Choosing coilover spring rates for your EVO
Feb 17, 2009, 06:25 PM
#1
Choosing coilover spring rates for your EVO
First, if you don't really care about performance, then it doesn't matter. Get coilovers and get the drop you want. You can close this thread now.
But if you do care about performance, you're doing yourself and your car a disservice by getting the incorrect spring rates.
Second, I'm not going to spell out exactly what are the "best" spring rates. There are so many factors at play. Having a suspension tuner design a suspension for your specific needs and preferences is the best way to go.
This is intended to be a very rough guide people choosing spring rates for coilovers. It is not comprehensive, every car and driver is a little different, I am not a suspension master, and all that stuff.
Crappy spring rates are a problem with several brands of off the shelf coilovers. It does not necessarily mean the coilovers are bad, but the springs should be swapped out if performance is the goal.
Basics
Too stiff. You don't need 14kg spring rates on street tires. You don't have enough grip to generate enough body roll to need super stiff spring rates. You'll likely be overheating your tires. I would recommend R Compounds and stickier when you go past 10kg springs.
This is NOT a hard rule though and there are exceptions including auto-x, where physics do not really make as much sense (but they are still laws). Aero also changes thing, as does driver preference.
I disagree with the idea that vendors or companies keep their spring rates secret, because "trust" is funny thing when so many do it so wrong. It isn't a secret sauce. MANY people know what works and what doesn't so there's no excuse to hide it beyond trying to make it seem extra special. Even though many get it wrong, it's not rocket science or some witch's brew concoction of parameters. Maybe I'm a nerd or I care too much, but I really must know the spring rates for a set-up as it's the foundation for everything.
I know this section is short, but the majority of this post is dedicated to the section below, which shockingly is still a common occurance in the EVO world by even highly rated suspension companies.
Incorrect Spring Rates
What do I mean by incorrect?
Usually when you see spring rates for a set of coilovers it's something like 8kg/6kg. Makes sense, since the EVO is a little front heavy right? Well actually there's another factor at play.
The EVO has a multi-link rear suspension. The rear EVO spring is mounted inwards on the arm just a little bit. Unlike the front strut which is pretty much mounted at the hub, a 5k spring rate in the rear does not mean a 5k rate at the wheel. Essentially the suspension arm is a moment arm....placing the spring inwards towards the pivot point means it needs to be stiffer. Sometimes you'll see people mention the "motion ratio." For the front EVO with the strut based suspennsion, it's close to 1 (almost). For the rear, it's somewhere between .6 to .9.
To get the "wheel" rate, you multiply the spring rate by the square of the motion ratio.
So that 8kg/6kg is really more like 8kg/3.7kg ish.
Can you make it work? Kinda. A lot of front camber and a rear swaybar help. But a lot of front camber reduces your contact patch in a straight line and a rear swaybar reduces suspension independence. And then there's the suspension frequency to look at too. So basically yes....you want a slightly stiffer rear spring than the front for the EVO 8 and 9. At the very least, even front and rear spring rates. Top auto-x setups are generally like 12k/16k, but i wouldn't do that for the street or track since that can be a handful.
- Andrew
But if you do care about performance, you're doing yourself and your car a disservice by getting the incorrect spring rates.
Second, I'm not going to spell out exactly what are the "best" spring rates. There are so many factors at play. Having a suspension tuner design a suspension for your specific needs and preferences is the best way to go.
This is intended to be a very rough guide people choosing spring rates for coilovers. It is not comprehensive, every car and driver is a little different, I am not a suspension master, and all that stuff.
Crappy spring rates are a problem with several brands of off the shelf coilovers. It does not necessarily mean the coilovers are bad, but the springs should be swapped out if performance is the goal.
Basics
Too stiff. You don't need 14kg spring rates on street tires. You don't have enough grip to generate enough body roll to need super stiff spring rates. You'll likely be overheating your tires. I would recommend R Compounds and stickier when you go past 10kg springs.
This is NOT a hard rule though and there are exceptions including auto-x, where physics do not really make as much sense (but they are still laws). Aero also changes thing, as does driver preference.
I disagree with the idea that vendors or companies keep their spring rates secret, because "trust" is funny thing when so many do it so wrong. It isn't a secret sauce. MANY people know what works and what doesn't so there's no excuse to hide it beyond trying to make it seem extra special. Even though many get it wrong, it's not rocket science or some witch's brew concoction of parameters. Maybe I'm a nerd or I care too much, but I really must know the spring rates for a set-up as it's the foundation for everything.
I know this section is short, but the majority of this post is dedicated to the section below, which shockingly is still a common occurance in the EVO world by even highly rated suspension companies.
Incorrect Spring Rates
What do I mean by incorrect?
Usually when you see spring rates for a set of coilovers it's something like 8kg/6kg. Makes sense, since the EVO is a little front heavy right? Well actually there's another factor at play.
The EVO has a multi-link rear suspension. The rear EVO spring is mounted inwards on the arm just a little bit. Unlike the front strut which is pretty much mounted at the hub, a 5k spring rate in the rear does not mean a 5k rate at the wheel. Essentially the suspension arm is a moment arm....placing the spring inwards towards the pivot point means it needs to be stiffer. Sometimes you'll see people mention the "motion ratio." For the front EVO with the strut based suspennsion, it's close to 1 (almost). For the rear, it's somewhere between .6 to .9.
To get the "wheel" rate, you multiply the spring rate by the square of the motion ratio.
So that 8kg/6kg is really more like 8kg/3.7kg ish.
Can you make it work? Kinda. A lot of front camber and a rear swaybar help. But a lot of front camber reduces your contact patch in a straight line and a rear swaybar reduces suspension independence. And then there's the suspension frequency to look at too. So basically yes....you want a slightly stiffer rear spring than the front for the EVO 8 and 9. At the very least, even front and rear spring rates. Top auto-x setups are generally like 12k/16k, but i wouldn't do that for the street or track since that can be a handful.
- Andrew
Last edited by GTWORX.com; Jun 27, 2011 at 10:43 AM.
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Feb 17, 2009, 06:27 PM
#2
There are many other factors that go into determining spring rates....yes weight distribution of course plays apart, as does wheelbase, roll centers, unsprung weight etc. Motion ratio is just one part of it that is used to figure out the spring rate "at the wheels."
Last edited by GTWORX.com; Mar 30, 2011 at 07:39 AM.
Feb 17, 2009, 07:15 PM
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Thanks for posting.
I have a set of coilovers sitting in the box waiting to be installed. They came with Eibach springs, not sure what the actual spring rate is.
The fronts read (H200-065-T100 25,06) and the backs (H200-065-T100 43,06)
Does anyone know what spring rate is that?
I have a set of coilovers sitting in the box waiting to be installed. They came with Eibach springs, not sure what the actual spring rate is.
The fronts read (H200-065-T100 25,06) and the backs (H200-065-T100 43,06)
Does anyone know what spring rate is that?
Feb 17, 2009, 07:36 PM
#6
Evolved Member
Thanks for posting.
I have a set of coilovers sitting in the box waiting to be installed. They came with Eibach springs, not sure what the actual spring rate is.
The fronts read (H200-065-T100 25,06) and the backs (H200-065-T100 43,06)
Does anyone know what spring rate is that?
I have a set of coilovers sitting in the box waiting to be installed. They came with Eibach springs, not sure what the actual spring rate is.
The fronts read (H200-065-T100 25,06) and the backs (H200-065-T100 43,06)
Does anyone know what spring rate is that?
H200 = 200mm long
065 = 65mm ID
T100 = 100 N/mm (10.2kg/mm)
Don't know what that last one is, could be date of manufacture, possibly 25th week 2006 for fronts 43rd week 2006 rears.
Feb 17, 2009, 07:47 PM
#7
Very nice...now you need to explain the external reservoir characteristics, mainly Kw's. 7kf/8kr might seem low for KW's in all actuality its effectively a little higher? Oh and high and low speed dampning and a few shock dyno's of made in tawain coilovers for chuckles 
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Feb 18, 2009, 08:48 PM
#9
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Very good thread andrew, I have a question I have the KW's and when I bought them I bought a set of 8k springs to replace the 6k. So right now I have 9k front and 8k rear do you
think it's worth the time and effort to swap them around?
think it's worth the time and effort to swap them around?
Feb 19, 2009, 09:51 AM
#10
Very good write up Andrew. Thanks! I've been telling people for 4 years that you need to have higher rates in the rear.
I do believe that there are times when spring rates can be significantly higher than 10K even for street tires - for some people. On my '04 daily driver (25000 miles per year) I ran somewhere around 13k/15K and they were awesome. BUT - I had a perfectly valved set of Ohlins and was on 285/30/18 Advan Neovas . . . .
If you throw that high of spring rate on your average cheap set of coilovers it would probably destroy them before too long. . . so it is definitely NOT for everyone. But with a good set of coilovers and proper dampening . . . I don't think spring rates in the teens are out of the question. . .
EVOlutionary
I do believe that there are times when spring rates can be significantly higher than 10K even for street tires - for some people. On my '04 daily driver (25000 miles per year) I ran somewhere around 13k/15K and they were awesome. BUT - I had a perfectly valved set of Ohlins and was on 285/30/18 Advan Neovas . . . .
If you throw that high of spring rate on your average cheap set of coilovers it would probably destroy them before too long. . . so it is definitely NOT for everyone. But with a good set of coilovers and proper dampening . . . I don't think spring rates in the teens are out of the question. . .
EVOlutionary
Feb 19, 2009, 10:07 AM
#11
^That's true....especially for auto-x on good dampers. Auto-x kind of throws conventional thought out the window in my opinion. 
Dave Mac's old auto-x car actually rode pretty well with really high rates from what he tells me (although he never did give me a ride with the final set-up.....hmmmm).
- Andrew
Dave Mac's old auto-x car actually rode pretty well with really high rates from what he tells me (although he never did give me a ride with the final set-up.....hmmmm).
- Andrew
Feb 19, 2009, 01:05 PM
#13
What do you think about the 10k / 10k spring rates for a good street setup that occasionally does 5-6 auto-x's a year.
with yok. A048 R comps
Reason why I ask, Is because I have a set of coilovers on order and got them with 10k/ 10k..
Just wondering if this will be good enough for my intentions.
thx andrew..
with yok. A048 R comps
Reason why I ask, Is because I have a set of coilovers on order and got them with 10k/ 10k..
Just wondering if this will be good enough for my intentions.
thx andrew..
Last edited by SICK9; Feb 19, 2009 at 01:07 PM.
Feb 19, 2009, 02:33 PM
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Very good write up Andrew. Thanks! I've been telling people for 4 years that you need to have higher rates in the rear.
I do believe that there are times when spring rates can be significantly higher than 10K even for street tires - for some people. On my '04 daily driver (25000 miles per year) I ran somewhere around 13k/15K and they were awesome. BUT - I had a perfectly valved set of Ohlins and was on 285/30/18 Advan Neovas . . . .
If you throw that high of spring rate on your average cheap set of coilovers it would probably destroy them before too long. . . so it is definitely NOT for everyone. But with a good set of coilovers and proper dampening . . . I don't think spring rates in the teens are out of the question. . .
EVOlutionary
I do believe that there are times when spring rates can be significantly higher than 10K even for street tires - for some people. On my '04 daily driver (25000 miles per year) I ran somewhere around 13k/15K and they were awesome. BUT - I had a perfectly valved set of Ohlins and was on 285/30/18 Advan Neovas . . . .
If you throw that high of spring rate on your average cheap set of coilovers it would probably destroy them before too long. . . so it is definitely NOT for everyone. But with a good set of coilovers and proper dampening . . . I don't think spring rates in the teens are out of the question. . .
EVOlutionary
Personally, I love it.
