Greetings!
This post is in continuation with our previous post about 'Design of Suspension and Steering System for a Formula Student Car (FSAE) using 3DEXPERIENCE Platform'. This post will be explaining about the process of selection and design of shock absorbers for front and rear suspension.
Shock absorbers are selected based on design requirements, market availability and cost. The minimum wheel travel permitted in the competition is specified as 50.8mm; 25.4mm jounce travel and 25.4mm rebound.
In selecting a shock absorber, it was thus important to find a model that provided enough stroke travel in both directions after sag from the static weight of the vehicle and motion ratio were taken into consideration.
Motion ratio should be close to 1 for better handling and the motion ratio should not stray too far from 1:1 as this allowed a relatively small shock absorber but also affects the handling capability.
The shock absorbers selected are Mountain Bike Shock Absorbers as these types of shock absorbers are cheap, readily available, compact and light weight. And also, it was decided to use a shock absorber model common at the front and rear of the car because it was expected that using the same shock absorbers in front and rear suspension would improve the predictability of the vehicle under various handling manoeuvres thus enhancing the feel experienced by the driver.
When selecting shock absorber locations, the design’s packaging, simplicity, accessibility, and applicability were all considered.
The “Natural Frequency Method” was used as a goal and starting point in selection of the vehicle’s springs. According to this method, “if the iterated coil rate is equal to the available coil rate, then this available coil rate is best suitable for given suspension geometry”.
For deciding the actual value of coil rate (spring stiffness) and respective wheel frequency, the iterations were carried out using mathematical formulas given in Staniforth book and also using the Bump/droop DBM Model of front and rear suspension. The mathematical formulas are:
While performing the iterations at different value of available coil rate, once the iterated coil rate gets matched with available coil rate spring, then this selected coil rate was considered as best suitable spring for the vehicle.
Final Suspension Actuation Specifications:
After performing iterations, the 200 lbs/in coil rate spring at front static motion ratio 1.602:1 and rear static motion ratio 1.665:1 was satisfying the Natural frequency method and hence consider as the final spring geometry for the vehicle.
Parameters | Front | Rear |
Type of Suspension Actuation | Outboard, damper to lower wishbone | Outboard, damper to lower wishbone |
Shock Absorber Model | 2022 DHX FOX 200 x 50 | 2022 DHX FOX 200 x 50 |
Spring Stiffness | 200 lbs/in | 200 lbs/in |
Static Motion Ratio | 1.602:1 | 1.665:1 |
Wheel Frequency | 148 CPM | 145 CPM |
Wheel travel at 1mm compression | 1.5196 mm | 1.562 mm |
Wheel travel at 25mm compression | 40.3321 mm | 41.752 mm |
Motion Ratio at 1mm compression | 1.5196 | 1.562 |
Motion Ratio 25mm compression | 1.6132 | 1.67 |
Wheel rate at 1mm compression | 86.61 lbs/in | 81.972 lbs/in |
Wheel travel at 25mm compression | 76.851 lbs/in | 71.712 lbs/in |
The above readings were taken using Bump/Droop Model in Dymola Behavior Modelling app. The mathematical calculations were verified by the results obtain from Dymola Behavior Modelling app. The graph obtains between Spring Travel Vs. Wheel Travel for both front and rear suspensions are shown below:
The above figures 1 and 2 shows CAD Modelling of front and rear suspension assembly along with spring and damper assembly which was created in CATIA Part Design app and Assembly Design app.
Edu 3DEXPERIENCE CATIA Behavior Modeling Dymola Simulation Work Showcase
Students Involved:
@AK
@PP
