Making changes and modification to the car is a fine art, just like driving it on the track. Very small a change can make very great a difference. Note that we refer to "difference" rather than "Advantage", because everything related to suspension geometry modifications is a balance between different things: Stif vs. Soft, etc. However, with the driving enviorment being preset, it is possible to adjust the car in an advantagous manner accordingly.
For the sake of efficiency, one must isolate twicks and drive the car smoothly after each seperate change. This will enable to approach the results of the changes in a siecentific way.
The idea of a succesfull suspension geometry is to positively effect the car's stability and Grip according to the needs of the driver.
Suspension geometries create compromizes between road isolation (dampening of bumps), road holding (a flat and large tire contact patch) and road handling (car balance).
Suspension geometry, weight transfer and load transfer:
One important concept in that regard, is weight transfer, which is a distribution of the car's mass (as if the car's weight transfers across it) according to physical forces acting on the car. These forces, act upon two different parts of the car apart: The unsprung weight, which are the tires, wheels and brakes, not supported by springs, and are less relevant to our subject, and sprung weight, which results in chassis lean, creating effects of weight transfer known as "dive" (forward weight transfer), "pitch" (or "Squat", rearward weight transfer) and "roll" (sideways weight transfer). Supension geometries cannot effect the unsprung weight transfer (also called "load transfer"), only change the speed of weight "transition" and the relation between the sprung and unsprung weight. However, it has an active effect over sprung weight. Keep in mind, though, that the physical forces could not care less of your suspension stiffness, and the load not dealt with by the chassis, will be put over the tire. More information on weight transfer can be found in Grip.
Struts are dampers with a fitted spring. In fact, what is normally called a "damper" or even a "shock absorber", is typically a strut.
Tires and Rims
The tire is the final station of the suspension and is the most important one. A well-dampered wheel with a bad tire is worst than a bad damper with a good tire. The importancy often associated to the rim is also nothing in comparison to the tire's quality. Moreover, contraty to a common belief, rims with a greater radius do not nessecarily improve performance. Actually, it will generally decrease performance. The deciding factor is the tire's width. Wider rims stabilize the tire, enabling less side-collapse, and also generally weight less. This is particularly important because the speed of the wheel's travel increases the load generated by the rim. This weight, to differ from that of the car, is not laid on the springs, and results in additional demands of the dampers.
A rim of a greater radius would in theory allow for less side collapse of the tire, but in reality, width preceds radius.
Contrary to a common belief, the spring, being loaded over the damper, is the one absorbing shocks, unlike the damper, which unjustly earned the name "shock absorber".
a car, compression of the suspension spring is caused when the wheel travels across the front side of a raised bump. A portion of the energy used to cause forward motion of the vehicle is redirected causing the wheel to travel up, which compresses the spring. The spring stores the vertical energy, and as the wheel travels down the backside of the bump, the energy stored in the spring pushes the wheel back down. For safety and handling, this has the significant benefit of keeping the tire in contact with the road surface as the tire travels over a bump. A similar process occurs for dips, except that the spring elongates rather than compresses to start with.
Without springs, the wheels would transfer the redirection of the vertical energy into the vehicle chassis and cause the vehicle to bounce off the bumps. This would both be annoyingly uncomfortable to the passengers (a concern to street car manufacturers), and the driver would momentarily lose some or all of the ability to steer, accelerate or brake as traction from the tires would be lost (a safety concern for all, and a maximum performance concern for racing).
With springs, the vehicle body can maintain a relatively linear path (providing comfort for the passengers), while the wheel travels up and down over the bumps (allowing for continuous safe vehicle control, and continued traction for maximum racing performance).
Therefore, the purpose of the spring in an automobile is to isolate the wheel assembly from the body, and allow the tire to maintain contact with the road over surface imperfections.
Coil-Over Springs: There is increasing availability of coil-over suspension setups for street cars. The coil-over is a combined assembly of the shock and spring where the spring rests in a height-adjustable base. Coil-over setups were designed to optimize racing performance, and so several characteristics of coil-overs seem to make them the ultimate choice in suspension modifications.
First, coil-overs allow the car to be lowered more than conventional shock and progressive rate spring upgrades. This of course is a good thing for weight transfer control, but as we mentioned earlier, can cause real comfort and damage-potential problems for a street car.
Secondly, the spring rates are usually much stiffer than even progressive rate springs, and they're usually linear. On the race track, there's no need for a progressive spring rate, and coil-overs with their racing purpose, aren't intended to use them. This also allows the spring to be shorter which is where the greater lowering potential come from.
The main purpose and advantage of the coil-over design is to allow independent ride height adjustability of each corner of the car. This ride height adjustability allows manipulation of the center of gravity of the car. In particular, by manipulating the front to back or side side to side location of the center of gravity, weight transfer can be controlled to influence specific tires on the car. If you can pre-dispose the car to distribute weight transfer a certain way, you can optimize the grip of certain tires and improve the overall speed of the car through corners.
Raising the back of the car puts the CG more forward. Raising the front of the car pushes the CG more rearward. The left or right side could also be independently raised or lowered. With these adjustments, you can reduce the amount of grip lost to specific tires during weight transfer.
For example on a race track with a high percentage of high-speed right turns, you could set the car up to reduce the weight transfer to outside left tire, and increase the weight that remains on the right rear. Compared to the same car with fixed ride heights, the one with the adjustable coil-overs will be able to increase the overall grip through more even wieght distribution during the dynamic load of the right turns.
So, this sounds like an awesome modification to run out and get, right? Well, yes it is -- for racing. Like we say, for the street this is an extreme modification. You may be die hard, and live with the harshness of the ride, but you'll likely have very grumpy passengers.
It is possible to minimize the harshness by using softer springs, or even progressive rate springs for that matter. However, those springs will tend to be longer, will not allow the car to be lowered as much negating some of the advantage. Some people will use stiff springs for the track, then swap to softer springs on the street between events.
Coil-overs are less recommended.
Relative to its shock absorbing function, the spring must be stiff enough to prevent full compression or elongation in large bumps and potholes. However, it must also be soft enough maintain good contact with the road. The softer the spring the better the road contact over bumpy surfaces. However, the stiffer the spring, the better the resistance to bottoming out on large bumps. Somewhere between these extremes is a range of good spring rates (stiffness) to work for the expected environment. Contrary to a common belief, a sports car is not nessecary a stiff and unpleasant car. Stifness generates less obedience from the respective part of the automobile. Stiff dampers combined with stiff springs and high quality tires can improve handling. However, without the entire combination, the result achieved will not be better than that of a soft composition.
Body roll, along with "pitch" and "dive", are the tendencies of a car's body to recieve an angle in relation to physical forces acting upon it. Roll, in particular, is the tendency of the chassis to lean sideways in response to the centripetal force working on the car. It is, in fact, a lateral transfer of sprung weight. As such, it is being effected by springs more than any other mechanical component. Note, however, that springs cannot change the amount of roll, but only it's rate and the speed of the weigt transfer.
Roll (lateral sprung weight transfer) is not to be confused with an (unsprung) lateral weight transfer, which is the difference in load on the tire. Even though, these different phenomena are related: The stiffer the spring, the more of the physical centripetal force it can absorb. With the spring being softer, it will only absorb a small amount of force, and the rest of it will be loaded on the tire, reducing it's ability to cope with the different demands.
In order to handle bumps and dips, the entire wheel assembly is designed to have a certain amount of vertical travel length from full extension to compression. The rougher the road, the more wheel travel is needed, and the longer the overall spring length needs to be. Factory passenger cars are designed to function well over a broad range of conditions, and the suspension system in particular must be prepared to compensate for potholes, freeway expansion joints, rutted gravel roads, and other less than ideal road surfaces. Therefore, a street car is designed with quite a bit of suspension travel length (think of how far you have to jack up a car's body to get the wheel off the ground--that's about half the wheel travel)). In a high performance sports car (i.e. of the Porsche, Ferrari, Viper, and NSX type), manufacturers assume a more limited range of road surfaces, and design in less wheel travel by a of couple inches. In the typical sports car or sports sedan of the Mustang, Camaro, Eclipse, Integra, and BMW 3 types, the suspension is a little better than the general sedan, but it's really not a great deal different.
In racing, we can assume a certain degree of ideal conditions, or at least more ideal than public roads. In a stock street car, even notoriously "bumpy" race courses feel glass smooth compared to most public roads. In these conditions, the purpose of the spring can be focused to maintain maximum and consistent contact of the tire with the relatively much smoother road surface. Under these conditions, very little wheel assembly travel is required. The spring can be optimized for smaller wheel travel conditions. For example, a CART or Formula 1 race car driven on smooth courses may only have 1/4 to 1/2" of total suspension travel!
How does wheel travel impact handling? Well, just from the CART example given above, we might assume that shorter wheel travel is better. And, of course, it is. Though the wheel assembly travels up and down, it does not do so on a linear path. The wheel assembly is at some point fixed, and the wheel assembly actually travels in an arc. Whether the body stays put, and the wheel travels (through bumps), or the wheel stays put and the body travels (body roll), this has impact on the camber angle of the wheel which changes the tire contact patch shape.
Therefore, for racing conditions, limiting the wheel travel distance is a desirable thing. For street cars, the use of lowering springs (shorter and stiffer) is one method to reduce wheel travel. In extreme cases, it will also be necessary to use shorter shocks.
Stiffness vs. Roll
So far we have used bumps in the road to illustrate how springs behave. Springs are also acted upon by the forces of acceleration, braking, and cornering. The momentum of the vehicle body in cornering, braking, and acceleration transfers into the springs causing compression and elongation. This is an easy to see effect of weight transfer as it results in visible body roll -- both the side-to-side roll we're mostly familiar with during cornering, but also front-to-back roll -- particularly the "nose dive" under hard braking.
Body roll by itself is not necessarily bad. If the four tires remain flat on the road surface with balanced downforce, who cares whether the car body is parallel to the road or not (aerodynamics aside). What body roll does though is change the angles of the suspension components to the wheel assembly (which we call suspension geometry).
This is pretty much the same thing as we discussed above with wheel travel, except this is from the opposite perspective. With the wheel on the fixed plane of a smooth road, the body now travels, and causes the wheel assembly to travel in the arc we described. This changes the camber and tire contact patch particularly of those tires which are unloaded and the suspension elongates.
Aside from bump absorption, the spring also contributes to the roll stiffness of the car--the ability to resist dive under braking, squat during acceleration, and body roll during corning. The anti-roll bars also play a roll in this, and the two combined create the total roll stiffness of the car. Stiffer springs will resist body roll more, reduce change in the suspension geometry, and maintain a more consistent tire patch size.
Note: many people are under the misconception that body roll causes weight transfer. This is not true. See the weight transfer article for details about this.
The spring's roll resistance characteristics helps to resist the forces during dynamic changes, and make the car more stable during the transition. This implies a stiffer spring is needed to minimize the compression and elongation, and therefore minimize the change to the suspension geometry.
However, even purpose-built race cars cannot simply use the stiffest spring available. If we return to the case of having no springs at all (the ultimate in stiffness), even a "smooth" race track would be violently bumpy without some suspension dampening. At some point the spring becomes too stiff for the road surface, and the vehicle will lose traction as it bounces over surface imperfections. The race technician and driver have to find the most effective balance between being soft enough to allow the tire to stay in contact with the road surface over bumps, and being firm enough to control suspension geometry and keep the tire as flat as possible on the road surface.
A driving enthusiast's car which does double duty on the street and the track has a larger window to find compromise in than does a race car. Putting full race springs on your street car may seem the macho thing to do, and though your car should be faster on the track, it will make your life miserable on the street. In fact, it is quite likely to cause damage to other suspension components when you come across that surprise pothole.
Today, after-market springs offer features that not too long ago would have been found only on race cars. The research done in sports car class racing has resulted in several manufacturers producing high performance progressive rate springs for virtually all sport enthusiast cars that allow an acceptable comfort level on the street yet significantly increase handling performance over the stock springs.
Most factory stock springs have a constant or nearly constant factor of stiffness called the spring rate. As the spring is compressed or elongated, the force required to change the spring's length stays the same. The spring rate is linear as the spring goes from full elongation to full compression. This provides greater comfort across minor and major bumps, but does little to minimize body roll under hard cornering.
Progressive rate springs have a softer spring rate during some initial portion of compression or elongation, but then get progressively stiffer as continued force is applied. This is typically accomplished by changing the shape of the spring. This ability to start soft and get firmer with higher compression allows the spring to accommodate typical street bumps with satisfactory comfort. On the track under high braking or cornering forces, the spring's stiffer region comes into effect to reduce the body roll compared to the stock spring. Compared to a full race spring, there is a little more body roll before the spring takes a firm set, but that's the compromise of a dual purpose car.
Most after-market progressive-rate springs start out about 15% firmer than the stock part, and get stiffer from there. Though they offer acceptable bump absorption, they do give the vehicle a noticeably rougher ride, especially with larger bumps where the spring becomes stiffer. However, given the success of these springs, the comfort for performance trade off is considered well worth it by sports car enthusiasts.
In selecting an after-market spring set, you should know how much stiffer than stock it is, whether it is progressive or linear, and how much it will change the car's ride height. If you're concerned about losing too much ride comfort, you should ride in another car as closely prepared to yours as possible. Some people stiffen their suspensions for periodic racing only to discover they really don't care for it the remaining 97% of their driving time.
You should also know what other suspension changes you're going to make to the car including wheel and tire sizes, and talk with a technician experienced with your car type. Certain combinations of springs, shocks and tire sidewall sizes will function better than others. A mechanic from a specialist shop or race team may offer some advice learned from experimentation and testing.
One other thing related to spring selection is that of vehicle ride height. On the street, the variety of road surfaces, speed bumps, drainage channels, and steep driveways requires the car's lowest point have a certain practical height above the road to avoid damaging the car.
In racing, ride height has significant impact on the vehicle's center of gravity ("CG") which is one of the major influences in a car's weight transfer characteristics. Ideally, the CG should be as close to the ground as possible, and race cars will be lowered as much as allowed by the rules. Open-wheel formula cars are lowered as much as possible without bottoming out while racing which often ends up being 1/2" or less on very smooth tracks.
The most straightforward way to lower the CG is to lower the car, and the most direct way to do that in a street car is with shorter springs. Most street cars can be lowered somewhat from their factory setting, but there are several practical limitations in the design of the suspension system. A realistic compromise needs to be made that considers the clearance needs for the street, and the suspension system of the car.
Extremely low cars ("slammed" in today's vernacular) may look good (or at least look like the racing sedans they seek to imitate), and if done right will handle better on the track, but there are some limitations on the street. Springs which are too short may cause interference problems with other suspension components such as the shocks. Additionally, the suspension geometry (the connection points, and lengths of it parts) are designed with a certain spring length in mind to keep the wheels in proper alignment. A severely lowered car that does not also alter the suspension will cause the wheels to have excessive camber for sure, and will likely also adversely affect the castor and toe. You might think it looks great, but this will severely reduce the handling performance of the car. The spring rate is more important than the slight reduction of the CG's height.
You should consult someone experienced with your car type before just buying the spring which seems to lower your car the most. Such a spring may also require a specific matching shock or other suspension changes to actually improve the handling performance.
Speaking of shocks, it is generally necessary to buy stiffer shocks at the same time you change the springs. Springs alone will lower the CG, and will reduce body roll, but neither is the primary function of the spring. For road imperfections, shocks work in conjunction with the spring, and are designed with each other's ratings in mind. Going over bumps, a stiff spring may resist the first compression well, but without sufficient shock capacity, the car will bounce more than it should afterwards which ultimately reduces the car's handling performance. Also, stiffer springs will prematurely fatigue stock shocks. They'll last a while, but will eventually get weaker and decrease the handling performance.
If you can only afford shocks or springs, either keep saving to get both, or start with the shocks. Performance shocks alone which provide firmer bump and rebound control, and greater control over weight transfer rate, will improve handing performance more than stiffer springs alone will.
Some Great information regarding shocks and tuning can be found here http://www.se-r.net/suspension/shock_tech.html
We mentioned that a spring retains energy to allow it to return to its original shape after being compressed or stretched. Unfortunately, a spring will not just return to its original shape and stay there. You've probably witnessed yourself that if you compress or bend a spring it will oscillate back and forth in smaller and smaller increments until finally coming to rest. If you have ever seen an old car bounce endlessly after going over a bump, you have seen what springs will do in a car with ineffective dampers. This is not good for safe control of the vehicle, and it's certainly not any good for effective handling while racing.
The dampers primary purpose is to control this oscillation. In a passenger car, the designer has the choice of just how fast the damper dampens the spring. If the dampening is immediate, the car will have better weight transfer rate control, but a harsher ride. If the dampening is a little slower allowing perhaps 2 to 3 oscillations, the ride will feel much smoother.
In racing, we want the dampening to be almost immediate. A vehicle's bouncing on the springs creates erratic shifts in the tire contact patches and mechanical downforce on the tires. Both of these conditions reduce the effective grip the tires have. Any bounce in the body of the vehicle must be eliminated quickly to allow full grip to return as fast as possible to the tires.
However, like springs, it is possible to have too stiff a damper. First, if the damperss are stiffer than the springs, the springs will be overpowered, and will not actually fulfill their bump absorbing function.
Secondly, a damper has a major affect on how quickly weight transfer occurs in the dynamic changes of accelerating, braking, and cornering. The stiffer the shock, the faster the weight transfer occurs. This will help the vehicle have very responsive steering, but the transfer can be too fast for the driver.
During cornering in particular, the driver must be able to induce smooth weight transfer and feel the tires reach their maximum grip. If the weight transfer occurs too fast, the driver will not feel the tires approach that peak grip, and will likely overshoot the traction capacity of the tires causing excessive sliding or spins.
When modifying your car, don't immediatly go for the full professional racing gearing. Full-race dampeners are going to be too stiff for the street, and will likely cause the car to bounce off of bumps. Additionally, you'll probably not have the sensitivity to feel the grip level of the tires when cornering at maximum speed.
To help with the dual purpose street/track car, and to provide some adjustability for tuning handling performance, there are several after-market "shocks Absorbers" that are adjustable. A manual (or even electronic) dial allows selection of several settings which are progressively stiffer. These shocks can be turned to their softest setting (they're still going to be stiffer than stock) for a smooth ride on the street, and their firmest for the track to minimize body roll and increase the steering responsiveness. The adjustability also allows finer tuning of handling performance for a given track. As discussed in the weight transfer and the handling tuning sections, adjustable dampers can be used to help adjust out small amounts of oversteer and understeer.
If you can't afford adjustable dampers, don't assume that stiffer is better in a fixed rate Damper. Shocks should be selected knowing the springs they will be used with. Too stiff a dampener will overpower the spring reducing its effectiveness. If the shock is not adjustable, then matching the damper to the rating of the springs is even more critical. You should consult a shop experienced in this matching.
Talk with a technician familiar with your car, and find out what shocks offer the best performance for you car's degree of modification. The "killer" damper for someone else's car may not be the best one for yours.
Bound or "bump" is the stiffness of the shock in the donward direction. A shock with a softer bound reacts faster, but can hinder lateral adhesion in a corner.
Rebound is the stiffness of the damper when pressure is applied upwards. A stiffer rebound would delay the car's response and make it more gradual and smooth. Read this story by Ross Bentley:
Anti roll-bars are bracing connected between the lower portions of parallel wheel-arms, decreasing body roll while driving. However, unlike springs, they do not have any other physical work to do, and can therefore achieve this purpose more efficiently. They add to the roll resistance of the suspension springs for a higher overall roll resistance Because the primary purpose of the spring is to maintain maximum contact with the road surface over imperfections, we must settle for the roll resistance provided, and it is rarely enough. The anti-roll bar adds to the roll resistance without resorting to an overly stiff spring. A properly selected anti-roll bar will reduce body roll in corners for improved cornering traction, but will not increase the harshness of the ride, or reduce the effectiveness of the tire to maintain good road surface contact.
So, how does limiting body roll improve handling? The suspension system geometry (the lengths and connecting points of its parts) of a street car is designed to keep the bottom of the tire parallel with the road for maximum contact patch. At rest, the car's suspension has a particular geometric relationship to the road surface. Body roll changes that relationship, and reduces the suspension's ability to keep the tire parallel to the road.
During body roll, the car body is no longer parallel with the road, and neither is the suspension geometry. Even though the suspension allows the wheel to be somewhat independent from the body, the high cornering forces, and resulting large body roll of a factory car, on the track take the suspension close to its limits where it affects the angle of the wheel.
Large amounts of body roll cause the wheels to tilt away from the corner which lifts the edges of the tire and reduces the contact patch size. While this can be compensated for by having the wheel purposely tilted inward to start (adding negative camber), there is a practical limit to this which is not enough in most cars to compensate entirely for the body roll. The anti-roll bar reduces the amount of body roll, and therefore helps to maintain as much of the contact patch as possible.
As with all good things, more is better only to a point. Because the anti-roll bar connects the left and right sides, this reduces the independence of independent suspension. Too stiff a bar, and you can cause too much loss in the ability of the left or the right wheel to independently respond to road surface imperfections. The purpose of suspension is to maintain maximum tire contact with the road. The purpose of independent suspension is to allow the left and right wheels to each seek that contact separately. The left wheel may need to be going down when the right needs to be going up. If they were tied together as with the old floating rear axles, one or both of the wheels is not achieving maximum contact. In fact, too stiff an anti-roll bar can actually cause an inside wheel to lift completely off the ground during hard cornering.
When cornering, the bar will twist with the outside end being pushed down, and the inside end being lifted (just like the body of the car). On the outside tire, this downward pressure helps increase tire traction. However, on the inside tire, the anti-roll bar is pushing up on the suspension reducing the downward force the spring is trying to place to keep the tire on road. If the anti-roll bar is too stiff, it will overpower the spring, prevent it from extending enough to keep the tire on the road, and the wheel will actually lift off the ground. This is not an optimum situation, but it is common in several racing classes. The cause is not so much poor engineering, but the limitations of the class rules that allow the engineer to compensate for it.
A sway-bar has another slightly different purpose, which is roll coupling. Roll coupling is the relationship of the roll resistance of the front of the car and the roll resistance of the rear.
The balance of the roll coupling, because of its effect on traction, has influence on whether the car has a tendency to understeer or oversteer. While this can be caused by several factors, the anti-roll bar (especially, an adjustable one), can be used to compensate.
As we mentioned, the anti-roll bar helps increase the mechanical downforce of the outside tire during cornering. This increases the traction of that tire, and that end of the car (front or rear). An increase in traction at that end, may leave the opposite end with too little traction. An imbalance of traction occurs, and one end of the car will lose traction before the other end. If the front tires lose traction before the rear tires, the car will understeer. If the rear tires lose traction before the front tires, the car will oversteer. Changing the anti-roll bar stiffness can adjust this out.
Types of Geometry
More information herein: http://www.carbibles.com/suspension_bible.html
Camber is the lateral incline of the wheel. I.E. A tire is not straight upside-down, it is tilted to that the top of it is pointing "inside" towards the body of the car. This is designed to decrease the negative effect of body roll while cornering. When turning right, the car leans left, forcing the outside wheel to lean out too, giving it more positive camber, which is contradicted by the existing negative camber, to keep that particular wheel (front outside wheel) flat against the surface.
Castor is the angle of the wheel when looking at it's profile. This angle dictates the "height" of the contact patch. I.E. the point in which the tread elements face the greatest loads and adhesive demands, after which they begin to get away from the surface untill becoming airborne as they rotate back around the wheel untill they again meet the road surface. This retreat of the tread is what gives us the feeling of the steering wheel, the feedback to it, and it's "will" to straighten back up. Positive castor (there is no negative castor) leans that peak (the center of the contact patch) forward slightly.
The effects of more castor are heavier steering, but also more feedback and even more grip. It also creates more wear. One of it's main effects is it's relation with camber. This angling of the wheel in castor, will generate more negative camber during a corner. One must be aware of this when adjusting either of the two, or when he has to decide which of them to adjust to recieve the nessecary effect.
"Toe" can be in or out, and is defined as the lateral angle of the tire when looking from above. I.E. The tires can be angled to face away from the car, or face towards the car's body and "back" towards the outside. It effects the car's stability. Toe-in results in more understeer, hence a slower response turning-into a corner, less front grip inside the corner and even less stability on the straight. Toe out would generate a quick response, and some oversteer. Rear-wheel toe is only adjustable in cars with an autonomous rear suspension, or can be changed via rear-steering or through "bump-steer". The first is often desireable, the latter is not. Also, rear wheel toe should never be set to "out".
The suspension has an effect on following handling characteristics: