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Front-view Suspension Geometry

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Preface

Roll center is quite possibly the most misunderstood aspect of car setup. This guide is designed to accurately explain what is going on in a "just the facts ma'am" fashion, without inaccurate generalizations that get thrown around and without diving too deep on the subject. Before we begin, since roll center adjustments affect chassis roll, it is important to understand what chassis roll is.

Chassis Roll

As you go through a corner, your car experiences a right/left weight transfer towards the outside of the corner. For example, as you turn left, weight is transferred to the right side of your car and the car leans to the right. This lean is chassis roll. Even with no chassis roll, weight is transferred to the outside wheels, but chassis roll increases the amount of weight (force) applied to the outside wheels and is an important part of proper handling in corners. Too little or too much roll will cause a less than idea traction situation—more on this later and how it presents itself.

What is Roll Center?

The book definition of roll center is confusing. Suspension is complicated—there's a lot going on. The whole point of the concept of roll center is to make understanding chassis roll easier, yet for most, it does the exact opposite. This is not the fault of the concept, rather it is the fault of the way the concept is explained and the fact that most guides end up showing a complex diagram with a bunch of different color intersecting lines—as if it makes things clearer. It does not—not for those hoping to grasp the concept just enough to tune with it and not design cars. Let's start off with a useful definition. This is the way I explain it to people at the track who ask me for assistance.

Roll center is an imaginary point under the car that it teeters left/right from to create chassis roll. The lower this point is, the further it is away from the car's weight, and the more the car will want to teeter, or chassis roll left/right.

A Visualization

After spitting out the above definition, I present this real world analogy. Imagine you're holding a large broom vertically with the broom head up. If you hold near the broom head, it is easy to keep it from teetering. If you hold it toward the other end, putting a good deal of distance between the broom head and your hands, it is much more difficult to keep it from teetering. The car's weight is the broom head, where you grip the broom handle is the roll center.

What Does This Mean?

The thing to nail into your head is this: The lower the roll center is, the more then car will want to roll. Any adjustment that lowers roll center will make the car roll more, and any adjustment that raises the roll center will make the car roll less. This is where roll center ends. Honest. That's it. Anything further is just tuning chassis roll, which can be done in MANY different ways including roll center, and is NOT specific to roll center. Changing roll center is much like changing ride height for different corner handling without changing the front/rear weight transfer properties of your car.

How Do You Find Roll Center?

Who cares. Are you designing a new car? It's under the car somewhere close to the ground. Half of what scares people off is talk finding the roll center and all the concepts that go along with it. That's like requiring a carpenter to learn how to design a circular saw together before cutting a 2x4—useless. Move on, let's get tuning.

What Does it Affect?

In most cases, tuning roll center is a final "fine tuning" adjustment that "lives inside" other settings such as springs, sway bars, and shock down travel. If you can't get the chassis roll right with roll center adjustments, it is likely something else needs adjustment such as ride height, spring choice, sway bar choice, or shock down travel, which all affect chassis roll as well. I like to think of small roll center changes as "fine tuning my spring, piston, or oil" selection in the corners. On 1/8 scale cars, since some roll center adjustments are so huge, the roll center selection may prevent proper sway bar selection.

A very important thing to remember is that roll center affects chassis roll over the ENTIRE range of roll from initial roll during low speed operation to high speed operation where the car is leaning hard in a corner. Other adjustments such as sway bars, springs, and shock down travel have more of an effect on TOTAL chassis roll, or high speed extreme roll—since the forces exerted by a spring increase as they are compressed and shock down travel puts a hard stop on how far the car can roll. This means that, like ride height and shock oil, roll center is one of the few settings you can use to adjust the low speed roll behavior of your car and greatly affects delicate low speed sections of the track.

What Does This Mean On the Track?

First and for most, the rear roll center is king. Much like you can sorta drive a car that has a front wheel or shock broken off, you can drive a car that has its front roll center all wrong—its just less than ideal. If the rear roll center is wrong, it is much more of a problem—corner exit and infield traction will be wrong and the car will be a hand full. Look at the B4—the team is constantly keeping up with track surface, shock damper, and tire/insert trends by releasing new rear hub options, grinding their u-braces down, and generally offering a ton of options to tune that car's rear roll center, while the front has remained the same for years, never moving more than a ball stud washer or two. That demonstrates the relative effect and importance of the front and rear roll centers. The front is definitely more in play on a 4wd car, for what its worth, but the rear still takes the cake.

Rear Roll Center

The lower the rear roll center is, the more off power and low speed corner grip the rear end will have, but the less on power corner exit the car will have (and it will get worse the harder the car is pushed out of the corner). The higher the rear roll center is, the more it will be able to be pushed on corner exit and maintain grip, and the more willing it will be to rotate in low speed corners or any time the rear is unloaded under corner entry or mid corner transition. An extremely high roll center will make for a car that can punch out of any corner extremely hard but will slide around willingly in low speed corners. An extremely low roll center will make for a car that is too hooked up and unwilling to rotate in the infield and very difficult to exit corners with any speed, causing you to wait on the car to get moving or else it will come around on you.

If you are unsure as to which direction you need to go on rear roll center tuning to address an issue, ask yourself this: When I apply throttle, does it get better or worse? Applying throttle will cause more weight to transfer to the back and more chassis roll. If the throttle improved the situation, and more grip was had, you likely needed to lower the roll center to get more roll. If it made it worse, and the additional roll from throttle application broke the car free, it was probably rolled to far to begin with, dictating a higher roll center to prevent the excess roll.

If you have trouble finding a balance on rear roll center, another setting is likely to blame. On a recent track change, I found that I needed to lower my rear roll center for better traction in the infield technical sections, but it hurt my ability to maintain grip at the end of a sweeper's on power exit to a straight away. My rear spring choice was as stiff as I felt I could run, so my solution was to add another limiter to my front shocks to reduce total weight transfer to the rear on acceleration.

In my opinion, if you have to err on the side of high/low in the rear, pick high. This will leave you with a car that doesn't punish you if you drive it too hard out of corners and at worst has a little more rotation at low speed, which can be fast in and of itself. There is nothing I hate more than a car that breaks free in the rear once it feels hooked up and you get on the throttle.

Front Roll Center

The higher the front roll center is, the more responsive (or aggressive) the initial steering will be, but mid corner steering may be reduced. The lower the front roll center is, the softer the steering response is and more mid corner steering may be achieved. The reason I say MAY on the mid corner steering is because mid corner steering is affected by many things and the addition or subtraction of roll in that situation could make things go either direction. This is a generalization. Sorry. Unfortunately it is true more often than not, especially when the rest of the front end is setup well.

I find myself tuning front roll center as a function of steering response and balance with the rear roll center on mid corner and corner entry—how linear and predictable the car steers.

For example, after selecting tires and dialing in the rear roll center on the B44 platform (one, two, or zero washers), my next step is to adjust the front roll center so the car has a predictable corner entry. The B44 platform has a tendency to "hook in" and turn in dramatically at corner entry under deceleration as you approach the apex of a low speed corner, especially under brake, or to "see saw" in mid corner in sweepers as you try to meter out throttle if the front and rear roll centers don't play nice with each other.

An excessively high front roll center will make the car overly responsive and twitchy and very non forgiving on less than perfect landings—a high front roll center tends to make a car more willing to go into acrobatics where as a lower roll center will allow the car to push out of a crazy landing where the front would otherwise grab and punish you. An excessively low front roll center will make the car push significantly on corner entry but the car will steer very well when the car slows down mid corner.

Ways of Adjusting

Lowering Roll Center

  • Raise inside camber link position at brace/tower
  • Lower outside camber link position at hub/caster block
  • Raise hub/caster block mounting hole at arm
  • Lower the inside arm mounting location
  • Lengthen camber link

Raising Roll Center

  • Lower inside camber link position at brace/tower
  • Raise outside camber link position at hub/caster block
  • Lower hub/caster block mounting hole at arm
  • Raise the inside arm mounting location
  • Shorten camber link

A Warning

Certain changes are pretty dramatic for roll center AND have an effect on other areas. Use finer adjustments first.

  • Changing the length of camber links affects camber rise.
  • Changing arm mounting locations change wheel down travel.
  • Large changes up or down ANYWHERE (such as tower holes on 1/8 scales) affect camber rise.

Unless you're looking for a dramatic change, fine tuning is best left to ball stud washers and hinge pin pills. You should be able to feel a 1mm up/down change on the track. If not, you may be barking up the wrong tree by changing roll center at all.

Wayne

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