Book 2: Mobility and You

Introduction

GURPS is one of the few RPGs to look realistically at encumbrance, but the rules only go as far as simple penalties for carrying too much stuff. That's too bad. Extending the rules to consider the effects of any body weight – not just carried items and excess fat – adds a lot of realism to PCs' capabilities.

It also wonderfully simulates the agility of very large or small creatures. Size lets big animals run faster than a man, but somersaults, nimble dodges and ballet leaps should be out of the question for rhinos or Giants. Conversely, cats and monkeys, despite those short legs, put on amazing bursts of running speed and feats of acrobatics. If you were the size of a mouse, you too might be amazingly agile. (You don't even have to be small, just notably strong for your weight. Think of Spiderman, or the creature from the Alien movie series.)

Simulating quickness

Adjusting GURPS' DX score doesn't model this weight-related agility well. Losing fat should make you quicker on your feet and able to climb and jump better than before, but shouldn't make you a better rider and more accurate with guns and swords.

Below is a better way to add a power-to-weight factor to characters: natural encumbrance. GURPS already uses this concept (counting extra body weight toward encumbrance) in limited situations: characters with the Fat or Density traits, or under high gravity. Book 2's only real changes are to logically extend this to all characters, and to consider any encumbering weight, not only that from fat, density, gravity, plate mail, and treasure sacks.

Book 2 also introduces a new concept: negative encumbrance, the result of a heap of strength moving a light body. Additional rules look at power vs mass for realistic results when varied gravity, static aerial lift, and underwater action come into play. But all this new stuff isn't a reinvention of the game; it only expands upon familiar GURPS encumbrance rules.

Your Starting Stats

To get reasonable results from the natural encumbrance rules, you need reasonable weight and power stats for your design.

Human-sized PCs generally have workable stats, but other off-the-shelf GURPS designs usually don't. Big designs tend to have modest ST scores that are fine for damage and other combat purposes but are far too low for lifting and carrying ability. Small designs tend to sport ST stats that play well in combat but are too high for lifting purposes: in GURPS a cat can shoulder 90 lbs., and a six-inch, squirrel-like Cidi as much as 120 lbs.!

These creatures need ST scores that are realistic and play well, an ideal that requires dealing with some quirks of the game system. If you lack stats for an odd-sized design or aren't sure about the stats you have, see extensive coverage of the topic in Book 1 before continuing.

Unreal stats

The above doesn't mean you can't use incredible stats for fantastic or superpowered creatures! Quite the opposite: just how would you move if a radioactive spider bite gave you the strength of ten men? Use these rules and find out!

Notes on ST

The rest of Book 2 makes reference to "Load ST"; this is your ST score for lifting and carrying purposes.

Fliers and swimmers use "thrust" to move. This is the same as Load ST by default, though it can differ. Additional wings or fins are likely to boost thrust. Thrust would also seem to scale with the size of the fins and wings, but small wings or fins can beat much faster than large ones, complicating the net effect. In the end, set thrust as you like (and just keep it at its default of Load ST if you're not sure).

The only point cost of modified thrust is that of the natural encumbrance you end up with

Encumbrance Basics

So you've got your weight and Load ST. Oh, and then there's your battleaxe collection, sacks of treasure, and Iron Rod of Badger Summoning... What's your encumbrance?

Natural Encumbrance

Encumbrance with no carried items is "natural encumbrance". Let Load ST x 15 lbs. be the body weight that leaves you with no natural encumbrance, negative or positive. That's 150 lbs. for the Load ST 10 person, 180 lbs. for Load ST 12, etc.

Many characters will have a body weight equal to or close to that norm. Others will have notable extra weight (positive natural encumbrance), and others will have a notable surplus of Load ST (negative natural encumbrance). Positive encumbrance is a disadvantage. Negative encumbrance costs points. (Diet or no diet, a PC doesn't get it unless he pays for it.)

The Encumbrance Table

It's more than a table; it's a tool. Much of it's seldom used, too. The three most important columns are in red figures for visibility; everything else is extra goodies.

Note: The Table renames "Max" encumbrance "Super-Heavy".

Here's what the columns mean:

• WSR (Weight-to-ST Ratio): Total weight (body weight plus carried items) in lbs. divided by Load ST. The WSR numbers for "None" to "Super-Hvy" levels of encumbrance are standard GURPS encumbrance breakpoints plus 15.
  
• Encumbrance Level: Names for encumbrance levels. "Negative" levels bear the unimaginative names "Neg 1", "Neg 2", etc.
  
• Modifiers: Modifiers to physical abilities, named "Full" and "Half".
  
• Move Modifier: A multiplier that adjusts Move for encumbrance.
  
• Cost: Suggested point costs for natural encumbrance.
  
• Encumbrance Factor: This and MSR let you pull fancy tricks when gravity, buoyancy, and other things alter weight but not mass.
  
• MSR (Mass-to-Strength Ratio): Total mass (body mass plus carried items), measured in lbs. at 1-g, divided by Load ST.

Figuring Encumbrance from WSR

Divide your design's body weight by its Load ST. There's your naked Weight-to-ST Ratio (WSR). Find it on the Encumbrance Table to get your level of natural encumbrance.

What about carried items? Toss 'em in: add their weight to yours and figure WSR.

Example: With Load ST 22 and weighing in at 400 lbs., an Ogre has a WSR of 18.2. Use the WSR column and read across to get Light encumbrance.

If he picks up 50 lbs. of treasure, weight is 450 lbs.; WSR becomes 20.5, for Medium encumbrance.

Example: A teen gymnast has Load ST 7 and weighs 88 lbs. Her WSR is a low 12.6, for Negative 1 encumbrance. (She'd have to slim down to less than 77 pounds to get into the Neg 2 range. Whether she can do that healthily is up the GM.)

The system is far more similar to classic GURPS than it may appear. See the Appendix for details.

Borderline cases

As in GURPS, a weight level right on the border allows you the lighter encumbrance: WSR 21 just qualifies for Medium encumbrance, and any additional weight becomes Heavy.

Effects of Encumbrance

Here's how that weight affects you:

Encumbrance and Move

Instead of subtracting encumbrance level from Move as GURPS does, multiply Move by the Move Modifier. (Even GURPS offers the option on CI p.14.)

Forget special modifiers, maximums, and minimums. Throw out GURPS' separate systems for animals, fast superheroes, and flying beings. Use the Move Modifier for everything and anything, fast or slow, on land, air, or sea. This gives realistic results for all characters.

You might want a calculator, but only during character creation. Jot down Move scores for all likely encumbrance levels before play starts, and you're set to go even as encumbrance and Move Modifier change during the game.

Example: The Ogre begins play with Light encumbrance and a Move Modifier of x0.8. When he picks up those extra 50 lbs. and hits Medium encumbrance, Move Modifier drops to x0.6 and Move is reduced accordingly. If you compute and jot down the weights that give him Medium and heavier encumbrance levels, and the associated Move scores, before play begins, you'll be ready for any weight changes in the game.

Encumbrance and fatigue

Encumbrance level greatly affects energy expended in movement and other physical activity. Book 6 details fatigue and how encumbrance fits in.

Encumbrance and Skills

Encumbrance modifies skills as follows:

• Half modifier: Affects Dodge, Stealth, Jumping, Dancing, many Sports skills (such as Skiing or Skating), balance rolls, and DX rolls to stop or change position suddenly.
  
• Full modifier: Affects Acrobatics, Climbing, Flight, and Swimming.

GURPS already uses such modifiers, without the names: it uses the Half modifier for Move, Dodge, and Climbing, and the Full modifier for Swimming.

Example: The Ogre above moves like an Ogre should: heavily. He suffers a -1 penalty on activities like dancing, dodging, moving quietly, and maintaining balance, and a bigger -2 penalty on attempts to climb or perform acrobatics. Overall, though, he's not bad off.

Example: The young gymnast above gains a +1 bonus to her Acrobatics, Climbing, and Swimming skills from her combination of strength and low weight.

Other affected skills

GULLIVER suggests applying the Half modifier for positive (not negative) encumbrance to martial arts skills that rely heavily on quick full-body movements. In general, these are powerful combat skills to which GURPS awards some advantage over more "generic" skills, and include: Fencing, Karate, Judo, Main Gauche, Katana, Short Staff, Tonfa, Boxing, Wrestling, Sumo Wrestling, all combat art or sport skills, and dynamic combat skills like Flying Leap.

See Book 5 for further detail on the above. Also see the Appendix for additional ideas on encumbrance and athletic skills.

Skills that affect Move

Some skills that are modified by encumbrance, such as Swimming, also affect Move in GURPS. But under Book 2 rules, Move Modifier already handles the encumbrance factor. So let encumbrance adjust these skills for control roll and other maneuverability purposes, but not for Move purposes.

That puts swimming and flying rules on equal footing: you adjust your base speed using Move Modifier, and your control rolls using skill modifiers.

Example: You have Swimming-12, but a -4 penalty for Medium encumbrance in water. Use unadjusted skill 12 to determine your base Swimming Move, and multiply that by Move Modifier. But use encumbrance-adjusted skill 8 for control rolls (keeping your head above water, etc.).

Extreme Encumbrance

Extreme encumbrance from weight is trouble. A wizard's spell, Neutro-Man's "density beams", or a landing on a high-gravity world can leave an adventurer crushed under his own weight.

Mobility effects

With Extreme encumbrance from weight, you collapse and may not move. Athletic actions are out of the question. The GM can rule on whether you can operate a radio or fire a gun.

Systemic effects

Roll immediately vs HT and again every hour, with a penalty equal to your level of Extreme encumbrance. Damage is the amount the roll was missed by, times the level of Extreme encumbrance! Damage accrues over the hour, so divide it into appropriate time periods if the character has any hope of rescue. DR does not protect, but let Toughness protect against hourly damage with half its value.

Example: The GM goes crazy with that 18 you rolled: instead of teleporting across the street, you end up on a faraway 5-g planet. Your 200 lbs. of body weight and equipment become 1000 lbs. – with ST 10, that's a WSR of 100! You've got Extreme 6 encumbrance.

Not even able to crawl, you're sprawled and roll vs HT with a -6 penalty. Say you miss the roll by 5; that's 5 x 6 or 30 points of damage, which works out to 1 point of damage every 2 minutes. After only 40 minutes or so, you may be at negative HP and making rolls vs death!

Protection: If your muscles are weakened by fatigue or illness and you collapse under your lowered Load ST, your internal resistance to crushing won't necessarily be as weak. In general, use full Load ST to check for Extreme encumbrance in such situations.

The GM may reduce systemic effects for a soft surface beneath you that helps support your load evenly. Reduce effective levels of Extreme encumbrance for such support, by up to 50% for a custom-fit cushion or sling.

Extra Load-Bearing Ability

You'll notice that it's hard to build a big design without it collapsing. Nature manages to build them; why can't these rules? Because Nature has a few tricks up her sleeves:

Extra Encumbrance

Not all supporting structures are created equal. Thick blocky bones, multiple legs, or a loosely-defined superior load-bearing design, as in FF's Dwarves, will support more weight.

If your design concept includes any of these, consider Extra Encumbrance (CI p.54), which increases resistance to positive encumbrance. The "None" to "Super-Heavy" levels of WSR become 17, 20, 25, 35, 45, and (presumably) 60, using the GURPS description.

However, GULLIVER suggests an expanded Extra Encumbrance advantage available in levels. This is very useful when building large creatures, and a lifesaver on visits to high-gravity worlds:

Extra Encumbrance (revised)	5 points/level

Extra Encumbrance increases the WSR limit for levels of non-negative encumbrance. One level multiplies the amount of WSR over 15 by x1.5. Two levels multiplies this by x2, three levels by x3, five levels by x5, and so on. The Table below makes this clear:

Extra Encumbrance Table

levels->	none [0]	1 level [5]	2 levels [10]	3 levels [15]	X levels [5X]
encumb.	WSR	WSR	WSR	WSR	WSR
None	17	18	19	21	15+2X
Light	19	21	23	27	15+4X
Medium	21	24	27	33	15+6X
Heavy	27	33	39	51	15+12X
X-Hvy	35	45	55	75	15+20X
S-Hvy	45	60	75	105	15+30X
Extreme 1	55	75	95	135	15+40X
Extreme 2	65	90	115	165	15+50X
Extreme 3	75	105	135	195	15+60X
Continue progression for additional levels.

The advantage comes from extra strength and stability in your supporting structure, especially legs. While your legs and back can carry much, your arms are no stronger; you cannot lift more than Load ST determines (BS p.89).

Extra Encumbrance has no effect on creatures in midair or floating in water, or on levels of negative encumbrance.

How many levels to give a design is your choice. Suggestions:

• One level for 3 to 5 legs (or other limbs supporting weight), two levels for 6 or 7, and three levels for 8 or more. A legless body flat against the ground also qualifies for up to three levels.
• One level for Size +1 to +3, two levels for Size +4 to +6, etc., for thicker-than-human bones.
• One level for an efficient load-bearing design (ox, elephant, Dwarf, etc.).
• One level less for a wide, splay-legged stance. (Legs straight under the body support weight better.)
• Additional or fewer levels for exotic structure, magic, etc.

Example: A four-legged creature's Extra Encumbrance might range from one level for small designs (a cat), to two levels for large sizes (a horse), and three for large size and a hefty load-bearing frame (an ox or elephant).

Higher ST

You can also boost load-bearing ability by adding Load ST. Increase base ST before scaling ST up for size (see Book 1), especially for muscle-rippled Ogres and the like. Real-life big mammals don't particularly have a huge proportion of weight devoted to muscle, though. GULLIVER boosts base ST a bit in its big animal designs, but Extra Encumbrance is more important.

Other Load-Bearing Modifications

Multiple legs

CI offers new rules for animals and encumbrance, but GULLIVER ignores those and uses the same encumbrance rules for humans, mice, and twelve-legged cows. Its only special consideration for multi-legged creatures is the suggestion to give them the Extra Encumbrance advantage.

Invertebrates

Some creatures have unusually poor support for weight, especially those without skeletons (endo- or exo-). Cut Load ST to a fraction of its value – the one-fourth value suggested in GURPS, or any other value. Captain Earthworm will find the weight of both carried objects and his own body very encumbering. That's why Earth invertebrates remain small, slow, or aquatic!

How Heavy Can You Get?

Good question. Even using all levels of Extra Encumbrance suggested by the guidelines, jumbo beasts can be hard to design. Take Brachiosaurus, easily Size +5. Using a low estimate of a 30-ton weight, a strapping Load ST 1000, and four levels of Extra Encumbrance, the creature barely squeaks by with Heavy encumbrance. Use other paleontologists' estimates of 50 tons, or build the larger Ultrasaurus, and things get rough.

That's fine. Scientists still debate how the larger dinosaurs could have moved about. Old theories assume brontosaurs bobbing in lagoons, yet recent evidence points to terrestrial lifestyles despite that perplexing weight. Many researchers shrug and ignore the problem. (A few lone voices on the fringe of science point to sauropods as evidence that Earth's gravity itself must have been lower during the Age of Reptiles!)

Until there's a definitive answer, Extra Encumbrance lets you play it either way. Be stingy with the advantage and find some other means of reducing encumbrance, or lay the trait on thick to let Ultrasaurs rattle the earth.

Mass vs Weight

Your weight can change with the environment, spacefarers. It'll go up or down with local gravity, and you can even become weightless on Earth – just jump into a pool.

Then there's lift. Dragons fly thanks to a store of lighter-than-air gas (hydrogen, of course; that explains the flame!). If the buoyancy of that gas just offsets the beast's poundage, then it has zero weight – it's a winged blimp.

But WSR 0 shouldn't mean infinitely negative encumbrance. Your power still needs to move an unchanged mass.

This is outlined below. Yes, it looks more complex than the GURPS-like encumbrance rules above. But rest assured that all the below is for getting realistic results only in those odd game cases involving air, water, space, or unusual combinations of weight and mass. For the vast majority of situations involving normal PCs on dry Earth, the rules revert to the simple WSR lookup above!

Encumbrance from Weight vs Encumbrance from Mass

Two types of encumbrance are in effect at once:

From weight

Your WSR indicates the burden you feel, pushing you down against the ground. With a low or zero WSR, you're comfortably unburdened, no matter how high MSR may be. At WSR 15, you feel the normal burden of human weight. At higher WSR, you feel heavy; at a WSR that indicates Extreme encumbrance, you collapse. (Think beached whales struggling to breathe.)

From mass

It's your effective MSR that determines how mobile you are. Names like "Super-Heavy" or "Extreme" are misleading when the encumbrance comes from mass alone; you're slowed by the mass, but it's not a burden on your system. (Think giant whales crusing comfortably through the ocean.)

Sinking

In air or water, Super-Hvy encumbrance from weight is the point at which exhausting flapping or paddling barely keeps you level; you struggle to hover, with no power left over for mobility.

At Extreme encumbrance from weight, gravity takes over: you're on a one-way trip down, with a few floating pinfeathers marking your wake. (One consolation: although your thrust wasn't enough to keep you aloft, it might slow the fall; see Book 4. And you're not being crushed by this Extreme encumbrance at all, as there's no ground below you – for another two seconds, that is.)

Encumbrance part by part

These rules focus on your supporting structure and overall mobility, but how you carry a load may matter. Items carried in the arms will tire your arms based on their weight; the closer weight gets to your maximum lift, the more tiring it is. In 0-g, you can "lift" any mass, though perhaps very slowly. Details are left to the GM.

Computing Encumbrance

Choose one of the below "engines" as your basic way of combining effects of both weight and mass, then apply all Environment-specific rules that follow.

Simple rule

Find your encumbrance level from WSR, as above. But also look up your MSR on the right side of the Encumbrance Table and read to the left to get encumbrance level from MSR. Use the worse of those two encumbrance levels.

Detailed rule

If power and mass are what determine mobility, where does weight fit in? You need to divert some of your power to handling your weight – to keep from sinking in water or falling in air, or to resist gravity and keep your body in its walking position. The power that's left over – if any – is what moves your mass.

In other words, trying to move a given mass with a burdensome body weight is like trying to move that same mass with a lot less power. It's like having a much higher MSR. This is easy to simulate.

The Mass vs Weight system:

1. Take your WSR and read across on the Table to get Encumbrance Factor. This is an easy x1 for any WSR of 0.7 or lower.
2. Multiply your MSR by Encumbrance Factor. That result is the effective MSR you're forced to deal with.
3. Look up effective MSR on the right side of the Table, and read to the left to get your encumbrance level.

And that's the magic procedure that gives you a unique mobility rating for an average human Christmas shopper with 20 lbs. of gifts in hand, or for an Earth-born, 32-ton bio-blimp with 28.5 tons of lift from hydrogen gas and a ST 1100-equivalent flight thrust, transferred to a new 1.3 g planet with atmospheric density of 0.006.

Advanced rules

The only relevant advanced rules are those in Book 4 that decrease effective motive power for aquatic creatures on land, terrestrial creatures in the water, and any flier in the air. These rules may further strand beached fish, reduce maneuverability in swimming humans, and prohibit large fliers.

On Land

Standard gravity

For the 95% of game situations that involve land and standard gravity, playing with the Mass vs Weight system gets you the same result as just looking up WSR. The rules were designed that way; try some examples and see. So forget anything fancy, and just look up WSR. There's nothing else to do.

Nonstandard gravity

This is where the Mass vs Weight system comes into play. Multiply your WSR by local gravity and proceed from there:

Example: You're an astronaut, ST 13 and 180 lbs., with 80 lbs. of gear. MSR and WSR are 20; you'd have an uncomfortable Medium encumbrance on Earth.

You land on a 0.8-g planet; WSR drops to 16. Read across from WSR 16 to get Encumbrance Factor x2. Multiply your MSR 20 by that to get effective MSR 40; find MSR 40 and read to the left to get Light encumbrance.

You feel a comfortable WSR of only16, but you're still slowed a little by a hefty mass to move around.

Extra Encumbrance

While Extra Encumbrance reduces the amount of weight encumbrance you feel, it doesn't change your mass. This leads to an exception to the rule that the Mass vs Weight system doesn't change things for land creatures at 1-g.

Example: The earlier 30-ton Brachiosaurus, with Load ST 1000 and four levels of Extra Encumbrance, feels only Heavy encumbrance from his WSR 60. But using the Mass vs Weight rules, MSR 60 times Encumbrance Factor x7 leaves an effective MSR of 420, or net X-Hvy encumbrance.

Although the sauropod's amazing load-bearing ability stands up to a crushing weight, it doesn't reduce the incredible mass that has to be moved. In short, the sauropod feels the burden of Heavy encumbrance, but moves even more slowly. X-Hvy is a more correct mobility, and the level of natural encumbrance that a Brachiosaurus PC would purchase.

Book 7's land designs properly consider mass and weight in this way, but it only makes a difference when big designs have lots of Extra Encumbrance. Ignore if you wish.

In Space

Encumbrance Factor will be x1 in 0-g, so you only need to look up your MSR.

Example: You're a human in space with WSR 0, MSR 15. Your Encumbrance Factor is x1, so effective MSR remains 15. You just qualify for Neg. 3 encumbrance.

Example: You're a bioship orbiting the planet. You have10 times the length of a blue whale, a weak thrust of only 30,000 lbs. (jets, or solar "wings"?) and a mass of 150 million pounds. WSR is 0; MSR is 5000. Read across to get Extreme 2 encumbrance, a Move Modifier of x1/15, and serious penalties on Dodge and fancy acrobatics. You maneuver like the Love Boat. But that slowness is all from mass; there's no encumbrance at all from weight, so you're under no strain.

Life in low-g

While low gravity won't change much for creatures that are already light and strong, it can greatly improve mobility in larger designs. But refer to CII p. 140 for rules on the difficulties in adjusting to your new mobility. In particular, p. 142 suggests a DX roll to avoid trouble when you make use of a speed boost from lower gravity; the Free Fall skill is also vital to staying in control. Clumsy attempts to make use of your newfound agility may send you spinning out the airlock!

In the Water

Swimmers often don't have to worry about their own body weight – if they have the same density as water, they'll be weightless! But they have no support from the ground below, so a little weight is very hard to hold level. And water itself is a more difficult medium than air to move through, which reduces mobility.

Weightless encumbrance

If you're weightless in water, Encumbrance Factor will be x1, so you only need to look up your MSR. But double MSR, as water is a more difficult medium to move through.

Example: You're a web-footed, gengineered aquatic humanoid. If you're neutrally buoyant, you have no weight; take your MSR, double it for water, and look it up. There's your natural encumbrance.

Example: You're a whale with Load ST 1000 and a mass of 150,000 lbs. That's MSR 150, but WSR 0 as you're neutrally buoyant. Encumbrance factor is x1, so effective MSR remains 150. Double it to 300 for water and look that up: you have X-Hvy encumbrance from mass.

Question: why do whales beach themselves? To test game rules! Ashore with WSR 150, your Encumbrance Factor is infinite – you have no effective power to push your mass, so you might as well be pushing infinite mass. You're immobile. Meanwhile, grossly Extreme encumbrance crushes you slowly.

Your weight in water

Non-neutrally buoyant items have weight underwater. This can be negative weight, called positive buoyancy, or positive weight, called negative buoyancy. Treat net negative weight as normal weight, with a difference in effect: the life preserver tries to lift you instead of sink you.

You can set such weights arbitrarily: say, a life jacket confers 20 lbs. of negative weight. A small anchor might confer 20 lbs. of positive weight underwater. As negative weight will cancel out a like amount of positive weight, together they leave no weight. But that doesn't mean you're free to load up on both life jackets and lead diving belts as long as they balance out. They'll both add to your mass – a couple of pounds for the life jacket, about 20 lbs. for the anchor – and increase your drag (not covered here, but some Reduced Move for lots of drag is realistic). Swim without the extras.

See the Appendix for all about buoyancy and its effects, including density and weight in water.

Computing encumbrance

Figure encumbrance normally, with one exception:

Multiply WSR by x5 in water for encumbrance calculation.

You have no ground underfoot to support that weight; nothing stops you from moving up or down but your exertions!

Cinematic option: Reduce the x5 WSR multiplier to x3 or less for a more cinematic ability to swim with heavy loads. (GURPS uses no multiplier, which lets an average human hold a 200-lb. anchor level in water!)

Sinking freely

If you let yourself sink freely with weight (or let yourself rise freely, or swim on the surface, with negative weight), you don't divert any power to fighting weight. You are essentially weightless, and computer your encumbrance from MSR only (doubled for water, of course). See the Appendix for more detail.

In the Air

CI suggests a new encumbrance table for fliers. Ignore it. GULLIVER uses the same rules for everyone.

Your flight power and weight can leave you zipping like a hummingbird or barreling like an overgrown turkey. You have no ground below to support your weight; as with swimming, WSR is multiplied here. Unlike swimming, your body weight is also going to count. Flying is a tough job.

See Book 3 for detailed flight ability and wing design.

Computing encumbrance

Figure WSR and MSR using flight thrust. Ignore Bio-Tech's Requires Low Gravity limitation; reduce weight itself for low gravity instead.

Now multiply your WSR for flight purposes:

Multiply WSR by x5 in air for encumbrance calculation.

That's for a realistic setting, in which the size of fliers is greatly limited in Earthlike gravity. They need every advantage they can get to stay aloft. A bird might have weight cut to 2/3 or so what it would be otherwise, thanks to weight-saving measures (right down to foregoing teeth and a second ovary). It'll also have oversized pectorals to give thrust a big boost. Its flight thrust-to-weight ratio might easily be doubled by these modifications, helping offset that x5 weight.

But even with such a freakish body, a birdman will be slow or grounded under earth gravity. To fly, you'll need to stay small, resort to magic, develop superhuman wing muscles, make use of airfoils, or stick to low gravity.

Cinematic option: Alternately, reduce the x5 WSR multiplier for cinematic settings, right down to no multiplier at all to let winged humans fly as easily as they walk! That's the GURPS default flight ability. These rules, however, stick with the x5 multiplier as the default.

Example: You're a human with bio-grafted "angel wings". MSR is a powerful 10, thanks to uber-sized lumps of wing muscle. But treat airborne WSR as WSR 10 x 5 = 50; Encumbrance Factor is infinite. Sorry, you can't quite lift off.

You move to a 0.5-g world. WSR is only 25; Encumbrance Factor is x7. MSR is 10 x 7 for Encumbrance Factor = 70; you fly with Medium encumbrance (ignoring the effects of thinner air on thrust).

Example: The flight thrust of a bee at human size is anybody's guess; let's imagine it'd have a weak MSR 30 (bees don't have the chunky muscles we big creatures have). But at roughly x1/150 the length of a human, a real bee is likely to have x1/150 this MSR. Net MSR 0.2 sounds fine. WSR in flight is 5 times that, or 1, which is just high enough to become relevant: MSR is 0.2, x 1.1 for Encumbrance Factor = MSR 0.22, for Neg. 8 encumbrance. The bee can stop on a dime, hover beautifully, and zip away incredibly fast – even at very high gravity.

Free fall

As in the earlier water rules, you can ignore weight by not fighting it, i.e., by going into free fall. You're weightless and can figure encumbrance from MSR only. It's a cute trick until you hit the ground.

Unpowered flight

Gliders are in (slow) free fall, which means they don't expend any effort to offset weight. That means they have no encumbrance from weight and could compute encumbrance from thrust and mass only – but they don't have thrust. All they can do is adjust the angle of their wings to change direction.

For maneuverability purposes, figure MSR normally, then multiply by 10. This simulates the very weak maneuvering force allowed by twists and position changes, relative to stout beating wings. Gliders will generally be less maneuverable than a reasonably competent powered flier. But no matter how bad the maneuverability, that encumbrance generally won't be too tiring, unless your load rating itself is strained.

Creatures with only parachuting ability are even less maneuverable: multiply MSR by 50, not 10.

Book 4 covers gliders' movement in detail. While encumbrance doesn't affect forward Move, weight will affect speed of descent.

Example: You have a gliding human design, ST 8 and 120 lbs. MSR is 15; use MSR 150 to determine gliding maneuverability. You maneuver as if encumbrance were Heavy.

Buying Natural Encumbrance

If your encumbrance from weight and your encumbrance from mass differ, pay for natural encumbrance from mass.

In other words, you pay for actual mobility. The separate burden of weight felt by the creature is a special effect of its strength, Extra Encumbrance, the environment, etc.

Example: If the earlier Brachiosaurus were a PC, it could take the full -40 points for X-Hvy encumbrance. The fact that it feels a lesser crushing weight equivalent to only Heavy is a benefit of its Extra Encumbrance.

Cost of Negative Encumbrance

You can buy negative encumbrance up to the level your naked body weight and Load ST allow. Each odd level of negative encumbrance adds to a few physical abilities, while each even level adds to the same abilities plus a few more, including Dodge. That's why the even levels cost so much more.

Fortunately, the PCs most likely to have negative encumbrance – small races – will have some points to pay for it, thanks to low ST and HP. But if you just can't come up with the points, see the Appendix for budget-shopper tips, as well as a cost breakdown of what goes into negative encumbrance.

Cost of Positive Encumbrance

Equipment is something you choose to carry, so you get no points for encumbrance caused by it. (An item you paid points for and will almost always carry, such as a heavy suit of armor in a Supers game, might be an exception.) But natural encumbrance is a big disadvantage that really hampers mobility. Allow a -10 point disadvantage for each level greater than None, up to -50 points. Additional levels of Extreme encumbrance are worth only -1 point each.

The disadvantage cost of positive encumbrance includes skill, Dodge, and movement penalties. The only way to get rid of the disadvantage is to lower your body weight, a process that's either slow (make those Will rolls, dieters!) or drastic (what to lop off first?). You'll have to buy off the disadvantage point cost too.

The cost for Super-Hvy encumbrance is the same as the cost given for the Sessile disadvantage. Super-Hvy encumbrance is better than Sessile in that the character can stagger very slowly, but is worse than Sessile in that not only moving but just standing will rapidly cause fatigue! Better lie down and stay down.

Varied Levels of Encumbrance

You may have one level of encumbrance on land, and another in the water or air. Adjust the costs as below:

You have one primary mode of movement: land, water, or air. This is land for most characters and water for aquatic ones. (If you have Amphibious, designate either land or water as your primary mode of movement, for purposes of encumbrance costs.) Air as a primary mode of movement is extremely unusual; Earth fliers have land as their primary mode, with air as an added mode.

Purchase encumbrance as follows:

• Base your cost of encumbrance on your encumbrance in your primary mode of movement.
• Worse encumbrance in a non-primary mode is worth only x1/5 the cost difference for one mode, or x2/5 for two. (A non-flier has no mobility in air, and gets no points for this.)
• Better encumbrance in a non-primary mode costs x1/2 the cost difference for one mode, or x2/3 for two.

See Book 3 for much more on your modes of movement.

Example: You're a flier with Neg 3 encumbrance on land and in water, and Light in air. Land is your primary mode of movement. Pay 35 points for the Neg 3 encumbrance, and take one-fifth the cost difference between Neg 3 and Light, or -9 points, as a disadvantage for your lowered flight mobility. Net cost: 26.

Example: You're amphibious, with Light encumbrance on land and Heavy in water. Pick water or land as your primary mode of movement for point cost purposes. Say you choose land; take -10 points for Light encumbrance. Your worse encumbrance in water is worth an additional -20 points, x1/5 for a non-primary mode of movement = -4. Net cost: -14.

If you also had flight with No encumbrance, you'd pay the 10-point difference between Light and None, x1/2 for non-primary mode = 5, for the improved mobility. Net cost: -9.

Unpowered flight encumbrance: If you can only glide, buy your aerial encumbrance level normally, based on unpowered agility.

If you also have Powered Flight, buy your aerial encumbrance based on powered agility. For simplicity, your calculated gliding encumbrance, whether better or worse, has no additional cost.

Finishing Up

Try a few designs using these rules. Build some small, light creatures, and some big, ponderous ones. (Don't forget extra ST and frame modifications – Extra Encumbrance – to help the big bodies move.) Run the creatures through some combat or obstacle courses, and you'll see how natural encumbrance makes each "feel" more alive.

Book 2 APPENDIX

1. Book 1 Recap: Size

To set realistic creature weight and ST, first guess how heavy and strong your creature might be if it were human-sized – say, ST 12 and 170 lbs. for a sturdy dog, ST 15 and 200 lbs. for a heavily muscled Giant, etc.

Now guess how many times larger or smaller than a human it actually is in height or length. Call that Linear Scale. Multiply human-scale strength for lifting and carrying purposes by Linear Scale squared, and multiply human-sized weight by Linear Scale cubed. (Or if you know the weight of a real creature, use that.)

Example: If the above dog were about one-half human length, Linear Scale is x1/2. Multiply base ST by x1/4 to get ST 3 for lifting and carrying purposes, and multiply base weight by x1/8 to get about 20 lbs.

If the above Giant were three times human height, Linear Scale is x3. Multiply base ST by x9 to get Load ST 135, and base weight by x27 to get 5400 lbs. (Pretty heavy; better add the Extra Encumbrance advantage!)

For ideas on treating ST for combat purposes, and lots more on size, see Book 1.

2. Optional Encumbrance Effects

The skills affected by encumbrance in this Book are just suggestions; change or add more affected abilities (Bicycling? Body Sense?) as you see fit.

If you think most physical activity should be affected by encumbrance, here's an option:

Apply half the Half modifier (might as well call it the "Quarter modifier"), rounded down, to all athletic skills not already affected. Medium or Heavy encumbrance hit you with a -1 on all remaining athletic skills – Broadsword, Knife Throwing, Riding, Bicycling, Fast-Draw, many more – as well as general athletic DX rolls not already mentioned. Extra-Heavy or Super-Heavy encumbrance give -2.

Physical skills without a full-bodied athletic component – shooting a gun or bow, piloting a vehicle, tasks of manual dexterity – wouldn't take a penalty.

If you use this "Quarter modifier", add -5 points to the cost of positive natural encumbrance for each point of penalty. If you let the "Quarter modifier" work as a bonus with negative encumbrance, add 5 points to the cost of the advantage for each point of bonus.

3. Advanced Encumbrance Table

This is a super-detailed version of the Encumbrance Table, with lots of "in-between" levels. Use it if you've got a thing for precision.