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CDL · Antilock Brakes

When driving a doubles combination on a slippery downgrade, what braking technique reduces jackknife risk most?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Smooth, progressive brake application that maintains rolling wheels on all axles — combined with appropriate gear selection and engine braking to reduce reliance on service brakes

  1. A Use only the trailer hand valve
  2. B Maximum brake force on the front axle only
  3. C Pump braking at random intervals
  4. D Smooth, progressive brake application that maintains rolling wheels on all axles — combined with appropriate gear selection and engine braking to reduce reliance on service brakes

Why this is the answer

Progressive braking maintains wheel rotation on all axles, preventing the lockup that triggers jackknife. On downgrades, engine braking (selecting a lower gear before the descent) reduces service brake reliance — the engine's compression does the primary retarding work, leaving service brakes for final speed control. The trailer hand valve (which applies only trailer brakes) is dangerous on slippery surfaces as it can lock the trailer wheels while tractor wheels continue rolling — a guaranteed jackknife setup.
Source: FMCSA CDL Manual, Chapter 7, Doubles and Triples, Downgrade Braking

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About this topic: Antilock Brakes

ABS prevents wheel lockup during heavy braking, preserving steering and reducing stopping distance on slick surfaces. Drivers must brake firmly, not pump, on ABS-equipped vehicles. This is one of 5 Doubles and Triples questions on antilock brakes, each with a full explanation on a single study page.

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