Analysis: F1 Suspension Geometry

I’ve talked a lot about the aerodynamic and power unit components of a Formula 1 car on this blog, but rarely touched on the raw mechanical systems that are also critical to performance. There’s a reason for this, though – it’s all a bit voodoo. There are plenty of theories behind proper suspension geometry for a race car, however it becomes much more complex to analyse these mechanics at F1 level as downforce – the biggest performance differentiator in the sport – plays an important role in the design calculations.

For this blog post I am going to run over some of the important aspects of suspension geometry and the factors involved (e.g. centre of gravity, aerodynamic downforce).

Camber

Camber is the angle between the centreline of the tyre tread face and the vertical.

camber

For racing purposes we only really see negative camber, i.e. the tops of the tyres are pointing inwards. A generic setup in F1 shows higher angles of camber at the front and little-to-no camber at the rear.

There are two advantages to running camber, one being improved stability and two, more grip at high speed. As the speed of the car increases, the tyres are pressed further downward as downforce generated by the wings and underfloor rises. When the driver turns into a corner at speed, the car will lean over to the opposing direction of travel, which consequently shifts the position of the tyre’s contact patch. Running optimum camber allows for maximum contact between rubber and track during a corner on the outside tyre, increasing the grip available and thus the speed at which the car can take the turn.

By having little camber on the rear tyres, grip is consistent for right and left corners despite the inside front tyre picking up off the ground a little (in extreme cases). Overall stiffness of the suspension and anti-roll bars also play a part in how camber enhances grip, but that’s more experimental work than theory.

There are some key disadvantages of camber angle, though. The teams optimise the aerodynamics of the car around a specific camber angle in the wind tunnel and on CFD design software. Adjusting the camber too much either side of this reference can be detrimental to aerodynamic performance as the position of the tyre’s wake can play havoc on how airflow is manipulated over the car, as well as disrupt the properties of the front wing aero structures (e.g. the Y250 vortex).

It is for this reason that we tend to see higher levels of camber at circuits with longer straights. If top speed has a greater emphasis on laptime than downforce, low-drag wings and bodywork will be installed. However this takes away downforce, so grip is recovered in the corners by increasing camber. The new aero packages will also be designed to work with the resulting camber angle increase.

Typically you will see more camber at venues such as Monza (Italy) and Circuit Gilles Villeneuve (Canada).

However if the camber angle is too aggressive it can cause severe tyre wear problems on the inside shoulder, hence why Pirelli set a maximum limit for each track.

Toe

Toe is defined as the angle between the centreline of the tyre tread and the horizontal when viewing the car from above.toe

Toe-in is as above – the tyre’s leading edge is pointing in towards the centre of the car, whereas toe-out the tyre points away from the car.

Adjusting toe angle alters both the balance and responsiveness of the car. Toe-in is generally used to dial out oversteer as it makes the car less pointy, whilst also reducing the twitchy feeling of the car down the straights. Toe-out helps get the nose of the car into the corner as the inside tyre will already be angled towards the apex.

In general, an F1 car will have about 1 degree of toe-out on the front tyres and 1-2 degrees toe-in on the rear tyres.

Toe angle is often a last-resort setup change for single seater racing as doing so increases the scrubbing effect of the tyre against the track, worsening tyre wear. Wing angle and other mechanical changes will be made before more toe angle is applied, but for tracks such as Monaco I would imagine it’s an absolute must to experiment with.

Caster angle

Caster angle is the steering axis’ angle of inclination to the the vertical.

Caster

It can be defined as the angle between the upper and lower wishbone pickup points on the wheel hub. This can be relatively easy to spot on a single seater racing car as the push/pull rod is exposed. The angle at which it is leant back provides a rough indicator of how much caster angle the car has, as it has to squeeze through the gaps in the wishbones from the base of the hub to reach the chassis.

All F1 cars run with at least a few degrees of positive caster angle – as far as I am aware there is no need to use a negative angle – and in some cases it can be over 10 degrees. Positive caster is when the steering axis is in front of the tyre, so looking at where the contact patch of the tyre is we can see that the steering axis extends beyond it in the forward direction.

Although the mounting point of the wishbones are fixed at the chassis and cannot be changed, the caster angle can still be altered by adjusting the point at which the wishbones attach to the hub (I would presume offset bushings of some sort or, the more expensive route, entirely new hubs).

The reason why caster angle is always positive is that it has a self-centering effect on the steering as torque is applied. The higher the angle the more self-centering the steering feels. This is because the tyre is, if you like, leaning over on itself slightly as it is rotated, with the offset from the upper wishbone wanting to pull it back in.

Positive caster is beneficial for stability – think of it almost like the angle the forks are at on a bicycle. On downhill mountain bikes the forks are at a higher angle of recline, providing greater control at a cost of responsiveness.

Roll Centre

RC (roll centre) is pretty simple in theory but when applied to a real-world situation this subject can be a bit more complex. Its design is one of the most influential for mechanical setup as any amount of roll affects the car’s handling. There are devices that control roll (dampers, torsion bars, anti-roll bars etc.) but they require a certain amount of movement from the car before their effect occurs.

The RC of the car is defined from a number of steps. First, draw and extend lines running in parallel with the upper and lower wishbone arms from the vertical centreline of the tyre until they meet at a point. Secondly, construct a line from the centre of contact between the tyre and ground, to the intersection point from step one. Finally, mark the point where the vertical centreline of the car and the previous line meet – this is your RC.

This is the point the car rolls around during weight transfer from side to side, i.e. when cornering. This is because all centrifugal forces are directed through the CoG (centre of gravity), which is directly above the RC, therefore the car’s mass will push away from the direction the car is travelling in and rotate around the RC point.

Suspension

The RC is marked as a black and white checkered circle above, whilst the CoG is a checkered yellow and black. Remember that the roll centre is always below the CoG.

The CoG appears relatively high for a car that has virtually all of its weight sat just a few millimeters above the ground, but we have to consider CoG when the driver is in the car. Considering that the driver is sat at an incline (their toes are in line with their upper chest) and their head (a fairly large region of mass relative to the rest of the human body) is positioned quite high up relative to the rest of the car, the CoG is raised by some margin.

rollcentreside

The height of the RC will change with both the angle of the wishbones and also camber. Generally speaking, a higher wishbone angle will create a higher RC. Flatter wishbones will have the reverse effect.

The front and rear roll centres often differ from eachother, as you can see in the illustration above. The roll axis is the line between the front and rear roll centres. Note that both the RCs and CoG are three dimensional coordinates.

The application of roll centre in F1 varies significantly from road cars. Whilst more roll translates into extra weight on the outside tyre and therefore more grip, this knowledge does not translate too well due to the high speeds F1 cars are going.

Lowering the CoG of the car increases the speed at which the car can take a high speed corner, as the tyres are less likely to slip as weight is transferred from one side to the other. As the RC and CoG are brought closer together, the car will roll less as there is less leverage for the CoG to rotate.

As the CoG is barely adjustable during the season (this is a set design characteristic of the car from conception), the only way of adjusting the RC is to play with the camber and wishbone angles. The height of the front and real roll centres have independent advantages and disadvantages to eachother, as the former is orientated around the responsiveness of the car and the latter related to traction under acceleration – F1 cars are rear wheel drive.

The higher the RC is at the front (i.e. moving it towards the CoG), the more responsive the car is and the less steering input required for a corner. This also reduces the overall roll of the car, which is important from an aerodynamic perspective – too much roll can disturb how the air passes over the car during a corner.

On the contrary, the RC is often much lower at the rear of the car. The tell-tale sign of this is that the top rear wishbone in particular is often angled slightly down towards the floor, and the lower wishbone at a little-to-no angle at all. Positioning the RC lower at the rear allows for more roll and squat – the transfer of weight heading towards the back of the car due to accelerative force. The lower RC will therefore improve traction as the driver picks up the throttle on corner exit.

Although there is only a small difference in height between the front and rear RCs, you can clearly see how the car’s roll axis is at an angle from the side of the track.

Lewis_Hamilton_2014_Singapore_FP1

The picture above is of Lewis Hamilton in his Mercedes W05 from the 2014 Singapore GP. Notice how, as he is taking a fairly tight right-hander, the car is perching on the left-rear corner slightly. The small inclination of the roll axis favours weight transfer towards the rear of the car, and, in this case, onto the left side, too.

With the amount of torque the current power units are producing, it is no wonder that there is an emphasis on gaining as much traction as possible in order to preserve the rear tyres and keep them within their best operating temperature window. As long as the car is producing sufficient downforce from the front end of the car, this roll axis setup should make for lovely driveability and excellent responsive handling.

Advertisements

9 thoughts on “Analysis: F1 Suspension Geometry

  1. Henry

    I think you have the toe section backwards. As a track enthusiast with 9 years under my belt, toe out at the front wheels has always helped cornering ability. This is because the inner wheel now has a decreased radius. However, straight line stability is compromised. Toe in helps combat oversteer in corners and improves straight line stability. Road cars come with a little toe in from the factory. Being that RWD cars are more prone to oversteer it makes sense that f1 cars run toe in on the front.

    Reply
  2. Nic

    This a great article. I’ve understood key principles of the ones you’ve discussed but never really grasped their merits in practice.
    Thanks

    Reply
  3. ALee in Birmingham

    Where have these kinds of articles been for ages!?
    Good work explaining it in simple terms. I can see this being useful, for those who starting to change setups in racing games.

    Two typos I noticed, though. Some Ctrl + F will help locate these.
    “height of the front and real roll centres” – front and rear?
    “Notice how, as he is taking a fairly right right-hander” – tight right-hander?

    Reply
  4. Pingback: Announcements 7… | theWPTformula

  5. Angad

    Was reading about the geometry to get a nice setup on F1 2016 game. now i can go back, change settings and see what difference they make

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s