Proper Spring rate for Evo 9

#1
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.




Is this true or false?
Why most aftermarket coilovers are still offering higher springrates for the front as compared to the rear still?
 

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