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Gangrene
Federated Suns
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 Posted: 05-May-2002 16:07 Post subject: mech stability |
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In my studies of vehicle dynamics it has been shown that the stability of a machine is often dependent upon the speed at which it is traveling. Stability in this case refers to the idea that if a small input is given to the system (the vehicle), the result should be that the system responds in a controlled manner. An real-life example of speed dependent stability is a car: if you travel at 5 mph and put in a sudden impulse by turning the steering wheel by 90 degrees, the car simply changes direction. However, if you are traveling in a car at 150 mph and suddenly turn the wheel 90 degrees, the rear end will probably spin around and the car will lack controled motion.
Because Battletech is coming out with so many speed enhancing technologies (superchargers, MASC, etc), I think it is time to consider mech instability due to high speeds. For a reasonable cutoff point that would not greatly effect gameplay and does make some physical sense, I would like to propose 100 mph (approx. 160 kph). This equates to 15 in movement points. The rule I propose is this:
If a Battlemech uses 15 or more mp's for its movement, a piloting roll must be made at the end of the movement phase. The modifier for this roll would be +1 for every mp over 15 spent.
If the pilot fails the roll the mech skids as per the rules of skidding in the BMR.
Ex. A Dasher engages masc and uses 20 mp in its run. Because it used more than 14 mp it must make a piloting skill roll. The modifier would be +5 (20-15 = 5).
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Talen
Capellan Confederation
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| Posted: 05-May-2002 20:10 Post subject: mech stability |
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But if I am running at an incredibly high speed I can turn on a dime almost. A 4 wheeled car has many weight transfering and suspension differences than a bipedal machine. I dont agree with your idea at all.
There are so many differences between the 2 styles of locomation and handling...
...and if a 'mech is turning after running, it has to slow down...thats one of the reasons a 'mech needs 1 MP to move 60 degrees to the left or right (this is only true at higher speeds really)...the 'mech is slowing down for the turn.
So, again, I dont agree with you on this one.
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Talen
Capellan Confederation
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| Posted: 05-May-2002 20:14 Post subject: mech stability |
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And one more point...when braking a car's tail spin is due to the fact that the motion of the tail exceeds that of the pressure of keeping it glued to the ground. For that reason you will usually not see, say a Formula 1 car, lose the tail end at the end of a braking zone...they have tons (literally) of downforce on the back end. The slip for an F-1 car is usally when accelerating out of a turn.
Mechs weigh enough (again tons) that they are gonna be glued tot he ground. If anything, the 'mech would try turning and the leg would snap...
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"Historians exercise great power and some of them know it. They recreate the past, changing it to fit their own interpretations. Thus, they change the future as well." - Leto II
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Gangrene
Federated Suns
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| Posted: 05-May-2002 21:37 Post subject: mech stability |
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But if I am running at an incredibly high speed I can turn on a dime almost.
No offense, but you probably can't run at an incredibly high speed, or at least the speed necessary to induce instability.
...and if a 'mech is turning after running, it has to slow down...thats one of the reasons a 'mech needs 1 MP to move 60 degrees to the left or right (this is only true at higher speeds really)...the 'mech is slowing down for the turn.
That's an interpretation I don't really agree with. If you look at the slip rules in citytech there is no indication that the unit actually slows down.
There are so many differences between the 2 styles of locomation and handling...
True, but the fundamental dynamics of the contact patch are pretty similar. While a direct comparison is probably too simple, some conclusions can be drawn from it.
And one more point...when braking a car's tail spin is due to the fact that the motion of the tail exceeds that of the pressure of keeping it glued to the ground.
In my example the car was not breaking. A car does not need to break to become unstable. Some vehicles can become unstable just travelling in a straight line and going too fast.
Mechs weight enough (again tons) that they are gonna be glued to the ground. If anything, the 'mech would try turning and the leg would snap...
That is probably a better way to go, but it seems like to would be harder to work into the rules.
Plus realize what kind of mechs this rule would effect. Assaults, heavies, and most mediums would never have a problem. It is the lights and ultra-lights that would be hurt. A 10 or 15 ton mahine trying to run in excess of 200 mph is not going to be real stable, IMO. This rule is just supposed to hurt the "let's supercharge my ultra-light" mentality.
Ideally the critical speed would increase with mech mass, also. But that seems like it would be a pain to implement.
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chihawk
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| Posted: 05-May-2002 21:40 Post subject: mech stability |
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You're forgetting something...a mech has a gyro to keep it upright.
That's why when a mech has taken a gyro hit it must make a pilot roll to avoid falling after a run or jump...
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Gangrene
Federated Suns
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| Posted: 05-May-2002 21:43 Post subject: mech stability |
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One more thing that comes to mind: do you know why most cars are moving towards rear-wheel drive? Its because when power transmission and turning forces are coupled to the same comtact patch slip is more likely to be induced. If a mech is running at a significant speed, it is safe to say that it has only one foot on the ground at any one time (with maybe a brief overlapse). So if it is turning then both turning forces and power transmission are acting on the same patch.
So I don't see instability as an unreasonable consequence.
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chihawk
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| Posted: 05-May-2002 21:45 Post subject: mech stability |
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Ummm, aren't the clear majority of cars front wheel drive?
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Gangrene
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| Posted: 05-May-2002 21:48 Post subject: mech stability |
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Quote:
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On 2002-05-05 21:40, chihawk wrote:
You're forgetting something...a mech has a gyro to keep it upright.
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I didn't forget that, chihawk. Gyros can only do so much, and can be overcome. Anyways, the initial instability would probably be a side-slip sort of motion, so a gyro for standing up really wouldn't help all that much.
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Gangrene
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| Posted: 05-May-2002 22:01 Post subject: mech stability |
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Quote:
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On 2002-05-05 21:45, chihawk wrote:
Ummm, aren't the clear majority of cars front wheel drive?
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A lot of modern and older peformance-based cars are rear-wheel drive, such as the Dodge Viper or Ferrari 550. A lot of the newer cars may have all-wheel drive, in which power transmission is actually controlled electronically.
I think you are right, though, in that a lot of regular cars are front-wheel drive (alhough I'm not sure if it's a majority).
As for the comparisons with mechs, I'm assuming that there are not a lot of Honda civic-caliber mechs in the militaries. Plus cars have 4 contact patches to work with instead of 2.
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[ This Message was edited by: Gangrene on 2002-05-06 00:51 ]
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Talen
Capellan Confederation
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| Posted: 05-May-2002 22:05 Post subject: mech stability |
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That's an interpretation I don't really agree with. If you look at the slip rules in citytech there is no indication that the unit actually slows down.
Whats not to see? If a mech traveling 16 hexes turns, they are spending 2 MP in ONE hex. That means they are moving half as fast through that hex if they were just running through. Example. Mech is moving at 10 hexes a turn. If a turn equals 10 seconds...they are moving 300 meters in 10 seconds. Now, they turn in one of those hexes...they are now only moving a distance of 270 meters, in the SAME amount of time. The ONLY reason why a 'mech would go less distance in the same amount of time is if they were going slower, right? As for the skidding rules...that is probably due to the fact that metal on cement gets less traction...and its onyl after turning anyways.
In my example the car was not breaking. A car does not need to break to become unstable. Some vehicles can become unstable just travelling in a straight line and going too fast.
Um...how so? Never heard of a car that could lose stability by going straight. Stability loss would come from side to side movement....
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Gangrene
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| Posted: 06-May-2002 00:05 Post subject: mech stability |
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Whats not to see? If a mech traveling 16 hexes turns, they are spending 2 MP in ONE hex . . .
Yeah, it is the best explanation for the speed of a Battlemech. Sometimes, though, I view mp's as more of a power output than a speed abstraction.
Um...how so? Never heard of a car that could lose stability by going straight. Stability loss would come from side to side movement....
It is possible at fast enough speeds for vibration modes to become unstable. In such a case a small disturbance in the steering could oscillate and grow out of control. I don't think that has happened to cars in a while (modern cars do not have a problem with this); it is more common on motorcycles and trailers (trains too, but they don't go fast enough to reach their critical speed).
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[ This Message was edited by: Gangrene on 2002-05-06 00:49 ]
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Talen
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| Posted: 06-May-2002 00:50 Post subject: mech stability |
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Yeah, but that has to do with the air affecting the car. Thats 200mph of wind affecting a 3000 lb car...not 150mph of wind affecting a 30 ton 'mech.
The speed instability would also have to do with air getting under the car, and at that speeds its dangerous...but again, a 3000lb car, 200mph of wind, not a 30 ton mech.
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Gangrene
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| Posted: 06-May-2002 01:01 Post subject: mech stability |
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Yeah, but that has to do with the air affecting the car. Thats 200mph of wind affecting a 3000 lb car...not 150mph of wind affecting a 30 ton 'mech.
The speed instability would also have to do with air getting under the car, and at that speeds its dangerous...but again, a 3000lb car, 200mph of wind, not a 30 ton mech.
That's one cause, but there are numerous others that depend on the surface of travel (bumps, pressure distribution, etc). And if a 15 or 30 ton machine is putting enough power into the ground to go 150+ mph the ground could shift under their feet.
Anyway, its not like 30 tonners would have a real problem with this unless you use L3 sprinting rules. If 100 mph is too unreasonable for a limit it can always be kicked up to something more desirable, say, 20 MP's. The only intent of the rule was to penalize turbo-charged ultra-lights and the like.
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Talen
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| Posted: 06-May-2002 01:43 Post subject: mech stability |
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Okay, I might be able to see the ground shifting, but you'll need to apply some physics here. A 30 tonner moving at 100kph will produce some 300tons of pressure (arbitrary number, really), and a 100 tonner moving at 30kph will be puting down 300tons of pressure. (see what i mean? a heavier mech can still produce the same pressure). The ground isnt really going to be gicing way any more...so i cant see that one used.
And I see what you are trying to do...but here is what you are really ddoing. You are taking away what light 'mechs rely on most, speed. A 30 tonner is meant to go fast, thats how it survives...
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Gangrene
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| Posted: 06-May-2002 02:55 Post subject: mech stability |
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Okay, I might be able to see the ground shifting, but you'll need to apply some physics here. A 30 tonner moving at 100kph will produce some 300tons of pressure (arbitrary number, really), and a 100 tonner moving at 30kph will be puting down 300tons of pressure. (see what i mean? a heavier mech can still produce the same pressure). The ground isnt really going to be going way any more...so i cant see that one used.
In order to create forward motion has to push laterally against the ground, just like cars, trains, etc. While your numbers are questionable, emphasizing them misses part of the picture. The reaction forces acting on the ground are going to be different between the two, with the fast light mech producing a more shallow traction vector (for lack of better description).
And I see what you are trying to do...but here is what you are really doing. You are taking away what light 'mechs rely on most, speed. A 30 tonner is meant to go fast, thats how it survives...
But the mp requirement is so high that it effects only a handful of FASA L2 designs. The Jenner, Spider, Oscout, Wraith, Hussar, Locust, and others are all unaffected. So the Dasher and Fireball take a hit . . . big deal.
Plus at some point the excess speed gets ridiculous. A 20 ton biped machine getting Dodge Viper speed on open terrain without any handling problems is BS. If my crappy targeting system cannot compensate for his movement, his control system sure as heck cannot achieve the fine level of adjustment that would be required for such maneuvers.
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[ This Message was edited by: Gangrene on 2002-05-06 02:56 ]
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