What I've Learned RE: Suspension Tuning

[Underdog]

During the last year and a half of ownership I have attempted to learn about suspension tuning for the purpose of modifying my '06 GT. This is my attempt to pass on some of that info to others.

 

Traction Vs. Load

 

The tires on your car have more effect on its handling than any other component. It is critical to understand how your tires work before judging why your suspension handles the way it does. In order to talk about this plainly, we must disregard what is happening inside the tire and focus on the input-output relationships that hold true to all tires. The input for tire performance is vertical load, or weight, on the tire. A car in motion will have the weight over each wheel changing dynamically. Through chassis tuning it is possible to adjust how the vertical tire loading will change, and by understanding how the tire responds to that change, we can predict the effects. The output of a tire from a handling standpoint is its traction, or how well it "sticks" to the ground. The traction between the tires and the ground determines how fast a car can accelerate, brake and/or corner.

 

In order to predict the effects of tuning we must understand how changes in vertical load (input) affect the traction (output). Different tires will have curves with different shapes and values, however, they will all show a smaller increase in traction as the vertical load is increased. We call this loss of relative traction a loss in the tire's cornering efficiency. The following graph and chart will illustrate this concept.

 

http://img362.imageshack.us/img362/5725/tireperformancecurvewg4.jpg

 

From this chart you can see how the cornering efficiency of a tire decreases as the vertical loading is increased. The cornering efficiency translates directly into the lateral g-force a car can handle in that condition. At 140%, a car would be able to pull 1.4g's, but with only 75%, the same car would corner at .75 g's.

 

A tire's cornering efficiency reduces quickly when it is asked to support more and more weight. This characteristic of any tire is a key element in understand why cars handle the way they do. With this knowledge, we can analyze the more complex conditions that a car experiences during actual driving.

Camber Angle & Contact Patch

 

A tire will provide maximum traction for any given load when it is perpendicular to the ground. This is called the zero camber angle. Its contact patch is bigger than when it's at any other angle. If the top of the tire is tilted out it has positive camber. This reduces the contact patch and the tire will have less traction. Negative camber is usually set to compensate for camber gain during loading but the idea is to have zero camber when maximum traction is needed.

 

Circle of Traction

 

The circle of traction concept is based on the fact that a tire has only a certain amount of available traction at any given time. This amount of traction is dependant on the weight on the tire, the conditions, the weather, etc. When studying the circle of traction, the total traction is considered constant. What the circle shows is that the total amount of traction is distributed between cornering forces and acceleration or braking forces. If you only have so much traction available, deciding how to sue this traction can have an important effect on how well a car handles.

 

If you could view the tire contact patch as it moves along the roadway, you could see how this concept works. The total traction can be represented as an arrow on a circular graph. This arrow represents the available traction and it can be pointed in any direction.

 

http://img168.imageshack.us/img168/8117/circleoftractionaq4.jpg

 

If our sample tire had 1000 lbs. of load and its cornering efficiency was 100%, it would provide 1000 lbs. of traction. This 1000 lbs. is available in any direction-pure cornering, acceelration or braking. Unfortunately, it is not available in any two direction at once at its full traction of 1000 lbs. The total amount is not additive, it is a vector amount that can be used in combination as shown on the circle.

 

This trait is very easily seen in RWD cars. Normally exiting a turn the car will have normal understeer but will transition to oversteer if you get on the gas. As you ask the rear tires to absorb acceleration force, there is less cornering force for the rear so the car has more oversteer as the driver applies more power.

 

For an extreme example, consider a car that is peeling-out. There is enough power to break traction such that all traction is being used for pure acceleration. As is shown in the circle of traction, this leaves zero traction to handle cornering loads and this results in the phenomenon "fish-tailing". Braking is similar in that locking up you brakes causes the tires to use their full traction for braking leaving none to allow the car to turn.

 

g-Forces & Skidpad Testing

 

Tire and handling performance is described in g-force. One g is simply the force equalt to gravity on Earth. A 100 pound object is said to weigh 100 pounds. The same object subjected to 80 pounds of force would have 0.8 g-force acting on it. This is convenient because it allows us to disregard the weight of the object. For example, a 3000lb. cornering force on a 3000lb. car would be a 1g load. The same force on a 4000lb. car would be a .75g load. By using g-force we can compare cars equally.

 

Lateral acceleration is most often measured on a skidpad and expressed in g's. A skid pad is a flat area of pavement with a painted circle. The circle is typically 200-300 feet in diameter. The car is driven around the circle as fast as possible without spinning out, the time is measured, and the lateral acceleration is calculated.

 

A simplified formula for determining cornering power on a skidpad is:

 

g=(1.225 * R)/(T^2)

 

where R = radius of the turn in feet, and,

T = Time in seconds to complete 360 degree turn.

 

For example, if a car takes 12 seconds to lap a 100 ft radius skidpad, the computation follows:

 

g=(1.225 * 100)/(12 * 12)

 

g=0.85

 

This means the car is cornering at a force equal to 85% the force of gravity.

 

We can now apply these tools to a discussion on weight distribution and dynamics.

[Underdog]

Weight Distribution & Dynamics

 

Now that we understand the principal relationship between vertical load and traction we can determine the performance of individual tires and the overall cornering power of the car. Best of all, we can learn how this translates in the the handling characteristic known as understeer/oversteer.

 

Remember the skidpad? Imagine you are driving your car around the skidpad circle as fast as you can. When you are right at the point where you are holding the line at the fastest possible speed you are at your maximum cornering force. If you were to try an accelerate on the same circle, or try to tighten your circle without shedding speed, you would exceed the available traction at one, or both axles, first. The axle which loses traction first determines the under/oversteer of your car. A car that loses traction at the front end first is said to understeer. When the rear axle loses traction first it is "loose" or oversteering.

Weight Distribution

 

A vehicles weight distribution is determined by the weight being supported by each wheel. These weights are constantly changing while the vehicle is in motion due to a variety of forces acting on the car. Using the tire performance curve we can analyze the effect of the dynamic load change on the traction available in a given situation.

 

Let us consider a car weighing 3000lbs, with an even weight distribution, 50/50. This car would have 750lbs on each wheel in a static condition. Using the tire performance curve we can make the following table:

http://img442.imageshack.us/img442/9316/example1ou1.jpg

 

If we apply the formula for total cornering force:

 

Cornering Force = Traction/Weight

 

Cornering Force = 3400/3000 = 1.13 g's.

 

This may sound pretty good but consider that this is a scenario in which the car is stationary. Cornering isn't much of an issue when you're standing still.

 

In the next example, consider a car beginning to corner. When a car turns it is acted upon by centripetal force. This force causes the weight transfer from the tires on the inside of the turn to the outside tires.

 

There are four factors that control the weight transfer of a specific vehicle: cornering force (g's), track width (T, inches), Center of Gravity height (H, inches), and the car's weight (W, lbs).

 

The transfer function can be described:

 

Lateral Weight Transfer = (W * g's * H)/(Gravity * T)

 

For our purposes we can factor in 1g of cornering force and simplify the equation to:

 

Lateral Weight Transfer = (W * H)/T

 

If, for example, our example car had a track of 60 inches and a CoG height of 20 inches, the lateral weight transfer would be:

 

(3000 * 20)/(60) = 1000lbs

 

An important thing to note here is that there are only three things a person can change to reduce weight transfer. If it doesn't 1) lower the CoG, 2) increase the track, or, 3) reduce the vehicle weight, then there will be no reduction in weight transfer.

 

To continue our examples, the total weight transfer would then be divided to the two axles based on the F/R weight bias. The car in the previous example would have 500lbs of vertical load transfered from each inside wheel to each outside wheel. We can run the same study as example 1 to find:

http://img211.imageshack.us/img211/6534/example2tk0.jpg

 

Total Cornering Force = 3160/3000 = 1.05g's

 

As you can see, the total cornering force is less than in the static condition, due only to the weight transfer from cornering loads.

 

To see the effect of having a front-heavy car, let us consider a 3000lb car with a 60% front weight bias and the same 1000lb cornering force.

http://img442.imageshack.us/img442/2108/example3ax3.jpg

 

Total Cornering Force = 3130/3000 = 1.04g's

 

Unfortunately, now that we are dealing with a car with a weight bias we must acknowledge that this total cornering force is the average between the two axles. When we consider each axle individually we begin to understand the effect of front-heavy cars.

 

For the front:

 

Front Cornering Force = 1750/1800 = 0.97g's

 

For the rear:

 

Rear Cornering Force = 1380/1200 = 1.15g's

 

From these results we can see that not only will this car corner slower than one with an equal F/R weight distribution but it will understeer at the limit. As soon as the car exceeds the 0.97g limit of the front axle, the driver will be unable to keep the car on it's line.

 

We can now draw the following conclusions regarding weight distribution and dynamics:

1. The highest cornering force is available when front-to-rear weight distribution is equal (given the same size tires at all corners).
   
2. Front-heavy cars will have the tendency to understeer during cornering.
   
3. In general, the best cornering force will be achieved when the loading on individual tires is as close to equal as possible. (Read: Wide-track, lightweight, and low)

The discussion of body roll and roll force distribution is next. We'll see if I get to it tonight.

[Underdog]

reserved #3

[Underdog]

reserved #4

[underscore]

reserved #5

[Richard B.]

isn't that why we have extra load rating s and stiffer sidewalls for performance orientated driving?

[rmcferon]

[ame=http://www.amazon.com/Chassis-Engineering-HP1055-Herb-Adams/dp/1557880557]Amazon.com: Chassis Engineering HP1055: Herb Adams: Books[/ame]

 

Some credit to the author would be nice, since most/all of what you posted is lifted directly from this book.

 

Chassis Engineering

Herb Adams

HP Books 1993

[whitetiger]

the tracton vs. load idea is why sometimes, upgrading sways can actually reduce grip at the limit.

[Underdog]

Chassis Engineering

Herb Adams

HP Books 1993

 

Thanks. Yes, it is a great text for understanding basic principles.

[JoeFromPA]

the tracton vs. load idea is why sometimes, upgrading sways can actually reduce grip at the limit.

 

One of my biggest concerns with adding stiff swaybars is the tendency to lose progressive loss of traction.....that's a very important safety factor.

 

From what I understand, you'll gain the ability to make transitions faster with stiffer sway bars (less body roll, more controlled motions) but you may trade-off some ultimate lateral traction.

 

Joe

[MilesA]

One of my biggest concerns with adding stiff swaybars is the tendency to lose progressive loss of traction.....that's a very important safety factor.

 

From what I understand, you'll gain the ability to make transitions faster with stiffer sway bars (less body roll, more controlled motions) but you may trade-off some ultimate lateral traction.

 

This is true, as far as it goes. Still, with a front MacPherson strut suspension, too much body lean leads to an undesirable camber angle change which reduces lateral traction, also. There is an optimal balance somewhere between too stiff and too loose.

[JoeFromPA]

yeah, and where that is I haven't a clue

 

I'm just slapping on some bigger bars and praying that it all winds up ok

 

Joe

[whitetiger]

One of my biggest concerns with adding stiff swaybars is the tendency to lose progressive loss of traction.....that's a very important safety factor.

 

From what I understand, you'll gain the ability to make transitions faster with stiffer sway bars (less body roll, more controlled motions) but you may trade-off some ultimate lateral traction.

 

Joe

 

Exactly. that why people who are not running stickier tires should get a thinner bar than people who will run Extreme perfomance summer tires or r-comps. Ou cars are fairly heavy, so sways generally help, but if you dont have the tires for them, dont get such a thick bar.

[rao]

Funny, I got yelled at for saying the same thing here a few years ago

 

Stiffer springs are not always better and larger sway bars are not always better. Larger sway bars actually reduce ultimate grip.

[whitetiger]

yep. Most people automatically think stiffer is better. Those that have uber stiff setups on economy AS tires are really hurting their perfomance.

[Underdog]

I'll get there guys... weight distribution and dynamics this evening, after work-time, during Guinness-time.

[whitetiger]

Hey underdog, Check this link for more awsome info. Its mostly about autox, but it has some very nice tech info on suspension tuning. http://farnorthracing.com/autocross_secrets.html

[Underdog]

^ Good read, thanks.

[JoeFromPA]

Most people automatically think stiffer is better. .

 

 

Yeah, well, that applies to a variety of areas in life...

 

Joe

[praedet]

Not to get this thread off topic, but when do you cover that the Spec-B is the pinnacle of all handling, and that it's traction circle actually goes to infinity in all directions due to the ginormous tires/contact patch and infinitesimally small unsprung weight?

[Underdog]

Actually, Newtonian physics do not apply to the spec.b. We can only use the Heisenberg uncertainty principle.

 

Off-topic is fine but I'm gonna ask for a clean-up when I'm finished posting actual info.

[praedet]

You can turn corners really well when you create your own gravity well to bend both space and time...

[whitetiger]

Not to get this thread off topic, but when do you cover that the Spec-B is the pinnacle of all handling, and that it's traction circle actually goes to infinity in all directions due to the ginormous tires/contact patch and infinitesimally small unsprung weight?

 

And dont forget 3lbs of each LCA means 50% increase in cornering.

[harman.khinda]

I'm have rallitek springs and KDW's on stock rims. How thick of sways should i run?

[rao]

So if I install lighter control arms (that means that they have less mass ) and then install heavier wheels which are located at the outer most point of the suspension arm travel, will the larger contact patch that I now have make my car handle as well as a Spec B