Choosing coilover spring rates for your EVO

GTWORX.com · Feb 17, 2009, 07:25 PM

First, if you don't really care about performance, than 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.

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 yes, but 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

GTWORX.com · Feb 17, 2009, 07:27 PM

There are many other factors that go into determining spring rates....yes weight distribution of course plays apart, as does wheelbase, unsprung weight etc. Motion ratio is just one part of it that is used to figure out the spring rate "at the wheels."

**SmikeEvo** · Feb 17, 2009, 07:33 PM

Well done!

tinkolby_25 · Feb 17, 2009, 07:39 PM

subscribed
awesome thread

18bora · Feb 17, 2009, 08:15 PM

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?

Yogi_B · Feb 17, 2009, 08:36 PM

Quote:

Originally Posted by 18bora

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?

Those look to be metric eibachs.

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.

dbsears · Feb 17, 2009, 08:47 PM

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

GTWORX.com · Feb 18, 2009, 08:50 PM

thanks guys, i'll try to update the thread when i can.

- andrew

Mike01gsr · Feb 18, 2009, 09:48 PM

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?

EVOlutionary · Feb 19, 2009, 10:51 AM

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

GTWORX.com · Feb 19, 2009, 11:07 AM

^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

donour · Feb 19, 2009, 12:19 PM

Quote:

Originally Posted by GTWORX.com

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).

A really good damper is the silver bullet. My zzyzx setup (koni 8611) rides awesome, even with 700 lb/in springs.

d

SICK9 · Feb 19, 2009, 02:05 PM

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..

kekek · Feb 19, 2009, 03:33 PM

Quote:

Originally Posted by EVOlutionary

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

My bsp car was driven to nationals on 12/16 setup with ohlins flag dampers. Not the smoothest ride, but it really wasn't terrible. This was done in one shot each way, 23 hours straight with one pit stop in the middle to rotate tires. This is a 2600+ miles round trip.

Personally, I love it.

EVOlutionary · Feb 19, 2009, 05:27 PM

Now THAT is dedication!

I am looking forward to more topics covered in this thread. It will be a great benefit to all evo owners.