Ok, i know i need to get moving on finishing the project i started for this section a few months ago, and moving is no excuse anymore, especially since i only work 3 days a week :roll:
So hopefully, i can now make some progress on these things...
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http://img172.imageshack.us/my.php?image=imgly3.jpg
The sheet in the link above is a printout of what the technician sees whenever you bring your vehicle in for an alignment. At first glance, it can be confusing and seemingly made in some kind of code deciphered by a lost language. But fear not. It is actually not that difficult to understand, and this guide is going to crack the code for you.
This sheet is divided into two parts. Front and Rear.
Front is the 3 angles on the side and some in the middle. How many and which ones are in the middle will vary from program to program. Rear is usually 2 angles on the side, and 1 or 2 in the middle.
And now, let the deciphering begin
Angles
These 3 are the measurements on the Left and Right side of the page, both front and rear.
Camber
If you look at a car from straight on, you will notice the wheels are not straight up and down. They have a certain amount of tilt to them. This is called Camber and it's measured in degrees and minutes (0.1 on the sheet is read as 0 degrees, 1 minute). If the top of the tire tilts in towards the car, this is Negative Camber. Leaning out is Positive Caster.
This is represented in the read out as -0.2 and 0.1 on the example.
Excessive camber is the main cause for alignment related tire wear and can eat a tire in as little as 1500 miles in the worst cases. Wear patterns on tires show up as one shoulder of the tire being worn down while the other side may still have tread on it. (see Oscar's tire thread in Gen. Chat for example)
Your vehicle will generally pull or drift to the side with the most
Positive Camber.
This is because, a negative cambered tire will want to roll in a circle if you pushed it along the floor. The more positive it is, the closer to going straight it will roll. So, if you have one wheel cambered and one not, the cambered tire will push against the uncambered tire. With no resistance, the negative wheel will win and the car will pull.
Camber is mainly designed to offset road crown. Road crown is designed to allow water to flow off to the ditch when it rains.
Caster
Caster is an angle used to increase forward stability. These days, Caster is not generally adjustable and if it is off, something bad happened
If you look at the car from the side and draw a line up from the road, through the center of the axle, this would be the centerline for Zero. Now, draw a line through the center of the strut to where it intersects with the first line you drew. This is your Caster Angle. Caster, like Camber, is measured in degrees and minutes.
Leaning the strut towards the back of the vehicle gives you Positive Caster. The more Positive Caster you have, the more stable the vehicle will be in a straight line, but the harder it will be to return the steering wheel to center from turning.
Positive Camber also increases low speed steering effort. Leaning it forward, obviously has the opposite effect as you go towards Negative Caster.
Caster is not a tire wearing angle.
Toe
Toe is the measurement of distance between the center of the front of the wheels, and the center of the back of the wheels. The difference between the two is your Toe. This is not a tire wearing angle in most cases, but if it is extreme enough, can feather the edges of the tires.
If the front of the tires are closer together than the rear of the tires, this is called Toe-In. If the front is further than the back, this is called Toe-Out.
During a turn, the outside tire will not be turned as much as the inside tire. This is called Toe-Out on Turns. This can really cause some understeer if not set correctly. In Go-Karts this can make or break your lap times. Toe is generally measured in fractions of an inch, but recently it has changed to degrees (as seen here) and i'm not quite sure where it's supposed to be measured from to obtain the reading.
Rear wheel drive vehicles will have a small amount of toe-in on the front. When the vehicle begins rolling, the forces will cause them to toe out, which will achieve a Zero Toe position.
There is a certain amount of sideways travel your wheels do as the vehicle is in motion. This is referred to as scrub, and it can affect your gas mileage quite a bit. There is a formula which i need to look up, but for every foot the vehicle travels forward, there is supposed to be a certain amount of lateral scrub. Toe is affected by Camber and should always be adjusted last.
These next angles are shown in the middle of the page.
Steer Ahead
This is a front end specific measurement. It is measure by the total toe and cross camber of the front end with the steering wheel centered.
When toe is correctly set, with the wheels straight ahead, the steering wheel is centered and Tie Rods adjusted until Steer Ahead reads 0.
With the wheels straight, if the steering wheel points to the right, you want to lengthen the left tie rod and shorten the right tie rod. This essentially turns the wheels to the right, which straightens the wheel. Do the opposite if the wheel points left.
Do not remove the steering wheel and put it back on straight. This will cause a huge problem in that you will now have an uneven amount of turns from center to lock between the left and right.
Thrust angle
Thrust Angle is the rear end's equivalent of Steer Ahead.
Draw a line down the center of the vehicle. You want your rear axle's measurements of total toe and cross camber to equal 0. If it's not zero, your axle isn't pointing straight and if bad enough, will cause the vehicle to "Dog Track". This baisically looks like you are drifting everywhere when going straight. Pretty funny if you ever see it
Cross Camber
I need to double check how this measurement is figured and what exactly it's good for, because in all honesty, i never use it.
Steering Axis Inclination Angle (SAI)
This is a front end specific angle and a combination of measurements obtained during the "Caster Sweep". The steering is turned form lock to lock and the changes in angles are measured by the machine. It's measured
at a point 6 inches below the road surface. It's the inward tilt of the strut assembly in relation to a vertical line on the raod surface. This is not adjustable. If anything is bent, broken or out of whack, then this is not going to be within spec.
SAI results in self correcting forces that help the wheels return to a straight ahead position after cornering
Total Toe
Add up the left and right Toe measurments and this is the Total Toe. It tells you which way the axle is essentially pointing. This works for both axles.
I hope you have found this guide helpful. Most of what i have explained in terms of Angles apply to MacPherson Strut suspension set ups. Wishbone type suspensions are measured from the ball joints at the ends of the wishbones. I will explain this more thoroughly when i put up the Chassis section of my Theory sections...which i have been slacking off on hardcore.....
Steering/Suspension Alignment guide (corrected/updated)
Linear Mode
