Chassis Calculator
Engineering calculations for automotive, motorcycle & heavy vehicle chassis
Torsional Stiffness
K = (G × J) / LK = Stiffness (N·m/rad) | G = Shear Modulus | J = Polar Moment | L = Length
Reference Chart
Typical shear modulus values for chassis materials
| Material | G (GPa) | Application |
|---|---|---|
| Mild Steel | 79 – 80 | General chassis |
| High-Strength Steel | 80 – 83 | Sports cars, trucks |
| Aluminium Alloy | 25 – 28 | Lightweight frames |
| Chromoly (4130) | 80 | Race / bike frames |
| Titanium Alloy | 41 – 45 | Motorsport, bikes |
| Carbon Fibre | 4 – 5 | Monocoque chassis |
Understanding Torsional Stiffness
What is Torsional Stiffness?
Torsional stiffness (K) measures a chassis's resistance to twisting under load. Higher stiffness means more precise handling and better suspension geometry accuracy.
Why It Matters
- Automotive: Reduces body flex, improving suspension geometry and NVH.
- Motorcycle: Frame stiffness affects steering precision and high-speed stability.
- Heavy Vehicles: Controlled flex allows load distribution across uneven terrain.
Polar Moment of Inertia (J)
- Solid Round Bar: J = π·d⁴ / 32
- Hollow Round Tube: J = π·(D⁴ − d⁴) / 32
- Square Section: J ≈ 0.1406 × a⁴
Beam Deflection
δ = F·L³ / (48·E·I)
Reference Chart
| Material | E (GPa) | Use Case |
|---|---|---|
| Mild Steel | 200–210 | General beams |
| H-S Steel | 200–215 | Structural members |
| Aluminium 6061 | 68–70 | Lightweight frames |
| Aluminium 7075 | 71–72 | High-stress parts |
| Titanium Ti-6Al-4V | 113–116 | Performance parts |
| CFRP | 70–150 | Racing monocoque |
Deflection Limits
| Application | Limit |
|---|---|
| Car floor crossmember | L / 500 |
| Truck frame rail | L / 800 |
| Race car beam | L / 1000 |
| Suspension arm | L / 300 |
Understanding Beam Deflection
What is Beam Deflection?
Beam deflection is the transverse displacement of a structural member under load. Controlling deflection ensures dimensional stability and correct suspension geometry.
Three Load Cases
- Centre Point Load: δ = FL³/(48EI)
- UDL: δ = 5wL⁴/(384EI)
- Cantilever: δ = FL³/(3EI)
Second Moment of Area (I)
- Rectangle: I = b·h³ / 12
- Hollow Rectangle: I = (b·h³ − b₁·h₁³) / 12
- Circle: I = π·d⁴ / 64
Weight Distribution
Front% = (W_front / W_total) × 100CG from front = (W_rear × WB) / W_total
Reference Chart
| Vehicle Type | Front % | Rear % | Layout |
|---|---|---|---|
| FWD Car | 60–65 | 35–40 | FF |
| RWD Car | 50–55 | 45–50 | FR |
| Sports Car | 50 | 50 | FR/MR |
| Mid-Engine | 42–48 | 52–58 | MR |
| Formula Car | 45–47 | 53–55 | MR |
| Motorcycle | 48–52 | 48–52 | — |
| Heavy Truck (laden) | 25–35 | 65–75 | — |
| Semi (unladen) | 50–55 | 45–50 | — |
Understanding Weight Distribution
Why It Matters
Weight distribution determines handling, acceleration, and braking balance. The CG position relative to the wheelbase affects tyre loading and understeer/oversteer behaviour.
Effect on Handling
- Front-heavy (60/40): Understeer tendency — stable but less agile.
- Rear-heavy (40/60): Oversteer tendency — faster rotation but less stable under braking.
- 50/50: Ideal balance — tyres share load equally improving grip limits.
Roll Center Height
RCH via instant centre of arm projections
Reference Chart
| Vehicle / Suspension | Front RCH | Rear RCH |
|---|---|---|
| Passenger Car (DWB) | 50–100 mm | 100–150 mm |
| Sports Car (DWB) | 20–60 mm | 80–130 mm |
| Formula Car (DWB) | 20–40 mm | 40–80 mm |
| MacPherson Strut | 70–120 mm | 80–130 mm |
| Solid Axle (Leaf) | — | 300–500 mm |
| Solid Axle (Watt Link) | — | 200–350 mm |
| Heavy Truck Front | 200–350 mm | 400–600 mm |
| Motorcycle (swing arm) | — | 250–400 mm |
Understanding Roll Center Height
What is Roll Center Height?
The roll center is the point at which lateral forces act on the chassis during cornering. Its height directly influences body roll and lateral load transfer.
How to Find Geometrically
- Extend lines through each suspension arm pivot
- Intersection = Instant Centre (IC)
- Line from IC to tyre contact patch crosses centreline at Roll Center
SFD & BMD
BEAM SECTION (for deflection curve)
Understanding SFD & BMD
Shear Force Diagram (SFD)
The SFD plots the internal shear force at every cross-section along the beam. It helps identify where the beam is most susceptible to shear failure. The shear force is zero at the point of maximum bending moment.
Bending Moment Diagram (BMD)
The BMD plots the internal bending moment at every section. The maximum bending moment location determines where tensile and compressive stresses peak — critical for material selection and cross-section sizing.
Sign Convention Used
- Sagging (+ve): Beam bends concave upward — tension at bottom fibre.
- Hogging (−ve): Beam bends concave downward — tension at top fibre (cantilever).
- Shear: Upward forces on left side = positive shear.
Key Relationships
- dV/dx = −w(x) (rate of change of shear = distributed load intensity)
- dM/dx = V(x) (rate of change of moment = shear force)
- Maximum M occurs where V = 0
Vehicle Application
- Car: Chassis rails, floor crossmembers, suspension subframes
- Motorcycle: Swing arm, handlebar, frame spars
- Heavy Vehicle: Main chassis rails under axle/cargo loads