Controller information
Even though many players play the game with keyboard, gamepads and other controllers are supported for menu navigation and in-game controls. Editors and customizer currently require using mouse.
Unfortunately, there is no one standard for controllers on PCs and it is very hard to support all devices. The most common ones (Xbox, Sony) should be supported, though, and more exotic ones can usually be calibrated to work with controller settings.
Fixing controllers not working
If your controller or one of your controllers is not recognized at all or does not seem to work as expected, it may be because of Steam Input capturing the input and not letting the game to get it (or change it to some unsupported format). There are several things that can be done to try to prevent that:
Option 1: In Steam Overlay (Shift+Tab by default), go to Controller Configuration at the top-right corner (behind browser), click Browse Configs at the bottom.
Make sure Templates is not set to "Keyboard (WASD) or Mouse". If it is, use "Gamepad" instead. This is done by first clicking Gamepad and then Apply Configuration at the bottom.
Option 2: Launch Steam in Big Picture mode (e.g. by right-clicking Steam icon in Windows an choosing "Big Picture", if the game is running, you may have to restart it afterwards)
Click Library, select Turbo Sliders Unlimited
Click Manage game on the left, then Controller Options
Click Steam Input Per-Game and select Forced Off
Option 3: When game is not running, launch Steam->View (menu on top)->Big Picture Mode, select Settings and Controller Settings
If e.g. Xbox Configuration Support is enabled and your Xbox controller is not working, try disabling it and see if that helps
Option n: Other games have similar issues. You can check how e.g. what helps in Rocket League:
If nothing helps, please come to TSU Discord (https://discord.gg/QCHEsuwQwT) and report your issue to #bug-reports. Also, if you find some other way to fix issues, others might be interested in hearing how.
Fixing false input
Some peripheral devices have drivers that make them pretend they are controllers, sometimes giving false input to the game. If, for example, your car is always turning to right, even when playing with keyboard, this could be the reason. There are online "controller test" pages where you can see if this is the case.
To fix this, you may first try to disconnect and/or disable drivers for devices you suspect are the reason. If you want to play with keyboard and disable all joystick input, you can also edit controllers.json in the game's AppData directory (AppData / LocalLow / Turbo Sliders / Unlimited / SteamId), changing "axis" to 0 for all input you want to disable. Note that you may have to do that while the game is running, and then restart the game afterwards, so that Steam cloud does not override your changes.
Feel free to join the Discord server to discuss issues further if needed: https://discord.gg/QCHEsuwQwT