# My robot doesn’t turn enough (less than calculations)

Hi everyone!

I’m currently facing a problem with my robot in my simulation: he doesn’t follow (enough) the theorical trajectory. It's a basic two-wheeled robot with a caster wheel (a fixed wheel with no friction, actually). Like this one:

His behavior is good in a straight line but when it comes to turn, he draws a circle 4% larger than the theorical one.

My calculation:

• For a turning radius R and a linear speed in turn V, I calculate a turning speed ω.
• With the length ‘l’ between the two wheels and V, I can calculate the speed of each wheel (Vg and Vd) with those formula:

But then, instead of having the good turning radius, for example: 10 m, I have 10,6 m. Consequently, I cannot control the good trajectory of my robot because it came wrong since he starts turning. I know that 6% isn’t catastrophic but it’s still too much. There is example of my measure :

Here are, the commands associated to this trajectory, published on the topics:

• I’m 100% sure of my calculation, about wheels speeds. I can show them if you want.
• My measurement point is between the two wheels
• If I let the robot doing many circles, it keeps the same wrong trajectory. It doesn’t evolute.
• The real robot I try to simulate doesn’t have a such large gap (Without any engineering control)
• I tried to increase or decrease the friction coefficient of the traction wheels, it’s never better.
• The mass and the inertia of the parts are very close to the truth.
• The error increase with the linear speed.
• I’m pretty sure of the distance between the two wheels too
• The wheels are made with gazebo cylinder and not with STL files (as it was for my firsts tests)