is a straight cut differential a possible thing to simulate currently? where the gear whine is consistent no matter your gear, and can be heard without using a transmission (EV single-speed race car).
I'm not aware of a way to do that, but that's not to say that it doesn't exist. Having another glance over the jbeam code, I'm not sure that there is currently anywhere that it would make sense to specify something like that. So I can't really help you on that side of things, but... If its not possible, you may be able to bodge it by sticking a single speed sequential gearbox in your powertrain. Then you can presumably adjust how it sounds by tweaking the gear ratio. The issue being that the vehicle controller may not like that? But its not unfeasible that it will ignore it, so it could be worth a try if all other options fail.
