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The Ultimate Guide to Suspension and Handling Part III: Balance the chassis By Mike Kojima

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The Ultimate Guide to Suspension and Handling Part III: Balance the chassis By Mike Kojima

Post by DTechR on Tue Mar 15, 2016 12:32 pm

The Ultimate Guide to Suspension and Handling Part III: Balance the chassis

Now that you have reduced body motion and improved steering response, we can work on the next major area of improvement, improving chassis balance. The goal for most of us is to have a car with neutral balance. Neutral balance, where all four tires slide the same amount, is the fastest way around a corner most of the time. This way the total maximum grip of each tire is being used. It might seem odd but experienced drifters prefer a neutrally balanced car because it allows them to have many control options for setting the car sideways. For example I set up Dai Yoshihara's Team Falken S13 in about the same way I would if it was a Time Attack car.

Unfortunately for the enthusiast, most cars are tuned to understeer from the factory. Understeer occurs when the front tires slide before the rears at the limit of grip. Manufacturers do this because it is the easiest handling mode for the average driver to control. Understeer is not 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 through chassis tuning we will 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 into an artform. A fast autocross car is often set to oversteer as are most rally cars. Any sort of race car that is driven mostly in very tight corners will probably be faster if it tends to have a bit of oversteer.  Due to consumer advocate Ralph Nader's highly publicized campaign against General Motors and the rear engined Corvair in the 60's which resulted in huge lawsuits against automakers, the entire automotive industry is adverse to producing a car with anything close to oversteer.  When an oversteering car exceeds its limits, it "spins out" which lawyers like to make sound especially heinous.  However this simply means that an understeeing car will plow off the road nose first and an oversteering car will go tail first.  You are gonna crash either way!

How do we tune a car's handling balance? By manipulating the tire’s slip angle. Slip angle is defined by the difference between the direction the wheel is pointing and the direction the contact patch of the tire is pointing. At extreme slip angles the contact patch actually slides across the pavement. A chassis' 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 and the car understeers. 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 car’s 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 is naturally more neutral handling than the other chassis layouts.

This is a classic example of grinding understeer in a nose heavy front wheel drive car.  I forget who this is but I know they are a member here who I have corresponded with.

Proper manipulation of tire loading 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 car oversteer? Sure, look at most successful racing front-wheel cars, they oversteer like crazy. To protect its customers, Porsche delivers most of its tail-heavy 911 models with a state of suspension tune that counters its natural tendency to overseer.

Another example of classic oversteer, this time in a tail heavy rear engine Porsche 911.

How does a tuner manipulate tire loading and slip angle? By tweaking the spring rates, anti-roll bar rates, tire sizing, tire pressure and to a lesser degree the shock damping. The first and cheapest option a tuner has is to increase the tire pressure. The harder a tire is inflated, within reason, the smaller slip angle it develops. For example, 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 will do quite a bit to reduce understeer.

In a corner, the car will roll toward the outside of the turn, transferring weight to the outboard wheels.  Suspension tuning has all to do with manipulating this weight transfer with springs, shocks and antisway bars.

Changing the spring rate and anti-roll bar torsional rate has a large impact on slip angle. Running a stiffer spring or anti-roll bar on one end of a car will cause more weight to be transferred onto the outside tire as the car tries to roll in a corner. The softer end of the car will compress and the more stiffly sprung end will resist compressing putting more load into that tire causing it to run at a bigger slip angle. So for your theoretical understeering, front-wheel-drive car you can run a bigger rear anti-roll bar, stiffer springs or both to tune out understeer. Conversely, stiffening the front suspension and increasing the rear tire pressures can help tame an oversteering car.

Selecting larger antisway bars or adding an antisway bar where none was before not only reduces roll on a corner but changing the ratio of stiffness allows tuning of the weight transfer affecting under and oversteer.  Adjustable sway bars are perhaps the most effective tuning tool for the average guy.  These Progress Group swaybars are three way adjustable.

Shocks can improve response and help with balancing the car.  Adjustable shocks can be a powerful chassis tuning device as well.  As an example soft shocks get the car to a steady point of weight transfer faster right after the initiation of a turn. When set stiff they will 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 car’s balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer down.  An analogy is that shocks are weight transfer capacitors.

Putting the endlink in the closer holes increases the bar rate and putting it in the further holes reduces it.  Unlike springs, adjustable bars can be quickly tuned at the track by someone of average mechanical skill.

Tires sizing can also affect chassis balance. Installing a wider tire on the end of the car that needs traction most is obvious. Rear engine Porsches have wider rear tires to help prevent oversteer. Powerful rear-wheel-drive cars also 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.

A major advantage of coilovers is that you can choose from a large selection of race springs to give you endless tuning options.  Hyperco, Eibach and Swift all make top quality racing style springs.

What’s considered to be ideal by most performance drivers and suspension tuners is a car that is basically neutral when cornering without brake or throttle input. At the limit of adhesion, the car slides with all four wheels. 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 and reduce understeer with slight throttle application is good. This applies to both front- and rear-drive cars. Rear-wheel-drive cars should also be able to invoke oversteer with large applications of throttle. Balance like this gives the skilled driver the most options to control the car at the limit of traction when driving.

 Stiffer Softer  
Front Antisway Bar More Understeer Less Understeer  
Rear Antisway Bar More Oversteer Less Oversteer  
Front Spring More Understeer Less Understeer  
Rear Spring More Oversteer Less Oversteer    
Front Tire Pressure* Less Understeer  More Understeer    
Rear Tire Pressure* Less Oversteer  More Oversteer  
Front Shock More Understeer, later in turn Less Understeer, later in turn  
Rear Shock More Oversteer, later in turn Less Oversteer, later in turn  

* This is a rule of thumb, extremes of under or over inflation will not act like this.  Very high tire pressures will reduce grip due to crowning and low pressures may increase grip due to flattening out the footprint.

Finding this interesting and useful?  Stay tuned as we will continue to unravel the mysteries of suspension tuning in the months ahead!


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