SAE Mini Baja Powertrain

Chris Gilson

Spencer Suggs

Southeastern Louisiana University

Mechanical Engineering Technology

Instructor: Cris Koutsougeras

Advisor: Ho-Hoon Lee

1

Table of Contents:

Abstract…………………………………………………………………3

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

Methods…………………………………………………………………5

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

Rear Differential & Suspension Selection………………………….…8-9

Engine Mount Design………………………………………………10-11

Engine Mount Stress Analysis…………..………………………….12-17

Transaxle Mounting Bracket……..…………………………………….18

Mounting Bracket Material Selection………………………………….19

Bolt Grade Selection………………………………………………..20-21

Torque Transfer to the Wheels……………………………………..22-23

Conclusion……………………………………………………………..24

2

Abstract

The goal of this project is 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 will be 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 determine the materials required to

assemble it.

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. One 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 budget without any major loss of

performance.

6

Fig. 1

Mounting the Transaxle underneath the engine at a 45-degree belt angle is what we

decided would be best. This positioning would help with easy installation and maintenance, be

a simplistic design, and enable us to rigidly mount both engine and differential. Although

mounting the engine directly above the transaxle would allow for the shortest possible belt

length and wheel base, it would make for a very cramped space between the two systems and

complicate maintenance.

Fig. 2

7

Rear Differential & Suspension Selection

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

rear suspension system. A solid axle would also not work with a belt driven system. As the kart

would travel over uneven terrain the axle would twist causing the belt to be damaged and

eventually fail. With our current resources and knowledge, we could not come up with

operational system that would keep the engine and differential perfectly inline without causing

serious damage elsewhere.

The independent rear differential would allow for the power train to be rigidly mounted in

unison together and enable both wheels to operate at different heights. An independent rear

suspension (IRS) system would also give the kart greater ground clearance, longer suspension

travel, better ride quality, and significantly better cornering performance.

Fig. 3

8

The Dana Spicer H-12 FNR rear transaxle is the kit we decided would be best for our kart.

The differential can range from $500 to $800 depending on axle grade. Polaris also makes an

acceptable transaxle, but while analyzing other schools’ mini-baja projects we have found many

of them found reliability issues with the Polaris model. So we determined the Dana Spicer was

the system to go with considering price, reliability, and efficiency.

Fig. 4

9

Engine Mount Design

Fig. 5.1

The engine mount is the designed to sit on the rear frame rails behind the driver seat. To

rigidly attach the engine to the mount we placed two 3/8” holes aligned with the pre-drilled

and threaded bolt holes already in place in the bottom of the engine along with two 3/8” slots

to allow for heat expansion of the system.

10

Fig 5.2

To rigidly attach the mount to the frame rails we also placed 3/8” slots horizontally along

the side of the mount to allow the engine to slide forward 3” for easy belt removal and

11

replacement. To apply tension on the belt for installation, you simply slide the engine towards

the front of the kart and slide the bolts through the mount slots and pre-drilled holes through

the frame. We designed the mount to wrap around the rail to decrease the possibility of

deformation of the frame rails from the bolt torque.

Fig. 5.3

Engine Mount Stress Analysis

Fig. 6

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

212

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.

13

Fig. 7

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 RB which 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=0RA+RB+29.1−246.5=0RA+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)=0RB=228.127N

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

14

Fig. 8

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

15

Fig. 9

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

16

Fig. 10

A model of the motor mount, as seen above, was created using the modeling program

SolidWorks. This advanced software allows for the simulation of forces being applied to an

object to determine if the selected material can withstand them based upon its known yield

strength. Considering T6 6061 aluminum has a yield strength of 2.75e8 N/m2 and the largest

concentration of stress is 1.102e5 the mount will not fail under the loads placed on it.

17

Transaxle Mounting Bracket

Fig. 11

The transaxle mount is designed to sit in between two frame rails to prevent vertical

movement. There will be two of these plates, one on each side of the transaxle. The single bolt

hole to the right will prevent horizontal movement with a vertical brace towards the rear of the

kart. The actual differential will bolt through to the six pre-drilled holes that sit around the

large 4” hole in the center. The large hole was put in place for the CV axles to slide into the

18

differential with easy maintenance and installation. Here is a similar example we found while

researching previous projects

Fig. 12

Mounting Bracket Material Selection

T6 6061 Aluminum is the material we 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 is perfect. Mild steel would

weigh too much and be difficult to machine to spec. CP grade 2 Titanium has a greater yield

strength than T6 6061 Aluminum and has low weight but cost about 20 times more.

Fig. 13

19

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. 14

20

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.

21

Fig. 15

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 chose is the Comet 780 series CVT, this CVT has a high 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.

22

Fig. 16

Formulas:

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

Complete Ratio (RC) = RCVT × RG × NCVT

Torque at Wheels = TOutput × NCVT × RC

RPM CVT Ratio (RCVT) Complete Ratio

(RC)

Engine Output Torque

(ft. lbs)

Torque at the Wheels

(ft. lbs)

23

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

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

RPMRange : Highest rpm – idle rpm = 2800 rpm

Fig. 17

Conclusion

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

work on this design project. There are still many unknowns and problems that are certain to

present themselves, but will be overcome through analysis and application of engineering

fundamentals. After a successful completion of this first phase we will be able to start the

process of building the mini-baja car throughout the Fall semester in the second portion of

senior design.

24

25