Your steering wheel shaking when you brake is usually caused by issues related to the braking system or related components. Here are the most common reasons:

• Warped Brake Rotors: The rotors are the discs your brake pads press against to slow or stop the car. Over time, heat and pressure can cause these rotors to warp or become uneven. When you brake, the uneven surface causes the brake pads to grip inconsistently, making the steering wheel vibrate.
• Worn or Dirty Brake Pads: If the brake pads are worn down or have accumulated dirt and debris, this can create uneven pressure on the rotors and cause shaking.
• Sticking or Seized Brake Calipers: Brake calipers hold the pads against the rotors. If a caliper sticks or doesn’t release properly, it can cause constant friction, leading to vibrations when braking or even when driving.
• Tire Issues: Unbalanced, worn, or unevenly worn tires can cause the steering wheel to shake when braking. Poor wheel alignment or damaged suspension parts can worsen this.
• Suspension Problems: Worn components like control arm bushings, ball joints, or tie rods may cause excess movement and amplify vibrations when braking.
• Brake Disc Alignment or Thickness Issues: Improperly installed brake discs or discs with thickness variations can create uneven braking surfaces causing vibrations.

The vibration typically occurs only when braking and often feels like a pulsation or wobble in the steering wheel. It’s important to address these issues promptly to avoid further damage and ensure safe braking performance. A mechanic can inspect the brake rotors, pads, calipers, tires, and suspension to diagnose and fix the problem. In summary, the shaking is most commonly due to warped brake rotors or related brake component wear, but tire balance and suspension issues can also contribute.