Attaching the 20S to a Schuberth C3/C3 Pro or BMW System 6 helmet using the standard clamp kit may be difficult because of the thickness of the outer shell. The thickness of the outer shell causes the headset to not sit firmly against the side of the helmet (as pictured below). An alternative option is to use the Glued Surface Mounting Adapter so that the headset will sit flush against the helmet.
Sena recommends the clamp unit over the glued surface mounting adapter. Even though the glued surface mounting adapter is provided for convenience, Sena does not assume any responsibility for use of it. Users may have to use the glued adapter at their own risk and responsibility.
If the Glued Surface Mounting Adapter is not your preferred way for attaching the 20S to the helmet, then then there is another possible option.
The 20S comes packaged with 2 rubber pads for altering how the headset sits against the outside of the helmet. You can swap out the thick rubber pad on the clamp kit with one of the two thinner pads or you can make the thick rubber pad even thicker by attaching the other pads to it.
However, for using the clamp kit on a Schuberth C3/C3 Pro or BMW System 6 helmet then you would need to attach the thinner rubber pads to the rubber pad that is on the back plate of the clamp kit (as pictured below).
Carefully insert the back plate and the rubber pads inside of the helmet behind the outer shell. Inserting the back plate where the biggest gap behind the outer shell is will make the installation easier (as pictured below). Removing the Allen wrench screws from the clamp kit will also make it easier to insert the back plate behind the outer shell.
Slide the back plate as close as possible to the sun visor switch. Reattach the Allen wrench screws and tighten the back plate to the other portion of the clamp kit.
Now the 20S will sit more firmly against the side of the helmet (as pictured below).