On rare occasions, you may see a vehicle named M4A2E4 roaming the World of Tanks battlefield. It’s an M4, but with obviously different suspension.
Like most, the US took some effort in testing different types of suspension and evaluating them. No country seemed to be entirely convinced of one method over another, as each had their advantages and disadvantages. The M4E4 was not the first produced vehicle that the US made with torsion bar suspension, but it did provide a very simple basis of comparison against other suspension types. Development of an independently sprung torsion bar suspension for the M4 series was authorized by the Ordnance Technical Committee in April and May of 1943
General Motors came up with the design, and two vehicles constructed, the first completed 22 July 1943, the second 15th August 1943. The vehicles were fitted with 26” wheels, with a 7” range of movement.
GM ran them around a little bit before handing them off to the Army, and over a combined distance of 3,500 miles between the two tanks, the vehicles suffered a number of bushing failures and several broken track links. As a result of this, additional types of track were sourced from both Ford and GMC. However, since T35E1 and T26E1 were using the same tracks, there was no durability testing of them.
Eventually, the tanks were split up. One went to the General Motors Proving Ground, and then after 200 miles, sent on to Fort Knox for some Armored Board testing. The other was delivered to the Tank Arsenal Proving Ground, where it was determined that greater spring capacity and improved bump springs were necessary. After being sent to the Fisher Tank Division in Grand Blanc, Michigan, for improvement, it was then sent on to the Aberdeen Proving Grounds for further testing, as TAPG had higher priorities.
A true weight comparison between the M4 bogie type and the M4E4 type was difficult to make, as the M4 type was completely detachable and fits onto a square hull. The M4E4 type was partly built into the hull and required a more complicated cross-section, making it difficult to determine what parts are intrinsically hull and what parts are suspension. The conclusion was that in spite of the heavier wheels, the M4E4 suspension was about the same weight as the M4 bogie type.
Once the vehicle was received at Aberdeen on 27 July 1944, the first thing they did was inspect the vehicle. It was discovered that the .50 cal barrel bracket was bent, the coaxial machinegun was missing the firing solenoid, and the bow MG mount had no spring. Worse, a resistor needed replacing in the stabilizer unit. Such things are important at Aberdeen when testing suspension.
The first thing they did was conduct tractive resistance tests, and compared to an M4A4 and an M4E8.
M4E4: 75lb/ton at 25mph. 45lb/ton at 3mph
M4E8: 104lb/ton at 25mph. 65lb/ton at 2mph
M4A4: 100lb/ton at 12mph. 62lb/ton at 4mph.
For further testing, they also compared with the T20E3 medium tank, also with torsion bars.
In general, the perfomance of the Medlum Tank M4E4 in the vibration study, was inferior to the other vehlcles tested on hard surface courses. T20E3 was the best of the lot. This distinction was explained by the wider track on M4E4, and the strength of the torsion springs, given that the M4E4 was about three tons heavier.
Torsion bar vehicles demonstrated better performance over the Perrymen cross-country course, mainly due to their lesser tendency to bottom severely and transmit jolts through the vehicle. However, over operations on the Belgian Block course at both 5 and 20mph, the old VVSS M4A1 showed more angular stability than any other vehicle. Both the HVSS and torsion bar vehicles showed very distinct wave-like pitching.
When they moved to the coarse washboard, the bogie and independent vehicles split into two at 5mph: The M4A1 and M4E8 basically stayed level, but bounced up and down, while the T20E3 and M4E4 saw the rear of the vehicle bobbing with a point of rotation around the front of the vehicle. “In this respect they are inferior to the other two vehicles when gun platform stability is considered”. By 20mph the angular movement had disappeared, and the hulls had all assumed a stable riding position. The proof officer opined in his report that the effect of a stabilizer* “would minimize, if not obliterate the difference in the angular motions” that the vehicles exhibited. He felt that the most important factor to be considered was the significantly less tractive resistance obtained through the torsion bar suspension.
The bottom lines came out as follows.
It is concluded that
a. The independently spring suspension, with torsion bar springs, is a practicable means for producing a satisfactory ride.
b. The medium tank M4E4 has ride characteristics which are superior to those of the Medium Tank M4, with either vertical volute, or horizontal volute, spring suspension.
c. High capacity shock absorbers and high capacity auxiliary bump springs are necessary for such a freely spring suspension
d. Release for production of the Medium Tank M4E4 would involve considerable maintenance problems without improving on the value of the Medium Tank M4 as a weapon.
It is recommended that:
a. Independently sprung suspensions with torsion bar springs and auxiliary bump springs be considered satisfactory in principle for application to tracklaying military vehicles.
b. Neither standardization nor production of Medium Tank M4E4 should be considered.
There are some interesting observations which can be made from this.
Firstly, it shows how a tank is an integrated weapons system. It’s not just a matter of ‘get the best possible suspension’ and ‘get the best possible gun’ and ‘get the best possible engine’ all in isolation. The torsion bar suspension showed it was more comfortable for the crew, and rolled more easily, but as a stable gun platform, which is pretty fundamentally important, it was actually inferior. However, the addition of more components (stabilizer) could mitigate the poor points to an acceptable level while maintaining the other benefits. Thus the US chose to move from bogies to independent. Then again, one must notice the British having moved from independent suspension to bogies at almost exactly the same time. Presumably they could do the same maths as the Americans could, but they came to a different conclusion, though differing priorities. Perhaps it was angular stability which was more important to them, or maintenance.
Secondly, you’ll note that even though the E4 was considered the best tested suspension type of the three, it was recommended not to be put into production to replace the other two. One of the realities of WWII (and any military endeavor, really) is that “’Perfect’ is the enemy of ‘good enough.’” One may think that as soon as they came up with the conclusion that “this is best”, they would immediately switch production over, but would the effort behind both changing production facilities and supporting a third suspension type in the field really be worth the 25% reduction in resistance and not bumping out as much when travelling at over 15mph cross-country?
So the end of it all was that M4E4 was an interesting exercise in scientific evaluation, but nothing really more
*The irony is that the next tank selected for production, M26, was not fitted with a stabilizer.