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SAE Mini-Baja West Design Report Ben Johnson
Brigham Young University-Idaho Dept. of Mechanical Engineering and Technology Copyright © 2005 SAE International
ABSTRACT A team of Brigham Young University-Idaho mechanical engineering students designed and manufactured an offroad vehicle. Efforts were focused on designing cost effective yet efficient systems including chassis, suspension, drivetrain, and controls. Design criteria included: originality and innovation, suspension, brake system, power train, structural design, craftsmanship, operator comfort, feasibility for mass production, and serviceability. The final product meets these criteria and provides the customer with a functional product, and the producer with a product worth manufacturing.
INTRODUCTION A team of Mechanical Engineering students from Brigham Young University-Idaho designed and manufactured a prototype Mini-Baja off-road vehicle. This report documents how and why the following components were designed: chassis, suspension, drivetrain, and controls. It also discusses the manufacturability and ergonomics of the BYU-Idaho Mini-Baja vehicle. The design goals for the 2006 Mini-Baja included: • Meet customer specifications • Create innovative and functional design • Minimize weight • Maximize control with suspension design • Optimize power efficiency • Maintain mass production feasibility • Minimize cost • Decrease time to market
Other aspects considered were operator comfort, innovation, craftsmanship, manufacturability, and serviceability. Consumers of this vehicle can range in height and weight. It was envisioned that users would be between 5’6’’ and 6’2’’ so the cabin was designed to accommodate both extremes. The frame was designed to decrease time to market. This was accomplished by making the frame modular and by using only simple processes to manufacture each component. The cabin and front suspension were designed and built simultaneously but separately from the rear suspension and engine compartment. The two modules were joined with nine bolts. By using a modular system this vehicle can be easily customized. FRAME DESIGN When designing the frame the primary goal was to keep the driver safe in the event of an accident. The shape of the frame was designed to maintain a safe distance between the driver and the environment in whatever position the car ended up after an accident. The frame was also designed to be aesthetically pleasing. In years past the frame looked like a bubble, this year the frame was built with more straight lines to give it a more aggressive look. This was done because most consumers tend to look for products that perform well and look good. See figure 1.
When producing an off-road vehicle the largest concerns are cost, maintenance, safety, and driver comfort. Each of these aspects has been optimized to produce the best product.
CHASSIS The design goal for the chassis was to build a lightweight, structurally sound, and aesthetically pleasing frame. The purpose of the frame is to protect the driver in case of a collision or rollover, and to support the components of the drivetrain, suspension, and controls.
Figure 1: Computer model of frame using Unigraphics software.
To ensure the safety of the driver, correct engineering design was used ensuring that all frame members would be in only tension or compression and minimal bending loads would be applied to the tubing. Material Selection It was required to make a steel frame using a material with equal or superior bending strength and bending stiffness of 1018 steel. Appendix A shows the analysis of bending strength, wall thickness, and density of various steel materials considered for the frame. The tubing chosen was 1.25” O.D., .065” wall thickness 1018 steel. This steel was chosen because of the low cost compared to the Chrom-Moly tested, it is available on short notice, and it is easier to weld and bend. The skid plate was made of .077” aluminum plate. The firewall was made of .03” aluminum plate. Aluminum was used to decrease weight and meet design criteria. The roof and other covers were made of a single layer of glass/epoxy composite to decrease total weight of the car wherever possible. Since these covers only provide a finished look for the car, structural integrity was not necessary in these applications. The side panels were made of a double layer because of the higher risk of impact, which increases the need for structural integrity, but light weight is still extremely important.
This is a common design that many teams and companies use. The upper A-arm was designed and manufactured on site to ensure that the shocks would have proper clearance. The lower A-arms came from a Polaris Predator. These A-arms were used because they have been tested and proven for the past four years on the four-wheeler. The cost is also low enough to justify using OEM parts instead of designing and manufacturing a new part. The front suspension uses a coil-over Fox Shox© Podium™ shock. This particular shock has an adjustable pre-load which allows adjustment of ride height, and initial stiffness. It also has adjustable rebound and damping to tune the suspension to all conditions. The frame was also designed with a fifteen degree incline where the front A-arms attach to improve the angle of attack and to improve suspension geometry. See figure 2 for more detail.
Fabrication Welding MIG welding was the welding process chosen for manufacture of this off-road vehicle for the following reasons: faster and easier than TIG welding, provides sufficient strength, and appearance. For the past two years TIG welding was used because the frame was made of Chrom-Moly and a specific wire or filler rod must be used when welding this steel. MIG welds can be as strong as the parent metal, while TIG welds depend on the parent metal and the filler rod. Since 1018 steel tubing was used, the MIG welding was the more viable option. Bending All the bends were done using a hydraulic mandrel bender, to maintain circular cross section and structural integrity of the tubing.
SUSPENSION The objective of the suspension system on any vehicle is to maintain control of the steering wheels while turning, braking, or traversing rough terrain, and also to increase traction of the drive wheels.
Figure 2: Side view showing the 15 degree incline of frame front end using Unigraphics software.
Travel The front suspension was designed to have 11.5” of clearance and ten inches of travel. Last year the car had eight inches of clearance, and five inches of travel, which is comparable to most four-wheelers on the market. Eleven and a half inches of clearance is an improvement, and test drives confirm the increase is needed. Ten inches of travel is also a great improvement. Alignment The front alignment is adjustable to allow changes in tire size and ride height. Caster and camber adjustments are made by adjusting the ball joint in the upper A-arm. REAR SUSPENSION
FRONT SUSPENSION
Design
Design
The rear suspension is less common than the front suspension. The original design on paper for the rear suspension was an independent swing arm.
The front suspension is an unequal length A-arm design.
Implementing that design proved to be too difficult, so the final design is a modified multi-link swing arm system with two radius rods. The rear suspension uses Fox Shox© 2.0 x 5/8 emulsion non-coil shocks, which use compressed nitrogen. The uncompressed length is 31.1 inches with 12 inches of travel. For the last two years the rear shocks used were custom air shocks. The major problem with these is the slow rebound speed, and they are starting to leak air pressure. Travel The rear suspension was designed with 14” of clearance and 12” of travel. Last year the car had ten inches of clearance and eight inches of travel. Last year’s design included a composite leaf spring, which unfortunately failed. That failure directed this year’s design process which produced a suspension far superior to the previous designs. More consideration was given to the amount of suspension travel this year to increase driver comfort, and control.
Torque and HP vs RPM 14 13 12 11 10 9 8 7 6 5 4 3 2 1300 1700 2100 2500 2900 3300
Pow er (Hp) Torque (ft-lb) Corrected Pow er (Hp) Corrected Torque (ft-lb)
RPM
Figure 3. Dynamometer results. Corrected Hp and Torque were found using a correcting factor for the altitude of 4200 ft. Data obtained using Dyno-Max 2000 software.
Alignment The rear camber and toe-in is set by adjusting the Heim Joints in the radius rods. The alignment was set according to specs from a local ATV dealer.
DRIVETRAIN The objective of the drivetrain is to transmit power from the engine to the wheels. The design team’s objective was to produce a drivetrain that was simple to operate and efficient since the rated power output from the engine is only ten horsepower. The drivetrain includes a ten horsepower Briggs and Stratton engine, Polaris CVT, Yamaha forward and reverse drive, custom chain drive gear reducer, GKN Visco Lok differential, and GKN cv axles. ENGINE Engine speed is limited to 3800 rpm. With the limited power, minimizing weight is an important factor in vehicle performance. Tests were performed using a dynamometer to measure actual performance. Figure 3 shows the results of the dynamometer testing. It was found that the optimal rpm settings were at 2900 for a maximum torque of 14.15 ft-lb at sea level, and at 31003400 to obtain a maximum power of 7.7 horsepower at sea level.
TRANSMISSION CVT Part of the transmission is a Polaris brand Constant Velocity Transmission (CVT). The Polaris CVT was chosen due to reliability, ease of use, and maintenance. This type of transmission uses two clutches, a primary and a secondary, which transfer power through a belt. The transmission gear ratios range from 6.3 to .7; this allows for slow driving needed in the rock crawl, and high speed driving in the endurance test. Forward/Reverse The forward/reverse drive comes from a Yamaha Timberwolf 250. This one was chosen over a Polaris brand forward/reverse, which was used two years ago, because of the lighter weight. Past experience shows that a reverse gear is desired by users. Test drives confirm their desire, as reverse is used regularly. Last year one of the problems encountered in the drivetrain was an unintentional shift into neutral. The custom shifter for this year was designed to ensure that changing from forward to neutral or reverse was intentional, and accidental shifting could not occur. Chain Drive Gear Reducer The objective of a gear reducer is to lower the final gear ratio. The chain drive gear reducer is custom made and uses Ramsey Silent Chain©. Silent chain was chosen for the following reasons: high efficiency in power transmission, quiet in operation, and ease of maintenance. The case is a prototype and only one was produced, it was made of machined aluminum sheet and billet and welded together. When the car is mass produced it will be cast which will reduce the manufacturing cost.
DIFFERENTIAL The differential’s objective is to provide better traction while driving straight, and allowing one tire to turn faster than the other while turning. The benefit of a locking differential manifests itself in low traction situations where both tires need to turn for maximum traction. The differential used is a GKN Visco Lok. This system provides the best of both situations described above. The differential automatically adjusts to the conditions using hydraulic pressure to lock both axles together when one tire slips excessively, but allowing one to spin freely of the other when turning. AXLE Due to the width of the rear suspension, it was necessary to modify the rear axles that had been ordered by extending them four inches. For mass production applications, it is recommended to purchase longer axles to remove this process. The axles were cut in half, center drilled, and the four inch tapered extension shaft was welded between the two pieces of the original axle. The axles have CV joints at each end. CV joints were used because they afford better performance at greater angles, which allows for more suspension travel.
CONTROLS The objective of the controls is varied. The controls includes the following systems: brakes, throttle, steering, and transmission shifter. BRAKES Disc brakes were selected for all four wheels due to weight and efficiency. The master cylinder was taken from a Geo Metro and a proportioning valve introduced into the system to optimize braking distribution between the front and back. It has two independent hydraulic systems, as required for safety purposes. This provides that at least two wheels maintain effective braking power in event of a leak or failure. The front brake rotors and pads are OEM parts from a Polaris Predator. The rotors are vented to reduce fade which occurs when brake rotors are too hot. The rear brake rotors and pads are inboard to protect from them from impact. They also are OEM parts, but come from a Bombardier four-wheeler. STEERING 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. The steering column was originally designed with only one u-joint. After test driving the vehicle the steering was too stiff at certain points so a second u-joint was added to decrease the angle in each u-joint. The Mini-Baja vehicle has rack and pinion steering. One of the benefits of this type of
Figure 4. Steering column geometry after inserting second u-joint drawn in AutoCAD.
steering is a lighter feel in the steering wheel due to a gear reduction which is inherent in the design. It also improves the feel of the road, and is more accurate and precise allowing the driver to rely more on feel than needing to see each bump in the road. It is also an inherently simpler design with fewer parts and pieces which leads to fewer repairs. The specific rack and pinion that is installed in the car has one and three quarter turns from lock to lock. The front suspension was designed to maintain excellent steering geometry at all suspension positions. This improves the handling of an off-road vehicle since the suspension is seldom at the normal ride height position. Bump steer is kept to a minimum because of this steering geometry also. The knuckles are OEM parts from a Polaris Predator. The correct geometry, mounting methods and positions, and cost were the reasons why custom knuckles were not designed and manufactured. All the ball joints on the vehicle are commercially manufactured to ensure safety.
MANUFACTURABILITY Part of the purpose of this project is to determine the feasibility of manufacturing 4,000 Mini-Baja vehicles per year. In order to accommodate manufacturability, several factors were taken into account. The frame was designed using bent-tube construction. All welds were MIG welded to simplify the manufacturing of this vehicle. Joints were fish-mouthed using a belt grinder with changeable dyes. This made grinding easier, providing a fast method to ensure a tight fit at the joints. The modular design decreases time to market. This method is proven in other markets, and if applied to the manufacture of this vehicle profits will increase.
ERGONOMICS In designing the Mini-Baja vehicle, ergonomic principles of aesthetics and driver comfort were considered. The vehicle is easily operable, due to the automatic transmission. Suspension was tuned to provide greater ride comfort.
Ease of service and maintenance were also considered. Durable parts were selected in all subsystems to minimize repair needs. In these ways, the vehicle was tailored to consumer preferences.
CONCLUSION Design and construction of the BYU-Idaho Mini-Baja vehicle resulted in the production of a reliable off-road vehicle which meets customer specifications. Design goals were met, resulting in a final product that will withstand the rigors of off-road travel while providing the driver with the necessary comforts. The vehicle is appealing to the customer in design, driver comfort and safety, and maintainability. The vehicle is appealing to the producer in manufacturability and reliability.
CONTACT Ben Johnson, email:
[email protected]
