REST LENGTH PROBLEM

Hi guys,

This is my first post, so I apology if my english is bad if I post wrong...

Well, there is my problem :

I'm trying to simulate an arch but I want to keep the segment between my points equal.

So I used the spring component but when I use a polyline with the particules out and after checking the length, the result is not equal to my first rest length and is not constant. Also, the length is linked to the amplitude of my vectors and I don't want it to be so.

To resume I want to keep my length constant.

Can some one help me pleaz?

THX