what is the rules for single like the upgrades

There isn't any rules with singles. The way I set it up on the new version is it all works off parts. So you can upgrade your pumps and batteries then assign a pump to certain wheels.

So if you upgrade all 4 of your pumps you can choose to assign 1 pump to both front wheels and that makes it a single or you can assign 1 pump to each front wheel and that makes it a double.

When hopping your car there are limitations.

Single pumps will automatically adjust to suspension 2.
Double pumps will automatically adjust to suspension 4