Despite being in general more conventional, in one aspect the Medium Mark I looked rather modern: instead of a high track run it possessed a low and flat suspension system with five bogies, each having a pair of small double wheels. The axles of these were too weakly constructed; as Major-General N.W. Duncan put it in his
Medium Marks I-III: "(...) a perpetual nuisance. The axles were continually breaking and the path of the Mark I tanks was littered with discarded wheels". This was cured by switching to a "box bogie" in 1931. To ease repairs the suspension was not protected by an armoured covering. There were two vertical helical springs of unequal length in each of the five bogie casings attached to the hull. In front and behind the normal ten road wheel pairs, there was a tension wheel pair. Ground pressure was very high, even though at the vehicle was not very heavy for its size. The engine was an air-cooled 90 hp
Armstrong Siddeley engine derived from an aircraft type. Surprisingly the engine and transmission was distributed throughout the hull - with the engine to the left of the driver, the gearbox underneath the commander and final drive at the rear, which Duncan describes as "an unbelievable retrograde step in view of war-time experience". The
Medium Mark B and the
Mark VIII heavy had introduced compartmentalisation to reduce the debilitating effects of engine noise and fumes on the crew. However, with the Medium Mark I considerations of ease of maintenance took precedence. The engine drove, via a multiple dry-plate clutch, a four-speed gearbox. It had no
synchromesh and switching between gears without excessive noise was a challenge to the driver. A propeller shaft connected the gearbox to a bevel box at the end of the tank which divided the power to a separate epicyclic gear for each track. These gears automatically provided extra emergency torsion to the normal first and second gear if the vehicle suddenly slowed down due to an obstacle or soft ground. The
petrol tanks were at the very rear of the hull, so the fuel lines had to run along the whole length of the vehicle, pumping fuel to a secondary tank that fed the engine by gravity. The engine was lubricated and partially cooled by oil; leakage was common and the original reservoir had to be replaced by a one. The tank could be electrically started, but only if the motor was already warm, so the first start had to be done by hand from the inside of the vehicle. Maximum speed was about and the range about . There was a cylindrical bevelled turret on top of the hull that carried a "Quick Firing" (shell and cartridge in one complete round) three-pounder gun (47 mm
calibre) and four ball mountings for Hotchkiss machine guns. A novel, unique feature was a three-man turret. This meant that commander was not distracted with performing either the loader's or gunner's tasks and could fully concentrate on maintaining situational awareness. This gave a huge potential combat advantage, but went largely unnoticed at the time. Except for the Lago prototype, a predecessor to the
Stridsvagn m/42, produced by
Landsverk in 1934 no other manufacturer constructed a tank with a three-man turret until the German
Panzer III. The practical importance of this feature is signified by the fact that later into the
Second World War, most of both sides tank designs either quickly switched to the three-man turret, or were abandoned as obsolete. There was no co-axial machine gun. There was only room to operate one machine gun from the turret; normally one gun was switched between the respective mountings as the guns were removable. The turret machine gunner doubled as main gun loader. In each side of the hull was a Vickers machine gun. There was one gunner to operate these, he also functioned as a mechanic. The shape of the Mark I Medium hull was very distinctive. The back was a simple armoured box; the front plate was high and perfectly vertical. Between them, from the armoured hood of the driver at the right of the vehicle six armour plates fanned out to the left, making for a complex hull geometry at that side. In all the tank made an ungainly squat impression. The crew of five was only poorly protected by 6.25 mm plating, rivetted to the chassis, barely enough to counter the threat posed by light machine guns. With its many shot traps the vehicle was unable to withstand even
anti-tank rifle fire and it had a high-profile. The internal lay-out worsened this vulnerability as the petrol tanks were inside the main compartment. ==Operational history==