Braking regulations — Minimum parking brake capability As a rule-of-thumb used in automotive practice, the minimum parking brake holding force is generally specified as what percentage of gross vehicle mass (GVM) when converted to equivalent braking force on level ground?

Introduction / Context:
Parking brakes (mechanical or electro-mechanical) must hold a stationary vehicle against rolling on level or modestly graded surfaces. Regulations and design targets convert vehicle mass into a required retarding force percentage to ensure safety and compliance.

Given Data / Assumptions:

• Gross vehicle mass (GVM) defines the maximum laden mass.
• Parking brake acts on rear wheels (typical) or integrated calipers/drums.
• Nominal design target referenced to level-ground equivalent force.

Concept / Approach:
Many industry practices cite a parking brake capability around one-fifth of GVM in force-equivalent terms, providing reserve to hold on specified grades once tyre-road friction and mechanical losses are considered. While regulatory details can vary by jurisdiction (grade-holding requirements), the 20% figure is a conventional benchmark used in preliminary sizing.

Step-by-Step Solution:
Express required holding force F_hold ≈ 0.20 * GVM * g (converted appropriately).Design mechanical advantage (lever, cables) and brake torque to meet or exceed F_hold at the tyre-road interface.Verify margin for wear, temperature, and park-on-grade conditions.

Verification / Alternative check:
Grade-holding tests (e.g., 20% or similar slopes) align with this sizing; certification procedures measure parking brake efficacy under load.

Why Other Options Are Wrong:
5% or 10%: typically inadequate for grade-holding with safety margin.15%: lower than common design targets.25%: higher than the widely used baseline and may be unnecessary for many classes.

Common Pitfalls:
Confusing grade percentage with force percentage; related but not identical due to tyre friction and wheel radius.

Final Answer:
20%