Spring Bias Trim system

[jchisolm]

I've been looking at various trim systems like Vans RV 10 spring bias system and rudder trim like Aerosport products. My question is do any of you know the secret sauce for selecting the spring? I ordered some springs from Vans and they seem to be about 4lb/in rate, 3 inch unloaded. The install stretches them to 6 inches which gives about a 24lb force. It's balanced with the 2 springs but I'm wondering why this spring rate and force. There seems to be a reason because Vans stuff is usually pretty simple and from reading blogs it seems these springs are a pain to install.

Then there is the Aerosport rudder trim. The springs on this thing seem very light weight.

I've googled everything I can think of and cannot find any real data on how to select a spring.