out of curiosity does extra weight (lets say a 200lb man) during highway pulls (getting up to speeds around 140) have more of a effect on performance then during drag racing? you can see my mods in my sig that and I have removed PS,AC, air pump, all cats, and all emmissions so I have taken out a good 100lbs so far.

 

No, it becomes less of a factor as speeds get higher, drag really starts to shoot up around 120.

 

Eh? It has the same effect. F=MA doesn't change because you're going faster. Yes there are additional forces that have exponentially larger impacts, such as drag as you mentioned, but weight still has the same requirement for X force to accelerate X mass by X MPH. One could actually argue that it has more impact because of the benefit of increased torque multiplication at lower speeds from closer ratio gears.

Kevin T. Wyum

 

Marshall is right.

Think of it this way. Motorcycles (crotch rocket types) can to 10s and 11s easily but they really don't go much faster top speed than a stock FD than runs basically 14 flat stock. The FD has a bunch of weight to hold it back down low but as speeds increase they both are fighting the good 'ol atmosphere.

 

Hmmm. would weight effect it more my car being auto and the crappy torque multiplication through gears?

 

Your car is slower from a roll than a manual simply because of gearing.

 

The problem with weight is overcoming inertia. From a dead stop this is obviously a greater challenge, but as velocity increases, it becomes less of an issue.

 

Weight usually only comes into play in cornering and immediate drags(1/4mile stuff).

 

SOB I had a nice long explanation typed out and F'ing tab key lost it all.

I'll try to give the cliff notes version.

It takes X force to accelerate an object from 0 to 30mph

It takes Y force to accelerate the same object from 100 to 130mph (includes drag etc.)

It takes Z force to accelerate a 100kg object from 0-30mph.

If you add that 100kg to the first object without changing the cross section and ignoring things like increased frictional load on wheel bearings etc. it will take X+Z force to accelerate the new 100kg heavier object from 0 to 30mph.

It will take Y+Z force to accelerate the same heavier object from 100-130mph.

It takes the same increase in force to accelerate the extra mass regardless of the frame of reference you are observing from.

The perception that it impacts it less at higher speeds is a result of proportion. The massive increase in other resistance such as drag and friction results in the force required to accelerate that extra mass being a much smaller total percentage of the force required to accelerate the object that 30mph, but that extra mass still requires the same amount all the same. So as I mentioned, it has the same impact, but you perceive it to be less with it's proportionate decline in the total force required.

Bleh the first time around was a better explanation : (

Kevin T. Wyum

 

thanks for the explanation. its always nice to have real hardcore facts

 

I agree with Kevin, the 200lbs person has the same effect on your acceleration in either case (high or low speed). The difference is that at high speeds you ALSO have many other factors like drag, ect which is one reason you cant accelerate as fast at high speeds as low speeds. Its not that the weight became less important its just that you added more additional factors besides weight.

 

Actually this statement is incorrect.

All the forces the car has to fight to accelerate, wind drag, rolling resistance, etc increase as velocity increases, wind drag increases dramatically, this reduces accleration dramatically. Since the accleration of the car is dramatically slower it doesn't use as much force to acclerate the mass of the car (including the 200lb person).

Look at lightweight cars like the Elise vs. heavy cars like the Viper. They both have similar low speed accleration, top speed however is dramatically different. Go karts would be another good example, great low speed accleration, no top end vs a car with the same/similar power to weight ratio.

 

Damn, Jeff types faster then I.
Anyhow, lets use the example of a 2800lb car and a 3000lb car (2800+200lb pass). And let's assume it takes 3x longer to accelerate from 100-130 (5ft/sec^2) then it does 0-30 (15ft/sec^2). Numbers picked for easy math, but it should be in the ballpark.

Force to accelerate:
lbs..........ft/sec2.........ft*lbs/sec2
3000.........15..............45000
3000..........5...............15000

2800.........15..............42000
2800..........5...............14000

The force does change proportionally; It has to by virtue of the formula. In a perfect world, the 2800lb car would out accelerate the 3000lb car by the exact same percentage at any speed.
Seeing as how it's not a perfect world, differances in drag coefficients between vehicles far outweighs a minor mass differance at high speeds.

 

High school physics doesn't explain a lot of real world facts

 

Heh, glad someone wrote it out. What I meant was, getting rid of that huge wing will have a greater effect than ditching 200lbs if you're running to 140mph. Other way around on the dragstrip bc, as you said, proportionality.

 

GRRR same damn thing again with the tab key.

Why did you use an observed acceleration that includes drag, friction, gear multiplication etc. That wasn't the point. You've just told us that it takes more force to overcome wind resistance etc. at 130mph than it does at 0mph. I think everyone got that. We also knew it takes more force to accelerate the object with more mass. I'm really confused about what you were trying to demonstrate.

Kevin T. Wyum

 

I think what he was trying to is that since your not accelerating as fast at higher speeds due to the agreed upon extra drag it takes LESS force to accelerate the weight.

If acceleration were to remain constant then the power required to accelerate the extra 200lbs would remain the same. Since we know the rate of acceleration drops off dramatically so does the effect of the extra weight.

Edit:

If you take a look at his example you can see the difference in power required to accelerate the two vehicles is 3000 lb*ft/sec^2, or

As the vehicle reaches speed that difference required to accelerate the two vehicles is 1000lb*ft/sec^2.

Therefore it takes less HP to accelerate the weight at higher speed. Weight does not have the same effect on vehicle accleration a 0mph as it does at higher speed, the faster you go the less it matters.

 

Time is a function of power though, not force. The same force is used to accelerate the object 30 mph regardless of from 0 to 30 or from 100 to 130 no matter the time taken to do it. (excludes drag etc.) The time it takes to accomplish that work is a function of power, shorter time to accomplish the same task = more power. Maybe it's best to put it this way. If there were no wind drag or mechanical friction and gear ratios were the same there would be no difference in the time that it took from 0-30 or 100-130.

 

If your accelerating you can't exclude time. Your dealing with power here.

Your right, if you excluded wind drag, rolling resistance, etc then there would be the same power required to accelerate from 0-30 and 100-130 in the same amount of time.

In the real world, which is exactly how the question was asked, you can't exclude any of those things. Bottom line, the extra weight doesn't make nearly as much difference from 100-130mph as it does from 0-30mph.

 

Does it have more of an effect on acceleration during "highway pulls" than during drag racing? No. Does it have a detrimental effect? Yes.

 

Additional weight affects a car at any speed, as Kevin stated. Mass is basically constant (with the exception of fuel consumption, unless you're tossing cargo or passengers out the window) for any calculation of force or acceleration for a given vehicle, so an extra 200 pounds affects the rate of acceleration of that vehicle from both 0-30 mph and 100-130 mph proportionately to how quickly it could have accelerated without the added weight.

Acceleration is defined by power and weight, as Newton's second law tells us. Everyone knows (or should know) that you must increase power or decrease weight to increase the rate of acceleration. Changing gearing simply increases or decreases the available power at the axles, so for all intents and purposes it can be considered to increase or decrease power also. Power at the axles is what we're concerned with, because it defines the rate of acceleration in any gear, as well as top speed.

The reason the rate of acceleration decreases at higher speeds is not only due to increasing air and rolling resistance, but because force is also decreasing while weight stays constant. Available engine (flywheel) power is finite and there's progressively "less" of it to do work with at the axles as gearing multiplication decreases in the higher gears. Remember, a vehicle's rate of acceleration follows the engine's torque curve, multiplied by the gearing.

The chart below showing drive power at the wheels in each gear for a stock FD illustrates this. The rate of acceleration decreases and the time to reach redline in each gear increases as speed increases, due to the factors mentioned above.

 

see now what iam wondering is if with everygear since there is less torque multiplication through the gears does weight and drag have more of a impact on acceleration.

 

No matter what all this mumbo jumbo says.

In a real car, in the real world weight has less of an effect on acceleration the faster you go.

No one can argue with that statement, well unless they are wrong.

 

Newton's second law is "real world" also. Let's review it...

F = M * A

For our purposes, it is best to rearrange the equation in terms of acceleration.

A = F / M

Acceleration is directly proportional to force. If you push an object twice as hard, it will accelerate twice as fast, assuming mass remains constant.

Acceleration is inversely proportional to mass. If you double the weight, the object accelerates half as fast with the same force.

At higher speeds, I have proven that Force at the wheels decreases, due to gearing, air, and rolling resistance. As Force decreases, so must acceleration. Mass, however, remains a constant.

As Newton's equation shows, Mass actually has an increasing effect at higher speed where Force is decreasing. Reducing Force has the same effect as increasing Mass; acceleration decreases.

How about a few CarTest 2000 numbers where weight is the only change to back this up?

0-30 mph
2,800 lb. FD, stock engine - 1.93 sec.
3,300 lb. FD, stock engine - 2.15 sec. (+0.22 sec., +11%)
3,800 lb. FD, stock engine - 2.36 sec. (+0.43 sec., +22%)

100-130 mph
2,800 lb. FD, stock engine, 4th gear start - 10.31 sec.
3,300 lb. FD, stock engine, 4th gear start - 12.24 sec. (+1.93 sec., +18%)
3,800 lb. FD, stock engine, 4th gear start - 14.36 sec. (+4.05 sec., +39%)

If weight had less of an effect at high speed, you would expect the variance between the 100-130 acceleration times to be less than the 0-30 times, but it isn't. The variance is actually significantly higher. Obviously Mass has even more affect at higher speeds.

I know it seems like weight would have less of an effect at higher speed, once the chore of getting the car into motion is over, but that is not the case. That is simply something Supra owners tell themselves to feel better about having to have far more power to accelerate as quickly as an FD owner can with less. Their gearing and the fact that they can make more power is their only advantage.

 

If you're talking about aerodynamic drag, that won't come into play until around 100 mph with an FD. Up to that point (all things being equal in a real world situation), less weight will have more of an effect.