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.

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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.

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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.

Basics

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. :D


- Andrew

So many people forget about motion ratio and try to compare spring rates between different cars (!), nevermind front/rear.

Jeff

Please, get technical. I always look forward to a good read.

Sticky? Or daily bump?

You guys have been unbelievably helpful on this forum.

^^agreed!!

Thank you.

On the EVO board there was one "tech" thread that was stickied that linked to a bunch of good threads. That might work well here.

- Andrew

in to read later.

This is pretty interesting, i was just coming back from a drive and felt so good driving the car, i plunged into a few corners and the car's front end just dives, it rotates so perfectly. The rear end of the car likes to go out but in a very controlled manner. What i don't really get is if its so great in stock form why don't they keep the same ratio of front to rear spring stiffness when upgrading to shorter stiffer springs.

That's a great question...

A short answer is that the front and rear of the react differently to changes in spring rate. Roll centers and dynamic alignment changes play a big part here too and change differently in the front vs the rear with ride height and spring rate changes.

- Andrew

I'm just afraid to really mess up the cars amazing handling characteristics, but it needs to be dropped lol. I'm seeing many companies coming out with 7/7,8/8 for front and rear rates and can't help but wonder if this will retain the stock character. The stock ride is nice and stiff, so maybe a set of coilovers with similar rates to stock(maybe a bit higher) would be great? I'll wait and see before choosing coilovers and spring rate, hopefully a few write up on different rates and their behaviors will come along

One important thing to keep in mind is the dampers play a huge role in ride comfort. Even if with higher spring rates, as long as the dampers can keep up, you could end up with a smooth, if not smoother ride than stock.

Yeah i think what contributes to the stiff feeling of the FR-S are the dampers. I read in a review that the FR-S has stiffer shocks than the BRZ. What i like is the match that the FR-S has between then dampers and the spring rates, would love to keep the same ratio/match.

I believe there have been more than one independent shock dyno's which show the dampers are the same between the two cars. Only the spring rates are different.

If there is any difference (and nothing has confirmed yet there is), it is small enough spring vendors are using the same springs for both cars. Most of the rates people are coming out with are somewhere in the middle between the two cars, when you look at the difference between front and rear. The North American FR-S has 40% higher spring rates in the rear, the BRZ is around 23%. Most of the aftermarket vendors are somewhere around 25-30% higher in the rear. Like Racecomp said, when you lower the car it has effects on the whole suspension so the rates may not be the same as stock for a lowered car.

Coilovers are an entirely different thing since the damping may be drastically different than stock.