Effects of Body Mass

[Varyon]

A new houserule I've been considering is formalizing the "special effects" of a character's body mass (weight) into actual effects, and I'm looking for a listing of relevant factors. This assumes divorcing mass and HP* - someone with higher HP than their mass indicates is someone who is particularly resilient, for example. Here's what I have so far.

Grappling: In Technical Grappling, mass is important for determining grappling encumbrance - light foes are easy to move around, heavy are more difficult. This also applies to more traditional encumbrance when someone needs to carry a character around, haul them up using a rope, and so forth. This seems a net Advantage for being heavier, although this is lessened if using the "base weight = 8xBL" alternate.

Knockback: Currently, Knockback is based on HP. It seems more appropriate to base it on mass, and someone in heavy armor is going to be a bit harder to move back as well. Advantage for being heavier.

Collisions: As with Knockback, damage from slamming or falling is currently based on HP, but mass makes more sense. This is a close call, but it seems a net Advantage for being heavier.

Blowthrough: Someone who is thin and wiry simply isn't going to absorb as much of the energy of a bullet passing through them as some hulking brute. While this makes the latter a more effective meatshield - he can absorb more damage - the fact he takes more damage in the process is problematic. Net Advantage for being lighter.

Carrying Capacity: Technical Grappling implies that an appropriate weight for a character is right around 8xBL, with extra at the very least counting against them while using mass-based moves while grappling. It may be appropriate to have this count toward encumbrance, possibly with lighter characters effectively starting with negative encumbrance. Advantage for being lighter.

Athletics: In theory, someone who is lighter but just as strong would be able to climb, jump, and so forth more easily, and may be able to run more rapidly. Using the Carrying Capacity notes, it may be appropriate to have a character who's current total weight is markedly less than 8xBL getting a bonus to such skills. The following table might work. Note weight is the character's total weight, including any carried gear, as a multiple of BL. As usual, if between two values, go with the highest.

Weight	Mod	Move	BL 20 (example)
3	+4	x1.8	60 lb
6	+3	x1.6	120 lb
7	+2	x1.4	140 lb
8	+1	x1.2	160 lb
9	0	x1	180 lb
10	-1	x0.8	200 lb
11	-2	x0.6	220 lb
14	-3	x0.4	280 lb
18	-4	x0.2	360 lb

The bonus would apply to most of the skills that Encumbrance Modifiers impact. It probably shouldn't apply to things like Karate and the like, although letting it apply to Dodge and the Parries of those skills may be appropriate. Yes, this would give a character of appropriate weight a bonus for running around stark naked. Obviously, this is a solid Advantage for being lighter.

Armor: Pyramid #3/52 has an optional rule for scaling armor surface area (and thus weight and cost) with character mass. When using this, a character's mass shouldn't modify armor by more than would be appropriate for a change in SM - that is, an SM 0 character's armor cannot weigh less than 1/2x normal or more than 2x normal, regardless of character mass. To make things more fair - and possibly more realistic - any change in surface area also modifies the maximum thickness (MaxDR) of armor the character can wear. A 50 lb SM 0 character would get away with half the weight per DR, but would also only have half MaxDR - leather armor for such a character wouldn't be able to exceed DR 2, for example. Net Advantage for being lighter.

*Optionally, HP probably shouldn't be able to vary too much from mass. A character with mass appropriate for an HP 10 character might only be allowed to have HP between 7 and 13, for example (+30%).


Anything I'm missing?

[McAllister]

You're trying to dissociate mass-based effects from HP and move them to character weight. Personally, I'd do it the other way around: force HP to match body weight with only a very small margin of choice. You can play a person who's 5 feet tall and weighs 90 points, or you can play a person with 15 ST and 18 HP, but they can't be the same person; it's just not how people are put together.

I don't know exactly how to determine weight for a human based on HP, but I think I'd use Skinny and Fat as disads that characters are forced to take if their HP is too low or high for their ST, instead of the +/-30% rule.

This doesn't help you at all... so, trying to help, I think the best thing to do might be to invent "negative encumbrance" as an advantage representing the fact that your ST is much higher than your mass suggests, have that as a separate advantage that super-light things can buy (I'm thinking about ultra-tech superlight robots as much as skinny people), and let the rest of the effects equal out to net 0.

[Varyon]

Originally Posted by McAllister

I don't know exactly how to determine weight for a human based on HP, but I think I'd use Skinny and Fat as disads that characters are forced to take if their HP is too low or high for their ST, instead of the +/-30% rule.

GURPS currently seems to assume an average ST/HP 10 human is right around 150 lb. This implies you can get HP for a human by taking the cube root of weight and multiplying by around 1.88. However, I think Douglas Cole got a better value by multiplying BL by 8, so I'd be more inclined to go with that. "Appropriate weight for a 12 HP character" would thus be 230 or so pounds.

Originally Posted by McAllister

I think the best thing to do might be to invent "negative encumbrance" as an advantage representing the fact that your ST is much higher than your mass suggests, have that as a separate advantage that super-light things can buy (I'm thinking about ultra-tech superlight robots as much as skinny people), and let the rest of the effects equal out to net 0.

I'm intending to try to work out estimates for what each aspect is worth, but the first step to doing that is determining what all those aspects are. As for negative encumbrance, that's a modified version of Payload (you can't store stuff inside of you, so call the Limitation "External" or something at somewhere around -20-50%), with boosted Basic Move and something like a Talent for the various affected skills. Set the Payload as an Alternate Ability to those traits, and give the traits a Limitation like "Cannot Carry Objects" and you've got it. Some weird "Already Encumbered" Disadvantage (for heavy characters) would be more difficult to work out, although simply inverting the point cost of "Negative Encumbrance" might work out.

[McAllister]

Originally Posted by Varyon

GURPS currently seems to assume an average ST/HP 10 human is right around 150 lb. This implies you can get HP for a human by taking the cube root of weight and multiplying by around 1.88. However, I think Douglas Cole got a better value by multiplying BL by 8, so I'd be more inclined to go with that. "Appropriate weight for a 12 HP character" would thus be 230 or so pounds.

Multiplying BL by 8 is really clean and I like it, but the problem is that it's based on ST, and if I were designing the system, I would base mass on HP. So, build a character, divide the HP by 2, cube that, and you've got the low estimate of your character's mass: you can increase that by up to 20% of that to determine your character's mass. If you want a weight outside that range, buy a different HP. I might even have Skinny and Fat exist as leveled traits, and you must buy 1 level of Skinny/Fat for each 10% beyond the first that your HP varies from your ST.

[Varyon]

Originally Posted by McAllister

Multiplying BL by 8 is really clean and I like it, but the problem is that it's based on ST, and if I were designing the system, I would base mass on HP.

If you like the 8*BL system, then simply use HP instead of ST in the equation - so weight is equal to (8/5)*(HP^2).

Skinny and Fat certainly should still get some mention, although I'd be more inclined to just do those as things like Distinctive Feature (easier to pick out in a crowd) and possibly situational skill bonuses (like being Fat making you more buoyant). I'd go with your suggestion - HP being too much lower than it should be means the character becomes Fat (to some degree), too much higher means the character becomes Skinny (to some degree). I'll have to decide on actual breakpoints, although +20% then every 10% isn't bad.

[Crakkerjakk]

In my games you can just purchase HP with the massless +0% modifier. Only your regular HP counts towards calculating mass-based effects which is a hindrance about as often as it is a help. Think I stole that from Bruno.

[whswhs]

Originally Posted by Varyon

GURPS currently seems to assume an average ST/HP 10 human is right around 150 lb. This implies you can get HP for a human by taking the cube root of weight and multiplying by around 1.88. However, I think Douglas Cole got a better value by multiplying BL by 8, so I'd be more inclined to go with that. "Appropriate weight for a 12 HP character" would thus be 230 or so pounds.

I use weight = (HP/2)^3, which gives 125 lbs. for 10 HP, and 167 lbs. for 11 HP. I don't quite like having weight be a multiple of BL, because BL varies as an area (cross sectional area of limbs) but weight should vary as a volume.

Bill Stoddard

[Flyndaran]

That would give me 12 HP on the dot with my normalized strength of probably 11 and slight pudge, that comes out quite nicely.
But there must be some variation. Biology is never that predictable.

[whswhs]

Originally Posted by Flyndaran

That would give me 12 HP on the dot with my normalized strength of probably 11 and slight pudge, that comes out quite nicely.
But there must be some variation. Biology is never that predictable.

What does "predictable" mean? If you mean that you can't predict one real, physical trait exactly from another real, physical traits—say, body surface area from body mass, or brain mass from body mass—well, yes, I'm right with you. But HP are not a real, physical trait. They're a game stat and nothing more. There is no real, objective physical trait that measures what HP are used to represent. So it's perfectly reasonable to define HP in terms of some physical trait. It's not an independent measurement of a different but correlated variable; it's a mathematical transformation of an already measured variable.

Bill Stoddard

[Varyon]

It has occurred to me that setting default ([0]) body mass based on HP, ST, or any other trait is inherently flawed. Rather, it seems more appropriate to have an absolute mass as the default, and any variance from that either costs or gives points. Yes, this does mean being a cat is now worth more points, and being an elephant is worth less. For campaigns where the default assumption isn't a human (or equivalent), feel free to set the default mass differently.

For most campaigns, a default mass of 160 lb* works well. Typically, to determine a character's weight, use 8x(HP^2)/5 (you may vary this as you see fit, of course). For Grappling, every +20 lb (+12.5%) is [1], every -20 lb is [-1]. For Blowthrough, every +8 lb (+5%) is [-1], every -8 lb is [1]. For Carrying Capacity, every +4 lb (+2.5%) is [-1], every -20 lb is [1]. For Athletics/Move, every +2 lb (+1.25%) is [-1], every -2 lb is [1], up to +40 lb (+25%), after which point every +6 lb (+3.75%) is [-1], every -6 lb is [1]. For Armor/Clothing, every +20 lb is [-1], every -20 lb is [1]. [EDIT]A raw change per +n lb is inappropriate. Rather, this should be handled as sufficient Lifting ST to negate the change, with the Limitation "Only for carrying armor/gear," and reversed. Factoring in price should be handled as doubling to value of the trait, based on Cast Iron Stomach being -50% for "price only." My next post has the relevant equation; note this eyeballs the "Only for carrying armor/gear" Limitation as around -16.7% (I originally had it as -20%, but felt an end multiplier of 5 in the equation worked better than 4.8)[/EDIT]. For Consumption, every +~26.7 lb (+~16.7%) is [-1], every -26.7 lb is [+1].

Overall, this means - leaving aside Athletics/Move for the moment - every +80 lb (+50%) is (4-10-20-4-3)=[-33], every -80 lb is (-4+10+4+4-3)=[11]. That means every +~2.4 lb (+~1.5%) is [-1], every -~7.3 lb (-~4.5%) is [1]. Athletics/Move, from above ([+1] per +2 lb until a change of 40 lb, after which point it's [+1] per +6 lb), is added to this.

Things can be broken up. Grappling effects, Knockback, and Slams/Falls (note Knockback, Slams, and Falls combine into a Feature) should always use your character's actual weight, but for anything else a "virtual weight" is acceptable - a human-sized 1 ton robot might be able to wear armor as though it only weighed 160 lb. Carrying Capacity and Athletics/Move should always both use the same virtual weight, however (for simplicity, simply ignoring both of these effects may be ideal; consumption is also probably a good idea to ignore, just grab Reduced Consumption or Increased Consumption if desired).

Another thing to consider are toxins. For simplicity, the size of a toxic dose should scale based on body weight, such that you need twice as much to affect a target that weighs twice as much as usual. This is either IT:DR or Vulnerability to Toxins, which is something like [+20] (Occasional), and a +2 to HT for the same, arguably around [+3], for a doubling/halving of body weight. That is, +160 lb (+100%) is [23], -80 lb (-50%) is [-23]. Of course, +320 lb (+200%) is only [35] or so, and -107 lb (-67%) is [-35]. The easiest thing to do here is just to keep in mind that a different weight than default justifies IT:DR or Vulnerability to Toxins (at [10] or [-10] per level; every +1 SSR to weight justifies a level), optionally with Resistant/Susceptible to Poison available as well.


Body mass does interact, after a fashion, with SM. For the most part, any mass increase due to SM should be handled as noted above. There are two volume effects that should also be kept in mind, however. First, one's virtual mass for purposes of armor/clothing cannot go beyond the average mass for the next SM up or down (for SM 0, this is +100 lb). Secondly, once SM comes into play Consumption shouldn't be ignored any longer - it should use the guidelines from Bio-Tech, but rather than being a Feature should be approximated using Reduced/Increased Consumption. As I outlined here, the non-mass-based effects of SM more or less work out as Features unless you are using a houserule that adjusts wounding modifiers based on SM. If that's the case, take the mass-based effects from here and tack on [25] for every +1 to SM, [-25] for every -1 to SM.

*Yes, I'm aware 160 lb is a weight, not a mass. If you'd like, think of it as "a mass that weighs 160 lb in 1G." Of course, in different gravities, Grappling and Falls end up being a bit different, but as Grappling favors higher-weight characters and Falls penalize them, call that a wash.