjustments (Ballast and Weight Relocation)

Ballast is often used in rally car setups to fine-tune the car’s weight balance and improve its
handling characteristics. When the car’s weight distribution is not ideal, ballast (additional
weight) is strategically added to specific areas of the chassis. For instance, if a car is too
front-heavy, ballast can be placed toward the rear to help balance the car, improving its
stability and handling, particularly on slippery or rough terrain. The strategic positioning of
ballast is critical; it is typically placed low and centrally within the car to minimize its impact
on the center of gravity (CoG) while ensuring an even weight distribution between the front
and rear axles, as well as across the left and right sides. This careful placement helps maintain
overall balance, ensuring the car remains responsive and stable during the dynamic
conditions of rally racing.
7. Chassis Adjustments for Rally Stages

Rally-specific modifications are essential to ensure that a car’s chassis can handle the
extreme and varied conditions of rally racing. These modifications often include reinforcing
weak spots in the chassis, adjusting the geometry for improved handling on rough surfaces,
and upgrading components to enhance durability and safety. Such changes are necessary to
withstand the constant stress and impact from racing on gravel, mud, snow, and other
challenging terrains. Additionally, height adjustments are commonly made to adapt to
specific rally stages. For instance, the chassis may be raised for gravel stages to provide
additional ground clearance, helping the car navigate rough and uneven terrain. Conversely,
the ride height may be lowered for tarmac stages to improve aerodynamics and handling on
smooth, flat surfaces. These modifications are crucial for maintaining optimal performance
and safety across the varying conditions encountered in rally events.