While discussing the laws of muktzeh – the rabbinic prohibition against moving objects whose use is forbidden on – the Gemara on today’s daf quotes a Mishna from Massekhet Kelim. This Mishna deals primarily with the laws of ritual impurity and discusses the relationship between a wagon and its undercarriage [mukheni], the system of wheels and the frame at the base of the wagon.
And the Sages said: The wagon’s undercarriage, when it is detachable from the wagon, it is not considered connected to it and they are considered independent units as far as the of ritual impurity are concerned. And it is not measured with it. This refers to calculating the volume of forty se’ah, as a vessel with a volume larger than forty se’ah does not have the legal status of a vessel and cannot become ritually impure. And the undercarriage likewise does not protect together with the wagon in a tent over the corpse.
A large wagon is considered a tent in and of itself and the vessels inside the wagon do not become impure if the wagon is over a corpse. However, the undercarriage is not included with the wagon in this regard. If a hole in the wagon is sealed by the undercarriage, it is not considered sealed with regard to preventing ritual impurity.
Regarding the discussion of muktzeh the Mishna concludes:
And, likewise, one may not pull the wagon on when there is money upon it.
Preventing objects from becoming ritually impure in a tent over a corpse can be accomplished in any number of ways. In this context, the discussion is with regard to an object covering a corpse like a tent. The object itself remains ritually pure and the objects above that object also remain ritually pure. Objects beneath it become impure. With regard to a ceramic vessel in a tent over a corpse there is a similar halakha. If it is sealed, it protects its contents from ritual impurity (Bamidbar 19:15). According to Tosafot, a large wooden vessel, which cannot become ritually impure because it has a capacity of more than forty se’ah and does not, therefore, have the legal status of a vessel, preserves the ritual purity of items inside it when it is closed on all sides. In the case at hand, if we understand the undercarriage to be the entire bottom part of the wagon and not just the wheels alone, as Rashi explains, then when there is an opening in the bottom of the wagon that is sealed by the undercarriage, the wagon is not considered sealed and does not protect its contents from ritual impurity in the case of a tent over a corpse.