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Suspension Tuning

Suspension Tuning

Suspension Tuning is a critical part of getting the ultimate performance package, it is even possible to have a more competitive car then a rival with superior upgrades, by having a well balanced set up. It all comes done to extracting the best from what components you have and making sure they are well set up for the road or track conditions. 

Anti-Roll/Sway Bars Tuning

Anti-Roll /Sway Bars as discussed in the Suspension Upgrades page,  can provide adjustable settings in the suspension set up. Especially useful in dialing out oversteer, or understeer handling characteristics and getting a better balanced car. They are the same effect as changing the springs, but their effects are only used in lateral cornering forces.

The reason this is so useful, is that you can make adjustment to front and rear independently with out affecting other suspension settings. Stiffer settings will reduce body roll, while softer settings will increase body roll. The relationship between the front and rear settings ( roll coupling), will also have a direct affect on the handling of the car. Like most suspension adjustments, it is best to take small incremental adjustments rather then going to one extreme to the other.

It is critical that the right Anti-Roll/Sway Bar is also selected to complement the other suspension components. I would suggest seeking profession advice on the overall suspension set as a whole and not just individual upgrades, when considering suspension adjustments.

 It is generally accepted that it is better to reduce the settings, rather then increase them to get a better balanced car. Softer settings will make weight transfer more gradual, with less abrupt loading, bending into corners rather then darting into them. Great care needs to be taken with too soft a setting as well, if the car has a low center of gravity, a soft setting could result in the car bottoming out. Also camber settings ranges could be affected with a soft setting, where the tyre/ tire exceeds the optimum set up.

Having too stiff a setting could result in poor handling in tight corners, with the inside wheel lifting off the ground. Also if either of the two wheels linked on the axle are on different road surfaces (one wheel on track, other on the side of the track), having a stiff set up will result in imbalances being transmitted through the Anti-Roll/ Sway Bar to the other driven wheel.

  • Understeer: reduce front or increase rear anti-roll/ sway bar settings.
  • Oversteer: increase front or reduce rear anti-roll/ sway bar settings.

 

Camber Angle

The Camber angle settings of the suspension can come in three different variations (vertically viewed from the front of the car), Positive, Neutral and Negative Camber. It is normally represented in degrees.

While it is true that a car's suspension will most likely pass through various degrees of all of them in the normal operation of the vehicle (depending on set up and design), having a good base in the beginning will be vital in having a competitive package for a given goal.

Negative Camber: increases the cornering grip of the tyre/ tire during cornering, helping to maximise grip and providing  better traction. Too much Negative Camber will increase the  inside tyres/tires wear and could result in handling imbalances.

Neutral  Camber: best suited for maximum acceleration and braking, this set up makes sure a flat contact patch is retained on flat road surfaces. The inside wheels may lift on the inner contact edge of the tyre/ tire duration extreme cornering.

Positive Camber: used more for off road applications, especially with agricultural vehicles as this setting helps the wheels to turn with lighter steering effort required.The outside wheel under extreme cornering loads will benefit, but camber levels are normally linked ( might be a consideration for oval tracks).

While double wishbone suspension is normally designed for camber angle adjustment, macpherson suspension might require aftermarket suspension mounts with various slots for adjustments. 

Camber Angle Diagram

Caster Angle

The Caster angle is the ability of the front suspension system to self center under cornering loads. Too much caster and the front of the car will understeer more ( positive caster), too little and you will get oversteer handling characteristics (negative caster).

Improper adjustment will result in steering inputs required both into and out of a corners, resulting in a car which is difficult to keep on a straight line.  A large positive camber setting (wheel facing forward of axis) is good for high speed stability but can make it more difficult for turning the steering, excessive amounts will increase tyre/tire wear. 

Caster Angle Diagram

Center of Gravity and Roll Center

The center of gravity can be seen as the balancing point of the car, if you hung a car from a cable above the ground it would be balanced perfectly. It can be hung from any dimension and would remain balanced, it can be viewed as the center point of where the majority of the mass of the car is located and is a 3-D location across different axis's.

It is critical to understand that all acceleration, braking and lateral g-forces go through this point and the whole suspension system is set up and designed around this point. Although in reality we have little adjustments of the center of gravity (unless we design a race car ourselves), understanding it's importance and effects is import on fine tuning suspension set ups. 

Generally speaking we want to have a car with a center of gravity as low to the ground as possible, this helps reduce weight transfer loads acting on the roll center. Less weight transfer downloads through high suspension loads, will result in better tyre/tire performance. This is because as the load increases on tyres/ tires, their ability of efficiently converting the vertical download decreases (coefficient of friction). The longer the wheelbase and wider the track of the car, in relation to the center of gravity, the less the weight transfer loads.

This also affects the cars dynamic roll centers, as the center of gravity acts as a lever on this point. The roll center is a point at which the suspension is designed to rotate around, this point changes fore and aft with acceleration and braking forces, also horizontally with left and right lateral forces in the opposite direction. The further the roll center is away from the center of gravity, the more roll or weight transfer happens, getting a balanced suspension set up which handles predictable past the limit is the ideal set up. 

Roll centers can be different both at the front and rear of the car, as the independent suspension components operate separately during different loading phases. It is important to reduce weight transfer through roll centers, because overloading a outside tyre/tire under extreme cornering for example, will demand more from the component then is available and reduce it's traction grip (see traction circle). They are determined by the suspension geometry, including length upper and lower arms, track width, chassis pick up points, tyre/ tire contact patch and other chassis settings. 

Center of Gravity and Roll Center Diagram

Damping/ Bump and Rebound

Dampers work in conjunction with springs to form the basis for car suspension, they are sometimes incorrectly referred to as shock absorbers. If a car was only fitted with spring and not dampers, then any movement in the cars suspension in the vertical plane, would effectively keep bouncing up and down until the kinetic energy is displaced. This would make the car very difficult to drive from a performance point of view, as the geometry would be constantly changing.

Dampers effective help to dissipate any vertical movement in the suspension and keep the springs movements controlled, also they help the wheel keep full motion under different loads, while keeping the wheel in contact with the ground.

  • 1-Way adjustable Suspension: rebound adjustable only.
  • 2-Way adjustable Suspensionbump and rebound adjustable.
  • 4-Way adjustable Suspensionhigh and low speed bump setting , high and low speed rebound settings (high speed for bumps, low speed for corners and braking).

It is possible to adjust the dampers to give different hydraulic resistance to fine tune the handling of the suspension for given conditions, the movement is broken down into bump (compression) and rebound (extension) motions. Dampers don't control load levels, but affect the speed of how fast the suspensions reacts to load changes and how fast the tyre/ tires contact patch receives these loads.

  • Stiffer front damping (bump and rebound), increases grip at the rear of the car, increasing understeer characteristics.
  • Stiffer rear damping (bump and rebound), increases grip at the front of the car, increasing oversteer characteristics.

 

Bump and Rebound Diagram

Bump

During bump, the dampers and springs absorb the upward movement from cornering or road irregularities (the springs store some of it), the dampers then goes into rebound. If there isn't enough damping then the cycle begins again until the car returns to the original ride height, with a bouncing motion to the car. Another trait of under damping is that loads go into tyre/ tire and suspension relatively slowly, this combined with the bouncing effect means a constant varying download force. Acceleration, braking or cornering in this state with also vary due to the various download rates, so it is important to have enough bump stiffness to be able to deal with uneven surfaces.

If there is too much damping, then it is effectively like running no suspension and any upward motion will be transmitted directly to the chassis.  Over damping will result in a increase in the loads acting on the suspension and the tyres/tires. The handling will feel very harsh and hard, this will effect street driving in terms of comfort levels, so might not be desired for a daily drive.

This is undesirable in both under and over damping settings as it will reduce the handling of the car and will affect acceleration, braking and cornering loads.

Rebound-

During rebound (following the bump compression phase) the dampers extend back to their original positions, using up the stored energy from the springs. The rebound stiffness needs to be set at a higher value then the bump setting as the stored energy is being released. If there is not effect damping on the rebound, the wheel will quickly return through the static level and start to bump again, with the bouncing effect unsettling the suspension with little control.

 If there is too much rebound stiffness, then the wheel could hold longer in the wheel arch then needed, effectively losing contact with the road as the force to push the wheel back down is slower to respond to the changing surface level. This state is again far from ideal and it is best to make sure a good level is set for optimal tyre/tire contact with the road.

Damping Effect On Cornering:

Bump

Front Bump Increase=   Understeer.

Front Bump Decrease=  Oversteer.

Rear Bump Increase=    Oversteer.

Rear Bump Decrease=  Understeer.


Rebound

Front Rebound Increase=   Understeer.

Front Rebound Decrease= Oversteer.

Rear Rebound Increase=   Oversteer.

Rear Rebound Decrease= Understeer.

 

Ride Height

Depending on the exact road or racing environment, generally speaking the suspension should be as low as possible to the floor with out the the car bottoming out ( wheel, chassis or ground making contact with each other).

There are good reasons for reducing the ride height:

  • Lower center of gravity-reducing weight transfer levels and aiding handling characteristics on the limit.
  • Increased aerodynamic downforce, the front should always be slightly lower then the rear to gain a rake effect (reducing lift) to aid downforce.
  • Reducing drag and increasing fuel economy.

Adjustments will require fitting of adjustable coilovers, dampers, or shorter springs. Care must be taken in lowering the car as it has a diverse effect on other suspension geometry and this could end up hindering the performance potential. 

Ride_Height_Diagram

Spring Rate and Wheel Rate

 Spring Rate:

It is a ratio indicating the resistance of a spring during bump or rebound (compression or expansion).Also known as suspension rate, it is critical for setting the correct ride height and is proportionate to the movement of the length of component travel in its stoke phases. As we know the whole job of suspension is to keep the wheels and tyres in contact with the ground at all times, for total performance.

By having the ability to change the spring rate, heavier vehicles can have a higher setting to stop the suspension bottoming out under extreme loads, or if the car has big downforce generation abilities. Softer spring rates could be an advantage in rougher terrains or raised kerbs at apexes. If have seen a car jump of a kerb at speed, than lower spring rates are needed. Unless of course you enjoy bouncing down the track, or even off the track in worse case examples.

Sometimes people complain when driving sports or track focused cars with competitive suspension on normal roads, this is because of higher spring rates.

Most springs will have ratings on after market upgrades and unless you have radically reduced weight of the car or revised the aerodynamic package, it is best to keep to the manufactures' recommendations. Spring rates can be measure on a machine or alternative the following formula may help:

k = \frac{d^4G}{8ND^3} \,

where d is the wire diameter, G is the spring's Shear Modulus (for example about 12,000,000 lbf/in2 or 80 GPa for steel), and N is the number of wraps and D is the diameter of the coil.


Wheel Rate:

Similar to spring rate but measured at the wheel instead of at the suspension linkage of the spring. Also it is important to know that the wheel rate would always normally be less than the spring rate, as the wheel will travel a larger distance through compression or expanding than the spring.

Wheel Rate is the Motion Ratio squared times the Spring Rate:

Wheel Rate = Spring Rate * (Motion Ratio ^ 2) * Spring Angle Correction


Toe In/ Toe Out

Toe In or Toe out is another important suspension setting, it affects the handling of the car in terms of tyre/ tire wear, straight line and cornering characteristics.

In terms of the best braking and acceleration capacity for the tyres/ tires, it is best to have neutral toe settings.

  • Too much toe in causes the outsides of the tyres/ tires to wear out.
  • Too much toe out causes the insides of the tyres/ tires to wear out.

 

Toe In/Toe Out Diagram

Weight Distribution-Static/ Corner Weights

 Weight Distribution is the final section we need to look at, we might of adjusted all the other settings listed above but the car still might be displaying understeer or oversteer characteristics during left or right cornering loads ( normally one or the other). The ultimate aim of suspension tuning is to try and get a perfectly set up car, which is balanced during cornering in both directions and maximises equal loads through the tyres/tires. Also cars with rear wheel drive layouts will have increased acceleration with more weight over the rear wheels to aid traction

Static Weight-

If we take a static car which is fully loaded with all fuels, oils and driver on board, ideally we want equal corner weights distributed equally across the car (50% left, 50% right at the front and rear of the car). While the overall car designs drivechain layout will play a fundamental importance in the total fore and aft static weights, it is possible to still make some changes to increase this ideal balance harmony (normally through moving components like batteries, fuel cells ect).

 It is always best to to get your static weight measurements in the first instants before making any radical suspension changes. This can be calculated with digital scales designed for the job, and will best be done when new suspension components are fitted to the car.

  • Front Wheel Drive= more weight over the front of the vehicle, normally 60:40 split.
  • Front Engined, Rear Wheel Drive= tend to be very balanced with 50:50 split possible.
  • Mid Rear Wheel Drive= more weight normally over the rear of the vehicle, 40:60 split.

As soon as the car is in motion, the car's weight will act through the center of gravity and affect the suspension's roll center during acceleration, braking and cornering loads. This is also when any uneven static load rates will transmit themselves into understeer or oversteer, when cornering.

Bear in mind the following rules:

  • If we increase the FL and FR spring perch (adjustment ring), then only ride height increases.
  • If we increase RL and RR spring perches, then only the ride height increases.
  • If we either increase the FL and RL or FR and RR ( one side of the car), we only increase the ride height.
Corner Weight Diagram

Corner Weights-

Before we adjust the corner weights to get a perfectly balanced car, or as close as is possible under the design constraints, we need to check that we have the following checklist:

  • Tyres/tires inflated to the desired pressures.
  •  Anti-roll bars are disconnected.
  • You have a method of recording the small adjustments , this is very important as it may take multi changes to get the desired corner weights.

Changing one spring perch will have the effect of changing the static loading and ride height on all four corners, so it is important to logically approach this.