At forty mph the steering shakes

Welcome to 2CarPros.

There are a few things that can cause this. First, inspect the tires for uneven wear or a broken belt.

https://www.2carpros.com/articles/how-car-tires-work

If they appear good, the next thing would be tire balance. At the same time tires are being balanced, the tech should check if you have a bent or damaged rim.

If tires are good, balanced, and there are no problem with the rims, then you need to inspect all steering components for excessive play as well as alignment.

Tell me, this isn't related in any way with the brakes, correct? Also, can you drive through the vibration. In other words, it happens at 40 but does it go away at 50? Does it always happen or is there something that seems to cause it such as a turn or bump?

Let me know.

Joe