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Suspension

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Suspension

If you haven’t read tires and grip yet, please do so before proceeding!

Suspension

If we were to look over the wheels, but below the chassis of the car, we would detect the system of springs and dampers that sits in between. This system carries the weight of the car’s body (referred to as “sprung weight”) and gives the driver a certain amount of “road isolation”, which gives the driver the comfort in not having to feel every bump of the road. When a tire moves at speed over a bump, it is pushed upwards, because some of it’s forward motion has been rendered vertical (upwards acceleration). The goal of a suspension is to minimize that and keep the tire stuck against the road surface.

The above statement only refers to softening bumps on a straight road. The same task has to be performed over both big bumps and during weight transfers. A soft spring might be better in softening bumps, but when a big bump is contacted it would compress too easily. It takes a certain balance to keep the car grippy. Also, springs are tasked with softening the bumps created by the weight transfers (generated by driver’s inputs). Springs tend to create an interesting effect, which is the isolation of the weight transfers working on the wheels, from the load transfers working on the body of the car. When we formerly expressed the weight transfer during braking as the “nose-diving” of the car, we were slightly misleading you: This is the reaction of the springs and the weight they carry, to the weight transfers acting upon the unsprung weight of the wheels.

Are load transfers desired? This is one of the complex questions of car engineering. Softer springs and dampers allow for better road isolation (the car “jolts” less over a bump), but react more dramatically to weight transfers (the car jolts more when braking hard or during sharp cornering). You should know this carries no effect to the amount of weight transferred, but it does change the speed in which weight is transitioned over the car. A faster weight transfer makes for a more “disciplined” and “quick” car, but overdo it and you will get twitchy and treacherous handling. So, like with managing your weight transfers while driving, body “roll” must be used in the correct dosage.

A tire must face a certain net sum of load. The spring cannot decrease it, but it can “share” some of it and make the transition more progressive and smooth. If the car is made to be overly stiff, it would suffer from less “body roll”, but this does not change the amount of load, the load not supported by the spring has to go to the tire, which would have to sustain all too much too soon. So, we don’t want to be “too stiff.”

What about too soft?

The negative impact of body roll is that with large angles, the wheel cannot remain in the similar position. As the body of the car leans aside into a corner or nose-dives forwards when braking, the wheel is forced to change it’s angle relative to the road too. This change is normally reducing the grip levels (making the tire grip the road with it’s less-grippy “shoulder”) and is unwanted. If the geometry, contact patches and wheel-arms are designed to compensate for this, the effect can be reduced, but never removed. Hence, performance cars are indeed stiff in general.

It’s a certain compromise: Stiff means the car bounces over bumps. Not only is this unnerving for passengers, it unsettles the tire contact patches (when it exceeds a certain point). However, when the car is turned, accelerated to braking, it’s body tends to move less (albeit more sharply). And this relates to springs only.

What about the damper?

The damper is made to stop the spring from oscillating (i.e. from moving up and down after decompressed). If you brake hard the front dives down. Then, lift off quickly and the front will push back up. An undampened spring would bounce all the way up, than back down, up, down, and so on. This effect is not wanted but, on the other hand, nor do we want a damper that will break the spring and make the car non-responsive.

Anti-Roll Bars (Sway Bars)

Other suspension components include an anti-roll bar that ties both ends of the front or rear axle to one another and makes them stiffer, but here the trade-off is for suspension independence: The ability of each side of the car to contact bumps without pressing the other side down. Too stiff a sway bar and you might lift a wheel in the air when one of your wheels goes onto a curb mid-corner.

The role of the suspension:

To be comfortable
To keep the tire pressed against the ground when driving over bumps and when making inputs of steering, brakes and throttle.

The parts of the suspension and their role:

Springs:

Soften the bumps on the road;
sustain blows from large bumps without compressing fully;
allow the body of the car to move during weight transfers,
making the weight transfer (and the response of the car) gradual;
Allowing for feedback to the driver through the body movement of the car;
change the vehicle ride height.

Dampers:

Stop the spring from osciliating, which makes it bounce back when depressurized and over again.

Anti-roll bar:

Reduce the leaning of the car (and hence the leaning of the tires) during a corner by locking both sides of the car to one another.

All in all, we want the car stiff to:

a) Avoid the soft spring being thrust by a big bump
b) Avoid the movement of the car’s body, which changes the angle of the tire, when the spring is too soft
c) Avoid the soft damper from letting the spring bounce up and down after depressurized.
d) Avoid the car having a slow response due to soft springs.
e) Avoid the car from having bad feedback, through soft springs.

But, we don’t want it to stiff. We want it as stiff as possible without yield to any of the following:

a) The car bouncing over bumps because of over-stiffened springs.
b) making it be too sharp in response to inputs by over-stiffening the springs.
c) making it irresponsive by over-stiffening the dampers.
d) making it too unstable when only one side of the car goes over a bump, by over-stiffening the anti-roll bar.

A well-balanced race car would:

Would be very grippy, but could use and overcome it’s grip levels sharply.
Not bounce over little bumps, but not feel very smooth over the surface.
Not be dramatically shocked when going over small curbs, but will jolt noticeably.
Not lean too much into a corner or nosedive under braking, but will lean faster
Not necessarily react instantly, but feel responsive and quick
Not bounce back when you lift off of the brakes, but will respond quickly.
Will give you the feel of the car, but might be harder to sense where the limit actually lies

Suspension geometries

Remember me saying how a tread element reaches full distortion when at the bottom of the tire, vertical both to the ground and the car’s body? Well, that is only partially true. Modern suspensions are built so that the wheels are placed in an angle, and this affects the shape of the contact patch. Tilt the tire forward and what have you got? A given tread element would contact the ground later, reach the peak of vertical load (along with longitudinal and lateral loads) faster, and then begin to wind off the ground earlier, giving you more “returning torque,” with a harder steering feel and more feedback. Tilt a tire slightly aside (upside of the tire facing towards the car’s body) and you will get more of the tire’s shoulder to contact the road. These settings are known as Camber, Castor and Toe.

Camber

Camber is the angle in which the contact patch “attacks” the road. In most cars, the tire’s contact patch is facing outwards from the car’s body and the tire’s upper portion is pointing towards it. When the body of the car leans sideways in a corner, it will change the angle of the wheels. When going straight, it would make the tire contact the road somewhat with the “shoulder”, but when you turn and the car “leans”, the tire will also “lean” and the inside tire will be flat against the road.

Castor

Castor is the angle when looking at the tire’s profile. When the tire is leaned backwards (clearly visible in the wheels of a supermarket trolley), it will change the shape of the contact patch and the location of the point where a tread element is under maximum load. This changes the amount of grip supplied by the front of the contact patch versus the amount of aligning torque produced by it’s back. Castor also effects camber: When turning the front wheels, their camber angle changes and this change relates to the amount of castor.

Toe

Toe is the angle the tires point “away from or into” the centerline of the vehicle. When the front tires are angled towards one another, they want to roll towards each and this gives more straight line stability, but make it more resistant to turning aside from that straightforward direction.

Chassis movement

The chassis is a part of the car’s body (in modern cars, an integral part) that, like the tires and springs, also has to endure some loads. Being the main mass or the holder of the car’s mass, most of the force of inertia and centripetal forces (with rotation and other forces that go along with it) works on it. A tire’s grip (and slip) therefore has to “drag” or “push” the chassis during acceleration and “force” it into corners. As these opposing forces take their toll on the chassis, the softer portions of it’s metal begin to deform also.

Like with everything we witnessed before, this movement creates angles that change the angle of the springs and of the tires. This effect is wanted (as it gives less resistance from the body to turn), but only when decreased to a minimum. Chassis bracing is used to give it additional resistance.

Another important thing besides the stiffness of the chassis, is the dynamic coefficient of the car. On an equally long car, it would be best for the wheels to be placed as apart from one another as possible. The closer the wheels are together, the more unnecessary load works unto them, the more unnecessary body roll is being generated by the protruding parts of the the car’s body and the less stable the car is. It can be said that the closer the wheels are together, the more they operate as a single wheel with more load and less grip. This is worthy as long as the car’s body itself does not have to be stretched, much like more downforce is worthy without making the car heavier.

Driver’s issues

This relates to drivability. This is a concept formed by the car’s handling, when combined with it’s feedback. Good drivability is when the car does what the driver wanted in a quick but predictable manner, and gives him the necessary feedback. Drivability, grip and power are what makes a car.

The first thing a driver should learn when driving a car is that creating friction is harder than maintaining it. Hence, if we were to record one’s use of his tires’ grip in a graph, it would have certain “spikes”. When we turn the wheel, the levels of friction (also expressed as grip of slip angles) would increase parallel to amount of turning. When he would get the wheel turned to the necessary degree, the graph would go yet higher and than settle back down to a static position as long as the wheel is still turned in the same angle. As the steering is returned to a straight line, the side force will progressively decline and, upon returning to the straight forward position, there will be slight side-force generated towards the opposite direction (one of the variations of the “pendulum effect”).

The moral here is that being smooth and progressive with your inputs allows for these “spikes” to be minimized. Getting to the limit of grip is often not worth it if reached too quickly. This of course does not mean slow inputs that fail to trigger the necessary response. Decisiveness is crucial when driving. In some sharp and slow turns, the car must be made to change direction faster and “settle” towards an upcoming straight or a successive turn, in which case you might find “quick” steering input, with little “progressiveness” to be better than your normal progressive input. This gets the front wheels turned to the necessary degree before the rear wheels begin to experience a slip angle, resulting in an often delayed response and yet with better handling, allowing the car to get back to a stable state and induce throttle earlier. Accuracy is crucial here, to have the patience (derived from knowledge and experience) to let the car turn without adding more steering (increasing the slip angle), rather than force it in due to eagerness.

One of the means of giving a driver the confidence and knowledge as to what the car is due to do, is seamless feedback and seat time. We have talked about light steering as generally “non-compliant”, and of heavier steering being more “grippy”. This is just that feedback. It is related to a force created in the back of the front contact patches. This is the place where tread elements begin to wind away from the road-surface and the slip angle is thus being “winded” back. Because of this, that portion has has more grip and it wants to return to it’s straight-forward position. This is what gives both the feedback and retracts the steering if we let go of it. The rear slip angle is only felt by the movement of the rear axle (through the seat) or by it’s effect on the front slip angle, making the steering heavy and the “aligning torque” larger. In fact, if the wheel is let go in that phase, it would turn the front tires the other way!

Now, what about speed?

When people learn about grip and vision in performance handling courses, they tend to develop a certain doubt to the effects of speed on safety. These doubts are only half true. As we have seen, grip and adhesion are affected by changes in friction (i.e. by speeding up, slowing down or turning, and not by the speed). A car driving 60mph in a constant speed, would have the same grip levels in 40mph in a constant speed.

So what does speed effect?

Speed effects the ability of the car to accelerate/decelerate and/or change direction. How?

An increase in speed is an increase of the moment of inertia working on the car. This force effects all three elements:

Acceleration:

Torque vanishes with speed. A car can accelerate faster at a slower speed, than it can at a high speed. In a standard manual car, reverse gear gives you the slowest maximum speed, but the highest amount of torque, enabling the car to accelerate faster, carry more load, “feel” faster and more powerful (more weight transfer), handle better uphill, and also cause more engine wear. As such, at a slower speed, the accelerator is capable of generating a greater tractive force onto the tire, making the potential of overcoming a tire’s total grip level, greater. You cannot spin the driven wheels in fifth gear.

Deceleration:

When people need to stop from a high speed (e.g. 60mph, or 90 on the track), they tend to hesitate. The car’s response to the brakes (in the form of weight transfer) would appear to most people as frightening, dangerous and – at a high speed – hazardous. They would therefore, instinctively, apply the brakes weakly to begin with and, as the hazard they try to avoid grows closer, their fear of impact would increase and the pressure on the pedal would increase progressively.

A skilled driver, however, would know that at a higher speed, when the car carries a greater moment of inertia, the brakes work harder and their ability to slow the car down effectively is decreased, not increased. This is sought to be compensated by braking faster and harder at a higher speed, and progressively releasing pressure as speed decreases, and not the other way around.

This human conclusion is not entirely off, because if the tire does slide and does lose lateral stability and seek to rotate, speed will increase this rotation.

Cornering:

With acceleration and deceleration, the forces are longitudinal and work parallel to the moment of inertia, which makes their effect smaller and requires compensating by giving it more gas/brake pressure. With cornering, the cornering force (“lateral acceleration”) works to divert the car from where the moment of inertia is pushing it (straight ahead), so the effect is multiplied by speed.
In this case, we want to make a smaller and smoother input, as speed increases, in order to avoid a response stronger than required, which will overload the tire’s grip.

This knowledge is important because, as people enter a corner too fast, the body of the car is reluctant to turn and this effects the tires. In each cornering scenario, a certain wheel works harderst. Normally, it’s the front inside wheel (it is both tilted into the corner and has the sharpest route). This causes the front axle to slide (have a larger slip angle) and, since this axle has to turn the car into the corner, it results in the car not responding accordingly to the turning of the wheel. The car would thus run wide and understeer (and not oversteer and spin or/roll, as most people think would happen when turning at course speed).

The normal response would be to think of cornering like a longitudinal force. If we drove into a corner too fast, and the speed (inertia) antagonized the car’s ability to turn, we must therefore compensate by turning more. This is not true. The speed increases the centripetal force and makes it use more of the car’s grip and in addition to the turning degree of the front tires, only gives it more force to handle, and less grip.

Again, this instinct is not fully wrong: Understeer is selected as a car’s natural handling, because it restricts itself: It makes the turning arch wider and wider, so eventually the force is decreased and the car grips the road surface again. More steering = more sliding = more understeer = more deceleration = more load being transmitted forward = potentially more grip. This equation is not always true, sometimes the car has exceeded the limit of grip and an addition of steering would only helplessly overload the front tires.

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