Chain / Chain wheel: D type vs sealed type

Question

I have spent a good 30 minutes on this site but couldn't find anything pointing a clear idea.

I bought a 26 inch, Dual suspension bike. It has a chainwheel of "sealed type." I asked the salesman, and he said that the other model has a "D type."

I know that a sealed type means a rubberized or a plastic cover, but the two bikes share the same cover. So it is not about the outer shell made of plastic, I suspect. Is it something about the sprokets or the teeth of the chainwheel?

(Link - https://www.singersl.com/product/tomahawk-26-mirage-multispeed-mountain-bicycle-by-tmk-26mirage)

Can anyone explain this difference?

Answer from google bard AI (possible speculation:

A sealed chainwheel has a rubberized or plastic cover, and rubber seals between the inner links and side plates. The rubber seals, also known as O-rings, seal in grease and prevent dirt from corroding the links. Sealed chains are longer lasting than standard chains. D type chainwheels are sprocket-type chainwheels used with single loop cut chains. Sealed chains: Are tighter due to the O-rings sealing the plates. Unsealed chains: Stretch more as the pins and rollers wear inside.

Answer 1

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The "D-type" wording describing the chainrings and crankset is not a common usage term for most bicycle enthusiasts. In fact, what it probably means here in the Tomahawk ad can actually be misleading. The most modern cranksets and their chainwheels for mountain bikes sold today are described as "Direct mount." This means that the chainwheels mount onto the drive side crankarm by means of a splined interface and secured with a large nut threaded on. Here is an example of a direct mount crankset and a compatible chainwheel:

It doesn't appear (& it would be fairly unusual) that the entry level Tomahawk has this type of crankset. I believe the D-type wording describing the Tomahawk crank refers to "Direct mount." In this case, it may be referring to the fact the chainwheels and plastic chaincase ("rubberized plastic cover") are permanently riveted or perhaps bolted to the spider of the crankarm.

Here, the seller describes a chainwheel as D-type because it attached to the crank spider via a 5 bolt pattern:

Yet another possibility that just increases the ambiguity of the term is that they've borrowed a description from industry to describe chainwheels. It again means Direct mount by virtue of mounting directly onto a drive shaft.

Finally, there is another possibility for what they mean by D-type. That would be "disc" referring to the round shape of the chainwheel. This, as opposed to "oval," which is a shape of chainwheels that have a history in the bicycle industry and are again a resurgent trend becoming more popular. My guess is that this has a low probability of what they mean compared to some meaning of "direct mount."

Obviously this probably isn't the precise answer you are looking for but as I've noted the "D-type" terminology is uncommon in normal usage and can be very ambiguous when it is.