Suspension and Tires 101
Tires
The tire is the most important component and the only one between your vehicle and the ground. However, it is not usually looked at in a detailed manner by the weekend performance enthusiast. Suspension is only there to control the contact between the tire and the ground and should be thought of as such. To begin EVERY good suspension design, a thorough knowledge of the tires you will be using should be attained. This is the weak link in nearly every racers vehicle. To accomplish this, you must have tire data and to attain tire data you must send a tire or multiple tires to be destroyed at a tire testing facility. This is VERY expensive and usually only achieved by high end race teams.
What are you looking for? A vast number of things: Normal loads slip angles, slip ratios, lateral force, coefficient of friction, camber sensitivity, Load sensitivity, and how they all interrelate. A thorough explanation would take an entire book to explain, so I will simplify and just explain the basic parameters.
Normal load: The weight applied to the wheel in the vertical direction (i.e. corner weight)
Slip angle: The angle between the direction the wheel is pointing and the direction the wheel is actually moving. “slip” is not a correct term since the wheel is not actually slipping, just the deformation in the rubber allows the wheel to have a slightly different path than intended.
Slip ratio: The ratio between the angular velocity of the driven wheel and the free rolling wheel. (Just look at drag slicks.. very high slip ratio)
Lateral force: The force the wheel can produce in the lateral direction. Varies for each circumstance.
There is much more info on tires, for a more thorough explanation, please read “racecar vehicle dynamics” by Milliken and Milliken or one of the many other books on tires. SAE bookstore has a great selection
Suspension
There are many different types of suspensions in vehicles however I will be talking about the “double A Arm” suspension as found on the front of the F-bodies and in multiple other vehicles. This, in my opinion is the best design for racing applications, however, different designs can achieve the same goal.
Static Suspension
I will start with some terminology that should be known to continue this discussion.
FVSA: Front View Swing Arm. Looking at the suspension from the front or rear
SVSA: Side View Swing Arm. Looking at the suspension from the side
Camber: Angle between vertical and the tire centerline axis in a FVSA view
Caster: Angle between vertical and the upper and lower ball joint in a SVSA view
Kingpin angle: Angle between vertical and the upper and lower ball joint in the FVSA
Instant center: The extension of the upper and lower A-arm until the point they meet. This is the point at which the wheel pivots about at any given position in the FVSA.
Roll Center: A line connecting the instant centers to the center of the wheel contact patch for each side is drawn. Where these lines cross is the Roll center (as shown: a box with an X)
Roll Moment: The distance between the roll center and the CG times the lateral acceleration force acting on the CG. This will determine what roll stiffness is needed.
Suspension with roll of 2 Degrees
Suspension with Bump of 1.5 inches
As you can see, in the 2 situations above, the contact patch with the ground has changed significantly. This is why all suspension design is a compromise!!! You can optimize it for bump but then in a roll condition your camber will go positive. Or you can optimise it for roll, but then in bump your camber will go too far negative (i.e. under braking..loosing stopping power)
Just spend some time and look at the situations above to compare the values. Just visualizing what is going on is half the battle.
Now you need to know what the end goal is for the vehicle.. road racing, off road, etc.. that will be the starting point. For example, the Baja trucks have VERY long instant centers to allow for small camber change over the 22+" of travel. However, on road race vehicles you would want to monitor your instant center length much more closely. but i digress...
Knowing what environment the vehicle will be in and the tires it will be using is the first step. Then optimising the suspension so the contact patch is always maximised (as well as possible). Sounds simple but the previous two sentences have cost billions of dollars in research for professional race teams.
If there are specific areas of interest, Steering geometry, G-G diagrams, etc.. let me know and i can elaborate. or just buy the books below.. there are hundreds of pages of explanations! more than i could ever type here.
Also, a side note. The debate between the solid axle and IRS can be handled quite easliy just by looking at suspension geomretry. The Solid axle effectively has an infinate length instant center which means no camber gain in bump/squat. This allows the optimal contact under zero lateral acceleration conditions. With IRS, to accomodate more ideal lateral conditions, the camber gain in roll is increased which means in bump/squat the camber gain is much more excessive than nessecary for pure straight line acceleration (i.e. it compromises contact patch in straight line for gains in lateral). Now i wont even go into the whole wheel hop issue.. thats totally a damping issue and beyond the scope of this kinematic discussion. I will follow up with that in a later discussion.
The basis of my information comes from two sourses.
Racecar Vehicle Dynamics By Milliken and milliken
Tune to Win by Carrol Smith
http://images.google.com/url?q=http:...une_to_win.jpg
** Also, if someone has suspension points for the F-body or Vette i can run them through the analysis software. The images above are just a generic selection of points**
Linear Mode
