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The Ultimate guide to Suspension and Handling Part 2, Controlling Body Motion

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The Ultimate guide to Suspension and Handling Part 2, Controlling Body Motion

Post by DTechR on Tue Dec 15, 2015 3:22 pm

The Ultimate Guide to Suspension and Handling: Part II - Controlling Body Motion

by Mike Kojima

Previously, we addressed the most important aspect of handling, the tires. The next important suspension trick to increase grip is to reduce body motion such as roll under hard cornering, dive under braking and squat under acceleration.  In later editions of this series we will get into the technical aspects of chassis dynamics but for now we are covering the basics and will gradually get into the harder to understand parts of the subject. Contrary to popular belief, body motion does not harm grip during cornering due to weight transfer to the outside wheels in a hard turn. Other things are the major contributors to weight transfer and we will get into that later. You want to reduce body motion because it slows how responsive the car is to inputs from the steering, brakes and accelerator, things that you, the driver use to control the car.  Body motion also gives the impression that the car is not handling well, roll, dive, squat and wallowing do not give a feeling that inspires confidence that a high performance car should have; instead these attributes are more appropriate for a boat or an aging baby boomer’s ride to a long dirt nap.  Watch an F-1 car in a turn; it nimbly darts around the corner with hardly any body motion.  Now watch an SCCA showroom stock racer, it leans squirms and squeals its way around the track at a much slower pace.

This Lamborghini Countach cornering at the limit shows that older exotics were not perfect.  Note the extreme turning angle indicating that the front tires are sliding in understeer.  This is because the outside front suspension has bottomed out causing the front tire to lose grip.  However notice that the tread of the outside front tire's tread is flat on the ground due to the Lambo's more sophisticated suspension geometry than the old 60's car above.

Sorry to use this photo again but here is the Dog II under heavy cornering load.  The Dog II is based off of a Nissan Sentra, an Economy sedan with relatively crude suspension, however the suspension geometry has been reworked to optimize it as best as possible. It also uses pretty sophisticated dampers, sway bars and high rate springs.  Look how the body roll is under control and the camber is keeping the tires' tread flat, even though they are distorted under side load.  By looking at the tire steering angle and attitude, you can tell that the car is well balanced without excessive understeer. The distortion shows how hard the tire is being used. This little Sentra is pulling much more G's than the Lamborghini above.  Surprisingly, if someone would be willing to spend the money, the Dog II's suspension is pretty streetable.

More insidious are some of the other bad side effects that excessive body motion can produce.  Many softly sprung vehicles will roll over and bottom out the suspension on one end or even both ends of the car when cornering hard.  This shocks the tires and will cause an instant loss of traction on the end of the car that first bottoms out resulting in unpredictable handling. If the suspension bottoms under a large amount of roll, a lot of weight will get transferred to the outside wheel where the suspension has bottomed causing a loss of grip.

Stroking the suspension through a wide range of travel dynamically in a turn can also result in the suspension geometry doing bad things.  Most factory vehicles have compromised suspension geometry due to packaging reasons and usually two things can happen when the car really heels over in a turn, neither of them good.  First the suspension can gain positive camber under roll on the outside wheels.  This is worse in cars with the very common McPherson strut suspension.  This when the car rolls, the tires don’t, making the outside of the tires tread tire heel over onto its shoulder, not using its full tread width effectively and tearing up the outside edge of the tread.

The other evil effect of roll is bump steer and Toe Steer.  Bump steer is caused when the steering linkage and the rest of the links of the suspension travel in different arcs as the suspension moves.  This is due to the steering linkage not being placed in the same geometric plane as the rest of the suspension’s control arms.  Racecars are designed not to have much bump steer but production cars often have the location of the suspension’s control arms and steering links compromised by design. The result of this is that the tires can get steering input even if the steering wheel is not moved when the car heels over. The driver feels this as a car that is twitchy and unstable at the limit. Dive and squat when combined with roll can make all of these issues worse.

The images above illustrate tie rod and control arm pick up points for both McPherson strut and unequal length A arm suspension for minimal bump steer.  Now go look at your car, chances are it's not going to be that close to this.  When the control arms and the tie rods have different lengths and pick up locations, the steering is going to move independently of steering wheel movement and you get bump steer.  This makes the car feel twitchy and unpredictable.  Too much body motion means more bumpsteer.  We will cover this phenomenon in more detail in future installments.

Toe steer is like bump steer but regarding the rear tires.  Excess roll causes the rear tires to be steered in a direction other than straight ahead.

Toe steer is geometric movement of the rear wheels steering them way from the direction of travel when the suspension moves.  Here are some examples of toe steer caused by trailing arms on a beam axle and semi trailing arms.

Moton Motorsport 3-way adjustable shocks as found on our EVO IX are an extreme and expensive example of stiffening the suspension to reduce body motion.  The sophisticated valving of the Motons allow spring rates 3x stiffer than stock while maintaining a near stock ride.  To better the already excellent EVO we went this way.  The Moton's also have a lot of low speed damping which also controls body motion well.

Okay, so now that you know body motion is bad, what can you do to control it?  The first thing to do is to run stiffer springs, stiffer springs will resist roll and bottoming out under roll and combinations of roll, dive and squat.  Of course stiffer springs have more rebound energy to them and to prevent you from bouncing like a pogo stick or floating in more mild cases, you need shocks with more damping, especially rebound damping.  Shocks generally do not affect how much a car rolls but they do affect how the suspension responds to bumps and steering input. More rebound damping keeps the car from bouncing and floating over bumps and undulations.  More damping also makes the car much more responsive to steering input.  Too much damping is not good; it can prevent the suspension from returning once compressed, causing it to pack down, lose travel and gradually bottom out.  We will get into the subtleties of shock tuning later.

Ground Control Advanced Design 2-way adjustable shocks as found on our 300ZX.  We mostly use coil overs on our projects as we feel that most stock replacement performance springs are designed for cosmetic street lowering and are too soft and low for the kind of track use that we subject our cars to.

Another way to cut body roll is to add anti sway bars (also known as anti roll bars) or to install bars of a bigger diameter.  Anti sway bars are torsion bars that interconnect the wheels.  They do not come into play until the car starts to roll over in a turn. When this happens they resist the roll, as the bar must be twisted for the car to lean over.  Anti Sway bars do not affect the ride as much as stiffer springs and have no affect on dive or squat.  Generally the shock absorbers damping doesn’t need to be altered when the anti sway bars diameters are changed.

Robispec EVO antisway bars are not only stiffer than stock but adjustable as well.

Stiffening the suspension will degrade the ride to some point and if stiff is good, more is not always better.  It is possible to go too stiff to the point where the car will not be able to deal with bumps and will hop and slither its way around turns instead of compliantly absorbing the bumps and finding traction.  Even racecars can be made too stiff. Some cars that are good from the factory with stiff suspension such as the EVO VIII, IX and the R35 GTR are notoriously hard to improve upon without seriously compromising overall drivability.

Whiteline's rear STI swaybar on bottom is a lot thicker and stiffer than the black stock part above.  It is also adjustable.

In  future editions of this series, we will cover chassis balance and how to tune it.


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