Can a Giant Wolf Spider climb upside down while being mounted?

Question

Imagine a beastmaster ranger halfling who mounts his giant wolf spider beast. The giant wolf spider has the spider climb feature, which allows them to climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Can a giant wolf spider climb upside down while being mounted? This is a tricky question to me since I imagine the halfling and his gear would weigh 100-250lbs. Do I simply use the carrying capacity of the spider (15*12STR=180lbs?) and the type of surface is irrelevant? It seems strange to me that a giant wolf spider could be upside down on a smooth surface with 200lbs on its back.

I am the DM. I’m curious about whether they can climb along a vertical wall while mounted, which should be easier than upside down.

Answer 1

A Giant Wolf Spider can climb upside down while being mounted.

The Giant Wolf Spider (MM 330):

Spider Climb. The spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Doesn't have to make ability checks. If you are curious about whether the rider makes ability checks, you may find guidance here. There is no difference in vertical or upside-down climbing.

Carrying Capacity (PHB 176):

Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry, which is high enough that most characters don’t usually have to worry about it.

The Giant Wolf Spider (MM 330) has a strength of 12 and is a medium-sized creature, so it can effortlessly carry 180 pounds.

If you are using the Variant Encumbrance rules (PHB 176):

If you carry weight in excess of 5 times your Strength score, you are encumbered, which means your speed drops by 10 feet.

If you carry weight in excess of 10 times your Strength score, up to your maximum carrying capacity, you are instead heavily encumbered, which means your speed drops by 20 feet and you have disadvantage on ability checks, attack rolls, and saving throws that use Strength, Dexterity, or Constitution.

First, the spider will become slower, then severely impacted by disadvantages, but it doesn't become immobile, and it will not fall downwards merely because it is encumbered.