My model does not going straight

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My platform: OS: Ubuntu 16.04 LTS with 4.15.0-52-generic kernel. Gazebo Version: gazebo-7

I have a differential drive model that I've created from scratch. It has 4 wheel and it's a differential drive robot. I have just create a box for chassis. It has cylindrical wheels. I didn't specify any mass or inertial info.

I put the model to the an empty gazebo world. I applied velocity to the hinges. For all hinges I applied 0,2 m/s Velocity with P(1) I(0,1) D(0,01) gains. The problem is, it is not going straight perfectly. I've tried to apply force as well. And I tried to give velocity with using a model plugin.

I've discovered that the contact points are not symmetrical. I've tried to make the wheels too thin to get rid of this thing. But it didn't work. Can you see the problem or a mistake that I did?

Here is my sdf file: C:\fakepath\model.sdf

Here is the contact points, It should be like this:0

But I see that contact points