Both the Petzl Basic and Petzl Croll S are compact rope-clamping devices, but their geometry and intended connection points make them behave differently when used in a ‘home-made’ knee ascender setup. The most important distinction lies in the cam's orientation and how each device aligns on the rope during ascent.
The Basic, according to Petzl, is a rope clamp, whereas the smaller Croll S is a ventral rope clamp. That subtle wording points to a major design difference. The Croll S includes a stainless-steel wear protector on the rope channel—absent on the lighter Basic—and is optimised for a chest or ventral connection.
When attached to a taut rope and the harness, the Croll S cam faces away from the climber’s body, a layout that suits right-hand operation and keeps the device running smoothly in line with the rope. The Basic, by contrast, functions more like a hand ascender; its cam faces forwards, toward the climber, when clipped in the same rope orientation.
If you’re using either device as the upper unit in a knee-ascender system, your connection will typically face towards the climber on the rope. At that point, right-handed climbers will generally find the Basic easier to operate, while left-handed climbers may prefer the Croll S.
In practice, both units perform well, but understanding the differences in cam orientation and wear protection will help you choose the right configuration for your climbing style and knee ascender setup.
