If I cut the knee bolster just a little, must I use the FMVSS No. 208 exemption? At what point is the knee bolster compromised?
When it is necessary to use the exemptions is up to the judgment of the modifier. Each vehicle model and each modification is unique, so NHTSA cannot make any blanket statements about what modifications will cause a vehicle to no longer comply with a safety standard. Therefore, if you do not use an exemption, then the vehicle must comply with the standard after modification, so when in doubt, use the exemption. Using the exemption includes labeling the vehicle, giving documentation to the client and saving a copy of that documentation, according to the regulation. In the case of mechanical hand controls, NHTSA has a research report available at http://www-nrd.nhtsa.dot.gov/pdf/nrd-11/handcontrol.pdf that can give you guidance on whether cutting the knee bolster would affect the injury criteria used in FMVSS No. 208.
