fl tech frame

8/12/2019 Fl Tech Frame

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Chassis

Introduction

Purpose and GoalsThe chassis is the backbone of the Mini-Baja; it must support all the cars

subassemblies as well as protect the driver. The chassis design is crucial to the success of

the project because if the chassis fails, that puts the Baja and the driver at tremendous

risk. The goal of the !!" Mini Baja frame will be to protect the driver, offer sturd#

mounting for all subs#stems, maintain all $%& rules and regulations, and still be

lightweight.

Background'hassis design in the $%& Mini Baja competition is critical for a winning car.

These cars are all powered b# (! horsepower Briggs and $tratton engines. To e)tract

ma)imum acceleration from this engine a lightweight chassis is necessar#. %t the same

time the chassis must undergo the rigors of off-road racing. To anal#*e a structure that

will undergo such loads, finite element anal#sis +&% is often a viable solution. &%

breaks the structure into smaller elements and anal#*es each element as a bod# and can

calculate the stress, deflection and other reactions of an# structure. % Transient &% will

be the main simulation done to optimi*e the weight and strength of the !!" Mini Baja

'hassis.

rom here forward a few assumptions have been made to aid design and anal#sis.

The total weight of the Mini Baja car, without a driver, is estimated to be !! lbs, with

the lightest car weighing /("lbs. 01 This is the average weight of the most competitive

schools cars from !!2. The driver will be referenced as a 3 foot / inch tall male

weighing 4!lbs, as per the $%& rules. 01

Design Objectives

To build a chassis to meet all of the previousl# mentioned goals the frame must5

&ndure the ma)imum d#namic load according to $%& Technical paper !!3-!(-

/3301 with a factor of safet# of (.4

6eep a driver alive during a 2.7g front impact

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6eep a driver alive during a 2.7g side impact

6eep a driver alive during a 2.7g 8oll over situation

%bide b# all $%& rules and 8egulations

9eigh less than "!lbs

:ave mounting structures for all subs#stems that will withstand the loads

produced b# those subs#stems.

SAE Rules and Regulations

%ll $%& rules and regulations can be found in %ppendi) %. The rame rules are

specificall# in $&'T / 8=?? '%@&, $A$T&M$ C8


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Figure 2: Frame Members

Maximum Dynamic Load

The loading conditions used to anal#*e the 'hassis are just as important as the

anal#sis itself. The more accurate the load the smaller the factor of safet# can be used.

The !!" Mini Baja 'hassis team purchased $%& technical paper !!3-!(-/3301,

$tructural 'onsiderations of a Baja $%& rame.


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Figure ": Vertical Imact #emo$stratio$ []

This data can be directl# used on the frame during a transient &% anal#sis. Because the

material that will be used for anal#sis, chromol# steel, is a well-known material and the

loads and stresses that are being used for the shock force anal#sis are from actual test

cases a factor of safet# of (.4 is used for the anal#sis of the frame. +$ee %ppendi) B forfactor of safet# justification

7.G !mpact

Fsing data from The Motor


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The mass of the Mini Baja car is !. slugs, therefore the g-force that will be used for

anal#sis is 2.7gs+the car with the closest mass+=9/2(". These values are highlighted

in Table below.

Delta V MPH pulse(ms) g-force Mass of Car(slugg)

6.337986158 9 3 81.5!"9"15!

6.835"8311 68 !.! 99.35656"7

8."7785!96 8! !.! 13!.6!57"

8.6!3685! 8" !.9 99."1395187

8.9!77!5165 7 5.7 9.988188

9.58!3"761 19 3.3 1".3"9966

1".31!76179 13" 3.6 1".3"9966

10.62544738 65 7.9 69.20698366

1".65!!738 1"3 !.9 9!.83!1!1!

1".68758!5 89 5.5 96.73"813

1".68758!5 7! 6.6 91.75"6!!67

1".68758!5 116 !.1 99.35656"7

11.!681858 65 7.9 13.3391788

11.!33993 8 6.! 119.913"9"5

11.!33993 93 5.6 98.6"817

1."5!6"11 93 5.6 1"".11"3""1

%able 2: Vehicle Mass a$d 'cceleratio$ []

"rame #eig$t

To calculate the ma)imum frame weight, the estimated values of each subs#stem were

added up and subtracted from an estimated total vehicle weight of !!lbs. This resulted

in a frame weight of "!lbs as shown in Table /.

Com ponent Wei ght (lb)

Tir es 50

Suspe nsi on 50

Stee ri ng 15Engin e 60

Transm issio n 60

Re ar Housi ng 40

Bo ! "anel s 10

S#i "late 10

Ele $tro ni $s 5

Br a#es %0

Total &%0

Tar ge t 400

'ram e eight 0

%able 3: (stimated Vehicle )eight

Design and Analysis

Material Selection

To build the Mini Baja frame steel must be used according to the rules. There are

man# different t#pes of steel available to the public. The frame of the Mini Baja will be

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made from tubular sections. Tubular sections offer superior loading capabilities per

pound when compared to solid sections or sJuare sections.

The material selection criteria are ver# similar, so the suspension material

selection chart will be used to choice a material for the chassis. %ccording to that

material selection chart +Table (3 on page 37 (/! steel is the preferred material for use

in the Mini Baja frame.

Roll %age &u'ing

%ccording to 8oll 'age Material $pecifications /(.4 in the $%& 8ules 0%ppendi)

%1 the roll cage must +% 'ircular steel tubing with an outside diameter of .4 cm +(

inch and a wall thickness of /.!4 mm +.(! inch and a carbon content of at least

!.("IH, or +B $teel members with at least eJual bending stiffness and bending strength

to (!(" steel having a circular cross section with a .4 cm +( inch outer diameter and a

wall thickness of /.!4 mm +.(! inch.H01 rom the material selection chart (/! allo#

steel was chosen as the material to be used for the entire frame. There is a note in the

rules about the use of allo# steel5 >=T&5 The use of allo# steel does not allow the wall

thickness to be thinner than (.42 mm +.!3 inch.H To allow the use of this allo# steel an

eJuivalenc# calculation must be made to that of (!(" steel. This calculation is

demonstrated below in &Juation (.

& K The modulus of elasticit#H 01< K The second moment of area for the cross section about the a)is giving the lowest

valueH 01

$# K The #ield strength of material in units of force per unit areaH 01

c K The distance from the neutral a)is to the e)treme fiberH 01


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c K(inch

( )

( )2("!(.7"!4

(

!//444"/.!(!2.4/

7733!".(!//444"/.!(!72!!

/

/

=

==

===

c

SyIB

EIB

strength

stiff

(*uatio$ &: +e$di$g tiff$ess a$d tre$gth


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driving position as in igure 4. 9hile this driver was in the frame measurements were

made to make certain all of the $%& safet# rules were satisfied.

Figure .: /.0 Male i$ Mi$i +a1a Frame

Su'system Mounting

To mount the subs#stems that subs#stem was first placed in space in relation to

the frame with mounting tabs at the mounting location. inall# piping was routed to the

mounting tabs of that subs#stem. This two step process is demonstrated with the

engineLgearbo) assembl# in igure 3.

Figure : %o ste subassembl4 mou$ti$g rocess ste & o$ the left a$d ste 2 o$ the right

!nitial Design

Fsing the rules, subs#stem dimensions, and a 74I male as the driving factors a

frame was designed using the minimum frame tubing specified in the 8oll 'age tubing

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section above. The total weight of the initial frame design is 7."24lbs. This frame is

displa#ed below in the left of igure 2without subs#stems, and the right of igure 2with

subs#stems.

Figure 5: I$itial Frame ithout subs4stems 6left7 a$d ith subs4stems 6right7

Analysis

S$ock Load

%s stated before, a transient &% anal#sis is preformed on the frame using the

force vs. time chart shown previousl# in igure / on page (. This force is broken up into

and A values because the shock is not mounted verticall#. The ma)imum and

minimum angle of the shock +theta in relation to the frame is !.2( and /.!2 degrees

respectivel#. The calculation of the and A forces are shown in &Juation , and the

resulting forces are listed in Table 4. This force will be applied to the front and rear

shock mount points individuall#.

2


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Figure 8: hoc9 a$gle i$ relatio$ to the frame

-cos+

-sin+

FF

FF

Y

X

==

(*uatio$ 2: Calculatio$ of a$d ; como$e$ts of the hoc9 Force

T*+E(s),oa(lb)T*+E(s),oa(lb) T*+E(s),oa(lb)

0-05 11%-5 0-05 44-0&6.5 0-05 105-%&04.5

0-0.5 11%-5 0-0.5 44-0&6.5 0-0.5 105-%&04.5

0-1 400 0-1 156-.4% 0-1 &.4-15%

0-1%5 1%0 0-1%5 501-5.44 0-1%5 11/.-%/6

0-15 00 0-15 &1&-44 0-15 .4-&056

0-1.5 1460 0-1.5 5.%-10& 0-1.5 1&65-65..%

0-% &/0 0-% 15%-%&45 0-% &64-.//

0-%%5 450 0-%%5 1.6-&&4.5 0-%%5 4%0-/%1/

0-%5 150 0-%5 5-..%5 0-%5 140-&0.&

'or$e 2 'or$eTotal 'or$e

%able .: hoc9 Force a$d ; Como$e$ts

or the &% anal#sis the mass of the driver and engineLtransmission are also

modeled in the &% program. To satisf# the $%& rules section !. vehicle

configurations the driver is assumed to weigh 4!lb. 01 The engine and transmission was

weighed and found to be appro)imatel# (!!lbs. The densit# of the frame tubing

+!."lbLinN/ is also modeled with acceleration due to gravit# taken into account.

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The order of frame tubing used is listed below in Table 3. These are all readil#

available sections according to our online retailer. 01 The# are listed in order of increasing

weight pre foot.


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*ni ti al C9SE *ni ti al Test it h +inimum Tubing Thi$#nesses +9 St ress : 4%066-6666.

Time +a;imum


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Figure &&: Left - I$itial Vo$-Mises tress at time =>&.sec, Right - tress after i$creased IM si?e

%t time K !.(24sec the ma)imum von mises stress was 4!7/psi and is located at the

cross bar that connects the left and right front shock mounts as shown in the left ofigure

(. To decrease the stress value in the cross bar the tubing section was moved up to

cross-section from cross-section (. This reduced the ma)imum von mises stress to

/(psi as shown in the left of igure (. This is still above the ma)imum stress of

!33psi, so to further reduce the stress the cross bar section was increased to cross-

section /, which reduced the ma)imum von mises stress to /"/2psi as shown inigure

(/.

Figure &2: Left - I$itial Vo$-mises tress at time =>&5.sec, Right - first reisio$ at time =>&5. sec

/(


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Figure &3: Fi$al reisio$ at time =>&5.

%t time K !.!!sec the ma)imum von mises stress was 42!psi at 'ross Bar as shown

in the left of igure (. Cue to previous changes in the 'ross bar and $2==sec, Right - first reisio$ at time =>2==sec

/


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Se$on Test ith Changes at +a;imum


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Figure &8: Rear hoc9 Force

The results are shown in Table 7, with the graph of ma)imum von-mises stress shown in

igure (7.

C9SE 1 *ni ti al Test it h +i nimum Tubi ng Thi $#nesses +9 St ress : 4%066-6666.

Time +a;imum


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The ma)imum von mises stress at !.(4sec is (/33psi, and is located on the %B at

the 88: as shown in the left of igure !. The %B is in need of a bracing bar to

transmit the force from the shock. % bracing bar of cross-section ( was added at shock

point to the 88:, this lowered stress to 372/3psi and moved ma)imum stress point to the

%B at the shock point. This is shown in the right of igure !. The tubing section of

the %B was increased until the ma)imum stress went down to /""psi with cross-

section . This is shown in igure (.

Figure 2=: Left - I$itial Vo$-mises stress at time =>&2.sec, Right - first reisio$ at time =>&2.sec

/3


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Figure 2&: Fi$al reisio$ at time =>&2.sec

%t time K !.(4!sec the ma)imum von mises stress was (73!psi and was located at the

88: on the %B as shown in the left of igure . Cue to previous changes in the

frames tubing sections the ma)imum stress reduced to /7"4psi as shown in the right of

igure .

Figure 22: Left - I$itial Vo$-mises stress at time =>&.=sec, Right - Fi$al reisio$ at time =>&.=sec

%t time K !.(24sec the ma)imum von mises stress was (!27psi and was located at the

88: on the %B as shown in the left of igure /. Cue to previous changes in the

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frames tubing sections the ma)imum stress reduced to 4//psi as shown in the right of

igure /. The %B tubing was increased to cross-section 4 to reduce the stress to ((34

as shown in igure .

Figure 23: Left - I$itial Vo$-mises stress at time =>&5.sec, Right @ first reisio$ at time =>&5.sec

Figure 2": Fi$al reisio$ at time =>&5.sec

%t times !.!!, !.4, !.4!sec the ma)imum stresses were reduced to 2, (4!/,

and /!23psi respectivel# due to previous changes. These results are displa#ed in igure

4andigure 3respectivel#.

/"


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Figure 2.: Left - Fi$al Reisio$s at time =>2==sec, Right - Fi$al reisio$ at time =>22.sec

Figure 2: Fi$al reisio$ at time =>2.=sec

The changes to the frame to withstand the rear shock load resulted in a gain of 3.(4" lbs

giving a total weight of 3.(lbs. The new stress vs. time graph is shown below in

igure 2and the data in Table (!.

/7


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C9SE % Se$on Test ith Changes at +a;imum


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Rea$ti on +oment s(lb?in)

99rm +ount ing 9rea 2 @ +2

'ront 56 0 0 0

Rear 4 0 0 0

'ront 116-.15 /0&-/5 .-& 0

Rear 116-.15 /0&-/5 .-& 0

Rear Apper Single +ount 616 0 0 1560

'ront 4-5/5 &&-515 &%-115 0

Rear 4-5/5 &&-515 &%-115 0

'ront 4-5/5 &&-515 &%-115 0

Rear 664-5/5 1%4%-465 110-4/5 1560Rear Total

Rea$tion 'or$es(lb)

Rear ,oer

'ront ,oer

'ront Apper

%able &&: use$sio$ Reactio$ Forces

Figure 28: Rear use$sio$ Mou$t

Figure 2/: 6Left7 Fro$t Aer '-arm tress, 6Right7 Fro$t Loer '-arm tress ith suort

(


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Figure 3=: 6Left7 I$itial Rear use$sio$ tress, 6Right7 Fi$al Rear use$sio$ tress

The stress values were initiall# above the ma) allowable ,!32psi for the front

lower anal#sis, so a bar of cross-section ( was added as shown in right ofigure 7. The

rear suspension section was changed from section ( to section to sta# below the

allowable stress.

Impact Loading

To calculate the forces used to anal#*e the 2.7g impact and newtons second law

is used. The force calculation is shown below in &Juation /.

( ) ( ) lbf

ftfta

slugsm

maf

23.4(/"/".4.!sec

/".4

sec

./7.2

.!

==

==

=

=

(*uatio$ 3: Beto$s eco$d la for Imact Force

The 4((/.!/"lb force will be used in a transient &%. The force will be ramped over a

pulse of !.!34sec. 01 The impact anal#sis is done to verif# that no frame member will fail

during the impact and the driver will therefore remain safe. The ultimate tensile strength

of the chromol# steel being used is 7,2!!psi. %s long as the stress remains below this

value the frame will be considered safe enough. The displacement of the pipes will be

also be monitored to verif# that the driver will not be harmed b# an# protruding bars

during the impact. $%& rules 01 state that the drivers arms etc must be at minimum three

inches from all structural members of the frame. Therefore an# displacement values over

three inches will be deemed harmful to the driver.


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Front & Side Impact

=nl# one modification was made to the frame during ront and $ide impact

anal#sis. Curing the side impact force the part of the ?$ that meets the 88: needed

e)tra bracing of ( inch =.C. and !.!/4 wall thickness to keep the stress below the

ultimate tensile of 7,2!!psi. The front and side impacted are summari*edbelowin

Table (.

+a;imum


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&Juation /the impact force is calculated as /lb. The resulting stress is shown in

igure /.

Figure 3": 6Left7 Roll Eer Force, 6Right7 Roll Eer tress for %e$ Foot #ro

The stress values for the ten foot drop is (!,"!psi which is above the FT$ of

7,2!!psi. The frame is ne)t anal#*ed at a eight foot drop. or a eight foot drop the

impact velocit# is calculated to be .2 ftLs and impact acceleration and pulse is

interpolated as 3./gs and 2.4 miliseconds respectivel#. Fsing &Juation /the impact

force is calculated as (!lb. The resulting stress is shown inigure /4.

Figure 3.: (ight Foot #ro tress

The stress value for the eight foot drop is "!,!4psi which is below the FT$ of

7,2!!psi. $o our frame will be rated for a ma)imum roll over drop of eight feet. Curing

the Mini Baja race the car should not have to undergo a roll over drop over five feet. 01

Design )or Manu)acturing

Curing initial design, design for manufacturing was considered. The lorida Tech

Machine shop has all of the necessar# tools to cut, bend, cope, and weld chromol# tubing.

The shop onl# has a limited si*e of dies for the tube bender. Therefore tubing bend

4


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"udget

es$ription "ri$e DT Total


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igure /25 'hassis 8ear &nd Before +left and after +right transmission reselection

Scheduling

Plan )or %ompletion

%s of now the car is in racing condition, and has passed initial testing. urther

testing will be done to prove the chassis has satisfied all nondestructive design

constraints.

Gantt %$art

The @antt 'hart for the chassis section can be found in %ppendi) C.

#onclusions

%fter shock load, and impact anal#sis on the frame it is read# to endure a Mini

Baja race and fulfill all of its goals. 9eighing in at a manufactured total of "!lbs +e)tra

weight added due to welding the frame will help the overall performance of the car

during competition. The chassis is completed and has gone through initial testing. This

includes low and high speed testing over rough terrain and small +( foot jumps. There

are plans for high speed e)tremel# rough terrain and larger + foot jumps testing. %s ofnow the chassis is read# for the $%& competitions. The final frame is displa#ed in igure

/".

"


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Figure 38: Fi$al Frame after Comlete '$al4sis

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