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Earlier today I was driving behind a double-trailer dump truck configured something like this:

enter image description here

As we approached a red light, the driver obviously applied the brakes, and for the last 50 or so feet before coming to a stop, the rear wheels of the rear trailer were all very obviously locked up as evidenced by clouds of smoke and a nice skid mark from all 4 tires on that axle.

This was during normal driving conditions. I don't think the driver was making any kind of emergency stop.

Are the rear wheels locking up due to some malfunction, or is this perhaps happening simply because the rear trailer was possibly unloaded?

Out of an abundance of caution I made sure to leave an extra comfortable distance. However, other than the spectacular smoke and skidmarks, the vehicle appeared to be under control. Is there any danger to either the truck or to other vehicles (me) when this happens?

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Truck tyres/tires are expensive and this sort of abuse doesn't help them last. Retreads are much more common in trucks too. So this bouncing will make the retread casing more likely to separate and fling off sooner. Picture the chunks of rubber you might see on the side of the road. – Criggie Feb 4 at 5:56

The issue most likely is, the trailer was empty. The brakes are designed to work well when the trailer is full. As you can imagine, if the dump was full of loose gravel there'd be quite a load in it. When the trailer is empty, the same amount of force on the brake pedal can cause the brakes to lock up easily. This is something more for the driver to worry about than you ... this gets expensive on tires!

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It doesn't if the trailer is loaded or not. The air brakes pressures are regulated by relay valves that adjust the pressure according to the load. If every empty trailer on the road after being unloaded after every customer started locking at every stop sign trucking would spend a lot money replacing tires. – resident_heretic May 25 at 2:18
    
@resident_heretic - Whether the auto adjuster is there or not, I've seen it happen a ton of times with empty trailers: the driver hits the brakes hard and the trailer brakes lock up. This doesn't happen as often (but still can) if the trailer is full because the tire to road contact is much more secure. – Pᴀᴜʟsᴛᴇʀ2 May 25 at 21:00

My first thought is that the trailer brakes are correctly adjusted and the trucks are out of adjustment. Excessive pedal pressure (movement) is required to engage the truck brakes. This over brakes the empty trailer causing the lock-up.

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With trucks whether they are tandem trailer (A or B trains depending on the size of trailers) or not have air brakes. Sometimes they develop leaks in the air lines. When the pressure drops below a certain value the trailer brakes lock up suddenly. I notice you didn't mention smoke from the front trailer. I suspect it was normal stop but the rear trailer had catastrophic air pressure loss.

Air brakes have 2 air line each- one to apply the brakes one to release. The air pressure works against a spring in the brake cylinder so when the air pressure dumps the the brakes lock up. If massive air loss happens at highway speed the brakes lock up and the brake lights do not activate. Anybody who tailgates at this moment is seriously injured or dead.

There is strong possibility of tire failure where the tire explodes violently because unlike car tires which are inflated to 32 psi-Truck tires 120 psi and are more numerous. This is one reason why cars should keep a respectable distance from trucks. In most jurisdictions it is illegal to follow a commercial vehicle closer than 30 meters (approx 100 ft).

When empty a truck can stop a lot faster than car can. Basically truck brakes are much bigger and more numerous and a designed to stop a helluva lot of weight. In this scenario it is braking overkill and then some. Cars have smaller brakes and only 4.

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