RE: Skymaster F-15 Steering Stability
Here is what I did to prove to a customer that the struts were too soft. Get it rolling straight with your current set up. Then, wire one strut as far up as you can. Block the other down as far as you can. Then roll it again. Does it still roll straight? If not, you need more spring.
If you were able to stretch the springs out, they sound mariginal. If you can stretch them out, they will pound back down just as easy.
Also, watch out for too much expo. If you are a little behind on your steering inputs, you have a tendency to grab more stick and get into the big throw part of the expo and over compensate, then you overcompensate the other direction and get into a pilot induced occillation condition.
I hope some of this helps. It is one of the more frustrating things to watch your plane trying to drive its self off of the runway.
Doug