Sejuani and Rammus have AD as their adaptive stat...

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Quite simply it's their adaptive stat because early the AD helps with their clears more than the AP would. After that they build full tank and the miniscule amount of adaptive stat they get from runes doesn't matter really. A faster and healthier early clear is more valuable than a negligable amount of damage during ganks where they mostly rely on the laner for damage anyway

You are correct, except for two things.

First is that Rammus and Sej often take precision secondary. While AD would help they don't gain any AD from that tree, it's health and attack speed split.

If they go domination tree or sorcery tree second you don't get much AD to start anyways, and the AP would be far more valuable particularly if I am opting to take something like storm or eyeball. Both Rammus and Sejuani have very high AP scalings that benefit a lot from having those stats. Plus they just added a higher AP scaling to Sejuani's W.

So, basically I agree with you, but just pointing out that it's not so cut and dry.

^ This. AD is just better for them with tank builds anyway. Building any bit of AP swaps it.

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Riot think that because Rammus gets a little attack speed from one of his abilities, that that makes him an AD champ.

You have to buy at least one AP item on him to trigger it to switch to AP adaptive.

Gooooooooooo RIOT!!!!

Don't forget his passive buffs his autos.

{quoted}

Riot think that because Rammus gets a little attack speed from one of his abilities, that that makes him an AD champ.

You have to buy at least one AP item on him to trigger it to switch to AP adaptive.

Gooooooooooo RIOT!!!!

Rammus' AA damage is actually pretty terrifying. His passive keeps him in the high AD range all game long, and late game he's able to hit high enough levels to seriously menace squishy / punch holes in tanks who have reduction effects applied.