BMW speed limiters

[2o6]

It doesn't?

[SVT_Power]

[Soup DeVille]

It doesn't?

No, it doesn't.

Weight effects the rate of acceleration, but top speed is determined by the point at which aerodynamic and mechanical drag can no longer be overcome by the power sent down the driveline.

Take a look at this:



That's the world's fastest diesel truck. Can you think of a reason why the frame rails have been filled with lead?

[Soup DeVille]

I'm technically not wrong when I say top speed is only limited by power right?

No, you're not.

[2o6]

That's the world's fastest diesel truck. Can you think of a reason why the frame rails have been filled with lead?

To keep it on the ground?

[Soup DeVille]

To keep it on the ground?

Yep. On a race car, you would add a wing to create downforce at speed so the vehicle remains stable, but that also creates drag; and land speed racers are after every last ounce of speed.

So, they typically add weight.

They also pay very close attention to the aerodynamics of their vehicles: often taping up door seams and spraying the surface down with WD40 to keep the salt from sticking and fouling the airflow. Obviously, they also add as much power as they can.

What they don't wrry about is weight: weight is their friend.

[MX793]

More weight = more rolling resistance.  On a 3200 lb car with low resistance tires, rolling resistance will be eating up over 10 hp at 120 mph.  Significantly lower than what aero drag is consuming (which would probably be close to 100 hp on a typical family sedan), but it's not entirely negligible either.

[SVT_Power]

So, add more power

[Soup DeVille]

More weight = more rolling resistance.  On a 3200 lb car with low resistance tires, rolling resistance will be eating up over 10 hp at 120 mph.  Significantly lower than what aero drag is consuming (which would probably be close to 100 hp on a typical family sedan), but it's not entirely negligible either.

As you go faster and faster, it becomes less important. Rolling resistance pretty much stays constant, but aerodynamic resistance increases exponentially.

[Minpin]

As you go faster and faster, it becomes less important. Rolling resistance pretty much stays constant, but aerodynamic resistance increases exponentially.

I like to think of it as terminal velocity, but you can fix our problem. POWER!!!!! (Jeremy Clarkson reference off) 

[MX793]

As you go faster and faster, it becomes less important. Rolling resistance pretty much stays constant, but aerodynamic resistance increases exponentially.

True, but its role is greater when you're talking about vehicles that couldn't exceed 150 mph with or without rolling resistance.  Although when all is said and done, even if it is eating up 10% of your available horsepower, that would only equate to a couple of mph difference even if it was removed from the equation altogether, let alone merely reduced by 10 or 20 percent via a weight reduction.

[Raghavan]

No, you're not.

What about drag?

[Autobahn]

There was much dispute about cars going too fast on the unlimited parts of the Autobahn some time ago. The big German automakers thus issued a "gentleman's agreement" that they would limit the topspeed of their cars to 250 km/h ~155 mph. This prevented at that point in time that speed limits on the Autobahn were further discussed. However, some manufactureres such as Porsche never joined in that's why Porsches generally are faster than 155 mph. M cars and AMG cars also have the speed limiter at 250 km/h but can be ordered without that. Some manufacturers require high-speed training for the owner in order to remove the limiter.

Today speed limits on the Autobahn are under heavy discussion with regard to the CO2 emission discussion. It's a debate that doesn't make sense with gas prices as high as they are today not many people are going full-throttle from Munich to Hamburg anyway (this is not possible because of heavy traffic and speed-limits and cameras anyway)...

Your 330 probably has "American" gear-ratios tuned more for 0-60 than for top speed. The German spec 330s should reach 250 km/h as far as I know.

[Galaxy]

I think one reason for the limiters is that most americans use all season tires.

[omicron]

What a crisp convertible, that 330Ci.

[FordSVT]

Top speed is really only determined by these things.

The horsepower of the engine and the necessary gearing to overcome some mechanical but primarily aerodynamic friction.

Weight has little to do with it, other than the small role it plays increasing your rolling friction. That's not much, and it's a relatively constant value whereas aerodynamic forces can start climbing into the hundreds or even thousands of pounds at higher speeds and chew up a massive amount of horsepower.

Weight primarily influences acceleration. The car has to overcome the vehicle's static inertia. So it will definitely take a heavier car longer to get to top speed than the same car with 500 pounds of it removed, but their top speeds should stay fairly close.

[Raza]

I'm technically not wrong when I say top speed is only limited by power right?

And gearing. 

If your gearing doesn't allow you to reach a speed, you can't get there.

As far as weight is concerned, it does matter less and less as speed increases, but I'd say that if you took two cars, given the same aerodynamic properties and power, with one weighing 1000 pounds and the other weighing 5000 pounds, the 1000 pound car would have a higher top speed.   

[SVT_Power]

Oh yes I forgot about gearing.

[FordSVT]

And gearing. 

If your gearing doesn't allow you to reach a speed, you can't get there.

As far as weight is concerned, it does matter less and less as speed increases, but I'd say that if you took two cars, given the same aerodynamic properties and power, with one weighing 1000 pounds and the other weighing 5000 pounds, the 1000 pound car would have a higher top speed.   

There would be a few percentage points different due to a difference in rolling friction, but that's it.

The point remains that in the real world, under real conditions, and once you put your physics book and calculator away, weight is overwhelmingly a secondary factor when determining top speed. It's all about the power/drag ratio. Weight can provide stability when faced with the absence of aerodynamic down-force aids like spoilers, and because those spoilers usually decrease potential top speed by increasing horizontal drag, it's often a better idea to make a car a bit heavier than lighter if you're shooting for a top speed crown.

[Soup DeVille]

And gearing. 

If your gearing doesn't allow you to reach a speed, you can't get there.

As far as weight is concerned, it does matter less and less as speed increases, but I'd say that if you took two cars, given the same aerodynamic properties and power, with one weighing 1000 pounds and the other weighing 5000 pounds, the 1000 pound car would have a higher top speed.  

I doubt it. It would howveer, take a lot longer for the heavier car to get there.
And every body always brings up gearing.

Gearing is just another factor that determines how much force can be apply at a given speed, or to be more precise : it's just another thing that determines power at speed.

[r0tor]

I doubt it. It would howveer, take a lot longer for the heavier car to get there.
And every body always brings up gearing.

Gearing is just another factor that determines how much force can be apply at a given speed, or to be more precise : it's just another thing that determines power at speed.

accept when your car's top speed is limited by gearing and you find your self bouncing off the rev limiter in top gear

[Soup DeVille]

accept when your car's top speed is limited by gearing and you find your self bouncing off the rev limiter in top gear

At that point it cannot make any power at any higher speed.

[hounddog]

Sorry, Trep.
No matter how much you wish your car to be free of a limiter, it has one. 
The police versions have one, so you can bet your scrawny little ass that the civilian version has one. 

Read this link, page four, the fine print at the bottom of the speed graph.
http://www.nlectc.org/pdffiles/msp2003bulletin.pdf

[TheIntrepid]

Sorry, Trep.
No matter how much you wish your car to be free of a limiter, it has one. 
The police versions have one, so you can bet your scrawny little ass that the civilian version has one. 

Read this link, page four, the fine print at the bottom of the speed graph.
http://www.nlectc.org/pdffiles/msp2003bulletin.pdf

Scrawny little ass,

I'll see it when i hit it.

[hounddog]

Gearing is just another factor that determines how much force can be apply at a given speed, or to be more precise : it's just another thing that determines power at speed.

Not force, energy.  Otherwise, I have no problem with your arguements in this thread.

Actually, I do have one slight problem; there will always be rolling friction involved.  The reason is tires always contact the road, and that contact does create friction, which does affect power and speed.  How much friction depends of course on the surface of the road or track, and must be taken into consideration in concert with drag.

[hounddog]

Scrawny little ass,

I'll see it when i hit it.

Your ass, or the limiter?

[TheIntrepid]

Your ass, or the limiter?

Easy there, officer

[hounddog]

Easy there, officer

Well, I have been drinking.  And, my team lost.  At least they put up a good fight!

[Soup DeVille]

Not force, energy.  Otherwise, I have no problem with your arguements in this thread.

Actuall, I do have one slight problem; there will always be rolling friction involved.  The reason is tires always contact the road, and that contact does create friction, which does affect power and speed.  How much friction depends of course on the surface of the road or track, and must be taken into consideration in concert with drag.

I never said that wouldn't be rolling friction: what I said was that it's effect is minimal at high speeds.

The energy/force thing depends on whether or not you want to use time as a part of your calculations: an answer can be found either way.

[hounddog]

I never said that wouldn't be rolling friction: what I said was that it's effect is minimal at high speeds.

The energy/force thing depends on whether or not you want to use time as a part of your calculations: an answer can be found either way.

Well, if you are going to have any calculations to determine speed, you have to have time and energy. 

But, I still say energy because at top speed the engine is no longer providing force as it does in acceleration, it is providing energy for constant speed.

As we all know two things needed to calculate speed are energy and momentum.  The others are time and distance.