A Study on Pitching Characteristics of the Passenger Car in Braking Process

Abstract

The passenger car will take on a strong pitching movement in the process of emergent braking, which makes a great impact on the ride comfort. The paper aims at resolving the practical problems of vehicle pitching. First, the variation characteristics of swing arm instant center of the front and rear suspensions are analyzed, and the suspensions adapted in the passenger car are under the combined effect of braking force and vertical force. Secondly, the calculation model of pitching characteristics and braking deceleration is established. Thirdly, test data of suspension K&C characteristic are analyzed so as to get the vertical and longitudinal displacement variation characteristics of the wheel center. Based on the calculation model of pitching characteristics and the tests of brake pitching characteristic, two conclusions are obtained. One is putting forward two evaluation indexes of vehicle pitching characteristics, the relationships of pitch angle versus braking deceleration and changing rate of longitudinal deceleration versus braking deceleration are used to comprehensively evaluate the pitching characteristics of vehicle; the other is taking advantage of four process parameters to control the pitching characteristics of vehicle in braking process, and the four parameters are respectively. The anti-dive control of front and rear suspensions, the variation of vehicle wheelbase, the displacement of the centre of gravity in vertical direction. According to the aforementioned study results, the ideal brake pitching characteristics can be reached through rationally matching the longitudinal inclination balance characteristics of front and rear suspensions.

F2012-G06-033

Appendix

1.1 Notation

C _sf :

Damp coefficient of front shock absorber

C _sr :

Damp coefficient of rear shock absorber

K _sf :

Spring stiffness of front suspension (N/m)

K _sr :

Spring stiffness of rear suspension (N/m)

K _tf :

Stiffness of front tire (N/m)

K _tr :

Stiffness of rear tire (N/m)

F _Buf :

Braking force acting on centre of under spring mass of front axle (N)

F _Bur :

Braking force acting on centre of under spring mass of rear axle (N)

F _Xof :

Braking force that car body suffers acting on swing arm instant center of front suspension (N)

F _Xor :

Braking force that car body suffers acting on swing arm instant center of rear suspension (N)

F _Zof :

The force resisting brake dive that car body suffers acting on swing arm instant center of front suspension (N)

F _Zor :

The force resisting brake dive that car body suffers acting on swing arm instant center of rear suspension (N)

F _Bf :

Braking force that acting on front tyre contact point (N)

F _Br :

Braking force that acting on rear tyre contact point (N)

F _Zf :

The ground supporting force that acting on front tyre contact point (N)

F _Zf0 :

The static ground supporting force that acting on front tyre contact point (N)

F _Zr :

The ground supporting force that acting on rear tyre contact point (N)

F _Zr0 :

The static ground supporting force that acting on rear tyre contact point (N)

F _Vf :

The spring and damping force of front suspension (N)

F _Vf0 :

The static spring and damping force of front suspension (N)

F _Vr :

The spring and damping force of rear suspension (N)

F _Vr0 :

The static spring and damping force of rear suspension (N)

r _dyn :

Rolling radius (m)

h :

Height of centre of gravity (m)

h _of :

Height of swing arm instant center of front suspension (m)

h _or :

Height of swing arm instant center of rear suspension (m)

L:

Wheelbase between front axle and rear axle (m)

L _f :

Distance between front axle and centre of gravity (m)

L _r :

Distance between rear axle and centre of gravity (m)

L _of :

Distance between centre of gravity of front axle and swing arm instant center of front suspension (m)

L _or :

Distance between centre of gravity of rear axle and swing arm instant center of rear suspension (m)

m ₁ :

Under spring mass (kg)

m _1f :

Under spring mass of front axle (kg)

m _1r :

Under spring mass of rear axle (kg)

m ₂ :

Sprung mass (kg)

J _y :

Rotational inertia (kg.m²)

Z:

The original displacement of the centre of gravity of the car (m)

Z _f :

Vertical displacement of the centre of gravity of front sprung mass (m)

Z _r :

Vertical displacement of the centre of gravity of Rear sprung mass (m)

φ:

The pitch angle around the centre of gravity Slope angle (rad)

β:

Braking force distribution ratio