Are all cogs of the same tooth count the same diameter?

[Shrike]

Think I'm missing something obvious here. Basically took a wheel off my bike which had an 11-28 on it off, and stuck on another wheel, with a 11-32 cassette on it thinking that it should be able to stretch into the 28 as normal without any B screw adjustment. Not so!

This sort of thing really messes with my head. Why the Jesus H Christ would that happen

To measure them properly I'd need to remove both cassettes so thought I'd ask before getting into it.

Just looking at them, the 28 on the DA 9100 looks smaller than the 28 on the Ultegra 6800 11-32, but that could easily be a visual trick.

[BdaGhisallo]

Yes, all 28T cogs must be the same diameter due to the common chain pitch. If they weren't the chain and cog wouldn't mesh properly when switching between different sized 28T cogs.

Assuming the cassettes are both 11sp or 10sp, the 28 in the 11-32 is now closer to the dropout since it is not the final cog but the penultimate cog now. So that diameter cog is now further up in the RD's parallelogram movement if you follow. The RD moves both laterally toward the hub and vertically down when you down shift. The B bolt adjusts the distance between that down and in arc of the RD and the axle center line. So it's not unreasonable to have to adjust the B tension bolt when you put on a bigger cassette.

To really picture this, the RD may clear a 25 cog in the final position, but if you move it to the first cog position, right next to the dropout, no RD would likely clear it due to them not being designed to handle such a large diameter cog next to the dropout.

[Shrike]

BdaGhisallo, thanks yes, that's clear! Not an easy thing to describe but I got it first go. Cheers!

Fine, I'll just have to adjust the B screw when switching these wheels around

[Rick]

I would suspect that if you adjust the B screw such that it will work on the 32, it will still work on the 28; but not vice versa.
The B screw opens up the chain wrap for larger cogs, but it will still work in a given setting on smaller cogs.

[2lo8]

Just to be needlessly and obnoxious pedantic I believe some Shimano Tourney and SRAM Horizon (and older non-slant derailers) could clear a 25t in the first position, given your frame has enough clearance. It's only slant parallelograms that care about the cone shape of cassette. Shimano's servo pantograph (dual sprung, now obsolete with 9100) used to even have a graphic where they showed the guide jockey following a cassette with cogs in a random order.

Also Shimano did make some 10mm pitch chains and parts for a while.

As far as your actual concern, after making sure the chain in long enough, set the B screw on the close side for the 32, and it should be able still shift the 28 cassette acceptably well.