Question: How to measure the contact force?

Hello guys, 

im not an expert of maplesim, I'm working on one of the default quarter car models (first pic).

 

 

I set a probe to measure the instant contact force between the tire and the post, but the graph shows that the force reaches both posite and negative values (as if the post were pulling back down the tire, is that right?)

What l have to do in order to see the real contact force graph? i just want to see the real positive value or 0. 

 

 

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