Cycloheptane
Cycloheptane has only a small amount of torsional strain and its overall strain is comparable to cyclopentane. It is a fairly common ring system.
Cyclooctane
Cyclooctane has a little more strain than cycloheptane. It has some angle strain and some torsional strain. In addition, it has a new type of strain called transannular or cross-ring strain. This is a type of steric strain caused by hydrogens on one side of the ring which point across the ring and bump into hydrogens on the other side of the ring. In this model, the hydrogens causing the transannular strain are colored red and green.
Cyclononane
Cyclononane has a little more strain than cyclooctane, althought the types of strain are the same. In this model, the hydrogens causing the transannular strain are colored red and green.
