Forces Acting on a Steering Knuckle
Nabeel Ilyas
B.Tech Student
Department of Mechanical Engineering
National Institute of Technology Karnataka, Surathkal
Introduction
The steering knuckle is one of the most critical components of a vehicle which connects the brake,
suspension, wheel hub, and steering system to the chassis. The knuckle is free to revolve on a single axis;
hence, it converts the linear motion of the tie rod into the angular movement of the stub axle. It is affected
by various loads subjected to different circumstances, while not distressing vehicle steering performance
and other desired vehicle characteristics. The knuckle is the vital component that delivers all the forces
generated at the tier to the chassis by means of the suspension system. The design of the knuckle is
usually done considering the various focus acting on it which involves all the forces generated by the road
reaction on the wheel when the vehicle is in motion. Considering the importance of the knuckle in the
steering system it is important to have a proper understanding of all the various forces acting on the
knuckle for design and optimization purposes.
Forces Acting on The Steering Knuckle
The various forces acting on the steering knuckle due to the motion of the vehicle are as follows:
Lateral Force due to Turning
Lateral force or side force is the cornering force produced by a vehicle tire during cornering. It is
equivalent to the centrifugal force generated due to cornering.
Brake Force Due to Torque Required for Braking
Brake force is generated on the knuckle at the points of the brake caliper mountings when the brake is
applied to retard the motion of the vehicle. It is calculated as the product of the pressure generated in the
fluid lien with the net area of the caliper piston and the coefficient of friction between the brake pads and
the brake disc.
Force Due to Steering Gearbox During Turning
This is the force exerted by the steering gearbox on the steering arm mounting the knuckle through the tie
rod while turning.
Weight Transfer During Breaking or Turning
During acceleration and braking, the inertia of the vehicle chassis causes a load transfer in the
longitudinal direction of the vehicle, i.e. the load from the rear is transferred to the front while braking
and load from the front are transferred to the rear while accelerating. This load transfer (also called weight
transfer) exerts a force on the knuckle. A similar effect takes place while negotiating a corner. Inertia
causes load transfer in the lateral direction, i.e. the load from the right side is transferred to the left side
while taking a right turn and vice versa.
Bump Force
When the vehicle undergoes a ground disturbance in the form of a bump or a hole a force is exerted on the
knuckles.
Force on Impact
When the vehicle is subjected to a front, rear or side impact an impulse force is exerted on the knuckle.
Calculations
Acceleration
While accelerating, an inertia force acts on the center of gravity, and as a result of this, a weight transfer
takes place from front to rear. Considering a gravity force of 0.5g and the forces acting on the vehicle
along the wheelbase, hence reaction forces will generate on both the wheel: front and rear. The reaction
force for the front tire is given by the
Braking
An inertia force acts on the center of gravity during braking and results in a mass transfer from rear to
front. Considering a gravity force of 1.1g, the equilibrium equation is given by Smith (1978) shown in
Equation 2.
Bump Load
The most damage that could happen to the steering knuckle happens when the vehicle goes through a
bump. According to Smith (1978), the lower A arm connection point suffers a load of about 2.5G ( 1G =
1*m*g, hence 2.5G = 2.5*m*g ). While the upper connection point or the strut mounting point takes a
load of about 1.1 G. Unlike acceleration and braking there is no effect of center of gravity on the force
accumulation on the steering knuckle.
Cornering
During cornering, the vehicle takes a turn of a certain radius of curvature about a center, this makes the
load transfer in the lateral direction from the outer wheel to the inner wheel. Smith (1978) proposed an
equilibrium condition (Equation (3)), given by a gravity force of about 1.2g while cornering.
Conclusion
Almost every type of motion of the vehicle such as turning, acceleration, and braking exerts some kind of
force on the steering knuckle. A crisp understanding of these forces and the calculations surrounding them
is necessary for designing and optimization of the steering knuckle.
References
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L. Rajesh Kumar, S. Sivalingam, K. Prashanth and K. Rajkumar, 2020, “Design, analysis and
optimization of steering knuckle for all terrain vehicles | AIP Conference Proceedings”, AIP
Conference Proceedings 2207, 020003
Mahendra L. Shelar, Prof. H. P. Khairnar, “Design Analysis and Optimization of Steering
Knuckle Using Numerical Methods and Design of Experiments”, 2014 IJEDR, ISSN: 2321-9939
Ameya Bhusari, Aditya Chavan, Sushrut Karmarkar, “FEA & OPTIMISATION OF STEERING
KNUCKLE OF ATV”, International Journal of Mechanical And Production Engineering, ISSN
2320-2092
Syed Hassan Abdullah, Mahad Moiz Ullah, Obaidullah Sharief, 2018, “Design and Multi-Axial
Load Analysis of Automobile Steering Knuckle”, International Journal of Innovative Science and
Research Technology, ISSN No.: 2456-2165