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Running, Steering, and Braking Systems (Chassis)

Automobile Maintenance · 自動車整備分野 · Written by Tokutei Pass based on the official study texts; not copied verbatim.

Suspension System, Shock Absorbers, and Stabilizer

The suspension system softens road shocks to improve ride comfort while maintaining tire road contact and driving stability.

A rigid axle suspension (connecting the left and right wheels with a single axle beam) offers high strength and is used on trucks, whereas an independent suspension (such as MacPherson strut, allowing each wheel to move vertically independently) has low unsprung weight and superior ride comfort, making it common on passenger cars.

Types of springs include coil springs (common on passenger cars), leaf springs (used on trucks, providing some self-damping through inter-leaf friction), torsion bar springs (utilizing twisting reaction force), and air springs (used on large buses and trucks).

Because a coil spring has no damping action of its own, the vehicle would continue oscillating up and down indefinitely. Therefore, a shock absorber (damper) is paired with it, using the flow resistance of oil passing through small orifices and valves as a piston moves to rapidly dampen spring oscillations.

A stabilizer (anti-roll bar) is a U-shaped spring-steel bar connecting the left and right suspension arms. When only one wheel compresses during cornering, the bar twists and uses its torsional reaction force to suppress body roll (tilting).

Suspension System, Shock Absorbers, and Stabilizer

Key points


  • Independent suspension allows left and right wheels to move independently, reducing unsprung weight and improving ride comfort and road holding

  • A shock absorber uses hydraulic flow resistance as oil passes through orifices to quickly dampen up-and-down spring oscillations

  • A stabilizer uses torsional reaction force to suppress vehicle body roll during cornering (it has no effect when both wheels move up or down equally)
WordMeaning
Independent suspension and rigid axle suspension
Coil spring and leaf spring
Shock absorber (damper)
Stabilizer (anti-roll bar)

Tire Markings, Air Pressure, Wear, and Special Tire Phenomena

Tire size markings such as 195/65R15 91H are molded on the tire sidewall.

In this marking, 195 is the section width W (mm), 65 is the aspect ratio (%), R indicates radial construction, 15 is the wheel rim diameter (inches), 91 is the load index, and H is the speed symbol indicating maximum allowable speed. The aspect ratio is calculated as: (Section height H ÷ Section width W) × 100(%).

If tire inflation pressure is too low, both shoulder areas wear rapidly, rolling resistance increases, and fuel economy worsens. Furthermore, high-speed driving on underinflated tires causes a wavelike deformation called a standing wave phenomenon, generating intense heat that can lead to a tire burst.

Conversely, if inflation pressure is too high, the center of the tread wears prematurely and road shocks are poorly absorbed, hurting ride comfort. Therefore, check and adjust tire pressure with a tire gauge when the tires are cold before driving.

The legal safety limit for passenger car tire tread depth is 1.6 mm; when remaining groove depth wears down to 1.6 mm, wear indicators (slip signs) appear across the tread.

Driving at high speed on wet roads with worn tread prevents water from escaping fast enough, causing the tire to float on a film of water so that both steering and braking become ineffective—a phenomenon called hydroplaning.

Key points


  • 195/65R15 means 195 mm width, 65% aspect ratio, Radial structure, and 15-inch rim; the passenger car tread depth limit (slip sign) is 1.6 mm

  • Underinflation causes shoulder wear and high-speed standing wave phenomenon, while overinflation causes center tread wear

  • Hydroplaning occurs during high-speed wet driving when tires float on a water film and lose steering and braking control; worn tread makes it much more likely

  • Example: When tire section width is 200 mm and section height is 120 mm, the aspect ratio = (120 ÷ 200) × 100 = 60%
WordMeaning
Aspect ratio (profile ratio)
Tread wear indicator (slip sign: 1.6 mm)
Standing wave phenomenon
Hydroplaning phenomenon

Wheel Alignment (Camber, Caster, Toe-in, and Kingpin Inclination)

Front wheel alignment refers to the geometric mounting angles given to the front wheels to lighten steering effort, ensure straight-line stability and returnability after cornering, and prevent uneven tire wear. The four primary elements are camber, caster, kingpin inclination, and toe-in.

Camber is the vertical tilt of the front wheels when viewing the vehicle from the front.

Tilting the top of the wheel outward is called positive camber, which reduces steering effort. Kingpin inclination is the inward tilt of the kingpin (steering) axis when viewed from the front; together with camber, it reduces the scrub radius (kingpin offset) at the road surface to lighten steering effort and generate straight-ahead return force.

Caster is the fore-and-aft tilt of the kingpin axis (strut axis) when viewing the vehicle from the side. Positive caster (tilted rearward at the top), like a bicycle front fork, imparts straight-line directional stability and self-centering returnability to the steering wheel after a turn.

Toe-in is the condition when viewing the vehicle from directly above where the front distance A between the left and right tires is narrower than the rear distance B (B - A > 0). It counteracts the outward rolling tendency caused by camber (camber thrust) to prevent sideslip and uneven tire wear.

Wheel Alignment (Camber, Caster, Toe-in, and Kingpin Inclination)

Key points


  • Camber is wheel tilt viewed from the front, while caster is kingpin axis tilt viewed from the side (providing straight-line stability and steering returnability)

  • Toe-in is when the front tire spacing A is narrower than the rear spacing B viewed from above (Toe-in = B - A), preventing sideslip and uneven wear

  • Example: When the front center-to-center distance A between the front tires is 1,446 mm and the rear distance B is 1,450 mm, toe-in = 1,450 - 1,446 = 4 mm
WordMeaning
Wheel alignment
Camber
Caster
Toe-in
Kingpin inclination

Steering System and Power Steering

The steering system transmits rotation of the steering wheel through the steering shaft to the steering gear, and then moves the tie rods and knuckle arms to change the direction of the front wheels.

Passenger cars widely use a simple, lightweight, high-rigidity rack-and-pinion steering gear, whereas trucks commonly use a ball-nut (recirculating-ball) steering gear capable of a large reduction ratio.

Power steering uses hydraulic pressure or an electric motor (EPS) to assist the driver's steering effort. Electric power steering (EPS) does not need to drive a hydraulic pump when no steering assist is needed, offering superior fuel economy.

In speed-sensitive power steering, assist force is made large during parking (stationary steering) and low-speed driving to make the wheel easy to turn, and made small during high-speed driving to maintain firm steering feel and directional stability.

When inspecting the steering wheel, check steering wheel free play (rotational play at the rim from straight-ahead until the tires begin to move), looseness or play when shaken vertically and laterally, and inspect the tie-rod end ball joint boots for tears.

Key points


  • Rack-and-pinion steering directly converts pinion rotation into linear rack motion, providing a simple structure and sharp steering response

  • Power steering provides large assist force when stopped or at low speed for light steering, and small assist force at high speed for stability

  • Electric power steering (EPS) controls a motor based on torque sensor and vehicle speed sensor signals, improving fuel efficiency compared to hydraulic systems
WordMeaning
Rack-and-pinion type
Tie rod
Power steering
Stationary steering (turning wheels while stopped)

Hydraulic Brake Principles and Drum Brakes vs. Disc Brakes

Hydraulic foot brakes apply Pascal's principle: pressure applied to an enclosed fluid is transmitted equally in all directions. A brake booster (vacuum servo) uses the pressure difference between engine intake manifold vacuum and atmospheric pressure to multiply pedal force.

The master cylinder uses a tandem master cylinder with two pistons in series that split the brake lines into two independent hydraulic circuits. If a fluid leak occurs in one circuit, the other circuit still secures minimum braking force.

A drum brake uses wheel cylinder hydraulic pressure to press brake shoes (with friction linings) outward against the inside of a rotating brake drum. Because the rotating drum wedges the leading shoe inward to multiply braking force—known as self-servo (self-energizing) action—it produces high braking force and easily incorporates a parking brake, but it tends to trap heat.

A disc brake uses pistons inside a brake caliper to clamp brake pads against both sides of a rotating disc rotor. Although it has almost no self-servo action, the exposed rotor dissipates heat well so fade rarely occurs, water is thrown off by centrifugal force for quick recovery when wet, and piston seal elasticity automatically adjusts pad-to-rotor clearance.

Hydraulic Brake Principles and Drum Brakes vs. Disc Brakes

Key points


  • Hydraulic brakes use Pascal's principle, and a tandem master cylinder uses two independent circuits to maintain braking if one circuit leaks

  • Drum brakes have self-servo (self-energizing) action for strong braking force, but heat builds up easily inside the drum

  • Disc brakes excel in heat dissipation and water shedding, resist fade and uneven pulling, and automatically adjust pad clearance via piston seal deformation
WordMeaning
Pascal's principle
Tandem master cylinder
Brake booster (vacuum servo)
Self-servo (self-energizing) action
Brake caliper and brake pad

Fade Phenomenon, Vapor Lock, Stopping Distance, and ABS

If the foot brake is used continuously on a long downgrade, frictional heat overheats the brake pads or linings, lowering their coefficient of friction and reducing braking effectiveness. This is called the fade phenomenon.

If brake overheating continues, the heat transfers to the brake fluid inside the caliper or wheel cylinder and causes it to boil, forming vapor bubbles in the brake lines.

When this happens, pressing the brake pedal merely compresses the vapor bubbles without transmitting hydraulic pressure, leaving the pedal spongy and the brakes ineffective. This is called vapor lock; to prevent both fade and vapor lock, downshift to a lower gear and use engine braking on long descents.

ABS (Anti-lock Brake System) uses wheel speed sensors during sudden braking or braking on slippery roads to detect wheel lockup and automatically modulate (reduce, hold, and increase) brake fluid pressure. Preventing wheel lockup maintains directional stability and preserves steering control to steer around obstacles.

The distance traveled from when the driver perceives danger until the brakes begin to take effect is the reaction (free-running) distance, and the distance from when the brakes take effect until the vehicle stops completely is the braking distance; their sum is the stopping distance (Stopping distance = Reaction distance + Braking distance). Doubling vehicle speed doubles the reaction distance and quadruples (2 squared = 4 times) the braking distance.

Key points


  • Fade is a drop in friction caused by overheated pads/linings, whereas vapor lock is loss of hydraulic pressure transmission caused by boiling brake fluid forming bubbles

  • ABS prevents wheel lockup during hard braking to maintain straight-line directional stability and steering control around obstacles

  • Stopping distance = Reaction distance + Braking distance; when vehicle speed doubles, braking distance increases 4 times

  • Example: When the reaction distance is 15 m and the braking distance is 25 m, the stopping distance = 15 + 25 = 40 m
WordMeaning
Fade phenomenon
Vapor lock phenomenon
ABS (Anti-lock Brake System)
Reaction distance, braking distance, and stopping distance

Parking Brakes, Heavy-Duty Auxiliary Brakes, and Brake Inspection

The parking brake uses a mechanical wire cable (or electric actuator) to lock mainly the rear wheels so a parked vehicle cannot roll. During inspection, pull the lever (or press the pedal) and verify that the travel (number of clicks/notches) is within specifications.

Large trucks and buses use full air brakes powered by compressed air from an air compressor. Because condensed water collects in the air tanks, drain water daily from the drain cock to prevent freezing in winter and valve corrosion.

Large diesel vehicles also use auxiliary brakes such as an exhaust brake or retarder to prevent foot-brake overheating on long downgrades. An exhaust brake closes a valve in the exhaust pipe to increase exhaust back-pressure and generate strong engine braking.

When inspecting the brake pedal, check pedal free play, pedal-to-floor clearance when depressed firmly, and ensure the pedal does not feel spongy (which indicates air trapped in the hydraulic lines).

On disc brakes, as the brake pads wear, the caliper pistons extend outward and the fluid level in the reservoir tank drops correspondingly, allowing pad wear or fluid leaks to be inferred from a low fluid level.

Key points


  • The parking brake holds the wheels mechanically via cable, and its lever travel (number of notches) is checked during inspection

  • An exhaust brake on a heavy vehicle closes a valve in the exhaust pipe to raise exhaust resistance and strengthen engine braking

  • Air trapped in the brake lines makes the brake pedal feel spongy and reduces pedal-to-floor clearance when depressed
WordMeaning
Lever travel (number of notches)
Air brake
Exhaust brake
Pedal-to-floor clearance

Frequently Tested Points and Memory Tips

For suspension and tires, match each pair accurately: Shock absorber = dampens vertical spring oscillation; Stabilizer = suppresses body roll in turns; Low tire pressure = shoulder wear + standing wave phenomenon; High tire pressure = center wear; Worn tread (1.6 mm slip sign) + high speed in rain = hydroplaning.

For wheel alignment, focus on the viewing direction: Tire tilt viewed from the front = Camber; Kingpin axis tilt viewed from the front = Kingpin inclination; Kingpin axis tilt viewed from the side = Caster (straight-line stability and returnability); Front-to-rear spacing difference viewed from above = Toe-in (Rear B - Front A).

A common power-steering trap question claims that assist force is increased at high speeds. In fact, assist force is large at low speeds/parking (for light effort) and small at high speeds (for stability).

For brakes, memorize the difference between Fade = overheated friction material (pads/linings) and Vapor lock = boiling brake fluid creating vapor bubbles, and contrast Drum brakes = self-servo action but retains heat vs. Disc brakes = good cooling, fade-resistant, and automatic clearance adjustment.

Key points


  • Shock absorber = dampens vertical oscillation / Stabilizer = suppresses body roll / Slip sign = 1.6 mm remaining tread depth

  • Low tire pressure = shoulder wear & standing wave / Floating on water film at high speed in rain = hydroplaning

  • Camber = tire tilt from front / Caster = axis tilt from side (stability & return) / Toe-in = viewed from above (Rear B - Front A)

  • Power steering provides large assist force at low speed and small assist force at high speed

  • Fade = overheated friction pads/linings / Vapor lock = boiling brake fluid bubbles / Disc brake = good cooling and fade resistance
WordMeaning
Body rolling
Uneven tire wear
Spongy brake pedal

Running Gear, Steering & Brakes

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