3 | P a g e Abstract Formula SAE is a student designed competition, organized by SAE International to take studentsout of the classroom and allows them to apply the textbook theories to the real work experiences.This project is to study and design the suspension system for Formula SAE racecar. Thedesigned suspension system must follow all the Formula SAE rules and regulations thuscompete with other racecar around the world. All the required steps in designing the suspensionsystem are conducted in this project in order to produce a racecar with optimum handling andcornering performance. At the end of this project, the designed suspension system must becompetitive enough which can be used for further development.
4 | P a g e Unsprung Mass Assembly 1. Introduction Formula Student (or Formula SAE (F-SAE)) is a worldwide university competition, organizedby the Society of Automotive Engineers (SAE), which encourages university teams to design,build, and compete with a Formula 1-style racecar. The main goal is providing students with anopportunity to gain experience in design, manufacturing, management, marketing and peopleskills by designing, building and racing a single seated racecar. Nowadays, four European racesare available in the United Kingdom, Germany, Italy and Austria. Teams get a lot of freedom todesign their own car. The competition is split into static and dynamic events. Static eventsinclude vehicle presentation, cost, and design analysis, while dynamic events include four racingcontests: acceleration, skid pad, autocross, and finally the endurance and fuel economyevent.(SAE rules 2013)To participate in the competition the vehicles must comply with the FSAE’s strict rules. Themost important rules a Formula student car has to comply with state that it has to have are:• A chassis that is designed in accordance with a number of safety regulations.• A four-stroke engine with a maximum displacement of 610 cc.• An inlet restriction with a maximum diameter of 20 mm.• A fully operational suspension system.The suspension system that will be designed must be able to improve the car cornering abilityand handling performance in order to make sure the car to be competitive with other team aroundthe world. This paper presents the design procedure that was followed in order to re-design,reduce the weight by 30 percent and still maintains strength for a fully operational unsprungmass assembly, which includes:• The Hub• Brake disc• Upright
5 | P a g e The main objectives of the project were as follows:• To re-design the unsprung mass assembly for DIT Formula SAE race car.• To reduce the weight by 30 percent and still maintain strength to withstand all brakingand cornering loads.• Replace the existing brake calipers with AP motorbike calipers-CP4227-2SO.• Redesign the hub so that there is no need for the stub axle, which will reduce the need forone component and reduce the weight.• To understand the concept of upright, brake disc and the hub and its applications, in thiscase it is specific to the Formula SAE racecar application.• To select appropriate bearing for the redesigned brake disc.• The application of self-technical knowledge, Solid works and FEA analysis, understandrelated high-tech material and its production process, and the application and theadvantage of the designed item.• To understand the sustainability of the selected materials.2. Literature Review The suspension design was mainly focused on the constraints of the competition. First, thedesign parameters were set and then using the old upright, designed previous year as a template,a new upright was designed keeping the same camper angle and king pin offset. Before themodeling began the ball joint locations were set using the old upright. A new hub was designedand an appropriate bearing was selected using the NSK catalogue provided and an new brakedisc was designed fitted with a given brake caliper. The main design requirement put forward forthe design of the unspring mass assembly was that it should have the ability to have manyadjustments, since different races often require alternative setups, and in this case it was decidedthat the upright design should include the following adjustments:• Camber angles• Under steer and over steer• Roll angle• King pin angle
6 | P a g e • Caster roll stiffness• Spring rate• King pin offset• Squat• Wheel base and• Track2.1 Scrub RadiusThe scrub radius, or kingpin offset, is the distance between the centerline of the wheel and theintersection of the line defined by the ball joints, or the steering axis, with the ground plane,which is illustrated in Figure 1. (Douglus 1986) Figure 1
7 | P a g e Scrub radius is considered positive when the steering axis intersects the ground to the inside ofthe wheel centerline. But here it was considered negative. The amount of scrub radius should bekept small since it can cause excessive steering forces.2.2 Kingpin inclination (KPI)It is viewed from the front of the vehicle and is the angle between the steering axis and the wheelcenterline. To reduce scrub radius, KPI can be incorporated into the suspension design ifpackaging of the ball joints near the centerline of the wheel is not feasible. Scrub radius can bereduced with KPI by designing the steering axis so that it will intersect the ground plane closer tothe wheel centerline. The drawback of excessive KPI, however, is that the outside wheel, whenturned, cambers positively thereby pulling part of the tire off of the ground.2.3 CasterIt is the angle of the steering axis when viewed from the side of the car and is considered positivewhen the steering axis is tilted towards the rear of the vehicle as shown in figure 2. With positivecaster, the outside wheel in a corner will camber negatively thereby helping to offset the positivecamber associated with KPI and body roll. Caster also causes the wheels to rise or fall as thewheel rotates about the steering axis that transfers weight diagonally across the chassis. Casterangle is also beneficial since it will provide feedback to the driver about cornering forces. Figure 2
8 | P a g e 2.4 CamberCamber is the angle of the wheel plane from the vertical and is considered to be a negative anglewhen the top of the wheel is tilted towards the centerline of the vehicle. Camber is adjusted bytilting the steering axis from the vertical, which is usually done by adjusting the ball jointlocations. Because the amount of tire on the ground is affected by camber angle, camber shouldbe easily adjustable so that the suspension can be tuned for maximum cornering. For example,the amount of camber needed for the small skid pad might not be the same for the sweepingcorners in the endurance event. The maximum cornering force that the tire can produce willoccur at some negative camber angle. However, camber angle can change as the wheel movesthrough suspension travel and as the wheel turns about the steering axis. Because of this change,the suspension system must be designed to compensate or complement the camber angle changeassociated with chassis and wheel movements so that maximum cornering forces are produced.Static camber can be added to compensate for body roll; however, the added camber might bedetrimental to other aspects of handling. For example, too much static camber can reduce theamount of tire on the ground, thereby affecting straight line braking and accelerating. Similarly,too much camber gain during suspension travel can cause part of the tire to loose contact withthe ground.2.5 Understeer and oversteer They are driving characteristics that involve sliding of either the front or rears tires. Excessiveundersteer and oversteer can result in an out-of-control car. A car’s tendency to understeer oroversteer is most commonly attributed to whether it’s front- or rear-wheel drive. Front-wheel-drive cars characteristically understeer, while rear-wheel-drive cars tend to oversteer. Both front-and rear-wheel-drive cars, though, can experience both understeer and oversteer in the rightconditions. The more power a car has, the more likely understeer or oversteer will show theirfaces.(Driving Fast 2013)Understeer happens when the front wheels start to plow straight even if you have the steeringwheel turned. Front-wheel-drive cars are susceptible to understeer because power is being sent tothe same wheels that steer the car, and when the tires start spinning there’s no grip to steer.Oversteer is the tendency for the rear end to slide out or fishtail.
9 | P a g e 2.6 WheelbaseIt is defined as the distance between the front and rear axle centerlines. It also influences weighttransfer, but in the longitudinal direction. Except for anti-dive and anti-squat characteristics, thewheelbase relative to the CG location does not have a large effect on the kinematics of thesuspension system. However, the wheelbase should be determined early in the design processsince the wheelbase has a large influence on the packaging of components.2.7 Track widthIt is the distance between the right and left wheel centerlines, which is illustrated in Figure 3.This dimension is important for cornering since it resists the overturning moment due to theinertia force at the center of gravity (CG) and the lateral force at the tires. Figure 33. Design Summary Using the old upright as a template on the solid works package carried out the design of the newupright. First the ball joints were placed at the appropriate places so that machining of the partwill be easier as camper angle and king pin offset will be already set.Those design summaries are as follows:
10 | P a g e Double arms wishbone of unequal length was designed with a top to bottom ratio of 1:1.1. Then,appropriate bearing was selected using NSK chart and a whole was cut and the bearings wereplaced. One of the objectives of the project was to reduce the size of the brake disc and hub sothat there is no need for the extra joints on the disc, and that’s what was done after that. Figure 4 Brake Disc
11 | P a g e Figure 5 HubLater the hub was resized for interference fit. Then the design of the upright was started, the sizewas reduced and much iteration was done until reaching a satisfactory design.
12 | P a g e Figure 6 UprightAll the parts were later assembled together appropriately and was analyzed using FEA.
13 | P a g e Figure 7 Assembly4. Design Calculations 4.1 Braking force and Torque Upon braking, the mass of the car and driver is decelerated by converting the combined kineticenergy into heat, by pressing the brake calipers on to the brake disc at high pressure to generatefriction. As the caliper is mounted on to the upright all braking force induced by the caliper aretransmitted through it in the form of torque through the center of wheels rotation. Assuming60:40 braking ratio and maximum deceleration of 60 mph to 30 mph in 3 seconds the torque wascalculated as below.(Bearing 2010)
14 | P a g e Therefore, each front tyre will produce 0.9 kN of force and a torque of 242 Nm.4.2 Bearings Due to the higher radial load and axial loads single row tapered roller bearings were used.Bearings of this type use conical rollers guided by a back-face rib on the cone. The rollers areincreased in both size and number giving it an even higher load capacity. They are generallymounted in pairs in a manner similar to single-row angular contact ball bearings. In this case, theproper internal clearance can be obtained by adjusting the axial distance between the cones orcups of the two opposed bearings. Since they are separable, the cone assemblies and cups can be
15 | P a g e mounted independently. Depending upon the contact angle, tapered roller bearings are dividedinto three types called the normal angle, medium angle, and steep angle. (Bearing 2010)4.2.1 Bearing Selection Due to the fact that there was no special selection procedure and the minimum external diameterwas given it was decided to select a single row tapered roller bearing of the following boundaryconditions:• External diameter (D) = 80mm• Internal Diameter (d) = 55mm• Thickness (T) = 17mm• C = 14mm Figure 84.2.2 Bearing Life Bearing life, in the broad sense of the term, is the period during which bearings continue tooperate and to satisfy their required functions. This bearing life may be defined as noise life,abrasion life, grease life, or rolling fatigue life, depending on which one causes loss of bearingservice. The life of the bearing was calculated as below:
19 | P a g e 4.2.4 Bearing Interference/ tolerance Bearing tolerance is needed to get a tight fit (Figure). Figure 9
20 | P a g e Figure 10 Using the table above it was calculated as follows:
21 | P a g e 5. Design analysis An FEA analysis was carried out on the upright. Initially a safety factor of 7 was obtained withweight equal to 825g and the selected material was aluminium 7074 T6. Figure 11 It was again redesigned and then a safety factor of 10 was obtained, with a final weight of 735g.Factor of safety (FoS), also known as safety factor (SF), is a term describing the structuralcapacity of a system beyond the expected loads or actual loads. Essentially, how much strongerthe system is than it usually needs to be for an intended load. Therefore, a safety factor of 10obtained below is sufficient enough to with stand a torque of 242Nm and an axial load of 950N.Thus by reducing the weight from almost 1500g to 750g the strength was still intact.
23 | P a g e 6. Sustainability A sustainability test was carried out on the upright and brake disc in terms of carbon footprint,total energy consumed, air acidification and water eutrophication.6.1 Brake disc for carbon steel Milled Figure 14 -‐ Sustainability Brake Disc (Carbon Steel)
24 | P a g e 6.2 Brake disc for Cast Iron Figure 15 -‐ Sustainability Brake Disc (Gray cast iron) It can be noted that it is more sustainable to manufacture the brake disc with cast iron, as all thevalues are half compared to carbon steel. The finished product will be transporting from Asia to Europe.
25 | P a g e 6.3 Upright Die Casted Figure 16-‐ Sustainability Upright (Die Casted)
26 | P a g e 6.4 Upright Milled Figure 17-‐ Sustainability Upright (Milled) It can be noted in here that there is not much of a difference between the sustainability factorbetween die casting and milling of the upright, although it will be slightly more sustainable tomill as the air acidification level Is a bit higher for die casting. The finished product will betransporting from Asia to Europe.7. Comments and Conclusions Overall the design of the assembly went well. As there were no right or wrong, the designer wasable to make as successive iterations until a satisfactory compromise has been reached. Thishelped the design to achieve a glimpse of what the actual world design projects function like.
27 | P a g e During the design process the designer was able to achieve all the objectives so that the overallweight of the car could be reduce but still maintaining it strength. The timeline of six weeksalong with the rigorous schedule of college did some limitation on the number of designiterations. However it was understood that many iterations would take to converge on to asatisfactory design.Overall it was challenging and at the same time interesting. Figure 18-‐ Exploded View
28 | P a g e 8. References Bearing, N., 2010. NSK Bearing Catalogue.Douglus, Mi., 1986. Race car vehicle dynamics, S.E International.Driving Fast, 2013. Understeer. Available at: http://www.drivingfast.net/car-control/understeer.htm#.UUeDsVTviQs [Accessed March 18, 2013].SAE rules, F., 2013. Formula SAE rules.