Anti Dive Geometry

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Anti-Dive Front Suspension

As we all know, a car has a tendency to compress the front suspension under braking. Anti-Dive Front Suspension can be used to limit this characteristic. This can be achieved by tilting the upper front Control Arm (A Arm) so the rear mount is lower than the forward mount.

For this vehicle I have made the following assumptions:

  • The front / rear braking bias is 60/40% toward the front
  • The Centre of Gravity (CofG) of this vehicle is 600mm above the ground

Anti-dive (For a double A Arm setup) is calculated using the following formula:

%Anti-Dive = Front Brake Bias (Tan (α) (Wheelbase Length / CofG Height) X 100

α is the angle between the ground and a line drawn from the centre of the tyre contact patch up to the Longitudinal Instant Centre.

I have read Herb Adam’s book ‘Chassis Engineering’.

Whilst discussing independent rear suspension, he states: “it is possible to get some anti squat, but a value of about 25% is the practical limit”. Any more can cause the wheels to hop under hard acceleration. Instead of accelerative forces transferring into the frame, they are transfered vertically causing the wheels to bound or walk, stressing the A Arms in the process.

This is a 4×4 vehicle, so I have applied the same theory to the front suspension. Logical right?? After all, if you give it the beans in some front wheel drives they hop around all over the show.

Taking the above equation:

25% = 60% x (Tan (α) x (2550 / 600) x 100

α = tan-1(25 / (60 x (2550/600)))

α = tan-10.098

Therefore, α = 5.6

Originally I was going to fabricate my own uprights, but now I have decided to use the standard Ford Sierra 4×4 front uprights and mushroom adaptors. Hence I needed to ditch all my previous workings and start from scratch. On a side not did anyone realise that the 4×4 upright needs different diameter mushroom adaptors to the 2wd? I didn’t!

Sierra Front Upright CAD

I used Kangaloosh to work out my front suspension and viewed the results in SuspEdit. Whilst I was there, I used my new found knowledge of the 25% maximum anti-squat to re-visit my rear suspension mounts. Currently I have 80% anti-squat, these will be removed and remade to give me 25%.

I am using 235/35/19 tyres for which the rolling radius is 314.2mm.

From Kangaloosh my bottom mounts are 228mm from the ground.

5.6 = tan-1228 / β)

Where ‘β’ is the distance between the Longitudinal Instant Centre and the centre of the front tyre’s contact patch.

β = 228/0.0980 = 2325.6mm

From Kangaloosh my top mounts are 391mm from the ground.

Vertical distance between top and bottom mounts = 392 – 228 = 164mm

Inclination of top A Arm(γ) = tan-1(164/2325.6) = 4.03°

I got these graphs from  Kangaloosh :

Roll Centre Migration

If you consider the CofG is the point on the vehicle where all mass may be assumed to be located. Now, the vertical distance between the roll centre and the CofG is the length of a lever arm, between where the wheels actually apply the forces to the chassis; then the greater the vertical distance between the two, the greater the leverage these forces will have on rolling the chassis in a turn.

A dynamic roll centre will create changes to the load transfer rate as lateral acceleration increases. In theory, the more constant this point is, the greater the feel of predictability a car will have.

Potential Roll Centre Solutions: The best solution, is one that is that leaves the driver feeling confident, yet minimises the moment arm that rolls the body.

There are several ways to accomplish one or both of these criteria:

  1. Increase the height of the roll centre. This could be accomplished by increasing the angle between the two control arms in the rear view. You could shortening the control arms or use taller lower ball joints. Both would also result in greater camber change. Furthermore, a higher roll centre causes jacking effects, which cause an increase in ride height while cornering;
  2. Decrease the height of the CofG: This can be accomplished by lowering the power-train and/or the seats, and/or cutting the roof off the car for example. Another very effective means is to use drop spindles (rather than drop springs).
  3. The use of stiffer springs will prevent the suspension from moving into the upper ranges of travel. This therefore limits the amount the body can roll in a turn. Stiffer springs however, mean that car will not absorb irregularities, this not only causes a harsh ride but can cause the car to loose contact with the ground, reducing grip.
  4. The use of sway bar(anti-roll bars) partially solve the problem of a harsher ride by allowing the use of softer springs but work like stiffer springs when the wheels try to travel in opposite vertical directions, such as in a turn. The suspension therefore travels less in bump, than it would have.

You want the roll centre close to the CofG in order to reduce the roll moment induced by lateral load transfer mid corner.

If the roll centre were higher than the CofG, it creates a negative roll moment and Jacking forces. This makes a car feel less predictable. The front tyres would transfer load to the into the suspension and cause the body to rise. Keeping the roll centre constant allows for a predictable load transfer in roll.

A low roll centre amplifies lateral acceleration forces at the centre of gravity, increasing body roll and therefore dynamic outside wheel load transfer.

For more information on roll centre click here

image: Roll Centre
Roll Center Migration

Roll Center Migration

Camber Gain

As you compress the suspension upward into jounce, camber becomes more negative, and in rebound it becomes more positive. This means that top tilts inward as the wheel rises, and tilts outward as it falls below ride height. Camber changes in pure bounce aren’t a good thing because it wears the outer and inner edges of your tires faster. However, not much time is spent in pure jounce or rebound. Camber change however, is very useful when cornering. For example, whilst turning left, the car body naturally rolls to the right, causing the suspension on the right side to compress up into jounce. This forces the right wheel to tilt in at the top in relation to the car body. This keeps the tyre flatter in relation to the ground, keeping the tyre contact patch more constant and hence providing much needed cornering grip. On the left side of the car, the suspension extends, tipping the wheel into positive camber which again keeps the contact patch more constant, during roll. The left wheel in a left is less important since most of the car’s weight has transfered to the right wheel.

For more information on camber click here

Camber Gain

Roll Camber

As a vehicle rolls each side of the car will see a different camber gain / lose.

For more information on camber click here

Roll Camber

Roll Camber Rate

Disclaimer….

I don’t pretend to be any kind of expert in suspension. Indeed, I change my own designs regularly as I read more books. I even get confused by books published by different authors, that offer different standpoints and conflicting equations. I often get a little way into calculations only to find, despite a stack of books and the Internet to call on, I hit dead ends.

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