ADAMS - Project - Hardpoint tuning to achieve pitch gradient targets using suspension “anti” characteristics
Pitch is a phenomenon that occurs in response to the acceleration and braking forces. Pitch is the front and rear motion of a car about an axis that extends from the left to right of a vehicle and through the center of gravity i.e. the transverse axis. Pitch angle is the angle that the body makes in side-view. Pitch is typically taken to be positive (+ve) for upward movement of the vehicle nose and negative (-ve) for downward movement of the vehicle nose.
This pitching movement occurs due to load transfer from front to rear and vice versa leading to extension and compression of springs. Anti characteristic is a term that describes the longitudinal to vertical force coupling between sprung and unsprung mass. It is a geometry setup that uses the longitudinal forces (acceleration or braking) to eliminate the spring deflection caused due to vertical load change.
Anti-dive: Reduces the compression travel in the front suspension during braking.
Anti-squat: Reduces the compression travel in the rear suspension during acceleration.
Anti-lift: Reduces the rebound travel in the front suspension during acceleration and the rebound travel in the rear suspension during braking.
These anti characteristics are driven by side view instant center location i.e its angle and distance from wheel center. This instant center location is optimized during hard point tuning to achieve the desired anti characteristics. It denotes the fraction of longitudinal load transfer that occurs through suspension links and through springs. The net longitudinal load transfer remains the same as it is purely governed by the center of gravity height, vehicle weight and wheel base.
PROBLEM:
To begin with, the excel sheet was filled with given inputs. Using the values of wheel base, CG height and vehicle weight, the load transfer value of 3585.72 N was obtained. If we consider the acceleration, the rear axle weight would increase after load transfer and front axle weight would decrease.
From the given spring rate and the motion ratio, we find the wheel rate and the ride rate of the front and rear suspension. Using this, the front and rear ride rate is calculated which gives us the pitch gradient.
The values of front and rear side view swing arm length and SVSA angle are obtained from ADAMS. For this, the double wishbone assy at front and multilink assy at rear is used. As given in the inputs, the tire radius, spring stiffness and the installed length are tuned and the required values of SVSA length and angle are obtained.
Then, from calculations, the anti-characteristics at front and rear are found. These values are confirmed using the ADAMS parallel wheel travel simulation.
Similarly, as required, the hard points are tuned to get the required anti-characteristics.
The images below shows the setup of suspension parameters:-
Double Wishbone Assembly at Front -
Rear Multilink Assembly at Rear -
The suspension parameters are as follows:-
Front--
Rear -
The tire radius was changed to 350 mm and the front spring stiffness was changed to 100 N/mm and rear spring stiffness was changed to 125 N/mm.
The initial hard points are:-
The hard points were tuned and the final hardpoints are as follows:-
The results obtained are as follows:-
A) Overlay curves for Front Suspension Assembly ----
1) Ride Rate -
2) SVSA Length -
3) SVSA Angle -
4) % Anti-Dive Braking -
5) % Anti-lift Acceleration -
6) Front end deflection under braking load with compensation -
7) Front end deflection under acceleration load with compensation -
B) Overlay Curves for Rear Multilink Suspension Assembly -
1) Ride Rate -
2) SVSA Length -
3) SVSA Angle -
4) % Anti-lift braking -
5) % Anti-squat acceleration -
6) Rear end deflection under braking load with compensation -
7) Rear end deflection under acceleration load with compensation -
The above results show that the required constraints i.e.
Front anti-characteristics < 50%
Rear anti-characteristics > 0%
are satisfied. Satisfying these constraints, we obtain the required targets of
Acceleration pitch gradient ~2.25 deg/g
Braking pitch gradient ~1.6 deg/g.
The results and the calculations are shown in the attached excel sheet. The ADAMS results very well match with the hand calculations.
The calculations can be found in the sheet below:-
https://drive.google.com/file/d/1hx2ruPB6Oo8FVti_klJT2_JmJu-ZBA-_/view?usp=sharing