Steering system forces and moments

1. Submitted to Presented By
Dr. V Suresh Babu Liril D Silvi
(143716)

3.  The forces on steering system are originate
from tire road interface.
 The forces are measured at centre of
contact
Lateral
Force
Overturning
moment
Tractive
Force
Overturning
moment
Normal
force
Aligning
Torque

5. The vertical upward force is
considered as positive force (SAE).
Fz has a component acting to produce
moment attempting to steer the
vehicle. The moment arises from both
the caster and steering lateral
inclination angles.
Mvλ = Fzr sinλ ∗ d sin𝛿 …..(1)
where
Mvλ = Vertical moment on wheel
Fzr = Vertical load on the wheel
d=lateral offset at the ground
λ = king pin inclination angle
𝛿=steer angle
v=caster angle

6. The caster angle resulted in sine
angle force component Fzr sinv which
nominally acts forward on the
moment arm “d cos𝛿”
M 𝑣𝛿=Fzr sinv * d cos𝛿 … … . (2)
Where
M 𝑣𝛿=Moment due to caster angle
Mvλ = Fzr sinλ ∗ d sin𝛿 M 𝑣𝛿=Fzr sinv * d cos𝛿
Mv= -(Fzr+ Fzl) sinλ ∗ d sin𝛿+(Fzl –Fzr) sinv * d cos𝛿
Load and caster angle may affect toe in and imbalance due to load or
geometric asymmetry may result in steering pull.
Steering
Pulling :-
Problem

7. The lateral force acting at the tire centre
produces a moment through the
longitudinal off setting from caster angle.
The net moment produced is :
ML=(Fyl + Fyr) r tanv
Where
Fyl,Fyr = lateral forces left and right
wheels
r = tire radius
It is major contributor to understeer.

8. The tractive force Fx acts on the king pin offset to
produce a moment :
MT = (Fxl - Fxr) d
Where
Fxl, Fxr = tractive forces on left and right wheels
The left and right moments are opposite in
direction and tend to balance though relay linkage

9.  The aligning torque acts vertically and
may be resolved into a component acting
parallel to the steering axis.
 MAT=(Mzl+Mzr) cos( λ2 + v2)
The aligning torque always act to resist
any turning motion, thus their effect is
understeer.

11. 𝑆𝑡𝑒𝑒𝑟𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =
𝑠𝑡𝑒𝑒𝑟𝑖𝑛𝑔 𝑤ℎ𝑒𝑒𝑙 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝑎𝑛𝑔𝑙𝑒
𝑠𝑡𝑒𝑒𝑟 𝑎𝑛𝑔𝑙𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑡𝑖𝑟𝑒 𝑤ℎ𝑒𝑒𝑙
15 - 20 ͦ steering wheel
rotation
1 ͦ steer angle tyre
wheel rotation
For
car
20-36 ͦ steering wheel
rotation
1 ͦ steer angle tyre
wheel rotation
For
truc
k
Steering wheel angle VS front wheel
angle (FOR TRUCK)
The actual steering may vary and influencing manoeuvrability of the vehicle Because
there will be variation in torque due to tire loads, tire pressure, coefficient of friction.NOTE:-

12.  Fig shows the steer angle gradient measured at the
steering wheel and left road wheel of a loaded truck
with manual steering.
 The magnitude of the steering system contribution is
dependent on the front wheel load and caster angle.
 For a simple analysis for the understeer influences in
which the lateral forces and aligning torques are
dominant.
 The understeer gradient :
Kstrg =
Wf(r v + p)
Kss
Where,
Kstrg = understeer increment (deg/g) due to steering system
Wf =front wheel load
R =wheel radius
P =pneumatic trail associated with aligning torque
V =caster angle
Kss =steering stiffness between road wheel and steering wheel

13.  Braking is a special case in which steering
system design play an important role in
directional response.
 The tire aligning torque which effectively act like
4-8 degree of caster angle under free-rolling
conditions can also reverse in direction during
braking.
 At low braking coefficient the aligning torque acts
in the tire caster effects. Acts in the direction of
travel which on the steered wheels attempts to
steer the vehicle out of the turn.(understeer
influence).
At high braking coefficient the aligning torque
reverses direction and may reach elevated negative
levels which will attempt to steer the tire into
direction of turn (an oversteer influence).

14. Thank You