Current TRL: 2 (12/17/21)

Suspension System

The current team has inherited fully integrated front and rear suspensions with Ohlins
TTX25 shocks. We have been working on the rear suspension primarily over the last few months. We found that the rear suspension was not rules compliant and also does not comply with our objectives that can be found below. We also found our shocks need to be updated for a lower spring rate and higher damping constant to attain desired dynamics over bumps in the track. Moving forward, we will need to do a similar analysis of the front suspension and also look at wheel orientation through ride. Our Next stage will focus on the geometry of the suspension points to reach our objectives of camber, bump steer, and roll stiffness.

 

 

Main Documents

  • Part Inventory (Google Drive) (Last Updated: 2020)
  • Full Model Inventor Files (Google Drive) (Last Updated: 02/29/2021)
  • Lafayette Kinematic Software (Github)(Last Updated: 10/29/2021)
  • Ohlin Shock Links (Google Drive) (Last Updated: 02/29/21)
  • Preliminary Characteristic Measurement Process (Google Drive) (Last Updated: November 1, 2021)
  • Relevant Suspension Rules for the Formula Hybrid Competition (Link Here)
  • Mid-Year Steering and Suspension Poster (Google Drive)(Last Updated: 12/12/2021)

Literature Review

The suspension on a racecar needs to be integrated with the chassis and steering system to get the intended results. The suspension attachment points integrated into the chassis largely affect the dynamics of the system. Additionally, our front suspension must be designed to create safe steering conditions as shock deflection can change wheel steer angle, camber, caster, etc. These same angle effects of the rear suspension deflections can cause instability in corning as well. While each subsystem has an individual function, we see that they are very interconnected, making cross subsystem study and integration is of vital importance. 

The Lafayette formula hybrid car currently contains a double A-arm configuration with a pushrod operated bell crank front suspension and a double A-arm with a linear coil over shock design. It is key that we identify key parameters for intended performance, test the current suspension to see if these parameters can be met, and modify the suspension system to reflect these parameters within reason. It is also our intention to begin design of a new suspension that can more closely represent our ideal performance expectations. This is why suspension development tools will also be a focus of this review.

Full Literature Review

 

Design Proposal

The suspension will be handed off to the club, with our subsystem teams guidance at TRL level 3. (See Subsystem TRL Chart)

The current state of the suspension system is relatively untested. We must first determine pertinent characteristics of the current suspension through static measurements of wheel deflection and computer modeling that are already under way. Then we will identify geometric changes that can be made to non-invasively modify the current design while improving key characteristics. Ideal modifications will be validated through computer modeling. Testing equipment will be developed to effectively confirm our model once implementation on the chassis has been achieved.

 

Motivation

A race car’s suspension is critical to its overall performance and safety. This system’s goal is to maintain maximum wheel area on the ground for maximum traction, especially through cornering. The suspension should maintain optimal wheel alignment despite forces due to uneven roads and high-speed turns. It is responsible for the handling of the car. Not only does this system impact performance, but it is integral to the safety of the driver by ideally ensuring maximum possible traction of all four wheels and mitigating rollover risk.

 

Conceptual Solution

Preliminary measurements of key characteristics like toe in/out, camber, and shock deflection are in the process of being completed. The next step for this system is to generate computer simulations of the kinematics of both the front and rear suspensions. These can be compared to our preliminary measurements. These tests will give the team an idea of the performance and handling of the car through parameters like variable roll center and wheel position. These computer simulations can then be iterated through varying geometries to find the ideal modifications we can make to suspension. It is the intention of our team to preserve last years design by making minor changes to improve performance. We do not plan to make massive chassis design changes to modify the current pick up points. This will be a constraint on our computer simulation iteration. We would like to get camber, toe, and roll center as close to ideal as possible, but the main goal is to get the system into a yet to be determined acceptable state for the club, such that designs for an improved suspension can begin for next years car.

We would like to develop more user friendly, accurate, and effective measuring devices for pertinent suspension characteristics. This may include skid pads with an integrated encoder for steer angle and toe measurements, and string potentiometers developed in house to accurately find shock deflection, camber, and toe characteristics. Once, we have an ideal geometry validated through computer modeling, we will hand off detailed documentation to the club to implement this onto the car. Testing information of these modifications will also be included. The suspension team is prepared to work with the club to confirm static tests and make additional modification suggestions as needed. These static measurements by the club will be aided by the measurement equipment mentioned above.

 

Codes and Standards:

Rollover Testing Methods Standards (Link Here)

This document provides insight into standard techniques used for rollover testing and rollover crashworthiness evaluation at both the vehicle and component levels. These tests are vital for safety in our vehicle 

 

Balanced Suspension (Link Here)

A discussion on the significance of having a flexible yet stiff suspension so that the car can maintain high levels of grip with the track while also minimizing ride height changes.

 

Steering and Suspension Technology (Link Here)

This technical paper collection covers both steering and suspension technology. This includes rear suspension design, composite material suspension arm as well as other related information.

 

Constraints

From the formula hybrid competition rules: The car must be equipped with a fully operational suspension system with shock absorbers, front and rear, with usable wheel travel of at least 50.8 mm, 25.4 mm jounce and 25.4 mm rebound, with driver seated. The judges reserve the right to disqualify cars which do not represent a serious attempt at an operational suspension system or which demonstrate handling inappropriate for an autocross circuit.

 

Metrics

A suspension must keep the wheels of the car on the ground and in the correct orientation through cornering and from perturbations due to  inconsistencies in the road. It should be able to give the driver maximum control over the vehicle. This means it should provide stability. The suspension must counteract upward forces on the wheel without causing excessive motion of the car or lifting of the wheels. Changes of the wheel orientation from these forces should keep the vehicle stable at the very least. Wheel orientation can also be controlled to increase tire to road contact and generally improve the driver control of the vehicle.

 

Ride Height

This is the height of the bottom of the chassis above the ground where the wheels contact. Static ride height refers to this distance when the suspension is only under the load of the physical car and its driver. The suspension will be under some compression under this weight. This is important to avoid contact of the chassis with the ground and gives a neutral suspension position. The travel of the chassis cannot cause the suspension to bottom out.

 

Jounce 

The term jounce refers to the suspension’s displacement under vertical forces that compress the suspension in excess of static ride height. This can be seen as downward force on the chassis

 

Rebound

Rebound is the opposite of jounce. It refers to the displacement of the suspension under motion that reduces compressive forces of the suspension. This is an extension of the suspension system where the chassis’ height is greater than static ride height.

 

Bump Steer 

As the suspension is under jounce or rebound, the wheel tends to ‘steer’ itself. This is a rotation of the wheel about its vertical axis due. It is generally stabilizing to the vehicle if the wheel turns towards the center of the car, especially in jounce. It is ideal, however, to remove bump steer altogether in that there is no steering in the wheel due to wheel displacement, however it is impossible to eliminate all together.

 

Natural Frequency

The natural frequency of the suspension is an indicator of its stiffness. Race cars generally have a higher natural frequency than passenger cars as a softer suspension improves driver comfort but takes longer for motion to settle after a bump in the road. Driver comfort is not important in a race car as driver control takes precedence here. If the vehicle takes too long to settle after some perturbation, stability through turns and bumps in the road is sacrificed. Chassis roll is also controlled by suspension stiffness.

 

Roll Gradient

Under lateral acceleration through corners, forces downward on the chassis will be greater about the wheels on the outside of the corner than the two wheels on the inside. This creates unequal suspension deflection on the left and right sides of the car. This causes the chassis to rotate or roll about its longitudinal axis. Roll stiffness is the torque per degree roll of the chassis and is largely determined by the track width of the car and the stiffness of the suspension. The roll gradient is a normalized metric roll stiffness that is defined by the degrees of roll per unit of lateral acceleration. It is generally necessary to reduce roll gradient in race cars like our formula style vehicle to keep the inside wheels at maximum contact with the ground through high speed corners.

 

Camber

Camber is the angular position of the wheels about their center horizontal axis. Negative camber is defined when the base of the wheel is turned away from the vehicle and is generally desirable as it provides stability in drive and cornering, maintaining maximum wheel contact on the ground. Optimum camber is determined largely by the tire slip angle and steering forces.

 

Objectives for Suspension Design

  • Ride height greater than two inches with a jounce and rebound of 1-1.5 inches for total wheel travel of 2-3 inches depending on the ride height we are able to achieve
  • Natural frequency to be between 2 to 3 Hz with the rear suspension to be about 15% faster than the front
  • Roll Gradient should be less than 3 deg/g and as low as possible
  • Bump steer should be reduced to a maximum of 3 degrees toe in while always maintaining inward toe through full wheel travel in jounce and rebound.
  • More calculation regarding tire data is necessary to find what camber is optimal. Generally we want to always maintain negative camber throughout wheel travel with a camber at ride height around 2-4 degrees with a camber gain around 2-4 degrees

 

Current Achievements

  • Ride height analysis of the rear suspension that showed we need to decrease our shock spring rate from 350 to 300 lbs/in. This gave us an acceptable rid height and allowable wheel travel over 1 inch in jounce and rebound
  • Natural frequency, stiffness, and impulse analysis showed us that we needed more damping than our current shocks are capable of achieving in the rear suspension

In Development and Future Directives

  • Ride height, natural frequency, stiffness, and impulse analysis of the front Suspension
  • Analysis of tire data to find optimum camber angles to update geometry
  • Bump Steer analysis of rear suspension and updates to geometry to get the toe in we want

 

 

Components:

Front Suspension

Front View

Hardware Assembly

The current front suspension design incorporates a double A-arm design with a pushrod attached to the lower A-arm at the ball joint. Under load this actuates a bell crank that ultimately deflects the Ohlins coil over shock. There seems to have been no documented testing on this design since 2019, in which no clear results were found. This style of suspension is commonly used in FSAE competition cars as documented in the literature review above. The attachment point on the bell crank is however a design we did not come across in our research. For these reasons, testing on the system must be done. Preliminary measurements of camber angle and shock deflection have been done using photo analysis. While effective at gaining preliminary data, these measurement are not perfectly accurate.

 

The imperfections in these measurements should be clear with a lack of steady progression. They do however give a baseline for to validate upcoming computer modeling and more accurate measuring equipment.

 

Front Suspension Documents 

Rear Suspension

Coil-over shock tower on the rear

 

Natural Frequency and Ride Height Investigation

We used measurement tests to find shock deflection under wheel displacement. this allowed us to find the motion ratio of the rear suspension. This is the amount of shock displacement per unit wheel displacement. Due to the the nonlinear geometry of the suspension, we found this ratio to be about 0.75.

Visual Animation of Motion Ratio

 

Using this motion ratio, we were able to linearize the suspension system into a vertical spring- mass system. From this we interpreted the ride height to be about 4.6 inches with the current 350 lbs/in spring. This only allows for 0.9 inches of allowable wheel travel in rebound. It was decided to use a spring rate of 300 lbs/inch instead.

 

Over the formula hybrid track, the ground will not be totally smooth. The suspension will be faced with various bumps in the track. We can model out now linearized suspension not over a 1 inch bump in the road as an impulse response. A state model was created for a quarter mass of the car.

We obtained a shock spring rate from our ride height investigation, the tire spring rate from the manufacturers data, and modeled the tire damping as very small. Because our Ohlin’s shocks have adjustable damping, the manufacturers data was viewed to show a maximum damping constant of 13.3 lbs-s/in. When running this through our impulse response of the state model, we found this damping constant would not comply with our objectives. A damping constant of 20.6 lbf-s/inch gave us our desired damping ratio of 0.92 and natural frequency of 2.5 Hz.

This data has effects of the wheel spring rate

removed to emphasize the shock dynamics

 

 

Subsystem TRL Chart

TRL What does this look like? Expected Completion Date
9 Validated through pre-competition tests and successful handling in competition 05/05/22
8 Successful full speed testing of handling at Metzger 04/20/22
7 Kinematics are tested at low speed on rolling chassis and confirm static measurements 3/20/22
6 Integrated shock functionality under load validated through wheel travel measurements such that it conforms closely to our computer model 02/20/22
5 Ride height and dynamics confirmed on rolling chassis 12/15/21
4 The suspension has been validated on the car through static deflection measurements of wheel travel and position without shocks on car such that it conforms closely to the computer model 12/5/21
3 The suspension design has been validated through computer modeling, such that it exhibits desired characteristics 11/15/21
2 Initial suspension design completed Previous Teams
1 Suspension concept created Previous Teams

 

Suspension Team

Kyle Picut