the United Kingdom equipped itself with a number of vehicles with improvised armour, such as the Bison concrete armoured lorry, intended to be used for defending airfields. Later in 1944, some Cromwell and Churchill tanks had sections of tracks attached to their existing armour to provide yet more extra protection. US M8 Greyhound armoured car crews would sometimes line the floors of their vehicles with sandbags to provide extra protection against landmines. The addition of improvised armour to tanks was performed by both Axis and Allies forces due to the arms race between the designers of antitank weapons and the designers of tank armour. In some cases, a tank that was effectively protected against existing antitank weapons at the time of its manufacture ended up, once finally tested and delivered to the battlefield, being vulnerable to newly designed antitank weapons. As such, tank crews would ask field repair workshops to increase their protection, using a wide range of armouring principles, including welded or bolted on metal "skirts" around treads and turrets (spaced armour) and welded screens (slat armour). Some German improvised armour was designed to protect weak points, such as sandbags added by Afrika Korps tank crews to the turret joint. On the Eastern Front, some tank crews added sandbags due to fears of magnetic mines. The German military became aware of these improvised armour approaches used by their troops and issued a recommendation against using most of them in 1944 in the Nachrichtenblatt der Panzertruppen (Newsletter for the Armoured Forces). While the German military was aware that improvised armour boosted tank crews' morale (by giving a sense of increased security) the analysts argued that many improvised armouring techniques were not effective. For example, welding spare tank treads to a turret was not effective, as treads were not armour-grade steel, and concrete was found to offer little protection while also leading to excess fragmentation. Some improvised armour, such as adding concrete or welding on tank treads on an 80 to 90-degree angle, actually made enemy weapons more effective, and both approaches overtaxed the tanks' powertrains from the extra weight. Welding on improvised Schürzen (skirting) was not permitted, due to concerns that welding the original factory plate armour could weaken it; however, using brackets to mount turret-side and back skirts or side skirts was permitted. Side skirts were permitted because the Soviet 14,5 mm antitank rifles could penetrate the less-armoured sides