This isn't entirely true. The US was fully aware of torsion bar suspension systems like those used in the Tiger, Panther and, Pz III. The US also knew about the Christie system using large diameter vertically coil sprung suspensions. The US Army's ordinance department however settled on the use of the volute spring suspension system as their standard demanding its use on designs up through 1942.
Thus, the Stuart, Grant and, Sherman all used the volute suspension system. For vehicles capable of speeds up to 20 mph cross country this system was adequite.
On the other hand, claiming the Germans that "the Americans [were] in the in the war with a tank far inferior to the high standards set by other tanks..." is an absurd claim.
The Sherman was a well thought out for 1940. It had a large enough turret ring to allow easy up gunning and new turrets that could take larger guns. The track system was far superior to anything the Germans used on their tanks in terms of life (about triple that of the German or Russian designs) and at road speeds reduced gasoline consumption giving tanks better mileage when travelling out of combat areas.
The US also continually improved their metallurgy making their armor perform better for a given thickness than that of Germany or Russia too. US tanks also had excellent radios, the best turret power traverse systems, the first operational gun stabilizer systems (and, yes, they did work when crews bothered to learn to use them). The only real drawback to the early US designs was too narrow a track (something the Germans were also guilty of on their early tanks) and too high a silouette due to the original choice of radial aircraft engines for power plants.
With the second generation vehicles like the M-24, and M-26 these problems were rectified and these vehicles were as good as anything the Germans produced. The M-25A1, of which 300 were produced prior to June 1944 but never saw combat, was essentially a US 'Panther'tank. It had equal or better armor, a 90mm gun, equal speed and cross-country performance and, was better equipped in details than the Panther. The ordinance department decided to by-pass its production in favor of the M-26 based on promises of the manufacturers to get the later in production within a few months (which, unfortunately, did not happen).
The same went with a plan to equip M4A3 Shermans with the M-26 turret as an interm vehicle just before D-Day (both tanks have the same diameter turret ring....thanks to good design and engineering).
As for the T-34 it had good armor a decent gun and good automotive performance on paper. The reality was it was severly handicapped by its 2 man turret and lack of vision devices which left it virtually blind except to events happening right in front of it. It's tracks and drive train were riddled with bugs and took several years to make reliable in service. The ammunition layout left much to be desired. All but 3 rounds were stored under the floor of the turret (which also lacked a basket) making loading slow and difficult. Lack of a radio and intercom system also made the crew less efficent.
Basically, there is alot more to a tank than just its gun and armor. It is a complex weapon system that has many more parts that make it efficent than just its gun and armor. On that basis, the US didn't do any worse than any of the other major combatants and, in many ways did alot better.