Steering System And Suspension Design Mechanics Essay

Steering System And Suspension Design Mechanics Essay The aim of this project is to work with a project team to design, build and optimize the running of a Formula SAE-A racecar, with particular interest in the Steering and Suspension systems. The Formula SAE-A project team aims to produce a competitive racecar that will compete in the Formula SAE-A competition in December. To achieve this I was required to, research the important aspects of steering and suspension systems used frequently in a nonprofessional racecar and select a suitable steering and suspension system that is within the motorsport teams limits. This project includes suggestions for the design and construction of these systems, the installing and optimising(or tuning) the steering and suspension systems and future recommendations to provide the most cornering and handling ability. Identifying the critical areas that are important for competitive steering and suspension systems, I can improve the effective handling and cornering capability of the racecar. Improving the handling and cornering power of the racecar will allow faster speeds into and exits out of corners, which will result in quicker lap times, better performance and higher overall standing in the 2006 FSAE-A competition. Adhering to the rules and regulations for the 2006 FSAE-A competition I aim to select suitable systems that are within the project teams limits by considering the financial cost versus benefit or performance to the car, complexity and time to design and 1.1 Cornering Ability and Handling 2 manufacture of each system. Critically analysing the 2005 teams racecar enables me to evaluate the cars steering and suspension setup performance and find any flaws or ways to improve them. This will give me a better understanding of the steering and suspension systems and how to find the optimum settings to perform with the 2006 car at the FSAE-A competition. Using a suspension geometry computer program developed by Wm. C. Mitchell software, I can model the 2005 teams racecar to compare the accuracy of the program, and then apply the program to optimise the 2006 racecar. The ideal outcome of this project will see that this years FSAE-A racecar have a working and well-tuned or optimised steering and suspension system that has high cornering ability and handling. Most of this projects work will become evident once we have manufactured our design and are able to test the car by running it on a test-track. If all things go to plan, I should be able to make small adjustments to improve and finally optimize the handling and cornering ability of the car which will be paramount to the performance at the FSAE-A competition. 1.1 Cornering Ability and Handling The cornering ability and handling of the racecar is very important to the overall performance of the racecar. Having excellent acceleration and braking power is good but without sufficient cornering ability and handling, the racecar will not be able to use the full potential and is more likely to run off the racetrack than take a podium position. Cornering ability and handling will be discussed in detail and how the steering and suspension systems affect it. 1.2 Explanation and definition of terminology 3 1.2 Explanation and definition of terminology Here is a number of terms and names that will be used in this dissertation to avoid confusion with other names and meanings. Ackerman Is both a principle and definition, where the principle is that the extended axis of the steering arms projected rearward meet at the centre of the rear axle (shown in figure 1.1). This allows the tyres to traverse an arc without skidding, which would otherwise oppose the steering forces making it harder to steer. The definition is described as the difference in the angle of the front tyres when turned. This dissertation will only refer to Ackerman as the principle from herein. Camber Is the angle between the vertical plane and the centre angle of the tyres (shown in fig 1.2), which can be positive or negative. This changes the size and shape of the tyres contact patch during a corner which in turn affects the amount of lateral acceleration or force it can produce (cornering and handling ability). A small amount of negative camber is ideal (around 1.5 degrees) to induce camber thrust and ensure a good contact patch during cornering (smith. C. 2004). 1.2 Explanation and definition of terminology 4 Camber Gain Or the rate of camber change in roll (or as the chassis rotates laterally). Caster Is the angle between the steering axis and the vertical from the side plane (see fig 1.3). Positive caster improves straight line stability but makes it slightly more difficult to steer, while negative makes it easier to steer with less stability. Jacking Is an upwards reaction force generated by the tyres when the racecar is accelerated during cornering and has its roll centre above ground level. Where the upwards force on the outside tyre is greater than the inner tyre having a 1.2 Explanation and definition of terminology 5 net resultant force that lifts or Jacks the sprung mass. This is unwanted and unsettling to the driver and should be avoided. The roll centre Indicates the point at which the chassis rotates (at the front and rear respectfully) during lateral acceleration. The two moment arms between the roll centre, the CG and the ground plane determine the racecars sensitivity to lateral acceleration by the production of rollover movements and jacking (Smith. C, 2000). The roll axis Is the straight line joining the roll centres of the front and rear tyres The roll moment Is the distance between the roll centre and the mass concentration at the front or rear of the car. The mass concentration is the equivalent mass or point of the CG if it were split into 2 points, one front and rear. Steering Axis Inclination and Scrub Radius Steering Axis Inclination or Kingpin Axis, is the angle between the vertical and the steering axis (figure 1.4). This helps the car to exit a corner by naturally trying to align the wheels back to centre. The SAI works with caster to allow more directional stability but less effort on steering (more sai and less caster). Scrub Radius Is the pivot point for the tyres footprint or the distance between the centre of the contact patch, to the extended SAI to the ground (figure 1.4). This allows more feel in the steering, a little is good, too much can be detrimental due to the increased steering effort for the driver. 1.3 Overview of the Dissertation 6 Slip angles Are the angles between the direction that the tyres are facing, and the direction that the tyres want to go. Deformation is due to the elastic nature of rubber when a vertical load is applied. This will be explained in detail in Chapter 2 and its effect on cornering and handling. 1.3 Overview of the Dissertation This dissertation is organized as follows: Chapter 2 Discusses cornering and handling of a FSAE-A racecar and describes various steering and suspension systems. Chapter 3 Explains the rules and regulations of the FSAE-A competition and how it affects the steering and suspension systems. Chapter 4 Introduces Wm. C. Mitchells suspension geometry software, describes its uses and strengths for this project and how it will be used to improve the steering and suspension systems. Chapter 5 Describes the analysis of the 2005 FSAE-A racecar and documenting areas that can be improved and implemented into the 2006 car. Chapter 6 Describes the analysis of the 2006 racecar and recommendations for improving the cornering and handling ability. Chapter 7 Discusses testing methods and ways to document and record actual performance of the racecar, followed by processes for optimisation of the steering and suspension systems for the best cornering ability and handling. Chapter 8 Outlines the projects achievements, findings and future recommendations. Chapter 2 Steering and Suspension Systems for a FSAE-A Racecar 2.1 Chapter Overview This chapter discusses the steering and suspension systems that are commonly used in cars on the road and in professional racing, their benefits and limitations, the ease of manufacture and complexity of design. This chapter also discusses cornering and handling in detail and how the steering and suspension can improve its cornering and handling ability. 2.2 Cornering and Handling Handling defines the racecars ability to maneuver around a corner at maximum speed without losing traction. C. Smith (1978) remarks that being able to travel around a corner faster reduces the overall lap time on a circuit for 2 reasons. First is simply that the car traverses the distance in less time, secondly, if the car exits the corner at a faster speed, there will be no time lost from having to accelerate from a slower speed. Smith (1978) also says that the factors that determine the cornering power of a racecar 2.2 Cornering and Handling 8 include the cornering capacity of the tires, which is influenced by: Vehicle gross weight Vehicle downforce Height of the vehicles centre of gravity Vehicle load transfer characteristics Suspension Geometry Size and characteristics of the tyres So you can understand, the tyres are arguably one of the most important parts of the racecar because all the moments and forces that the car undergoes is transmitted through the tyres. The acceleration and direction of the car is passed through the small footprints or contact patches of each tyre. Understanding what happens here will help to get the most out of both the tyres and racecars handling ability (Smith, C. 1978). 2.2.1 Tyres and slip angles The tyres ability to grip the road is a combination of vertical load applied to the tyre, the coefficient of friction between the tyre and the road, adhesion between the road surface and tyre, and slip angles developed between the tyre and direction of travel. The vertical load that is imposed on each tyre is changing continuously on a racecar maneuvering around a racetrack due to the load transfer from acceleration, deceleration and cornering. As the racecar travels around a corner, the tyres are subject to forces which result in deformation in the compound that the tyre is made of, this elastic deformation results in the contact patch pointing in a different direction to the angle of the tyre (Smith, C. 1978). Shows the deformation of the tyre compound in the contact patch and the slip angle developed. The path of the rolling tyre defines the actual direction of the tyre as it continues around the corner.There is a relationship between the slip angles and the potential grip that the tyre has to the road. Some tyre data has shown that 2.2 Cornering and Handling 9 Shows the generated slip angles in the tyre contact patch as slip angles increase, the lateral or cornering force increases up to a maximum which then either begins to drop or plateaus then drops, usually sliding occurs soon after the drop in force. The flat portion of the curve at or near the maximum is the optimum range of tyre grip that experienced drivers remain in to maximize the cars cornering potential. shows the relationship between tyre grip and the developed slip angles. shows the relationship between tyre grip and developed slip angles, picture from http://www.donpalmer.co.uk/cchandbook/modelgrip.htm 2.2 Cornering and Handling 10 2.2.2 Factors influencing tyre cornering capacity The other factors as mentioned before, vehicle gross weight, downforce, height of the CG, tyre size and characteristics, suspension geometry and load transfer characteristics, all can be factored into the design or used to improve cornering and handling. The cornering force is proportional to the increase of the vehicle gross weight and generated downforce from wings or aerofoils. The increased pressure on the contact patch generates a higher lateral force component (Smith, C. 1978). The height of the vehicles centre of gravity from the ground affects the moment between the vertical force on the tyre and the CG, this will affect the lateral load transfer during a corner. The lateral load transfer changes the vertical loads from one wheel to another due to the CG tendency to move sideways during a corner, which will decrease the total amount of cornering force generated from the tyres. For example, a 400kg car with a 50-50 weight distribution front to rear will have 100kg vertical weights at the two front tyres. Assuming the CG height is 250mm above the ground, the track width is 1300mm and during a corner the car is subject to a cornering acceleration of 1.4gs we can determine the load transfer. LoadT ransfer = 1.4 Ãâ€" 200kg Ãâ€" 0.25 1.3 = 53.85kg So this gives us 46.15kg on one side and 153.85kg on the other and is a 53.85% load transfer to the outer wheel. Obtaining tyre data in the form of Tyre cornering force versus Vertical load will allow us to determine the total cornering force with this load transfer, however getting the tyre data is difficult. Generally the tyre data is curved with less tyre cornering force as vertical load increases, so measuring the data of each vertical load and summing together will be less than the equal load distribution. Reducing the load transfer is done by lowering the height of the CG and widening the track width which will improve cornering ability. The suspension geometry determines the location of the instantaneous centres and roll centres of the racecar, these control how much the chassis rolls or pitches during cornering and accelleration, which moves the CG and hence affects the lateral load transfer. 2.3 Steering Systems 11 During roll, the suspension geometry also controls the amount of camber gain in the wheels during a corner, the change in camber affects the contact patch (increase or decrease in proportion) which changes the cornering capacity of the tyres. Ensuring that an optimum contact patch is maintained through the control of camber gain and good roll centre location is key to good handling and cornering. 2.3 Steering Systems Common types of steering systems are: Rack and Pinion basic steering system Recirculating Ball Bearing more complex system Power Steering fluid assisted steering 2.3.1 Rack and Pinion The rack and pinion steering system is a simple, cheap and relatively easy system to implement. It comprises of a rack, or toothed bar/rod which slides left and right due to the rotation of a pinion gear that sits on the teeth (Fig 2.3). The steering wheel turns the steering shaft which rotates the pinion gear, resulting in the rack pushing/pulling the steering rods. The rods are attached to the wheel hubs which turn the wheels to the desired angle (Gilles, T. 2005). The most difficult parts to design or manufacture are the pinion and the rack, the pinion defines the turning rate of the steering wheel which affects the responsiveness of the steering. The rack need to have hardened teeth which could be difficult to manufacture to some groups or would involve a significant cost to have it done. Besides these two parts the rest of the system is relatively simple, as a whole the rack and pinion setup is a cheap and common system that is reliable and resiliant. 2.3 Steering Systems 12 2.3.2 Recirculating Ball Bearing A typical Recirculating ball-bearing steering system uses a worm gear to shift ball bearings that are located within a channel such that when moved, pushes or pulls the housing in which they sit. The housing has teeth located on the outside which are in line with a sector gear that rotates a pitman arm (Fig 2.4). The pitman arm is attached with the track and tie rods, which aligns the wheels. This system can also be described as a parallelogram steering linkage system in which the linkages trace a parallelogram (Gilles, T. 2005). Figure 2.4: Recirculating ball bearing steering, picture from www.imperialclub.com/ Repair/Steering/terms.htm A Recirculating Ball Bearing can also be used in a similar setup to aRrack and Pinion gear system, where the recirculating ball bearing housing replaces the pinion gear with a sector gear that pushes/pulls the rack to align the wheels. The recirculating ball 2.3 Steering Systems 13 bearing system is significantly heavier than the rack and pinion system, due to the extra linkages, housing and gears. Friction needs to be managed in the design stage, i.e. including grease input points, dust covers etc. However the Recirculating ball bearing steering provides more sensitivity to the steering and minimum slack or loose feel in the steering wheel. Costing is also increased due to the extra material and the complexity of design makes the recirculating ball bearing system less attractive. 2.3.3 Power Steering Power steering systems are the same systems as rack and pinion and recirculating ball-bearing but with a significant modification. In a rack and pinion power steering system, the rack contains a cylinder with a piston inside it, driven by fluid supplied by a pump (see Figure 2.5). The fluid lines run to a rotary valve controlled by the steering shaft which determines the sides of the piston that the high pressure fluid acts on. This pressure assists the steering action which requires less force to rotate the steering wheel. Similar to the rack and pinion power steering, the recirculating ball housing is assisted by the pressure respectively in the ball-bearing steering (Gilles, T. 2005). Rack and Pinion power steering, picture adapded from www.cars.com/ carsapp/boston/?srv=parseract=displaytf=/advice/caradviser/steering_ fluid.tmpl 2.4 Suspension Systems 14 2.4 Suspension Systems There are two common types of suspension systems used frequently today, dependant and independant systems. The various types of both are similar but have their differences and functions. Some of these sytems are described below. 2.4.1 Dependant Suspension Systems Solid or Beam Axle Panhard Rod Watts Linkage Dependant suspension systems are variations of a simple beam axle that holds the wheels parallel with each other. So when the vertical angle of one wheel (camber) changes, the opposite wheel also changes (Gilles, T. 2005). Examples of the Panhard Rod and the Watts Linkage are shown in Figures 2.6 and 2.7, these types of suspension are generally different ways of attatching the solid axle to the chassis. 2.4.2 Independant Suspension Systems Double Wishbone, A-Arm or Four-Bar link MacPherson Strut Multi-link 2.4 Suspension Systems 15 Watts linkage suspension. Independent suspension systems allow the wheels to move independently of each other, e.g. if one wheel were to move up or down, the other would not be affected directly. It is common for racecars to have all four wheels with independent suspension as this usually provides the most customizable setup options to maximize the handling potential of the racecar. Double wishbone suspension systems are also known as double A-Arm or Four-Bar link systems. They all comprise of equal or unequal parallel links from the chassis to the wheel hub, with the shock absorbers configured in a Push or Pull rod setup, as Figure 2.8 illustrates. Unparallel and Unequal double wishbone suspension with Push or Pull rod shock absorber setup. 2.4 Suspension Systems 16 The MacPherson strut suspension system (Figure 2.9) is very popular with passenger cars and some sports models since it is a relatively cheap system to produce that provides reasonable camber control (Smith. C, 1978). The MacPherson strut suspension is good for everyday commuting but does not provide sufficient stiffness to avoid movement within the components (compliance or slack) and would not fit comfortably with wide tyres (Smith. C, 1978). Multi link suspension systems are simply Four-Bar link systems with one or more extra links to attain extra control. MacPherson strut suspension, from www.autozine.org/technical_school/ suspension/tech_suspension2.htm The objective of the independent suspension is to provide enough vertical wheel movement to absorb surface bumps and compensate for the accelerations of the sprung mass, prevent changes in the distance between tyres (static toe) as they are moving, control the change of wheel camber angle and change of track distance with the wheel and/or sprung mass movement, and to ultimately allow the most grip or traction available out of the tyres while minimising weight and maximising stiffness in the links (Smith, C. 1978). 2.5 Chapter Summary 17 2.5 Chapter Summary Having discussed the cornering and handling ability in a Formula SAE-A racecar and what factors can influence the performance, helps to have an understanding of what is happening when a racecar traverses around a corner. With this in mind we can apply this knowledge into the design to maximise the cornering and handling ability of the racecar. Also selecting an appropriate steering and suspension system that will provide the best cornering and handling but also takes into account the motorsport teams resources (time, materials and complexity of design). Chapter 3 Rules and Regulations of the FSAE-A Competition 3.1 Chapter Overview This chapter covers the rules and regulations that will affect the steering and suspension sytems. Starting with the more specific rules that affect the steering and suspension systems, then moving into the general rules and regulations like material strength. These rules and regulations have been put into the competition to give the entry teams maximum design flexibility and the freedom to express creativity, but also to ensure that a safe and working car that minimises chances of damage and injury. 3.2 Steering Requirements The specific steering system rules and requirements are as follows: The steering must affect at least two wheels The steering system must have positive steering stops that prevent the steering linkages from locking up. 3.3 Suspension Requirements 19 Free play is limited to 7 degrees measured at the steering wheel. Steering must be mechanically connected to the wheels i.e. steer by wire prohibited These requirements do not severely limit the steering system design at all as for most of the previously mentioned systems, none of which include steer by wire and all affect at least 2 wheels. The rules that need to be kept and monitored is the free play in the steering wheel and steering stops, otherwise the design is virtually open. 3.3 Suspension Requirements The rules state that the car must have a fully operational suspension system with springs and shock absorbers, front and rear, with a minimum useable wheel travel of 50.8mm (2 inches), 25.4mm (1 inch) in jounce and rebound with the driver seated. So the rules again do not restrict the specific suspension system but merely sets a benchmark that it must perform to. 3.4 Other Requirements Other requirements set out in the rules define that the wheelbase must be of at least 1525mm (60inches) and that the smaller track must be no less than 75% of the larger track. The minimum material must be; either round mild or alloy, steel tubing (min 0.1% carbon) with minimum dimensions as outlined in table 3.3.3.1 in the FSAE rules handbook; or an approved alternatice material that is tested and proved to meet the alternative material guidelines in section 3.3.3.2 of the FSAE rules handbook. The wheelbase requirement affects the suspension geometry design, setting a minimum length for the suspension linkages. 3.5 Chapter Summary 20 3.5 Chapter Summary Knowing and understanding the requirements and rules set out by the Formula SAE competition provides a starting point for our design, also talking with the previous team and the performance will help to identify areas needing improvement and investigaiton. Once finding sufficent information a start can be made to get the ball rolling on design and construction of the steering and suspension systems. Chapter 4 WinGeo3 Suspension Geometry Program 4.1 Chapter Overview This chapter introduces Wm. C Mitchells suspension geometry software, Racing by the Numbers and shows its most useful power of calculation and display of steering and suspension geometry of any four wheel vehicle. The information it can tell us will greatly improve the time taken to analyse steering and suspension set-up and will allow fast optimisation when the time comes to testing. 4.2 WinGeo3 Geometry Program The steering and suspension geometry can be modeled on Wm. C. Mitchells software which is quicker than manually measuring all the various important values repeatedly for the various settings you wish to try during testing. This enables a comparison with the originally intended design parameters of the 2005 racecar and an indication of how well the car will react while cornering. It also allows a comparison of the initial 2006 cars design and actual geometry after construction and allows us to optimise the geometry to provide the best cornering and handling ability of the racecar. By 4.3 Set-up and initial measurements 22 measuring the data and entering into Wm. C. Mitchells software, we can critically analyse the racecar with regard to the handling and cornering characteristics. The software requires actual measurements taken from the car which will be done and recorded according to the geometry software requirements. Once recording all the information that the software needs, we can analyse the way the steering and suspension reacts with the chassis. Moving up or down (ride) or rotating (roll) we are able to observe the change in camber, steering angles and caster at each of those changes. This is useful since during a corner, we may model the changes that the chassis will go and can see the result on the tyres (and contact patch) and get an indication of how well it will perform. Wm. C. Mitchells software can also be used to aid in the design of steering and suspension systems, through its design and build functions you may specify various values and the software will convert it into the required lengths of the arms and rods. 4.3 Set-up and initial measurements I strongly recommend allowing at least half a day to measure up a car for the first time and someone to help. It will save alot of time that would otherwise be lost dropping things, re-setting the origins and other fiddly jobs that are not normally accounted for. Once installing the program, printing out some forms will make things much easier for entering information into the program once the measurements have been taken, as the forms sets out the required information neatly and in similar format to the program screen. Open the geometry program and from the help menu open quick start. The help tree is on the left side column, from there open the Files menu and then Blank Forms, here is all the blank forms that is needed. Click on Blank forms: Measuring cars for some general information and hints, for a double wishbone suspension with a push/pull damping system, click on the Blank forms: Double A-arm and Rocker Arm option and print. Also click on Blank forms: Pull-rod / Push-rod form, Blank forms: Auxiliary points, and Blank forms: Swaybar form and print them all out. These all will be 4.3 Set-up and initial measurements 23 sufficient for the front suspension and rear (remember to print a second batch of forms for the rear) unless you have a control arm / panhard rod rearaxle suspension setup, for which there is a seperate form. First you need to make sure that the car is set up already with the correct alignment and on a flat surface as it would on the racetrack. Ensure that access to the suspension points is possible and that they are locked in place so they do not move if you lean on the car (within reason). Then determine a baseline or origin accurately and place strings on the surface plate or flat floor or tie to appropriate point, to represent the centerlines of the car (front to back, side to side). Once an Origin for each Axis has been made, where the X-Axis is the fore-aft longitudinal dimension (front to rear of the car). The Y-Axis is the lateral dimension, or left and right sides of the car (drivers side passenger side) and the Z-Axis is the vertical dimension from the ground up. Care must be taken when selecting an origin due to common suspension adjustments, such as changing caster, can move the tire contact patch. Each such change requires a careful remeasurement (or re-calculation). When the car is ready to be measured, follow these steps: Measure the track width of the front and rear tyres by taking the centre points of each tyre as low to the ground as reasonable, the WinGeo3 program measures track at ground level at the center of the tire contact patch. The easiest way is to measure to the middle of the tire,but this can be misleading if the tire has significant static camber, so as long as you are aware of the settings you should be fine. Measure the static toe for the front tyres while measuring the track at the front and do a quick calculation of the static angle pointing inwards or outwards that