The Fundamentals of Suspension

Written by Mike M in FeEb 2009 - this is agreat overview of the need for suspension and the basics of how and why moderm motor car suspension works.
Thanks Mike.

This is all about conservation of energy in this case mainly kinetic energy. A subject I found interesting in science at school. This is my thought on how it all fits together.

Inertia is an objects general dislike of moving, it wants to stay inert. It takes a force to get it moving and the amount it moves for a given force will be dependent on its mass. Remember the old saying “for every force these is an equal and opposite”. Once it’s moving it has momentum. Momentum and inertia work hand in hand. We’ve put energy in to moving something it’s going to want to keep going in that direction. In a vacuum the object would keep going but a car has a bunch of forces working against its momentum, friction of the road surface and wind resistance etc.

The spring’s main job is to keep the wheel on the road. A car is heavy and has a lot of inertia so it’s not going to want to vary from the direction its going. That’s not just left and right but up and down. Imagine a car with no suspension. You go over a bump and this is going to try and lift the whole car, working against its inertia. The road is not going to move down because it’s got a hell of a lot more mass than the car so the car has to go up. Its forward momentum has changed into an upward direction. That upward momentum is now working against gravity (another force) pushing it back down. All this time the wheel has lifted off the road along with the car and we’ve used up a lot of our energy bouncing up and down rather than going forward.

This is all very inefficient as it takes a lot of energy to move something with a lot of mass. So how about moving something with a lot less mass, just the wheel and what connects it to the chassis. By connecting the wheel to the chassis via a spring we now only have to move the mass of the wheel up and down. Springs are cool in that they store kinetic energy. So when the bump in the road pushes the wheel up the force that the car applies to that bump would have lifted car is now stored in the spring. The wheel stays in contact with the road and the car with all its mass and momentum keeps moving in the direction it wants to.

Unfortunately to complicate all this we like to change direction and accelerate and brake. When we do these things we are putting a bunch of other forces into the equation. A car has mass and there is a middle point to this mass that’s called the center of gravity. This is the point around which the car would balance if we could poke a stick through it. The position of the COG is dependent on the shape of the car and where the weight it.

When you push something along the point where you apply the force is important. Imagine pushing a heavy fridge, if you pushed it at the top it would tend to want to fall forward. If you pushed it at the bottom it’ll want to tip back on you. We’re applying the force above or below the center of gravity and its going to want to rotate around that center. So in a car all of our forces going into making the car turn accelerate or stop are applied where the wheel meets the road. This point is below the center of gravity. When we accelerate it’s like pushing the bottom of that fridge, the car will want to tip back and put its weight on the back wheels. When we brake it tips forward putting the weight on the front, the same goes for turning left and right.

So a car with a spring on each corner suddenly has a varying amount of mass working against each it. A spring that keeps the car at ride height for a given weight is suddenly subjected to more weight so it’s going to compress. As that corner of the car drops it moves the mass even further as the car rotates around its center of gravity. The sway bar helps us in corners because it lets the spring on the opposite side of the car assist the one under load. It’s a lever to transfer some of the load across to the other spring. The Sway bar has some spring itself so the thickness and amount of leverage will determine how much load is transferred.

The amount of mechanical grip a tyre has in a corner is dependent on how much weight there is on it. If we take a big proportion of our cars weight and put it all on one side of the car the other tyres aren’t going to have the same level of grip and we will slide. The sway bar lets us transfer some of the cars weight to the opposite side of the car. A car with a low center of gravity isn’t going to transfer as much weight as a car with a high center of gravity (remember that pushing a fridge analogy). I think this is why sway bars aren’t such a big factor in a Cobra as we aren’t transferring the same weight as say a Commodore. However Sway bars are still a useful tuning aid that will let us dial in how much understeer or oversteer a car has. We don’t need such a big heavy bar as we aren’t transferring as much weight as a 1600KG sedan with a higher COG.

The job of the shock absorber is to stop the spring oscillating as the forces move in and out of it. We want the spring to absorb the bump and return the energy, keeping the wheel on the road. Unfortunately the spring itself has mass along with the wheel and suspension components so they are subject to inertia and momentum. The shock damps out the forces these apply to the spring. Remember the equal and opposite statement, the shock gives us a way to tune the force the wheel has on the spring by applying some resistance to movement. The amount of resistance we need to apply depends on how heavy the spring is. The shock also lets us tune the speed the suspension reacts to a bump. By stiffening up the damping the wheel movement has to overcome the shocks resistance to movement. In sophisticated motorcycle shocks they have low speed and high speed damping settings to help fine tune this. Low speed damping is the resistance to small movements where high speed damping resists large movements. Small ripples in the road surface can be damped differently to a big dive inn the front when the rider puts the brakes on.

Tuning weight transfer left to right is pretty straight forward. We can hook the left and right side together with sway bars. This is a bit more difficult to do with front to rear weight transfer. The only real tuning aid we have here is the choice of spring weights and damping adjustments. Fortunately our shocks have some neat tools to help with this. Compression and rebound damping control how much resistance the shock has when the spring compresses and relaxes. By dialing up one or the other we can let the car squat on the rear when accelerating to add more weight to the rear tyres or stiffen up in the front and reduce the initial dive when we jump on the brake pedal. The AVO shocks we mostly use have a single adjustment for both. Double adjustables let you set the compression damping separate to the rebound so you can fine tune how the car transfers weight.

So which spring or sway bar? Well a tracks surface is pretty smooth so you can sacrifice ride comfort for stiffer springs which will reduce the amount of rotation around that COG. You don’t want them so stiff though that the shock cant damp them correctly or the spring isn’t soaking up the bumps and keeping the tyre on the road. Some weight transfer or chassis rotation is desirable though. If you can put more weight on the rear tyres as you accelerate you will have more grip. However since all the systems interwork you cant have your cake and eat it too. Getting the suspension right is a compromise and it’s all about finding a balance that suits your driving style.



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