Recommended Spring Rate?
 

I want to get coils for my car (BC Racing RM Series), and it gives me different spring rate options. However, I have no idea what spring rate actually is. I know it’s how much weight it takes to drop it, but don’t know which spring rate to get. I daily it so nothing too firm, but do like to push the car every once in a while. Is there a certain spring rate range I should go for to achieve this? And would I feel the difference between 8k and 7k? Any help is appreciated, thanks.


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My rates are 6kg front and 4.5kg rear, on MCA street performance coilovers.

Performance and comfort for the street is good.

BUT, matching springs to damper is valving black magic.

It's easy to get wrong, and tricky to get right.

Whilst higher speing rates with matching valving works well on smooth race track type surfaces, they will be garbage on average road conditions.

Have a read > https://mcasuspension.com/technical/29/Suspension-Setup

I leave this stuff up to the experts, I tell them what I the car is used for, my driving style and the characteristics I like, and give them money, they give what I want.

It's usually a comprise of sorts, but typically, if they know what they are talking about, you get a good result.

Opinions may vary.

I wasn't able to find the RM series available for the 86 on the BC website but one of the listed features of the RM series is simultaneous compression/rebound adjustment. For a street car, even one that visits a "turning" competition event from time to time, this would have me choosing the softer of the two examples you gave. I didn't find their offering for the car, so I'm only going off your 8k/7k example. The spring rate for any car also would take into consideration the spring length being used (which I also don't know for this kit), but in general when you put a street car onto any length of spring the spring should compress 25%-35% of it's free length. Think of the springs job as only supporting the weight of the car at ride height and the shock/struts job is to control the ride quality. On a street car I would go with the softer rate. From there you can use the valving adjustment should you need firmer/aggressive handling.

Somethings immediately come to mind.
1/ Everything's a compromise.
2/ Different people have different impressions of ride. That is, what I consider firm you might consider too harsh or too soft.
3/ If you were to compare a car with 7k front and rear and then immediately compare a car with 8k all round hopefully you would be able to tell a difference.
I could certainly tell the difference between 6k/6k and 9k/10k but I don't remember a difference between 6k/6k and 7k/6k.
4/ Given your use I would suggest something around 5k-6k. I'm thinking over 6k might be too much for your intended use.
5/ There is no best spring rate. See 1/.

I have 5K front and back and I think it’s too soft

Which are "best" depends on intended goals/use and tires. For mostly daily driving, i guess comfort is more valued. Then again, "to drop it", of course further reduction of limited shock travel will need compensating with higher rates, to not bottom out.

I suggest, if lowering, then to not go overboard, to not lower more then >1", and if for dailying, again to not go overboard with spine-crushing spring rates. And if you haven't yet bought those coilovers, rather consider something of better quality. On your side of pond i'd think of RCE superstreet-1 or CSG Flex A (or "normal" F/A) or Bilstein B6/8.

There are a lot of variables with damper valving, sway bars, tire of choice, use, etc.

I'd say that 99% of people that have to ask what spring rate they want would find 7k too stiff.

If you feel like getting into the details on spring rates just a little bit, here's some light reading.

Spring rates and the motion ratio:
https://www.instagram.com/p/B3cDzqOD..._web_copy_link
Spring rates and suspension frequencies:
https://www.instagram.com/p/B6TKG6Un..._web_copy_link

That's just meant to get your started....there's a lot more to get into on spring rates. But I haven't gotten to part 3 yet.

8 kg/mm springs on the front of a BRZ/FRS/GT86 means a suspension frequency of about 2.5 Hz, which is a lot for a daily driver street only car. The usual guidelines/ranges on what frequency for what type of car or what type of use are fuzzy and depend on a variety of factors. But 8 kg/mm and 2.5 Hz is stiff/high.

- Andrew

Most guys in the autox world daily their car and give good indication on a starting point.

I settled on a 6k/6k split as it kept the car nicely balanced but then used the sway car to fine tune how it felt. I know some guys use a bigger rear sway but a softer spring at like a 6k front and 5 or 4k back.

Long gone are the days of S chassis tuning when we thought 9k rates where cool.