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Old 06-26-2012, 07:57 PM   #1
Racecomp Engineering
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Basic tech info: spring rates, rear suspension, motion ratios, and you.

I wrote this before on an EVO messageboard and it helped a lot of people figure out what to aim for and motivate them to educate themselves on suspension. I thought it would help here too so I modified it a little and here you go. Multiply 1 kg/mm by 56 to get the rate in lbs/in.


Suspension Tech: Motion ratio and choosing spring rates

First, if you don't really care about performance or ride quality and just want a drop, close the thread now and don't sweat it too much.

But if you do care about performance and function, it helps to know what's going on! So keep reading.

Second, I'm not going to spell out exactly what are the "best" spring rates. There are many factors at play that it's impossible and wrong to say what's best for everyone and every application. Ideally you'd have a suspension tuner design a suspension for your specific set-up, needs, and preferences. It's also a little early for that IMO and many of us are still testing things out.

Third, I'm also not going to talk about specific brand names. This is just intended to be a rough guide for people looking at and comparing spring rates for springs and coilovers. Really this is one stepping stone for you to read more authoritative material on suspension.


We often see spring rates that are just plain wrong with several brands of off the shelf coilovers for a variety of cars. It does not necessarily mean the coilovers are bad, but the springs should be swapped out if performance is the goal.


1. Too stiff. You don't need crazy 14 kg/mm spring rates on narrow street tires. You don't have enough grip to generate enough body roll to need super stiff spring rates. You'll likely be overheating and overloading your tires very quickly. With R Compounds and sticky street tires you will end up on the firmer side of things but there is still much to test for the BRZ/FRS.

Autocross sometimes makes things a bit more interesting and firmer than usual often feels good to some people. Aero also changes thing, as does driver preference, and swaybar sizing. That's a topic for another thread.

2. Secrets. I strongly disagree with the practice of vendors or companies keep their spring rates secret. It isn't a secret sauce, it's just physics. Many get it wrong, but it's not magic and it's up to them to prove they got it right. Physics. Even if you're not a suspension nerd, non-disclosure agreements and secrecy should be a red flag. I really must know the spring rates for a set-up as it's the foundation for everything and I would refuse to purchase anything when that's not provided. Physics.

Failing to account for the motion ratio

Sometimes you see spring rates for a set of coilovers and it's something like 7kg/mm front and 5kg/mm rear. Makes sense, since the BRZ/FRS is a little front heavy right so we can make the rear a little softer? Well actually there's another factor at play.

The BRZ/FRS has a multi-link rear suspension. The rear spring is mounted inwards on the control arm just a little bit. Unlike the front strut which is mounted at the hub, a 5 kg/mm spring rate in the rear does not mean a 5 kg/mm rate "at the wheel". Essentially the rear control arm is a moment arm....placing the spring inwards towards the pivot point means it needs to be stiffer to have the same effect as it would if it were mounted at the end of the arm or at the hub. Sometimes you'll see people mention the "motion ratio." For the front BRZ/FRS with the strut based suspennsion, it's close to 1. For the rear, it's around 0.75.

To get the "wheel" rate of the spring, or the spring rate at the wheel, you multiply the spring rate by the square of the motion ratio.

So that 7k/5k is really more like 7 kg/mm front and 2.8125 kg/mm rear.

Is that bad? Well, if it's not what you were planning on then, yeah, that's not good. It has it's advantages and disadvantages. Some of these can be tackled through swaybars and alignment settings but my philosophy is to get the spring rates right first as a foundation. Softer rear rates CAN work.

Sooo....this doesn't mean that you should go stiffer in the rear, or that softer in the rear is "bad." But it's just something for everyone to understand when looking at spring rates for the various springs and coilovers out there. I'd encourage everyone to read up on what springs actually do for the car and how changes affect a car. I do plan on writing on a few more topics eventually but uh....we've been a little busy lately.

- Andrew

Last edited by Racecomp Engineering; 07-16-2012 at 11:03 AM.
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