|
|
By Alan Gruner To respond with comments or to add chapters from your own experiences please click here to enter the e-Book - Organizing a Formula SAE Team Forum" Or, send your thoughts by e-mail to mitchl@sae.org. We'll make sure they are posted to the correct section of the e-Book. If you have not yet reviewed part one click here. PART II: Design, Construction and PreparationWhat is a racecar? One simple, useful definition is this. A racecar is a vehicle that has nothing on it that is not required by the rules or that does not make it go faster. This definition gives you two important questions to ask about every design decision. Is it required by the rules? Will it make the car go faster? The third question for FSAE is, will it get us more points? Asking these will help when deciding between alternate sets of ideas or solutions. Common Misconceptions Another misconception is that FSAE is a good place to develop new technology. No FSAE team will have the knowledge and resources to develop brand new technology and still meet eligibility requirements for members. You can be innovative. But the place to do it is packaging existing parts and technology. Some teams use new technology developed at their universities. Some teams use new materials or parts that are donated to them by manufacturers who want to showcase their products. Very little in a modern racecar design is new. The first double-overhead-cam four-valve-head race engine appeared in 1916. The first stressed-skin monococque chassis was built around the same time. Electronic fuel injection was developed in the 1950s. Tubuler space frame chassis were state of the art in 1920's aircraft. Composite materials date back to at least the 1940s (several hundred years if you count adobe as a composite material). Experiments with anti-lock brake systems were being carried out in the early 1950s. CNC machining has been around since the early 1970s. Much of what we think about as "high tech" has been around for longer than a lot of you who are reading this. It takes a long time and a lot of development money for technology to reach the point where it is reasonable to put it into a $5500 racecar. The skill that is most important to develop is to be able to use what is available to you. If you are interested in developing really new technology, there are design competitions better suited than FSAE. A final misconception is that competitiveness in the FSAE comes from having the best car at the competition. Competitiveness comes from having the best combination of car, drivers and presentations. The team with the most points wins the competition. If you want to win, make decisions that add more points to the total at the competition. The majority of the points are available in the driving events. The driver is the biggest variable in determining how well a car will perform. It is no accident that Formula 1, CART and NASCAR drivers are the highest paid professional athletes in the world. In FSAE race preparation, driver training and selection are routinely neglected. At the time of this writing, only a handful of teams have come up with effective ways of getting drivers trained. For a lot of teams, good drivers happen by chance and not by design. Hopefully more teams will develop a systematic approach to driver training and selection. Lessons learned the hard way Lesson Number 1 - Make it SIMPLE First, keep in mind that you will not be able to come up with a completely new design each year and get it built and running in nine months. One key is to reuse any good designs you have from previous years. If you have a system from last year's car that works and works well, use it again. Make minor improvements if appropriate but keep major redesign work at a minimum. Save your time and energy for the systems that were less successful in previous years. For any system or part that requires major redesign, keep it simple. You realize after you have designed and built a few pieces and systems that it is harder to build something simple than to build something complex. It is easy to make things that are complex and heavy. It is harder to build things that are complex and light. It is hardest of all to build things that are simple and lightweight. To be competitive you make things simple and lightweight. Simplicity is hard to achieve because it takes the most thought, planning, and design iterations. Simplicity pays handsome rewards. I have read that William Kettering, the head of GM engineering when such landmark designs as the small-block Chevy engine were conceived and developed, had a sign on the wall of his office that said, "Parts left off weigh nothing, cost nothing, and don't cause service problems." One of the non-technical reasons for keeping designs simple and parts count low is that for a manufacturer there is less inventory to keep track of and inventory = $$$. This becomes important in your Sales Presentation. For you, the racer, it means carrying fewer spares to the track. You want to minimize, not just the total number of parts, but also the number of different kinds of parts. Another concern is that more pieces take longer to assemble. Assembly time has to be counted in the cost report. Longer assembly means more time in the shop building the car and less time driving it. At the competition, things break at the absolutely worst times. Often you have to tear half the car apart to fix it and there is only an hour until you have to be at the starting line. This DOES happen. The more pieces to assemble, the less likely you will make the race. You also have more opportunities to assemble it wrong. You will be embarrassed and could be injured when this happens. I have seen a car fail because students in a hurry forgot to put nuts on the bolts that hold suspension arms on the car! Lesson Number 2 - Force = Mass x Acceleration (F=MA) Lesson Number 3 - Packaging and Integration Drive 90% of Design Lesson Number 4 - Read the Rules Lesson Number 5 - Allow Enough Time for Details Novice car builders always grossly underestimate the time it takes to have a vehicle on the ground and running well. When I was working on my first FSAE project, an experienced member told me to use the rule of pi when planning any part of the car. The rule of pi states that when you have made your longest possible estimate of time needed to do something, multiply the estimate by pi (3.14) and that is the minimum amount of time that it will take to get it done. It works! Things always happen that you can't plan on. A machine shop is closed at a time when it is supposed to be open. You need data from someone who just found a new boyfriend or girlfriend or, worse, broke up with one. Someone breaks a tool that is critical to your project. Your professor decides to make a course more "challenging." You get called for an interview with your dream company. Your parents see your grades and demand changes. All of these things happen to students while trying to finish a simple part. A misconception of many newbies is you can design, build and install a part and it's done. It is never that simple. A racecar is always a work in progress as is every part on it. It is not unusual to go through three or four iterations of a basic design before you have something good. It is also not unusual on your first three or four projects to get halfway through building a part or system and then think of a substantially better way to solve the problem. It is easy to design something that can't be built with the tools you have. A typical sequence of events goes like this. Design a part. Redesign because you had better idea. Start over because you can't buy the right material for the new design. Redesign because the parts you attach to have changed or you realize you forgot something. Redesign and build, test. Find problems. Make more changes. Retest and make final tweaks. All of a sudden the part you thought was going to take two days to finish has taken two weeks. Lesson Number 6 - Get the Electronics Working Early The bigger problem areas are the crank position and (if you are running one) cam position sensors. These usually consist of magnetic or Hall-effect sensors that generate a signal from their proximity to a trigger wheel. The trigger wheel is a piece of metal attached to the crank that has teeth machined on it. Both the sensor and the trigger wheel have to fit someplace on the end of the crank where the engine designers didn't leave room for it. This takes awhile. This is where you need electrical engineers. Sensors and triggers have to be in exactly the right proximity to each other to get a signal the computer can read. You have to start installing, tweaking and debugging the engine electronics as early as is humanly possible. This has to start before you have your intake and exhaust systems designed. Teams lucky enough to have a dynamometer need to get the engine installed on it as soon as the engine is in your possession. If you don't have one, build a test stand. It need not be fancy. It just needs to be something you can mount the engine on to run it long before the car is finished. It is going to take quite a while to design and build the final intake and exhaust for your car. Don't wait until they are done to get the engine running; have it running before your school breaks for Christmas/New Year. Create some temporary intake and exhaust manifolds. They don't have to be legal or even practical for the final car. You need to do system debugging. Assuming your electronics are working correctly, chances are very good the fuel and spark maps are nowhere close to what will start the engine and have it idle reliably. Again, barring electrical problems, you can plan 20-60 hours of trial and error just to get the engine to start and run reliably when cold or hot. Remember the rule of pi. Lesson Number 7 - Avoid $2 Part Failures Suggestions to Get You StartedInventory Chassis There are three sets of critical "hard points" that you need for the design. They are the cockpit dimensions and control locations, the engine/powertrain dimensions, and the suspension points. Engine dimensions are the easiest, assuming you have an engine. As soon as the team has formed, put a crew on the job of measuring the engine in every dimension, especially the mounting points. Next, work up measurements for the driver's cockpit. To test ergonomics, a mock up of a cockpit can be something box shaped that various people can sit in to try different positions for the steering wheel, pedals, shifter, and switches. Find an arrangement that is comfortable, makes every thing easy to use and puts the driver's weight as low as possible. You will need adjustments for different size people, such as a combination of padding and adjustable pedals. Measure how much room is needed for the driver to move, paying particular attention to the elbows, knees, and feet. The suspension points often take the longest to specify. The suspension geometry has to balance a lot of conflicting elements and it takes time to get to a workable solution. By the time you have gotten the driving position together, the suspension points need to be ready also. From there you sit down with the rules and your knowledge of structures and play "Connect the Dots." It is usual for these points to come into conflict. The steering rack regularly appears between the driver's feet or legs. The only thing you can do is compromise. The temptation is to compromise toward the functioning of the machine and away from driver ergonomics. However, if the driver can't use the controls easily, the driver is not going to go as fast. The driver is the biggest variable in dynamic events. Make the job as easy as possible. You have to be able to use all the controls, throttle, brakes, shifter and clutch, AT SPEED! One thing that helps to focus on your task is to have a picture of what the car will look like when it is finished. This is difficult without a frame, but it is possible to guess within about a centimeter where most of the hard points will be and create a picture from that. You can certainly get close enough to begin designing the major structures before all the hard-points are set. The sooner people know what the space looks like, the sooner they can eliminate ideas like three-foot-long intake manifolds and focus on something that will fit and do the job. In 1996, as this is being written, you need to build a $6000, 500-lb. car. You need at least a month of driving to tune and debug it and for the drivers to get seat time. If you want to be a contender for the overall win, this is the price of admission. Powertrain & Electrical The question that the group almost always starts with is, "What kind of engine are we going to use?" It is an important question. The car doesn't go anywhere without an engine. However, there is a lot of work that is independent of the engine that can and must be started while engine decisions are still being made. An engine needs to be connected to a differential. The differential has to be connected to C.V. joints and half shafts to get the power to the wheels. If you use a chain or belt to transmit power to the differential, the chain is going to stretch and will need to be adjustable somehow. A sprocket has to be attached to the differential. You will have to engineer that. The pieces that connect the differential to the C.V. joint almost always have to be designed and built by the team. You can be 99% sure the half shafts will need to be a length not commercially available and the team will have to design and build those also. All these pieces need to be strong enough to transmit the engine torque, and brake torque if you are running inboard brakes, and yet be light and not cost too much. It takes thoughtful design with attention to detail and a lot of precision machining. This attention to detail separates the top teams from the also-rans. A project that can be started before the engine decisions are finalized is the cooling system: This is VERY important. The racecar that can't cool can't do anything else. The final specs of the system will depend on the engine decisions, but there is plenty of groundwork to be done. Cooling system calculations are no mystery. There are probably a hundred million vehicles on the road each day that don't overheat. However someone on the team has to learn how it is done. Someone has to build or borrow a test rig to measure all the relevant temperatures, pressures, and flow-rates. If you are using a liquid cooled engine, you have to source the heat exchanger or radiator from somewhere. Someone has to make sure it is mounted in the chassis in a way that it won't be damaged, has sufficient cool air coming into it and has somewhere for hot air to escape. Don't make any assumptions that a stock bike radiator will be sufficient. Countless races have been lost because of this. One last item, which needs thought and planning, is gear ratios. The final drive ratio is the only one that you can change easily. If your team has the resources to custom configure the internal ratios of the gearbox by the time you read this, congratulations! You have a level of sophistication that exceeds anything I could do. Whether changing internal ratios or just the final drive, the engine torque has to be usable to the drivers in whatever event they are driving. There are good books on the subject. One mistake that is often made is gearing the car for the acceleration event only. If you are only going to run one gear set at the competition, optimize it for the endurance event. That is where the most points can be earned and the most driving will be done. Suspension General Hints Remember that racecars are inherently high maintenance machines. It is entirely appropriate to use or design a part that will wear out quickly. This is not an excuse for being shoddy or not doing your homework. It means that parts that wear out regularly like brake pads and disks, suspension bearings and bushings, and drive chains can be sized for a short life between replacement. You have to be really anal-retentive about checking the whole car each time before it is run. Rebuild or replace anything that is questionable. A final hint. Where possible, iterate off of an existing design. It is much easier to take an existing part or system and analyze the strengths and weaknesses than to start from scratch, especially if this is the first car project a student has worked on. Your school has a lot of tools that can help you figure out where to reduce cost and mass. Full Scale Model Lessons Learned From FSAE CompetitionsStatic Events Cost Analysis & Presentation The bulk of the work is the cost report. The presentation is to clarify and/or defend things that the judges don't understand. It is mostly reactive for the presenters. The presenters should ideally be the people who compiled the report. Between them, they need to know where everything on the car was included in the cost report. It is hard to argue and negotiate with judges when you are staring at each other saying, "Uh... didn't we report the pedals in the chassis section? Maybe they're in with the brakes." It looks unprofessional. The judges expect you to have your stuff together. They might be inclined to be charitable to college students except that other schools put on the kind of presentation they are used to seeing at their jobs every day. The cost score comes from the price of the car and how easy it is to understand the information. They take specific numbers and compare them to the average or median prices from all the reports for the particular item. If it is below it is adjusted upward with an associated penalty until they see the car and verify that the item is indeed cheaper. If you don't include an item, then they adjust to the average with a penalty unless they can verify that the car doesn't have something. Even then it is discretionary. There are some things they require on every car. For example, you will be penalized for not including paint for a steel chassis. It can be a $10 item if you put just enough Krylon on to keep the thing from rusting. The judges know what many things should cost. If you deviate from that, then you'd better have documentation in the form of receipts or price quotes from suppliers. Every novice team tries to save money by under reporting assembly time for each subsystem and the whole car. The judges are wise to that scam and won't fall for it. It is a good idea to begin each section of the report with a short paragraph describing the pieces of the car and how they work, something like, "The brake system has dual master cylinders attached to the pedal through a balance bar mechanism. The pedal is of our own design and construction. The balance bar components were purchased. There are three brake calipers, one on each front wheel and one acting on the differential in the rear..." The goal is to make it clear exactly how the thing is put together. A good test for whether the report is complete enough would be to hand it off to someone not familiar with the car and see if that person understands it and what questions are asked. The easier it is for the judges to read the report and understand what you have done, the higher the score will be. These are relatively easy points to get. It takes time to get a good cost report together. Start it at the beginning of the year. Since 1995, the cost judges have been running a seminar at the competition on how to prepare a good cost report, and as a result, the quality of the reports submitted has improved dramatically. What was an outstanding report in 1995 became just average two years later. The trend is toward more and more complete documentation. Filling a three ring binder with a three to five inch stack of documentation is not uncommon. Try to get logistics management, accounting, and industrial engineering students involved. They will be able to add insight. Anyone reading your report should have no questions as to what any part looks like, who is going to manufacture it, how it will be manufactured, how long it will take to assemble, and how much the materials cost. Design Competition There are really two things that you are being judged on in the design competition. The first is the actual car itself. The second is how well you understand it. The first and most important thing in the design event is for the presenters to be assertive. Typically the judges will come and start looking over the vehicle as a whole and at certain details. A lot of them don't ask questions. They look at what you have done and evaluate it. They are human beings, not gods. They make assumptions about what they see and their assumptions can be incorrect or right on. They may not see the best thinking or understand why something is the way it is unless you explain it to them. They are also evaluating some 80 plus other cars and may just not have time for the logic of some particularly good idea to leap out at them. Judges also have preferences. They know a lot about what does and what does not work. You have to be ready to explain clearly what you designed and why you made those particular choices for your car. Presentations put together at 4 a.m. on the way to the competition after a week of all-nighters will not get you much. Once you have a conversation going with the judges, you have to demonstrate knowledge and reasoning. The underlying question a judge has is why. Why did you do it that way? What was the thought process? Do you really understand what you have? Engineering is, at its heart, an art form, the art of choosing compromises that best solve the problems presented. It is not only important to intelligently present the advantages of each aspect of the design but to know its limitations as well. The judges want to see that careful thought and analysis went into the decisions made. It is necessary to do your careful thinking about the design decisions up front. How will you explain it to the inquiring mind of someone who has been racing all his/her life? The judges also want to see data. You may have had the most brilliant thought process in the history of mankind to make the design decisions you did. The judges are going to be skeptical unless you can show them real data that proves your point. Again, it is important to have visual aids. When the car is put together, some of your best work may be buried in the middle of an assembly somewhere. Pictures of how things fit together help a lot. Photographs of the car during construction, exploded assembly drawings, graphs of horsepower and torque can all help to communicate how thoroughly you have thought out your design choices. You need to have most of the team on hand so the presenters can refer a question to the person who designed and built a particular item. It is also useful for junior members of the team to see and hear everything so they can be better prepared for presentations in future years. You also need to have tools there in case anything needs to be taken off or taken apart to show some particularly ingenious bit of engineering. Sales presentation The concept of the competition is that you are making the sales pitch to a manufacturing company that is considering building and selling 1000 of these racecars. If you are the manufacturer buying a design, what will you want to know? You will ask everything that the non-professional weekend autocrosser would ask because you have to sell cars to these people. Additionally you are going to want to know how difficult it is to build this car, what inventory is required for construction and service, and so on. A manufacturer will want to know the processes used to make your decisions. What computer modeling was done? How much testing has been done? No one wants to buy a design that is going to break regularly or is untested. When you have thought through the customer side of things, start collecting and organizing the information you will need. Think about what makes your car unique: Your original design features? Your creativity in using readily available, inexpensive parts in new, appropriate and innovative ways? How you saved money, inventory or assembly time by the simplicity of the design of part x or system y? Finally, as with all the other presentations, be visual! Make sure you have charts, graphs, photos, and spare parts to illustrate the message. Again, make sure the message is clear and the visual aids add to that clarity. It is easy to get caught up in making charts, graphs and photos that look cool and forget about the message. In 1994, MSU showed up with a 4-minute professional video that outlined the processes used to create the car that year. The presentation judges said afterward that their first reaction when they saw that our presenters had a video was, "Oh no." A previous team had shown up with 10 minutes of home movies of their car driving around in circles. Dynamic Events One of the most heartbreaking and often embarrassing things you see at every FSAE is someone with a great run that ends because of something simple that was overlooked before the car hit the track. You have to develop a systematic and consistent way to check all the critical nuts, bolts, hose clamps, fittings and electrical connections each time before the car goes on the track. This is a safety issue. You do not want critical parts of your car, like wheels, falling off when your are at speed. One of the simpler ways to keep track of your preparation is to create a set of checklists for each part and system of the car. This gives you a written record of what has been checked over before you drive and you don't have to rely on the memory of someone who has not had enough sleep for the last three or four weeks. Notice: "SAE" and "Formula SAE" are registered U. S. trademarks owned by SAE International To respond with comments please send your thoughts by e-mail to mitchl@sae.org. We'll make sure they are posted to the correct section of the e-Book. If you have not yet reviewed part one click here.
|
|||||||||||||||||||||||||||
|
|
||||||||||||||||||||||||||||