Also, while these spreadsheets do a great job of meeting the parameters you set them, there may be times when you are better off favoring other aspects of your car. For instance, front-engine, rear-wheel drive cars often benefit from a stiffer front sway bar -- even though you might think it would cause the car to understeer excessively. The reduction in body roll often helps the grip of the front tires more than the increased weight transfer hurts because the tires are flatter to the ground. Plus, the stiffer front sway bar can help "get the power to the ground" better exiting turns.
The first section "Existing Suspension Design" explains how to measure the characteristics of your current suspension. It also explains some of the basic theories behind suspension tuning.
The second section "New Suspension Design" helps you determine how you want your new suspension to behave.
"New Suspension Component Design" is similar to the first section. It is there to give you a workspace to design new suspension components using the numbers derived in the "New Suspension Design" section.
Then, you can measure your car (forms are available which make it easier to make the correct measurements, and organize them for easy entering into the spreadsheet), open a spreadsheet and start entering your numbers. The spreadsheet will calculate the characteristics of your suspension automatically, using equations built into it and the numbers you've entered. At the end of this manual there is a section "Special Applications" which assists you in using these spreadsheets to go beyond their usual limits.
All dimensions entered MUST be in either decimal inches or pounds, except when otherwise indicated.
After downloading the appropriate spreadsheet file for the suspension type you'll be working with, make a duplicate of the downloaded file, give it a suitable name and you're ready.
Cell protection is used to prevent accidental corruption of the spreadsheet's cells. In some cases, as will be explained later, you may wish to lift that protection. In these cases registered users ONLY can simply access the protection feature of Excel and use the password issued to them to unlock the spreadsheet (see Purchasing Spreadsheets And Manuals). Make sure to lock the cell protection when you are done to prevent accidental deletion of other cells. Note that when you enter numbers in the cells which ordinarily are used by the spreadsheet to automatically display results, you are deleting the equation within it. That is why it is important to always keep backup files.
The spreadsheets are arranged in two columns so that all information pertaining to the rear suspension is in the left-hand column, that for the front suspension is in the right-hand column. Figure 1 shows a sample spreadsheet with some of the numbers already entered. Note that the right-hand column has no numbers entered yet, so the result isn't displayed.
It is important to understand that you often MUST enter ALL the numbers pertaining to a given calculation before a result can be displayed by the spreadsheet. There are times when you will have to enter numbers for parts your car doesn't even have -- as is the case for sway bars. The reason for this is usually that the spreadsheet has to divide a number by "0" which causes an error message. Instead you may have to enter arbitrary numbers in the place of actual ones -- this causes no problems and is merely a limitation of the host programs. This will be explained in greater detail later.
Read this ENTIRE section before you start working with the car. This section involves making careful measurements of it. Some, like coil spring "free length," may require disassembling parts of the suspension to accurately make them. You should make those not requiring disassembly first, such as the car ride height etc. That way you won't have to disassemble the suspension more than once and the car will be ready for you to install the new parts.
To make it easier to keep track of all the required measurements you need to make of your car, forms are available for downloading which you can fill out as you work with the car's suspension. You must first download the Adobe Acrobat® Reader application to view and print the Acrobat "PDF"-file formatted forms. They are named similarly to the corresponding spreadsheets so it is easy to determine which form you should download. Note that ALL dimensions must be in either decimal inches or pounds unless otherwise noted.
When entering numbers in the spreadsheet it is best to do them in the order they are listed, but not required -- it will fill in the blanks as soon as it has received ALL the necessary numbers from you.
If you don't have access to a tester, you must make ALL the measurements shown in Figure 2 for coil springs, Figure 3 for leaf springs and Figure 4 for torsion bars. Then enter the resulting numbers in the corresponding cells of the spreadsheet. The wire diameter and leaf thickness dimensions are very critical -- use a micrometer or good calipers to make this measurement. The spreadsheet will automatically calculate the spring rates for you as you enter the numbers. See also "Special Note About Leaf Springs."
Suspensions usually have leverage on the springs, shocks and sway bars and this affects the effective stiffness of them. It is best to measure leverage on the car since this eliminates some sources of errors. You then enter the resulting numbers in the cell normally used to display the results (after unlocking the spreadsheet first -- see "Using The Spreadsheets"). The following method is easy and works great (make sure the car is on a flat, level surface):
Measure the normal ride height of the car at the end you are working with as close to the wheel center line as possible as shown in Figure 11. Then measure the length of the installed spring AT RIDE HEIGHT and, if measuring sway bar leverage, the distance of its end link from a suitable point on the BODY of the car as directly above it as possible. Now, add weight to that end of the car centered directly over the axle. Measure the new ride height, the new installed length of the spring and the new distance of the sway bar end link from the body. Subtract the second numbers from the corresponding first numbers to find the amount each distance changed. Then, find the ratio of leverage by dividing the change in ride height by the change in the other two dimensions respectively. The resulting numbers are the leverages of the wheel over each part as a ratio to "1."If you can't measure it that way, you must instead let the spreadsheet calculate the leverage by making several measurements and entering the numbers in it. Figure 8 shows where to measure some common suspension types. For some suspension types, like leaf springs and some torsion bars, the leverage is always 1:1. In those cases, you MUST still enter the numbers other than inner pivot to spring pick up so that the leverage on the sway bars can be calculated.
In a manner similar to that used above, if your car has 2 sway bars at the same end of the car, you'll have to first measure one, calculate its WHEEL rate, then measure the other's wheel rate and add the 2 rates together. Enter this total value in the results cell for the sway bar wheel rate.
Measure the ride height at one end of the car (Figure 11). Then, add a known weight to that end of the car, centered directly above the axle, sufficient to lower its ride height by at least one inch (make sure you don't bottom out the suspension!). Now measure the new ride height and subtract it from the first. Then, divide the known weight by that number. This gives you the total spring wheel rate at that end of the car. Dividing this rate by two gives you the spring wheel rate per wheel. Note that sway bar rates are NOT measured in this test since they don't resist vertical motion of both wheels together -- they resist only motion of the wheels in opposite directions (as in a turn). Then measure the wheel rates at the other end of the car by repeating this test on the other end of the car in the same manner.Take care to make the measurements as accurately as possible. If this test indicates a large discrepancy between the wheel rates calculated in the spreadsheet and those actually measured, go back and double check ALL the numbers you've entered so far, then repeat this test. Usually, if there is a problem, it is a mistake in measuring leverage or wire diameters or the part may be made using a special alloy or heat treating.
If you have access to wheel scales, which are placed under each wheel to measure the total weight on each wheel, you can weigh the car exactly as it is driven with the optimum level of fuel in the tank, spare tire removed or whatever. You can even weigh the car with the driver seated and design springs that take into account the resulting weight distribution (see "Special Applications"). Enter the weights measured in the appropriate cells minus the unsprung weights PER WHEEL after unlocking the spreadsheet (see "Using The Spreadsheets").
If you don't, you can use the numbers from magazine tests, manuals etc. to get pretty close. Enter the total weight of the car first, then the percent of weight distribution at each end, then the unsprung and removed weight PER END. "Removed weight" is simply a place for you to enter the amount of any weight you may have removed that would make it weigh less than the stock "curb weight" you entered earlier. If you add weight, make the number negative -- to do so it may be best if you precede the number with a wordspace (one stroke of the space bar), then the minus sign (-).
For circle track and other cars that use "weight jacking" (unequal weight distribution from one side of the car to the other), see the "Special Applications" section.
Measure the ride height of the car as shown in Figure 10. You must also enter the outside diameter of the tires you are using. These numbers are important if you will design new coil springs in the "New Suspension Design" section because they are used to calculate their free length.
If you really don't know where to start the following table can be used. It is meant ONLY as a GUIDE and gives typical ratios that I have found work well on competition cars I have personally tested and driven. These are here ONLY as examples and you should NOT assume that they will work in the same way for your car. This is, unfortunately, the least scientific part of suspension tuning. Too many things can affect how a car handles to make it easily reduced to a number, but at least you'll have a good starting point. Front engine/rear wheel drive cars often shift from extreme oversteer under power to severe understeer in turns. For that reason I don't feel it would be appropriate to list any value -- you'll have to decide on a ratio that best suits your driving style, track conditions etc.
|Car Type:||% Weight Front, Rear:||Total Wheel Rate Ratio Front/Rear:|
|Front wheel drive|
|.4 : 1 to .8 : 1
1.2 : 1 to 1.4 : 1
1.4 : 1 to 1.6 : 1
Take some photos of your car at maximum cornering speeds to find out how much it leans now. A good rule of thumb is to limit body roll to between 1 and 4 degrees, depending on the suspension type. If you have lots of room for negative camber -- or have a suspension with favorable geometry -- you can get away with more body roll. If you use a "tire pyrometer" for testing (as described in many books about suspension tuning) you can use it to determine whether you have enough room for negative camber or not, and whether the car leans too much. Body roll becomes less tolerable with wider wheels, because you can't get the maximum benefit out of them if they are running at an angle to the ground. If your car leans twice as much as you want it to, double your existing total wheel rate. But there is a good chance that your car will be able to corner faster with its new suspension -- which increases the body roll -- so it might be a good idea to make the total wheel rates higher than you think they need to be, especially if your existing suspension allows a lot of body roll.
The advantage of selecting a high natural frequency is that it allows you to rely less on sway bars and more on springs to achieve high total wheel rates. Sway bars have the disadvantage of causing the inside wheels to lift -- which can cause wheel spin exiting turns under power if the bar is on the driving wheels, as well as worse braking into turns etc. On the other hand, sway bars have a greater effect on handling balance (understeer and oversteer) than springs do. This is because sway bars reduce the grip of the tires at one end of the car to balance the handling. They not only can reduce the grip of a tire facing the outside of a turn by loading it more, but can also reduce the grip of a tire facing the inside of a turn by lifting it to the point where friction -- and grip -- is reduced. Also, sway bars have little effect on the natural frequency, so if you want a smoother ride with less body roll you can enter a low natural frequency and the spreadsheet will achieve the total wheel rate you entered earlier by recommending stiffer sway bars than it would if you entered a high natural frequency.
The rear natural frequency you enter will be used by the spreadsheet to calculate a natural frequency for the front suspension based on the car's wheelbase and the desired favored speed you entered.
If you don't want to change your ride height, you MUST enter the same numbers you entered in the " Existing Suspension Design" section. You MUST also enter the outside diameter of your new tires, or the outside diameter of your old tires if you don't change them. Entering these numbers allows the spreadsheet to calculate the free length of new coil springs, based on not only the new spring rates, but also the new tires, new leverage and new car weights. Don't forget that the free length will be different for stiffer springs even if you want the same ride height -- because they will compress less under the same weight than the softer springs will do.
If you end up with negative wheel rates, which can happen with sway bars, you'll have to go back and change one or more of your parameters until all numbers are either "0" or higher. A negative number means that you're asking the impossible of your car. If you have to change any parameters it is best to start by changing the least important parameters first, like the favored speed. The total wheel rate ratio front/rear should be the last thing you compromise. You can use this technique of changing the parameters for other reasons too. For instance, if you don't want your car to use a front sway bar, you can keep changing your parameters until the spreadsheet reads "0" in the cell for the new front sway bar wheel rate. Don't go too far in changing your parameters, though -- it is easy to compromise them to the point where you might as well not use these spreadsheets. The whole point is to determine the OPTIMUM suspension for your car.
Chances are that you will merely re-enter a lot of the numbers from the first section into this section. What you will be doing is choosing a few key numbers to change -- like the wire diameters of the springs and sway bars. Keep trying new numbers until the spreadsheet displays a wheel rate value in the results cell that is as close as possible to the value recommended in the previous section for the same part. The spreadsheet then calculates a free length for the new coil springs, all the new wheel rates and ratios, natural frequencies and the new favored speed.
If you order custom-fabricated springs, you should give the fabricator all the dimensions the new springs should have -- except the wire diameters or leaf thicknesses. Spring fabricators often use sophisticated alloys and heat-treatments to achieve specific spring rates, so the wire diameters or leaf thicknesses they choose might be different from those you'd expect. Just give them the spring rates you want the new springs to have. If you are ordering coil springs that have the end coils shaped differently to fit the spring seats, be sure to let the fabricator know what they are shaped like, and the dimensions of those end coils. This could include the "pitch" of them (the distance from one coil to the coil above it) and their inside diameter, if different. Tune to Win, by Carrol Smith has some good advice on this subject.
To do this, you'll have to measure ride heights and weights on each wheel (using wheel scales) with the driver seated in place. First, enter all the numbers for the side of the car the driver sits on in the spreadsheet, design your new suspension etc. until that spreadsheet is completely finished. Copy the spreadsheet and rename it so you know it's for the other side of the car and change the weights per wheel and the ride heights to those measured for that side. The spreadsheet will then design new spring rates that achieve the same natural frequency as those on the driver's side (and free lengths for coil springs) taking into account the different "corner" weights (ignore the sway bars, total wheel rates and total wheel rate ratios front/rear in the spreadsheet for the side the driver DOESN'T sit on). Sway bars make it impossible to achieve the same ratio of total wheel rates front to rear, on each side since they cannot be made in such a way as to have different rates on either side of the car. But the handling should be more nearly equal in both left and right turns using this technique since the springs are designed for the different loads they are subject to.
Measure the weight on each wheel using wheel scales, with the car set up exactly the way you want to run it it, just as you would ordinarily do, or make adjustments to these weights based on what you want the new weight distribution to be. Making calculations based on these weights would favor bumps on the straights if the weight is measured with the car on level ground. If you want to favor bumps in the turns, you'll have to find a way of determining the weight distribution while in a turn. The amount the right side springs compress in a turn should give you the clue to find these numbers. Take some pictures of the car in a full-speed turn from head-on to measure the total compression of the right-side suspension. You can then calculate the approximate amount of additional weight transferred onto the right wheels by multiplying the total wheel rates of the right-side suspensions by the change in right-side ride height in a manner similar to that shown earlier -- but in reverse. The result is the total amount of additional weight transferred onto the right wheels. You can then change the car weights in the spreadsheet to reflect this weight distribution in turns by adding half of this weight to the existing weights on each right wheel and subtracting the same amount from each left wheel. If the track has banked turns, additional weight may be transferred onto the left wheels as well.
Now, enter the numbers for the right side of the car in the spreadsheet, design your new suspension etc. until that spreadsheet is completely finished. Then copy the spreadsheet, rename it so you know it's for the left side of the car and change all the numbers in it that are different for the left side of the car. This might include the different weight distribution, different ride heights and different tire sizes. The spreadsheet will then design new spring rates that have the same natural frequencies as those on the right side (and free lengths for coil springs) and take into account the different weights, ride heights and tires for each side (ignore the sway bars, total wheel rates and total wheel rate ratios front/rear in the spreadsheet for the left side).
If your trailer has only one axle, you don't have the natural frequency problems that a two axle car has. What you should do is match the natural frequency of the trailer's suspension to that of the rear axle of your tow vehicle. But when measuring the "wheelbase," measure the distance from the rear axle of your tow vehicle to the rear axle of your trailer. Then calculate the favored speed and the natural frequency of the rear suspension of your tow vehicle. Next, enter the tow vehicle's favored speed and different numbers for the trailer's "desired rear natural frequency" in the "New Suspension Design" section until "front" springs are displayed that have the same natural frequency as that of the rear suspension of your tow vehicle. The result is springs on the trailer that are matched to the tow vehicle at the tow vehicle's favored speed.
If you have a two axle trailer, do the same as above to calculate the frequency the front axle of the trailer should have, then calculate the rear trailer axle frequency based on the trailer wheelbase, just as above, but matching the rear trailer springs to the front trailer springs. Most trailers have axles that are very close together, though, so you may not have to even make that last calculation since the difference in recommended natural frequencies would probably be very small -- smaller than even the normal production
Sometimes numbers refuse to display anyway. Some of the host programs won't display numbers above a certain size without you specifying this, so you should consult the manuals of the host program to determine how to make them display.
Another problem could be that you have deleted the equation in a cell normally used for displaying a result after unlocking the file. If this is what happened you'll have to copy your original spreadsheet and start over. That is why it is important to always keep backup files. Be very careful to only unlock the spreadsheet as long as necessary to enter your numbers, then lock it again.
Registered users ONLY may direct any questions about this manual or the spreadsheets to me by e-mailing me at firstname.lastname@example.org
© 1991 and 1995 Dennis Schøler Pedersen. This manual and all its contents are copyrighted. Any unauthorized copying or duplicating is forbidden by law. Authorization is given for registered users to print one copy of this manual per spreadsheet purchased.