Frame Design Report

Phone: +91-8986682423, +91-8102303112 E-mail: [email protected], Website: www.teamsrijan.bitmesra.ac.in

CHASSIS DESIGN REPORT

(2014 – 2015)


INTRODUCTION:

The objective of the frame sub team is to design a chassis in accordance with the FSAE rulebook

along with providing a lighter, stiffer frame keeping in mind the aesthetics and ergonomics of the

driver. The design started with a thorough study of the rulebook. The areas which needed a

change were identified keeping in mind last year’s design. After that a mock chassis was made and

the dimensions of the cockpit and foot well area were taken. It was ensured that enough room

was given to the driver for egress and that he had improved visibility. The 95th percentile male

rule was kept in mind while deciding the heights of the roll hoops.

KEY POINTS:

1. Type - Tubular space frame.

2. Material – AISI 1018 steel.

3. Frame weight (bare weight including brackets) – 40kg.

4. Torsional Stiffness - 2250 N-m/deg (Analysis was done in Solidworks 2013 using a FEA

beam model).

5. Welding used - GMAW (ER 70-S6 filler).

6. Crush Zone Material (Impact attenuator) – Dow impaxx 700 foam (Standard Impact

attenuator Type 13).

7. Firewall and Floor Material – Aluminium.

CRITICAL ANALYSIS OF PREVIOUS YEAR’S DESIGN:

Analysis of last year’s design concluded that the cockpit was too compact. It was difficult for the

driver to egress within 5 seconds. Bending of roll hoops was not properly done. There were

vibrations in floor and firewall. More importantly, the use of MDF fixtures lead to inaccuracy in

manufacturing of frame as maintaining wooden plates at 90 degree to base plate using L brackets

is difficult, irrespective of strength of brackets i.e. wooden plates swayed from top end.


CORRECTIVE MEASURES:

This year frame incorporated four key dimensional changes. Cockpit length has been increased by 50mm for comfort of driver. Rear section of cockpit has been increased by 25mm for packaging purpose. The front roll hoop has been raised by 30 mm to aid in driver egress. Height of main hoop has been raised by 25 mm for accommodation of 95th percentile male. Improvements were made in the packing of different components like dashboard, shifter, engine and exhaust. Rubber bushings were used while mounting floor and firewall to minimize vibrations. This year we moved to laser cut fixtures for high precision and accuracy of frame coordinates. The modelling and FEA of the frame has been done using Solid Works 2013. Standard Impact Attenuator Type 13(DOW IMPAXX 700 Foam) is being used, as it was felt that the cost incurred in manufacturing and testing an IA, could be better utilized in other areas.

DESIGN SPECIFICATIONS:

MATERIAL SELECTION:-

For the design of an FSAE chassis, the cost and ease of manufacturing are very important issues. It was necessary for the chassis to be structurally sound, but it was also essential that the chassis is relatively simple and inexpensive, as limited time, money and resources were available.

We had to decide the tubes to be used between SAE 4130 or AISI 1018. Comparisons are as follows:

AISI 1018

Mechanical Properties Metric English

Hardness, Brinell 126 126

Hardness, Knoop 145 145

Hardness, Rockwell B 71 71

Hardness, Vickers 131 131

Tensile Strength, Ultimate 440 MPa 63800 psi

Tensile Strength, Yield 370 MPa 53700 psi

Reduction of Area 40.0 % 40.0 %

Modulus of Elasticity 205 GPa 29700 ksi

Bulk Modulus 140 GPa 20300 ksi

Poissons Ratio 0.290 0.290

Machinability 70 % 70 %

Shear Modulus 80.0 GPa 11600 ksi


SAE 4130

Density (×1000 kg/m3) 7.7-8.03 25

Poisson's Ratio 0.27-0.30 25

Elastic Modulus (GPa) 190-210 25

Tensile Strength (Mpa) 560.5

25 annealed at 865°C

Yield Strength (Mpa) 360.6

Elongation (%) 28.2

Reduction in Area (%) 55.6

Hardness (HB) 156 25 annealed at 865°C

Impact Strength (J)

(Izod) 61.7 25 annealed at 865°C

From the above tables we can see that SAE 4130 grade is more versatile than 1018 steel tubes.

Comparing them on above parameters we find that 4130 tubes has more tensile strength, yield

strength, hardness due to addition of molybdenum and chromium. But 4130 tube requires heat

treatment for its welding and we don’t have heat treatment facility in our college. Moreover 4130

is more expensive than 1018. Therefore we have decided to go with 1018 tubes as it provides

sufficient strength which is required to make a rigid frame.

WELDING OPTIONS:

Team had to choose between Metal Inert Gas welding (MIG) and Tungsten Inert gas welding (TIG). MIG welding was chosen due to following reasons -

TIG is inherently more difficult (requires more operator skill) than MIG.

Controlling the TIG arc becomes difficult.

CO2 gas used in MIG is much cheaper; hence operating costs of MIG is less.

Gas consumption is more in TIG, as care needs to be taken to protect the tungsten

electrode.

TIG is much slower than MIG, requires more practice time and wasn’t that easy to get

satisfactory results.

http://www.efunda.com/units/convert_units.cfm?From=190&mrn=7700#ConvInto







http://www.efunda.com/units/convert_units.cfm?From=339&mrn=560%2E5#ConvInto







Good weld finish can be obtained using MIG.

The final welded frame was painted. This is essential as it protects the steel from corrosion and

gives the judges a good impression that the car was finished on time.

MOCK CHASSIS:

Next step was to construct a mock model to validate the dimensions of cockpit and footwell.

Dimensions that were to be identified from the mock chassis were:

Main Hoop and Front hoop height, Cockpit dimensions (lateral distance between side

impact structures), Footwell dimensions, Shoulder harness location.

Inputs required for proper mock model usage:

Seat location, driver’s visibility, pedal assembly dimensions, approx. steering location.

A simple wireframe model of frame from bulkhead to main hoop was made in Solidworks, keeping

members to a minimum. Plywood and MDF boards were used for construction of Mock chassis. It

provided a very rigid structure and relatively it was easier to work on. Besides we also had the

flexibility of adjusting the dimensions as per the need of the driver. This provided us with accurate

dimensions.

Fig: Mock chassis construction


SOLIDWORKS MODELING:

After the minimum required dimensions of the footwell was decided, the same was given to the

VD team for calculation of suspension A arm end points. After receiving the suspension geometry,

design of frame was started from the front end of the frame.

It was kept in mind to keep the frame well triangulated so that only tensile and

compressive forces acts on tubes.

Sketches were added for suspension brackets.

The end points of bracket sketches were the primary frame nodes where suspension

components would be linked. Same was done for the rear.

The requirements of the rules were kept in mind as the design proceeded (Front bulkhead,

Front bulkhead support structure etc.)

The cockpit and foot well dimensions from mock were used to generate the final

wireframe model of the front end.

For the rear, we made used of Engine CAD model to design the engine box. The differential

was added after the engine box.

Fig: Wire Frame Model


After having a complete sketch of the frame, we started adding weldments to the sketches. Points

that were kept in mind:

Tubing sizes as according to rules for roll hoops, bracings etc.

Tube end profiling. Aim was to obtain simpler profiles as compared to last year, and avoid

mighter ends and sharp profiles where ever possible.

Before proceeding any further, rules were checked to ensure that the frame complied with them.

Next Initial simulation was done to check whether the design basics were correct, i.e. the load

paths and stress concentration in critical members. Test done was a torsional stiffness test, as any

other test (frontal impact test, off axis frontal impact, side impact etc.) are not needed as we are

following rules laid out in Driver’s Cell section of the Rule Book.

Fig: Final Frame CAD model


Fig: Well triangulated space frame

TORSIONAL STIFFNESS ANALYSIS:

The Torsional Stiffness analysis was done in Solidworks 2013 using a FEA beam model. The loads

and constraints applied are as described below:

The front wheel centres were connected to the suspension hard points of the frame using truss

members. This was to simulate wishbones and pushrods which transfer loads to frame axially.

LOADS: 50 N force was applied to the front wheel centres (in opposite directions) so as to

constitute a Force couple.

CONSTRAINTS: The rear suspension hard points were constrained in all degrees of freedom.

This setup is equivalent to the frame being twisted by an applied torque at the front while it is

fixed at the rear.

F : Force applied

Τ : Applied Torque d : Distance between wheel centres δ : Deflection of wheel centre φ : Angular deflection of wheel centre


d= 1.154m

For left wheel:

δL = 0.4703 mm

φL= 𝐷𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓𝑙𝑒𝑓𝑡 𝑤ℎ𝑒𝑒𝑙 𝑐𝑒𝑛𝑡𝑟𝑒

𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑙𝑒𝑓𝑡 𝑤ℎ𝑒𝑒𝑙 𝑐𝑒𝑛𝑡𝑟𝑒 𝑓𝑟𝑜𝑚 𝑣𝑒ℎ𝑖𝑐𝑙𝑒 𝑐𝑒𝑛𝑡𝑟𝑒𝑙𝑖𝑛𝑒

= δL

𝑑/2

= 0.257𝑚𝑚 577⁄ 𝑚𝑚 = 4.466 x 10-4 rad

For right wheel:

δR = 0.4748 mm

φR = δR

𝑑/2 = 0.258𝑚𝑚 577𝑚𝑚 ⁄ = 4.464 x 10-4

φav = (φL + φR)/2 = 4.465 x 10-4 rad = 0.02564 °

Torque applied, Τ = F x d = 50 x 1.154 = 57.7 N-m

Torsional Stiffness = Τ / φav = 57.7 N-m / 0.02564 ° = 2250 N-m/deg

Fig: FEA analysis of the space frame


MANUFACTURING OF FRAME:

Frame was manufactured in a time frame of 20 days. Steel fixtures were used this time to increase

accuracy as steel did not sway from top unlike MDF which was used last year. Steel plates were

welded on a steel base plate using gussets. Steel plates had holes of required dimensions and

tubes were allowed to pass through these holes. Tubes of required length were cut and profiled to

create joints and nodes without any difficulty. MIG welding was performed to join tubes and

create a strong joint.

Fig: Steel fixtures mounted on the base plate

OTHER ASSEMBLIES IN FRAME:

IMPACT ATTENUATOR:

The rules requirement for an impact attenuator makes it a component that needs a lot of time

and resource to be devoted to its design and testing. The monetary resources required for its

testing ruled out its in house development. The alternative was to go for a Standard Impact


attenuator. This would save us man hours, allowing us to concentrate on more critical

components. The cost of A standard impact attenuator was comparable to what it would cost to

test a self-made IA. The fabrication costs would have been extra and were not calculated.

ANTI INTRUSION PLATE:

Steel was chosen as the thickness was specified in rule book and aluminium of required thickness

hardly gave any weight advantage, and had the added problem of welding it to the bulkhead.

FLOOR AND FIREWALL:

Aluminium Plate was chosen as it provided significant weight reductions and stiffness. The added

components were brackets welded to the frame to which floor and firewall were bolted using M6

bolts.

Fig: Manufactured Frame

Documents

Frame Design Report