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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: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
FRONT PERCENTAGE AND TURN-IN
Does more front weight help turn-in? Some people I talk to claim it does, but doesn’t more front weight add understeer, therefore making the car less willing to turn?
Turn-in is the most complex aspect of car behavior to analyze. So much is going on at once, and so many things affect it, not least driving style and subjective perception.
We can’t even necessarily say that in all cases we are trying to make the car rotate more readily, or achieve higher yaw acceleration or jerk. On high-speed paved ovals, or when trail braking into a large-radius turn on a road course, it is entirely possible for the car to yaw more readily than the driver wants – to be too free, or loose, or disposed toward oversteer. So we can’t necessarily assume that a car that yaws with more alacrity on turn entry really does have “better” turn-in properties.
However, when people speak of “better” turn-in, usually they mean that the car initiates a cornering maneuver more readily – that it is easy to get the car rotating.
If that is what is meant, we can consider good turn-in as being the greatest possible yaw and lateral acceleration and jerk, in response to a step-steer input (say, the initiation of a test-track J-turn) at constant vehicle speed, taking the wheelbase/track midpoint as the car origin. To maximize this, at least in theory we want the center of mass as far rearward as possible, and we want the polar moment of inertia in yaw about the center of mass to be as small as possible. The reason for the latter should be pretty obvious, but the former may require some elaboration.
In response to an abrupt steering input, the front tires run up slip angle faster than the rears, as the car accelerates in yaw. As yaw velocity builds, so does centripetal velocity, but at a lower rate. As the car settles into steady-state, centripetal acceleration builds to a maximum and holds it, while yaw acceleration falls back to zero (yaw velocity becomes constant). During the first part of this process, the front tires can be thought of as accelerating the car in yaw about the c.g., and at the same time accelerating the c.g. laterally or centripetally about the rear axle. That’s a bit simplistic, because the
rear wheels start building slip angle and cornering force at the same instant as the fronts – just at a slower rate – but it is useful to imagine the front wheels trying to rotate the car about the rear axle. For a given aY at the track/wheelbase origin, the c.g. sees a smaller aY if the c.g. is further aft.
Theoretically, at the other end of a J-turn, we have a step de-steer input, and then the effect reverses: the rear wheels can be imagined trying to generate yaw deceleration about the front axle, while still maintaining centripetal acceleration at the origin. Here, a c.g. further aft should theoretically hurt yaw acceleration.
But this isn’t how people actually drive race cars. In the real world, people usually enter an abrupt turn either on the brakes or using engine braking to toss the car. That means we have influence from brake balance, differential behavior, and driving technique. People also exit under power, with speed building. A real race car enters a turn under combined lateral, yaw, and rearward acceleration, the mixture of which is highly variable.
So if, for example, a driver says the car turns in better with a light fuel load in the rear, is that because the rear tires are closer to lockup as he trailbrakes? Is it because the car’s polar moment of inertia in yaw is reduced? Is it because the static front percentage is increased? All of those changes are occurring at once, but theory would suggest that the first two are producing the effect reported.
If a car turns in more readily when ballast is moved forward, we need to ask: where was the ballast before, and after? Was it far to the rear, and moved closer to center? Or was it near center, and moved away from center, to the front? Was the brake bias changed when the ballast was moved, or not?
Then there is the effect of the differential (or locker, or spool, as the case may be). If the device in the middle of the rear end has a tendency to resist differences in rear wheel speed and make the car go straight, that may in some cases have less effect when there is less rear wheel load. When the front tires make more force, even if it is true that they have more inertia to overcome, their forces may be better able to overcome the understeer-inducing effects of the diff, locker, or spool.
Finally, it is possible that if a car has more front percentage, the suspension may have more geometric roll resistance at the rear, relative to the front, to prevent excessive steady-state understeer. This may in some cases enhance turn-in, as discussed below.
ADJUST ROLL RESISTANCE DISTRIBUTION WITH GEOMETRY OR SPRINGS/BARS?
My newly finished track day car, based on a Porsche 914 chassis, has a front RC height adjustment separate from the ride height adjustment. Track testing is showing me that the car wants a more rearward biased roll stiffness distribution than I had anticipated when selecting the road springs and ARB’s. I could add roll stiffness to the rear but I am considering lowering the front RC as an
alternative. It was during that thought process I remembered that you had addressed the pros and cons of that relationship. Is my memory correct?
I have discussed various things that relate.
One in particular is the possibility of roll center height changes having unintended or unanticipated effects in strut suspensions, especially in cars that have strut suspension only at one end. The 914 has strut suspension in front and semi-trailing arm suspension in the rear.
To lower the front roll center in such a system, without changing the ride height, one has to either raise the ball joints or lower the inboard pickup points for the control arm. Either of these increases the length of the front view projected swing arm, and reduces camber recovery in roll.
In general, strut suspension forces us to have less camber recovery in roll than we would like, just to keep roll center height and steering axis inclination within reason. Adjustable ball joint or pickup point height is useful primarily to correct for the effects of lowering the car for competition, and it does provide some limited freedom in choice of roll center height. However, it cannot provide an escape from the fundamental limitations of the design.
Lowering the front roll center will also increase the amount of roll, absent other changes. If the front suspension has less camber recovery in roll than the rear, and we increase the amount of roll, the poorer cornering camber at the front will tend to dilute the understeer reduction from the change in roll resistance distribution. It may even happen that the effect from camber may outweigh the effect from load transfer distribution, and we may get more understeer rather than less.
A theory that one sometimes hears is that the geometric roll resistance distribution has a particularly large influence on entry characteristics. This is claimed to be due to the fact that the sprung mass takes time to roll, which delays the effect of elastic load transfer, whereas geometric anti-roll moments are present as soon as there is lateral ground plane force at the tires.
Let’s examine that. Two things make the roll displacement take some time: friction and inertia.
The friction consists of (largely unintentional) Coulomb friction in the pivots and sliding elements of the suspension system, and (intentional) viscous friction from action of the fluid in the dampers. The friction force is anti-roll (resists roll) when roll is increasing, does not resist roll when roll velocity is zero, and is pro-roll (acts to maintain roll) when roll is decreasing. The frictional forces will add or reduce wedge according to their front/rear distribution.
The inertia is the sprung mass roll inertia. It acts in opposition to roll acceleration. It can become highly significant in tall, softly sprung, lightly damped vehicles subjected to abrupt inputs. If the car rolls rapidly to some angle, holds that roll angle for a time, then de-rolls, roll velocity:
1. starts at zero,
2. increases to some value outward with respect to the turn,
3. then decreases to zero,
4. stays at zero for a time,
5. then increases inward with respect to the turn,
6. finally decreases again to zero.
Consequently, roll acceleration:
1. starts at zero,
2. is outward with respect to the turn for a while, first increasing and then decreasing,
3. passes through zero and then becomes inward with respect to the turn, again increasing and then decreasing (roll velocity is outward but decreasing, so roll acceleration is inward),
4. stays at zero for a time,
5. is inward with respect to the turn, first increasing and then decreasing,
6. passes through zero and then becomes outward with respect to the turn, again increasing and then decreasing, finally to zero at the conclusion of the maneuver.
During phases 2 and 3 above, the frictional forces act against roll. Since they occur within the suspension system, they must react through the tire contact patches, and they contribute to lateral load transfer in proportion to their front/rear distribution. More rear damping dynamically de-wedges the car and adds oversteer. The frictional forces act in parallel with the front and rear elastic and geometric anti-roll components. Therefore, their influence depends on their comparative magnitude, relative to the elastic and geometric moments.
During phases 5 and 6 above, the frictional forces act against de-roll: they act to maintain the roll displacement, and are thus pro-roll forces. More rear adds dynamic wedge, and adds understeer.
During phase 2, roll inertia is anti-roll in direction. During phase 3, it’s pro-roll. During phase 5, it’s pro-roll (anti-de-roll). During phase 6, it’s anti-roll (pro-de-roll).
The contention that geometric anti-roll has a disproportionate effect on entry behavior is based on the idea that the geometric forces are directly related to ground plane lateral force and therefore are not diminished by the anti-roll effect of roll inertia during phase 2.
I think that makes some sense, qualitatively. But then what about the effects of roll inertia in phases 3, 5, and 6? And how big is the effect really, in race cars as actually driven on road courses or ovals?
In a tall, softly sprung, lightly damped sedan or truck, undergoing a J-turn test at the test track, we might see the effect of roll inertia as a reduction of roll in phase 2, followed by an exaggeration of roll as the vehicle approaches steady-state cornering. There should in fact be a degree of roll overshoot and subsequent de-roll as the vehicle approaches steady state. With really light damping, the vehicle might exhibit roll oscillation as the driver tries to get the vehicle to steady-state cornering.
But do we see roll overshoot during late entry in race cars, ordinarily? Do we see noticeably higher roll velocities during late entry than during early entry? Not usually.
Race cars have relatively small roll displacements and velocities, and relatively stiff damping. The main thing that slows roll displacement during entry in a race car is damping, not roll inertia.
So, what of the effect of lowering the front roll center, versus adding rear anti-roll bar? For identical steady-state understeer gradient, will either option produce noticeably freer entry in a road race car? Probably not – but the lower front roll center will produce larger roll displacements and velocities, and will therefore make the car more responsive to damper settings than adding rear bar.
And, returning to the first question, if greater front percentage is accompanied by more geometric roll resistance at the rear (e.g. higher Panhard bar), will the car turn in more readily? Maybe, if the rear is damped less in roll than the front, which is common with beam axle rear ends.