Carmaker's Cribsheet
for GURPS Autoduel
By S. John Ross; Copyright © 1999

Whenever I design something with GURPS Vehicles, I scratch my head over the drivetrain power. I like to get it exactly right on the first go, but that isn't possible without knowing ahead of time what the final vehicle (including the drivetrain!) will weigh. This plagued me as I began work on some GURPS Autoduel campaign material a while back, and that's where this article began.

While I was at it, I also wanted to lock down a working "arena definition" of "car" for game purposes. Since I like letting my players design their own vehicles, I wanted solid parameters defining what kind of vehicle the AADA (or any other duelling authority) would let into an arena to compete. Chapter Six of GURPS Autoduel gave me a good start, defining typical frame strength, material quality, and volume (p.AD103), and I took it from there, adding a few more limits to keep the "Car Wars feel" in GURPS.

What's a Car?

For "legal" cars (from subcompacts to vans), I suggest the following limits be adopted:

A car may weigh in at no more than 8,000 lbs (4 tons), and no more than 1,800 lbs per wheel, may have a weight/volume ratio no greater than 50:1, and have no more than six wheels. All cars must have HT 12. All cars except vans are typically built on light frames.

Vans: Vans are the exception to the normal Car limits. Vans are normally built on extra-light frames instead of light, and have a maximum per-wheel weight of 1,650 lbs, a maximum weight:volume ratio of 23:1. However, Vans may have HT as low as 10 and still be acceptable.

Note that these limits are meant to represent those imposed by duelling arenas and local governments for "civilian" vehicles. Police and military vehicles would not be constrained by them.

Determining Expected Vehicle Weight

For a car with HT 12, the maximum loaded weight of a car (in pounds) is [Hit Points x 30.769], rounded down. Typical body hit points (and maximum weights) are listed here, followed by the minimum drivetrain power (in kW) to get acceleration 5, 10, and 15 (mph/s).

Body Size Hits Max Weight Drivetrain
Subcompact 75 2,307 lbs. 45/102/281
Compact 94 2,892 lbs. 57/128/355
Mid-Size 113 3,476 lbs. 68/153/425
Sedan 131 4,030 lbs. 79/178/494
Luxury 150 4,615 lbs. 91/204/564
Pick-Up 188 5,784 lbs. 113/255/706
Van* 113 5,022 lbs. 99/221/614

These figures (except those for the van*) assume light frames and a target HT of 12; a medium frame doubles these numbers across the board. The figures take all GURPS Vehicles rounding conventions into account (they're munchkinized up to the razor's edge).

*The Van figures assume a very light frame and a target HT of 10.

"Advanced" Drivetrain Power

The drivetrain numbers above make the simplifying assumption that any car will be designed to carry armor and/or cargo that will take it as close as possible to the limit of what the frame can handle while maintaining HT 12. If you don't want that much weight on your car (for a fuel-efficient model, perhaps), or if you need a more precise value because you're using the exact-surface-area option, use the following:

  • For Accel 5: 39.0625 kW per Ton (approx. Weight divided by 51.2)
  • For Accel 10: 87.890625 kW per Ton (approx. Weight divided by 22.7555)
  • For Accel 15: 244.14062 kW per Ton (approx. Weight divided by 8.192)

Round up to the nearest whole kilowatt; these are absolute minimums.

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