Lewis & Short

substrāmen, ĭnis, n. [substerno],

  1. I. what is strewn under, litter: molle, Varr. R. R. 3, 10, 4; 3, 6, 5; 3, 9, 8.
  2. II. Transf., supports: lubrica roboreis aderant substramina plaustris, i. e. rollers, Sil. 12, 444.