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Old 11-23-2021, 03:50 AM   #1
whswhs
 
Join Date: Jun 2005
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Default translating vehicles from 3/e to 4/e: Maneuver Rating

In the campaign I'm getting ready to run, set on the future Mars of an alternate timeline, I'm borrowing some material from GURPS Mars, which I consider one of the best 3/e supplements. Its Martian vehicles were (naturally) designed using GURPS Vehicles. Now, I plan to run it using 4/e, so I need to convert those vehicles to 4/e statistics. For the most part this is straightforward; weight and cost and occupancy, for example, just transfer over, and I can refigure hit points based on vehicle weight using the 4/e rules. But I was kind of perplexed by what to do with Maneuver Rating (MR).

Looking at GURPS Vehicles, I see that a vehicle's MR is compared with the Gs it takes during a turn; for example, a vehicle taking a 30° turn at 50 mph takes 1.25 G. This seems to be fairly accurate physically. A vehicle turning 30° per second will complete a full circle in 12 seconds; 50 mph equates to 22.35 meters per second, or 268.2 meters in 12 seconds, and dividing by 2π gives 42.69 meters; plugging that into the formula A = v^2/r gives 11.7 meters per second squared, and dividing by 9.806 gives 1.19 G, which is close enough for gaming.

GURPS 4/e uses Turning Radius instead of MR. It assumes that vehicles make a succession of 60° turns, and must travel some particular distance to do so. If a vehicle turns every second, it will complete a full circle in 6 seconds, and if it travels a distance D each second, that circle will have a circumference of 6D, and a radius of 6D/2π = 0.95D; it seems to be rounding this to 1.00, to say that the circle's radius equals to distance traveled in one second. (Another way to look at this is to envision the vehicle as traversing a hexagon, with "radius" meaning the distance from a corner of the hexagon to its geometric center.) A vehicle that travels less than its TR between turns must make a control roll. The standard Turning Radius is Move/Basic Move, which is hard to relate to the 3/e rules, but there's a proposed more realistic formula of Move^2/10. This seems to reflect solving the formula for Gs above to find r, giving r = v^2/A; setting A = 10 equates to 9.14 meters per second squared, which is fairly close to 1 G, so this seems to be an approximate formula for a vehicle with MR of 1.00.

Now, working in yards and seconds, if we take A = 10, and set r = v (that is, assume a 60° turn every second), we get v = 10, or Move 10, which GURPS approximates to be 20 mph. And plugging that into the table in GURPS Vehicles (Speed 20 mph, Bend 60°) does give 1 G, so it appears that GURPS 4/e is implicitly assuming that vehicles have MR 1.00.

What if a vehicle has a different MR in 3/e? Well, say it has MR 0.75. When it tries to make a turn at 1 G, it exceeds its MR by 0.25 G, which is a penalty of -1.

GURPS 4/e has a penalty of Move/Basic Move -1, and I'm not seeing any obvious way to equate that to an MR-based penalty. A formula based on the "realistic" formula might work better, but no such formula is suggested.

It looks as if, for a 60° turn, the G-force is proportional to the speed; a 10-mph increase equates to 0.5G, which is a -2 penalty. So the 3/e penalty would be -1 per 5 mph over 20 mph, and -1 per -0.25 to MR. That first part is even simpler than -1 for a speed increment equal to Basic Move (and also more physically realistic, a double bonus!); the second part lets us say that Handling Modifier = (4MR -4).

Does all that make sense?

[There's a minor problem in that the "family car" in 3/e Vehicles has MR 0.75, and the TL7 "sedan" in 4/e has Handling rating of 0. But I'm willing to say that 4/e is overoptimistic and the actual modifier in 4/e should be -1. It's not going to come into play in the Martian campaign anyway—no one will be driving TL7 sedans across Mars.]
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Old 11-23-2021, 06:15 AM   #2
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

I'm not familiar with 3e rules, and it's early in the morning so I may be misinterpreting some of the equations, but first off, considering this is a game you're running rather than a 4e book you're writing, why not just houserule that you'll be using 3e's MR rules instead of 4e's TR ones? You appear to be conversant with the former rules, and you seem to be a sufficiently-skilled GM that your players being unfamiliar with them won't really be a problem at the table.

Failing that, you mention the "more realistic" equation of (Move^2)/10 appears to be using the acceleration due to gravity (which GURPS rounds to 10 yards/sec^2) as the denominator, and is assuming an MR of 1G. So... why not just use 10xMR as the denominator? A vehicle with an MR of 1.25G would use (Move^2)/12.5, while one with an MR of 0.75G would use (Move^2)/0.75. Higher MR leads to a tighter TR, which should be the effect. As for the penalty, that looks to just be -(TR-1) (the normal equation for TR is Move/Basic Move, round down, and the equation for the penalty is -1 per full increment by which you exceed Basic Move... which works out to the above equation), which would be compatible with any means of calculating TR. However, I honestly find the Pushing the Envelope rules a bit odd, in that there doesn't seem to be any scalability - if you've got Move 3 and are moving at 23 yards/second (as in the example), you'd have a Turning Radius of 7, and you'd take a -6 to your roll if you tried to turn 6 yards after your last turn... or at only 1 yard after it. I think it's more appropriate if you take a -1 penalty per hex that you are "early." So, with TR 7, you can turn after traveling 7 yards without issue, turn after traveling 6 yards at -1 (with a DX+3 roll, that's net DX+2), turn after traveling 5 yards at -2 (DX+1), 4 yards at -3 (DX), 3 yards at -4 (DX-1), 2 yards at -5 (DX-2), or 1 yard at -6 (DX-3)... or 0 yards (a 120-degree turn instead of a 60-degree one) also at -6 (I'd be inclined to boost this to -7, for consistency).
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Old 11-23-2021, 11:20 AM   #3
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

Quote:
Originally Posted by Varyon View Post
Failing that, you mention the "more realistic" equation of (Move^2)/10 appears to be using the acceleration due to gravity (which GURPS rounds to 10 yards/sec^2) as the denominator, and is assuming an MR of 1G. So... why not just use 10xMR as the denominator? A vehicle with an MR of 1.25G would use (Move^2)/12.5, while one with an MR of 0.75G would use (Move^2)/0.75. Higher MR leads to a tighter TR, which should be the effect. As for the penalty, that looks to just be -(TR-1) (the normal equation for TR is Move/Basic Move, round down, and the equation for the penalty is -1 per full increment by which you exceed Basic Move... which works out to the above equation), which would be compatible with any means of calculating TR. However, I honestly find the Pushing the Envelope rules a bit odd, in that there doesn't seem to be any scalability - if you've got Move 3 and are moving at 23 yards/second (as in the example), you'd have a Turning Radius of 7, and you'd take a -6 to your roll if you tried to turn 6 yards after your last turn... or at only 1 yard after it. I think it's more appropriate if you take a -1 penalty per hex that you are "early." So, with TR 7, you can turn after traveling 7 yards without issue, turn after traveling 6 yards at -1 (with a DX+3 roll, that's net DX+2), turn after traveling 5 yards at -2 (DX+1), 4 yards at -3 (DX), 3 yards at -4 (DX-1), 2 yards at -5 (DX-2), or 1 yard at -6 (DX-3)... or 0 yards (a 120-degree turn instead of a 60-degree one) also at -6 (I'd be inclined to boost this to -7, for consistency).
The lack of scalability is something that bothers me about the Pushing the Envelope rules as well.

Refiguring the TR based on the vehicle's MR and current speed has two problems. A minor one is that 4/e doesn't use MR; it uses Handling, which is a bonus or penalty to the relevant skill and an enhancement or limitation to Enhanced Move. A greater one is that it requires me to square current speed and divide by 10xMR (or 10+2.5xHandling), and that slows things down a bit—possibly at an inconvenient time, during a high-speed chase scene. Being able to work directly with bonuses or penalties to skill is quicker and simpler.

As for how to figure it, I'm not sure I understand you correctly. I think that what you're saying is that if the speed is 60 MPH (Move 30) and the Handling is +2 (giving 1.50G and a divisor of 15), then I figure a turning radius of 30^2/15 = 60, which says I have to go forward for 2 turns before turning 60°. How would you figure the penalty if (a) I try to turn after going forward for only one turn or (b) if I try to take the same turn at 90 MPH rather than 60 MPH?

My own guess was that the penalty would be (Speed^2)/10 -1 = (Speed^2 -10)/10. But then, for example, if Speed is 15, we get (225-10)/10 = 215/10 = 21.5. I think that's a -22 penalty, which is really catastrophic and which seems far too high for driving at 30 MPH.
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Old 11-24-2021, 07:00 AM   #4
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

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Originally Posted by whswhs View Post
A greater one is that it requires me to square current speed and divide by 10xMR (or 10+2.5xHandling), and that slows things down a bit—possibly at an inconvenient time, during a high-speed chase scene. Being able to work directly with bonuses or penalties to skill is quicker and simpler.
Yeah, that is indeed a problem. If you don't have issues using peripherals at the table, it's easy to setup a spreadsheet to do the work for you - but I'm well aware having a computer/tablet/etc at the table can be an unwanted distraction.

Quote:
Originally Posted by whswhs View Post
As for how to figure it, I'm not sure I understand you correctly. I think that what you're saying is that if the speed is 60 MPH (Move 30) and the Handling is +2 (giving 1.50G and a divisor of 15), then I figure a turning radius of 30^2/15 = 60, which says I have to go forward for 2 turns before turning 60°. How would you figure the penalty if (a) I try to turn after going forward for only one turn or (b) if I try to take the same turn at 90 MPH rather than 60 MPH?
I'll take your word for it that +2 Hnd corresponds to MR 1.5. And, yeah, that means you need to travel 60 yards (as you note, 2 turns worth of travel). For determining the penalty, it's based on how many yards you've traveled since your last 60-degree turn (unless I've misinterpreted things, GURPS lets you make a 60-degree turn at any time, but then you have a "cooldown" of some number of yards before you can make another turn). So, using my suggestion, trying to do a 120-degree turn is at -60 (net -58, considering the +2 from Hnd) if you try to do it immediately, and gets a +1 for each yard traveled between your two 60-degree turns. For a cinematic driver with Driving 30, they couldn't even attempt a turn with less distance traveled than 31 yards (which would be a roll against 3), and need to travel 38 yards to have a 50% chance of pulling it off. And note you'll want to use yards traveled, rather than turns, as the former gives better resolution.

Of course, there are ways around that. First off, the above assumes you're maintaining 60 mph - slowing down will give you a tighter turning radius (for an extreme driving sequence, you'd probably use Pushing the Envelope to slow down more rapidly than is safe, do a tight turn, then immediately accelerate as much as your vehicle can manage). Additionally, this seems like a prime candidate for a Technique - call it Tight Turning (or something more colorful) - which would default to Driving and reduce (or even eliminate) the penalty for turning too early. You could also adapt however it was 3e handled turning too quickly, although I'd assume that's more something where the penalty depends on how much the G's pulled by the maneuver exceeded MR, and that's probably not something to quickly calculate at the table (at least in terms of translating into 4e).
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Old 11-24-2021, 07:50 AM   #5
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

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Originally Posted by Varyon View Post
Yeah, that is indeed a problem. If you don't have issues using peripherals at the table, it's easy to setup a spreadsheet to do the work for you - but I'm well aware having a computer/tablet/etc at the table can be an unwanted distraction.
I'm going to be running this via computer, using Zoom or Meet or something. But I think if I were going to do that I'd just pick up my calculator; I can remember the formula well enough.
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Old 11-24-2021, 08:07 AM   #6
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

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Originally Posted by Varyon View Post
Yeah, that is indeed a problem. If you don't have issues using peripherals at the table, it's easy to setup a spreadsheet to do the work for you - but I'm well aware having a computer/tablet/etc at the table can be an unwanted distraction.
Apologies for the double-post*, but this just occurred to me and I didn't want it overlooked on account of being a late EDIT instead. I think there's a way to get a "good enough" simplification for the calculation of Turn Radius, here. When you create a vehicle, use the (Move^2)/(10+Hnd*2.5) method to calculate the TR at half the vehicle's nominal maximum speed (or if you expect some other speed to be the average of what it actually uses, calculate for that instead). Now divide that speed by the TR you just calculated; the result is the vehicle's Basic Move for purposes of maneuverability (you still use its normal Basic Move for acceleration). Now you can just divide your current Move by that value (call it Maneuverability) to get TR. It's not as precise, but should work.

As an example, let's say we have a Hnd+2 vehicle with an average speed of Move 15 (30 mph). At that speed, Turn Radius is 15 yards. This gives us Maneuverability of 1, which makes things extremely easy - whatever speed we're going, that's our Turn Radius. If the vehicle instead had an average speed of Move 30, the more-complex equation gives us a Turn Radius of 60 yards, for a Maneuverability of 0.5 - double current speed to determine Turn Radius.

As you can see from the example, the further you are from the speed for which you calculated Turn Radius, the less accurate the model is, but perhaps not egregiously so. Technically, you can correct the model by dividing Maneuverability by the quotient of the average speed you calculated for and your actual speed, but that's just bringing back the squaring in a roundabout way.

*EDIT: OK, looks like it's not a double-post after all...
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Old 11-25-2021, 09:07 AM   #7
whswhs
 
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

Well, let's do some math here.

Assume a vehicle with MR 1.00 (to correspond to 4/e) making a 60° turn (to correspond with 4/e) at 20, 30, or 40 mph. We'll give it Basic Speed/Accel 2 to correspond to the sedan in 4/e.

Using the 3/e rules, at 20 mph, it experiences 1.00 g, which is within its MR and inflicts no penalty. At 30 mph, it experiences 1.50 g, and is at -2. At 40 mph, it experiences 2.00 g, and is at -4.

Using the 4/e standard rules, its Move is respectively 10, 15, and 20. Its turning radius is 5, 7, and 10. Its Move is always greater than its turning radius. So it doesn't seem that it will ever have to make a control roll.

Using the alternate rules, its turning radius is respectively 10, 22, and 40. Its Move is equal to its turning radius in the first case (no control roll) but less in the other two. We don't have an explicit rule for penalty, but if we assume a penalty of TR-1 (which I believe you suggested), it has to make control rolls at -21 and -39. That's pretty much impossible. Even at 25 mph (Move 12.5), its turning radius is 15, which gives a -14 penalty.

What if we use your rule? The sedan's maximum speed is 30, so we figure its TR at 15, which is 22.5. Dividing average move of 15 (30 mph) by 22.5 gives 0.67 Maneuverability. So TR at other speeds is 1.5x Move. At 20 mph, it needs 15 yards; at 30 mph, 22.5 yards; at 40 mph, 30 yards. Since its actual Move is respectively 10, 15, and 20, it always has a penalty, which is respectively -9, -14, and -19. (Did I do that right?)

At least that's linear in velocity, like the 3/e rule. But it looks like it will never exempt you from control rolls, and the penalties are much steeper. (Conversely, for your example vehicle with MR 1.50, it looks as if there can never be penalties.)

(I think that in your treatment, MR may define the reference speed for which your vehicle experiences no penalties. That doesn't seem intuitively right.)
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Old 11-25-2021, 09:58 AM   #8
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

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Originally Posted by whswhs View Post
Using the 4/e standard rules, its Move is respectively 10, 15, and 20. Its turning radius is 5, 7, and 10. Its Move is always greater than its turning radius. So it doesn't seem that it will ever have to make a control roll.
If you're only looking at things on a turn-by-turn basis, correct. But you can certainly turn more than once per second - at 40 mph, the driver may want to do a tight turn, turning 120-degrees over the course of, say, 8 yards (turn, move 8 hexes, turn again); doing so is at -9 (TR-1). Of course, the character could actually do a much tighter turn, turning 120-degrees instantly, also at -9.

Quote:
Originally Posted by whswhs View Post
What if we use your rule? The sedan's maximum speed is 30, so we figure its TR at 15, which is 22.5. Dividing average move of 15 (30 mph) by 22.5 gives 0.67 Maneuverability. So TR at other speeds is 1.5x Move. At 20 mph, it needs 15 yards; at 30 mph, 22.5 yards; at 40 mph, 30 yards. Since its actual Move is respectively 10, 15, and 20, it always has a penalty, which is respectively -9, -14, and -19. (Did I do that right?)
If you use a non-scalable penalty, you're looking at -14 (TR of 15), -21 (TR 22), and -39 (TR of 40), but only if you try to turn early - you can make a 60-degree turn, then wait about a round and a half before making the next one, without any penalty or need to roll.

Of course, I recommend against a non-scalable penalty. As I suggested, it will work at least a bit better if you set it to be a penalty equal to TR to do an instant-120 (turning 120 degrees in place), then add a +1 for each yard traveled between the two 60-degree turns (at +0, you no longer need to make a roll). So, above, the sedan moving at 20 MPH (Move 10) and with a Turning Radius of 15 can do an instant 120-degree turn at -15, can do the 120-degree turn over the space of a yard at -14, 2 yards at -13, and so forth - if it turns at the start of one round, then attempts to turn after moving for a full second (moving 10 yards in that time), the driver is at -5.

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Originally Posted by whswhs View Post
At least that's linear in velocity, like the 3/e rule. But it looks like it will never exempt you from control rolls, and the penalties are much steeper. (Conversely, for your example vehicle with MR 1.50, it looks as if there can never be penalties.)
These control rolls are only necessary if you're making two 60-degree turns too close together. If you're going straight, or are only turning after crossing the requisite distance since the last time you turned (which may well take more than one second to cover), you won't need to make a control roll. At least, not for this - road hazards, rapid deceleration, etc may all call for them, of course.
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Old 11-25-2021, 07:22 PM   #9
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

I was (implicitly) assuming that we had just turned before the start of the move. So it seems to me that we would have TR of respectively 15, 22, and 30, and we would move 10, 15, or 20, which would give a penalty of -5, -7, and -10.

I don't think that rule is equivalent to the old rule (3/e), I don't think it's equivalent to (either version of) the new rule (4/e), and I don't think it's simpler to apply. And it seems to give fairly steep penalties. They're not impossibly steep, and I might consider it if the required calculation were simpler, but I don't think it is.

Suppose our vehicle is traveling at 40 mph (Move 20). Making a 60° turn gives it a penalty of -4, which lowers effective Driving from, say, 14 to 10. That seems severe enough to give most drivers pause.

But say we wait one turn before making that turn. Now it's facing half as many Gs per turn (averaged). That takes it down to 1.00g; it's as if it were traveling half as fast. So it has no penalty.

Or for a fancier maneuver, the driver could decelerate to 30 mph, and then make a 60° turn at only -2. Effective Driving drops to 12 rather than 10, which is still pretty manageable.

I think I do like the 3/e rules better than either version of the 4/e rules; the cinematic version is too generous, and the alternate version is too harsh. And I think I can apply the 3/e version without holding up play too much, at least if I interepret it in terms of penalties to Driving (or Boating or Piloting) rolls.

Thanks for all the discussion and for your proposals.
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Old 11-25-2021, 08:37 PM   #10
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Default Re: translating vehicles from 3/e to 4/e: Maneuver Rating

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Originally Posted by whswhs View Post
Well, let's do some math here.

Assume a vehicle with MR 1.00 (to correspond to 4/e) making a 60° turn (to correspond with 4/e) at 20, 30, or 40 mph. We'll give it Basic Speed/Accel 2 to correspond to the sedan in 4/e.

Using the 3/e rules, at 20 mph, it experiences 1.00 g, which is within its MR and inflicts no penalty. At 30 mph, it experiences 1.50 g, and is at -2. At 40 mph, it experiences 2.00 g, and is at -4.

Using the 4/e standard rules, its Move is respectively 10, 15, and 20. Its turning radius is 5, 7, and 10. Its Move is always greater than its turning radius. So it doesn't seem that it will ever have to make a control roll.

Using the alternate rules, its turning radius is respectively 10, 22, and 40. Its Move is equal to its turning radius in the first case (no control roll) but less in the other two. We don't have an explicit rule for penalty, but if we assume a penalty of TR-1 (which I believe you suggested), it has to make control rolls at -21 and -39. That's pretty much impossible. Even at 25 mph (Move 12.5), its turning radius is 15, which gives a -14 penalty.
For this alternate I would suggest a penalty of something like -2 x TR/Move (rounding away from zero), for -0, -2, and -4. This gives similar penalties to 3e in this case at least.
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