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TECHNOLOGY – CONSULTANT


Using inboard brakes, you remove the action of brake torque on the uprights and control arms, with only the retardation forces going through the uprights


With regard to the lateral (y-axis) forces, it is necessary to remember that there are usually tension and compression loads on the upper and lower arms in static condition, just from holding the car up and holding the wheel in position. Ordinarily, in a front suspension the balljoints are inboard of the wheel plane, and the ride spring acts on the lower control arm. That means there is a bending load and a tension load on the lower control arm and a compression load on the upper control arm, when the car is not doing anything but resisting gravity. The loads from cornering or braking are additive to (or subtractive from) the static loads. On the outside wheel, when


cornering, the y-axis ground plane forces will reduce the tension load on the lower arm and the compression load on the upper control arm. There will also be some increase in the normal, or z-axis, force, which will have an opposite effect. As long as the vector sum of the z and y forces has a line of action


that is outboard of the upper balljoint, the lower control arm sees tension and the upper control arm is in compression. When the vector sum line of action is inboard of the upper balljoint but outboard of the lower balljoint, both control arms are in compression. If the vector sum line of action passes inboard


action passes through the lower balljoint, there is no compression or tension load on the upper arm, and there is a compression load on the lower arm.


JACKING COEFFICIENTS But when we are considering jacking coefficients for x and y-axis forces, for purposes of


“the loads from cornering or braking are additive to the static loads”


of the lower balljoint, the lower control arm is in compression and the upper is in tension. When the vector sum line of action passes through the upper balljoint, the lower arm sees neither tension nor compression, and the upper arm sees compression. That is, there is no moment about the upper balljoint to generate a force at the lower balljoint, but there is a moment about the lower balljoint that can generate a force at the upper one. When the vector sum line of


44 www.racecar-engineering.com • May 2012


determining geometric anti-roll and anti-pitch effects, we are concerned with the changes from static conditions. For y-axis forces, for an outside wheel the changes due to ground plane force are always in the compression direction for the lower control arm and in the tension direction for the upper control arm. For an inside wheel, the changes are always in the tension direction on the lower arm and in compression on the upper.


For braking, if the brake is


outboard there will be a rearward force at the lower side view projected control arm and a forward force on the upper side view projected control arm. The rearward force on the lower will be greater than the ground plane force, and the sum of the forces on the upper and lower (which will be subtractive from each other) will equal the ground plane force.


But if the brake is inboard,


there will be rearward forces at both the upper and lower side view projected control arms. The torque of the brake will not act on the upright and the control arms. It will react directly through the caliper and rotor (disc) mounts on the sprung structure. Only the retardation force will act through the upright, and it can be thought of as acting on the upright at hub height. The upper and lower forces will each be less than the ground plane force (and additive to each other). Their sum will still equal the ground plane force.


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