Simple and Accurate Alignment Method for a Porsche 996, Part 5: Calculate Adjustments
|Part 1: Introduction|
|Part 2: Preliminaries|
|Part 3: Camber measurement|
|Part 4: Toe measurement|
|Part 5: Calculate adjustments|
|Part 6: Front adjustments|
|Part 7: Rear adjustments|
The Heart of this Method is Calculating the Adjustments
My claim to the simplicity of this method is that it consists of basically three steps:
- Measure while the suspension is fully loaded, and measure only once
- Calculate all the adjustments
- Make all the adjustments
The key point is that there is no iteration as is usually done. All alignment methods I have seen rely on measuring, tweaking, measuring, tweaking, etc. There are many problems with this: for the DIYer it means expensive tools or settling the suspension after each adjustment. My thought was to completely short circuit all of that and find a way to do this procedure without any iteration.
The only way to achieve this is to model all the important geometric details of the suspension and the measurement process, and then calculate the adjustments including all the interactions between them. This is what I have tried to do in the spreadsheet which I’ll provide a link to in a moment.
The spreadsheet contains all the key suspension geometry factors for the Porsche 996, and compensates for a number of other issues. For example, when measuring the toe in Part 4, we should have measured it with the laser beam perfectly horizontal. But we didn’t, we measured it with the beam pointing down a bit to hit a tape measure lying on the ground. Is this is a big factor? YES! When dealing with toe of a few minutes of arc (5 minutes for Porsche specs), and a camber angle in the 1-2 degree range (12-24 times the toe angle), even a small deviation from horizontal affects the measured toe angle by much more than the toe angle itself.
I must warn anyone using this method that although I have spent a considerable amount of time testing and improving the spreadsheet on my own 996, and it seems to work well, but only if the suspension setup and measurements in earlier parts are rigorously followed! I was having trouble getting my toe to come in where it should be (the steering wheel was pointed just slightly off center when going straight). Finally I discovered — after not following my own advice — a loose bolt holding one side of the rear diagonal suspension brace, probably from the mechanic not having torqued it after dropping the transmission for the IMS bearing replacement 6 months ago (this is when I bought the car and the IMS issue was found in the pre-purchase inspection).
Here is the 996 alignment spreadsheet. It is a Google Docs spreadsheet. It is free for anyone to use for non-commercial purposes, licensed under a straightforward Creative Commons license. You are free to view the original and copy it and modify it as long as its origin is appropriately attributed by maintaining the license notice.
I will be updating the spreadsheet based on my own and others’ experiences with it. There are still a few minor geometry effects I want to do a little bit better, so it will be changing. I will track the version number on the spreadsheet.
The spreadsheet has four parts:
- Geometry, both of the suspension and the unchanging parts of the measurement setup. This section should not be changed without thorough understanding of how the spreadsheet and the suspension geometry works.
- Alignment Setup: how far away from the wheel the tape measure is placed, and whether it is in front of or behind the wheel
- Camber and Toe Targets: Porsche 996 spec, tolerance, and your own targets for toe and camber
- Measurements and Adjustments: once the above information is entered and measurements are made, all the adjustments are calculated here
Cells are color coded as follows:
- White (no color): should not need to enter or modify or check these
- Blue: cells requiring input from the user
- Red: cells containing calculated adjustments
Proceed through the spreadsheet from top to bottom as you work through the steps in the process. Adjustment calculations are always immediate, but will not be correct until you have filled in the blue cells with the correct data from the previous steps.
Measurement and Adjustment Directions
Any measurement or adjustment can be either forward or backward, or left or right. We have to be very careful they are entered and interpreted correctly.
I have settled on the following for describing directions:
- Positive numbers are toward the front or outside of the car (Positive = Up or Out). Examples:
- If the tape measure is placed in front of the wheels, the distance will be entered as a positive number
- If a positive toe adjustment is calculated, the toe control arm must push its part of the wheel out, away from the car’s centerline.
- Negative numbers are toward the rear or centerline of the car (Negative = Back or In). Examples:
- Placing the tape measure behind the wheel being measured means the distance must be entered as a negative number.
- A negative toe adjustment means the toe control arm must pull its part of the wheel in toward the car’s center.
Maybe somewhat confusingly, toe in is defined as positive, and toe out is negative. Porsche’s toe spec is 5 minutes positive, which is 5 minutes toe in.
Please be certain you have fully internalized these conventions, because reversing any of them will cause major alignment errors.
Step By Step: Overview
The front and rear wheel alignments are independent of each other so the spreadsheet will be gone through once for the front wheels and once for the rear wheels. For the first set of wheels, half of the entries will be meaningless, but they will be filled in later.
Please note that all distances are entered in cm (centimeters). This was very convenient for me just because my tape measure had an easily read cm scale on it. I’m sorry but you will have to enter all your measurements in cm also. To convert from inches to cm, just multiply your measurements by 2.54.
You can enter different values than I have in the Target section. My Targets are for a slightly aggressive street alignment for a normal 996 (not GT3) with the X74 suspension. Refer to the alignment specs appropriate to your car and modify the target values as needed.
If you have followed the instructions so far, you will have the following 5 measurements available for each wheel:
- From Part 4:
- Distance from the laser level’s laser hole to the tape measure lying on the ground either in front of or behind the wheel when you made the toe measurements
- Two measurements of where you found the laser dot to hit the tape measure (we will enter their average value)
- Distance from the laser level’s laser hole to the car’s centerline
- From Part 3: Camber angle
We will first enter the distance to the tape measure in the Alignment Setup section, then enter the other four measurements in the Measurements and Adjustments section.
Once the measurements for a wheel are entered, all the adjustments required for that wheel are calculated and displayed. We will then take out the wrenches and get to work, described in Part 7 (rear wheels) and Part 6 (front wheels).
Step By Step: Alignment Setup
The Alignment Setup section has only two numbers to fill in. These are the distances from the laser level’s laser hole to the tape measure when it was lying on the ground with its end at the car’s centerline and the laser dot shining on it. One number is for the front wheel measurement, and the other for the rear. I assume that the distances are the same for right and left; if not you may need to enter values separately for the left and right. But the precise value of this distance is not so critical; if you are within 1-2% it is totally fine.
Notice that I measured this distance by putting the tape measure behind the wheels, with the laser shining backwards. For me the rear wheels got in the way of the laser beam from the front wheels so the furthest back I could go was the rear wheel itself.
If you measured using the laser level shining forward, enter a positive number for your measurement.
It is critically important to get the positive vs. negative right! Your adjustment calculations will be way off if any of the signs are reversed.
Step By Step: Target
The Target section lets you fill in what you want the alignment angles (camber and toe) to be when you are done. Refer to the alignment specifications, a copy of which is here:
Step By Step: Measurements and Adjustments
Remember that blue cells indicate cells you need to fill in yourself. There are three blue cells per wheel:
- Camber, from Part 3.
- Laser Output to Centerline, from Part 4.
- Laser Spot on Tape Measure, from Part 4. Remember that there were two separate measurements made, flipping the laser level over between them. The average of those two measurements must be entered here.
Once the three measurements are entered for each front wheel, all the red cells for that wheel show the adjustments that must be made to bring the wheel into alignment. Even though there are four red cells per front wheel, there are only two adjustments — don’t panic! The three adjustments in the “Toe” row are actually all the same thing, just different ways of looking at it.
The camber adjustment is done by moving the top of the strut either in or out. The red cell for camber adjustment tells how far the top of the strut needs to move. Again, a positive movement is toward the outside of the car, and a negative adjustment is toward the inside of the car.
The toe adjustment is done by rotating the steering tie rod against the tie rod end that connects to the front wheel. The leftmost red toe cell labeled “Adjust, mm” tells how much the tie rod end needs to move; again positive is toward the outside of the car (resulting in more negative toe, an unfortunate confusion in signs) and negative is toward the inside of the car (resulting in more positive toe).
But an adjustment in mm is hard to judge in this case. Much easier is using the thread of the tie rod and tie rod end themselves to help us measure the adjustment. This thread is a 1.5 mm thread, in other words for one full turn of the tie rod, it moves 1.5 mm against the tie rod end. That is what the second red toe cell labeled “Adjust, turns” says. If it says 0.5, then the tie rod end must be pushed out by 0.5 turns of the tie rod thread.
In my experience, since these adjustments are generally quite small, an even more convenient way to measure the adjustment is in 1/6s of a turn. This is how much the tie rod turns when it is turned so that one of its adjustment faces that you place the wrench on goes around enough so the next face takes its place. This is very easy to judge when working underneath the car and have only a limited range of movement of the wrench. It is easy to know when you have turned the tie rod 1/6 of a turn. That is what the last red toe cell labeled “Adjust, 1/6 turns” says.
You can take your pick of any of these three ways of describing the adjustments. They are all the same. I like the 1/6 turn one the best.
After entering measurements in the three cells for each rear wheel, two red cells are calculated with camber and toe adjustments required to bring the wheel into alignment. These adjustments are in mm that the appropriate control rod needs to move. Part 7 shows how to do this with high accuracy.