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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.
WHY NEGATIVE CAMBER ONLY ON THE FRONT WHEELS, OR MORE ON THE FRONTS THAN ON THE REARS, ON SOME CARS?
I would like to ask you about negative camber. I have an MG Midget that runs on 8in wide slicks. The ones used before (crossplies) are no longer available and some that fit are radials. I have been told that if I run radials I will need more negative camber. I notice that most classes from Australian V8s to F1 have a lot of negative camber on the front. An RX7 owner I know runs about -7deg for road racing. I also note that the negative is only on the front; the backs run flat or minimal negativeve. If this is required to get cornering grip with radials, why is it not also applied to the back. As the backs always have (almost) 0 camber should this not give severe oversteer as
the front would have much more grip than the back? How is it that the backs can grip with minimal negative but apparently the fronts can't?
In a fair number of cases, the reason for not having negative camber on the rear wheels, or having very little, is simply that it's not mechanically possible to run much negative on the rears because the rear of the car has beam axle suspension.
If the car does not have a full-floater axle (rear wheels mount to hubs that turn on bearings running on snouts at the ends of the housing tubes; axle shafts have splines on both inboard and outboard end), but rather a conventional style axle (wheel attaches to a flange on the end of the axle shaft; axle shaft has splines only at the inboard end), we can only get a small amount of negative camber. We have to bend the middle of the axle housing down to do this, and we are limited by friction and wear at the splines, and in some cases by inability to insert the axle into the diff at all.
With a full-floater axle, we can do this and can also typically get half to three quarters of a degree at the outboard ends, with ordinary straight splines. Additionally, where legal, we can often obtain special axles with barrel-shaped splines, and special cambered snouts. But even with this hardware, it is uncommon to see more than about two degrees.
With independent rear suspension, it is, at least in theory, easy to get all the negative camber we want, although in production cars we may be limited by the adjustment range afforded by the stock hardware.
Negative camber is a mixed blessing, with any tire. For cornering, what we really want is some amount of inclination into the turn, meaning negative camber on the outside wheel and positive camber on the inside one. Exactly how much we want depends on the tire, the rim width, the inflation pressure, the grippiness of the road surface, and the normal force on the tire. The reason negative camber on both sides of the car is helpful is that, up to a point, it's worth giving up some cornering power on the inside tire to get an increase on the outside tire, since the outside tire is more heavily loaded and consequently more important: the gain on the outside tire is greater than the loss on the inside one. Ordinarily, with passive suspension we can't get greater than 100% camber recovery in roll, or even anything approaching 100%, so we sacrifice inside tire inclination to improve outside tire inclination.
The more load transfer we have, the greater the gain on the outside tire becomes, relative to the loss on the inside tire. Taking an extreme case, if there is no load at all on the inside tire, its camber doesn't matter at all.
But of course we don't just want lateral force from our tires – we need them to make longitudinal force as well. And for that, we want them straight up: zero camber. So camber settings are always a compromise between lateral grip and longitudinal grip. At the front, running a lot of negative camber will help front cornering power up to a point, but this will come at the expense of braking ability. At the rear, both braking and propulsion will be adversely affected (assuming rear wheel drive).
When we see really extreme static camber settings, that usually means the suspension's camber control properties are less than optimal. All Mazda RX7's have strut front ends, which have very poor camber recovery in roll when lowered for racing. The first version of the RX7 has a beam axle rear end, with flanged axles, so it isn't mechanically possible to get a lot of negative camber at the rear, but the camber recovery in roll is much better than at the front. Later ones have a form of semi-trailing arm independent that can be set with quite a lot of static negative camber, but has better camber recovery in roll than the front suspension has.
When the front end has much poorer camber control in roll than the rear has, we can't necessarily assume that front wheel inclination is more favorable than rear when cornering, even on the outside front wheel, just from the static camber. It may be, or it may not be. And if the front end does have more favorable cornering camber than the rear, it does not necessarily follow that we will have oversteer. The camber does create a tendency toward oversteer in such a case, compared to a different camber picture, but other factors also enter in. Those factors include front/rear roll resistance distribution, front/rear weight distribution, and the front/rear tire size relationship.
If we do manage to achieve better cornering camber at the front than at the rear, that means we can use more spring and/or anti-roll bar at the front, and get balanced cornering with slightly greater limit adhesion, plus better ability to put power down while cornering, due to greater inside rear wheel loading. So we usually want to set the front camber for best front cornering power, provided we don't hurt braking too much. If the car is too loose, we just add front spring and/or bar. We could balance the cornering by taking front camber out instead, but that results in lower limit lateral acceleration, and poorer ability to put power down on exit.
With a spridget, we at least have a front end that benefits from being lowered, in terms of camber recovery in roll, rather than deteriorating with lowering as a strut suspension does. With radials or bias-plies, give the front end as much negative camber as will improve front cornering grip, tempered by attention to braking, and then add front roll resistance as needed to eliminate any oversteer.
ROLL CENTER/FRONT VIEW GEOMETRY WITH DUAL BALL JOINTS
On cars with double ball joint strut suspension (Hyundai Genesis Coupe, Pontiac G8) I understand that the steering axis operates through the 'virtual' joint at the intersection of the two control arms when viewed from above. How do you calculate the roll centre position for a suspension like this? Do we use the same 'virtual' joint in the same way as regular strut suspension? Or is there some other point?
With any strut suspension, there is a virtual upper control arm plane that is perpendicular to the strut axis. The strut axis is often not the same as the steering axis, even with single ball joints. Often, the ball joint lies somewhat outboard of the strut axis. The steering axis is then a line through the ball joint center of rotation and the upper strut mount center of rotation, and the steering axis inclination is greater than the strut inclination.
With dual ball joints, the virtual ball joint can be even further outboard, but that only affects the steering axis, not the strut axis, and not the virtual upper control arm plane.
If the two lower links are in a common plane, or very nearly so, that common plane can be taken as the lower control arm plane. The front-view projected control arms are then the lines where the upper and lower control arm planes intersect the front axle plane (vertical, transverse plane containing the front wheel centers). The front-view instant centers are the points in that axle plane where the upper and lower front-view projected control arms meet.
The front-view force lines are then definable as lines from the contact patch center to the instant center. The slopes of these lines, in combination with the ground plane forces at the contact patches, determine the linkage-induced support forces in the system, and in turn the anti-roll or pro-roll moment produced by these, and accordingly the roll center height.
The front-view force lines are always instantaneous perpendiculars to the front-view projected motion paths of the contact patch centers. The motion path's instantaneous inclination from vertical, or the dy/dz of the contact patch as the suspension moves, is the same as the instantaneous slope from horizontal, or dz/dy, of the force line.
It is possible to create multi-link
suspensions that do not have pairs of links in common planes, or even close to
common planes. In such cases, it becomes problematic to try to define
projected control arms. It may only be possible to find the motion path of the
contact patch with a 3-D computer program. But even then, the front-view and
side-view projected force lines are instantaneous perpendiculars to the contact
patch motion paths.