Suspension tuning 101 (Making it Stick)

[taosracer]

I'm sure a lot may already be familiar with these articles from the archives of Sport Compact Car Magazine (RIP) These are a very well thought out overview of suspension concepts. Here is a link to the first installment (copied below) Links to the rest of the series are at the bottom.

There are a lot of things to consider before attempting to "improve" the handling of a vehicle. Since the BRZ is engineered with a low polar moment of inertia, I would strongly recommend learning as much as possible before spending money on things that could potentially screw it up.



http://www.modified.com/tech/0506_sc...1/viewall.html


Suspension tuning is the black art of compact performance. With the majority of the world concerned about making horsepower, handling has traditionally taken a back seat. However, as all serious geeks know, every fast, well-rounded street car has as much suspension tuning as it does power tuning.
With the popularity of drifting, time attack contests and racetrack hot lapping on the rise, suspension tuning and handling are becoming popular with enthusiasts who previously spent all their efforts making power.
Finding straight-line horsepower gurus to help you is relatively easy, but it's much harder to find an expert who can make your car corner well. The solution? Make yourself the guru. If your automotive interests are greater than a one-dimensional urge to blast straight down the 1320, then it's time to get to work.
In this series, we'll uncover the mysteries of car handling one at a time. This month, we begin with the four fundamental first steps.
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Step one: Sticky tires
Tires by far are the biggest contributor to finding more cornering force. By bolting on a set of stickies, you-in minutes-make the biggest possible gain in cornering power. Generally, putting the biggest tires that will fit inside your wheel wells without rubbing is the way to go. Choosing an ultrahigh-performance tire is also important.
The list of sticky street tires is miles long. Below are a few of our favorites organized by cost. The cost-no-object tires will ultimately yield more grip than the value-priced tires, but in most cases, the value-priced choices offer 90 percent of the performance of the more expensive tires at 60 or 70 percent of their cost.
Some of our favorite ultrahigh-performance street tires
Cost No Object
Michelin Pilot Sport and Pilot Sport2
BFGoodrich g-Force KD
Bridgestone Potenza S-03
Value Priced
Falken Azenis
Kumho ECSTA MX

If you're attending track events, autocrossing or simply wanting the most grip possible, try a set of DOT-approved racing tires. Some can be used for everyday driving, while others grip almost like racing slicks and last only slightly longer. These tires produce more road-sucking grip than any suspension mod you can make.
The drawbacks are many. First, these tires can be expensive; second, they wear quickly; and third, the number of heat cycles their rubber formulations can withstand before losing significant grip is limited. Many of them don't do well in the wet and none work in the snow or ice. It's possible to end up with expensive, fast-wearing and not-so-grippy tires by using them on the street and by subjecting them to too many heat cycles. Most users of these race tires use them only on the track.
DOT-Approved Street-Legal Race Tires
Durable enough for street use
Yokohama A032R, Yokohama A048
Toyo RA-1
Nitto NT 555 RII
Hankook Z211
Pirelli P Zero Corsa
Michelin Pilot Sport Cup
Avon Tech R
Kumho Victoracer V700
Track only
Hoosier A3S04, Hoosier R3S04, Hoosier Radial Wet
Kumho ECSTA 710, Kumho ECSTA V700

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Generally, it's possible to stuff a tire two sizes larger than stock into most cars' wheel wells. For instance, a car that came with a 185/70-14 tire on a 5-inch-wide wheel can usually accommodate a 205/50-15 on a 7-inch-wide wheel. Putting the tires on a wheel of the recommended width is important as well.
Going up an inch in wheel diameter and running a lower profile tire is a good thing. A lower profile tire has shorter, stiffer sidewalls, which improve response to steering inputs and hold the tread flatter to the road surface during cornering load. However, it can be overdone.
Ultralow-profile tires are more sensitive to suspension tuning and camber changes. And stiff sidewalls don't conform to road surfaces easily. This makes ultralow-profile tires sensitive to shock, as the short, stiff sidewalls have very little compliance. Harsh surface inputs can make these tires skip and hop across the surface instead of digging in and finding grip. Large wheel and tire combinations also increase rotating and unsprung weight.
For example, most enthusiasts driving small-bore four-cylinder front-wheel-drive cars run 17x7-inch wheels with a 205/40-17 tire. The big wheel and low-profile tire look cool, but this combo is too large and too heavy for optimal performance. Hard-core track geeks driving these same cars almost always fit a lightweight 15x7-inch wheel with a 205- to 225/50-15 tire.
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Huge wheels also increase your car's final drive. Their added weight increases the flywheel effect, slowing the car's acceleration and increasing load on the brakes. This means wheels larger than 18 inches are rarely used for performance, so even for large cars, 18 inches is the maximum practical wheel diameter. There are few choices for DOT-legal race tires larger than 18 inches anyway.
Huge wheels can greatly increase unsprung weight-the weight of the components that aren't suspended. This includes the suspension arms, brakes, half of the shock absorber and the wheel and tire.
For the suspension to work well, the ratio of sprung to unsprung weight must be kept low. The best example of unsprung weight hurting handling is found on monster trucks. Even though they have wheel travel measured in feet instead of inches, it doesn't provide a significant measure of driver control.
This is why monster trucks have nearly as much unsprung as they have sprung weight, thanks to their huge tires and axles. This makes the shocks work much harder to damp wheel movement. Reduce unsprung weight and the suspension doesn't work as hard, the ride improves and the tires stay in contact with the ground.
The obvious way to make up for the disadvantages of increased wheel diameter and width is to use a light wheel. Light wheels are easier to accelerate and brake. They also reduce unsprung weight. Some examples of our favorite light wheels can be found in the table below. The mostly expensive wheels listed on this page are forged or semi-solid forged. These processes improve the grain structure and provide superior strength for the weight.
The value wheels are all cast. Casting is a less expensive way to make wheels and nearly all low-priced wheels are cast aluminum. Some of the Enkei offerings have MAT (Most Advanced Technology) formed rim sections. MAT helps improve the metal's mechanical properties using a spinning process that yields almost as much strength as forging for a fraction of the cost.
Light Wheels
Cost No Object
Volk TE37, Volk CE28N
Rays Gram Light 57F
SSR Competition X, SSR GT2, SSR GT1, SSR GT7
5Zigen FN01R-F
Motegi Track Lite
Centerline Impulse
BBS RC
Value Priced
Axis Mag Lite, Axis Reverb
Kosei K1 Racing
Almost all Rotas
Enkei RPF1, Enkei NTO3-M, Enkei RPO3, Enkei RS+M
Rays Gram Light 57C
Team Dynamics Procomp, Team Dynamics Pro Race

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Beware of some of the other lightweight, low-cost wheels offered on the market; we've found many of these bend like butter, sometimes just from normal track driving antics, like hitting FIA curbs or dropping a wheel off the track. The value wheels we have listed either have a history of surviving race conditions or are wheels we've had personal success with on the track.
Step two: Reduce body roll, dive and squat
The most important basic suspension trick is to reduce excess body motion. Roll under hard cornering, dive under braking and squat under acceleration all create problems for a driver.
Contrary to popular belief, roll doesn't cause weight to transfer to the outside wheels. Rather, it hurts handling by slowing chassis response to steering, braking and accelerating-all critical inputs for controlling the car.
Body motion also gives the impression the car isn't handling well. Roll, dive and squat all contribute to a lack of confidence behind the wheel. Watch an F1 car in a turn; it nimbly darts around the corner with no excess body motion. Now watch an SCCA showroom stock racer; it leans, squirms and squeals its way around the track. Extreme example, sure, but exactly the heart of the problem.
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More insidious are the other side effects excessive body motion produces. Many softly sprung vehicles will roll and bottom the suspension on one or both ends when cornering hard. This shocks the tires and will cause an instant loss of traction on the end that bottoms first. The result usually involves a tow truck.
Moving the suspension through a wide range of travel can also result in another problem. Most factory vehicles have compromised suspension geometry and several problems can occur when a car heels way over in a turn. First, the suspension can gain positive camber. This is worse in cars with MacPherson strut suspensions. With strut-type suspensions, the car rolls, but the tires don't. This forces the tire to roll onto its outside edge and reduces its contact patch-clearly not the best way to use the tire.
The other evil effect of roll is bump steer. Bump steer is caused when the steering linkage and the rest of the suspension travel in different arcs throughout the range of motion. As a result, the tires can give steering input even if the steering wheel isn't moved when the car heels over. This translates to the driver as a twitchy and unstable chassis. Combine dive and squat and all of these problems add up to a serious lack of control.
Now that you know body motion is bad, what can you do to control it? The first thing to do is 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 prevent your car from bouncing like a pogo stick, you need shocks with more damping. Shocks don't 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 more responsive to steering input. Too much rebound damping can prevent the suspension from returning once compressed, causing it to pack down and gradually bottom out.
Another way to reduce body roll is to install larger anti-roll bars. Anti-roll bars are torsion bars that connect the wheels. They don't come into play until the car starts to roll in a turn. During roll they must be twisted for the car to lean over. Anti-roll bars don't affect the ride as much as stiffer springs and have no effect on dive or squat. Generally the shock damping doesn't need to be altered when the anti-roll bar diameters are changed.
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Stiffening the suspension will degrade the ride, and it's easy to make your car too stiff. If this happens, the suspension will not be able to deal with bumps and will hop its way around turns instead of compliantly absorbing the bumps and finding traction.
Step three: Balance the chassis
Now that you've reduced body motion and improved steering response, we can work on the next major area of improvement. The goal for most of us is to have neutral handling. Neutral balance, where all four tires slide the same amount, is the fastest way around a corner most of the time. This way you use each tire's maximum grip. It might seem odd, but many experienced drifters prefer a neutral car because it allows them to have many control options for getting sideways.
Unfortunately for the enthusiast, most cars are factory tuned to understeer. Understeer occurs when the front tires slide first when at the limit. Manufacturers do this because it's the easiest handling mode for the average driver to control. Understeer isn't efficient for extracting maximum lateral acceleration because the car will use the front tires excessively, while the traction contribution of the rear tires is wasted. It's also the slowest and most boring way around a corner. Bottom line? Understeer sucks.
If we go too far in the quest to eliminate understeer, we'll inevitably create oversteer. Oversteer occurs when, at the limit, the rear tires slide before the fronts. Drifters work at controlling and driving in a state of continuous oversteer, raising it to an art form. Oversteer can make you a hero or a douche bag. Do it well and everyone will love you. Do it poorly and they'll laugh when you're riding home in the flatbed.
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How do we tune a car's handling balance? By manipulating the tire's slip angle. Slip angle is defined as the difference between the direction the tire is moving and the direction the contact patch of the tire is pointing. At extreme slip angles, the contact patch actually slides across the pavement.
The primary dynamic contribution to slip angle is the load placed on each individual wheel while cornering. A greater load on a given wheel/tire results in a greater slip angle of that wheel/tire when subjected to a sideways cornering force. A nose-heavy front-wheel-drive car has more weight and thus cornering load on the front tires, which causes them to run a larger slip angle than the rear tires. The front tires start to slide first, causing understeer. A rear-engine car has a larger proportion of its weight on the rear tires. The rear tires run a larger slip angle so the natural tendency is to oversteer. A mid-engine car usually has the most even weight distribution with near-equal slip angles from the front and rear tires. This creates more neutral handling.
Properly manipulating tire load and slip angle by controlling weight transfer is key to balancing the chassis. By altering weight transfer and tire loading during cornering, much can be done to change a car's natural handling tendencies. Can you make a nose-heavy front-wheel-drive car oversteer? Sure. Look at most successful front-drive racecars; they oversteer like crazy.
How does a tuner manipulate tire loading and slip angle? By tweaking the spring rates, anti-roll bar rates, tire sizing and pressure, and to a lesser degree, the shock damping. The first option a tuner has is to increase the tire pressure. The harder a tire is inflated, within reason, the smaller slip angle it develops. In the case of a nose-heavy front-wheel-drive car, if you add several psi to the front tires and take some pressure out of the rear, the front tires will run a smaller slip angle while the rear tires' slip angle will increase. This alone can do quite a bit to reduce understeer.
Changing the spring and anti-roll bar rates has a large impact on slip angle. Running a stiffer spring or anti-roll bar on one end will cause more weight to be transferred onto the outside tire as the car tries to roll in a corner. The softer end will compress and the more stiffly sprung end will resist compression, putting more weight into the tire and causing it to run at a bigger slip angle.
The best thing to do for your understeering, front-wheel-drive car is run a bigger rear anti-roll bar to tune out understeer. Conversely, stiffening the front suspension and increasing the rear tire pressures can tame oversteer.
Shocks can improve response and help balance the car right after the initiation of a turn; soft shocks get the car to a steady point of weight transfer faster. When stiff, they can delay weight transfer. Thus, shocks affect how the car feels at turn-in and also how it feels past mid-turn. A car with the shocks set fairly hard will turn in sharply. If the shocks are set too hard, the balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer.
Tire sizing can also affect chassis balance. Installing a wider tire on the end that needs traction most is obvious. Rear-engine Porsches have wider rear tires to help prevent oversteer. Powerful rear-wheel-drive cars tend to have wider tires in the rear than in the front. Many front-wheel-drive autocrossers and road racers install a wider front tire to get more front grip.
At the limit of adhesion, a car that slides all four wheels without brake or throttle input is considered ideal; it also doesn't exist. Being able to provoke slight oversteer by lifting the throttle and more aggressive oversteer with slight braking while cornering at the limit is useful as well. Being able to slow rotation with slight throttle application makes front-wheel-drive cars easier to control.
Rear-wheel-drive cars should also be able to invoke oversteer with large applications of throttle. This kind of balance gives the skilled driver the most options.
Step four: Reduce weight transfer
Weight transfer is the movement of weight from the inside to the outside wheels during cornering. Excessive lateral weight transfer hurts handling. It's caused by centrifugal force working on the chassis' center of gravity, which loads the outside wheels and unloads the inside wheels.
Contrary to popular belief, very little weight transfer can be attributed to lean in a corner. Even at large roll angles, weight transfer due to roll is quite small. So lowering a car's center of gravity and widening its track width will reduce weight transfer more effectively than reducing roll angle.
Lowering is best accomplished with shorter springs. The smartest approach is to use shorter springs and shorter-bodied shock absorbers or struts that maintain stock compression travel at a lower ride height. Excessive lowering can change suspension geometry, causing positive camber during roll and contributing to increased bump steer.
The easiest way to increase track width is to use wider wheels and tires that fill out the wheel wells. This also increases the amount of rubber on the road. Using wheel spacers and wheels with a more positive offset can also increase track width. Any positive change in track width, and therefore offset, increases the scrub radius. Scrub radius (see diagram on page 128) is the distance from the centerline of the tire's contact patch to the point where the steering axis intersects the ground, also known to regular readers as "The Dave Point." Increasing the scrub radius allows forces generated by the tire more leverage to act on the steering. To the driver, this translates as torque steer under acceleration and braking.
To minimize the change in scrub radius, it's important to try to increase wheel width to the inside as well as the outside by paying close attention to the wheel offset. This puts more rubber on the road and increases the track width while maintaining the same scrub radius.
Increasing track width also changes the motion ratio of the suspension, which effectively reduces spring and anti-roll bar rates. Lastly, a very positive offset wheel puts a large strain on wheel bearings, ball joints and steering linkage, making them wear much faster. All of these are good reasons not to go overboard with this method of increasing track width. A good rule of thumb is it's safe to use the largest wheels and tires you can stuff in your stock wheel wells by rolling the inner fender flange.
A good guideline is to increase the track width and lower the car more on the end that slides first in a corner. An understeering, nose-heavy, front-wheel-drive car can use more track width and a lower ride height in the front. A powerful rear-engine car can be lower and have more track width in the rear. This play on physics will help reduce weight transfer in both cases.
In the next installment, we'll discuss more basic mods you can do to improve handling, some of the common deadly sins of modifying your suspension and basic tips on suspension geometry.

Other Installments:

Making It Stick Part 1: Four basic steps to better handling

Making It Stick Part 2: Four more steps to better handling

Making It Stick Part 3: It's all in the geometry

Making It Stick Part 4: More lessons in suspension geometry

Making It Stick Part 5: Damper fundamentals

Making It Stick Part 6: More advanced dampers

[Dimman]

Whoever wrote that doesn't understand the inter-relation of body motion and weight transfer? Sheesh...

[Matt Andrews]

if memory serves, those were written by Mike Kojima. Who does indeed know his stuff. One of the smarter engineers I've run into track side. He now runs the site www.motoiq.com . I doubt that body motion and weight transfer relation is missed on him. But admittedly that article was too long to reread for an introduction to the basics...

[7thgear]

that's the problem

suspension tuning is a big pill to swallow if done right, like a week long antibiotics course.

but people don't want that, they want to pop a lil blue pill and go apeshit.

[Dimman]

Originally Posted by Matt Andrews

if memory serves, those were written by Mike Kojima. Who does indeed know his stuff. One of the smarter engineers I've run into track side. He now runs the site www.motoiq.com . I doubt that body motion and weight transfer relation is missed on him. But admittedly that article was too long to reread for an introduction to the basics...

He said that the mounting of the FR-S's front struts affected the motion ratio on MotoIQ. Hearing he was well respected, he threw me for a loop given that MacStruts are almost universally 1:1, until I asked some legit guys on here, and diagramed it out myself.

He was wrong.

He is not helping anyone by separating weight transfer from body motion.

[jamal]

Originally Posted by Dimman

Whoever wrote that doesn't understand the inter-relation of body motion and weight transfer? Sheesh...

So what is he missing?

My only complaint is the tire recommendations, which are WAY out of date.

For further reading, I'd suggest the Motoiq Ultimate Guide to Suspension and Handling:

http://www.motoiq.com/tech/the_ultim..._handling.aspx

[Dimman]

Originally Posted by jamal

So what is he missing?

My only complaint is the tire recommendations, which are WAY out of date.

For further reading, I'd suggest the Motoiq Ultimate Guide to Suspension and Handling:

http://www.motoiq.com/tech/the_ultim..._handling.aspx

He's separating weight transfer and pitch/roll. He states that roll doesn't cause weight transfer, but doesn't identify that weight transfer causes roll.

He also limits weight transfer to centrifugal force and roll (lean). All acceleration forces cause weight transfer, so it affects pitch (dive and squat) as well. So his tuning advice to limit body control by jacking up spring rates first, and then dealing with the cause after, doesn't really make sense.

He uses the lack of motion in an F1 car as an example of why to use high spring rates, but ignores the fact that they are operating in a completely different environment, and the fact that it is the tires that are designed to handle most suspension duties in F1. We are not running our cars on glass-smooth tracks with huge downforce, and tires that are tuned for their own spring and damping rates.

So to me his explanation of jacking up spring rates is: because racecar.

He may know what he's talking about, but his explanations are not the greatest.

[pyro530]

This just makes me miss Sport Compact Car. Modified mag doesn't even come close in their level of tech articles.

Thank you for the post!

[taosracer]

Glad those old articles got some responses. Thanks for the links to the MotoIQ guide.

Here's another good read for those that are wondering about micro-cellular jounce bumpers. What the

http://www.hrsprings.com/technical/d...jounce_bumper/

[jamal]

Originally Posted by Dimman

He's separating weight transfer and pitch/roll. He states that roll doesn't cause weight transfer, but doesn't identify that weight transfer causes roll.

He also limits weight transfer to centrifugal force and roll (lean). All acceleration forces cause weight transfer, so it affects pitch (dive and squat) as well. So his tuning advice to limit body control by jacking up spring rates first, and then dealing with the cause after, doesn't really make sense.

So to me his explanation of jacking up spring rates is: because racecar.

He may know what he's talking about, but his explanations are not the greatest.

Well it's a basic article geared toward improving the handling of a street car, and when you put on sticky tires and push the limits of your average sedan things go wrong with the suspension geometry pretty quickly. There's not really much you can do aside from more spring rate and very minor alignment and geometry changes.

And I think what he was trying to do is make it clear that body roll doesn't mean weight transfer. It's a pretty common misconception and I recently got in one of those annoying internet arguments with someone who was trying to say that a stiffer spring reduces roll and pitch and therefore weight transfer.

[SmartedPanda]

Aren't Rotas heavier then stock wheels?

[fatoni]

Originally Posted by SmartedPanda

Aren't Rotas heavier then stock wheels?

rota is a brand and not a product so some rotas are and some arent