How to Choose Spring Rates

Spring Rates Calculator

Using Suspension Frequency to Help Determine Ideal Spring Rates for your car.

In the right margin of this blog I have posted a spring rate calculator.  It looks like a small excel spreadsheet and it will help you determine what spring rates will work best for your application.  Most consumers who are upgrading their suspension don’t think about spring rates they choose a coilover kit or brand of springs and bolt them on.  They may be basing their decision on opinions or reviews they read online and they may not be getting the best setup for their application.  When I say most consumers, a large majority of consumers are modding their cars for cosmetic reasons and aren’t actually intending to participate in any motor sports.  So for them, the spring rate is inconsequential because they are looking for either a good brand they can brag about or something that is inexpensive.  

If, on the other hand, you are interested in actually participating in some sort of motor sport like AutoX, track days, or racing, then a properly tuned suspension will give you an advantage over your competitors who may have only bought a recommended coilover package.  This isn’t necessarily a bad way to go since there are a lot of good coilover kits out there that are tuned for the car and application.  But understanding spring rates and how they can be used to custom tune your suspension to very specific tracks or your own driving style can give you an edge on the competition.  At the very least it will give you a better understanding of how the suspension works. 

You don’t need coilovers to adjust spring rates, but one advantage of coilovers is that the springs have a standard diameter and you will be able to find a variety of spring rates and lengths online.  Usually the rates will be available in 50lb increments.  This gives you much better suspension tuning options than an aftermarket spring kit that is designed to fit the stock spring locations.  These kits are cheaper since you don’t need to upgrade to coilovers but there won’t be a variety of spring rates available to choose from.

The standard spring rate measurement I will be using is lbs per inch which represents the amount of force in pounds it takes to compress a spring one inch.  For example a 400lb spring will compress one inch when 400lbs of force is applied.  The same spring will compress two inches when 800lbs are applied and four inches when 1600lbs are applied.  To give you some perspective a Ford Focus might have 80-150lb springs, a mildly built Miata might have 300lb springs and a full track car may have 600-2000lb springs.  Obviously the higher the spring rate the harder the ride. Bigger cars will also have stiffer springs since the springs will have to carry the weight of a heavier vehicle.  

It seems obvious to many why stiffer springs would be more desirable, but it may not be obvious to everyone.  A brief explanation is that a stiffer spring maintains to road contact, the car won’t lean as much in a turn, and it won’t bow as much under heavy braking.  The compromise is that the ride quality will be much harsher and that means less comfortable for daily driving. This will be covered in more detail in future articles.

Suspension frequency (SF) describes the natural frequency of the spring in relation to wheel motion and can be used to estimate the appropriate spring rate for various applications.  The calculation isn’t particularly difficult but will require either finding data or taking measurements of the car.  You can also use this to evaluate spring rates of various coilover kits and stock spring packages.  Most passenger cars will have a rate of approximately 1 Hz.  A fairly aggressive suspension setup will be around 2.0 Hz and a track dedicated car will have a frequency of around 3.0 Hz or more.  So you can use this information to figure out where on this spectrum of suspension frequencies will fit your application.  If you drive your car to the track or AutoX event you might want to try a SF a little over 2.0.  It is important to note that you want your rear SF to be slightly higher than your front otherwise the car might start to porpoise; rock forward and backward.  Also keep in mind that you will need to upgrade your dampers (aka shocks) to handle springs that are stiffer if you haven’t done so already, especially if you choose a very aggressive spring rate.   

The spring rate calculator on the right is fairly simple to use.  Figure out what suspension frequency you want, update the vehicle data for your car in the white cells and the ideal spring rate will update in the yellow.  Everything in gray should be left alone and the calculated spring rates will be yellow.  For now none of the cells are protected so you could go in and modify any of them but none of the changes will be permanent.  If you make any mistakes, refresh the page and it will reset the calculator.  The spring rates are calculated for individual corner of the car but since the left and right side should be essentially the same the calculator will only calculate front and rear. If you want some more background information check out the Eibach website.  Please note for a live axle or solid beam axle this calculator will not work.

User Inputs:

Race Weight - This is the weight of the car with fully suited driver, and fuel.  If the car is stock, take the curb weight, add your own weight when fully suited, and a little extra for incidentals.

Weight Distribution Front - percent of the total weight of the car in the front.  You can use stock data to get you close.

Unsprung weight - this is the weight not supported by the springs, so this is the weight of the wheels, tires, hubs, brake caliper, half the control arm, etc.  You can estimate this weight and it will probably be somewhere between 80-100lbs per corner.  Keep in mind that particularly large wheels will be significantly heavier and may push your unsprung weight well over 100lbs per corner.  Since some cars have staggered wheels the unsprung weight from front to rear may be very different.

Spring Angle - if you have a McPherson Strut, you can estimate the angle at 5-10 degrees.  Otherwise you need to determine what angle from vertical the spring/coilover/strut is mounted in the car. 

D1 & D2 - D1 is the distance from the spring mount on the control arm to the control arm pivot point. D2 is the distance from the ball joint to the control arm pivot point.  The units aren't important as long as they are the same (mm, inches, feet, furlongs or whatever). If you have a McPherson strut, then leave these numbers alone since 98/100 is the approximate ratio.  You will see you need to do this for the front and rear.  If you are a Mustang owner and have a live axle, just do it for the front since this is not setup for solid axle setups. 

Everything in gray should be left alone since it is either calculated for you based on the info you entered in the white cells or cell descriptors but I will explain what the various measurements are.

MR - Motion Ratio, this is the ratio of the motion of the wheel to the motion of the spring.  This is calculated by taking D1 and dividing it by D2.  That is why the units of measure aren’t important, since we are using them to calculate a ratio.

ACF (Angle Correction Factor) - This is needed to accurately calculate the how much the spring compresses in relation to the wheel motion.

Front and rear Sprung Weight - In this formula, this is calculated based on the weight distribution and unsprung weight.  In this current configuration, the unsprung weight is considered to be the same for front and rear wheels.  I will update it shortly so that you can put unique values for the front or the rear.

Spring Rates - These are the required spring rates to achieve the desired suspension frequency.  You probably won’t be able to get a 212.3356 # spring, so you will have to decide if you want to with a 200# of 250# spring.  When deciding which way to go, remember that you want the rear SF to be slightly higher than the front, so look at both the front and rear SF before deciding. 

If you have any questions please feel free to leave a comment, and feedback would be appreciated.