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Shock Testing & Maintenance

The Care & Feeding of Dampers

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5 things you MUST know

about YOUR shocks

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PleaseAsk questions

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First, some questions

for you

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Do you knowif your shocks

have the right travel?

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Did youdyno test your

shocks this year?

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Did you force match your shocks?

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Who checked the gas pressure in their shocks this

morning?

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Who checked the settings this morning to confirm they

were correct?

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What is the least maintained component in

an FSAE car?

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Besides the driver’s suit?

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Course Outline

Shock Travel Damper Testing Choosing the Correct Damping Keeping Them Working Right The Last Word

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Shock Travel

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Travel Considerations

Packaging Must fit on the car! No travel interference

Wheel travel requirements Motion ratio Wheel travel vs. shock travel

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Typical Packaging Smallest package for shock Least space Least weight

“Convenient” motion ratio Whatever fits

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Proper PackagingDesired motion ratio Higher shock to wheel travel ratios yield: Lower spring rates

Lower spring and shock friction Lower component loads

Higher shock velocities Packaging considerations

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Motion Ratio Comparison1:1 0.75:1 0.5:1

Wheel Travel 25 mm 25 mm 25 mmShock Travel 25 mm 18.8 mm 12.5 mm

Wheel Velocity 50 mm/sec 50 mm/sec 50 mm/secShock Velocity 50 mm/sec 37.5 mm/sec 25 mm/sec

Wheel Rate 25 N/mm 25 N/mm 25 N/mmSpring Rate 25 N/mm 44 N/mm 100N/mmSpring Preload 50 kg 67 kg 100 kg

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Shock Velocity Comparison

260

704

445360

840890

Bleed Disc Stack

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Shock Velocity Comparison

Bleed Disc Stack

260206155

324

550704

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Shock TravelMotion Ratio Considerations Maximize use of shock travel Anticipated wheel travel Anticipated shock velocities

Remember extreme events Curbs, pot holes

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Fit Check on Vehicle

Make sure shocks fit correctly! Correct travels Shock and spring

Jounce bumper engagement No binding or interferences

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Positioning Shock TravelMust properly position shock at ride height Typically mid stroke at ride height Determine shock travel on track Use data acquisition Check for bottoming/topping

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Damper Testing

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Dyno Graph Basics

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BDC TDCComp mid Reb mid

CompAccel

CompDecel

RebAccel

RebDecel

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Force – Displacement Graph

BDC TDC

Rotation

Compression mid-stroke

Rebound mid-stroke

Reb Decel

Comp Accel Comp Decel

Reb Accel

Compression Open Compression Closed

Rebound OpenRebound Closed

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Force – Absolute Velocity Plot

Quadrant 1

Quadrant 4

Quadrant 3

Quadrant 2

Comp Accel

Compression Open

Comp Decel

Compression Closed

Reb Accel

Rebound Closed

Rebound Open

Reb Decel

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Damper Dyno Curves

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Useful Dyno Plots Force – Velocity Force – Absolute Velocity Force – Displacement PVP or Peak Velocity Plot

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Force - Velocity

Rebound

Compression

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Force - Absolute Velocity

Compression

Rebound

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Force - Displacement

Compression

Rebound

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PVP – Peak Velocity Plot

Curve connectsIndividual points

17 Peak Velocities

Rebound

Compression

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PVP – Peak Velocity Plot

What is a PVP Plot? A collection of average forces at peak

velocities Each average force is plotted as a point A line is drawn through each point to generate the

curve Representation of “average” force at each velocity

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PVP – Peak Velocity Plot

Curve connectsIndividual points

17 Peak Velocities

Rebound

Compression

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Curve at a Single Velocity

Compression

Rebound

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Single Speed Plot

Typical Plot 1 or 2 inch fixed stroke 10 in/sec peak velocity Only one speed

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Single Speed Plot

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Single Speed Plot

Pluses Has become the default standard Because the standard, easy comparison

between teams Easy to compare your data to another team’s

Good overall view of damper forces

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Single Speed Plot

Minuses Only one speed Ignores acceleration affects

Shows poor representation of low speed controls Poor for comparison of damping performance

at low velocities

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Multiple Speed Plot

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Multiple Speed Plot

Pluses Able to see effects of acceleration Better analysis at each specific velocity Focus on velocity car is working Observe how valving is working at each specific

velocity

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Multiple Speed Plot

Minuses No standardized set of velocities Hard to compare results with other teams

Analysis is more difficult

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Dyno Curve Analysis

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Dyno Curve Analysis

Want smooth, uniform curve No lags No signs of cavitation No incongruities Minimal hysteresis

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Hydraulic Lag

HydraulicLag

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No Cavitation

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Minor Cavitation

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Slight Cavitation

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Cavitation

Loss of damping force from cavitation

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Massive Cavitation

Loss of damping force from cavitation

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Incongruity

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Hysteresis

Hysteresis

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Proper Curve

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Dyno Curve Analysis

Use opening side of curve This is the initial force seen by the vehicle

during any event Analyze entire cycle – NOT PVP Use Force/Velocity and Force/Displacement

Analyze multiple speeds

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Force – Absolute Velocity

Compression Open

Rebound Open

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Force – Displacement

Compression Open

Rebound Open

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Choosing the Correct Damping

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ButHow Much Damping?

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How Much Damping?

Under Damped Over Damped

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How Much Damping?

Damped

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How Much Damping?Effects of Different Damping

Courtesy of Kaz Technologies and GM Racing

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How much Damping?

Tire Force Variation Considerations Damping is only required at resonant

frequencies Primarily sprung & unsprung mass resonance Damping required to control resonance

Damping at other frequencies increases tire force variation

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How much Damping?

Courtesy of Kaz Technologies and GM Racing

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How much Damping?

So, how much damping?The correct amount To control resonances To control pitch and roll rates To control suspension travel Minimize tire force variation

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Keeping YOUR ShocksWorking Right

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Damper Care & Feeding

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Damper Care & Feeding

Main objective is repeatability and durability Dyno test shocks at least once a year Tune adjusters so forces match Check gas pressure before each test Look for oil leakage Check for rod and seal damage

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Same Adjustments – Not Force Matched

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Same Adjustments – Force Matched

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Oil on the Shock RodIf there is oil on the rod, is it leaking? Must have oil on rod to oil seal No oil, seal will quickly wear out

Will often see “halo” of oil on rod If you see a puddle, shock is leaking!

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Professional Damper Care

How race teams do it Dampers rebuilt and forces matched after

each race Dampers curved before race Gas pressure checked each day Damper settings checked with each setup and

set down

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The Last Word

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5 things you MUST know

about YOUR shocks

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5 Things You Must Know My shocks have the correct travel They fit correctly on the car The shocks are performing properly They have the correct damping My shocks are ready to race!

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Questions?

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Jim KasprzakKAZ TECHNOLOGIES

jkaz@kaztechnologies.com248-855-3355

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Thanks!