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In association with. Introduction. We are a team of year 12 students from Wootton Upper School just outside Bedford. Our team consists of four members of which the roles were allocated as such below. Jeffery Koh – Team Manager / Driver
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Introduction We are a team of year 12 students from Wootton Upper School just outside Bedford. Our team consists of four members of which the roles were allocated as such below. • Jeffery Koh – Team Manager / Driver • Mark Heard – Technical Designer (Chassis and Construction) / Driver • Seb Hensman – Marketing (In charge of publications) / Technical Designer • Haider Zaidi – Artwork Designer (Body Shell and Logos)…and car breaker! • (Honorary member) James Cleave – Supervising Teacher / Slot Car World Champion We assigned the roles based on particular skill levels and areas of expertise to ensure, not only the car was running to its optimum efficiency, but also every team member was contributing to the end product. In this presentation we will take you through every aspect of our car’s design explaining processes used and modifications made to achieve what we believe is a winning formula.
Chassis Design Exploded Pro/Engineer View • The chassis was identified right from the start as the major part for design progress. The only restrictions imposed on us were the dimensions within the rules. We made sure we kept within these rules but adapted most of the other components. We had a team meeting and sat down and drew up a chassis specification. This included points such as : • Must have a low centre of gravity • Long wheel base • Strong, but flexible material to reduce vibrations • Guide pin location and thickness • All of these would be crucial in assisting the car to achieve high stability on the corners and a good average speed over the whole lap. • The chassis itself was designed using 2D Design and then exported to a laser cutter which then cut the sections out of High Impact Polystyrene. A material that met our specification and was readily available in school. Using the laser cutter proved a valuable tool as it enabled us to achieve very accurate parts which could be adapted easily and quickly if necessary. These parts where then taken forward to the assembly stage where they where all glued together using a chemical adhesive and assembled with some of the required parts included in the kit provided by Scalextric. Assembled Pro/Engineer View This shows how two chassis tessellate reasonably well to save material. You can also see all the component parts, including the most recent modifications.
Chassis Design continued... Balance We aimed to design the chassis so that it was perfectly weighted and balanced. This meant we had to keep it completely symmetrical right down to the location of the holes for the axels. We also added a cable guide which would keep the wires that fed from the motor nice and tidy within the case and also to assist in the guide pin returning to its rest position. Guide Pin Hole This was the single biggest problem we encountered with the chassis design, one day before our heat race we were testing and training with the car and our ‘Designer’ decided not to brake at the end of the straight forcing the car to slide sideways and broke the guide pin hole. This meant that a modification had to made and implemented in a very short time, overnight for the Regional Final, the next day. The logical approach was to strengthen the plastic around the hole by making a double layer. This proved very effective and enabled the chassis to withstand larger turning and impact forces on the guide, experienced when turning the corners. Ground clearance at front on 21mm Diameter wheels = 0.5mm Body shell pins Fixing the body shell to the chassis, was a bit of a last minute thought. We had originally decided to just tape the body to the chassis. On first testing, this proved to work, but we could see the body shell shaking and vibrating all the time, so provision was also made within the chassis design to include two tubes that run the width of the chassis base which are used to secure the shell the chassis itself. This also allows easy access to the internal parts if minor changes need to be made on race days. Separating the body shell from the chassis would help reduce vibrations by allowing the body shell and the chassis to vibrate independently from one another without effecting each other. Ground clearance at rear on 21mm Diameter wheels = 1mm Wide track and low centre of gravity This is one of the most crucial parts of the design as it effects so many different variables on the car and its performance. The wide track will help to spread the weight of the chassis over the largest possible area. The low centre of gravity helps to reduce the amount that the car will ‘tip over’ in corners. This gives the car much more stability over a lap.
Body Shell Design Original Body Shell designed in Pro/Desktop Extrude Body Shape Extrude Wing Cut Out Sketch Body Shape Sketch Wing Cut Out Sketch Roof Section Extrude Roof Section The body shell design was originally based on a Touring Car style racer. Our team designer spent sometime developing the original shell on Pro/Desktop before we got access to Pro/Engineer. Although a good looking design, the original shell would be difficult to vacuum form. When we got hold of Pro/Engineer, a much simpler design was created which was inspired by a Drag Racing Funny Car. The shell is designed much like a big wing, to produce down force in the form of drag. Apply Draft Angles to relevant sides • We had to consider the following specification points when designing the shell. • Maximum and Minimum dimensions stated in the rules • Clearance of the tyres under the body shell • Flexibility of the shell to absorb vibration • Correct mould design to ensure ease of forming Apply Rounds Warp body sides
Body Shell Manufacture Completed Body Shell design .stl (Stereo Lithography) file, imported into CAM software Roughing paths calculated Finishing paths calculated Due to the constraints of our school equipment, not having a milling machine, we had to source a contact in industry that would mill out the mould and form the body shell for us. We send the plans away to “3-axis Machining”. The shell itself is moulded from a thin layer (0.007”) of polyurethane which makes it very light adding almost no mass to the car and therefore having very little effect on the cars centre of gravity. Slight modifications had to be made to some of the dimensions by Trevor Crout, of 3-axis machining, to allow the body to be machined out of the polyurethane material available for moulding. Other small changes included a larger draft angle on the rear of the shell so that it can be removed with relative ease. Roughing pass completed Finishing pass completed Hand Finishing completed Completed Body Shell
Assembly Chassis We started off the assembly by making some chassis parts which would be used as experiments so that we could test fit the components and adjust sizes accordingly. We ended up making quite a lot to make sure that all of the pieces fitted together! Once we had found a chassis design that worked, we then put the motor and all the other vital equipment onto the chassis and set about testing it. We had some issues with flexibility of the HIPs and had to add a simple locking mechanism to stop the chassis from flexing too much in the middle. It was at this point at which the part of the chassis that holds the guide got broken. So we had to redesign this part of our chassis. What we did was add another layer to the guide hole. This added a slight amount of weight over the front axle, but not so much that the handling became compromised, as we found out when we tested it on the track. The car seemed to perform really well and was easy to drive. Body Shell Once we had completed the chassis we set about attaching the body shell. We planned to attach the body shell to the chassis with tape, as mentioned earlier, but during the chassis testing stage we identified the need to regularly remove the shell for chassis adjustment and we also wanted to dampen vibrations through the body from the chassis. Mr. Cleave showed us his Slot Cars and we developed a method where pins would be pushed through the body shell and locate in tubes that are loosely retained by holes in the ribs of the chassis. This seemed to be a very effective method of attaching the shell to the chassis. This picture shows a spare body shell on the chassis that we made, to enable testing to take place, before we were able to source someone with a CNC Mill The arrows show where the pins are that hold the body shell to the car.
Assembly - Final Parts Assembly for Tuning We assembled the running gear by starting with a set of gears that could be easily changed so that we could work out the optimum gear ratio for the track and motor. These are the most up to date parts of the chassis, ready to laser cut. They include all our modifications including the strengthened guide hole. You can just see the version number that is engraved into the chassis. This is version 1.81 We also curled the wires from the guide to the motor so that the wires would create a springing force and would help to make the guide straight if the car went off the track. This would make the car easier to put back in the slot if it came off the track. Pinion Spur Gear We had included a hole for a bearing which is shown by the arrow. This was done because, without a bearing, the axle would rub against the Hips, inside of this hole, and wear it out very quickly. We got the bearing from a Scalextric car. The bearing will also retail some oil and help reduce friction in the drive train.
InitialTesting Testing for all race teams is essential. We went through various different stages of tests for lots of different aspects of the car’s performance which is shown below. Test 1 - Driver We began this process by holding a school qualifying session to see who could post the fastest lap time over a set track. It resulted in two clear favourites appearing, both from the design and technology class, in the form of Mark Heard and Jeffery Koh. Once we had identified the drivers they put in countless hours of training to prepare for the event. This involved becoming familiar with the track layout and learning the limitations of the car. (This meant practicing racing Scalextric at home. What a terrible bit of homework to have to do!) Test 2 - Cornering This identified weaknesses in the chassis and strengths within the body shell. We identified that when the car goes round a corner, it is subjected to large forces which put large amounts of strain on parts of the chassis. This was learnt the hard way, during the test when one of the team members cornered too fast and snapped the ring securing the guide pin. This caused us to go back to the drawing board and redevelop this area of the design. We came up with the solution of doubling up the ring which would mean that we could corner slightly faster without the risk of having a recurring breakage. This caused more weight to be placed over the front of the chassis, reducing the grip at the rear. To combat this, we redesigned the guide holder to remove more material where it wasn’t necessary. New Parts, showing strengthening plate and removed material Original Part, Version 1.6 Removed Material
Testing Continued Test 3 - Grip Grip is essential to keep our car glued to the track and to ensure we start and corner to the best of our ability. We determined the grip of the car through endurance testing. This involved running the car for approx 50 laps and recording the lap times as we went. It was clearly visible that the times began to drop as we continued to race. This was later found out to be caused by the wheels picking up dust and residue from the track. We came up with a simple solution to this problem in the form of rolling the tires over a piece of electrical tape, which removes all particles from the tires surface, cutting a few 10ths of a second off of our recorded times. Another method we used to enhance the grip of the car was to true the tyres. When the tyres were originally places onto the wheels, it could be seen that they were not entirely round! During testing, it could be seen that the tyres were only running on certain parts and the high and low spots could be seen. These sometimes matched the moulding marks on the tyres from where they had been injection moulded. By carefully sanding the tyres, to make them round, the grip of our car improved, quite dramatically. Test 4 – Gear Ratio As we mentioned on the assembly page, we looked at different gear ratio’s between the motor (Driver gear) the Axle (Driven gear). We discovered that a small gear was needed on the pinion which would mesh with the larger spur gear to create a car that had maximum acceleration. The smaller pinion gear would allow the motor to get up to speed faster than if the pinion was larger. A larger number of teeth on the pinion would have provided us with a higher top speed but this was not necessary as the standard course did not offer any long straights to take advantage of the top speed. Also the good acceleration of the car would mean that the motor would create a gyroscopic effect because of the motor being mounted sideways in the chassis, think of it like a bike wheel being spun. The faster it goes the harder it is to turn. The motor in the car works n the same principal. Velocity Ratio = Driven ÷ Driver
Future Modifications An advertisement to encourage new teams in school Wheel base + Guide pivot to Back Axle With our current car, we had quite a long wheel base. Which means that our car is very stable at high speed cornering, but all of us in the design team feel that we could have made a car with a slightly shorter wheel base which would mean that our car could change direction much more easily. This would mean that it could come become less stable and more likely to de-slot. Seeing as the track we would race on didn't have any long straights, it could have been advantageous for us to have used a shorter chassis. This area could be cut out of the chassis to shorten the wheel base and the guide to back axle distance. This is the easiest place to do this because it doesn't require as much changing is size of all the other components of the chassis Weight We could have experimented with weight in the car. This could have been done by adding weight onto the chassis in different places. For example if a track was predominantly right turns then some weight could be added on the right hand side of the car so that the car would be less likely to fly off the track. Also by putting weight inside the car you could effect the grip of the car. This could be done by putting weight at the back of the car to stop the back sliding out in fast corners. This would work because the tyres would be pressed into the track harder, requiring more force to move them. Weight could be added in these areas to improve handling.
