Wisconsin S2k

I have seen people argue this before, and recently I read a thread on another forum that said weight will not affect your top speed.

To me this doesn't make sense. maybe some physics wizards can answer the question for sure, so correct me if I'm wrong.

To my understanding, there are multiple forces working against a car's forward acceleration. Rolling resistance, air resistance, mass, etc. A car's forward acceleration force is obviously from the engine making power and turning the wheels to make it go. As a car goes faster, it's forward force gets smaller and smaller until it reaches top speed. A car will reach it's top speed once it's forward driving force is equal to it's resistant force.

Now my question is this. The top speed is reached when the car is no longer accelerating. Since acceleration directly involves the mass of the car (F=ma), how can the top speed of the car not be affected by weight?

In other words, let's say a car can go 150mph. If you take out 200lbs, would this not reduce the force resistance working AGAINST the car's forward acceleration? Therefore giving it an overall lower resistance force. Then maybe the car can top out at 155mph?

YellowS2kPwr

This is a really good question. I'm not 100% sure, but yes I think weight does have something to do with top speed. I think a Turbo Suzuki Hayabusa can hit 220+ mph, hoewever that might be due to the hp/weight ratio. I have also seen the very heavy Mercedes Benz SL600 hit 189 mph. The top speed is ultimately dependent on gear ratios (in a perfect driving enviornment) the way I see it. So if a car tops out at 150 mph (rev limited), then no its not going to go 155 if you lose 200 pounds, I know you know this but I just wanted to state it. I'm no physics wizard, but the weight of the car doesn't matter nearly as much once it's rolling.

oknessad

hmm i see your argument but I can also see it going the other way. For instance: If you take that 200 lb out of the car, perhaps you can GET to 150 mph quicker due to there being less mass but once you reach 150 perhaps wind resistance is just as strong on the object as it was before.

However, in the end I would think that because there is less mass, the powerplant has to exhert less power in order to move the object at the same speed, therefore you could get to 150 mph and still have some power left?

Seems like the more I type I start second guessing my point by the end of the sentance... Physics majors? show your stuff!

YellowS2kPwr

I was the same way when I was typing

Wisconsin S2k

I know that top speed has to do with aerodynamic resistance.

But what always confused me is the top speed is the speed at which the car no longer accelerates. Yet accleration involves mass directly. (the force working AGAINST the car is derived from this equation.... force=mass*acceleration) So technically this means that once the car reaches top speed, it no longer accelerates, which means force=mass*0, which equals 0.

haha, i'm probably just confusing myself even more. i'll shut up and wait for the physics majors to chime in.

oknessad

I think the key factor in considering that equation is that the only variables are force, mass and accel. And it is assuming this happens in a vacuum and seriously guys... the last time I drove my S in a vacuum I noticed a serious lack of power . So it could be that we are thinking about top speed in a way that is too elementary for us to really understand what is happening for top speed to be reached.
Maybe in a vacuum if you reduce the mass the top speed could be increased?

PilotKD

Weight effects acceleration, not top speed. Top speed will either be drag limited or gear limited. Drag limited meaning that you don't have the hp to overcome drag. You could have a 5000lb car with .28 coefficient of drag (very good) with 200hp and it may take 5 minutes to get up to top speed, but it will eventually get there. If it were not gear limited, it'd top out wherever drag equaled hp available. If you could some how take 1000lb off the same car without changing anything else, it'd get to its top speed a lot quicker, but would still top out at the same speed.

oknessad

that makes sense. Nice explanation KD. I think I was en route to that answer in my first post saying it would take a shorter time to GET to top speed with a lighter car, however I kinda got lost in if in fact it would increase top speed. And I guess it couldn't since air is always there and it doesn't get any easier to go through the air no matter how heavy the object.

Wisconsin S2k

edit: i read KD's post.

now how about top speed up an incline.....

if you have 2 identical cars, then load one up with an extra 300 pounds, the one with more weight will accelerate more slowly up the hill. but again, does that affect top speed?

woohoo, i love continuing to confuse myself. muahaha.

SilverKnight

play GT4 and find out