How to measure spring rate
10th March 2005
Normally a machine to measure spring rate is not available, hence TiTAN Suspension founders have found a brilliant way to acquire useful figures / number related to spring rate.
A simple hydraulic press machine is required with a gauge to read psi or kg/cm^2 or whatever unit.
Make a bottom seat for the coil and a top holder.
Mark the bottom zero (0) and one inch then 2 inches then 3 inches and so on. Maximum 6 inches will do.
Place the coil between the bottom seat and secure the top holder without putting too much pressure. Just tight will do.
Start to compress the coil up to 3 inches travel.
Read the psi value while holding the pressure lever down. Do not let it go. The pressure reading is not the same between holding the pressure and letting go. Read is at exactly 3 inches around 76.2 mm
Now, you may compare the psi pressure of various coils but all must be using 3 inches compressing travel. Use a known coil spring rate as reference to find out it's psi value. Or you could even be creative to do this: -
I checked my abandoned coil suspected 8 kg/mm and found it's pressure extremely strong and didn't dare to compress it to 3 inches because worry the arms to hold the coil would break. It was only compressed to 2.5 inches and read 600 psi.
Using basic arithmetic and assume linear spring rate it is found to be 600 divide by 2.5 multiply by 3 = 720 psi / 3 inches.
So we assume a 8 kg/mm coil is 720 psi / 3 inches.
If you measure a coil with 400 psi / 2 inches then perform this: -
400 / 2 * 3 / 720 * 8 = 6.66 kg/mm
Please realize that this method is only applicable to LINEAR spring rate and not progressive. And in order to meet the "linear" category the spring pressure of psi / travel inches must be same through usefull stroke.
i.e. my spring will sink 65 mm from lifted height to rested / stationary / equilibrium. The absorber stroke are design to give a minimum of doubling of the lifted to rested travel. i.e. The absorber can safely travel down another 65 mm or more.
I've made sure that my coil remains at constant spring rate for the complete travel / compression from zero (0) to 130 mm or more. And that the spring does not saturates. In order to meet this non-saturation requirement, the spring is generally long and have at least 25% more travel than the absorber's stroke.
i.e. if absorber stroke is 150 mm (standard on allot of TRD short stroke for AE86 or similar RWD of pre 90s era) then the coil must be longer by no less than 50 mm. i.e. between 200 mm to 250 mm is good. With 250 mm you are guaranteed the spring is always working within it's designated rate. Very good.
After that read this to continue: - How to choose Spring Rate