Need help picking Springs? This tool will help.
 

Hey guys, I'm not much of a poster but I figured that I'd share some information that has been helpful for me.

When picking the springs for my car I wanted to eliminate the wheel gap and at the same time maintain the stock feel for the handling of the car.

Below is a link to a spreadsheet I made with various springs and their advertised spring ratios. I've also calculated the "actual" ratios specific for the twins and how springs will affect the under-steer/over-steer of your car when accounting for the motion-ratio.

If there are any mistakes or things that I've missed I'll be more than happy to make the corrections.

The top portion is for the BRZ and the bottom for the FRS
Link:
https://docs.google.com/spreadsheet/...VE&usp=sharing

Thanks for your work

dang a lot of work to almost be like this thread http://www.ft86club.com/forums/showthread.php?t=8739

[edit] just noticed the "actual" column. thats pretty helpful to know!

btw, how were these actuals calculated? kinda weird how all the rear spring rates for all the brands are around the same range but their advertised numbers are all over the place

So to calculate the actuals I took the information from here:
http://www.ft86club.com/forums/showthread.php?t=9936

In the "Failing to Account for the Motion Ratio" section the front of the BRZ/FRS the motion ratio is close to 1 and for the rear it's closer to .75.

You can think of our suspension in the shape of an upside-down trapezoid instead of a rectangle. The rear struts sit closer to the middle of the car than the front struts, hence the .75 number.

Towards the end of that section he mentions, "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."

The Square of the motion ratio is .75 * .75 = 0.5625

Taking this into account, the spring rate of the front is essentially the advertised spring rate. However, for the rears, you want to multiply the the square of the motion ratio (0.5625) to the advertised spring rate of the rears.

So for a spring rated at (F 3.16k/R 3.88k) the Front remains the same 3.16k, but for the rears it's 3.88k * 0.5625 which gives you an "actual" spring rate of 2.1825k.

On a side note, I decided to go with the RS-R Super Downs because they have a spring rate bias that's approx. 6% stiffer than stock and with better wheels I'll be even closer to that stock feeling.

[edit]Racecomp Engineering if I'm missing anything please let me know. :)

Two things I'll mention:

1. Good work!

2. There's more to understeer/oversteer balance than just the main spring rates. This car is very bumpstop active, so lowering an inch up front with soft rates means you're getting a LOT of spring rate from the bumpstop itself. So the rates on the springs are almost secondary once you're in a corner and the body is rolling...the spring rate of the stock bumpstops gets progressively very firm. So lowering 1 inch with the same spring rates as stock will definitely change the handling (and ride) of the car. Even lowering with the same % increase in rate front and rear can change balance. Maybe simply

Suspension geometry is also a factor and you did note this fact in the spreadsheet.. Lowering an inch up front has a different effect on geometry than lowering an inch in the rear. Maybe mention the bumpstop thing too and/or change the wording (the 2 examples especially...putting a number on how much % more/less understeer there is than stock is very tricky). You could say the change the spring rate bias X% or something.

I would also note which springs include replacement bumpstops (our RCE springs and Eibach).

- Andy

Anyone have any data on the TRD lowering springs yet? I want a lowering spring that will keep the ride quality and handling as close to stock as possible. I love the way the stock suspension feels but I would like to get minimize the wheel gap.

Thanks for your input! I'll definitely update the spreadsheet later tonight, and add more information regarding bumpstops and correct word usage.