Springs, Spring Rates, and Choosing the Ideal Spring Rate
by Alan LaFrance, find him on Google or read more articles at www.subaruwrxsti.org
http://www.vikingspeedshop.com/suspension-102-choosing-the-ideal-spring-rates/
Spring Nomenclature
The key components in determining the rate of a particular spring are the free length, wire diameter, mean diameter, and the total number of active coils.
What is a Spring Rate, and what is the difference between 8 kg and 500# springs?
Spring rate is expressed as the amount of force required to compress the spring a certain distance. This measurement will be expressed in metric (kg/mm) or in standard (lbs/in).
Using the examples above you can determine that Spring A has a rate of 8 kg/mm. Meaning if you apply 8 kilograms of weight to the top of the spring it will compress 1mm. If you apply another 8 kilograms (16kg total) of weight it will compress another 1mm.
Spring B on the other hand is measured in pounds per inch. If you apply 500 lbs. of weight onto the top of the spring it will compress 1 inch. Likewise if you apply another 500 lbs. of weight it will compress another 1 inch (1000 lbs. = 2" of compression).
To convert kg/mm to lbs/in you multiply the kg/mm rate by 56.
To convert lbs/in to kg/mm you divide the lbs/in rate by 56.
So in this case, an 8 kg/mm spring is equal to a 448# spring, and a 500# spring is equal to a 8.9 kg/mm spring.
How do I calculate the rate of a Linear Spring?
For the sake of simplicity I'll just put up this equation for determining the rate of a traditional linear spring.
Calculating spring rates for dual-rate springs is simply done by plotting the spring as two independent springs which gives you free lengths, travel, and stage rates. Calculating the spring rate for a progressive spring is much more complex.
What is the difference between a Linear Spring, Dual Rate, and a Progressive Spring?
Linear springs have the same spring rate through their intended travel range. The spring force increases linearly until the spring nears it's limit. This makes for a very predictable and steady handling experience at the cost of some ride harshness.
Dual Rate springs have two distinct spring rates. The spring force starts out with the linear soft rate until it fully compresses, from that point the spring now acts like stiffer linear spring. This is a good medium ground for a street car. You can enjoy the harmonics and benefits of the softer rate over small bumps, but during heavy cornering loads you will get the benefits of the stiffer rate. Although it's not as ideal as it sounds, it's closer to the best of both worlds than a linear rate spring.
Progressive springs can come in many flavors. They can be multiple rate progressive springs, or they can be a fairly standard progressive profiles like the yellow spring pictured above. You'll notice that the profile of the spring from top to bottom starts out narrow and gains width as it reaches the base of the spring. This has the effect of steadily increasing the spring rate for every inch of travel. This makes for a car that seems timid at low speeds but very rigid during strong cornering loads. While this seems to be an ideal situation for a street car the actual result is a car that seems to be somewhat unpredictable, especially during those last few inches of compression length.
So how do I choose the correct Spring Rate for my car?
Thankfully most aftermarket companies do the homework for you and offer their suspension kits with proper spring rates, or at least spring rates that are a good starting ground. To use an example, we'll take the 11' WRX with stock and an aftermarket coilover suspension; the KW V1 coilover setup.
Part 1 - Know your Motion Ratio to Determine Spring Effectiveness
The motion ratio is a mechanical ratio that gives you an idea of how much the spring compresses as the suspension compresses. It is expressed often as "the spring is XX% effective" when discussing motion ratios. Here's more insight into motion ratios using visual cues, and how to calculate them.
This is the front of the WRX / STI suspension. As you can notice Subaru does something very insightful, they put the spring on a more effective axis angle that is not inline with the strut. This makes the spring more effective and less prone to shear issues. One of the benefits to the MacPherson suspension design is that the strut and spring are very effective. That is, when the wheel moves, the spring and shock move about the same length.
http://i0.wp.com/www.vikingspeedshop.com/wp-content/uploads/rearmotionratio.jpg?zoom=1.5&resize=439%2C282
This is the rear of the GR style multi-link suspension. The shock / spring placement is moved inboard slightly which makes the spring less effective since it does not move at the same ratio as the wheel.
You can calculate this for your own vehicle but for this article we are going to use 90% for the front and 75% for the rear.
Step 2 - Spring needs to Fit your Needs
The stock 11' WRX uses a 217# front spring and a 194# rear spring. These are very compliant and quite soft for the road from a performance standpoint. Most of us want to lower the car, and that is going to take away travel length and require a MUCH higher spring rate to maintain suspension travel distance. Especially considering that travel distances for the front and rear suspension are no more than 3" in length. Lowering your car 2" would leave you with 1" or less travel for the suspension to act and control your car.
For the interest of this article let's try to setup the car for 1.0G of lateral acceleration at stock loaded weight (3400 lbs.) with a stock weight distribution and a 1" lower ride height.
With the higher lateral acceleration (read: more tire grip) we are going to have about 10 - 15% more weight shift to the outside tires. The amount of weight that shifts is related to the center of gravity, wheelbase, total vehicle weight, and weight distribution. The amount of roll depends on the weight that shifts outwards, instant center, motion ratio, and total wheel rates. Instant center affects the motion of the lower control arm which in turn dynamically affects how effective the spring is. If you don't fix your roll center after lowering the car, then you'll actually have MORE roll since the lower arm has a different length and angularity.
With the 1" lowered height we are going to take the height and multiply it times the motion ratio. This will give us how much compression length we've removed. Here we can see the downside to lowering a MacPherson strut design like the WRX and STI; the reduction in 1/3rd or more of the shock travel. This will require approximately 40% more spring rate in the front and 30% more spring rate in the rear to maintain similar suspension movement as OEM.
We started with a front rate of 217#, so we are going to increase this by 50% because of the added weight transfer and the lowered height. This gives us a spring rate of 325#.
We started with a rear rate of 194#, so we are going to increase this by 40% because of the added weight transfer and the lowered height. This gives us a spring rate of 271#.
Since the WRX is prone to understeer from the factory we want to correct this by tightening up the rear. When you increase the spring rate there will be a shift of weight to the outside tire in the rear, this in turn makes the car want to pivot more at the limit. It's a slight shift, but this helps to keep the front tire from washing out and creating understeer. We're going to increase the rear rate by at least 10% to 300# to help neutralize the chassis.
So, to maintain full suspension travel and keep to minimal NVH we've come to 325# front and 300# rear. This is a minimum factorization. Ideally I'd recommend moving up to ensure you don't limit the travel during dynamic movements. Something in the 350# range is ideal for a street tire'd Subaru, going to even higher spring rates is not going to gain you much since the tires themselves are limited, or you can lower the vehicle more with a higher rate.
Step 3 - Higher isn't Always Better
There is a point where you can have too high of a spring rate. When you have too much spring rate the car will be harsh riding and will begin to understeer/oversteer sooner. It might feel very responsive and feel like it "handles" better, but ultimately you've made the car less capable. Bear that in mind before you run out and slap some 550# springs on your WRX.
by Alan LaFrance, find him on Google or read more articles at www.subaruwrxsti.org
http://www.vikingspeedshop.com/suspension-102-choosing-the-ideal-spring-rates/
Spring Nomenclature
The key components in determining the rate of a particular spring are the free length, wire diameter, mean diameter, and the total number of active coils.
What is a Spring Rate, and what is the difference between 8 kg and 500# springs?
Spring rate is expressed as the amount of force required to compress the spring a certain distance. This measurement will be expressed in metric (kg/mm) or in standard (lbs/in).
Using the examples above you can determine that Spring A has a rate of 8 kg/mm. Meaning if you apply 8 kilograms of weight to the top of the spring it will compress 1mm. If you apply another 8 kilograms (16kg total) of weight it will compress another 1mm.
Spring B on the other hand is measured in pounds per inch. If you apply 500 lbs. of weight onto the top of the spring it will compress 1 inch. Likewise if you apply another 500 lbs. of weight it will compress another 1 inch (1000 lbs. = 2" of compression).
To convert kg/mm to lbs/in you multiply the kg/mm rate by 56.
To convert lbs/in to kg/mm you divide the lbs/in rate by 56.
So in this case, an 8 kg/mm spring is equal to a 448# spring, and a 500# spring is equal to a 8.9 kg/mm spring.
How do I calculate the rate of a Linear Spring?
For the sake of simplicity I'll just put up this equation for determining the rate of a traditional linear spring.
Calculating spring rates for dual-rate springs is simply done by plotting the spring as two independent springs which gives you free lengths, travel, and stage rates. Calculating the spring rate for a progressive spring is much more complex.
What is the difference between a Linear Spring, Dual Rate, and a Progressive Spring?
Linear springs have the same spring rate through their intended travel range. The spring force increases linearly until the spring nears it's limit. This makes for a very predictable and steady handling experience at the cost of some ride harshness.
Dual Rate springs have two distinct spring rates. The spring force starts out with the linear soft rate until it fully compresses, from that point the spring now acts like stiffer linear spring. This is a good medium ground for a street car. You can enjoy the harmonics and benefits of the softer rate over small bumps, but during heavy cornering loads you will get the benefits of the stiffer rate. Although it's not as ideal as it sounds, it's closer to the best of both worlds than a linear rate spring.
Progressive springs can come in many flavors. They can be multiple rate progressive springs, or they can be a fairly standard progressive profiles like the yellow spring pictured above. You'll notice that the profile of the spring from top to bottom starts out narrow and gains width as it reaches the base of the spring. This has the effect of steadily increasing the spring rate for every inch of travel. This makes for a car that seems timid at low speeds but very rigid during strong cornering loads. While this seems to be an ideal situation for a street car the actual result is a car that seems to be somewhat unpredictable, especially during those last few inches of compression length.
So how do I choose the correct Spring Rate for my car?
Thankfully most aftermarket companies do the homework for you and offer their suspension kits with proper spring rates, or at least spring rates that are a good starting ground. To use an example, we'll take the 11' WRX with stock and an aftermarket coilover suspension; the KW V1 coilover setup.
Part 1 - Know your Motion Ratio to Determine Spring Effectiveness
The motion ratio is a mechanical ratio that gives you an idea of how much the spring compresses as the suspension compresses. It is expressed often as "the spring is XX% effective" when discussing motion ratios. Here's more insight into motion ratios using visual cues, and how to calculate them.
This is the front of the WRX / STI suspension. As you can notice Subaru does something very insightful, they put the spring on a more effective axis angle that is not inline with the strut. This makes the spring more effective and less prone to shear issues. One of the benefits to the MacPherson suspension design is that the strut and spring are very effective. That is, when the wheel moves, the spring and shock move about the same length.
http://i0.wp.com/www.vikingspeedshop.com/wp-content/uploads/rearmotionratio.jpg?zoom=1.5&resize=439%2C282
This is the rear of the GR style multi-link suspension. The shock / spring placement is moved inboard slightly which makes the spring less effective since it does not move at the same ratio as the wheel.
You can calculate this for your own vehicle but for this article we are going to use 90% for the front and 75% for the rear.
Step 2 - Spring needs to Fit your Needs
The stock 11' WRX uses a 217# front spring and a 194# rear spring. These are very compliant and quite soft for the road from a performance standpoint. Most of us want to lower the car, and that is going to take away travel length and require a MUCH higher spring rate to maintain suspension travel distance. Especially considering that travel distances for the front and rear suspension are no more than 3" in length. Lowering your car 2" would leave you with 1" or less travel for the suspension to act and control your car.
For the interest of this article let's try to setup the car for 1.0G of lateral acceleration at stock loaded weight (3400 lbs.) with a stock weight distribution and a 1" lower ride height.
With the higher lateral acceleration (read: more tire grip) we are going to have about 10 - 15% more weight shift to the outside tires. The amount of weight that shifts is related to the center of gravity, wheelbase, total vehicle weight, and weight distribution. The amount of roll depends on the weight that shifts outwards, instant center, motion ratio, and total wheel rates. Instant center affects the motion of the lower control arm which in turn dynamically affects how effective the spring is. If you don't fix your roll center after lowering the car, then you'll actually have MORE roll since the lower arm has a different length and angularity.
With the 1" lowered height we are going to take the height and multiply it times the motion ratio. This will give us how much compression length we've removed. Here we can see the downside to lowering a MacPherson strut design like the WRX and STI; the reduction in 1/3rd or more of the shock travel. This will require approximately 40% more spring rate in the front and 30% more spring rate in the rear to maintain similar suspension movement as OEM.
We started with a front rate of 217#, so we are going to increase this by 50% because of the added weight transfer and the lowered height. This gives us a spring rate of 325#.
We started with a rear rate of 194#, so we are going to increase this by 40% because of the added weight transfer and the lowered height. This gives us a spring rate of 271#.
Since the WRX is prone to understeer from the factory we want to correct this by tightening up the rear. When you increase the spring rate there will be a shift of weight to the outside tire in the rear, this in turn makes the car want to pivot more at the limit. It's a slight shift, but this helps to keep the front tire from washing out and creating understeer. We're going to increase the rear rate by at least 10% to 300# to help neutralize the chassis.
So, to maintain full suspension travel and keep to minimal NVH we've come to 325# front and 300# rear. This is a minimum factorization. Ideally I'd recommend moving up to ensure you don't limit the travel during dynamic movements. Something in the 350# range is ideal for a street tire'd Subaru, going to even higher spring rates is not going to gain you much since the tires themselves are limited, or you can lower the vehicle more with a higher rate.
Step 3 - Higher isn't Always Better
There is a point where you can have too high of a spring rate. When you have too much spring rate the car will be harsh riding and will begin to understeer/oversteer sooner. It might feel very responsive and feel like it "handles" better, but ultimately you've made the car less capable. Bear that in mind before you run out and slap some 550# springs on your WRX.
