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Suspension Design Advice

Even those who have already read the manual and started working with their spreadsheets may still be unsure how to decide what springs and sway bar rates they should choose. This page is meant to help you with that decision. First, you must have a good idea what you're trying to accomplish with your suspension modifications. Most modifications are made to achieve one of several general goals. Below are some of those goals, and how best to achieve them, and what problems to watch out for.

Don't understand a lot of these terms? Check the FAQ for some recommended reading that can help you.

Custom Lowering Suspensions

Tuning Racing Suspensions

Wheel and Tire Fitment Tips

Custom Lowering Suspensions

By far most people who modify their car's suspension do so for looks. The lowered "look" is very popular, and most aftermarket springs are designed to allow people to cheaply and easily do that. If that is what you are interested in doing be aware that, while there are advantages to doing this, it can cause a number of different problems; some are minor, some can be major.

  • Advantage: Lowering your car looks cool.
     
  • Advantage: Lowering your car also lowers its center of gravity (CG) which decreases "weight transfer" to help improve handling. And it may also lower your car's "roll center" and change other aspects of your car's suspension geometry in ways that help handling. But sometimes these changes hurt.
     
  • Advantage: Lowering your car improves aerodynamics at high speeds.
     
  • Problem: Lowering changes your car's wheel alignment. Usually the car's "negative camber" and "toe-in" are increased. Extra negative camber may be a handling advantage.
     
    Solution: After you have lowered your car, make sure to get the car's suspension aligned immediately at a good wheel alignment shop.
     
  • Problem: Lowering the car makes it more susceptible to "bottoming out." This is when your suspension compresses enough, usually over bumps (and especially bumps in turns), to cause your suspension's "bump stops" to contact. This pounding can damage your car's suspension and chassis if it is excessive. It can also cause your car to "skip" over bumps in fast turns, or even increase the likelihood of your car flipping over! This can be dangerous and isn't the way to win races either.
     
    Solution: Make sure that you select a spring rate that is stiffer in proportion to the amount you lower your car. If you plan to lower your car so much that it only has half the "wheel travel" it currently has (the distance between the bump stops with the car at normal ride height), you should double the spring rates. This will ensure that your car will not bottom out any more than it currently does.
     
    Example: This example assumes that your car currently has springs that have a linear (not progressive) rate of 120 lbs/inch, and has 4 inches of "compression" ("jounce") wheel travel (at normal ride height) before contacting the bump stops:
     
    1. First calculate the amount of weight needed to compress you current springs four inches:
          4 inches x 120 lbs/inch = 480 lbs.
    2. Then calculate the rate necessary to cause the new springs to compress 2 inches with the same 480 lbs of weight on it:
          480 lbs / 2 inches = 240 lbs/inch.

    Your new springs should be 240 lbs/inch minimum. That is a 100% increase (200% of the original rate). If you lower your car only 1 inch (so it has 3 inches of compression travel), the rate should be at least 160 lbs/inch (480 lbs / 3 inches = 160 lbs/inch). That is a 33% increase. If you lower your car by 3 inches (so it only has 1 inch of compression travel), the springs would have to be 480 lbs/inch (480 lbs / 1 inch = 480 lbs/inch). That is a 300% increase.
     
    The problem with most aftermarket springs is that they are usually only slightly stiffer than the factory springs — usually between 10% to 25% stiffer. This isn't generally enough when cars get lowered by even one or two inches. So be very careful when buying springs and don't buy them if the vendors won't tell you what the spring rate is, and definitely don't buy them if the spring rate is lower than what you calculate they should be — unless you plan on driving only on smooth roads!
     
  • Problem: The lower ride height can also cause your car to scrape "speed bumps," and when crossing over sidewalks into driveways and parking lots, and to bump into curbs when parking.
     
    Solution: None, except to avoid those places or not lower your car so much.

With the above information you should be able to either design your own lowering springs using the spreadsheets, or analyze aftermarket lowering springs from different manufacturers using the "solution" above to determine which ones are stiff enough to accommodate the amount of lowering.

Tuning Racing Suspensions

Many people compete in car racing that allows them to modify their car's suspension to improve handling. The Sports Car Club of America, for instance, allows lowering springs, different shocks and struts, and different sway bars in its "Solo II" "Street Prepared" category, and in its "Improved Touring" series, as well as the various "Production" racing classes. This usually involves lowering the car, like above, as well as substantial stiffening of the suspension — often on the order of 300% or even higher. Some people want to race-tune their car's suspension for street racing. That isn't entirely practical, or recommended, but if that's what you want to do you can follow the same procedures as racers do, but limit the spring stiffness to something you can live with on the streets (usually around a 200% increase in stiffness, or a "natural frequency" of between 1 and 2 cycles/second — see the manual for more information).

  • Advantage: The stiffer suspension helps improve response and limits body lean in turns, and "dive" and "squat" under braking and acceleration.
     
    Complication: Going too stiff can ruin handling on bumpy tracks.
     
  • Advantage: Lowering your car improves aerodynamics at high speeds.
     
  • Advantage: Lowering your car also lowers its center of gravity (CG) which decreases "weight transfer" to help improve handling. And it may also lower your car's "roll center" and change other aspects of your car's suspension geometry in ways that help handling. But sometimes these changes hurt.
     
  • Problem: Every car and every track seem to require a different setup, and every change affects other aspects of the car.
     
    Solution: These spreadsheets allow you to instantly calculate the effect that, say, stiffer front springs will have on the car — how that change will affect the car's "favored speed" (a function of the front springs relative to the rear springs, and the car's "unsprung weight"), and how much softer the front sway bar should be to retain the current handling balance. Or, if you want to change the handling balance, simply change one number in your spreadsheet, and it will calculate what components need to change in order to accomplish that.
     
  • Problem: The track time (and parts) required to "sort out" suspension modifications is expensive.
     
    Solution: Use these spreadsheets to calculate your best estimate of what your car's setup should be before you get to the track. You will very likely be far closer to your car's ideal setup than you would be from just guessing. Make the mods before you go to the track. Then load the spreadsheet into your laptop computer and you can make quick re-calculations at trackside to suit conditions, helping you save even more valuable time in car setup!

In addition to the above concerns, the questions of handling balance and body roll are important to racers. The manual has more info on determining how much body roll is acceptable, and how to determine how much body roll you have now, as well as suggested values to use for various types of cars when deciding the handling balance (oversteer and understeer). But here is a quick summary:

Body Roll

Every car is different, but in general body roll is excessive if:

  • You can't adjust the suspension far enough to achieve adequate negative camber. This is determined through tire testing. If the outside shoulder of the tires wears too quickly, or gets too hot, yet you have already adjusted the suspension to obtain the maximum negative camber possible, you need to decrease body roll through stiffer suspension components.
     
  • The response of the car when maneuvering is too slow. This is more subjective, but in general somewhere between 1 and 4 degrees of body roll is normal in racing. Just don't go so stiff you lose handling in bumpy sections.
     
  • The car doesn't make good use of the tires facing the inside of turns. They tend to run at more extreme angles to the ground than the outside tires do. This is because manufacturers favor the outside tires since they are capable of performing more work than the inside tires. But the ultimate cornering speeds are achieved when both the inside and outside tires are working at their best levels. An absolute minimum of body roll is required for this, unless you have a solid axle suspension in which case this isn't an issue. In any case, this is less important than the above concerns.
Handling Balance

The spreadsheets describe your car's handling balance with a ratio called "Total Wheel Rate Ratio Front/Rear" stated as a numerical ratio to "1." If you like the handling balance of your current suspension, don't change this number in the ratio displayed by the spreadsheet. If you think it under- or oversteers too much, you should decide on a new number. If it understeers too much, make the "front" number smaller (which signifies softer front sway bars and springs, or stiffer rear sway bars and springs). If it oversteers too much, make the number higher (which signifies softer rear sway bars and springs, or stiffer front sway bars and springs). The best thing to do is to experiment with your current suspension. Try removing the sway bar at the end you want to have stick better, then drive it that way. If you like it, calculate the resulting ratio using the spreadsheet and enter that value here. If not, make a bigger change and test that setup.

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
Mid-engine
Rear engine
61%, 39%
44%, 56%
39%, 61%
0.4 : 1 to 0.8 : 1
1.2 : 1 to 1.4 : 1
1.4 : 1 to 1.6 : 1

 

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