Negative camber provided by cornering forces allows to take out the best of the tyre. Thisapproach has led engineers to investigate an innovative development for current suspensionsystems.

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Some of vehicle performance largely depends onthe tyre’s performance, which is related not only toits own design (architecture, materials, etc.) butalso to the operating conditions (temperature, posi-tioning on the ground, etc.). Optimum conditionsfor tyre operation and related performance canthus be determined independently of the technicallimitations of state-of-the-art suspension systems.

Operating Principle

In order to enable a vehicle to take a curve andcounteract the forces of inertia (lateral acceleration),tyres have to develop lateral forces called „slipforces“. These cause the tyre, and especially its con-tact patch, to deform. Under the effect of centrifugalforce, the vehicle tilts by a certain „roll“ angle,

Suspension Systems: Optimising the Tyre Contact Patch

Figure 1: Straight-line and cornering conditions. Figure 2: Symmetrical deformation achieved with suitable oppositecamber linked to each slip force.

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which means that the wheel „copies“ the camberthat depends on the vehicle’s suspension systemtechnology. Values of between 4° and 6° of camberper g with respect to the ground are common fortorsion or multi-link axles. This rolled configurationalso results in deformation of the contact patch.

In summary: when a vehicle is cornering, slipand camber together produce a trapezoidal defor-mation of the tyre’s contact patch, as well as non-homogeneous pressure distributions between thetyre and the ground. Figure 1 shows these distribu-tions for straight-line and cornering conditions.

Optimum ConditionsThe optimum conditions for tyre function, in partic-ular for the transmission of forces between theground and the tyres, are those that maintain ahomogeneous pressure distribution over the wholearea of the contact patch. The mechanism of slipforces will always produce deformation, and there-fore it is necessary to counteract this by applying asymmetrical deformation of the same amplitude.This objective can be achieved if sufficient oppositecamber is applied to each slip force. By this means,a contact patch with the same shape as that foundin straight-line driving is obtained (in the case ofzero static settings), Figure 2.

Tyre PerformanceAs explained above, a tyre’s maximum lateral gripis obtained by combining positive drift and count-er-camber. The effect of the camber reversal can beobserved by comparing measurements of the tyre’spotential longitudinal and lateral forces under ver-tical load and drift, thus representing a bend. Thenegative camber (solid curve) provides a significantincrease in the tyre’s transversal and longitudinalforces. Figure 3 shows that this is true for both theinner and the outer wheels. These gains are espe-cially important in the case of combined brakingand cornering. In low vertical load conditions, forinstance, these gains can provide a greater safetymargin when braking in a curve (ESP or ABS oper-ation) or if the pedal is lifted off during the curve.

The non-homogeneous pressure distribution due tothe trapezoidal shaping also increases tyre wear. Ifpressure homogeneity can be restored for straight-line driving as well as for all lateral accelerationconfigurations, wear can be reduced by between10% and 35%.

Technological SolutionsIn order to benefit from the tyre’s whole potential, itis necessary to have a suspension system that pro-vides counter-camber during cornering that is inrelation to the slip. Current suspension systemsallow for this by acting on two levels:

• by limiting part of the camber linked to roll bymeans of the axle kinematics. Current valuesrange from 0°/° with respect to the ground for arigid axle to 1°/° for „trailing arm“ type axles.Negative values are also conceivable, but thesewould reduce the overall vehicle compromise(bumpy road, driving when fully loaded, etc.).

• by introducing non-zero static camber initial set-tings to the vehicle, to the detriment of wear dur-ing straight-line driving, Figure 4.

The literature also provides examples of suspen-sion system concepts that generate counter-camberwhen cornering:• by using active systems.• by means of kinematics (negative camber under

roll, negative camber linked to steer, etc.).

The Michelin Optimised Contact PatchSystemMichelin develops a passive system that provides anegative camber which is directly linked to the lat-eral force supplied by the tyre when cornering. Theoperating principle is illustrated in Figure 5. Thefirst system is balanced under the action of the lat-eral forces, while the second tilts in the direction ofthe negative camber under the action of a momentaround the pivot point. Michelin is able to imple-ment this system by adding an extra oscillating

Figure 3: Estimation of the gain in lateral force on the outside wheel. Figure 4: Current compromise

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frame linked to the box by two links, thus providinga virtual pivot point below the ground, which isnecessary for the system to function, Figure 6.

This Michelin suspension system provides elasto-kinematic adjustment options that were previouslynot achievable using current systems, Figure 7.

The kinematic operating principle, which offersan extra degree of freedom, not only provides neg-ative camber under lateral force but also allowsindependent adjustment of roll camber take-up andvertical bump camber. This technical solution allowsus to design kinematics that enable zero verticalbump camber (stability, wear, etc.) while maintain-ing cornering camber take-up.

ResultsBy making use of the lateral forces, this passive sys-tem allows the camber to be matched to externalstresses. What is more, this takes place in real time.The tyre’s contact patch remains regular (rectangu-lar shape) at all times, so that the contact patchpressure distribution is homogeneous, thus ensuringmaximum potential. This increased performancepotential can be developed, according to the needsof our automotive customers, in terms of:

• Handling: improved sports performance of thevehicle without increasing wear.

• Safety: increased degrees of freedom of ESP orABS systems without downgrading corneringspeed.

• Wear: improved tyre lifetime without downgrad-ing performance.

• Comfort: improved comfort for the same han-dling, Figure 8.

by Loïc Serra, suspension systems analysis engi-neer and François Andre, suspension systemsdesign engineer, both Michelin SuspensionSystem/France.

Figure 7: An elasto-kinematic tuning field.

Figure 5:MichelinOptimisedContact PatchSystem.

Figure 6: Addingan extra oscillat-ing frame.

Figure 8: Improvements