Your 18 wheeler truck steering wheel shakes and knocks when you get at 40mph what is causing it?

There are a number of possibilities:

• Poorly balanced, out-of-round, or bad mount steer tires
• Mud, dirt, or other debris accumulation on the rims
• Out-of-round brake drums making infrequent contact with the brake shoes
• Worn kingpins
• Loose tie rod ends
• Worn wheel bearings
• Worn Pitman arm
• Worn drag link
• Misaligned steer tires