P3D-R
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Eibach indicates to do that. I know shops who have avoided it and it rode horribly. By trimming the bumpstop it is going to increase bump travel prior to engaging the bump stop. I can only tell you that it works and makes a massive diffference. If you don't trim it the shock will likely be resting on the bumpstop at static ride height. I've attached their PDF. If it works for Eibach it will work for the rest as well. It's critical in the rear from my experience.