So, another basic question - what's the difference between the speed of a vehicle and the acceleration ratings? I know that the first accel. number is walking rate (move that rate without an action), and the second is running rate (complex action to move that speed), but how do those numbers relate to speed? Or do those acceleration numbers stack from previous turns? Or am I completely off base?

Speed is velocity at the moment, acceleration is the rate at which you increase/decrease speed.

The rulebook uses the term "Acceleration" but the definition on page 159 gives two versions each in terms of speed

The same page defines "Speed" as the vehicles maximum reasonable speed.

As far as I can see at the moment, the book does not really clarify how to use the two together. I would suggest that you use the first "Accel" number as automatic when desired, the second "Accel" number as possible if the driver succeeds at a Vehicle Test, and the "Speed" number as the max without a penalized test.

I was confused by this as well when I first read through SR4, but I think I have a handle on it now.

The Speed rating is the maximum velocity a vehicle can travel without "suffering real difficulties" (as the BBB puts it).
You are correct that the first acceleration number is the walking rate and the second is the running rate. However, the statement of requiring a "complex action to move that speed" is not strictly true as I read the rules. The key thing to remember about SR4 is that Vehicle movement rules now follow the same pattern as metahuman movement rules. I'll try to explain in detail here. When a vehicle is stopped, its current speed is 0 (obviously), so if a character wants to get the vehicle to go somewhere the vehicle needs to accelerate. A vehicle may increase its current speed by an amount up to its walking rate with no test, and no expenditure of an action (not even a free one). Bear with me, this will make sense. Just like a character takes all of the IP's in a round to travel their walking distance, a vehicle takes all of the IP's of its driver in order to increase its current speed by the walking acceleration rate.

Let's take the GMC Bulldog Step-Van, its Acceleration ratings are 5/10. So, with no test and no action it can go from 0 to 5 meters/turn (if it started at 0), or if it started with a speed of 15 it could go to 20.

A driver may make a Free action to accelerate a vehicle's current speed by its Running rate. Note that it is the act of accelerating itself that takes the Free Action.

Using the GMC van example again, a driver could take the vehicle from 0 to 10 meters/turn, or 15 to 25, and so on, using a Free Action. Theoretically, this acceleration takes places over all IP's of the combat (it will reach it's target speed after all actions in the round are complete), but if you want to avoid some record-keeping craziness, assume that the vehicle reaches its new speed instantly and continues to travel that speed for all remaining IP's. So if you want to determine how far the vehicle moves with each IP you'll need to divide its current speed by the highest number of IP's present in the battle. I've seen many DS people simplify this by assuming there will always be 4 IP's in a round even if no characters have 4 IP's of actions. So if a vehicle has a current speed of 25 it will travel 6.25 meters per IP.

But isn't there a Complex Action involved in there somewhere? Yes. On pg. 160 of the BBB under Tactical Combat it states that (parts in parenthesis I'm assuming):

The way I read it, if the vehicle is moving at any speed above 0 the Driver must spend a Complex Action in order to maintain control that round. Extra IP's that the Driver has can be spent on other actions. The Driver of a vehicle can also spend a Complex action to accelerate the vehicle beyond its Running Rate that round by making a Vehicle Test. Every hit on the Vehicle Test adds 5 extra Meters to the distance the vehicle is going to move this turn (its current speed).

Also, the answer to your "do those values stack from previous rounds" question, the answer is: yes. If the vehicle had accelerated to 10 the previous round, it could accelerate up to 20 this round.

That is how I read the rules, and after thinking about it, they did make sense to me. Other interpretations would have semi-athletic Trolls regularly outpacing a vehicle driven by a regularly-skilled person.

There are standard definitions for those terms that work too.

Acceleration is change in speed. Speed is speed.

If you are going 35 units of speed and accelerate 10 units of speed, you end up at 45 units of speed.

Speed is how fast you are going (or how fast you CAN go in this istance) acceleration is how fast you get going.

Speed is a number given in the vehicle stat blocks that has no clear rules defining its use / usefulness. Acceleration is a number given in the vehicle stat blocks that has no clear rules defining its use / usefulness.

That clear things up for you? >.>

Really it is pretty clear. Speed is how fast your vehicle can safely travel measured in meters per combat turn(this can be converted to km/hr with some simple math, the dodge scoot for instance can travel safely at 72 km/hr or about 44 m/hr, not highway speed but definatly reasonable.) Acceleration is how fast the vehicle can change it's speed, once again in meters per combat turn. The first number is the basic acceleration, the second number is the "run" number if you take a free action to book it.

So, to take an example with the dodge scoot, that has an acceleration of 10/15, and a speed of 60. John the shadowrunner is trying to get the hell out of dodge on his scoot, he jumps on(having wirelessly started the engine before getting there) and starts to accelerate. At the end of the first combat round he is now moving 10 meters/turn, and has traveled 10 meters from his starting point. During the next turn some gangers catch up and start shooting at him, so he spends a free action to punch it to go faster. He is now going 25 meters/turn(15 over the 10 from before), and has traveled 35 meters from his starting point(10 from last turn, 25 from this turn). He does the same in the third round, now traveling 40 meters/turn, and having traveled 75 meters. On the next turn he continues going 55 meters/turn, and having traveled 130 meters. He continues to accelerate on the 5th turn, but only moves up to 60 meters per turn, the maximum safe speed for his vehicle. He could travel faster by making a vehicle test, but he would risk crashing/blowing the engine.

What about slowing down then?

Very few vehicles use their main drive to decelerate and many may, by design, be inclined to stop more suddenly than they speed up.

That's a perfectly reasonable set of rules - wasn't SR3's rules something like that? After rereading the rules (I was going from memory before), I see that there's absolutely no RAW justification for this interpretation, but I like it, and it matches real-world physics.

Thanks OOB and Malachi!

(Edited to give credit to Malachi too. Forgot to quote him when I posted)

Malachi, I like your take. I'm going to use that in the future. Thanks!

*tips his hat*

My very first SR character was a Rigger (back in SR2! You think drone rules are bad now?), and I continue to have a special place in my heart for vehicle related stuff. When SR4 hit the shelves I made sure I had a solid understanding of how vehicle stuff worked.

EDIT: An addition to the post I made above: When I GM, I count a Complex Action used to Accelerate as the 1 Complex Action per round spent "driving." It seemed unnecessary to me to have to spend a Complex Action to "go really fast" and then spend another Complex action just to "maintain control." Since the extra Acceleration test requires a Vehicle Test to be made, I rule that this Vehicle Test pulls "double duty."

If anyone else doesn't follow the math: 1 Combat Turn is 3 seconds, so 20 in a minute, 1200 in an hour. There are 1000 meters in a kilometer. So converting meters/turn to km/hour means multiplying by 1200/1000 or 1.2. The Dodge Scoot with a speed of 60 * 1.2 = 72 km/hr.

Speed is the maxiumun velocity of the vehicle, acceleration is the walk/run rate of the vehicle, expressed in meters/combat turn.

Very good legwork here indeed.
Do anyone mind if I tag along DireRadiant with a question? (Like anything is going to stop me;))

Do anyone know if there are rules for breacking?

There are no published rules for braking. I would play it as a simple vehicle test with the Threshold set by how fast the vehicle is going and how quickly (in what sort of distance) the character needs it to stop.

To go along with what is already there, look at page 159 and the Vehicle Threshold Table. If you are decelerating more then the Run speed, then do a Vehicle Test. Use the Sudden Stop Threshold, which is one.

Thats how I've always handled it. It is actually quite difficult to slow down quickly from a fast speed without losing control of your car. You can usually safely break at about your acceleration rate(if it takes you 3 seconds to go from 0 to 35mph in your car, it will take about 3 seconds to go from 35 to 0). You can stop faster, but even with power brakes you are going to skid and have trouble driving straight unless you are good.

Thank you, most helpful.

Not sure how I missed that...

Truth be told, the vehicle rules require a little more...interpretation then most the other rules of the game, what I and malachi suggested is more a reasonable interpretation created from what we are given then something that is clearly defined. I find that several points in the core rule books are not defined clearly enough for my tastes. The way posted above is most likely what was intended by the rules, though I find that rarely does it matter in a game. Faster cars win drag races, and street chases are more about opposed driving tests then speed.

Agreed on the vagueness and needing interpretation. The "chase combat" rules are one of the few sections of SR4 that I completely despise. I have never found it difficult to keep track of relative vehicle speed and distance in a "chase" scenario, so I've never had need of any of the bizarre abstractions that have appeared in SR rules for riggers ("Maneuver Score" I'm looking at you. Get back in that hole I stuffed you in).

Having rules for exactally how fast a car accelerates can be useful though, I had a troll adept once who could run almost highway speeds, so he could outrun most vehicles that hasn't gotten to full speed yet, which made it very difficult for people to run away.

It's also useful to see if you get out of range before they launch that last burst of fire off at you as you speed away, for slow vehicles (the 5/10 ones) to see how long it takes them to get up to a reasonable speed, and for getting up to ramming speed

Completely agree. The problem with the "chase combat" rules as presented in the BBB is that they don't take vehicle speed into account... anywhere. By those rules a kid on a bicycle could catch up to a Eurocar Westwind if the kid rolls a better vehicle test. Oops.

The first step in combat involving vehicles is to choose between Tactical Combat and Chase Combat.

If you use Chase Combat mode for what should be Tactical Combat, then you should not be surprised by the results not being what you like.

If you choose Tactical Combat instead of Chase Combat, then do not be surprised if you don't get the results you want.

My opinion is that the "kid on the bicycle chasing a Eurocar Westwind" is a Tactical Combat, in which case the Eurocar Westwind is going to run away from the kid on the bicycle very quickly. I wouldn't even allow the scene to be Chase Combat. Since I don't use Chase Combat rules for the scene, I don't need to worry about using all the chase rules and how they don't work in Tactical Combat.

Of course if the "kid on the bicycle" has the right bike and cyberleg options there may be an opportunity for Chase Combat.

edit: One way to identify if Chase Combat may apply is if the vehicles involved can all attain the appropriate speed for the environment in which the scene is occurring. The clue that they shouldn't be in Chase Combat is when it's reasonable that the vehicles don't operate in the same order of magnitude in speeds.

Yup, basically the chase combat rules should only be used with both/all vehicles have little trouble reaching the highest speed possible for the enviroment and it boils down more to driver skill and maneuverability. When you are in a motorcycle chase darting between shipping containers at the docks, darting through alleys and dodging cars, as long as you are not completely outclasses in terms of speed, speed is largely irrelevent. However, once the chase leads to a straightaway with no obstacles, then the faster vehicle will quickly outrun the slower one.

Another words, don't use the chase combat rules until someone starts weaving in and out of traffic and other difficult spots to lose a tail.

I have a quick question, and this seemed the place for it. If the driver slams on the brakes do any of you think the passengers who are not wearing restraints should have to make some kind of check to keep from being in the front seat/plastered to windshield?

The reason I ask is that it seems funny to me to think of the hacker (still jacked in) playing suction-cup Garfield on the inside of the windshield without even being aware of it.

I would say that the hacker (if not knocked inconscious) would at least have the feeling that something is not completely right, while in VR there's a penality to perception tests, but I would say that being trown on (if not trough) the windshield it something that has a negative treshold (you need an effort to not notice it).

Ok, but would they have to make a test if not strapped in, hacker or not? Or does everyone just assume they automatically adjusted?