Basically the title.
In: Engineering
Trucks drive a lot more than cars, and they’re designed to be able to work at high driving range with high weight loads without failing.
Most importantly, air brakes will stop a machine by *default*. This means if a truck’s braking system fails, the truck will slow to a stop.
Liquid brakes for a truck would and add a significant failure point that could cause accidents, plus it would require tons of liquid and much more vehicle maintenance.
So why don’t cars use air brakes if they’re so awesome? Well, they’re a lot more complex and expensive than liquid brakes. They’re also heavier — relative to the weight of a car, air brakes would really kill your fuel efficiency.
Trucks, however, have massive weight loads and get used a lot more than most cars, so the extra weight and expense is fairly trivial for them.
Air hoses are easier to connect to trailer, and air brakes will work even with leaks in system.
As trailers are added and disconnected, brake lines are too. Traditional brake lines with fluid in them perform very poorly with even a small amount of air in the line, and repeated connecting/disconnecting will inevitably result in some in the lines.
Air is simpler in that you just need a sealed system, and the medium that controls braking (air) is added after the system is closed.
A further advantage is that brakes work on friction, and friction produces heat. Under heavy braking, brake fluid can boil and produce voids in the brake lines, which reduces pressure on the brake pads and reduces braking power. This is known as brake fade. Getting water in the brake lines is even worse because its boiling point is lower, creating steam voids. This affects all vehicles, including passenger cars travelling down steep hills, and is why you’re advised to use engine braking rather than the foot brake. Very heavy vehicles going downhill are ripe for brake fade and why escape lanes exist. Air brakes are already gas so cannot boil, and indeed the extra heat will increase the air pressure in the lines, improving braking force slightly.
Ability to connect a trailer or multiple trailers quickly. When you remove air for the system (like leaving a trailer somewhere) the brakes are locked. If you lose air pressure they fail with the brakes applied. If there’s a small leak the trucks air compressor can compensate until it’s fixed. Repairs are much faster and can be performed roadside. You can hear air leaks. Air lines don’t rust.
Many other systems are controlled by air. The truck and trailer suspension, automatic transmission, cab suspension, axle lockers, radiator fan. All controlled with air pressure.
Trailers are often disconnected and reconnected.
If you spill some air, no big deal. If you spill some hydraulic fluid, that’s somewhere between a mess and a pollution standards violation.
If you need more air, it’s free. You just pump it out of the atmosphere. If you need more hydraulic fluid, that’s $20-50 per gallon.
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