4/7/15

In This Session 2015 Webinar 4 • Review: How a car turns a corner • Cornering forces Weight (Load) Transfer • Springs and shocks plus • Tuning for grip and balance Care and Feeding of Your Tires • Load transfer timing • Care and feeding of tires There is STILL no free lunch • Questions 4/7/2015

1 2 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

How a Car Turns a Corner Car in a Corner Like a Weight on a String

• Steering input creates a slip angle at the front tires resulting in a lateral force that starts to turn the car • The rear tires stop the car from rotating with a slip angle and lateral force • The combined lateral forces • F = MA = MV2/R, an inertial force opposes an of the tires act at the CG accelerating force accelerating the car toward • Flateral (tires) = MV2/R the center of the arc of the path • What you already know, more weight or higher 3 speed needs more Flateral or bigger R 4 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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The Tires Are Turning the Car Inertial Force Causes Weight Transfer

• Tires are load sensive, less incremental grip with more load The sidewall distoron you see is evidence of the lateral forces generated by each re

5 6 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Inertial Force Causes Weight Transfer Inertial Force Causes Weight Transfer

• Tires are load sensive, less incremental grip with more load • Tires are load sensive, less incremental grip with more load • Suspension sffness at each wheel and suspension geometry • Suspension sffness at each wheel and suspension geometry determine the weight distribuon among the re contact patches determine the weight distribuon among the re contact patches • Back seat analogy to load ming on contact patches 7 8 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Static Forces on a Car Forces on a Car in a Turn

Inertial reaction to acceleration causes lateral weight transfer and chassis roll on springs

9 10 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Calculating Lateral Weight Transfer Dynamic Tire Loading

• Sum moments around Z: WR x t = W x t/2 + W x A x h • Tires are load • Fractional weight transfer, FWT = WR - W/2 = A x h/t sensitive, gaining • Actual weight transfer, WT = A x W x h/t smaller increments of • Bottom line: weight transfer is proportional to lateral grip with increments acceleration and the ratio of CG height to track width of increasing load • Goal is minimum change in contact patch forces • Tire is spring/damper also, low damping

11 12 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Spring Rate, Wheel Rate, Tire Rate Roll Resistance • Roll resistance determines individual contact patch force • Spring rate is force/deflection gain or loss from load transfer during roll for the spring alone, lb/in • Ride springs, ARBs, dampers provide roll resistance • The wheel rate is the force/ deflection for the wheel • Dampers are speed sensitive, generate force only when determined by the spring the shaft is moving movement and leverage • Tires are load sensitive: the front or rear axle-pair of tires • The spring rate of the tire is a with more roll resistance transfers more weight losing function of design, and more grip than the other pair inflation pressure • What you already know: adjustments to roll resistance • Tire-to-tire spring-rate distribution create balance changes variation is due to manufacturing tolerances and • Tire roll: tires are springs, weight transfer compresses is usually small outside tire, inside tire recovers some height

13 14 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Damping Benefits Interactions: Springs/Bars/Dampers

• Springs are displacement sensitive, • Springs support the car, resist roll force increases linearly with deflection • Anti-Roll Bars add roll resistance, allowing • Dampers are speed sensitive, force softer springs, avoiding a harsh ride increases with faster movement • Front ARBs are sensitive balance adjusters • Dampers allow softer springs to absorb and generate wedge curing power oversteer larger bumps and control the release of but also transfer more weight hurting grip at stored energy in the spring that end of the car • Dampers prevent spring oscillations, C • Dampers are speed sensitive allowing softer = 1.0 is critical damping, C = 0.1 is underdamped, C = 1.2 is overdamped ride springs and controlling roll timing

15 16 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Tuning for Grip & Balance Timing of Forces and Roll

• Basic car and suspension design variables determine lateral weight transfer: track width, CG height, RC heights, roll stiffness-distribution • Total grip levels are determined by tire design, tire attitude (suspension geometry), tire inflation pressure, tire temperature, and track conditions • Balance depends on static front/rear weight distribution and front/rear roll stiffness distribution. • Aerodynamic forces add to tire loads producing increasing incremental traction with speed

17 18 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Tires Are Sensitive to Temperature

• Poor grip below some threshold temp: 180-200 deg F • Tire temps monitor camber, inflation, driver • Adopt a measurement technique, be consistent • Three readings, middle and both shoulders of active Care and Feeding of Tires patch, looking for even distribution • Make sure the probe reaches the belt plies, that’s where the heat is generated but don’t penetrate belts

• Autocross runs are short, scuff rather than roll, don’t heat much, IR gages should work, surface cools quickly • Keep good records including ambient conditions, track temperature and track conditions • Straight-line acceleration and braking heats tires and brakes for starts/restarts, swerving is unnecessary for heating, might help cleaning the tread surface

19 20 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Temperature Effects & Adjustments Inflation Pressure Trade-Offs

• DOT steel-belt radials usually higher temp in center, Pressure Too High Too Low doesn’t change with pressure Tread Center Tread Shoulders • Consider air cooling effect, inside of tread can be hotter Wear on a fendered car Traction Worse Worse • Inner edge way too hot, too much negative camber Quicker Slower • Outer edge hot, too much toe-in or needs more negative Response camber Fatigue stress Better Worse • Front tires hotter than rears, understeer • Rear tires hotter than fronts, oversteer • Driver style/skill is an influential factor in determining the correct inflation pressure • Inflation pressure increases with tire temperature

21 22 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Moisture Increases Hot Pressure Finding the Right Pressure • Proper hot pressure is what you want • Start with a pressure range recommended by your tire • Need to find a cold pressure that gives you the right hot supplier pressure a few laps into the track session • Bias-ply easy, find pressure for uniform temperature • Tire gets hot, inflation gas gets hot, inflation pressure across contact patch goes up • DOT radials are more difficult, wear pattern is critical • Pressure gain during a track session is important • Tread wear should NOT reach the outside edge of the • Moisture in inflation gas causes higher hot pressures tread • Multiple-cycle dry nitrogen purging is the best solution, • Low pressure can cause construction failure even after pressure is increased but a desiccant in the air line works • Start high and bleed down • At least drain the compressor tank regularly • Over inflation decreases grip, increases response • Camber change begs an inflation adjustment

23 24 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Tire Data: Spring Rate vs. Pressure Blistering

• Load wheel, measure • Heat comes from tire deflection deflection in the tread • Static, not dynamic plies and belts, hysteresis • Rolling tire rates are different • Rubber is an insulator, holds heat • Sensitive to inflation pressure, camber • Rubber gets too hot, • In use, temperature reverts, loses rise increases strength, gases inflation pressure expand, pops off tread rubber

25 26 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Graining on Dirt, Front Tire Graining on Pavement

• Left photo, sliding cold tires • Right photo, too much negative camber overloading inside shoulder of rear tire

27 28 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Abrasion & Graining Scrubbing Tires, Break-In

• Break-in allows the cord/rubber structure to even out stress concentrations, conditioning the tire for best long- term wear and grip • Scrubs off mold release, roughens shiny tread surface • 90% run for a few laps (10 min), cool tires off the car, reduce pressure, overnight rest is best • Racing tires generate higher temps during first run, could be 20 to 30 deg F, use 4-6 psi higher pressure for scrub • New tires - avoid sliding, overheating, practical for solo? • Buying pre-scrubbed tires saves track time, if done right Sliding causes surface waves that bend over • Modern tires are more precisely made so scrubbing is and wear then pop back when sliding stops not absolutely necessary • tirerack.com is great source of tire info, can “heat cycle” tires you buy from them, saves track time

29 30 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

Storing Tires Give-Up, Heat Cycles • All rubber compounds soften with repeated cycles of • Tg, avoid temperatures below freezing!!! strain, especially rubber reinforced with carbon black • Some tires continue to vulcanize with each heat cycle, • Store in black plastic (garbage) bags harder, less grip • Avoid sunlight, heat, ozone, oils, solvents • Stress/heat generate mechanical and chemical changes that also hurt grip, “dead layer” • No matter what you do rubber ages, the • Broken polymer chains can reattach or not, producing tires might not be as good next season different changes in the rubber • Additives in the compound can change a tires reaction to heat cycles and mechanical stress • Tire companies’ tread compounds are very different, don’t wear in the same way • Taking durometer readings can provide useful information but requires consistent technique and consideration of ambient temperature 31 32 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

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Tire Treatments Directional Mounting • Always carefully read manufacturer’s instructions for mounting your tires • Diesel fuel and wintergreen is one mix I’ve heard • The splice used to close the tread of, commercial products probably contain toluene rubber joint is a scarf (angled) joint • All hydrocarbons are soluble in all other • Front tires-scrubbing due to braking cannot be against the thin edge hydrocarbons • Rear tires-scrubbing due to acceleration • Softer rubber generates more grip cannot be toward the thin edge • Tire sidewall will have an arrow, “Side • Not necessarily quicker lap times Facing Outwards” or some other indication of the desired direction of • Helps soften worn (harder, thin tread) tires , get rotation more use from them • The line across the tread shows some • Safety issue, tread delamination wear due to scrubbing against the joint • The tire is not going to come apart but mount them correctly before the next event 33 34 Copyright Paul Haney, 2015 Copyright Paul Haney, 2015

The Racing & High-Performance Tire

Subtitled: Using the Tires to Tune for Grip & Balance Available on insideracingtechnology.com for $55 postage paid to U.S. addresses SpeedSecrets.com I sign all books bought directly from me SpeedSecretsWeekly.com Also available on sae.org and amazon.com HPDE-Instructor-Tips.com to domestic and international addresses Web site: insideracingtechnology.com Facebook: www.facebook.com/Drivercoach Email: [email protected] Twitter: @SpeedSecrets YouTube: www.youtube.com/speedsecrets1

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