Size 1-4. The dormakaba (formerly known as Dorma) BTS75V is a highly adaptable hold open floor spring suitable for all doors with widths up to 1100 mm and up to 120 kg leaf weight. Thanks to the floor spring's adjustable spring strength, closing force can be individually regulated.. Size 1 - 4. Adjustable closing force. Concealed installation. Virtually maintenance free (other than routine dusting and cleaning)