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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
MORE ON REACTIVE ANTI-DIVE
Last month we considered the Lotus reactive anti-dive or ride height modification system that was recently outlawed in F1. I have been giving more thought to the details of applying this idea.
I have decided that Lotus made a fundamental mistake in putting the slave cylinder in series with the ride spring, by placing it in the suspension pushrod. For a reactive system to work in a beneficial manner, it needs to act in parallel with the spring, as conventional anti-dive does.
This might be done by arranging for a slave cylinder to act on the rocker in parallel with the main coilover or torsion bar. With an inboard brake, it could be done by having mechanical linkage to the rocker from the caliper floater.
When the slave cylinder is in the pushrod, or otherwise in series with the ride spring, the slave cylinder will not extend until there is sufficient braking to overcome the load on the pushrod or whatever the slave cylinder acts on. This means the system generates no anti-dive on light brake application, and then has to generate a lot of anti-dive on heavy brake application if it is to have a useful effect on ride height. If anything, it would be better to have the system generate lots of anti-dive on light brake application, and then not so much in the range of brake force approaching wheel lockup. At a minimum, we’d like the behavior to be roughly linear.
It might be possible to add preload springing to the system so that the slave cylinder was preloaded to the point where it was just short of extending at static condition. But then the preload would not have similar effect when pushrod loading was something other than the static value. When greater load was on the pushrod, we would still get anti-dive effect only beyond some braking force threshold. When lesser force was on the pushrod, we would have an undamped spring in series with the main ride spring.
If the mechanism acts in parallel with the ride spring, we can have it affect ride height from the smallest amounts of braking force. We can also create nonlinearities with preload or limiting springs, but we will inevitably have those acting in non-braking situations as well.
So to get the best from this concept, not only should we make it act in parallel with the ride spring, but we should also be wary of unintended consequences and avoid getting carried away by the temptation to introduce clever nonlinearities.
Even within those constraints, the concept still offers the possibility of obtaining anti-dive while letting the contact patch move rearward somewhat in compression when not braking, thereby improving the suspension’s ability to ride bumps without the usual penalties. It would be possible to arrange for the slave cylinder or perhaps a mechanical linkage to act as, or in series with, a third spring rather than individual wheel ride springs. This would mean that side-to-side differences in anti-dive force generated by the brake-reactive system would not affect dynamic diagonal percentage.
If the system is arranged so it acts in parallel with the ride spring, and so that it creates anti-dive for all levels of braking force, that means that the caliper bracket will always rotate whenever the suspension moves, although it may be possible to make the relationship somewhat nonlinear with respect to suspension displacement. This motion of the caliper will create some additional unsprung mass inertia, which is generally not desirable, but the effect will generally be small. The added inertia will be present whether the brake is inboard or outboard.
This also means the system will cause the contact patch to move forward in compression when the suspension is cycled with the brake locked on a K&C rig, and the rate of forward displacement with respect to vertical displacement will equate to the jacking coefficient in the usual manner. However, it still will not be possible to infer the amount of overall anti-dive purely from the suspension geometry. It will be possible to have a side view instant center that would normally create pro-dive, and still have some net anti-dive from the system as a whole.
Furthermore, when the reactive system acts in parallel with the spring, it does not have to generate forces in the same direction as the spring. It can generate a downward jacking force just as easily. That means we could have brake reactive anti-lift at the rear.
It would even be possible in some cases to have drive torque reactive anti-squat and anti-lift with independent suspension. This would entail mounting the final drive so it can rotate, and using that rotation to energize some sort of system acting in parallel with the springing.
This might not be particularly useful at the rear of the car, however, because it is possible to get anti-lift and anti-squat the conventional way by having the contact patch move rearward a bit in compression, unlike the front of the car, where it ordinarily has to move forward. Reactive anti-lift under power at the front might have some potential, when the front wheels are driven.
FORCES ON CONTROL ARMS WHEN CORNERING
In a double A arm suspension are the forces opposite for the upper A arm vs. the lower A arm? Example: in a left hand turn is the force for the right front A arms inward on the lower and outward on the upper? I hope I made sense because I really want to know. I have watched “Minding Your
Anti” [my video of a lecture I gave at UNC Charlotte in 2003, still available on DVD for US$50] a dozen times. It has helped me immensely. I just want to make sure I understand the forces correctly.
For this discussion, we will assume that the suspension is of a type that has an upper control arm and a lower control arm, or can be approximated as such for modeling purposes. We will assume that the lower control arm plane intersects the wheel plane below the wheel axis and the upper control arm plane intersects the wheel plane above the wheel axis. We will assume that there are no drop gears in the uprights.
If we are talking about the forces on the control arms induced by ground plane forces (longitudinal and lateral forces at the contact patch, the forces that create geometric anti-roll and anti-pitch effects), for the most part the answer is yes, the forces induced in the upper and lower control arms are opposite in direction. The main exception would be the case of longitudinal forces from braking or propulsion, where the torque is applied to the wheel through a jointed shaft and only the thrust acts through the suspension linkage – that is, propulsion with a sprung final drive (independent or DeDion suspension) or braking with inboard brakes.
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 ball joints 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 ball joint, 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 ball joint but outboard of the lower ball joint, both control arms are in compression. If the vector sum line of action passes inboard of the lower ball joint, the lower control arm is in compression and the upper is in tension.
When the vector sum line of action passes through the upper ball joint, the lower arm sees neither tension nor compression, and the upper arm sees compression. That is, there is no moment about the upper ball joint to generate a force at the lower ball joint, but there is a moment about the lower ball joint that can generate a force at the upper one. When the vector sum line of action passes through
the lower ball joint, there is no compression or tension load on the upper arm, and there is a compression load on the lower arm.
But when we are considering jacking coefficients for x and y axis forces, for purposes of 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 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 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.