I didn’t want 10mm vertical spacers to minimize the risk of damaging the integrity of the aluminum. 15mm will allow for perfect adjustment for me to move from level-to-bottom of chassis rails, to middle of chassis rails, to top of chassis rails, as the chassis is 30mm tubing
That risk could be offset by using a thicker cross section, or perhaps a different material like stainless.
higher cost though as well. doubt i will be doing much height adjustment at all. but it will be nice to experiment with