If one spring will support 4lbs. The other spring will only support 2lbs. With the two spring together it will support 6lbs. Does this help.

I have tried with 4 yellow springs (the yellow springs I use - not yellow in general) and the truck do not run so good - the silver springs I use are progressive springs - as more you push them as harder they are - and together with the yellow springs it works fantastic (in my opinion).

Greetings from Germany

harald-hans