I should state at the outset that I understand tank vs. tank combat is NOT a useful metric for studying the performance of armored forces in WW2. However, I've been curious what effect, if any, a diversity of tank chassis had on the effectiveness of armored forces in tank vs. tank combat (again: I'm not arguing that this is more important than it is). Development of the KV chassis, for instance, allowed Soviet designers to house larger guns such as the ML-20S or M1938 152 mm field howitzers and use them in direct fire anti-tank roles. The Germans followed a similar pattern with the Jagdtiger's PaK 44, the Jagdpanther's Pak 43, etc. The Americans, on the other hand, were limited to the Sherman chassis. It wasn't practical or perhaps even possible to mount anything larger than a 105 mm howitzer (obviously larger guns are fitted to the Sherman chassis, but they were artillery pieces that were not used for direct fire against German armor).
The reason I ask is that I've always wondered if one of the ignored advantages of the M26 Pershing was that it could have given American designers a heavier chassis and the capability to mount larger guns for anti-tank roles, just as the Germans and Russians had done. In other words, the Pershing, in addition to providing US armored forces with a heavier tank, might also have given American designers flexibility. I'm pretty sure the answer to my question is "yes," but I'm curious if anyone else on /r/AskHistorians has researched the topic.
Again: this question is very limited in scope. I'm not trying to place undue importance on tank vs. tank combat or argue that the Sherman was an ineffective/the deployment of the Pershing was critical. Thanks for reading.
I'll have a go at this.
Your observations about chassis size are accurate. The more basic issue is that armoured vehicle development accellerated so rapidly from 1939 that it left doctrine and forward planning in its wake. With limited engineering skill and resources, nations had to balance addressing short-term needs against attempts to deal with longer-term planning. Nations that were further behind in 1939 were at an especial disadvantage as their attempts to deal with the short term- eg, bigger guns, more armour, more reliable engines- ran into the wider, longer-term issues.... such as the chassis available.
What makes armoured vehicle development such a complex area, with simple answers lacking, is this knock-on effect. Most nations were forced to settle on a certain basic vehicle or two, this being forced by the stage at which they had to tool up for really serious mass production.
The Soviets had an advantage in that they had a powerful, tank-specific (albeit aircraft-derived) engine by the late 1930s. Having this engine allowed them to design large tanks with plenty of development potential. The US was fortunate to have British investment and federal government oversight to build huge production lines around the M3/M4 chassis, which was just large enough and also reliable.
The Germans of course went through their crash program of sticking-a-75mm-gun-on-anything-that-will-move in 1941/2, with the StuG being on hand for good fortune. In this they were both lucky (all those stolen Czech chassis...) and saddled with too many different designs, which led to various efforts to rationalise (such as the combined MkIII/IV chassis), which were never finalised.
The British were always behind, with a grossly flawed system of design, procurement, production and testing.
But of course size isn't everything. To be successful as a general-purpose chassis, said chassis had to be cheap. The Germans could never really use their Panther or Tiger for SP guns in any numbers because of their complexity and cost, and in any case outside the Russian steppe having a gun effective at 2500 metres was, well, being good at something that didn't matter. The Soviets took a relatively long time to have spare capacity for SP guns, but their simple designs were ready when needed. Whether they really needed their large SP guns is perhaps debatable, the British and Americans were able to batter their way into German towns without them.
So, in sum, while many Allied troops would have loved to have a 105mm high velocity gun on a Pershing chassis available in June 1944, the absence of such a vehicle was not critical. The Allies had co-ordinated air strikes on hand, seriously good artillery support, and for the British there were AVRE combat engineer vehicles for taking on concrete fortifications. They adapted.
Certainly when looking at the tanks of 1945- the M26 and Centurion were the basis of their respective nations' tank forces for decades- one wonders why they weren't available years earlier. The answer to that is in the detail of engineers dealing with inter-related design problems, while planners tried to guess which tank to mass produce, knowing their decision today could be a disaster in 18 months when those tanks were finally hitting the battlefield.
In defense of the Sherman's chassis, it was capable of mounting heavier anti-tank guns than it was usually equipped with. Notably the Firefly variants with 17pdr guns fitted and the M36B1 tank destroyer (turret of M36 with 90mm gun on M4 chassis) both carried more effective armament than the standard 75mm or 76mm guns, or 105mm howitzer. Post-war Israeli Shermans carried 105mm guns in modified 76mm gun turrrets.
In terms of providing a heavy assault gun for use against fortifications, /u/llordlloyd noted that Churchill AVRE were available for short-range demolition tasks, while the M12 (155mm howitzer on M3/M4-derived chassis) was used for direct fire support against large fortifications in Metz and the Siegfried line.