GURPS: Vehicle Hit Locations for Cars Analysis

Precis- I explore some of the wonky oddities surrounding Vehicle Hit Locations

First, this post on explosives is excellent (didn't know Roger had a blog until an hour ago).

I'm going to take a look at vehicle hit locations. This is something that has come up before in Agency 17.

Compared to helicopters (control surfaces), planes (ditto), spacecraft (who wants to inhale vacuum?), and boats (water), cars don't care too much about damage to the exterior. Shooting up the side panels, passenger compartment, or the trunk of a car does jack shit towards disabling the vehicle. The way I see it, a car only has a few real vulnerabilities:

• Engine
• Wheels
• Fuel Tank
• Driver
• Drive train (really starting to get pedantic here)

So let's look at the actual table, piece by piece. I'm going to focus specifically on cars. That means I'll be ignoring things like superstructure, wings, exposed riders, etc.

Body: Powered vehicles that take a major wound must check vs, HT or suffer halved move.

Guinea Pig:
The TL6 Roadster (B464) has ST/HP 42, HT 9f (flammable), SM +2 and DR 4, and counts as unliving.

A piercing attack needs to deal 21 points of injury to cause a check for the major wound. That would be 63 points for a piercing attack (unliving reduces incoming damage by a 1/3rd), 42 points for a large piercing attack (1/2 reduction via unliving), or 21 points of damage for huge piercing or impaling attack.

A quick look at TL5/TL6 rifles in High Tech shows a few that could manage to knock out a car in a single body shot, accounting for -4 damage due to DR:

The TL5 Greener Elephant Rifle (although admittedly extremely rare in Prohibition Chicago) can  pass the test with 6d+2 pi++ damage, a maximum of 34 (after applying for DR 4), 8 points of damage shy of the threshold large piercing attacks have to hit to cause a major wound. A quick look at http://anydice.com/ shows that 6d+2 will deal 21+ damage 36.31% of the time.

The TL6 H&H Royal Double-Express, .600 NE (also extremely rare in Prohibition Chicago) also easily does the job with 5dx2 pi++ damage, doing at least 21 damage roughly 3.24% of the time- before figuring in the x2 portion of the damage text which makes it a much more likely 77.65% chance.

The Steyr-Solothurn S18-1000 is more or less a souped up H&H Royal Express with 6dx2(2) pi++, dealing 22 damage 99% of the time.

As for machine guns:

The Maxim 1-pdr is an interesting case due to the armor divisor of (0.5), giving our poor roadster effective DR 8. Still, with 5dx2(0.5)pi++, it can manage to cause a roadster a major wound 39.97% of the time! This isn't even accounting for the followup explosions (which will wreck any occupants extremely fast).

The Browning M2HB makes the cut, dealing 42 points of large piercing damage roughly 50% of the time. The KPZ DShK-38 has identical damage numbers.

A 12-gauge 2.75" firing slugs (4d+4 pi++) can also hit the 21 damage threshold 2.7% of the time.

Verdict:

The body location mechanics make a good deal of sense- only a small handful of TL5-6 rifles can pump out enough damage in a single shot to potentially knock a vehicle to half move, and two of the three machine guns available are definitely WW2 offerings. Shotgun slugs can also do it, but with abysmally small chances.

Notably, some favorite TL8 anti-material rifles such as the Barret M82A1 don't pack enough wallop to disable a TL6 roadster with a single body shot.

 Glass windows, Large and Small

When you hit a glass window, you make an Occupant Hit check and if that succeeds your attack hits someone inside. Depending on if the window was up or down, they get half vehicle DR or no DR respectively on the attack.

First, there isn't much description as to the size different between a Large Window (a -3 penalty to hit, reduced to -1 for the roadster's SM) and a Small Window (-7 penalty to hit, -5 for the roadster). If a full windshield is a -1 to hit, would a normal car window really be at -5? I mean, the size difference between all the vitals for a human and the skull hit location is much smaller than a full-sized windshield to a car door window, right? It's a maddening bit of ambiguity.

Using our Roadster example, a single occupant would be hit on a 9 or less (37.5%), while two occupants have a 50/50 shot of either of them taking a hit on the 10 or less. Now, this is the straight attack, not the 5 points becoming 1d of cutting damage under Occupants and Vehicle Damage- your 7d+1 Springfield rifle shot plows into someone.

p.470 also includes the following on windows:

An attacker can sometimes target a
vehicle’s occupants directly. This is
only possible if the vehicle has an
exposed rider (E), glass windows (G or
g), or an open cabin (O)...  A rider has no cover; someone in
an open or glass-windowed vehicle
has partial cover (legs, groin, and half
the torso). There is an extra -1 to shoot
into or out of a window unless the
occupant is actually leaning out.

Verdict: Why shoot a window when you can shoot the guy behind it at a -1 penalty? Shooting through the vehicle DR and hitting your main target is a far better bet than relying on Occupant Damage.

Vital Area: 

Vital Area: A powered vehicle (anything with a ST attribute) has vital
areas: engines, fuel tanks, etc. The
wounding modifier for a tight-beam
burning attack is x2; that for an impaling or any piercing attack is x3!

This is where things get really wonky in a way that really rubs me the wrong way. The quote above seems to strongly indicate that Vital Areas do not benefit from Injury Tolerance: Unliving, a trait that most vehicles have. Hilarity ensues.

The M1903 Springfield rifle does 7d+1 pi damage. According to the entry for Vital Areas, it would receive a wounding modifier of x3. 14 damage after DR will hit the 42HP needed to reduce a vehicle to 0 HP immediately- 18 damage before DR. The M1903 will do 18 damage roughly 80.83% of the time with a single shot.

A 12-gauge 2.75" shotgun firing a rifled slug (4d6+4 pi++) will deal 18 damage before DR 15.9% of the time, also reducing the roadster to 0 HP in a single shot. 

I should also mention that dealing 42 points of damage to a vehicle in a single hit will cause 8d of cutting damage inside the passenger compartment due to the 5 points of damage -> 1d of cutting conversion as per vehicle damage and occupants.

Verdict: This is broken.

Wheels:

Damage over HP/(2*Number of Wheels) cripples a wheel. For the TL6 roadster, that's 6 points of damage. Effects of crippling a wheel are the same as a character with an equal number of legs losing one leg.

So uh, time to look up the rules for adding legs or losing legs in Characters? Because looking up extra legs to determine the properties of a vehicle losing wheels is intuitive and user friendly?

A quick look at B.54-55 shows that:

• For 3-4 wheels, crippling 1 reduces move by half.
• For 5-6, each missing wheel reduces move by 20% until only 3 or left (40% move at that point), and then any gone after that stops movement completely
• For 7+, each lost leg reduces move by 10% until 3 are left (40% move at that point), and then losing another stops all movement completely.

Additionally, any damage to a wheel forces an HT check. If it fails, it's immediately crippled.

Hitting the wheel of a SM +2 roadster is just a -2 penalty.

Verdict: Wheels are a primo target for ending car chases, especially since 4-wheel cars are the norm for most modern settings.

Insurance Claims:

Of the hit locations, the body hit location does the best job of mirroring reality- very few weapons available at the same tech level as the sacrificial lamb can reliably knock it out in a single hit, which I feel is very true to life.

On the other hand, the "Vital Areas" hit location is an abomination that seems to completely ignore  that an engine would have Injury Tolerance: Unliving, just like the rest of the car.

Wheels are an interesting case, and I feel that having one become crippled should force a control roll or  give control rating penalties and that the move penalty should be reduced to 25% until two wheels are crippled. 

Windows are another strange case because you can explicitly aim for a window and take your chances that an occupant inside gets hit, or you can just accept a -1 (unless you want to hang out your window like a chump to fire your gun) to try and hit someone inside, who gets partial DR for half their body.

Fix 'er up (my thoughts on potential fixes/solutions and balancing):

Body: Leave as is.

Wheels: The first crippled wheel on a four-wheeled vehicle reduces max speed by 25%. A crippled wheel requires a control roll to prevent a mishap/accident (I highly suggest applying speed penalties using everyone's favorite table to the driving check). A second crippled wheel reduces max speed by 50% and requires another control roll. 

A penalty to handling would probably also be an excellent idea as an alternative to maximum speed reduction or in addition to a maximum speed penalty. 

Windows: Attacking a window directly is a -3 penalty while attacking an occupant directly through a window is a -5 penalty. Choosing to attack an occupant directly dictates who is hit on a success, choosing to attack the window directly means rolling for Occupant Damage on a hit. If that subsequent roll is also a hit, a random occupant is hit by the original attack. If the Occupant Damage check is not a hit, the vehicle sustains damage from the attack, but do not make additional Occupant Damage Checks for the same attack.

If random hit location is used for a direct hit on an occupant, the legs, groin, feet and half of the torso (1-3 on a 1d roll) are protected by vehicle DR. A closed window provides half Vehicle DR.

Vital Areas:

Reduce the wounding modifiers for piercing attacks to a vital area:

• Huge Piercing: 1.5x
• Large Piercing: 1.25x
• Piercing: 1x
• Small Piercing: .5x

Impaling attacks get a wounding modifier of x1, because who is going to stab an engine block with a spear?

Attacking the engine has a -3 penalty and may only be done from the front or sides.

Attacking the fuel tanks has a -6 penalty (Exception: Externally mounted tanks, -3)  and may not be done from the front on most civilian vehicles. Damage over Vehicle Max HP/5 cripples the tank, causing the Vehicle's range to drop by 75% of its max.

Overall Vehicle HP and Damage:

Gurps currently doesn't have any stated rules (that I've been able to find with a more than casual less than decent search of various source books) on how to handle when a vehicle is disabled from generic damage. I'm willing to bet I've missed something, somewhere, but until I learn about something, I'd probably go with the following:

Once a vehicle reaches 0 or less HP, it needs to check against HT immediately and every turn afterwards or immediately come to a stop.

Vehicle Occupant Damage:

I would make a few changes:

• Called shots to the wheels and fuel tank do not check for occupant damage
• Shots that hit the engine from the side of the vehicle do not check for occupant damage
• Instead of randomly choosing who is hit, a GM may at their discretion cause Vehicle Occupant Damage to affect whoever in the vehicle is closest to where the damage to the vehicle occurs

Thanks:

I'd like to thank Douglas Cole for taking the time to read a draft of the post and getting in touch with David Pulver, who explained what lead to the vehicle hit table during development of 4th ed:

However, in the original draft of 4e, all hit points scaled with square root (like ST) rather than cube root. This had a significant
effect on vehicle hit points. A car, for example, would have had around twice as many hit points as it ended up with and a tank would have
about 3 times as many HP...  

The late change to cube root HP in the final draft, made after I had left the project and was thus unable
to comment, unfortunately affected some rules in subtle ways. The high multiplier for piercing damage was originally intended as a way of allowing
"golden BB" type effects, but with only half to a third as many HP in the final version produces some unfortunate overkill results.