Why do I get oscillating tractive force on non-driven wheels?

Hi!

I am trying to make a Simulink model of a one DOF 75 meter accelartion of a electric rear wheel driven formula student car. But I get this wierd behaviour of a oscillating tractive force on the front, non-driven wherels (FxF), when it should be zero. I have used MagicFormulaTireTool to get my parametrs for my tire model. Any help is much appriciated to solve this.

Best regards, Erik

Answer 1

[Jason Rodgers]

The auto-solver selection chooses ODE45 which does not play well with the relaxation length dynamics in the tire. Try switching to ODE23tb. Also avoid using the discrete delay blocks when feeding into continuous integrators; this can cause unwanted numerical issues and slow simulations. One could use a memory block for this but using a transfer function with a fast time constant is a bit better. See this blog post discussing this: Blog Post

[Erik Böhlin]

Hi Jason!

Sorry for my late response.

Thank you so much for your feedback. I took your edvice and switched to ODE23tb and switch from unit delay blocks to transfer functions, . It removed the oscillating tractive force. But I still get big a negative tracive force, as seen in the figure, on the front wheels when the torque is zero.

I will continue to work on this, any further help is much appreciated.

[Jason Rodgers]

Hi Erik,

The negative torque is due to the non-driving wheels having negative slip(due to the car accelerating) and is expected behavior. The large overshoot is due to the relaxation length of the tire. The setting was 0.5m (which represents the distance the wheel must travel before the tire forces reach some steady state). If this will always be a non-driven tire you can set the relaxation length to a small number. If the front tires will be driven you would want to return that to a more realistic value. I also noticed the time scale for the overshoot is pretty small. Is this actually effecting your results or are you just wanting to understand what you are seeing?

[Erik Böhlin]

Hi Jason, thanks for your great feedback. My main issue is that the car is too slow. I have used parameters to resemble our formula student car and it should theoretically accelarate faster. I thought the reason could be due to the negative traction force at the beginning, but as you said it is only for a fraction of a second. It should not affect the accelration as much as I first thought.