Vehicle Dynamics 1.2 - Front View Geometry

In my first Vehicle Dynamics blog, (www.ali-f1.blogspot.co.uk/2014/06/suspension-geometries.html) I discussed front view geometry. This blog will be a short follow up of front view suspension geometry, where I will discuss: camber change rate, tyre scrub and finally caster. (I understand that caster is a side view geometry component but I’ve included it in this blog due to requests.)

Camber Change Rate:

Camber change occurs when a wheel moves up or down, for example, when cornering. Camber change rate is a function of swing arm length only, which is the length from the wheel to its Instant Centre. The longer the distance of the swing arm, the smaller the change in camber. Camber change wants to be kept at a minimum so that an optimum camber angle can be maintained throughout a wheels range of motion. 

The regulations in F1 pre determine a high and far away Instant Centre, simply because of the high bulkhead. If we replace a front wheel’s two wishbones with a swing arm, and take it all the way to IC we will find that we have an incredibly long swing arm. This means that camber change is minimal which is highly desired by the teams. A changing camber whilst cornering would make the car extremely unstable as the lateral accelerations and load transfers occur.

Scrub

Scrub is another front view variable. Using the same swing arm to IC procedure as with camber change, the amount a wheel scrubs laterally against the floor can be seen. Scrub occurs during the jounce (upwards motion) phase of a wheel.

As a wheel jounces, it moves upwards following a radius from the IC, this causes the wheel to push outwards as it rises upwards. This outward lateral motion causes the tyre to scrub against the ground. Scrub is a function of IC height, where scrub is at a minimum when IC is placed on the ground.

In F1, IC’s are quite high and therefore scrub levels should be quite high. High scrub levels will cause unnecessary tyre wear, so you may ask why do teams have such high IC’s if its harming their tyre life?

The answer comes in two parts. The first is that there is such little suspension movement on an F1 car, scrubbing levels are kept low by minimal suspension jounce. Small suspension travels are used in racing to maintain ride height and therefore provide the best and most consistent handling. The second part is tyre sidewalls: the current 13” wheel rims provide a small amount ‘suspension’. This means that the whole wheel doesn't have to jounce to absorb energy and maintain ride height, the tyre side wall can flex and therefore scrub doesn’t occur.

With a likely move to 18” wheel rims, suspension geometries will again have to be changed to accommodate the new rules and regulations.

 

Caster:

Caster is a geometry setting used to help realign front wheel direction. When we drive round a corner in our own road cars, we do not have to put effort in to straighten the car’s steering up to head off in a straight line again. This is the outcome of caster.

Examples of caster and how it works are most easily understood from a computer/desk chair example, but the way it works is essentially the same for racing.

Positive castor:

Positive castor is achieved by placing the axis of revolution of the wheel in front of the tyre’s contact patch. With the axis of revolution in front of the contact patch, the wheel can’t help but to be pulled straight when the driver releases the steering wheel.­­­

From this image you can see that the wheel revolves around the highlighted kingpin. This king pin is not perpendicular to the ground; it is angled so that it creates a positive caster.

The axis of revolution (highlighted as a red line below) is an extrapolated line of the kingpin. This allows us to see the angle at which the front wheel revolves around as the steering is turned.

From the side view we can now see the angle at which the axis of revolution runs at (red - king pin angle) in comparison to the wheel centre line (blue), at the bottom of the wheel centreline is a blue dot, this dot represents the tyre contact patch with the ground. The angle between the two lines (bottom half of the tyre) represents positive caster.

Caster is an intricate part of steering geometry, it affects: wheel height during steering and weight shifts across the car which cause oversteer or understeer. The way it affects camber and other steering geometries is quite complex and requires thorough understanding by a race engineer.  

Thank you for reading this short follow up! The next vehicle dynamics blog will be on side view suspension geometry which lets us examine longitudinal behaviours of the car (braking and accelerating etc)

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Ali