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December 2007

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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:  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.





Your comments regarding Ackermann, or anti-Ackermann, or perhaps why bother with Ackermann, would be appreciated.  It occurs to me that the more heavily loaded outside front tire must operate at a larger slip angle than is possible for the opposite unloaded front while cornering.  The result is that the lightly loaded tire is pulled sliding across the pavement not doing much work.  I assume it is reasonable to distinguish between the slip angle of a tire which is being deflected by cornering forces and one which is sliding.  Would it be more efficient to use front roll stiffness to completely unload the inside front during hard cornering?  Does Ackermann aid turn in?  Would anti-Ackermann provide an advantage?  Is it possible that toe out in combination with anti-Ackermann might be effective?  Is the turning angle of the front wheels in most hard cornering racing situations so small that Ackermann is not a factor?  What are your thoughts?


For the benefit of newbies, Ackermann effect is a property of steering geometry that causes the front wheels to toe out as steering angle increases.  If the front wheels toe in as steering angle increases, that is called negative Ackermann or anti-Ackermann.  If the toe angle does not vary with steering input, that is zero Ackermann, or parallel steer.


Ackermann effect must not be considered in isolation.  The tires do not know what kind of Ackermann properties the steering system has.  They only know how much they are toed in or out, at a particular instant.  How much the wheels are toed in or out at a particular instant depends on a combination of Ackermann effect and static toe setting.


Additionally, there can be toe changes due to bump steer or compliance steer.  For simplicity, we will disregard these effects here.


The static setting provides a starting point when the steering is centered, and Ackermann effect adds toe-out from there, in a fixed relationship to handwheel (steering wheel) angle.




Trouble is, the optimal toe angle in terms of tire performance is not a constant, nor does it have a fixed relationship to handwheel input.  Unless we are prepared to engineer some sort of elaborately programmed steer-by-wire system that controls the right and left front wheels independently, we cannot obtain optimal geometry for all possible situations.  We are stuck with striking a compromise for a particular set of conditions.


The nature of that compromise depends in part on how much extra slip angle we wish to give the outside front wheel.  One can reasonably argue that the more heavily loaded wheel reaches peak cornering force at a greater slip angle than a more lightly loaded one, so the front tires achieve the greatest peak cornering force when the outside tire has a greater slip angle than the inner one.


The questioner asks whether there is a difference between slip angle of a tire that is sliding and slip angle of a tire that is not sliding.  More precisely, we might talk about a tire that is only partially sliding, in the rear portion of the contact patch, and one where sliding is occurring in the entire contact patch.  There is no difference in the definition of slip angle; it is simply the angular difference between the tire's instantaneous direction of travel and its instantaneous direction of aim: the difference between its bearing and its heading.  However, there is a difference in the effect of adding slip angle in the two cases.  If the tire is below the slip angle where its lateral force peaks, adding slip angle adds lateral force and also adds drag.  If the tire is above the slip angle where its lateral force peaks, adding slip angle does not add cornering force and indeed probably reduces it.  However, up to slip angles associated with total loss of control, drag continues to increase as we add slip angle.


One thing that makes all this a bit complex is that when a tire is near its peak cornering force, lateral force greatly exceeds drag force, yet moderate slip angle changes have a fairly small effect on lateral force, but a relatively large effect on drag force.  This makes it difficult to evaluate the effects of toe changes on cornering capability, purely by observing changes in car balance or amount of understeer.


Drag forces at the tires do not turn the car in the sense of accelerating it laterally, or centripetally (toward the center of the turn), but they do tend to steer the car: accelerate it in yaw, or rotate it.


This means that it is possible to have a case where we are adding moderate amounts of cornering force at the front by increasing outside tire slip angle, yet the steering trace and the driver feedback may show increased understeer, and the car may be slower!  In such a case, we can, at least in theory, dial a bit of oversteer in by juggling tire load distribution, and then we may have a slightly faster car than we started with.  The only way to know is to try this rather than immediately backing up on the toe or Ackermann change.


We have so far been assuming that what we're after is the highest peak cornering force.  We get that if both tires are at their optimum slip angle for lateral force at the same time.  However, one could also make a case for having the tires not peak together, to make breakaway gentler and make the car more driver-friendly.  This is somewhat analogous to the question of whether to tune the engine's


exhaust and induction systems for the same rpm, to get the highest peak power, or tune them for different speeds, to spread the power band.


Is this complicated enough yet?  We're just getting started.


Suppose we have sufficient information to decide what slip angles we want on the two front tires, or what difference we want in their slip angles.  Does that allow us to say what our toe-out or toe-in should be at a particular instant?  Nope.  Without knowing the geometry of the car and the turn, and without knowing what the rear wheels are doing, we can't even get close.


For simplicity, let's suppose we don't want any difference in the inside and outside tires' slip angles.  Let's take a look at what it would take to get that, in various situations.


Some readers will be familiar with the concept of a turn center.  This is the point about which the car's center of mass or c.g. (sometimes approximated as the midpoint of the car's centerline in plan view) is instantaneously revolving, as it negotiates the turn.  If the car has a constant attitude angle that is, if it is drifting or sliding a steady amount all points on the car are moving about the turn center.


At any given instant, the car has an instantaneous direction of travel, which is always a tangent to its path of motion.  If the car is traveling in a curved path, that path has an instantaneous radius r.  This radius is equal to square of the car's instantaneous speed along its instantaneous direction of travel, divided by its centripetal acceleration: r = v2/a.  In a totally unbanked turn, with the tires sliding only a little, these two quantities are approximately equal to the car's speed as read by a speedometer or wheel speed sensor, and the car's lateral acceleration as measured by an on-board accelerometer.  (If the turn is banked, or the car is sliding dramatically, these approximations become much poorer.)


If, in plan or top view, we construct at the c.g. a perpendicular to the car's instantaneous direction of travel, and define a point on that line a distance r from the car's center of mass in the direction of the turn, that point is the turn center.


In the car's frame of reference, the turn center can be anywhere from the rear axle line to well ahead of the front axle line.  (It could even be behind the rear axle line, if the rear wheels have a negative slip angle.  This could occur when negotiating a banked turn at low speed.  Normally we can ignore this case when studying behavior at racing speeds.)


The simplest situation is a small-radius turn, taken so gently that tire slip angles are negligible.  In this case, the turn center is on, or very nearly on, the rear axle line in plan view.  For zero slip angle at both front wheels, the front wheel axes should ideally meet in plan view at the turn center.  That implies that the front wheels will have substantial toe-out.





The larger the turn radius, and the larger the rear wheel slip angle, the further forward the turn center moves.  At some point, the turn center will lie on the front axle line in plan view.  In this situation, we have equal slip angles on the two front wheels when the toe-out is zero.


In high-speed, large-radius turns, the turn center is usually ahead of the front axle line.  We now have a condition where the front wheels need to be toed in for the slip angles to be equal.


As a broad generalization, the front wheels are steered more in small-radius turns than in large ones, and the turn center is further rearward in the car's frame of reference.  This argues for having at least some Ackermann in the geometry, but it is harder to come up with a general rule to calculate exactly how much.


In high-speed turns, steering inputs are generally very small, and consequently Ackermann effect has far less influence than static toe setting.  Ackermann has greatest influence in autocross and hillclimb cars.


Does Ackermann aid turn-in?  Basically, yes, and so does static toe-out, at least up to a point.  Really excessive toe-out, whether from static setting or Ackermann, will hurt front grip and the effect will reverse, but within a sane range, turn-in will at least feel quicker with some toe-out.  This is partly due to the early yaw moment from inside front tire drag as the handwheel is first turned.


Does using anti-Ackermann along with static toe-out make sense?  It's certainly done successfully, on winged single-seaters on high-speed ovals.  Logically, however, it makes more sense to use static toe-in with positive Ackermann.  This is conventional practice in passenger cars.


What about completely unloading the inside front wheel using front roll resistance?  Well, it does make Ackermann academic, at least during the time that the wheel actually is airborne, and it eliminates any concerns about tire drag due to the front tires fighting each other.  Unfortunately, in many cases using that much front roll resistance will create excessive understeer.