Technical tip

Before we start we gotta have an explanation of common vehicle dynamics terms. We had better understand the common ways to describe the different aspects of vehicle dynamics before we even turn a wrench, so here we go:

• Friction circle: This is basically a vehicles performance envelope. It's expressed in lateral G’s, accelerating and braking G’s. When graphed, the friction circle looks like an egg with the X axis lateral G’s and the Y access braking and accelerating G’s.
• Understeer: This is when, at the limit of vehicle traction, the front of the car slides first before the rear. Race car drivers call this "push". This is the way that many cars come set up to behave from the factory as it is the most predictable for average drivers. The crash mode for understeer is that when the limit of adhesion is exceeded, the car will plow strait ahead off the road nose first. This is not the fast way to have your car set up but if you are a dork mode driver. When the car understeers you should regain control if you let off the gas, unless of course you run out of road first. That is what air bags are for. Even my uncoordinated evil twin sister could get that right, maybe. It is not efficient for extracting maximum lateral G’s because the car will dynamically use the front tires excessively for turning, overloading them while the rear tires basically just hold the back of the car up. Front wheel drive cars like ours tend to exhibit understeer as the final terminal mode of balance.
• Oversteer: This is when, at the limit of vehicle traction, the rear of the car slides first before the front. Race car drivers call this "loose". The rest of us call this "spinning out", "spinning a shitty", "doing a brodie" or even crashing. The final crash mode of oversteer is backwards, tail first into the woods or in the worst case spinning round and round with the driver as a helpless passenger. Since the infamous days of Ralph Nader and the Corvair, most auto manufactures avoid oversteer like the plague. Oversteer is difficult for a dork to handle because recovery requires judicious use of countersteering and throttle feathering to control; fine motor skills that only some of us can deal with. Although oversteer looks neat and macho it is really a slow way to drive except in pro-rally on the dirt which I don’t know too much about. Oversteer is slow on the pavement because hanging the tail out bleeds off a great deal of speed going through a corner. Conserving the momentum is the fast way around as turn.
• Neutral: This is the fast way around a turn where all four wheels slide evenly. Since the total friction circle traction of each tire is being used, all the available grip that the tires have is being put to the ground. Racers call this "drifting". Neutral is the fast way around a corner most of the time. Neutral is also the hardest handling mode to achieve for the suspension tuner.
• Polar Moment of Inertia: Or PMI as we will refer to it, is a description of how a cars mass is distributed along the length of the vehicle. A car with a high PMI is like a rear engine, rear drive car like a Porsche 911 or a front engine, front wheel drive car like Audi. same thing only the poles are different, so to speak. A car with a low PMI would be a mid engine car like a Boxster. Low PMI cars have most of their mass about the middle, high PMI cars have the mass at one end or another. Low PMI cars are the easiest to get a neutral balance out of due to the balanced, centralized mass. High PMI cars like to oversteer, in the case of the 911 or understeer like our cars. To get a feel for this phenomena, hold a bowling ball in one hand and rotate it back and forth by twisting your wrist. Now get a set of dumbbells of the same weight, grab the middle of the bar and do the same thing. Bet the bowling ball wants to rotate easier right? Guess what type of car will be easier to get neutral!
• Slip Angle: This is the wonderful thing that allows us to tune our cars suspensions despite the design limitations caused by the PMI. Proper manipulation of slip angle is the great equalizer and is what suspension tuning is all about. Slip angle is the difference in which a cars wheels are pointed vs the angle that the tires contact patch is placed on the road. The main thing that affects slip angle is the manipulation of the individual load placed on each wheel while cornering. This is the key for suspension tuning. A front wheel drive car has most of the weight on the front wheels. So the front wheels run at higher slip angles and develop understeer. Conversly the same for a rear wheel drive, rear engine car developing oversteer. That is also a reason why a mid engine car with equally loaded tires will be more or less neutral. Slip angles, weight distribution and PMI are the main factors in how a vehicle will handle.

Because our cars are front heavy, front tire overloaded, front wheel drive cars, does that mean that we are condemned to econobox hell for driving fun? Heck no! By design we can not change the basic layout of our cars to significantly change the PMI or weight distribution but we can sure tweek the slip angles of the tires to achieve world class handling out of our killer econo transportation units.

The easy way to tweek the slip angles are with anti-sway bars and springs. Shock absorbers, going against what people think that they do, are not really for changing the handling balance. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient (which is big word for a change from straight line travel to turning) maneuvers like initial turn-in and zig zaging around slalom cones.

Changing to heavier springs changes the slip angle differential by resisting the cars tendency to roll on the end of the car that they are installed on. The resistance of the heavier spring to compression causes more weight to be transferred to the outside wheel of the end of the car that they are installed on as the car tries to lean over in a corner. This causes that wheel to proportionally run at a higher slip angle than it normally would. If you put heavier than stock springs in the rear of your car, while not changing the spring rate of the front, the car would tend to understeer less.

Antisway bars work in much the same way. Antisway bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Antisway bars offer resistance to independent side to side wheel movement. This is how these bars limit sway in the turns and hence their name. While limiting sway, the antisway bars also cause weight transfer to the outside wheels. By altering the diameter of the antisway bars or installing them where there were none before adds yet another chassis tuning element. If you were to increase the size of the rear antisway bar on an SE-R you would be increasing the amount of weight transfer to the outside rear wheel, thus causing it to run a bigger slip angle. This would give you more oversteer.

Tire pressure also can affect the slip angle. Higher pressures reduce the slip angle and lower pressures increase it. A great deal of suspension tuning can be done for free by adjusting the tires pressure.

Alignment also has a great deal of effect on a vehicles handling balance. Caster and camber affect how a tires contact patch is positioned on the ground by compensating for a tires tendency to flex and lift the inside tread while cornering, By helping keep the tread flat, these settings can increase or decrease the available friction circle traction on an end of a car thus affecting balance. Toe in or out can affect balance also by changing how a vehicle turns in.

Here are some general basic rules if you want to improve your cars handling:

• If you race in sanctioned competition, read your rule book before performing any modifications at all to your vehicle. It would be a pity to buy a fortune in trick parts only to have them declared illegal by an official. Beware of SCCA’s Street Prepared rules as many hot and popular street car mods would put you out of this class.

• Do not lower your car too much! This perhaps is the number one no no. It is a symbol of a true rice-boy (not a racial slur, means all show no go, typical car that has $5000 bucks of wheels and tires, a big 5 inch tip, aero kit, a thumping stereo, vinyl graphics, no springs and not much else). Lowering looks really cool and can make a significant improvement to a cars cornering capability but going too low is detrimental to both handling and even safety. Going too low can cause bumpsteer, where the tie rods and control arms are traveling different arcs resulting in the wheels steering themselves with no steering wheel input. When a car is so low that the suspension bottoms under cornering loads, the end of the car that bottoms first will violently slide out. Super low guys are convinced that they are driving super touring cars but if you take them out on the track they will suck incredibly. Our older Porsche 911 65-89 are cursed with short travel suspension as it is and cannot take being lowered more than 1.5-2 inches at the most. You want to maintain at least 1-3/4" of travel.

• Buy and install matched components from a single manufacture. For instance do not put H&R front springs in with Eibach rear or a Suspension Techniques front bar with a TMC rear. Suspension manufactures usually offer these parts as a tuned set with rates, etc. made to match each other. So unless you really know what you are doing don’t mix and match. Good stuff for our cars is made by Eibach, H&R, Suspension Techniques, Ground Control and Stillen.

• If you are using high performance springs, try to wait until you can afford the shocks also. Performance springs store more potential energy when they are compressed. They need a shock with more rebound damping to keep the car from bouncing all over the place after you hit a bump. Really good shocks like Koni or GAB’s are adjustable so you can tune your shocks to the springs. Performance springs with stock shocks usually feel floaty on high speed undulations. I find that that feels a little spooky. Performance spring’s quicker rebound characteristics also seems to wear out stock shocks quickly making them get super floaty.

• Limiting body roll is good. Install stiff springs and swaybars with matching shocks, and lower the car to a reasonable level. Limiting roll keeps the weight from transferring excessively, allowing the inside tires to work more in a turn. Limiting roll also helps keep the car from bottoming in a turn and keeps the car out of the bumpsteer zone. McPherson strut cars (> -89) like ours do not gain negative camber under roll either so limiting roll helps keep the tires from folding over.

• Having adjustably is good. Having the ability to adjust shocks, camber and toe is very useful when trying to extract G’s from your car.

• Align your suspension and optimize your tire pressures. In the previous paragraph, I said that alignment and tire pressures can make a huge difference. IT IS TRUE! If you are poor, you can still make big improvements in your cars grip by just playing with tires pressures and the car’s alignment. Try the poor boy technique and dial in some front negative camber, increase the front tire pressure, decrease the rear and set your toe. Boy will you see a difference. Try to find a place that does racing alignments near your house as alignment is where people really get ripped off as it is almost never done correctly. Most hacks just throw a car on the rack and if it falls somewhere within in the wide factory specs, don’t touch a thing. What you need is a blueprint type alignment where the suspension is adjusted exactly to spec. Most repair shop dorks don’t understand this and will argue and tell you that that is not necessary. A race prep shop will understand. When your car is aligned, it should be done with your weight in the drivers seat and with the technician bouncing the car after every adjustment to settle the suspension. Set your tire pressure before you take the car in. Remember that you must realign the car if you lower it!