Car ''tuning'' 101

[klorophil]

Hi there just thought that I might share with you a little guide that I made a while back for another forum.Well it's not exactly small but I think it might be useful to some of you if not just a good read.Back then I tried to cover the basics so I'll share this with you and try to improve on it.Since that time I learned an awful lot of things on car engineering theory so I'll modify my original text accordingly.Also if you do not agree with what I'm saying please feel free to reply.

For those questionning my experience and validity as a so-called tuning ''specialist'', well I have been tinkering with cars, especially import sport compacts since I was 15, so I've been doing this for the last 10 years or so.Also, I'm a trained car technician I graduated at Québec's Wilbrod-Bherer technical school in 2004, and I've worked at a BMW dealer for 1 year and am now working at a Subaru dealership.I also did 3 years of mechanical engineering.Like I said I repair cars as a living and also do many sidejobs by modifying rides.

So enough introduction into the meat of the subject.

[klorophil]

Improving your ride

So you want to make your ride faster, better looking or more comfortable.Well that's a good idea and it can be a lot of fun.It will also make your car unique and be custom tailored to your taste.Depending on what you drive there's an awful lot of parts you can install to make your car better.Even if you drive a rare one off, there are many things available in the car customising market.It's a huge place and if you don't want to loose money you need to be informed.That's what I'm trying to do here inform and help you decide what to do with your car.I will try to focus on the mechanical part of tuning, since that's what I know best and also because I'm not very interested in ICE (in-car entertainment).My background is also in mechanics so I don't know all too well body kits and the technology and craft behind them.Then again, most of the time what will be the most expensive are the mechanical bits so you need to know what you're doing.For example, a certain type of parts if not properly installed or if not chosen right can be detrimental not only to your engine but also to your safety.That's the whole purpose of this guide.To help you choose and research the right parts for your car.

One of the first step you should do is to come up with a budget and a plan.The budget part should be easy, and it's all up to you.With the vast choice you have before you, you can easily invest 500$ and make your car faster or spend 50k$ and have an undrivable monster.That's why you need a plan.That is you need to know what is the most important thing you want to change about your car.What is the purpose of your vehicle is it a comfy cruiser from A to B but with some added punch or an all out drag monster.This is very important as it will dictate what you need to do to your car to achieve your goal.This with a realistic budget should give you the ride you want and in a short amount of time and also hassle free, or almost.

To achieve the plan you need to know your car.What is it you don't like about it what is it you like.Every car that leaves the factory was built and designed with compromises.Suspensions, tires, engine everything was made to satisfy the biggest crowd possible.And do not ever forget that it was built with a budget.It's entirely possible that you do not associate with those boys and girls for which the car was built.So identify what you would have liked to have in your car.The feel of a car is entirely subjective so go with your tastes not those of others.

One very important thing to understand is that if you remove an OEM (Original Equipment from Manufacturer) part and replace it with an aftermarket one there's all the chance that it will show in your daily life with the car.If you're building an all out race car its not that important but for a daily driver it might be advisable to know the limits which you do not want to break.A triple plate clutch may reduce slippage but it will take the calf of Arnold Schwarzeneger to push the pedal and with a carbon/ceramic disc its either on or off.Not something I would recommend in town or during rush hour.Just remember that if you improve one aspect of a car there's likely to be another effect somewhere else and that it may not be to your taste.So that's why it's important to know what you're doing when modifying your car.

A word on warranties.Basically anything you do to the mechanical parts of your car will render the warranty null and void.I'm not a lawyer but thats the case most of the time.Also if you install a body kit and you have a body warranty regarding rust and perforations it's not too difficult to understand that it won't apply anymore.As for installing all these parts please try to find a trusty mechanic.You might be able to do a lot in your backyard but for certain things it's best to call in the experts.Mechanics are the only ones with the knowledge and manufacturer support to understand fully your particular car.Moreover, most of them will love you for making them install performance parts on a car.I should know because that's my job and I love it when someone asks me for a turbo upgrade. :D

So that's the first step next up is the Tires Section as I feel that's the first thing you should change in most cars.

[klorophil]

Tires

First, let me ask you this question what is it that makes you slow down when you apply the brakes.The obvious answer is the brake themselves but that is far from the truth.It is your tires.Sure the brakes stops the rotation of the wheel and it might be a good idea to change those if you are to add more engine power, more on that later.However, what really makes you slow down is your tires.It's easy to understand that when the wheel stops rotating your tires will also stop, then it should also be easy to understand that whitout proper connection to the ground all that stopping won't do anything as you will slide your way into the nearest lampost.So tires should be your top priority.A good set of rubber is what will make your car go fast, stop properly and corner with aplomb.

On a stock car changing for better tires makes the biggest difference.There are loads of makes and models of tires for your car you just need to find the one that fits your style.For example an extreme performance tire that is DOT-legal might be excellent on a dry track, but will feel like driving on ice on a greasy patch of wet road.So you need to know what you want to do with the car.See the plan is coming in again.Is it for the occasional track-day or for pure racing?Is it for some occasional run down your favorite road or for the quarter-mile?The answer to these questions is what will dictate your choice of tires.

[klorophil]

Tire size.

Every tire sold has a universal code on its side.I guess many of you are familiar with it.It might look like something this way: 225/45HR17.So I'll explain what all those numbers and letters mean.

225: That is the width of the tire from sidewall to sidewall in milimeters.This is handy when comes time to determine contact patch etc... Basically, the wider the tire the more grip it has.225 mm is 8,85 inches.

45: This is the height of the sidewall expressed as a percentage of its width or the tire aspect ratio.In this case 45% of 225 mm which makes 101,25 mm or 3,98 in.Theoritically, the lower the sidewall the more rigid the tire will be and thus the less contact patch deformation under load.Less contact patch deformation is good, it gives you more grip.I say theoritically because, there are certain everyday issues that become problems when you have too low a sidewall.

H: This is the speed rating of the tire.When a tire rotates it creates friction with the pavement this is what makes your car go forward or brake.However, this friction has some drawbacks.The one we are interested in here, is heat.Rub your hands together and they will heat up, well it's the same for the tire and the road.At high speeds this can be quite massive.Since your tire is inflated with air, this air will expand when its heated ultimately blowing the tire open.Not a good thing.Since, we are mostly concerned with performance cars here, it's a good guess to say that you will go fairly fast with it.Thus it's important to have the right speed rating.It's quite difficult to judge how faster your car will be after you tune it, that's a job for engineers.So as a rule of thumb, you should go for the highest rating available in the size you want.Most of the time that would be a Z rated tire.Note though, that not all manufacturers print the speed rating of their tires.As of 1991, tire manufacturers who sold their tires in North America were not obliged to print the speed rating on the tire.However, this is available in the tire service description.I'll touch this in a little bit.

R: This is the type of construction of the tire.R means radial, which in turn means that the layers of rubber and materials that compose the tire are laid radially around the center of the tire.There are 2 other types of construction and their associated letters.D is for diagonal which means that all the layers are criss crossing each other on top of one another.This is what is called a ''bias ply'' or ''cross ply'' tire.This is very uncommon for road tires.B is for belted.It's basically a ''cross ply'' tire but with metal belts under the tread of the tire.This is old-school, almost no road or race tires are constructed this way nowadays.R is what you will find and what I would recommend for road or track usage.

17: This is the diameter, expressed in inches, of the wheel that the tire can fit or its internal diameter.A WORD OF CAUTION! This coupled with the aspect ratio, will be used to calculate the outside diameter of the tire.It's important although not compulsory to keep that inside the OEM measurements.If not, your speedometer will read wrong.A bigger diameter, will make the speedo read slower than you really are, while a smaller diameter will do the contrary.While some modern cars can be calibrated correctly some of them don't.Also, ABS, variable steering, traction control and ESP all take into account the speed at which the wheel is turning.If you change the OEM diameter your wheels will turn at a different speed and it can mess with all of those.

[klorophil]

Tire Service Description.

Since 1991 new tires didn't show the speed rating as I explained earlier.This was replaced by 2 numbers followed by a letter at the end of the size code printed on the tire.It should look something like that: P195/60R15 87S.

87: This is the load index or how much weight 1 tire can support.In this case that would be 545 kilogramms or 1 201 pounds.

S: This is the speed rating of the tire.You have to understand that this is associated with the load index.The S here means that at 545 kg on each tire you can travel at speeds up to 180 kph or 112 mph.

545 kilogramms might seem really small but if you multiply that by 4 then you can put 2 180 kg of weight on those tires and still be able to traval at 180 kph.That's a staggering weight.Staying in the sport compact world, most sports cars weight in at 1 300 to 1 400 kg.Add maybe 100-200 kg to that for full passengers and fuel and you are still way far from the maximum weight permittable.I wouldn't recommend going 180 kph five up on public roads, while the tires, as demonstrated here, will be able to support the weight, that doesn't mean they will have the grip to turn or stop the car.

[klorophil]

UTQG.

UTQG is probably the most useful information you can find on any tires.UTQG means Uniform Tire Quality Grade, and it is just that.Each tire sold in North America has a UTQG number printed on it.It might look something like that: 140 A A.

140: This is the treadwear grade.Without going into the details of its determination, this is just how much the tread will wear over time and use.It's a comparative value, what this means is that the tire is compared to a test tire with known properties and then given a percentage of its durability over the test tire.In this case, it means that this tire will wear 40% slower than the test tire.The value of the test tire being 100 and this tire's value being 140.For anything used as a daily driver I would not recommend anything below 220.Normal all-season tires have a rating of 380 to 480.Note however, that in theory the softer the rubber compound of a tire is, the more grip it will develop but in turn will lose durability.Then, a low treadwear grade can promise more grip than a higher grade but it will wear faster.

A: The first A when there is a space between the other one, signifies the wet coefficient of fricition of the tire on wet concrete and asphalt test surfaces.There are 4 ratings, AA, A, B and C.AA is the best performing while C is the worst.While this is useful, it has to be taken with a grain of salt.Firstly, the way the test is done is by braking some wheels mounted with the tires until they lock, when this happens fancy electronic gizmos calculate the coefficient of friction.Thus, it could be said that this only reflects the capability of the tire to brake not turn.While this is true, in the real world two things happen.First, the same rubber compound is used for braking and lateral grip, so depending on tread pattern and tire construction we can assume that they will be fairly similar.Secondly, on a winding track or road, braking performance is the most important thing.You go faster by braking harder.

A: The other A is just another speed rating for the tire.A is good for above 115 mph, B is between 100 to 115 and C is between 85 and 100.Note that this is not very precise and I would normally use the service description to determine speed ratings.

[klorophil]

Tire Conclusion.

So that's about it for the tires.With the following information you can decide which brand and model of tires would best suit your needs.Also note, that living in Canada, I'm only familiar with what is available here.There might be other useful information elsewhere in the world.

Just a small note on the legal side of things.Here, in Canada, you cannot have a tire that is wider than the outside width of the car.There are ways of getting over this law and I will discuss them later when talking about wheels and chassis alignment.Also, no slicks are allowed on public roads.I know that there are DOT approved slicks available in the US but those are illegal overhere.A slick tire, is just a tire without any form of tread pattern or a very small one just to let water out, i.e. a racing tire.

A little word on tire pressure.On the side of each tire is marked the maximum inflation pressure.THIS IS NOT what is recommended by your car's manufacturer.This is only the maximum pressure the tire can hold.It is most useful for your mechanic when he mounts your tires on your wheels.Finding the right inflation pressure is fairly easy.This is written on a sticker, located either in the front driver's door or in the glovebox compartment.It's also available in the owner's manual.Having the right amount of pressure in your tire is of utmost importance for performance driving.While the tire itself provides grip, having it under inflated or over inflated will affect it's ultimate ability.It will also wear the tire faster.

So that's about it for the tire part.You can find lots of information here: http://www.tirerack.com as well as longer descriptions of what I just talked.Next up is the brakes.

[T-Bird]

are you going to talk about building forged engines and turbo upgrades/kits, fuel systems modifciations, ECU tuning and standalone operation etc... I'm in the process of doing all those things right now on a few different cars. just curious how in-depth you are planning on getting.

[79TA]

That all depends on how in depth his friend from the other forum originally was. So far it looks like basic tips for going after aftermarket parts.

[HeilSvenska]

Maybe there's more, but right now it seems more like tire buying guide. No offense, but it does seem like it.

[RC45]

And a word on the "warranty void" issue - this is not 100% accurate. And I would hope such a guide is quite accurate.

Maybe in communist canada they can void you warranty over trivial items, but the consumer int he US is protected - and the manufacturer can only void the warranty on the componet that was modified or componets reasonably affected by the mod.

So an airfilter mod wont void your tranny warranty, and a rear wing wont void the rust warranty on your hood ;)

Oh - and this paragraph is very misleading:


As for installing all these parts please try to find a trusty mechanic.You might be able to do a lot in your backyard but for certain things it's best to call in the experts.Mechanics are the only ones with the knowledge and manufacturer support to understand fully your particular car.

The dealership is going to be the first one likely to try void your warranty and most are ill equipped and less than enthusiastic about supporting any kind of aftermarket. Not to mention not every technician is, lets say - as detail orientated as one would want them to be.


Moreover, most of them will love you for making them install performance parts on a car.I should know because that's my job and I love it when someone asks me for a turbo upgrade. :D

Then you and your dealership ar equit eunique and not the norm, I would even hazard a guess you are in the 1% minority. :)

[79TA]

hmm, perhaps a useful topic would be scheduling your family and friends to have a Saturday free so that they might have time to help you drop in a built engine . . .

sigh, I just hope I can get my engine in before 2008 ends. :-(

[klorophil]

That all depends on how in depth his friend from the other forum originally was. So far it looks like basic tips for going after aftermarket parts.

That is effectively what I'm trying to do.Also I have to rewrite pretty much everything as I did this guide a couple of years ago.Back then a) my english wasn't as good as it is now, so I have to make sure the previous sentences etc... makes sense and b) I learned a lot since then and find that many if not all the parts that I included are not 100 percent accurate.

Also, view this as only my opinion on tuning and modifications.

[klorophil]

And a word on the "warranty void" issue - this is not 100% accurate. And I would hope such a guide is quite accurate.

Maybe in communist canada they can void you warranty over trivial items, but the consumer int he US is protected - and the manufacturer can only void the warranty on the componet that was modified or componets reasonably affected by the mod.

They can and do void it.Moreover, in certain province like Ontario modified cars are frowned upon.I remember something from a minister back there saying something like this:
"If you drive a modified car, we will find you, find your car, take it to the crusher and crush it and all your modifications.You won't have your car anymore and all your hard work will be gone."

That was their transport minister quite disconcerting I would say.:crying:

So an airfilter mod wont void your tranny warranty, and a rear wing wont void the rust warranty on your hood ;)

Oh - and this paragraph is very misleading:


The dealership is going to be the first one likely to try void your warranty and most are ill equipped and less than enthusiastic about supporting any kind of aftermarket. Not to mention not every technician is, lets say - as detail orientated as one would want them to be.


That is what I'm trying to explain here, you've got to watch out for those things.So to be safe I said that it will be voided.Where I work we do install performance parts for the cars we sell, especially WRXs and STis, and those are guaranteed.But if you were to install parts yourself or from another garage then the engine is not guaranteed or the suspension parts etc... Honestly, I can understand why the dealer won't guarantee it because when time comes for them to get repaid by Subaru Canada it's tough to justify the repair.Keep in mind though, that's what we as the mechanics are being told.

[klorophil]

are you going to talk about building forged engines and turbo upgrades/kits, fuel systems modifciations, ECU tuning and standalone operation etc... I'm in the process of doing all those things right now on a few different cars. just curious how in-depth you are planning on getting.

This is more like a basic guide I consider those mods quite advanced.I'll touch them but no there won't be anything like: "How to choose the right turbo for x engine?" or "How to calculate cam durations?" etc...

Although, if you have questions regarding all of this or what you are trying to do I guess it would be ok for you to start another thread and ask specific questions there.I will be more than happy to answer all of your questions.;-)

[RC45]

http://en.wikipedia.org/wiki/Magnuson-Moss_Warranty_Act

Looks like its about time Canadians got some consumer protection ;)

[klorophil]

AFAIK pretty much all warranty claims, problem with a dealer etc... that were put in front of a judge in the Quebec province were won by the customer.We have what we call "Cours des petites créances" in Québec, you pay 25$ and can enter a civil claim in front of a judge for anything you want.Most of the time it is the customer that wins.I should know it because I or my employer was victim of this tendency a couple of times.

It might not be as popular as the sue everyone and everything in the US, but it does get things done.

Also you have to understand that we do not have the same civil code in the Québec province as other provinces or the US for that matter.Since we were a french colony we kept pretty much the same civil code.AFAIK under that code you are guilty unless proven otherwise regarding civil matters.Though, I'm not a lawyer and I'm not the least bit interested in being one.;-)

[avocadoflavo]

This may be a lil bit off topic. I'd like to know how to upgrade a boxer4 ej251 to a boxer 4 turbo as the outback 2005 GT +++ . Will the crank survive? My valves? cams maybe.. I'm new to tuning,

[enzoferrari]

Thanks a lot for sharing,is very helpfull.

[klorophil]

This may be a lil bit off topic. I'd like to know how to upgrade a boxer4 ej251 to a boxer 4 turbo as the outback 2005 GT +++ . Will the crank survive? My valves? cams maybe.. I'm new to tuning,


I'll try to be brief.The EJ-series bottom end is quite strong and it should be fairly easy for you to swap parts from an EJ257 this is the engine that you find in a WRX STi.It's also fairly easy to swap in the entire engine as exterior dimensions do not change and the mounting place are the same.But I would need more info as to what car you inted to modify.Is it the Forester or the Impreza as both came with this engine.

[04RCSTI]

This may be a lil bit off topic. I'd like to know how to upgrade a boxer4 ej251 to a boxer 4 turbo as the outback 2005 GT +++ . Will the crank survive? My valves? cams maybe.. I'm new to tuning,

It can be done. Here's a detailed guide more or less...with pics...

http://www.rs25.com/forums/showthread.php?t=71639


Kloro, the guide is OK, but it sounds like a buying guide to us because a lot of us know all these things already..

I picked this up on the sti forums, could be of some help to you

I know there are a lot of people out there who hear those 3 words tossed around a lot, and have no clue what they mean. I came across this helpful page while I was online the other day looking up toe adjustments. I thought maybe some people could benefit from it, maybe it can be a sticky:

Camber, Caster and Toe: What Do They Mean?

The three major alignment parameters on a car are toe, camber, and caster. Most enthusiasts have a good understanding of what these settings are and what they involve, but many may not know why a particular setting is called for, or how it affects performance. Let's take a quick look at this basic aspect of suspension tuning.

UNDERSTANDING TOE

When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics.

For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tires, while too much toe-out causes wear at the inboard edges.


So if minimum tire wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. The illustrations at right show the mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results.

When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.


http://www.ozebiz.com.au/racetech/theory/image57.gif

With toe-in (left) a deflection of the suspension does not cause the wheels to initiate a turn as with toe-out (right).



The toe setting on a particular car becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-the never-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.

With four-wheel independent suspension, the toe must also be set at the rear of the car. Toe settings at the rear have essentially the same effect on wear, directional stability and turn-in as they do on the front. However, it is rare to set up a rear-drive race car toed out in the rear, since doing so causes excessive oversteer, particularly when power is applied. Front-wheel-drive race cars, on the other hand, are often set up with a bit of toe-out, as this induces a bit of oversteer to counteract the greater tendency of front-wheel-drive cars to understeer.

Remember also that toe will change slightly from a static situation to a dynamic one. This is is most noticeable on a front-wheel-drive car or independently-suspended rear-drive car. When driving torque is applied to the wheels, they pull themselves forward and try to create toe-in. This is another reason why many front-drivers are set up with toe-out in the front. Likewise, when pushed down the road, a non-driven wheel will tend to toe itself out. This is most noticeable in rear-drive cars.

The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition.

It should be noted that in recent years, designers have been using bushing compliance in street cars to their advantage. To maximize transient response, it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tires. By allowing a bit of compliance in the front lateral links of an A-arm type suspension, the rear axle will toe-in when the car enters a hard corner; on a straightaway where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tire wear and stability characteristics. Such a design is a type of passive four-wheel steering system.



THE EFFECTS OF CASTER

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative.

Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castering front wheels of a shopping cart (above). The steering axis of a shopping cart wheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as "trail."

Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved.

The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it.

Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.


http://www.ozebiz.com.au/racetech/theory/image59.gif

Like a shopping cart wheel (left) the trail created by the castering of the steering axis pulls the wheels in line.



WHAT IS CAMBER?

Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber (see next page). The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. It's interesting to note that a tire develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree. This fact is due to the contribution of camber thrust, which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch).

To optimize a tire's performance in a corner, it's the job of the suspension designer to assume that the tire is always operating at a slightly negative camber angle. This can be a very difficult task, since, as the chassis rolls in a corner, the suspension must deflect vertically some distance. Since the wheel is connected to the chassis by several links which must rotate to allow for the wheel deflection, the wheel can be subject to large camber changes as the suspension moves up and down. For this reason, the more the wheel must deflect from its static position, the more difficult it is to maintain an ideal camber angle. Thus, the relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in passenger cars presents a difficult design challenge, while the small wheel travel and high roll stiffness inherent in racing cars reduces the engineer's headaches.

It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. The illustration on the bottom of page 46 shows why this is so. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.

While maintaining the ideal camber angle throughout the suspension travel assures that the tire is operating at peak efficiency, designers often configure the front suspensions of passenger cars so that the wheels gain positive camber as they are deflected upward. The purpose of such a design is to reduce the cornering power of the front end relative to the rear end, so that the car will understeer in steadily greater amounts up to the limit of adhesion. Understeer is inherently a much safer and more stable condition than oversteer, and thus is preferable for cars intended for the public.

Since most independent suspensions are designed so that the camber varies as the wheel moves up and down relative to the chassis, the camber angle that we set when we align the car is not typically what is seen when the car is in a corner. Nevertheless, it's really the only reference we have to make camber adjustments. For competition, it's necessary to set the camber under the static condition, test the car, then alter the static setting in the direction that is indicated by the test results.

The best way to determine the proper camber for competition is to measure the temperature profile across the tire tread immediately after completing some hot laps. In general, it's desirable to have the inboard edge of the tire slightly hotter than the outboard edge. However, it's far more important to ensure that the tire is up to its proper operating temperature than it is to have an "ideal" temperature profile. Thus, it may be advantageous to run extra negative camber to work the tires up to temperature.



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(TOP LEFT) Positive camber: The bottoms of the wheels are closer together than the tops. (TOP RIGHT) Negative camber: The tops of the wheels are closer together than the bottoms. (CENTER) When a suspension does not gain camber during deflection, this causes a severe positive camber condition when the car leans during cornering. This can cause funky handling. (BOTTOM) Fight the funk: A suspension that gains camber during deflection will compensate for body roll. Tuning dynamic camber angles is one of the black arts of suspension tuning.



TESTING IS IMPORTANT

Car manufacturers will always have recommended toe, caster, and camber settings. They arrived at these numbers through exhaustive testing. Yet the goals of the manufacturer were probably different from yours, the competitor. And what works best at one race track may be off the mark at another. So the "proper" alignment settings are best determined by you-it all boils down to testing and experimentation.