Roll Center Understood

A long time ago I wrote an article titled “Caster Understood” which explained in simple terms how caster affects a car’s handling. The great response from the RC community made me realize the need for clear, simple explanations on car handling, which at some times can be quite daunting and confusing.

Since that article, I have written several “how to” articles that help you to fine-tune your RC cars, or at least help you to “de-mystify” concepts that are part of our sport. I hope that this article on roll center will help to clear up a lot of the confusion surrounding this topic.

What is a Roll Center?

A “roll center” (RC) is a theoretical point around which the chassis rolls, and is determined by the design of the suspension. Front and rear suspensions have different roll centers.

A “roll axis” is an imaginary line between the front and rear roll centers.

The amount that a chassis rolls in a corner depends on the position of the roll axis relative to the car’s center-of-gravity (CG). The closer the roll axis is to the center of gravity, the less the chassis will roll in a corner. Chassis rolling at one end of the car or the other gives more grip to that end of the car.

Roll center is one of the most under-utilized adjustments on a car, but one of the most powerful. This is because roll center has an immediate effect on a car’s handling, whereas anti roll bars, shocks and springs require the car to roll before they produce an effect.

For the purpose of this article, I have borrowed explanations from a variety of sources, and I will try to paint a clear picture of how roll center works.

Roll Center Basics

Here are some basic facts about roll center (RC) and center-of-gravity (CG).

* Roll center (RC) is the point around which the car rolls

* Each end of the car (front and rear) has its own roll center

* Center-of-gravity (CG) is where all cornering force is directed

* RC and CG are (ideally) in the middle (left-right middle) of the car

* RC is vertically below the CG in cars

* Rolling produces more grip

Where is the Roll Center?

Roll center is determined by the cars suspension geometry. Each end of the car has its own roll center, determined by the suspension geometry at that end of the car.

The following diagram shows how you can find a car’s roll center at one end of the car or the other.

This looks a little bit complicated, but here is a breakdown:

* Line ‘A’ is parallel to the upper arm.

* Line ‘B’ is parallel to the lower arm.

* Line ‘A’ and line ‘B’ intersect at point ‘IP’

* Line ‘C’ goes from the wheel contact point (WC — bottom center of the wheel) to point IP

* The point at which line ‘C’ crosses the car’s centerline (CL) is the roil center

Adjusting Roll Center

You can adjust roll center by changing the angle of the suspension arms.

IMPORTANT:

Always ensure that left and right sides of the car have the same settings!

Adjusting roll center can be done in a variety of ways, depending on the car. You can change either end of a suspension arm to change roll center. Sometimes it is easier to change the inner pivot point of the suspension arms (nearest the chassis), while other times it is easier to change the outer pivot point of the suspension arms (nearest the wheels).

Car manufacturers have come up with some innovative ways to do this on their car. Most modern, intermediate-to-pro-level cars have SOME way to adjust roll center.

When adjusting roll center, changes to the upper arms result in small RC changes, while changes to the lower arms result in larger RC changes.

(Since the lower arms are closer to the roll-center itself, making changes in the lower arm angle will have a very large effect on roll center position.)

* To give a lower roll center, make the suspension arms flatter (more horizontal).

* To give a higher roll center, make the suspension arms more angled. Upper arms would have more of an upward angle where they meet the wheels; lower arms would have more a downward angle where they meet the wheels.

How Does Roll Center Work?

When cornering, centrifugal force is applied to the car’s CG, which tends to push the car to the outside of a corner. This causes the CG to rotate around the RC. Since the RC is below the CG, cornering force causes the car to rotate AWAY from the force. Hence, the car rolls to the OUTSIDE of the corner.

* When the RC is far away from CG (lower RC), when the car corners the CG has more leverage on the RC, so the car will roll more.

* When the RC is closer to CG (higher RC), when the car corners the CG has less leverage on the RC, so the car will roll less.

* If the RC was right on top of the CG, when the car corners the CG has no leverage on the RC, so the car would not roll at all.

Depending on what the car is doing, you will want one end or the other to roll more or less. You change the height of the RC accordingly to make it closer or further from the CG (which for all intents is a fixed point).

Effects of Front Roll Center Adjustment

Front roll center has most effect on on-throttle steering during mid-corner and corner exit.

LOWER front roll center

* More on-throttle steering

* Car is less responsive

* Better on smooth, high grip tracks with long fast corners

HIGHER front roll center

* Less on-throttle steering

* Car is more responsive

* Use in high grip conditions to avoid traction rolling

* Use on tracks with quick direction changes (chicanes)

Effects of Rear Roll Center Adjustment

Rear roll center affects on- and off-throttle situations in all cornering stages (entry, mid, exit)

LOWER rear roll center

* More on-throttle grip

* Less grip under braking

* Use to avoid traction rolling at corner entry (increases rear grip)

* Use under low traction conditions

* Increases traction, reduces rear tire wear

HIGHER rear roll center

* Less on-throttle steering

* Car is more responsive

* Use in high grip conditions to avoid traction rolling

* Use on tracks with quick direction changes (chicanes)