stress analysis preliminary report - razors edge motorsports · 2011-09-09 · stress analysis...

Tubular K-Member: Final Stress Analysis Report

Razors Edge Motorsports

August 2nd, 2011

Report Contents

• Section 1: Outline and Assumptions

• Section 2: Preliminary Analysis & Findings

• Section 3: Final Analysis & Conclusion

• Appendix A: Diagrams

8/12/2011 Razors Edge Motorsports 2

Section 1

Outline & Assumptions

Project Outline

• REM has produced a tubular front suspension cradle for Chrysler LX/LC platform cars

• Need to generate 3D model of existing prototype part from supplied photographs/dimensions

• Determine real-world loading situations and perform FEA analysis of prototype

• Based on findings, generate recommendations, if any, to improve design


Vehicle Assumptions

• Vehicle gross weight 4600 pounds

• Static front weight distribution 55%

• Force on engine mount pads = 800 pounds

• Load balance front/rear under maximum braking = 70/30

• Load balance left/right under maximum right turn = 70/30

• Leverage ratio front lower control arm = 0.75

• Leverage ratio front spindle 0.80


Chrysler LX/LC Front Suspension

• Front suspension utilizes two independent lower control arms (LCA) per side

• Rear LCA is load-bearing member, front is primarily tension member

• Spring and sway bar mount to shock absorber housing @ 75% of rear LCA length*

• “Tall” spindle design means that lower control arms do 80% or more of lateral and longitudinal load work – assumed 100% for this analysis*

* See diagram in Appendix A


Materials Used

• Primary tubing is 1.25” OD x 0.058” wall 4130 chromoly steel

• 8 “bolt-through” tubes are 1.25” OD x 0.083” wall 4130 chromoly steel

• Sway bar mount brackets are 2” square x 0.125” wall mild steel tubing

• Sway bar corner gussets are 0.125” thick mild steel plate

• All mounting tabs are 0.188” thick mild steel plate• Tube ends (with and without hole) 0.125” thick mild

steel


Notes About Model

• Model Theoretical Weight = 17.4 pounds including 15.6 pounds of raw material and 1.8 pounds of weldment

• Welds modeled without actual volume – leads to some erroneous stress risers in welded corners


Notes About Images

• Images show exaggerated deformation of model to make visible where and how deformation occurs – no actual deformation was predicted greater than 0.050”

• Factor of Safety (FOS) is calculated based on ratio of Von Mises Stress compared to Yield Strength of each material used

• Color scales on FOS graphics are set with minimum FOS=1, maximum FOS=2. This is done to highlight the areas that require attention and it is important to note that “red” does not necessarily indicate failure


Loading Philosophy

• Wish to use “worst case” scenario for this analysis

• Heaviest car this part will fit on is a Dodge Magnum with a curb weight of ~4300 pounds. For these calculations, I added 300 pounds more for driver/fuel load (though these predominantly add rear weight, not front) for 4600 pound weight

• Performance figures exceed capabilities of a lightened, full-race car of the same platform using racing slick tires


Section 2

Preliminary Analysis & Findings

Loading Scenarios Overview

• Static Loading– Vehicle sitting still on ground

• 3G Vertical Compression– Same as static but with engine and suspension loads 3x as high

• 1.2G Braking– Straight line braking at 1.2G deceleration – likely unattainable in

full-weight vehicle– Straight line braking is worst case for tension on front LCA’s

• 1.5G Right Turn– Maximum right turning force, likely unattainable in full-weight

vehicle– Steering rack forces ignored


Static Loading Calculations

Vehicle Weight = 4600lbs

Front Static Weight Bias = 55%

Front Suspended Weight* = 0.55 * 4600lbs = 2400lbs

Rear LCA leverage ratio = 0.75, so Vertical Load on Suspension Mounts = (2400lbs*0.25)/0.75 = 800lbs

Engine Cradle Weight Assumption = 800lbs**

* Weight of unsprung mass ignored – all weight assumed sprung

**Engine/transmission assembly together weighs approximately 800 pounds, but weight is spread between engine mounts and rear transmission mount. Because rear mount was not included in this analysis, 100% of the weight was used to add safety factor to the analysis


Static/3G Bump Loads Visual

Note: Arrows are not scaled for load magnitudes


Static Loading Results 1

• FOS acceptable in all areas

• No concerns


3G Vertical Loading Results 1

• Simply tripled static loading values

• FOS generally acceptable

• Begin to see stresses in rear LCA mounting tabs, and near all four frame mount tubes


3G Vertical Loading Results 2

• All vertical loads on the rear of the cradle pass through the short section of tube in red here


Braking Loading Calculations

Desired Acceleration = 1.2G = 1.2 * 32.2 ft/s^2 = 38.64 ft/s^2

Weight = 4600lbs, mass = 4600lbs / 32.2 ft/s^2 = 143 slugs

F = ma = 143 slugs * 38.64 ft/s^2 = 5525lbsFront brakes do 70% of the work, so 0.7 * 5525 =

3868lbsIncrease vehicle weight bias to 70% front so front

suspended weight = 4600lbs * 0.7 = 3220lbsTriangulate using statics to get loads at each arm

(see diagram Appendix A)


Braking Loads Visual



1.2G Braking Loading Results 1

• Extreme tension on front LCA mounts causes overload of tabs and the tubing they’re attached to

• FOS below 1 in some places indicates possible failure


1.2G Braking Loading Results 2

• Primary areas with low FOS are around welded joints – lack of 3D “volumized” weld contributes to this but overstress conditions are real

• Tabs themselves are overloaded in tension on one side indicating potential failure


Right Turn Loading Calculations

Desired Acceleration = 1.5G = 1.5 * 32.2 ft/s^2 = 48.3 ft/s^2

Weight = 4600lbs, mass = 4600lbs / 32.2 ft/s^2 = 143 slugs

F = ma = 143 slugs * 48.3 ft/s^2 = 6907lbsFront wheels do 60% of the turning work

(conservatively), so 0.6 * 6907lbs = 4144 lbsTriangulate using statics to get loads at each arm

(see diagram in Appendix A)During triangulation, assume outside (left) tire does

70% of the work and divide loads accordingly


Right Turn Loads Visual



1.5G Right Turn Loading Results 1

• Includes swaybarforces +/- 1000lbs vertically

• Low FOS contained to rear LCA mounting tabs and immediate surroundings


1.5G Right Turn Loading Results 2

• Again, loading at frame mount tube is quite high

• Rear LCA mount tabs fail in tension on one edge, but the tube they’re mounted holds up relatively well


Preliminary Summary Of Results

Loading Scenario Result

Static Vehicle Passed – No Concerns

3G Vertical Bump Passed – Minor Concerns

1.2G Straight-Line Braking Failed – Front LCA Mount Tabs

1.5G Right Turn Failed – Rear LCA Mount Tabs


Preliminary Recommendations 1

• Change material on 8 tabs highlighted in blue from 3/16” mild steel plate to 3/16” 4130 plate

• Alternatively increase thickness of mild steel plates to 5/16”

• FEA on these two options will be completed for final report


Preliminary Recommendations 2

• Change tubing of 4 tubes highlighted in orange from 0.058” to 0.083” keeping same 4130 material

• Consider using 4130 welding rod to weld 4130 tabs to 4130 tubing


Preliminary Conclusion

• Design is generally sound, requires minor modifications as noted in recommendations

• Consistent weld quality is critical for acceptable performance on this piece – there is very little margin for error

• Consider adding additional gussets for LCA mount tabs depending on available space


Section 3

Final Analysis & Conclusions

Results of Preliminary Recommendations

• CAD model was updated with recommendations made in Preliminary Report

• FEA was re-performed to determine improvement

• Tube thickness increases performed as expected

• Tab material changes to 4130 improved FOS in tabs

• New problem became apparent with radial force “hot spot” where tabs connected to tubes


Radial Force Hot Spots

• Note high forces where end of tab meets mounting tube – present both inside and outside of tube

• High stresses in vertical “bolt-through” tube due to constraints used to represent through-bolt (not modeled)


Further Work on Mount Tabs

• It was determined that the cause of the radial forces on the tubing were from the mount tab trying to rotate around an axis below that of the tube centerline due to flex in the tab “ears”

• Because tab did not completely encompass the tubing diameter, torque load was not properly distributed around the tube leading to stress risers

• New tab design was conceived to alleviate the issue


New Tab Design

• Original design had misaligned center of torque load, causing radial stresses on tubing walls

• New design distributes load around perimeter of tube so all forces are tangent to tube surface – “shear” rather than “pressure” force

8/12/2011 Razors Edge Motorsports

LOAD

Pushes inTowards tubecenter

Pulls outAway from tubecenter

Original Design New Design

Effective centerOf torque load

LOAD

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New Tab Results 1

• 1.2G Braking Load

• “Hot Spots” where tabs mount completely disappear

• Rear arm, rear tab may not have sufficient clearance to replace with new-style tab, REM to determine if modifications possible


New Tab Results 2


• Tabs on front, drivers side (shown) are mild steel with 0.125” mild steel triangular gussets

• High stresses on weld joint are localized around singularities where triangular shape of weld model comes to a point – these can be ignored


New Tab Results 3


• Tabs on front, passenger side (shown) are mild steel with no gussets

• High stresses on tabs and tube show insufficient distribution of loads: gussets are needed


New Tab Results 4

• 1.5G Turning Load

• Tabs on rear, drivers side (shown) are mild steel with 0.125” mild steel gusset front tab only

• High stresses concentrated around weldmentsingularities and other “sharp” features where actual welding will distribute load


Final Summary Of Results

Loading Scenario Result

Static Vehicle Passed – No Concerns

3G Vertical Bump Passed – No Concerns

1.2G Straight-Line Braking Passed – No Concerns

1.5G Right Turn Passed – No Concerns


Final Recommendation

• Utilize new tab design with 3/16” mild steel (or stronger) tabs and 0.125” mild steel triangle gussets

• If possible, revise sway bar mounting tube to allow for rear LCA, rear tab to also be replaced with new style tab


Final Conclusion

• The Razors Edge Motorsports designed tubular front K-member offers substantial weight savings over the OEM component while maintaining an appropriate factor of safety under normal operating conditions

• Extremely hard braking appears to be the most stressful scenario on the K-member with increased drivetrain weight and tension on front tension arms adding torque in the same direction to the longitudinal tubes

• Suspension impacts from curbs, potholes, etc. can load the suspension beyond what has been evaluated here and this product is, therefore, recommended for off-road use only


Appendix A

Diagrams

Suspension Diagram


Note the close proximity of the wheel mount spindle to the lower control arm mounting point on the Knuckle. For this reason, upper control arm support was ignored for this analysis and 100% of forces were put through the lower control arms.

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Simplified Front LCA Model


This model was used to convert wheel loads into reaction forcesAt the front and rear suspension mount points. Vertical loads were ignored as both bars have two DOF joints on the inboard end.

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stress analysis preliminary report - razors edge motorsports · 2011-09-09 · stress analysis...

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