The frame is based on triangles, the fundamental shape for trussed structures. Triangles allow for simple, rigid, lightweight structures to be made. The idea is that all loads can be taken along the axis of the tubes, preventing bending loads, and allowing for a lighter, stiffer, and stronger frame. A triangular cross section, wide at the top and narrow at the bottom, makes for increasing camber in the wheels through the suspension travel, which is desired to ensure wheel strength. The wide top is also helpful to place the components of the frame, including the suspension, steering, and drivetrain elements.
Before this general geometry could be implemented, a few smaller issues had to be resolved. For example, it was generally agreed upon that last year’s frame was overbuilt, and further loading calculations supported this conclusion. In order to determine the minimum sizes of our tubing, stresses in the frame were analyzed after applying dynamic loading conditions while varying the frame and tubing parameters. The tubes were chosen such to provide a high factor of safety for the frame under the following loading conditions: a straight two-foot fall, a collision with a bump roughly the size of a curb at 20 mph. These calculations supported a reduction in tube size from 1” square to .75” square with a wall thickness of .035”. At this size, the estimated tensile and compression stresses fell within our factor of safety of 2. Torsional loading proved not to be an issue, as the frame would be trussed well enough to make it function as a single member during such loading. Torsional strength calculations put failure beyond concern
We chose 4130 cromoly tubing for its good welding characteristics and high strength. Although there are many steels with higher tensile strengths, none are as easy to weld as 4130. Since our team welding ability at the time was novice at best, this characteristic was of prime importance. Lighter materials, such as aluminum and titanium, were considered but were ultimately rejected because of the difficult heat treatment process that is necessary and the overall added cost. Likewise, we rejected carbon fiber as too costly and difficult to work with. The possible benefits did not justify its use. We noted that the weight of the frame was a small percentage of the total weight, and reducing weight there was not as useful or cost-efficient as in other areas. Although round tube was deemed slightly more rigid, the square shape is an easier geometry to work with in terms of mounting and welding. Also the total weight saved in switching to a round-tube frame of equal strength was a mere 2.92 pounds .
The new folding design required that one set of wheels must be inboard of the other set in the folded position so they could overlap. For stability reasons, we decided that the rear wheels should be narrower, by two inches on a side. In order keep the A-arms at reasonable length in the rear, we decided to make the entire frame narrower from 10” to 9” and to mount the top A-arms on top of the frame rather than on the side or inside of the frame.
Last year’s frame used the front internal hub shifter as a structural member, which imposed limitations on its placement and configuration. We decided to make the frame longer so a permanent structural member could be placed past the shifter. We also made the wheelbase shorter, because we used U-joints rather than CV joints at the outboard ends of the front half shaft. This significantly reduced weight. The shorter wheelbase allows the same angular deflection of the wheel to result in a smaller turning radius, reducing the sinusoidal velocity produced by U-joints running at high angles.
The need for a shorter wheelbase led us to investigate the possibility of turning the rear rider around backwards, so that their pedals extend out from the buggy rather than being inside the buggy. This turned out to have many other benefits, including a more symmetric design, a center of mass nearer to the center of the buggy, and it eliminated the problem of the rear pedals hitting the front seat. It also allows for a taller rider on the rear.
We decided to carefully jig the frame halves prior to welding for reproducibility, replaceability, and dimensional precision. We constructed a jig on an aluminum plate, and carefully cut and mitered all the tubes to the correct length and shape before welding. The diagonal tubes were made on the mill to produce the correct angle quickly and reproducibly. Finally, the frame pieces were tack-welded in the jig before being TIG-welded at all joints. The jig also helped prevent warping of the frame due to the intense heat and resulting metal expansion. We originally planned to heat-treat the frame to reduce the stress at the welds and the brittleness caused by molybdenum crystal grain growth around the welds. However, the effects of these welding byproducts were determined to be too insignificant to justify the long heat-treatment process. The connecting hinge was later added to the frame by mitering the ends of both frame halves to fit the interlocking hinge pieces. With the pin in place, the pieces were welded onto the frame while it was clamped together. The trapezoidal hinge plates, of 0.08” thickness were also welded onto the assembly at this time. Next, the top members were mitered to receive the T-handle apparatus, which was also welded into place while the two halves were clamped together in the unfolded position.
The frame performed perfectly, with no appreciable flexing or yielding during or after competition. It is strong and reliable.
In hindsight, there were only a few things that we would have done differently regarding the fabrication of the frame. For instance, we realized that the entire frame configuration could be greatly simplified if we had known beforehand where all of the components were going to be attached. This will require a complete computer model of the buggy if such a decision is to be made in the future. Also, the frame was designed so that the top members would just clear the minimum 15” that the rider must be supported above the ground. This proved to be too high, as we did not account for the additional height added by our seat design. In the future, the seats should probably be dropped into the frame to provide more stability with a lower center of gravity, while still satisfying the minimum height requirement.
The main thing we learned from the frame construction was the importance of a jig in any kind of fabrication. It proved most important when making multiple parts that needed to consistently have the same dimensions. It also helps speed production along and keep parts in order. In addition, the final frame was much more accurate to design and much less crooked than past products. This jigging process would be implemented in almost every other part fabrication process throughout the project.
In order to meet our goal of a sub-10 second assembly time we decided that the two buggy halves had to be hinged. The hinge would eliminate the need to align the two buggy halves during assembly, as was the case with last year’s buggy. Also, keeping the buggy halves connected allowed the brake cables to run from the rear of the buggy to the front, giving the front rider control of the brakes.
The hinge had to allow the buggy drivers to assemble the buggy quickly and easily while not adding appreciable weight to the frame. The other major consideration was the hinge’s strength, both in axial and torsional loading. This was crucial in order to keep the frame halves together and aligned when the buggy is assembled as well as during assembly.
In order for the buggy halves to fold easily, the buggy had to come up in the center while the wheels came together. This meant that the hinge had to be located on the bottom of the frame. A bottom-mounted hinge would allow gravity to hold the frame halves in a stable position. This is accomplished because as the frame is unfolded the ends of each frame half rest up against each other while the hinge holds the bottom edges together. A down side of a bottom-mounted hinge is that the triangular frame shape allows for only one hinge connection point versus the two the top would have provided.
To get an idea of what design would work best, we looked at commercially available prefabricated hinges. The hinge that seemed most likely to work was a typical fence gate style hinge composed of two mounting brackets attached with a pin. However, due to the torsional loads required of the hinge in the disassembled state, all commercial hinges were deemed too weak.
The main weakness in the strength of a conventional hinge is the fingers that hold the mounting brackets to the pin. The ends of the mounting brackets have interlocking fingers that curl around the pin. This design is very strong axially, but is lacking in torsional stability. This is why gates and other hinged members use two or more hinges spread out along the length of the hinged member. The single lower frame member does not allow for more than one hinge to handle the torsional loads, so a new stronger hinge had to be designed.
By benchmarking the old buggy’s top connection system, another hinge idea was spawned. A hollow steel rod cut into sections could be welded in an alternating pattern to two steel plates and a pin inserted into the alternating rod segments to form a simple hinge design. This hinge design would be similar to a standard hinge (as described above) with its fingers welded down to the plate to form loops. This would eliminate the likelihood of the pin deforming the fingers under torsional loads. Also, the hinge could be custom-built to exactly meet our strength requirements.
A three inch wide bottom hinge was chosen go give us a wide hinge point without extending too far out of from the lower section of the frame. Since the hinge on the top of the old buggy worked well and we still had remaining material, the same hollow (OD 5/8”, ID 5/16”) shaft steel and (OD 5/16”) pin were used as last year.
A Matlab program was devised to evaluate the number of minimum shear points necessary. In order to maintain symmetry, six rod sections with five shear points were used.
(Link to Chris’s Matlab Calculations)
The mounting brackets were designed to be as strong in tension as the bottom tube of the frame . To get the strength required, a 0.066” sheet was needed. Due to material availability, 0.080" sheet was used.
The final design consideration was the connection of the hinge to the frame. In order to eliminate stress concentrations in the thin-walled tubing, frame inserts were rejected. Clamping the hinge to the frame was rejected for its weight and complexity. Welding was chosen as the preferred method for its ease of implementation and strength.
With the hinge completely designed, the next problem was how to lock the frame in the unfolded position. The top connection on the frame would not have to handle strong axial loads because the top of the frame is in compression during most riding. The top connection would, however, have to handle the torsional frame loads between the frame halves. These torsional frame loads would actually appear as shear in the individual connection points because of the trussed frame. For this reason it was decided that the top connection would consist of two connections, one at each corner of the top of the frame triangle.
In an effort to maintain the quick assembly, a simple self-locking design was desired. Weldable T-handle plungers were found on McMaster-Carr. Although not completely automatic, these plungers would give the user a secure handle to guild the frame halves together and provide a very secure attachment. The plungers would then hold the frame halves together while the shear stress created would be handled by the tube that the plunger goes into. This design is very similar to the proven connection design of last year’s buggy.
The hollow rod was cut into six half-inch sections. The 0.080” plate was cut to the shape of the mounting brackets. Then the sections were placed on the pin material and welded to each mounting bracket and allowed to cool. This ensured that the pin would be able to fit into the hinge once welded. Then the frame halves were clamped together and the hinge was welded to the lower frame member and again allowed to cool with pin in to ensure proper alignment.
For the top connection the T-handle plungers were welded to one frame half. Then hollow steel tubes were welded to the other frame member next too where the T-handles align.
The hinge performed perfectly in competition during both assembly and the race. There is a slight amount of misalignment that occurs when the buggy is loaded when in the folded position, however this is minor enough that when the buggy is assembled the T-handles and hollow tubes align themselves when pushed together.
Everything proved strong and reliable. If the T-handles could be replaced with a self-locking mechanism, assembly time could possibly be improved.
One of the fundamental problems with the ’01 buggy was flexibility in
the crank supports. This caused a myriad of problems. To combat chain derailment,
the chain had to be over tensioned. This destroyed the internal hub shifter
bearings. The slots that were used to tension the chain slipped, causing
the chain to derail. Overall, the riders were not able to put their full
power into the cranks for fear of derailing the chain, or breaking part of
The design goals for this year’s crank supports were to create structures that deflected minimally in torsion while remaining foldable to fit inside the four-foot cube. This was to be done with a minimum of weight and complexity.
We decided that a triangular three-member support system would be ideal due to its inherent geometric rigidity. Using a single oversized member was considered but dropped due to the difficulty in maintaining constant chain length. A single member could not have a pivot concentric to the hub axle unless it was very bent or we rearranged the hub configuration. Y shapes and other complicated geometries were considered but dropped. The V shape support seemed simpler and better. As for the third support, we debated whether to have it point backwards and be in tension, or point forwards and be in compression. After geometric considerations, we decided that the tension member idea would not fit inside the frame as a rigid member, but would need to telescope or hinge to fit. We went with the compression member. Initially we thought that the compression member would still have to hinge to fit inside the box. This was not preferable due to the possible slop in the hinge. However, by moving the point of attachment we were able to leave the third member rigidly welded to the bottom bracket thus producing a more rigid support. To increase rigidity we enlarged the support tubes to 7/8” outside diameter with .035” wall thickness. After testing we decided that the cranks still had too much flex. We put in a triangular truss section atthe midpoint of the cranks where they would be most effective. This eliminated most of the flex.
We eliminated the need for slots in the supports by using an eccentric bottom bracket shell. See the drivetrain sectionfor more information.
In order to maintain constant chain tension, the pivots of the crank supports
had to be placed in line with the hub axle. We considered putting pivot
points beyond the axle but decided that it was much easier to ensure concentricity
if the supports pivoted on the axle itself. Since the axle was relatively
narrow without much length beyond the dropout bolts, the pivot mechanism
also had to be narrow. Also, since the tubes were much larger and stiffer
than the old ones, it was no longer possible to bend the tubes out around
the axle to get it on and off. Thus, a two-piece mechanism was used to get
the crank supports on and off the axle. As for the pivot itself, a piece
of 1/8” steel plate rotated about a flanged washer, slightly wider than 1/8”.
We went with a ball plunger locking mechanism to keep the crank supports in the riding position. This was this because it was a quicker assembly than pins or bolts.
Lengths / Positions
The criteria for crank position was for them to be able to fit a 6’6” front rider at a maximum and a 5’10” rear rider at a minimum. For simplicity we placed the back of each rider at the centerline of the buggy and measured from there. We estimated about 18” from the back of a rider to their crotch. We estimated the top of the seat would be about 3” above the frame (it is actually a little more). The clearance from the frame and hub shifter to the center of the bottom bracket needed to be at least 17” so that the heel of the rider’s shoes would not be obstructed. The rear is a little lower than the front since female shoes tend to be smaller.
From the positions of the seat in its maximum position and the location of the hub shifter we calculated where the bottom bracket needed to be and therefore the size of the support.
We built a jig to make the cranks so the connection points would be perpendicular and the correct width. The jig held the positions of the bottom bracket shell and the two pivot plates. Tubes were then mitered by hand to fit into place. On the front to ease mitering the pivot plates were bent. This gave a more structurally rigid member and reduced manufacturing time. The third member was mitered into place on the buggy and then tack welded.
The crank supports worked adequately. To date there have been no chain derailments on them, and riders are able to pedal as hard as they can without excessive deflection. The supports are significantly heavier than last year. Also, the deflection that does occur comes as much from the attachment point as from the supports themselves. The ball plunger locking mechanism worked much better than expected. Although it is difficult to disengage, the assembly of the cranks was nearly flawless.
The biggest area for improvement in the cranks is in the pivot area. If another team is to do the same pivoting style, some thought should be put into more rigid connection methods. Some method other than metal plate could potentially reduce weight and improve stiffness. Also, some type of automated mitering technique analogous to the A-arm fabrication could reduce fabrication time significantly.
Seats / Seatbelt
The design goals for the seats were to provide a sturdy support to press against and to keep the rider securely on the buggy. In addition we decided to make the seat position adjustable to accommodate different sized riders.
This year’s buggy went through multiple seat designs before the final solution was decided upon. Following the lessons learned from last year’s seat design, this year’s were intended to allow for greater pressure to be applied to the seat back so that the riders could pedal with greater force and stability. Also, adjustability was included to accommodate riders from 5’4” to 6’6”.
Originally, the seats were based around a lightweight, portable, folding chair that both contoured to the rider’s back and also had connecting straps from the top of the back support to the lower front of the seat section. These straps not only allowed for greater pedaling pressure to be applied, but also hugged the rider’s waist so that during turns a feeling of greater stability was achieved.
This original design had a plywood square base, cut to overhang the width of the frame by an inch on either side. The folding seats that were purchased from an outdoors store and were made of fabric with interior structural support and book-bag-type straps to adjust the tilt of the back support. Four holes were drilled in the bottom part of the seat and correspondingly four holes were drilled in the square wooden base. Nuts, bolts, and washers then connected the folding chair to the plywood. Originally a rip occurred in one seat and larger washers were needed to prevent further tearing. Before the chairs were attached to the plywood, eight ¼” holes were drilled, two in each corner. Each pair of these holes was then spaced so that the width of the frame (3/4”) could fit between them. Then four steel rectangular attachment devices were fashioned, with holes spaced to correspond to the pairs in each corner of the plywood. The plywood support was then held on the buggy by placing it on the frame and putting, in each corner, two ¼” bolts which sandwiched the frame between the steel rectangles and the wooden frame. These attachments held particularly well, and the bolts could be loosened so that the wooden frame/seat assembly could slide as far forwards or backwards as was required. The movement was limited by the suspension in front of the seats and by the break in the frame in the rear of the seats. This design, although structurally sound, was relegated to the retrofit of the old buggy and steel framed BikeE seats were chosen for the new buggy instead. This change over from being on the new buggy to the old buggy was easily achieved through simply re-drilling the holes in the wooden platform and widening the spaces between the holes on the rectangular steel attachment pieces to accommodate the 1” in steel frame of the old buggy.
BikeE brand seats were much like widened bicycle seats with two steel supports extending from the rear and connected with a mesh material for the back support. They came with connections on the bottom of the seats that could be snapped into place to hold onto an I-beam like piece of steel that was bolted to the frame with four bolts, two in the front and two in the rear. It connected to the frame on two widthwise pieces of steel, one being the widthwise beam at the center connection point of the frame and another added just before the suspension in the front/central region of each half of the frame. A major problem with the seats arose when it was realized that the seat flexed from side to side due to a lack of lateral support. This problem was solved with the addition of two plastic seat attachments. These attachments were milled out of polyethylene block. They had a semicircular hole at the top, which allowed them to snap onto the seat support bar and a runner cut into the bottom which allowed them to snap to the top frame member. They slid along the frame, allowing for adjustability and yet they transfered side to side loading of the rider to the frame members, reducing torque on the I-beam and providing stability for the rider.
The seats worked, but not as well as we hoped. The back supports were not strong enough, and bent under hard riding. Also, due to the mounting method and design of the seats, the people’s seated position was much higher up than anticipated. The main advantage of these seats was their comfort. They were very comfortable and looked good.
One major goal of future buggies should be to keep the center of gravity as low as possible. The rules require that the lowest part of a rider must be at least 15 inches off the ground. Right now the riders sit almost 20” off the ground. Also, the purchasedseats do look good but they need to be strong enough.
Dust Abatement Device
Fenders were added in the final days of assembly as our “dust abatement” assemblage. They were placed on the front two wheels of both the old and new buggies. For both buggies the design was similar except for the size of a single drilled hole that was attached to the uprights using the bolt that connected the upper a-arms to the front uprights. These bolts were separate sizes on each buggy.
The fender sections of the dust abatement system were plastic bike fenders that we purchased from the Bike Rack. Two holes were drilled widthwise through each of these fenders. These holes supported the steel rod that would attach the fender to the upright of the respective buggy. The rods were bent on the outboard side of the fender to keep the fenders from falling off. The hefty support rods suspended the plastic fenders directly over the wheels, towards the rear in order to reduce the dust being kicked up by the treads. These rods where then welded to 2” long rectangular pieces of .063” steel plate which were bent at approximately 80 degrees. These pieces had holes drilled in them on the bottom flat section that allowed attachment to the uprights using the bolts already there holding the A-arms in place. Cornell stickers were then placed on the tops of the black fenders, which added to the buggy’s intimidation factor.
During the actual competition, the fenders mostly served an aesthetic purpose, as the competition course was not very dusty. However, the fenders have proved extremely effective when the buggy was ridden on more dusty surfaces. The support rods were slightly deformed during transportation, but the problem was easily fixed upon arrival.
Fenders, when necessary, should be minimally intrusive, and designed to
attach to the buggy in a place where bolts are already in use for other reasons,
such as the uprights. The supporting rods were not the strongest components
on the buggy, and did deform some during transportation, but the trouble caused
by the lightweight supports did not merit larger, more rigid support members.
Overall, the fenders served their purpose nicely, and if compatible, should
be used on next year’s buggy to save time.