ET 493 | Senior DesignFinal Report

Alfred ShowersPrepare For: Dr. Cris Koutsougeras

Table of Contents

1. Objective..............................................................Page 3

2. Methodology

A. Steering System Decision..................................................Pages 3-4

B. Rough Schematic of The Frame........................................Pages 4-5

C. Front Suspension Design...................................................Pages 5-7

D. Steering Rods Design........................................................Pages 7-8

E. Dynamics of The Design.................................................Pages 8-11

3. Cost Analysis............................................................................ Page 11

4. Deliverables........................................................................Pages 11-12

A. Fall Semester

B. Spring Semester

C. Areas of Improvement

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1. Objective

The objective for this project is to design and build a steering system for a mini Baja car. The purpose of the steering system is to provide directional control of the vehicle which is the turning of the front wheels. Along with controlling the vehicle, the steering system has to provide good ergonomics and easily operated.

After researching some requirement in determined a steering system must offer sufficient precision for the driver to actually sense what is happening at the front tires contact patch as well as enough feel to sense the approach to cornering limit of the front tires. It must be structurally stiff to avoid components deflections. The steering must be fast enough so that the vehicle’s response to steering and to steering correction to happen almost instantaneous and it must also have some self returning action. The feel, feedback and self returning action are function of the kingpin inclination, scrub radius, castor angle and self aligning torque characteristics of the front tire.

2. Methodology

A. Steering System DecisionI researched and compared multiple steering systems. I desire a steering system that would provide easy operation, would be low in maintenance, provide excellent feedback, and be cost efficient. The table below is the decision matrix used to compare the different steering systems, in which 5 being high and 1 being low.

STEERING SYSTEM DECISION MATRIXHYDRALLIC SYSTEM

ROTARY CABLE

RACK AND PINION

FLAG AND SHAFT

RECIRCULATING BALL

LOCK AND LOAD RATIO

5 4 3 1 4

FEEDBACK 5 2 5 3 4

RANGES IN MANTENANCE

5 3 4 3 3

OPERATION DIFFICULTY

5 3 1 1 1

COST 1 2 5 5 5

Based off my decision matrix, I chose to use the rack and pinion system. The rack and pinion system chosen is a 14 inch rack and pinion. It has a 1.5:1 gear ratio which provides a good compromise between control and ease of use. If the ratio is high, the driver would have to turn the wheel several rotations to reach full lock. In the tight space of the vehicle, this is undesirable. If the ratio is too low, a slight movement will

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cause the wheel to turn. This is undesirable because a bump in the trail could cause loss of control. The rack and pinion will be connected to the knuckles using the rod ends. The 14 inch is measured from centre to centre of the adjustable rod ends. The 12:1 lock to lock ratio of the rack and pinion equals 1.5 turns of the wheel is 4.25 inches of rack travel.

Figure 1: Specifications of the Rack and Pinion

In a rack and pinion system, the track rod is replaced with the steering rack which is a long, toothed bar with the tie rods attached to each end. On the end of the steering shaft there is a simple pinion gear that meshes with the rack. When you turn the steering wheel, the pinion gear turns, and move s the rack from left to right. Changing the size of the pinion gear alters the steering ratio. It really is that simple.

B. Rough Schematic of FrameBefore I can begin with the designing of the steering system, I assisted the frame designer in designing the frame. This is because without the wheel base in the horizontal and vertical directions, I wasn’t able to design the steering mechanism.

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Figure 2: Rough Schematic of the frameFrom the frame, we were able to determine that wheel base in the horizontal direction (From rear wheel the front wheel) is 79 inches. Now I need to know the wheel base in the vertical direction of the front, which from the distance from the left wheel to the right. To find this I would have to design the length of the A-arms in the suspension system.

Figure 3: Horizontal Wheel design in AutoCAD

C. Front Suspension DesignThe overall purpose of a suspension system is to absorb impacts from course irregularities, such as bumps, and distribute that force with the least amount of discomfort to the driver; while providing the best handling. I completed this objective by doing extensive research on the front suspension arm’s geometry to help reduce as much body roll as possible. To insure those qualities, the proper camber angle will have to be applied.

Camber angle alters the handling qualities of a particular suspension design; in particular negative camber improves grip when cornering. This is because it places the tires at a better angle to the road or course, transmitting the forces through the vertical plane of the tire rather than through the shear force across it.

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Figure 4: Suspension design in AutoCAD

As you can see in figure 3 above the tires have a -1.24 degree camber at static ride height to help with steering and cornering. This is done by making the upper arm which is 19 inches shorter than the lower arm which is 16.5 inches and bringing their mounting locations closer together. To inquire the mounting location I had to measure the wheel assembly where the suspension will connect at the top, which was 5 inches from the centre of the rim and the bottom which was 3 inches from the centre of the rim. I had to know the front dimensions of the frame which was the other mounting points. I obtained this from the rough schematic of the frame design, which was 14 inches across the top 12 inches across the bottom, and 11 inches in height putting it at a 5.2 degree slope. So as you can see, from that design, I was able to determine the front wheel base to be 55 inches wide.

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Figure 5: Suspension design with shock resistance in AutoCAD

I also designed our camber angle to decrease to -.05 degrees when the shock absorber is compressed during turns (as you see in figure 4 above.) This allows our tires to stay in better contact with the ground while cornering which greatly improves the steering response.

D. Steering rods designNow by knowing the front wheel base I was able to determine how long my steering rods have to be to satisfy my design. Now I had to measure the length of the assembly which was 5 inches and how far it extends from the wheel which was 1.5 inches. From that I was able to determine its angle, which is 17 degrees.

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Figure 6: Steering Rod Design in AutoCAD

As you can see in the figure above, I placed the 14 inch rack and pinion 5 inches from the center of the wheel and also parallel to it. This required for my steering rods to be 15.5 inches to connect from the assembly to the rack and pinion.

E. Dynamics of design See the figure for an example. The conering radius of an outside wheel R1 is more than radius Ro. Therefore it is necessary for outside wheel to transit greater distance than on what it about turns. The cornering radius of the inside wheel R2 is less, than radius Ro. It is necessary for this wheel to transit smaller distance. I mean that if the inside wheel will transit a path smaller, than on which it will be scrolled, then wheel slips. If this doesn’t happen, therefore this will be an unsafe design.

If this problem arises, I will have make adjustments components to satisfy this theory.

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Figure 7: Scheme for Kart Turning

To apply this to my design, I did some research and determined that the 12:1 lock and lock ratio travelling 4.25 inches will give me a 30 degree turning angle. This is determined by simply dividing 360 degrees, indicating a full circle, by the ratio.

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Figure 8: Turning angle design of right tire in AutoCAD

Now knowing the turning angle of the inside turns, I was able to determine the turning Angle of the left tire using this formula:

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Figure 9: Turning angle design of left tire in AutoCAD

Now to find Ro that was spoken of in the scheme kart turning segment of the paragraph is used this formula:

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Ro

Ro

Ro

55

Figure 10: Ro design in AutoCAD

Finally as you can see above in figure 10 the radius from the left tire and the radius from the right tire connect to the point Ro and forms the same turning angles of the wheels. This verifies that the design of the steering system will not cause the tire to slip during concerning through a course.

3. Cost

A. Rack and pinion - $98.00

B. Rods- $40.00 each

C. Rod Ends – $14.00 each

Total: 206.00

4. Deliverables

A. Fall Semester

Oct. 4th- 18th - designing the frame of the mini Baja. By this time we should have a schematic drawing of the dimensions for the frame and the front wheels base

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Oct.18th-25th – research on the different types of steering systems. I will compile a decision matrix of the two weighing: Cost, operation difficulty ranges in maintenance, feedback, lock and load ratio.etc. Then I will choose the type of steering system best suitable.

Oct 25th –Nov 15th - designing of the components such as the: Ball Joints, male and female rod ends, spindles, steering shaft and hub assemblies, steering wheels, and the tire rods. With this I would create a schematic drawing of the complete steering system, to suspension system, and the wheel base.

o 2 conceptso Weigh the pros and cons of the two conceptso Then Choose from the two concepts

Nov 15th- Nov 29th – Calculations using my knowledge of strength of materials and dynamics. I will make adjustments as needed to insure that this will be a safe design. After all the needed calculations, I would then get with Golman to create a full schematic of the Baja.

B. Spring Semester

- TBA - Fabrication Process

C. If you read my deliverables, I have them all checked off meaning that they all were completed this semester. It was told to me by my advisor, Dr. Lee that the dynamics part of the deliverables was incorrect. Though they were incorrect, all my deliverables for this semester were meet. Due to the fact that I cannot start on the fabrication stage for next semester because of the mistake in theory, I will work on my break to improve this design.

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