Rashi, Targum Yonasan: It is like the wheels of Hashem's chariot (Yechezkel 1:16) - a wheel within a wheel. (Gra - there is an inner wheel around the axle; spokes connect it to the outer wheel.

Gra: It is like [a wheel of] our chariots.

Rashi: They are the holes in the middle of the wheels. I.e. yhey are the inner wheels (Gra, Malbim). Tzuras ha'Bayis (in Rashi 37) - they are called Gabeihem because it is the highest (thickest) part of the wheel when it lies on the ground. The center of any circle is called Govah, for all parts are drawn from it.

Rashi: They are rims around the wheels, which tie them. Tzuras ha'Bayis (in Rashi, verse 37) - they roll on the ground. Normally, one makes a rim of iron around them. Here this was not needed, for the entire wheel was of copper.

Radak: This is an expression of pressed and snug.

Radak citing Targum Yonasan: What covers them.

Gra: They are the spokes that go from the inner wheels to the outer wheels. ¹

Rashi: They are spokes of the wheels, which are inserted from the hole to the rim.

Radak: This refers to being tied.

Gra: They are the outer wheels ¹ .

Malbim: They were all molten together - the axles on the ground with the wall of the Mechonah, and the three parts of the wheels were molten together.