Topic: ROLL CENTER

Roll Center is often times under estimated on its importance in a race cars setup because it can appear complicated and too technical. In truth it is actually quite simple when broken down. The location of the front roll center is crucial in how your car will handle and how fast it will be able to maneuver through the corners. Take a few minutes to understand roll center and you may just find yourself rolling to the center of victory circle.

Your first step is to grab a pencil and piece of paper and head to your garage. You will need to draw a sketch of your front end alignment in order to be able to determine the roll center.

DON’T WORRY, this will not be as difficult as it sounds.

Place your car on a level

surface with the wheels

pointing straight. You will

need to measure the height

of both ends (at the center

of the pivot points) of the

upper and lower control

arms in relation to the

ground. Then map them to

scale. your drawing should resemble mine however the angles and therefore instant and roll centers could be in very different positions. At this point you will have drawn all the shows in black on my sketch.

The next step is to locate the “Instant Centers for both the left and right side control arms. These points are the intersection of the lines drawn from the upper and lower control arm. These are the blue and red lines on my sketch.

YOU ARE ALMOST THERE! The last step is to draw a line from the center of the tread of one tire to the Instant Center of the opposite side. (ie the center of the right tire to the instant center of the left side control arms.) Then do the same for the other side, creating an “X”. This is represented by the purple lines on my sketch. Where these two lines cross is your Roll Center.

See, now that wasn’t so bad. 

Next you need to DETERMINE WHERE ROLL CENTER SHOULD BE to achieve optimal cornering performance. There is no one correct Roll Center location. It varies based on the type of track and driver preference.

Here are just a few “rule of thumb” guidelines to consider.

* The roll center starts at the “Static Location” (when the car is at ride height) and moves to the “Dynamic

Location” (the chassis rolls as the car maneuvers through the corner).

* The roll center height has an impact on the camber of the front tires.

Ideally, you want it set where there is little or no camber change in the right tire as the car corners.

* Higher roll center allows less chassis roll, while lower roll center results in more roll.

  ~Tracks with low banking – higher roll center

  ~ Tracks with high banking – lower roll center

* In asphalt applications, the roll center should be located to the right of the centerline of the car. However, a

roll center located too far to the right will result in a “pushy” feel (much like having stiff springs on the

front of the car).

  ~ Cars with lower center of gravity- roll center can be further left (towards the center line)

  ~ Cars with higher center of gravity – roll center can be farther to the right.

The last step is to UNDERSTAND HOW TO CHANGE THE ROLL CENTER to get it to the ideal location

for you and the track(s) you race on.

* You want the lower control arms to be as close to as level as possible or only angled down slightly.

*Changing the angle of JUST ONE upper control arm will adjust the roll center (ie increasing the angle of the

right upper control arm should move the roll center to the right, decreasing the angle of the left upper

control arm should give you the same result).

This should give you a basic understanding of Roll Center. This understanding should allow you to improve the handling and corner speed of your race care. However, Roll Center is just part of the whole picture. During the off season we will try to explain other parts and how they work together to get you and your car around the track.

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TECH TALK CORNER

TECH  TALK

Topic: Caster, Camber and Toe

These three topics are crucial to the performance of  your

race car. You need to understand how the work together

and how to adjust them to maximize your cars handling and drivablility.

CASTER is the tilt of the upper ball joint. A line

drawn through the centers of the upper and lower

ball joint is called the pivot line. With this line straight

up and down, the car will wonder and be greatly

influenced by any little bump in the racing surface.

Caster deals with the amount of effort that goes into

steering the car. Positive caster, the upper ball joint

located slightly further back then the lower ball joint,

gives the car more stability. Caster Stagger is when

one side has more positive caster then the other. This

aids in the cars ability to turn one way or the other.

In our application, turning left requires more positive

caster on the right then the left side. On long tracks,

less caster is needed, only a degree or so, where

tighter bullrings could require more.

CAMBER is the tilting of the top of the front tires inward or outward. The top of the wheel leaned in towards the center of the car is negative camber, leaned outward is positive camber. It is basic knowledge that the more of the tire patch that touches the racing surface while in the corner, the better the car will perform.  Camber works with the movement of the suspension to keep the most amount of tread surface flat on the track. Camber is most crucial while the car in the the corner, therefore it is the Camber Change that is important to understand. Camber Change is how much camber is lost or gained from Static (in the straightaway) to dynamic (in the corner) chassis position. (This was explained in greater detail in the Roll Center article that can be found on the Tech Talk page.)

To simulate the transition from static to

dynamic, set the camber with your car

level in the shop. Then place a jack

under the tire and raise it until it

reached the maximum point of the

suspensions travel. You will notice that

there is approximately 1.5 to 2 degrees

of camber gain for each inch of

suspension travel or bump. A basic goal

with camber is less change. This can be

accomplished which longer spindles or

less upper control arm angle.

Determining the correct amount of

camber for your car is dependent on

both the track you race on and personal

driver preference in the feel of the car.

A rule of thumb for left-hand turns would be positive camber in the left front and negative camber in the right front.

Inadequate camber can be identified with tire temperatures. Too much or too less would show up as excessively high temperatures on either the outside or inside of the tire tread.

TOE is the alignment of the front of the front tires. Toe-in is the front of the tire points slightly inward while toe out points slightly outward. To figure toe, measure the distance between the front tires, first from the back of the tire then from the front of the tire and determine the difference. Toe is adjusted with the tie rods.

In the corner, the inside tire must navigate a tighter radius while the outside tire has further to travel. By toeing the tires, you are trying to neutralize this difference.

Too much toe, in or out will cause the front end to feel as if it is scrubbing the tires on the straightaway. Too little will make the car feel unstable. A little bit of toe is good, more is not always better.

One thing to be aware of is Bump Steer. This is the change in toe as the car corners. Some feel a little is okay, some say zero Bump Steer is best, too much is never good.

Now that we have an understanding of how these three critical aspects of front end geometry work independently, you should consider how they influence each other and work together. First of all, Caster and Camber go hand-in-hand. Caster can increase the amount of camber that is gained in dynamic chassis conditions. When adjusting camber by changing the upper control arm angle you could effect the toe in dynamic chassis condition or bump steer. The best thing to keep in mind is that they all are connected. Changing one can influence the other. Its a good idea to check all when a change to one is made.

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