Suspension

One of the most distinguishing features of the Churchill tank was the suspension system. 44 small diameter wheels arranged on 22 separately sprung bogies, 11 to each track. The overall appearance was quite unlike any other British W.W.II tank and whilst at first glance all bogies look appear identical there were a surprisingly number of subtle differences between some of them.

Description

Each suspension unit consisted of a bracket, frame, axle, wheels and springs. On the earlier Mks all units were independent being separately bolted to the pannier floor above. Each unit was bolted to the next by a tie plate attached to the outer faces and secured with lock nuts. The rear bolt on each plate was allowed a small end clearance but the front bolt clamped the tie plate firmly. With the introduction of the MkVII the suspension was strenghtened by replacing the tie plate between units 2 and 3 was with a welded plate producing a combined suspension unit.

This change was also retrospectively applied to some earlier Mks, especially those in service as AVREs, Bridglayers and ARVs which were required to carry heavier loads. Whilst many such 'upgrade' modifications occurred post war, it is interesting to note the AVRE at Greye Sur Mer which was recovered from the beach in its original condition also has the combined 2 & 3 bogie unit.

The bogie support bracket was constructed from welded plates and incorporated large boss on each side to which the fulcrum shaft was clamped. A bogie frame, mounted on bronze bushes, pivots about the fulcrum shaft which was hollow and acted as an oil reservoir, oil nipples at each end. One end of the bogie frame carried the axle and the other end formed the lug for the rebound stop. On the upper side of the axle shaft boss was a hard steel knife edge against which the lower spring seat pressed. Each bogie unit contained four springs with the exception of number 11 which is unsprung. The outer two springs carried the load, the next inner spring, the "bumper" spring, controlled the last 3/4 inches of bump through and the inner most spring was there simply to retain the "bumper" spring in position. The knife edge itself was used to minimise any buckling of the springs by giving them freedom to assume their own axial alignment. Due to the working radius of the wheel, some slight buckling of the springs was unavoidable and so the springs were arranged to be straight in line when they were at the point of maximum stress i.e. maximum bump.

The wheels themselves were 10" diameter and (perhaps surprisingly) were simply push fitted on to the axle which rotated in white metal bearings.

As with the fulcrum shaft, the axle was hollow and acted as an oil reservoir, being replenished via recessed nipples at the end of the axle. There was another nipple at the centre of the bogie frame which provided pressure relief.

Deflection & Rebound

Bogies numbered 1, 2, 3, 9 and 10 also had varying degrees of curtailed rebound. Bogie Number 1 only really came into use when either climbing or 'nosing' . Bogie numbers 4 to 8 were allowed a full spring deflection of 3 inches with a 2 inch rebound. Bogie number 11 was essentially a track tensioner rather than a strict suspension unitand had a rebound of 2 inches and came into play every time a gear change was made.

Fulcrum Bosses

The bosses into which the fulcrum shaft fitted were subject to some variation amongst the Churchill series. On the early Churchills (MkI and II) the bosses on all bogies were of the same pattern with the fulcrum clamping nut below.

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Later a strengthening rib was added to the boss which also became longer....

... and Bogies 1, 2, 9, 10 & 11 were modified to have shorter bosses with the clamping nut moved to above the fulcrum shaft.

Bogie Frame Rebound Lugs

Most bogie frame lugs were of a 'two pronged' as shown below..

However, bogies 1 & 2 were different pattern bogie incorporating an extra central strengthening rib.

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Bogie 11 was of yet another pattern being similar to 1 & 2 but without the central rib.

Bogie Frame Strengthening Strut

Bogies 1, 2, 9, 10 & 11 incorporate a strengthening strut which runs across the bogie frame. On some vehicles bogie 8 also has the additional strut (note this bogie has different pattern fulcrum bosses than the others with similar struts). I've also come across a strut on bogie 4 on one vehicle.

It would appear that the orignal Churchill suspension units may not have incorporated this strengthening strut as a report submitted to the Tank Board by the Director of Armoured Fighting Vehicles (A.C. Richardson) lists "Reinforced brackets Nos. 1, 2 and 11 bogies" as being and essential modification to ensure battleworthyness of the Churchill in all theatres of war by spring 1942. The report recommennded that such reinforced brackets should be included from tank No. 650.

Daily Maintenance

As one would expect, this suspension system required frequent maintenance. The daily schedule (or every 50 miles if the daily mileage was less than 50 miles) was to lubricate the bogie axles and fulcrum shafts with gear oil (C.600) using a pressure gun and to inspect and tighten the bolts on the on bogie bracket tie plates.

When things get mixed!

Despite the variations in details the bogie units were by and large interchangeable if the situation required. The photo below shows a ARV MkII which has some of the bogies positions mixed. Tthe combined 2 & 3 bogie unit fitted at stations 8 and 7. possibly this was to strenghten the suspension under the winch gear but more likely it is an accident of 'restoration" or maintenance.

So what would you expect to see at the number 2 & 3 stations ?

From the fulcrum boss we can see that bogie number 2 is not correct. Bogies 2 and 3 have been welded together via the tie plate which is actually the one that should fit between 10 and 11. Whether the welding was to create a combined bogie or whether it was necesary just because this plate does not have a bolt hole in the necessary place - who knows!