What could be causing my BMW 323i to shake?

I have a 2000 BMW 323i and I feel a shaking in the stering wheel and in the seat around 40 mph and over this sometimes happens at slower speeds too. It gets worse when i apply the breaks .

The following has been done to it in the past 3 weeks....

1. brand new tires put on

2. 2 of the rims have been switched out (due to 2 of them being bent)

3. the bushings have been replaced

4. front end alignment (the tech did not say that he seen anything that needed replaced)

Im not sure if the belt in the tire is broken or if its something more serious... what do you think?

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My guess it's still something with the tires...check to see if something is digging into the wall of the tire...i had a similiar problem with my car and so didn't my friend

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deff balance, alignment doesnt do anything for the car shaking. its the balance and if thats not it, then one of the rims could be cracked
The most basic thing to do would be to have the wheels balanced. But I assume this would've been done when you mounted the new tires.

Warped brake rotors would normally make your car shudder ONLY when you apply the breaks, and would shudder at any speed, not just at 40mph.