SAE Mini Baja Powertrain

Chris Gilson

Spencer Suggs

Southeastern Louisiana University

Mechanical Engineering Technology

Instructor: Cris Koutsougeras

Advisor: Ho-Hoon Lee

Table of Contents:1

Abstract…………………………………………………………………3

Introduction……………………………………………………………..4

Methods…………………………………………………………………5

Ideal Position for the drivetrain…………………………..…………...6-7

Drivetrain Frame…………………………………………………….8-15

Brake and Throttle Controls………………………………………..16-17

Engine Mount Stress Analysis…………..………………………….18-22

Mounting Bracket Material Selection………………………………….23

Bolt Grade Selection………………………………………………..24-25

Torque Transfer to the Wheels……………………………………..26-27

Accomplishments……………………………………………………...28

Conclusion……………………………………………………………..29

2

Abstract

The goal of this project was to design the placement and mounting of the engine,

transmission, and differential and to implement these items on a mini-baja car. This was

accomplished by utilizing the skills and knowledge we have gained while enrolled in the

Mechanical Engineering Technology Program. Upon completion of the second semester the

mini-baja car will be able to compete among other schools in time-trials and torture tests for

the international SAE competition. This has been accomplished by following the SAE guidelines

and regulations using the already provided Briggs & Stratton 305cc engine, a CVT transmission,

and an independent axle differential. In conjunction with the frame design and suspension

design teams, we completed a structural analysis and determined the materials required to

assemble it. In this final semester we have used the designs we have worked on thus far and

have fabricated and assembled the actual vehicle with the assistance of the participants who

have been working on other aspects of the design process.

3

Introduction

The SAE mini-baja is a college competition in which students from universities from around

the world design and then build small off road type vehicles. Standards which each team must

adhere to are in place to ensure fair competition. Annually there are numerous competitions

held across the US and as well as around the world. These events utilize numerous ways to

score a design. These include endurance, cost, acceleration, hill climb, and maneuverability.

Some of these schools have large budgets with which to work and spare no expense in the

design and fabrication process.

The power train plays an important role in the operation of a mini-baja car. Without it there

is no possible way to transmit the power produced by the engine to the wheels. The rotating

output shaft of the motor is where the torque originates and then travels through the CVT then

to the transaxle and finally to the wheels. In the CVT a there are no gears so the proper term to

use is speed ratio rather than gear ratio. In most CVT systems a belt and pulley are utilized to

achieve an infinite number of speed ratios based on the rotational speed of the input. This is

accomplished by varying the diameter of the pulley as the rpm of the input shaft changes. As

the rotation passes through each subsequent component a gear reduction occurs changing the

input speed to a slower output speed while having the same or greater output torque.

4

Methods

This design project used knowledge obtained in mechanical design, dynamics, statics,

strength of materials, physics, engineering graphics and project management. These principles

were applied to both the design and analysis of the mini-baja car drive train components

throughout the process to ensure the desired results.

5

Ideal Position for the drivetrain

After researching many previous mini-baja karts from other schools, we determined the

simplest and most efficient route to go would be to rigidly mount the power-train along the

frame rails behind the driver seat. Using an independent rear suspension system this would be

the simplest way. Another possible design was to use a swing-arm style system off a motorbike.

This would not work considering both wheels would not always rest at the same ride height due

to uneven terrain. This swing-arm style would cause the drive belt to twist and snap. Although

it would work with a chain driven system with one chain per wheel, it would be quite costly and

most likely cause us to go over our allowed budget. With simple CVT and a readily available

differential we can easily stay under our budget without any major loss of performance.

Fig. 1

6

After having a solid rear axle donated to us for our project we went with a swing-arm style

system. To do this we had to make a separate detachable frame from the main kart frame to

rigidly mount the engine and differential. The drivetrain had to be rigidly attached so the belt

length would not change causing belt slip. Mounting the transaxle in line with the engine is

what we decided would be best based on examining other builds and designs. This positioning

would help with easy installation and maintenance, be a simplistic design, and enable us to

rigidly mount both engine and differential.

Fig. 2

7

Drivetrain Frame

After much research on previous mini-baja designs, we initially decided to utilize an

independent rear suspension system but went with a solid axle that we obtained through a

donation. This solid axle system is best set up with a swing arm style suspension. It had to be

rigidly mounted to a separate frame.

Fig. 3

8

We designed the step rails to keep the shaft of the engine in line with the differential shaft

to keep the belt as parallel as possible. The tabs on the side are there to bolt to the axles. Also

we incorporated a panhard bar to give the frame two axis of rotation. The single joint in front

was to allow for sway and oscillation motions but due to cost and time constraints we opted for

a simpler two joint design.

Fig. 4

Fig. 5 Fig. 69

After obtaining the chromoly 4130 steel tubing we began fabricating the drivetrain

frame. Using a chop-saw we notched 45 degree cuts into the pipe to make a 90 degree turn.

Using magnets and straight edges we line them up and clamped the pipes to make sure they

did not move during welding. To make the 45 deegree turns on each side, we notched a 22.5

degree cut with the chop saw out of the pipe. When both ends had a 22.5 degree cut the total

angle equaled out to 45 degrees. This was designed to keep our engine shaft even with the

differential shaft.

Fig.7

10

Once the outer perimeter of the drivetrain frame was complete, we move onto to joints.

Using control arms meant for a heavy-duty truck that was made of the same material as the

pipe we were using we cut the ends off each side and welded them to the front of the

drivetrain frame. A pipe was placed through the center of the joints to keep them perfectly

inline during welding.

Fig.8

Fig.911

Fig. 10

12

Fig. 11

Once the joints were welded on we placed a brace bar as close to the engine placement

as we could. We cut a triangle out the end of the bar and grinded a halfcircle out of it to make

smooth contact between the bars. To help with the stress from the engine and to solidify our

frame. Then we added two more brace bars to run along side the engine for bracing and

solidification. The braces would also act as a mounting point for the engine mount later on.

Fig. 12

13

Fig.13

To make the axle mount tabs we drill two holes in .25” thick flat steel and shaved two

gussets per mount for added strength. To attach them to the drivetrain frame we bolted the

flat-plate to the axles and set the frame in place on top. After inserting the gussets, we then

welded everything in place.

Fig.14

14

Fig. 15

The final step in completing the powertrain frame was to attach it to the main kart

frame. For this we designed tabs in solidworks and had a local machine shop cut them out for

us. To attach them we bolted them to the drivetrain frame joints and pushed them against the

back of the main frame and welded them in palce.

15

Fig. 16

Fig. 17

Brake and Throttle Controls

Utilizing the integrated drum brakes of the differential along with brake cables we

obtained, the braking system was integrated into the frame to allow for pedal attachment and

proper operation. Pedals from our donor go kart were used along with fabricated mounting

system.

16

Fig. 18

The mounting system for the pedals utilized a piece of ½ inch all-thread inside of a ½

inch inside diameter pipe. This was mounted using nuts, lock washers and flat washers on each

side between two pieces of angle iron that were welded to the frame. Plastic bushings were

inserted into the pedals to allow for a smooth range of motion.

Fig. 19

The brake cables that attach to the differential had to run from the drums to the center

of the frame under the seat where they join at an adjustable shaft. The cables were secured by

welding a piece of angle iron under the seat while leaving clearance for the seat mounts. Two

holes with a diameter of 5/8 inch were drilled to rout the ends of the cable thru and they were

secured with C-clips. Abracket for the throttle cable was fabricated with a return spring and secured

with screws already attached to the motor in pre-tapped holes.

17

Engine Mount Stress Analysis

Fig. 20

Torque produced by the engine on the pulley in the counterclockwise direction is 19.7 Nm.

Rearranging the equation for torque T=f*d the force was found to be 107.065 N. A moment

equilibrium equation was then used about points A and B utilizing all information in the

diagram above which represents the motor, pulley and mounting plate with their respective

distances. These forces are present at the mounting bolts located in the plate.

The calculations of the resultant forces are:

ΣM A=−wa−Fb+2aRB=0

RB=FB+wa

2a=F B2a

+w2

RB=21.2

2∗.0975+275.6

2

18

RB=108.718+137.8

RB=246.52NΣM B=wa−Fb+2aR A=0

RA=FB−wa

2a=FB2a

−w2

RA=21.2

2∗.0975−275.6

2

RA=108.718−137.8RA=−29.1N

The shear and bending moments are utilized in the structural analysis process to assist in the

design of our motor mount. This ensures that we use the proper size and type of material which will

withstand the calculated forces exerted by the drivetrain components on the mount without failure.

Based upon the maximum shear and moment values we can select a material that has a yield stress that

is safely above the exerted forces.

Fig. 21

After finding the reaction forces at the bolts in the mounting plate we then applied those

forces to the entire length of the mount. A moment equilibrium equation about point A was

then calculated to find RBwhich is 228.127N. Subsequently an equilibrium equation in the Y

direction was used to plug in the known value of RB to obtain RA which is -10.727N.

+↑ΣFy=019

RA+RB+29.1−246.5=0

RA+RB +29.1−246.5=0RA+RB =217.4N

+↺ΣMA=0RB (0.225)+(29.1)(0.015)-(246.5)(0.21)=0RB (0.225)+(29.1)(0.015)-( 246.5)(0.21)=0

Fig. 22

RB=228.127NRA+RB=217.4RA +228.127=217.4RA =−10.727N

20

Using the resultant forces found, a shear diagram was calculated by using a force

equilibrium equation in the Y direction for each of the three sections in the motor mount plate.

The calculations for each section are:

0≤x≤0.015: +↑ΣFy=0−10.727−V1=0V=−10.727 N

0.015≤x≤0.21: +↑ΣFy=0−10.727+29.1−V=0V=18.373 N

0.21≤x≤0.225: +↑ΣFy=0−10.727+29.1−246.5−V=0V=−228.127 N

21

Fig. 23

Using the resultant forces found, a bending moment diagram was created utilizing a

moment equilibrium equation for each of the three sections.

The calculations are:

0≤x≤0.015: +↺ΣMb=0−10.727x−Mb=0Mb=−10.727x Nm

0.015≤x≤0.21: +↺ΣMb=0−10.727x+(29.1)(x−0.015)−Mb=0Mb=18.373x−0.436 Nm

0.21≤x≤0.225: +↺ΣMx=0−10.727x+(29.1)(x−0.015)+(−246.5)(x−0.21)−Mb=0Mb=−228.127x+51.328 Nm

22

Mounting Bracket Material Selection

T6 6061 Aluminum is the material we initially decided would be best for our kart with an

ultimate tensile strength of at least 290 MPa (42,000 psi) and yield strength of at least 240 MPa

(35,000 psi). At low cost, easy machinability, and minimal weight this material was perfect.

After having difficulties sourcing a machine shop to make the mount we decided to go with

3/16” A-36 plate for the mount which has properties within our requirements.

Fig. 24

23

Bolt Grade Selection

For the bolt grade selection, we used the highest force in the powertrain system as our

basis for selection. The highest concentration is 246.5 N located at Reaction Point B (RB) on the

engine mount stress analysis diagram above. For safety concerns we doubled the force at RB to

500 N and will use this as the basis for all mounting bolts throughout the powertrain.

Fig. 25

24

Using the pre-drilled and tapped 3/8” holes in the bottom of the engine as our bolt size we

then found a bolt grade spreadsheet and chose a bolt that could handle these forces.

According to the graph below a SAE Grade 5 bolt with a width of .375” = 3/8” can handle up to

9888 lbs = 43985 N of tension force and 8280 lbs = 36831 N of shear force will easily handle to

500 N of force the bolt would receive.

Fig. 26

25

Torque Transfer to the Wheels

To calculate the torque transferred through the powertrain we had to select a readily

available continuously variable transmission (C.V.T.). Investigating other universities, we found

that many of them were using a lower speed ratio CVT to increase the karts maximum velocity,

but the SAE competition only has one straight line acceleration trial. The other trials include a

30-degree hill climb, uneven terrain, and endurance event so we chose to go with a higher

speed ratio to give us more torque at the wheels and better bottom end performance.

The CVT we obtained through a donation an approximatehigh speed ratio (RH) of .69:1 and a

low speed ratio (RL) of 3.71:1. To find the Torque at the wheels we first had to find the engines

output torque at different RPMs which is provided by Briggs and Stratton. The engine idle speed

is 1800 rpm. The max engine output torque is at 2800 rpm. Also the max engine rpm is 3600 so

we will just use these points in the rpm range for now.

26

Fig. 27

Formulas:

CVT Ratio (RCVT) =RL−[ (RL−RH )× (RPM−RPM Engage )RPM Range ]

Complete Ratio (RC) = RCVT × RG × NCVT

Torque at Wheels = TOutput× NCVT× RC

Comet 780 CVT Specs:

Low Speed Ratio (RL)= .69 : 1 High Speed Ratio (RH)= 3.71 : 1

RPMRange = 2800 REngage = 800

CVT Efficiency (NCVT) = 88%

Gear Reduction Ratio (RG) = 12.58 : 1

RPMEngage: The RPM at which the CVT first engages = 800 rpm

RPM CVT Ratio (RCVT) Complete Ratio

(RC)

Engine Output Torque

(ft. lbs)

Torque at the Wheels

(ft. lbs)

1800 2.63 : 1 29.11 : 1 13 336

2800 1.66 : 1 18.37 : 1 14.5 235

3600 .69 : 1 7.64 : 1 13.8 98

RPMRange: Highest rpm – idle rpm = 2800 rpm

Fig. 28

27

Accomplishments

Spencer:

Obtained materials for joints that will attach to the main cage Fabricated frame parts Created digital and hand drawings of components and design Solicited donations Determined necessary components to be ordered Welded frame components Picked up donor go kart Worked on engine mount modifications Determined necessary components for correct engine operation Devised way to mount motor Assisted in frame mounting design Designed components for joint that we decided against using Tested components after assembly Collaborated with other mini baja groups to assist in design and building

Chris:

Obtained rims Determined necessary components to be ordered Fabricated frame parts Welded frame components Created digital and hand drawings of components and design Determined necessary components to allow for correct differential operation Devised way of mounting differential Assisted in frame mounting design Designed components for joint that we decided against using Solicited donations Determined proper tire size Tested components after assembly Collaborated with other mini baja groups to assist in design and building

28

Conclusion

Throughout this semester our capability and knowledge have been demonstrated by the

work on this design project. There are still many unknownsand problems that are certainto

present themselves. These will be overcome through analysis and application of

engineering fundamentals. After a successfully completing of the first phase of our project

we have now started the process of building the mini-baja and intend to complete the

drivetrain portion in time for the final presentation date.

29