Hello ClubWRXers, this thread is something I wanted to create for quite sometime, yet haven't quite been able to put together in a well thought out structure. Recently however, I ended up in a discussion that really cemented much the information I intend to discuss in a coherent format. So here goes...
Purpose of this thread: The information presented is designed to give members of our forum a general understanding of the basics of car setup. This is nowhere even remotely close to anything resembling a comprehensive guide. If you want that, people have written books. I would suggest looking up Fred Puhn. I will, however, discuss some fundamental concepts and ideas that I think are very useful to developing an understanding what you car is doing in a corner. My goal with this information is to give the reader the ability to develop a clear path to positively affecting the handling of their car on an asphalt or concrete surface.
Disclaimer: With the purpose in mind, please read, enjoy, and ask questions; but also understand that whatever changes you make to your suspension will effect the handling dynamics of your vehicle. If this is not done properly or with care, you can very well influence the handling and stability very negatively. Enjoy at your own risk.
Part 1: Tires
How does one go about making their car handle better? What needs to be accomplished when your car sticks to the pavement like glue?
The answer to this is pretty simple, better adhesion between tire and road. So the goal with suspension tuning becomes trying to make the best use of the available grip of your tires. So what's my best recommendation to make your car handle better? GET GOOD TIRES! Seriously. Once you have a good set of tires, please proceed with the following.
Making the best use of your tires:
I'm sure if you talked to Bridgestone, Yokohama, or Hoosier they would have a much longer, dorky, graph riddled answer for what makes a tire grip, as far as I'm concerned you should concern yourself with these three areas and little else:
1. Contact Patch. To use cliche, this is literally where the rubber meets the road. You must maximize your tire's contact patch to have any hope of getting the most grip from that tire.
2. Normal Force. This is force that will be used to press the tire into the pavement. It is this force, times the coefficient of friction between the tire and the pavement that produces the actual grip of the tire.
3. Slip Angle. This is a term used to describe the amount or angle that the tread of the tire has moved in relation to the wheel. This website goes into a real good discussion on slip angles and how they influence handling: Slip Angles and Handling
So from these items, you can quickly see that handling is, at its core, about maximizing your available contact area and increasing its effectiveness by controlling weight transfer.
Part 2: Vehicle Dynamics and Suspension Equipment
To look at the vehicle dynamics we will discuss the motion of the vehicle around three axes of rotation about it's CG (center of gravity). These are defined as:
Pitch and roll are both created by the transfer of weight in the vehicle. Our end goal with controlling pitch and roll is to control the weight transfer of the vehicle and positively affect the vehicle dynamics is to produce the desired yaw (steering).
So with all this discussion of the tires and the factors that influence their performance and the suspension components that allow us to control the vehicle dynamics to maximize the grip of our tires, what does that all mean for us?
By tuning our suspension we are attempting to make changes to the pitch and roll of the vehicle to accomplish the following:
Control the amount of weight on the front tires during braking. Too much weight on the front end during braking means that the rear end isn't doing much at all to assist with braking and the rear end has become unweighted and lost grip. This reduced rear grip can create handling issues for the car on corner entry.
1. Mitigate the loss of camber in the corner and maximize the contact patch of the tire.
2. Control the transfer of weight to each tire to maximize the relationship between the total grip and the slip angles produced.
It is this second concept which is a bit complicated and I will attempt to discuss in detail.
From the "Slip Angles and Handling" webpage:
MAJOR POINT: You can set your suspension to actually steer the car with the rear tires as well as the front tires. The more tires you have working to get you around a corner the better. In reference to vehicle dynamics, this is yaw, or more commonly referred to as "rotation" or how well the car "rotates" through the corner.Often times, in a corner, you will turn the steering wheel a certain number of degrees initially and then need to reduce the steering angle before you reach the apex of the turn. This is because the rear tires are generating a slip angle and some of the steering is taking place at the rear tires. It is even possible to travel through the turn with the steering wheel in the straight forward position because the rear tires are doing all of the steering.
More to this point it is actually the ratio of the slip angle of the front tire to the slip angle of the rear tires that will determine if a car will understeer or oversteer. Ideally we would like to have a car which handles perfectly neutral, with maybe a slight tendency for oversteer. So with the Impreza platform in mind, we have our work cut out for us in trying to dial out the understeer that is inherent in the vehicle for many different reasons.Increasing the slip angles at the rear of the car will make the race car looser (oversteer). Likewise, reducing the slip angles at the rear will often be interpreted by the driver as making the race car tighter (understeer).
To do this we should work to decrease the slip angle of the front tires and increase the slip angle of the rear tires.
Effects on the Front End:Weight transfer from the inside tires to the outside tires affects the slip angles. When your race car is in the turns, equally loaded tires will run at smaller slip angles. Conversely, a large difference between the inside tire's load and outside tire's load will cause the slip angle to increase.
From this we can see that adding more roll stiffness to the front end of the car will reduce the weight transfer and create smaller slip angles on the front tire. As this happens we also reduce the amount of weight which is applied to the outside tire in the corner. This is also the tire which should have the largest possible contact patch in the corner. By limiting weight transfer to this outside tire, we will be increasing understeer. With this interaction between camber loss, slip angle, and normal force; there becomes a maximum point in roll stiffness which will produce the most grip from the front tires.
Effects on the Rear End:
Increasing the weight transfer will produce more slip angle on the rear tires along with producing more camber loss. Both of these two will promote the car to oversteer. The slip angle will be very progressive and predictable in its loss of grip where the camber loss can produce very unpredictable results. Also a decrease in weight transfer will also produce oversteer as the normal force generated on the outside tire is no longer large enough to produce grip. Again you can see that there becomes a maximum point for roll stiffness where the maximum rear grip is produced.