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Baja SAE Collegiate Design Series Report—Brigham Young University-Idaho Chris Valentine Brigham Young University-Idaho, Student of Business Management Copyright © 2009 SAE International
ABSTRACT
Major Design Goals for 2009
A team of mechanical engineering and business students from Brigham Young University-Idaho designed and manufactured a prototype of an off-road recreation vehicle. Design efforts focused on reducing weight from last year, improving aesthetic appeal, beginning with the end in mind, and to complete computer design prior to any manufacturing. The prototype meets SAE competition criteria and provides the future manufacturer with a proven product that is worth selling. All systems outlined in the report explain the purpose for chosen designs, specific design objectives, and also improvements over last year’s vehicle. Concept design and build time totaled less than four months.
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Reduce weight from last year Improve aesthetic appeal Begin with the end in mind 100% design prior to manufacturing (fig. 1)
INTRODUCTION Starting December of 2008, four Brigham Young University-Idaho students from the mechanical engineering department began designing a solid model of an off-road vehicle to compete in the Baja SAE Oregon Collegiate Design Series. Collaborative efforts combined resources, student talent, and experience from across university departments to build BYU-I’s best baja vehicle ever yet made. Prototype building began in late January and will be completed in April. The following article outlines the engineering and design process that was used in developing each system of the vehicle. It also addresses other special concerns relating to other aspects of the design process such as: improvements over last year’s prototype, manufacturing processes, and team limitations and strengths. Note: in all “Design Objectives” sections, however not mentioned, SAE specifications were first considered before all listed items.
Figure 1. Final vehicle design.
CHASIS & FRAME Design Objectives • • • •
Protect the passenger Structurally sound Minimize weight Low center of gravity
The overall purpose of the frame is to support and protect the driver and vehicle components in case of a collision or rollover. The goal was to build a relatively
lightweight, structurally sound, and aesthetically pleasing frame that maintains an estimated center of gravity. By design, heavy components are mounted as low to the ground making the vehicle less prone to roll. The driver and component placement maintains a 60/40 weight ratio between the rear tires and the front, respectively; proving the vehicle more stable when jumping, and allows for sufficient front weight to steer. Other aspects considered during the design process were the interdependence of the suspension and steering systems, operator legroom, manufacturability, craftsmanship limitations, and serviceability. The frame was designed with independent wheel suspension in mind. The orientation of the engine output shaft needed to be properly aligned with the rest of the drivetrain. Also, front shock mount placement was designed to best absorb frontal impact by allowing the shock to rest on a slight angle. Special attention was given to mount the steering wheel as high as possible without being in the way of the driver’s view and being low enough to accommodate for safety equipment. Vehicle legroom was increased by 8’’ while shortening the overall length of last year’s cab by about 12’’. This was done by mounting the pedals as far forward as possible. FRAME SHAPE – When designing the frame the primary goal was to keep the driver safe in the event of an accident. The shape of the frame maintains a safe distance between the driver and the environment in whatever position the car lies after an accident. The frame was also designed to be aesthetically pleasing. In years past the frame looked like a separate piece from the rest of the vehicle. This year the frame was built with complimentary angles and maintained a golden proportion to be aesthetically pleasing (fig. 2). The golden proportion is a mathematical and artistic ratio that is pleasing to the eye.
skid plate, side panels, fire wall, and all other panels were made of .03” aluminum plate. MIG welding was chosen to manufacture this year’s vehicle due to the material choice; and because it is faster and easier than TIG welding and provides sufficient strength. All bends were done using a hydraulic mandrel bender, to maintain circular cross section and structural integrity of the tubing.
SUSPENSION Design Objectives • • •
Maximize maneuverability Clearance over obstacles Maintain geometric alignment in front tires
The main objective of the suspension system is to maintain control of the steering wheels while turning or traversing rough terrain. In last year’s model the front tires did not maintain geometric alignment when the suspension was fully compressed. In fact, the tires slightly pigeon-toed with suspension flex. This year design improvements in the front end corrected the problem. This year’s vehicle will have independent suspension opposed to a solid rear axle. Having each tire independently suspended allows for greater overall clearance and better traction over extremely rough terrain. A-ARM DESIGN – Both the front and rear suspension systems are designed with unequal length A-arms. There was special design attention given to the front Aarms to ensure the tires remained geometrically true as the suspension was either fully extended or compressed. This was done by offsetting the top A-arm from the bottom to create a calculated radius on which the A-arms pivot maintaining proper alignment when traveling up or down over terrain (fig 3).
Figure 2: Computer model of frame showing the golden proportion. MATERIAL CHOICE AND FABRICATION–The tubing chosen was 1” O.D., .12” wall thickness 1018 steel. This steel was chosen because of the low cost and ease of welding and bending compared to chrome moly. The
Figure 3. The circle shows the offset top A-arm from the bottom A-arm, bottom perspective
The rear A-arms are not offset because they have no measurable effect on vehicle steering. However, the bottom A-arms are wider than the top ones to absorb more stress from frontal impact. The width is limited by the plastic cover for the CVT (fig 4). Also, the front shocks sit 55° pitched into the front end to complement the swing arc of the A-arms, and have been tilted back 20° to transfer frontal impact energy into the frame. The rear shocks are mounted parallel to the A-arms on a 65° angle (from horizontal), oriented to face into the frame, so as to best absorb downward pressure from the arc of the A-arm swing.
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Optimize balance of low-end torque and top-end speed Proper weight distribution
The overall objective of the drivetrain is to transmit power from the engine to the wheels. One of the design team’s objectives was to ensure that each element of the drivetrain was easily accessible to work on if need be. Last year’s vehicle did not have reverse, and it was decided that reverse was a necessity. The balance of power distribution to allow for low-end torque and highend speed was also very important because of the limited 10 hp output from the Briggs & Stratton engine. All of the drivetrain components were positioned as low and forward as possible to ensure proper weight distribution. Several years ago the baja vehicle had a rear differential, however this year’s vehicle was designed to have locked rear axles to minimize weight and maximize rear tire traction. The entire drivetrain includes: the engine, a Comet 790 CVT, a SNOWNABSTEDT power transmission planetary gear, a sprocket reduction system, and independent rear CV drive shafts (fig. 5).
Figure 4. Circle shows the limitation to extend the bottom A-arm because of the plastic CVT cover. A-arm construction – Both of the front A-arms were taken from previous years’ vehicles and originally came from a Polaris Predator. However, the rear A-arms were designed and manufactured on site to ensure that they matched the measurement of the front wheel track and were able to support the greater proportion of weight in comparison to the front of the vehicle. Shocks – Both the front and rear suspension use nitrogen charged Fox© FLOAT AirShox. This particular shock has an adjustable interior pressure that allows 1 changes in rider weight and desired stiffness . CLEARANCE – The vehicle has an overall clearance of 12’’. Front and rear end clearance measures 14’’ and 12’’ respectively. Last year’s vehicle had only about 3’’ of rear clearance due to a protruding swing arm for the solid axle. This year, the limiting factor for ground clearance was the angle at which the CV joints on the rear independent axles could bend.
DRIVETRAIN Design Objectives • •
Easily serviceable Forward and reverse
Figure 5. The entire drivetrain less the CV jointed drive shafts ENGINE – All engine specifications are set and cannot be changed; and engine speed is limited to 3800 rpm. With the limited power, minimizing weight is an important factor in vehicle performance. The vehicle’s frame design allows for easy access to the engine and also mounts it as forward as possible. CVT – Part of the transmission is a Comet brand Continuously Variable Transmission (CVT). The CVT was chosen due to reliability, ease of use, and maintenance. The transmission gear ratios range from 3.76:1 to 1.04:1. PLANETARY GEAR – This year’s vehicle was specifically designed with forward and reverse drives. Although the usage of such a gear adds 20 lbs to the
overall weight, it was a calculated sacrifice. This part of the transmission also acts as a 2.47:1 gear reduction. If the vehicle did not include the planetary gear, further gear reduction would have been necessary within the custom sprocket reduction system.
Because the overall weight of this year’s vehicle will be less, the momentum needed to stop will be less as well. Also, with ability to switch the transmission into neutral the brakes will not have to counteract the momentum of the engine when competing in the brake test.
SPROCKET REDUCTION SYSTEM – The goal of this system is to lower the final gear ratio. The sprocket reduction system was designed to use off-the-shelf components. With a size 50 chain and 4 interchangeable sprockets it reduces final ratios to 15.98:1 (including the reduction in the planetary gear). The side walls of gear reducer housing were made of .25 steel plate to properly support the shafts on which the reducing sprockets rest.
Another design feature of the braking system is dual master cylinders. One controls the front calipers and the other controls the rear. In case of failure of one master cylinder the other will have sufficient braking power to safely stop the vehicle.
DRIVE SHAFTS – Each drive shaft has two CV joints. Unfortunately, the suspension clearance was limited by the angle at which the CV joints could properly operate. However, for this same reason the vehicle was designed using CV joints because they afford better performance and ground clearance than a solid rear axle. Each shaft was shortened to match proper A-arm length and maintain co-linear wheel track.
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BRAKING Design Objectives • •
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Safely bring the vehicle to a stop Decrease stopping distance from last year Reduce unnecessary momentum
The design goal of the braking system is to stop the vehicle in the shortest distance possible. Disc brakes were used on each front tire. The rear braking power comes from a single disc and caliper located inside the housing of the sprocket reduction system. On last year’s model the rear brake provided insufficient braking power. It was located on the rear axle. The design flaw in last year’s rear brake was location and size. By housing the brake inside with the sprockets the brake will be shielded from splashing water and dirt which dramatically reduces friction that the brake needs to stop the vehicle (fig. 6). Also, the brake is larger than last year’s.
STEERING Design Objectives Minimize turning radius Maintain proper tire alignment
The steering system contains the steering wheel, steering column, rack and pinion, tie rods, knuckles, and ball joints. The steering wheel is mounted with quick release mechanism to decrease exit time from the vehicle in case of an accident. All steering parts aside from the column were purchased parts. The steering system on last year’s vehicle was loose and unresponsive. This year’s vehicle was designed with rack and pinion steering. One of the benefits of this type of steering is a lighter feel in the steering wheel due to a gear reduction. It also improves the feel of the road allowing the driver to rely more on feel than sight. The specific rack and pinion that is installed in the car has one and three quarter turns from lock to lock. STEERING GEOMETRY – The front suspension was designed to maintain excellent steering geometry at all suspension positions. Last year’s vehicle had a problem with the front tires pigeon-toeing as the tires flexed with the suspension. This year the placement of the rack and pinion was deigned to maintain proper geometrical alignment while steering. TIRE ALIGNMENT – The front alignment is adjusted by lengthening or shortening the tie rods.
MANUFACTURABILITY
Figure 6. Rear disc mounted left of the sprockets inside the sprocket reduction housing, caliper above.
Part of the purpose of this project is to determine the feasibility of manufacturing 4,000 of this year’s baja prototype. In order to accommodate future manufacturers, several factors were taken into account. The frame was designed using bent-tube construction. All welds were MIG welded to simplify the manufacturing process. Joints were fish-mouthed using a belt grinder with changeable dyes making grinding easier and providing a fast method to ensure a tight fit at the joints. And, all replaceable parts are easily purchased at any motorsports dealer.
IMPROVEMENTS FOR NEXT YEAR • • • •
Lighter weight Fewer mounting bars and brackets in the rear Larger diameter tubing with thinner walls or different material altogether Narrower and shorter wheelbase for improved maneuverability and less weight
CONCLUSION Design and construction of this year’s prototype was very successful. With only one month to design and less than four to build and test, this year has proven to be the most efficient production for BYU-Idaho students yet. Budget limitations and time restraints have caused the BYU-I team to be resourceful and innovative with regard to their design plans and prototype construction. It has been a good exercise to work within limited means to produce the best vehicle possible to compete in the Baja SAE
Oregon Collegiate Design Series. Design goals were met, resulting in a final product that will withstand the abuse of off-road conditions and be a formidable competitor in this year’s competition.
ACKNOWLEDGMENTS The Design Engineers: Dax Wells, Adrian Lords, James Cheney, Ryan Wilding
REFERENCES 1. FoxShox informational booklet attached to the product.
CONTACT Chris Valentine, email:
[email protected]
