Straight-legs bench dip should have triceps as primary
Louis Deveseleer
Daniel Velkov It is often known as a triceps exercise, but I think it actually works the chest and front delts more.
Your triceps would work more if your elbows could move back a little more, but having the bench forces your body forward and the elbows stay mostly vertically above the hands.
You can check this by having someone poke into your triceps and your chest and front delts while you perform the exercise. For me, chest is fully engaged, triceps only partially. Please try that out and let me know what you think!