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What conditions should we
design camber recovery for?
It may seem simple to say roll at maximum anticipated g but usually we are just off the brakes at this point and the chassis will not have fully returned to normal ride height.
I am also confused that some experts recommend max roll plus 1" bump on the loaded wheel - gives great results for camber recovery but is it really relevant? - I am struggling to see the justification unless there just so happens to be a bump at a fortuitous point!.
Is there perhaps a sequence of chassis attitudes we can consider as the car progresses through the corner?
With independent suspension, for all applications other than the outside wheels in a highly banked turn, we can't get the camber recovery we want. We get what we can, without producing excessive camber change in conditions other than roll.
Some definition of terms is in order.
Camber is a tire or wheel's lateral inclination relative to the road surface, toward or away from the vehicle's longitudinal centerline, often denoted by φ (phi). Camber is conventionally positive when the top of the tire tilts outboard.
Inclination is a tire or wheel's lateral inclination relative to the road surface and to direction of travel, often denoted by γ (gamma). Inclination is conventionally positive when the top of the tire tilts to the right, as seen from behind. We may also speak of inward or outward inclination. Inward is toward the turn center. Sometimes inward inclination is taken as positive, which makes some sense, although it conflicts with right-positive sign convention if we are turning left.
Camber gain is the rate of camber change with respect to suspension displacement, as measured by moving the suspension, usually with a jack, with the car stationary on the shop floor. This quantity can be an instantaneous rate of change at a given point in the travel range, or it can be taken as the change over a chosen interval of displacement, such as the first inch of compression from static position. Camber gain is generally taken to be positive when the camber change is toward negative as the suspension compresses.
Camber gain is directly related to front view swing arm length. For a front view swing arm length of 180/π inches, or approximately 57.3 inches, camber gain is one degree per inch. Camber gain is inversely proportional to swing arm length. If swing arm length is half of 57.3", or 28.65", camber gain is 2 degrees per inch. If swing arm length is twice 57.3", or 114.6", camber gain is 0.5 degrees per inch.
Camber recovery refers to the suspension's ability to make the wheel lean less than the car leans, when cornering. Most commonly, camber recovery is expressed as a percentage, usually the difference between camber change and roll, divided by roll, times 100%. When discussing this purely in terms of suspension geometry, it is common to ignore deflection effects in the tires and other parts.
So if the geometry is such that camber changes at half the rate that roll displacement changes, that's 50% camber recovery. If camber doesn't change at all with roll, that's 100% camber recovery. If the wheel actually leans into the turn as the body rolls outward, that's more than 100% camber recovery. If the wheel leans the same as the body, that's zero camber recovery. If the wheel leans more than the body, that's negative camber recovery.
It is most common to consider camber recovery in pure roll, but it is also possible to look at a combination of roll and ride, and calculate camber recovery for that condition.
If we didn't have to worry about any other effects, ideally we would like the tires to be upright when running straight, and lean into the turns a degree or two when the car rolls in cornering. We would like the inward inclination to be sufficient to compensate for roll due to tire deflection, plus another degree or two. We can have no camber recovery for the tire deflection component of roll, because it involves no suspension motion. For example, a go-kart has no suspension other than tire and frame deflection, but it rolls a little bit on its tires. The wheel inclination always changes in an outward direction with this roll, by an amount equal to the roll angle. There is a similar tire deflection component to roll and camber change when we do have suspension.
The tire makes best cornering force with some inward inclination, so we would actually like well over 100% camber recovery.
But we can't get that, or anything close. To get even 100% camber recovery, the front view swing arm length needs to be half the track width. For a 57.3" track width, this implies that in any
condition other than roll, camber will change 2 degrees per inch of suspension displacement. That's basically the camber change rate of a swing axle suspension. To get 150% camber recovery,
the front-view swing arm length needs to be 1/3 of the track width. The camber change rate then would be 3 degrees per inch. Camber changes, as the car brakes or goes over humps, dips, and bumps, are unacceptably large when the front view swing arm length is that short.
So the game is not one of getting ideal camber recovery and letting other properties fall where they may. Rather, we have to strike a compromise between camber control in roll and camber control in other situations. We have to decide how much camber change we are willing to tolerate in ride – or in pitch, heave, and warp – to reduce camber change in roll. With long swing arms, we have poor camber control in roll, but good camber control in the other modes. This produces a forgiving car, which also stops well and puts power down well in a straight line. This comes at a cost in ultimate cornering power.
There is one way to get better camber control in all modes at once: widen the track, and increase the swing arm length by a smaller percentage than the percent increase in the track. Of course, there are penalties in other areas when we do that. In racing, we will generally be constrained by either a track width limit or an overall width limit in the rules. In autocross and hillclimbing, we may not
have a width limit in the rules, but we will want a reasonably narrow car due to the tight confines of the course.
Independent suspensions on banked ovals present a special case. There is so much "dive" or ride compression, that it is actually fairly easy to get more than 100% camber recovery on the outside wheel when the ride motion is included. Quite often, the inside front suspension actually compresses, meaning that to get camber recovery there, we need an instant center to the left of the car, and negative camber gain.
Current NASCAR setups are a further special case of the special case. The suspensions are designed to create maximum downward jacking, with little regard for camber recovery. The front end is intended to jack down to the bump rubbers on the first turn, and be held down on the rubbers even on the straightaways for the rest of the run, by a combination of aerodynamics and shock valving. The suspension is very nearly immobilized when the car is at speed, so camber recovery is not much of an issue. The static camber is set to whatever provides desired running camber once the front end is on the rubbers.
The reason this approach works is that the cars run at high air speeds, have splitters that work best when they are near the road, and yet have to drive over a barrier to get through tech. If it weren't for the ground clearance rule, the teams could just set the car at whatever height they wanted statically.
The cars often actually have negative camber gain on the right front wheel: front view instant center to the right of the car. The idea is to get a force line sloping below ground toward the middle of the car, to maximize downward jacking with lateral force. This could theoretically also be done with an instant center to the left of the wheel and below ground, but I guess this is hard to do with the
spindles the rules require. The left front has an instant center to the right of the wheel. The idea there is to get the smallest possible spring-to-wheel motion ratio, again to try to get that corner of the car down to the bump rubber and keep it there. It would be possible to just use a softer spring, or make the motion ratio at the lower control arm smaller, but again the rules get in the way.
So we end up with a front end whose camber control characteristics are diametrically opposite to what we'd want if the suspension worked normally at speed, just to work the peculiar rules. This illustrates what profound and counter-intuitive effects can arise from seemingly simple rules.
It's also a classic case of
"any suspension will work if you don't let it."