Auto & Shop InformationLesson 7 of 17
Brakes, Suspension and Steering
Disc and drum brakes, springs and shocks, steering linkage, and wheel alignment terms.
Table of ContentsShow
These three systems have nothing in common mechanically, and one thing in common practically: all of them are about what happens where the tire meets the road.
Brakes
| Disc brake | Drum brake | |
|---|---|---|
| Friction parts | pads | shoes |
| What they act on | a rotor, squeezed from both faces | the inside of a drum, pressed outward |
| Actuator | caliper with a piston | wheel cylinder |
| Heat | sheds it well, exposed to air | holds it |
| Where found | front wheels, and often all four | often the rear wheels |
The arrows in the figure are the memory hook. Disc brakes squeeze inward, drum brakes push outward.
Discs handle heat better, which matters because braking is nothing but the conversion of kinetic energy into heat. A drum encloses its own friction surfaces, so the heat has nowhere convenient to go, and overheated brakes fade - they lose friction and the pedal goes soft.
Front brakes do most of the work. Weight shifts forward under braking, pressing the front tires down and giving them more grip to use. That is why front discs with rear drums is such a common arrangement.
The hydraulic system
Pressing the pedal moves a piston in the master cylinder, and the brake fluid carries that pressure to a piston at each wheel.
Pressure applied to a confined fluid is transmitted undiminished throughout - Pascal's principle, from the hydraulics lesson. It is why one pedal operates four brakes with the same force, and why the system needs no linkage running to each wheel.
Brake fluid must not compress, which is why a liquid is used.
Air in the brake lines makes the pedal feel soft or spongy, and this is the question the topic asks most often. Liquid does not compress but air does, so a bubble absorbs the pedal's travel instead of passing the pressure on. The fix is bleeding the brakes: forcing fluid through until the air is out. A soft pedal is air; a pedal that sinks slowly to the floor is more likely a leak or a failing master cylinder.
A power brake booster uses engine vacuum to multiply pedal effort, which is why the pedal goes hard when the engine is off.
Antilock braking pulses the brakes rapidly when a wheel is about to lock. A rolling tire steers and grips better than a sliding one, which is the friction lesson's point about static friction exceeding kinetic friction, applied to a safety system.
Suspension
The job is to keep the tires in contact with the road while isolating the body from the surface. Contact is what matters: a tire in the air neither steers nor brakes.
| Component | Job |
|---|---|
| Springs (coil, leaf or torsion bar) | carry the weight and absorb bumps |
| Shock absorbers or struts | damp the spring so it stops oscillating |
| Control arms | locate the wheel while letting it move up and down |
| Ball joints | pivots that let the wheel steer and move |
| Sway bar | resists body roll in a corner |
| Bushings | rubber isolators between moving parts |
The spring and the shock absorber do different jobs and this is asked directly. The spring carries the load. The shock absorber does not. Without damping, a spring that is compressed by a bump keeps bouncing, and a bouncing tire spends part of its time barely touching the road.
Worn shocks show up as continued bouncing after a bump, as nose dive under braking, and as uneven tire wear.
Steering
| Component | Job |
|---|---|
| Steering wheel and column | driver input |
| Rack and pinion | converts rotation into side-to-side movement |
| Tie rods | push and pull the steering arms at each wheel |
| Power steering | hydraulic or electric assistance |
Rack and pinion is the common modern arrangement. A pinion gear on the column turns against a toothed rack, so turning the wheel slides the rack sideways - the same rotation-into-straight-line conversion a screw performs.
Alignment
| Term | Meaning |
|---|---|
| Camber | how far the top of the wheel tilts in or out, viewed from the front |
| Caster | the forward or backward tilt of the steering axis, viewed from the side |
| Toe | whether the front edges of the tires point toward each other or apart |
Bad alignment wears tires unevenly and pulls the vehicle to one side, which is the symptom a question describes. Toe is the commonest culprit for rapid, uneven wear across the tread.
What you can skip
Across the 59 questions on this topic in our bank:
- Brake fluid grades. DOT 3 and DOT 4 never appear. Know that brake fluid absorbs water and must be the type specified.
- Suspension designs by name. MacPherson struts never appear.
- Alignment angles in depth. Caster comes up once, camber four times and toe five. Know what camber and toe are and that misalignment wears tires unevenly.
Where people lose points
Swapping pads and shoes. Pads are disc, shoes are drum.
Saying drum brakes cool better. Discs do.
Thinking the shock absorber supports the vehicle's weight. The spring does.
Saying a soft pedal means worn pads. It usually means air in the lines.
Thinking antilock braking shortens every stop. Its purpose is keeping the wheels turning so the vehicle can still be steered.
Forgetting that brakes are hydraulic. The pedal moves fluid, not a cable.
Work one in under a minute
A driver reports that the brake pedal feels soft and travels farther than it used to before the vehicle slows. What is the most likely cause, and why does this symptom appear?
Air in the brake lines.
The system works because liquid does not compress, so pedal movement becomes pressure at the wheels immediately.
Air does compress. A bubble in the line squashes as the pedal is pressed, and that travel is spent shrinking the bubble instead of pressing the pads against the rotor.
The remedy is bleeding the brakes, forcing fluid through the lines until the air is expelled.
Note that the diagnosis came from one physical property. Compressibility is the whole reason the system uses fluid, and the whole reason air ruins it.
Where this leads
Pascal's principle runs the brakes, static against kinetic friction explains antilock braking, and energy conservation explains where the vehicle's motion goes.
Related lessonsReference
- Drivetrain and Transmission - what these systems stop and steer
- Pressure and Hydraulics - Pascal's principle in general form
- Friction - why a rolling tire grips better than a sliding one
- Work, Energy and Power - the kinetic energy that becomes heat
Practice this topic
Check that this lesson stuck. Answer questions on brakes, suspension and steering only, and see the right answer and why after each one.
Practice Brakes, Suspension and Steering questions