Quick answer: Unreal Chaos Vehicle sliding all over the road after upgrading to 5.4? Friction curve interpolation changed - reauthor the friction modifier curve or set FrictionForceMultiplier to 1.2-1.5.
Truck handling that felt right in 5.3 is a hockey puck in 5.4. The values on the wheel didn't change.
Bump friction multiplier
On the wheel, raise Friction Force Multiplier to ~1.3. The 5.4 friction curve evaluator uses linear instead of cubic interpolation; the effective grip dropped silently.
Reauthor the lateral curve
Open Lateral Friction. Add more keys around 0.0 - 0.3 slip ratio to match the cubic shape from 5.3. Five keys is usually enough; you don't need a smooth curve here.
Verify with the debug view
p.Chaos.Vehicle.Debug 1. The per-wheel friction force renders as a colored arrow - if it's visibly shorter post-upgrade, the curve is the cause.
“Engine upgrades that 'don't change anything' frequently change the parts you noticed first.”
After every major engine upgrade, re-record a single golden lap of your vehicle. Visual diffs against the previous build catch every tuning regression at once.