Emergency Braking and Jackknife Prevention
Jackknife in a doubles or triples combination can occur at the tractor-trailer coupling (tractor jackknife), at the first dolly-trailer coupling (dolly jackknife), or at both simultaneously. A tractor jackknife occurs when the tractor's rear wheels lock during braking, causing the rear of the tractor to slide out. A dolly jackknife occurs when the converter dolly wheels lock or when the weight of the following trailer pushes the dolly sideways through the coupling. Modern ABS systems significantly reduce jackknife risk but do not eliminate it, particularly for older dollies that may not have ABS.
The progressive brake threshold technique — applying brakes in a smooth, escalating pressure sequence rather than a single full application — is the primary preventive measure for jackknife during emergency stops. This technique allows the driver to maintain controlled deceleration while monitoring the combination for any signs of instability. If sway begins during emergency braking, the driver should ease brake pressure slightly rather than releasing fully (which allows the combination to regain speed) or increasing pressure (which worsens the instability).
On wet or icy roads, a doubles or triples driver must begin braking earlier and with lighter initial pressure than on dry roads. Ice reduces the coefficient of friction so dramatically that full brake application immediately locks all wheels in most conditions, triggering ABS cycling and potentially sway. A speed of 30 mph on ice presents the same stopping challenge as 60 mph on dry pavement for many double trailer combinations. Road condition awareness must begin before reaching the stopping point — not at it.
Blowout response in doubles and triples requires the driver to maintain steering control while the combination stabilizes. A blowout on the rear trailer can cause significant sway and must be managed by holding steady steering (not overcorrecting), releasing the accelerator to slow progressively, and moving to the shoulder gradually. A driver who brakes hard in response to a blowout-induced sway will almost certainly convert manageable sway into an uncontrolled situation.