Crossbar spread is determined by where the towers of your rack are placed on your vehicle. This spacing will determine which accessories and what load your vehicle can carry.
Our Q-Tower and BaseLine fits are specified for a fixed position and should not be moved from the indicated measurements (the 4 M's; the crossbar spread is the M4 distance.) You can find your specific measurements on the Q or Base Clip sheet that came with your Clips or by using our fit lookup tool here.
If you have towers other than our Q or BaseLine Towers, please refer to the attached "Crossbar Spread Table" for the proper crossbar spread for your accessories and load.