Related: automotive-mechanics · machine-design · mechanics-of-materials · cheat-sheet
Team: Klantech AutoSport — ~25 students designing/building a rugged two-seat buggy for BAJA 800. Philosophy: OEM parts where smart; design-build ownership on structure and vehicle systems; self-sufficiency and career prep.
Result to claim: 1st place BAJA 800 Hyderabad 2023. Your role: project lead for structure, steering, brakes, and full integration.
Performance targets (report list): effective strength; aerodynamically effective packaging; minimum weight; simpler weld assemblies; driver-friendly vision; safer/faster egress; any rough terrain; driver safety; smart packaging of battery/fuel/engine; higher power & torque; steep-road climb; sudden braking in hills; exceptional steering control.
Tools: SolidWorks 2022 Student Edition CAD + static FEA; Lotus Shark for suspension kinematics; Ansys mentioned in calc workflow for FOS framing.
External: BAJA SAE ↗ Engineering Toolbox ↗
Manufacturing kickoff: lock rim dimensions → fix wheelbase & track → frame in SolidWorks starting with Rear Roll Hoop (RRH), then front/rear supports, lateral cage members, and floor sized for driver + navigator access with rule spacing between seats.
Pedals & steering: wheel height set for clear vision and full rotation without striking the driver’s legs. Front hoop isolates steering/suspension/wheels from occupants and houses the steering column.
Design iterated for engine bay volume, dual cockpit, steering compartment, and force flow under unexpected multi-axis impact. Suspension arms, steering, and wheel alignment co-optimized. Triangulation used throughout for higher FOS at lower weight.
PVC 1:1 mock: 1-inch PVC pipes before steel — validated engine placement, driver comfort, component positions, and joint count at each node.
RRH nodal points: early design had too many nodes (one bad weld could condemn the hoop) → redesigned for fewer nodes and durability. Nose and RRH initially oversized → excess drag/aero instability → resized for lower height/width.
Rear body and nose redesigned around engine, battery, fuel tank, radiator. Gearbox and electrics packaged between the drivers. RRH inclined ~10° for seating comfort, lower car height/drag, easier seat housing.
Electrics / vision: light housing designed for night-drive illumination and safety rules; dashboard as switch hub and protective shield.
External: BAJA SAE ↗ Engineering Toolbox ↗
Space-frame options (Al, Ti, CFRP) rejected: cost, procurement, special fab. Prefer low-carbon seamless steel for weldability/ductility.
| Property | Value |
|---|---|
| Material | IS 3074 |
| Carbon | ≈0.16% |
| Outer diameter | 44.5–45 mm |
| Wall thickness | 2.5–2.8 mm (calc table uses 45 × 2.6 mm) |
| Yield strength | ≈311 MPa |
| Tensile strength | ≈423 MPa |
| Elongation | ≈36% |
| Moment of inertia (45×2.6) | ≈75586.9 mm⁴ |
| Bending strength | ≈1044.8 N·m |
| Bending stiffness E·I | ≈15495 N·m² |
| Mass per length | ≈2.62 kg/m |
Arms: AISI 4130 chosen as optimum for control arms (see suspension).
External: BAJA SAE ↗ Engineering Toolbox ↗
FEA on bare frame (not full assembly) for FOS. Three primary cases: front, rear, side impact. Least FOS across directions governs triangulation/sizing. Mesh noted ≈130725 nodal points (SolidWorks).
Calc assumptions: mass with driver+navigator ≈650 kg (powertrain GVW also 680 kg); max speed 60 km/h; frontal & rollover collision duration 0.15 s; side 0.20 s.
| Case | Method / notes | Force | FEA max von Mises (report) |
|---|---|---|---|
| Front | ½mv² / D; D≈v·0.15 s | ≈36.1 kN (36097 N) | ≈157.66 MPa |
| Side | Δt=0.20 s; D≈3.33 m | ≈27.1 kN (27089 N) | ≈189.32 MPa |
| Rear (FEA) | Static impact load case | — | ≈186.11 MPa |
| Rollover | Drop from 10 ft (3 m); v≈7.67 m/s; D≈1.15 m | ≈12.8 kN (12783 N) | — |
Interview: derive energy→force; justify Δt; compare σ_max to Sy≈311 MPa with FOS — don’t stop at “stress looked OK.”
External: BAJA SAE ↗ Engineering Toolbox ↗
Architecture: fully independent short–long double A-arm (SLA) on all four corners. Rejected solid axle (unsprung mass, little adjustability). Preferred over MacPherson / semi-trailing for camber control, handling, and adjustability.
Shorter upper arm induces negative camber as suspension rises — offsets body-roll positive camber on the outside tire so the contact patch stays nearer vertical under load transfer.
Dampers to upper wishbone for desired motion ratio over bumps/obstacles.
| Envelope spec | Value |
|---|---|
| Front track | 63 in |
| Rear track | 63 in |
| Ground clearance | 9 in |
| Rim | 12 in |
| Tire | 24 in |
(BAJA width constraint ≈70 in edge-to-edge guided track/tire choice.)
| Alignment target | Value / intent |
|---|---|
| Kingpin inclination (KPI) | ≈6° — scrub radius / road feel; too high → roll sensitivity |
| Static camber | 0° — maximize efficiency; hold through bump |
| Caster | ≈12° positive — upper BJ ~1 in behind lower; strong self-center, more steer effort |
Knuckle functions: hold spindle, mount caliper, steering link, set steering axis. Material trade: structural steel vs aluminium (density, strength, vibration, corrosion) → structural steel for strength/stiffness and lower vibration on bumps. Front and rear knuckles modeled in SolidWorks + FEA (stress/deformation).
Lotus Shark: hardpoints from SolidWorks → camber/toe/caster/kingpin change rates vs travel. Hardpoint cloud includes lower/upper front/rear/ball, damper wish/body, steer in/out, spring up/low, wheel spindle & centre (mm-class coordinates in report).
Lotus Shark hardpoints (mm, report):
| Point | X | Y | Z |
|---|---|---|---|
| lower front | −1168.4 | −353.06 | 266.7 |
| lower rear | −896.75 | −353.06 | 266.7 |
| lower ball | −1059.27 | −567.19 | 204.58 |
| upper front | −1168.40 | −353.18 | 494.68 |
| upper rear | −896.75 | −353.18 | 493.85 |
| upper ball | −1050.80 | −542.67 | 442.76 |
| damper wish | −1059.25 | −468.50 | 228.72 |
| damper body / spring up | −1001.48 | −403.47 | 787.73 |
| spring low | −1059.25 | −468.50 | 228.72 |
| steer out | −881.58 | −593.26 | 317.5 |
| steer in | −833.32 | −381 | 381 |
| wheel spindle | −1054.1 | −636.27 | 317.5 |
| wheel centre | −1054.1 | −685.8 | 317.5 |
External: BAJA SAE ↗ Engineering Toolbox ↗
Pugh of AISI 1018 vs AISI 4130 for A-arms:
| AISI 1018 | AISI 4130 | |
|---|---|---|
| Carbon % | 0.18 | 0.30 |
| Tensile (MPa) | 440 | 971 |
| Yield (MPa) | 370 | 856 |
| Hardness BHN | 126 | 217 |
| Cost Rs/m | 325 | 725 |
| Pugh net | −6 | 0 (selected) |
4130 chemistry (report): Fe 97.03–98.22; Cr 0.80–1.10; Mn 0.40–0.60; C 0.28–0.33; Si 0.15–0.30; Mo 0.15–0.25; S ≤0.040; P ≤0.035.
Arm tube: OD 25.4 mm; ID 23.9 mm; thickness 3 mm; Sy 856 MPa; Sut 971 MPa; cross-section area cited 58.08 mm² (report says mm³ — treat as area). Upper length constrained by existing chassis nodes.
External: BAJA SAE ↗ Engineering Toolbox ↗
Rule: four-stroke piston ≤810 cc. Compared Maruti Alto 0.8 vs Renault Kwid 0.8:
| Alto 0.8 | Kwid 0.8 (chosen) | |
|---|---|---|
| Power | 47 bhp | 54 bhp (~14% more) |
| Torque | 68 Nm @ 3500 | 72 Nm @ 4368 |
| Power/weight | 68 bhp/ton | 84 bhp/ton |
| Torque/weight | 99 | 113 |
| Cylinders | I3 | I3 |
Why Kwid: SCe AFR control; better power/torque-to-weight; cable-shift gearbox → less modification; stock mounts set correct inclination for oil/coolant circulation.
Layout: rear engine, RWD — weight on driven wheels (loose surfaces), cabin space, unitized drivetrain install, front-crash separation, cooling path. Manual 5-speed (not CVT): driver preference + engine braking downhill + gear choice on low-μ / mud.
Fuel: Kwid tank (ECU/fuel-pump compatibility), rear above engine, firewall + separate compartment vs cabin/engine. Radiator: rear, blowing toward engine. Battery: front — access + away from heat/fire. Exhaust: manifold (3 pipes to heads) + frustum + tailpipe — note + reduced backpressure.
Tractive effort inputs: GVW 680 kg; weight dist 1:3; tire static R 12 in / dynamic 12.5 in; frontal area cited 0.29 m² (elsewhere 1.87 m² — know which assumption you defend); Cd 0.8; fr 0.3; η_trans 75%.
| Load | ≈ Force |
|---|---|
| Rolling RR | 2001 N |
| Aero AR | 172 N |
| Grade GR (45°) | 3335 N |
| TTE / η | ≈7934 N |
Note on frontal area: calc sheet lists both ~0.29 m² and (in another summary) 1.87 m² with Cd=0.8 — if asked, state which value you used for AR and recompute consistently.
External: BAJA SAE ↗ Engineering Toolbox ↗
Choice: four-wheel discs over drums (fade, wet, serviceability). Independent dual-circuit via Bosch tandem master cylinder; single pedal locks all wheels via four lines.
| Element | Choice |
|---|---|
| Master cylinder | Bosch tandem; piston Ø 29 mm |
| Caliper | Maruti 800; piston Ø 40 mm |
| Rotor | Custom cast iron; OD 220 mm / ID 80 mm; thickness 11 mm; R_eff≈110 mm; 5 mm vent holes |
| Pads / lining | Steel-graded; μ_pad≈0.40 |
| Fluid | Dot 4 |
| Pedal | Custom; ratio 5:1 |
| Tire μ | ≈0.75 |
| Tire radius | 12 in |
Static weight: front 149 kg / rear 457 kg / total 650 kg → % front static ≈24.6%; CG height cited 48 cm; wheelbase 83.5 in. Dynamic front under braking ≈350.6 kg (34.7%).
Calc chain (30 km/h stop): KE≈22.55 kJ → foot force 20 kgf≈196 N → pedal 981 N → P_mc≈1.485 MPa → F_cal≈1865 N → F_clamp≈3730 N → F_friction≈1492 N → rotor torques with R_eff 110/40 mm → F_tire≈1492 N/corner → F_total≈5968 N → a≈9.18 m/s² → SD≈49 m → t≈1.35 s.
Design intent (report): rotor kept simple/efficient; cast iron for durability; large surface + 5 mm vents for heat so pads don’t overheat. Circuit: four independent lines from double tandem MC, one pedal.
External: BAJA SAE ↗ Engineering Toolbox ↗
Geometry: Ackermann (not reverse/anti) — BAJA speed limit ~60 km/h favors pure rolling / less scrub; reverse Ackermann is high-speed F1 territory. Davis gear rejected (sliding pairs, wear). Four-bar Ackermann: inner wheel more angle than outer.
Mechanism: rack-and-pinion over Pitman arm (complexity, space, couldn’t hit turning radius with fixed ratio). Steering ratio 10:1 for fewer lock-to-lock turns; quickener optional if needed.
| Spec | Value |
|---|---|
| KPI | 6° |
| Wheelbase (steering sheet) | 84 in (brake sheet also 83.5 in) |
| Track width | 63.75 in |
| Turning radius (avg) | 110.77 in |
| Lock-to-lock angle | 510° |
| Inward angle | 45° |
| Rack travel | 3 in |
| Rack length | 26.3 in |
| Tie rod length | 12.5 in |
| Pinion diameter / radius | 1 in / 0.8 in |
| Pitch | 0.2 in |
| Teeth length on rack | 11.8 in |
| Steering wheel | Partial ellipse; max radius 6 in |
Rack placed above chassis base so a line through instantaneous centre and tire centre supports Ackermann packaging. Shorter/longer tie rod → toe-in / toe-out (understeer / oversteer rule of thumb).
Materials: rack rod/teeth/pinion/rack = EN 24 (ρ 7850 kg/m³, E 205 GPa, Sy 710 MPa, Sut 1110 MPa — wear resistance); casing Al 6061.
External: BAJA SAE ↗ Engineering Toolbox ↗
Q. Walk me through the chassis from blank page to steel.
Rims/tires → wheelbase/track → SolidWorks RRH and cage → PVC 1:1 packaging mock → cut RRH nodes and resize nose/RRH for drag → IS 3074 tubes with triangulation → impact energy loads → FEA (~130k nodes) → fab. I owned structure through steering/brake integration to 1st place.
Q. Quote the chassis tube and why not 4130 for the whole frame.
IS 3074, ~45 mm OD, 2.5–2.8 mm wall, ~0.16% C, Sy≈311 MPa — weldability/ductility/cost for a space frame. 4130 reserved for A-arms where strength/hardness won the Pugh against 1018 despite cost.
Q. Derive the frontal crash load you designed to.
½mv² absorbed over D≈vΔt. With ~650 kg, 60 km/h, Δt=0.15 s → ~36 kN. Side Δt=0.2 s → ~27 kN. Rollover from 3 m → ~12.8 kN. FEA von Mises examples: front ~158, rear ~186, side ~189 MPa vs Sy 311 with FOS.
Q. Why SLA double wishbone and those alignment numbers?
Independent travel + camber control for off-road. Short upper arm cancels roll-induced positive camber. Envelope: 63 in tracks, 9 in GC, 12 in rims, 24 in tires. KPI ~6°, camber 0°, caster ~12°. Dampers to upper arm. Lotus Shark on SolidWorks hardpoints.
Q. Engine and packaging decisions?
≤810 cc; Kwid 54 bhp/72 Nm beat Alto on power and power-to-weight, SCe AFR, cable shifter. Rear engine RWD for loose-surface grip and cabin; manual for engine braking and mud gears. Kwid tank above rear engine behind firewall; radiator aft blowing to engine; battery forward.
Q. Walk the brake hydraulic chain.
Disc + Bosch tandem MC (29 mm) → Maruti 800 calipers (40 mm) → custom CI rotor 220×11 mm with 5 mm vents → Dot 4 → 5:1 pedal. At 30 km/h: ~981 N at MC rod, ~1.49 MPa, clamp ~3.7 kN, μ=0.4 → ~1.5 kN friction → ~6 kN total → ~0.9 g class deceleration in the calc. Rest bias ~25/75 F/R — discuss transfer.
Q. Steering — why Ackermann and what are the hard specs?
Ackermann for low-speed pure rolling. Rack 10:1, EN-24 rack/pinion, Al casing, rack 26.3 in, tie rods 12.5 in, travel 3 in, track 63.75 in, WB ~84 in, turning radius ~110.8 in, lock-to-lock 510°, wheel ~6 in ellipse. Pitman rejected on turning radius.
Q. What would you improve next?
Measured CG earlier, FEA-to-test correlation, instrumented brake bias, and a single consistent frontal-area assumption in the TTE sheet — same rigor we later used on AeroJC thrust/CG rigs.
External: BAJA SAE ↗ Engineering Toolbox ↗