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Suspension 101 - Components of Suspension & What They Do

Handling of a vehicle is determined by many variables. Some of these are tires, center of mass (a.k.a. center of gravity), chassis torsional stiffness, suspension geometry, suspension components (springs, shocks, anti-roll bars), etc., some of these are discussed below.

Springs are energy storing devices. They store energy by deflection. Most springs are constant rate springs, i.e., amount of the force that is stored is proportional to the deflection of the spring. It is usually expressed as:

F = k * X

where F is the force (in lb., at least in the U.S.), k is the spring constant (or spring rate) in force per deflection (lb./inch), X is the deflection (in).

There are also progressive springs. Progressive spring's spring constant (k) increases with deflection.

Springs absorb bumps, limits the motion of the vehicle due to acceleration, braking, cornering, etc.

Shock absorbers dampen the motion of suspension. Shock absorbers do not absorb impacts; springs do. Shock absorbers are dampers.

Shock absorbers also control the transient motion of the vehicle. Shock absorbers control HOW the car nose dive when brake is applied. Springs control HOW MUCH car nose dives. Shock absorber controls HOW the car goes into roll when the steering is applied. Springs and anti-roll bar control HOW MUCH car rolls.

There are some adjustable shocks available. Adjusting the extension of the shock is called "rebound," and adjusting other direction is "bump."

Anti-Roll Bars
Anti-roll bars are also known as roll bars, sway bars, anti-sway bars, etc. Anti-roll bars connect right and left wheel. They resist roll by twisting themselves, acting as torsion springs.

Weight Transfer
Weight transfer is a shifting of loading on tires due to acceleration. Increasing speed, braking, and cornering cause weight transfer. For transverse acceleration, weight transfer can be expressed as:

dW = (m * h * a) / t

where dW is the resulting weight transfer due to acceleration a (in m/s^2 for metric unit and G in U.S. unit when lb. is used for mass), m is the mass of the vehicle (kg for metric unit and lb. for U.S. unit), h is the height of center of mass, and t is the track width. For longitudinal weight transfer, use wheel base instead of t.

This equation calculates the total weight transfer but does not calculate how much weight is transferred by front or rear wheels. It required more complicated equation and a lot more details and assumptions to calculate the weight transfer by each wheel, and I am not about to get into that.

Just remember that, if you make on end of the car stiffer, there will be more weight transfer at that end and lose some traction capability due to increase loading on that end.

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.

Handling characteristic that, when car is turning on the constant radius circle, front end of the car pushes to the outside of the circle with increasing speed. Racecar drivers call this “push”. Turning radius of the car increases with increasing speed, or more steering is necessary with increasing speed. It is caused by front end of the car having less traction than rear end. 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 is best if you are an inexperienced driver. When the car understeers you should re-gain control if you let off the gas. 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 - scrubbing off significant speed.

Handling characteristic that, when car is turning on the constant radius circle, rear end of the car pushes to the outside of the circle with increasing speed. Turning radius of the car decreases with increasing speed, or less steering is necessary with increasing speed. It is caused by front end of the car having more traction by rear end. The 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. 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.

Ideal condition that is rarely achieved. Car turns on the constant radius circle with constant steering angle with increasing speed. As strange as it seems, some racing cars can do this, until you lose traction and get into severe oversteer. 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 (PMI)
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, or a front wheel drive car like an Integra Type R; 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. 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 what 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.

Suspension 102 – Tuning your car to Perform

Springs, Shocks, & Anti-Roll Bars – The Answer to Slip Angles
The easy way to tweek the slip angles are with anti-sway bars and springs. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient maneuvers like initial turn-in and zigzagging 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. 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. Anti-roll bars work in much the same way. Anti-roll bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Anti-roll 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 roll, the bars also cause weight transfer to the outside wheels. By altering the diameter of the anti-roll bars or installing them where there were none before adds yet another chassis tuning element.

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

Symptoms & Solutions of Tuning Problems
Entry Understeer - Increase the rebound of rear shocks. Decrease the bump of front shocks. Increase the rear brake bias. Brake earlier.

Entry Oversteer - Decrease the rebound of rear shocks. Increase the bump of front shocks. Decrease the rear brake bias.

Steady State Understeer - Stiffen rear spring and/or anti-roll bar. Soften the front springs and/or anti-roll bar.

Steady State Oversteer - Stiffen front spring and/or anti-roll bar.
Soften the rear springs and/or anti-roll bar.

Exit Understeer - Increase rebound of front shocks. Decrease bump of rear shocks. Stiffen rear springs/anti-roll bar. Soften front springs/anti-roll bar.

Exit Oversteer - Decrease rebound of front shocks. Increase bump of rear shocks. Soften rear springs/anti-roll bar. Stiffen front springs/anti-roll bar.

Car is slow to respond to driver's input - Stiffen the car by springs, anti-rollbars, and shocks. Increase tire inflation pressure.

Car hops over the bump - Soften the car. Decrease tire inflation pressure.

Basic Rules for Superior Handling

1. Do not lower your car too much! This perhaps is the number one no-no. It is a symbol of a truly ignorant person with regards to suspension dynamics. 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 most likely be slower then the rest of the pack. You want to maintain at least 1-3/4" of travel.

2. Buy and install matched components from a single manufacturer. 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. Performance springs with stock shocks usually feel floaty on high speed undulations. Quicker rebound characteristics on performance springs also seem to wear out stock shocks quickly, making them get floaty. Limiting body roll is good. Install anti-roll bars. 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.

3. 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. On a showroom stock racer optimizing the alignment and tire pressures alone can make the car corner a lot better and have faster lap times. As front camber is not adjustable on our cars (unless you purchase a kit) it is important to make it so. These will slightly harshen your ride but will sharpen turn in due to the elimination of squishy rubber with metal bearings. Rear camber adjustably is not critical on most FWD cars.

4. Align your suspension and optimize your tire pressures. 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!

5. If you haven’t, cut your bumpstops. It is important to cut the rubber bumpstops on your shock shafts to get a little more wheel travel out of lowered suspensions. Cut an inch or one segment out of the front and rear ones. Do not go crazy and remove the bumpstops or cut off more than half of them away. That can cause the suspension to bottom out possibly breaking it. If the car suddenly gets real bouncy, that is what probably happened. The only solution is to get new shocks.

46 Posts
nice post! but i have a "dumb" question...i always see Shocks & Struts used together...Shocks and Struts are the same thing and can be used interchangeably, correct? or is struts a shock but a shock is not a strut, vice versa? :confused:

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10,086 Posts
dabuda said:
nice post! but i have a "dumb" question...i always see Shocks & Struts used together...Shocks and Struts are the same thing and can be used interchangeably, correct? or is struts a shock but a shock is not a strut, vice versa? :confused:
Don't know if Dabuda has been looking lately (hope so -- howya doin', Dabuda??) -- but I didn't know the answer to this "dumb" question either, so I checked it out.

And look what I found (google sure is cool!) -- this is from Monroe's website (that's a shock company):

Q: What is the difference between shock absorbers and struts?

A. Struts and shocks are very similar in function, but very different in design. The job of both is to control excessive spring motion; however, struts are also a structural component of the suspension. Struts can take the place of two or three conventional suspension components and are often used as a pivot point for steering and to adjust the position of the wheels for alignment purposes.

So yes indeed, you could say that a strut is a shock (as well as being other things too), but a shock certainly isn't always a strut.

Hyperion's post is tremendous. This really does answer a lot of things about "handling" as well as about suspension.
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