Grappling at really low ST in GURPS

A recent post in the forums, which started off with a link to this very blog – specifically my old post on how ST rolls are problematic in GURPS, brought up a very interesting edge case that I really wish had come up explicitly in the Technical Grappling playtest.

It has to do with scale.

The Issue

Take a ST 1 or 2, DX 10 snake that is trying to make a Constriction attack against a ST 1, HT 10 pixie. Many of these contests will pit ST for the attacker against the ST or HT of the defender, whichever is better.

Huh. ST 1 or 2 will always roll vs. HT 10, and so the extrinsic nature of ST bites you hard here, since you’ll never have a creature contest like this end poorly for the HT 10 pixie. This is especially interesting since ST 2 vs. ST 1 is the same ST ratio as ST 20 vs ST 10.

The Solution


GURPS has buried within it in a few places a recommendation to re-scale oddball ST values centered around 10 for places where things get weird. So ST 1 vs. ST 1 or ST 2 vs. ST 1 would turn into ST 10 vs. 10 or even ST 20 vs. ST 10, which re-centers the contests around the norm of 10, which turns the equal-ST contest into something appropriate.

OK, so boom, rescale. We’re good, right?

Well, perhaps good enough, but in keeping with the concept of “ST rolls must die,” here’s another way.

Power Ratio Table


What we’ll do is take a look at the ratio of the Basic Lift of the two combatants, and assign a modifier based on that ratio to one or both of the combatants. Thus, Contests of ST (or ST-based skill) vs. the best of ST or HT will mostly turn into a Contest of DX (or maybe HT)-based Skill vs. HT, but with a modifier to ST.

This is basically the same as “normalize the defender’s ST to 10, and take that as a penalty

Another way to go – though similar – is to just take the ratio of the stronger to the weaker. Unfortunately, the math is a little ugly.

1. Take the log10 of the ratio of stronger to weaker. Call that PR
2. Modifier = PR * (9.5+10.8*PR)

That produces something like this:

I’d probably just modify the skill of the attacker – if he’s stronger (he’s on the favorable end of the ratio) he gets a bonus. If he’s on the weaker end of the ratio, he gets a penalty.

Snake vs. Pixie Revisited


The ST 1, DX 10 snake vs. the ST 1, HT 10 pixie is now at a 1:1 ratio, no penalty or bonus, and this becomes DX 10 vs. HT 10 (the fact that the snake is using Constriction Attack would probably be treated as a ST multiplier of some sort in this case).

This would be the same math you’d do for a ST 50, DX 10 giant putting the squeeze on another ST 50, HT 10 giant.

Icky Math. Why did it have to be Icky Math?


I just banged this out because I wanted to preserve the +1 bonus for each +1 to ST in the regime of most PCs. I could probably find a nicer functional relationship between the ST Ratio, BL Ratio, or Log of one or both of those ratios if we don’t want a +1 to the slightly stronger guy for each +1 to ST vs. a ST 10 baseline. We might also invert that, where you might get slightly more of a penalty or bonus from ST10 (no bonus) to ST 20 (currently +10 relative to a ST 10 guy).

Hopefully that would give nicer math. Naturally I’d see if I can use the Size and Speed/Range table here, since my thoughts on that are well known.

But that should help with normalizing oddball ST scores to the more 10-centric HT and DX. It also gets rid of a direct ST roll, and that makes me happy.