First off I should explain why this is in this forum. Its in no way an article or a teg tip and I am not asking assistance in working on my suspension (but I do have questions). It relates to racing/race cars (well all cars, but we only care because of racing/competition, right?) and fits in no other forums so here it is. 
I donât know if everyone read the tire stagger thread where I just argued with mythos EF/DA for 3 pages, but it was that thread that once again sparked my interest in the suspension tuning books I bought a while ago. In looking for quotes to counter mythos with I found that I missed a lot of stuff through the first read. That and, at the time, I didnât understand some of the things the authors were saying. I am not saying that I now understand everything because if I did I wouldnât have made this thread. I do understand more but there are some parts that are still a little greek. I am also making this thread because typing this stuff out makes me understand better. Gnome sayin? Yeah, so on with it, eh?
Iâm gonna start with the terms.
Center of Gravity â Also called the center of mass and Iâm gonna abbreviate it CG. The CG is the most center concentration of mass on the car. If you picked up the car in this spot on a worthy string all 4 tires would leave the ground at the same time. If you decided to balance the car on a pole, the car would rest on the top of pole at the CG. The CG also has a height and this is the most important axis of the CG. (man I need a pic, lol)
Mass Centroid Axis â If you sliced the car into little sections (like a loaf of bread) each section would have its own CG height. The mass centorid axis is an axis created if all these little CGâs were connected together.
Roll center â The front and rear suspension each have their own roll center locations. These roll centers are in the same plane as the respective suspension, and are determined by the suspension geometry. To find the roll center you get a front or rear view (an elevation, if you will, of whichever end you want to find the RC for) of the suspension, extend the upper and lower arms into the car until they intersect, then draw a line from the center of the tireâs contact patch to the intersection of the extended arms, and where that last line drawn intersects with the center of the car is the roll center (technically its where this line from the contact patch intersects the line from the other tireâs contact patch, but when the car is sitting the roll center is in the center of the suspension).
picture stolen from Fred Puhnâs âHow to Make Your Car Handleâ
Roll axis â Think of connecting the roll centers to each other by an imaginary line. This line is the roll axis.
Moment arm â This is the distance from the roll center to the CGâs height.
Roll couple â This is the torque generated by the moment arm about the roll center when weight is transferred laterally.
Now, when we go around a corner weight is obviously transferred laterally. It goes from the inside tires to the outside ones. This we can see and feel in the form of body roll. But I think it should be noted that body roll does not cause this weight transfer. The centrifugal force of the car trying to go off on a tangent causes weight transfer and weight transfer causes body roll.
We can calculate the total, overall weight transfer of the car by this equation:
Lateral load transfer (lb) = (Lateral acceleration (g) x car weight (lb) x CG Height (in))/track width(in)
As you can see, the equation doesnât take the type or stiffness of the suspension into account. Thatâs because it doesnât matter for the overall lateral weight transfer.
Now, in the words of Carroll Smith âLateral load transfer is a bad thing.â This is because âany transfer of load from one tire of a pair to the other reduces the total tractive capacity of the pair.â So we obviously want to reduce lateral load transfer.
That overall load transfer can be broken down into 3 different kinds of load transfer, unsprung weight transfer (wheels, knuckles, brakes, etcâŠ), weight transfer through the roll centers, and weight transfer of the sprung mass. If you add them all up you get the overall lateral load transfer. Each is important in its own respect but the weight transfer of the sprung mass is the most important to us because we can tune it.
Unsprung weight transfer â This is the least important of the three. Some weight is transferred because of it, and there isnât much you can do about it.
Weight transfer through the roll centers â When we go around a turn, there is weight transfer that goes directly through the roll centers to the other, outside tires. There is more of this kind of weight transfer as the roll centers go up. Also, the heavier end (ie the front) gets more of this kind of weight transfer. Our carsâ roll centers are fairly low, especially when lowered. So this isnât as bad as it could be.
Weight transfer of the sprung mass â Typically this type of weight transfer takes up most of the overall weight transfer, which is a good thing cause we can tune it. The portion of sprung weight transfer that each end gets can be changed by roll stiffness. The higher the roll stiffness is, the more sprung weight is transferred at that end. Some quotes on this are:
âThe ratio of front to rear sprung weight transfer is directly proportional to the ratio of front to rear roll resistance.â (âCompetition Car Suspensionâ by Allan Staniforth, P204)
âThe suspension with the highest roll stiffness will receive the largest portion of weight transfer caused by body roll.â (I think what he means by âweight transfer caused by body rollâ is âweight transfer of the sprung massâ because thatâs what he is talking about when he says that)(âHow to Make Your Car Handleâ by Fred Puhn P41)
Now is when the stuff gets interesting (and where I am a little and maybe even a lot confused). What happens is the remaining centrifugal force (the portion of the force that isnât creating the other 2 types of weight transfer) pulls the mass centroid axis outward and, because the tires stick to the ground, the mass centroid axis creates a torque around the roll axis. This torque is called the roll couple. The front and rear suspensions each have their own roll couple and the front and rear roll stiffness resist their respective roll couples. Roll couple equations from Fred Puhnâs book:
Front roll couple = (Front roll stiffness/Total roll stiffness) x Total roll couple
Rear roll couple = (Rear roll stiffness/Total roll stiffness) x Total roll couple
Total roll couple = Front roll couple + Rear roll couple
According to these, as the stiffness goes up at either end so does the total roll couple and the roll couple at that end.
Carol Smith says, âThe greater the resistance of the springs, the less roll will result â but there will be no significant effect on the amount of lateral load transfer because the roll couple has not been changed and there is no physical connection between the springs on opposite sides of the car. The same cannot be said of the resistance of the anti-roll bars. In this case, because the bar is a direct physical connection between the outside wheel and the inside wheel, increasing stiffness of the anti-roll bar will both decrease roll angle and increase lateral load transfer.â How is the roll couple not changed? If the roll couple is not changed, how do springs then change the nature of the car? Could someone please explain this?
I guess what I am getting at is the âratesâ people talk about. Smith says, âIf the roll axis at one end of the car is further below the mass centroid axis than it is at the other end, then that end of the car will have a greater roll moment and therefore lateral load transfer will take place more quickly at that end, and traction will suffer.â I assuming that when he says âmore quicklyâ he means âsooner.â Example: lets say the outside tires have infinite grip, if you increase the lateral acceleration to infinity both the inside tires are going to lift off the ground (lets say both at the same time) and eventually the car is going to roll over. If we add rear roll stiffness to our car, the rear tires will transfer their sprung weight more quickly and will come off the ground sooner than the fronts. Is this right? So if it is the rates of weight transfer that actually make the car more understeer or oversteer prone then adding rear stiffness just raises the rear roll couple. But the only way it could do that is by raising the centrifugal force. :shrug: I dunno.
If it is just about rates then my guess is that as the rear stiffness goes up so does the roll couple (regardless of type of roll resistance). The only way the roll couple can go up is if the force on the moment arm goes up (cause the moment arm doesnât really change). Force = mass x acceleration so when the force goes up the acceleration has to go up because the mass says the same. Acceleration == rate. So the more rear roll resistance there is the faster (sooner) the weight transfer at rear happens.
Also, if the roll stiffness tuning is just about changing the rates of weight transfer then does that mean that the other theory of transferring more sprung weight on the stiff end is wrong? Or does it just mean that it transfers the same amount it wouldâve eventually anyway, just sooner, so it thus transfers more at some specific instant (see example 2 paragraphs up).
This took waaaaaaaaaaaaaaay too long to type. I hope someone can reply with some help (marc!?!, GspeedR?, Johnny?, Robbie?).
Ben
