Containing Both the Vehicle Impact Analysis

And the Mechanical Systems Analysis

Friday, January 15, 1999

John Clarke and Adam Bennett

INTRODUCTION

Gideon's Torch is Taylor University's first generation solar powered vehicle. As a result, standard techniques and common materials were used in the construction of an aluminum space frame. While lighter materials are available, such as composites, the aluminum space frame offers a strong barrier of protection for the driver. As this report shows, our design displays a high regard for human life. A creator is always responsible for his or her creation; so must a solar car designer be responsible for producing a vehicle which does not compromise safety.

BASIC STRUCTURE AND GENERAL MATERIALS

OVERVIEW

An aluminum space frame provides the skeleton of Gideon’s Torch. Pairs of arched square tubes on the top and bottom run the length of the car and create the spine from which all other trusses are built upon. The driver compartment and roll cage is anchored from these four central beams. A bumper tube surrounds the outside edge of the car approximately 35 cm above the ground plane. The remaining space frame structure is composed mainly of elements forming truss structures that provide stiffness and strength. The floor of the driver's compartment will be of honeycomb sandwich plank.

DRAWINGS:

· Isometric View:

This drawing shows the car's frame (pink, blue, and burgundy), the driver (light blue), the battery box behind the driver (purple), and the ballast carriers at the driver’s sides (purple). The battery box attaches at eight points as described in the subsection Projectile Protection, a part of the next major section. The ballast carrier attachments to three lower truss members are also described in this section.

 

 

 

 

 

· Side View:

This view shows the double arch that defines the car’s shape.

SPECIFIC MATERIALS

The frame is composed of 6061 T6 aluminum tubing. All trusses were TIG welded at Taylor. None were fabricated. The 6061 T6 Aluminum tubing being used usually cures to a T5 or T4 temper. This relative weakness at the welds is taken into account in the safety factors. Several types of round and square 6061 T6 tubing is used depending upon the purpose and strength requirements. The strongest tubes surround the driver in a roll cage. The variety of tubing is displayed in the chart below.

CRASHWORTHINESS

The car has several distinctive features that influence how it would behave in a crash. A double arch is created along the length of the car by four square aluminum tubes. During a front or rear impact, these arches would tend to compress and bow. Several cross pieces between the arches resist the flexing action when the arches are placed into tension. An additional feature of the car is the existence of a bumper tube which surrounds the outside of the car. This bumper tube is not designed to withstand the full force of a crash, but to absorb the energy of the impact as it begins to deform. Such absorption will decrease stress loads on the main beams and thereby provide the driver with added protection. The driver's compartment has been designed to withstand a full crash load.

DRIVER'S COMPARTMENT

· Front View:

In this drawing the driver in light blue sits inside the driver’s compartment. The green tube represents the structural members. The blue tube represents the roll cage. The red nodes in this picture are representing the harness attachment points.

· Side View:

PROJECTILE PROTECTION

Battery Box: The battery box has eight nodes at which the box is connected to the chassis. The nodes are distributed around the eight corners of the box. The bottom of the battery box is attached to the chassis' main bottom beams. The top of the battery box is also immobilized. The top front is attached to the back of the role bar structure. The top back of the battery box is attached to the chassis via members running from a main brace to the top back corners of the battery box. The members between the battery box and the driver have been designed to isolate the box from the driver's compartment in a crash.

Ballast Carriers: The ballast carriers are mounted on the sides of the chassis just outboard of the driver's compartment near the roll cage. They will be attached to the bottom truss members. Using two ballast containers brings the maximum weight for each carrier to 35 pounds. The ballast carriers with their maximum loads would only exert a 175 pound load on the three truss members in a 5 G impact. This is well within the loading limits of the members. Therefore, the ballast carriers will not become projectiles.

Body: The body panels and solar array will be attached firmly to the car, however, they will not be designed to take a 5 G impact. A light, thin plastic will cover the bottom of the solar car, so it will not affect the driver in the event of a crash. The driver's compartment will be isolated from the bottom of the car by the sandwiched honeycomb plank. The upper body panels (consisting of the canopy panel and the leading and trailing edges) and the solar array, however, do have a greater potential of harming the driver. The first remedy of this situation is the full roll cage. The main roll bar protects the driver from projectiles in a forward crash, and the forward roll bars protect the driver from rear impact projectiles. In side impacts the driver is also protected by the roll bars. The protection of the roll bar is, of course, entirely dependent on the size of the projectile being larger than the roll cage openings. The body panels on the top level will wider than the openings, so they are not a concern. The solar array will be composed of 10 x 10 cm cells mounted to 10 cm wide aluminum strips mounted from left to right along the car. In the event of a front or rear impact, the strips will not be able to enter the roll cage. The direction of the strips will be oriented along the length of the car directly to the left and right of the canopy for side impact protection. The roll bars will also give adequate protection in roll over and an impact at an angle between front and side.

CRASH SCENARIOS

To analyze the space frame structure of Gideon's Torch, a finite element analysis program named Cadre 1.02 was used. We analyzed our frame in the different crash scenarios required. Our tests included static tests of front, rear, and side impact loads of 5 G, as well as rollover impacts of 3 G at several points on the structure. From the output data of our analysis we have learned that our space frame design will withstand the prescribed impact scenarios and give our driver the safety and protection necessary. Our data shows a very minimum amount of deflection or buckling in the structural members directly surrounding the driver as well as small amounts of buckling in other parts of the structure absorbing the impacts.

In all scenarios, excluding rollover, we are assuming the impact to be a bumper with an elevation of 35 cm and a height of 10 cm (as given in the Structural Report Guidelines). This assumption gives us the impact directly on the "bumper" of our car. We are assuming a 5 G impact for our car to be 3825 lb. and a 3 G impact to be 2295 lb. Our definition of the driver compartment is the structural members surrounding the driver that give the driver safe environment.

FRONT IMPACT:

Direct Frontal Impact: We analyzed the front-end of our frame with three crash scenarios in mind. The first scenario includes a direct frontal impact of 5 G. To simulate this using finite element analysis we first bound the rear end of the car so that it would not move. Next, we put forces on certain crucial attachment points in the frame (battery box, motor, driver's harness, etc.). These forces simulate the conservation of momentum. We then applied the 5 G load to the front of the car to get an accurate output for a front-end collision. The space frame of our vehicle was able to absorb the direct frontal impact with very little deformation in the overall structure of the frame and practically no deformation in any of the structural members composing the driver's box.

Off-Center Impacts: The second and third front-end crash scenarios applied 5 G impacts to our frame from two different angles off-center on the frame. These scenarios simulate non-centered collisions, which tend to do more damage to the frame structure. We treated the scenarios much like the direct, centered impact with our data input. This gave us an accurate analysis to gather data from. The structure of our car held-up well in these impacts. We did see deformation in some structural members, but nothing that would in any way injure the driver. Once again, the driver's box remained fairly intact.

SIDE IMPACT:

We conducted analysis of a 5 G impact at three different locations on our frame. The outputs of the simulations showed us that the frame structure would absorb the impact and deform, but not beyond an unacceptable limit that might endanger either the driver or someone in the vicinity of the impact.

REAR IMPACT:

Our rear impact simulation was very much like the direct frontal impact simulation discussed above. A 5 G impact load was applied to the rear end of the frame structure while several nodes on the front end of the structure were bound into place. The rear end of the structure employed a lighter construction because more space exists between the driver and the rear of the car than the front of the car. As a result of this larger crumple zone in the rear of the car, the design could be much lighter than the front without endangering the driver.

ROLLOVER:

For any application of our solar car a rollover accident would not include as much force as a solid impact. For this reason it was only necessary to apply 3 G impact loads in the rollover analysis. We conducted analyses for several different positions on the car frame to be sure to cover several possible rollover scenarios. In these simulations the impact forces applied on the body were directed toward the center of mass of the frame. Our frame was easily able to handle the different rollover scenarios.

CONCLUSIONS

None of the impact scenario caused our frame to distort or break to a point which would be considered unsafe. The Taylor University Solar Racing Team feels confident in the structure of Gideon’s Torch to protect the driver in case of impact. Testing will be continued, improved, and updated throughout the spring. The team intends to verify the Cadre 1.02 software results with MSC/NASTRAN 2.1 software.

INTRODUCTION

A solar car suspension/wheel systems accomplishes three purposes: it lifts the frame off the ground, it allows movement, and it provides shock absorption while on the harsh road. In addition to these purposes, the Taylor University Solar Racing Team has sought to design a reliable and simple suspension system. Included within this paper is an overview of both the front and rear suspensions.

FRONT SUSPENSION

Instead of using traditional products like dampers and springs to reduce vibrations, two multilayered beams will be used. While unique, this method has been successfully tried on other racing vehicles. We have modified a design created by Jeff Daily, who has used this suspension for electric vehicle racing in Michigan. Each of the beams will contain layers of quarter-inch aircraft plywood intermixed with a few layers of carbon fiber. The aircraft plywood provides the flexibility and the carbon fiber provide strength reinforcement. These wooden beams will be directly connected to the two lower square aluminum tubes which form the underside and backbone of the car. One beam lies under the aluminum tubes, and the other lies on top. Their connections to the beam are immovable. The wheels will be attached on either end of the beams through the use of an aluminum block.

WHEELS

The team is using Mag Moped wheels produced by Thomos. These wheels have a built in hub, disk brakes, axle, ball bearings, and mounting plate. As a result, they can be attacked to any structural member.

STEERING

The steering mechanism will be a basic steering wheel design, with the steering wheel rod running into a rack and pinion unit. This rack and pinion unit will be placed conveniently under the car and in front of the front suspension, providing a direct connection to the steering rod with little play. The steering rod is connected to the two blocks on either end of the suspension beams. The blocks are able to pivot around their connection point on the wooden beams. When the blocks turn, so do the wheels.

REAR SUSPENSION

The rear suspension is a simple swing arm design with the motor, belt, gear, and wheels all as a single unit. A rectangular box surrounded by strong structural trusses has been designed in the aluminum space frame to provide a secure location for the rear array. In this two-wheel design, the tires are separated by the minimum 15 cm. A TIG welded frame made up of aluminum trusses will provide the support for the wheels, gear, and motor. Because of the needed strength of the swing arm, the aluminum will be 1.25" wide square tubing with a wall thickness of 0.083". Such strong tubing is the same used to form the main supporting arches of the solar car. The general shape of a two-dimensional rectangular swing arm is pivoted at the end which allows for movement up and down but not sideways. Two adjustable Monroe air shocks will be connected from a shock mount on each of the rear corners of the swing arm to secure positions on the car frame.

MOTOR/GEAR ASSEMBLY

The motor will be an Animatics Smart Motor, which will be firmly bolted to the swing arm frame. Using a belt, the motor will turn a gear specially toothed to fit the belt. The gear will turn the axle, which will rotate the wheels.

TESTING

No testing has been completed on these designs. First, mockups of the front and rear designs will be constructed to visually determine feasibility of designs. Second, aircraft ply and carbon fiber will be purchased to create beams for testing. The beams will be tested for rigidity, flexibility, durability, and weight tolerances. Third, simulations for individual parts as well as combined systems will be conducted on a computer.

The simulations will test the front and rear suspension under 6G blows (estimated worst case scenario) to the wheels at various directions, with a G being equal to the full weight of the solar car. The weight is estimated below.

CONCLUSION

Design work is proceeding well. Some materials and items have already been purchased, and other materials will be purchased shortly. Construction on the body has begun. With continued vigilance and hard work, Gideon’s Torch will become a reality.