Team
Buggy
Competition
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History
The design of the '01 buggy is largely based on a very thorough analysis of our experience at the '00 competition. We carefully considered the performance of our own entry as well as the observed successes and failures of other entries. In this section, I will describe the design that we entered last year as well as our lessons learned at the competition and consequent design decisions for this year.
Although we began the design of the '00 buggy in late January, we didn't begin its construction until approximately two weeks before the competition the weekend of April 7th. Due primarily to lack of manpower, time, and money, our design was severely limited. Other factors were also against us, however. We had never participated in the competition before, and therefore didn't really know what to expect. Not only did we not know the extent of the course, but we didn't know the extent of the competition. Compared to our two week construction push, teams from other schools routinely spent more than a year with a dedicated team of 10-20 students in their own lab with their own tools building engineering marvels for the competition.
Our buggy was made up of essentially two identical halves. Each buggy
half consisted of a self contained two wheeled vehicle that, provided
each rider could balance on two wheels, contained everything needed to
operate independent of the other half. Each half had it's own drivetrain,
seat, brakes (in theory… the brakes never made it on the production
model), etc. Then, we simply attached the two halves together, one in
front of the other and allowed for an articulation between the two for
steering. Furthermore, the single member that we used to attach the two
halves was free to twist… meaning that the front half could rotate with
respect to the back half and vise versa. This provided our "suspension"
as it were.We had no gearing, the pedals were direct drive. Although we purchased bicycle freewheels to use as ratchets in each axle (to allow for the outside wheel of the buggy to spin freely as it went through turns), because of a miscommunication between the team and our advisor we only installed one in each axle… meaning we could only turn left! The frame of each buggy half was designed by plotting all the points that needed to be supported (i.e. the bottom bracket, wheels, brake mounts, seat, seat back) and then connecting them with the fewest number of truss members that didn't interfere with the chain line. This meant that the buggy was a very 3D thing. It occupied space. This may seem obvious, but it contrasted starkly to what we saw in Huntsville.
From observing other teams at the competition, we noticed a very prevalent trend. All the buggies in serious contention for first place looked very much like pedal cars. They had car-like steering, car-like suspension, and car-like drivetrians. The favorite frame style was to simply purchase a 6" square aluminum tube and weld stuff to it. Whereas our buggy was a 3D structure that we designed by considering where everything needed to be and then building the frame around it, other teams seemed to begin with a large central beam and then attach stuff where they needed it to be. They relied on the huge stiffness of this central member for strength… they had to since everything was cantilevered. In fact many schools outsourced the welding since they didn't feel confident welding the aluminum they chose as their construction material.
During the race, we encountered a large bump square on that caused the central member connecting the two buggy halves to bend. We had intended for the weight of each rider to be centered directly over his or her respective axle, however, because we were so rushed, we never actually calculated this… it was simply a best guess. As it turned out, our best guess was about 6 inches off, meaning that each rider's center of mass was about 6 inches in front of the axle beneath them. When the central member bent, the front half of the buggy dove down so that the nose of the buggy was digging a trench as the back person pedaled along.
Some argued that we had a good design; if we beefed up this central member and repositioned the axles under the riders, the buggy would have worked fine. Others saw the flaw as a lacking in suspension. While the pivot between the two halves allowed the buggy to take obstacles that were encountered one wheel at a time very well, it had no ability to absorb impacts that were faced by both front wheels at once.
Overview
Over the summer, we spent many long hours under a tree on the engineering quad discussing how to best approach this year's entry. Here is a summary of what we decided:
1. Suspension - it needs to be improved. Not only does it need to be able to handle torsional loads on the frame (as last year's did excellently) but it also needs to be able to absorb bump loads as well. Unlike automobile suspension, the primary purpose is not to provide a smooth ride. Rather, the goal of our suspension system is threefold. Fist of all, a suspension reduces and channels the forces applied to the wheels by the course. This means smaller lighter tubes can be used for the frame and the buggy can be made lighter. Second, suspended wheels will allow the wheels to remain in contact with the ground while traversing obstacles. More wheel contact means more pedal power can be put to good use. Finally, the drivers are the ones powering this thing, and humans respond very adversely to the large jostling that the course will give them. Although the course isn't long (about 1/2 mile) reducing the jostling the riders experience will reduce muscle strain and enable them to pedal harder. After discussing all the options, we settled on an independent A-arm suspension scheme.
2. Drivetrain - it needs to be improved. During the race a constant source of aggravation was the chain slipping off the sprocket. Even though we only had one gear, we saw other teams with a cassette of gears experiencing this same difficulty. We want a shiftable drivetrain that WON'T derail. This should be an easy thing to achieve... in theory. We want to keep four wheel drive. Although many successful buggies were made as two-wheel-drive designs, we feel this is a small but important advantage and it's something that we're capable of implementing. We've kept the concept from last year of having each person power their own axle. This greatly simplifies the folding of the buggy because there are no chains that go the entire length of the vehicle.
3. Steering - lots of debate here. Front chunk steering (as last year's design came to be called because the entire front half of the buggy would move in order to steer) had a lot going for it. It was simple to build, had few moving parts, and made the implementation of A-arm suspended, powered, front wheels a WHOLE lot easier (as opposed to car-style steering). It also provided an obvious spot to take the buggy apart in order to fit in the volume requirement. In the end we decided to go with car steering. The primary reason for this is the large moments caused on the steering system when a wheel encounters a bump on last year's buggy. We would hit a bump and the buggy would articulate even though the driver strained to hold it straight. A car-style steering system (with small wheel scrub) will have a much smaller force to counteract when traversing rough terrain.
4. Frame - This was primarily dictated by the decisions we'd already made. We wanted to stick with a trussed frame and we wanted it to be rigid along the entire length. We've moved to 4130 chromoly steel in order to make the buggy lighter (stronger steel means thinner tubes).