Weight has to do with top speed. It doesn't in a frictionless no gravity scenario, like space. Otherwise there is a certain amount of force needed to move an object. The counterforces are drag, gravity and friction.

 

i challenge you draw the free body diagram and to say exactly, in scientific and engineering terms and equations, where weight enters the constant velocity force balance in the direction of motion except through the rolling resistance as i stated before. compared to the air loads and the engine horsepower differnces, this difference could be ignored between the two cars here....

you're totally, completely, wrong.

 

I vote for flyboy2160. The only effect that weight reduction has on top speed is by lowering tire load and thus reducing available grip and rolling resistance. There was a book from the late 80's or early 90's Chasis Engineering by Adams (can't remember the first name) that talks about this in the first 1-2 chapters I believe.

Also, the tire rolling resistance quickly becomes a small component of total resistence after 60-65 km/h (That's usually where the two are about 50/50). Tire rolling resistance usually goes up by something like 25-50% between 0km/h and 150km/h. Aero drag will usually triple between 60km/h and 150ish km/h for most production cars. That should give you some reference points as to why tire rolling resistance isn't too much of an issue.

My bet is that getting rid of a cooper S decklid spoiler will almost definately give you more in top speed than shedding 200lbs. Since above 200ish km/h (About 124mph) aerodynamic drag on production cars becomes HUGE.

Patrick

 

my spoiler-less puppy, sans ecu speed limit, will be at the Glen on those 200+mph straights, end of the month. I will report whether or not it spanks the spoiled BlueMCS buggy, that might have a 200 lb advantage, with his modded bumper carrier.

Knuckles and know-how, free-body to free-body.

 

Yeah, but rolling resistance can be huge as well. You can't just say that weight has no impact, except rolling resistance, because that can be huge. Plus you are showing a calculation for constant velocity. Top speed is determined by acceleration. You have to accelerate and keep accelerating to keep increasing your top speed. When you can accelerate no longer than you reach your top speed. I agree with you to maintain a set speed, weight doesn't play into it. But your car doesn't instantly reach a preset top speed, it has to get there. Acceleration = Force/Mass. I guarantee that if you load 1,000 pounds in your car, your top speed would be lower (as well as your acceleration).

 

I will be at the Glen myself on 9/27. I know for a fact that under good conditions I have hit 129.1 from a standing start in a measured mile. I have also topped out somewhere around an indicated 143 but, it took a while longer to get there. My speedometer seems to be off around 5% so the numbers seem pretty good. I am not 100% convinced that there is anything electronic keeping you at 136. It felt more to me that it simply did not have enough power to go any faster.
As for all the argument about top speeds, I have seen a Mitsubishi EVO do 135 or so in a measured mile. The same EVO one year, $15,000 and many mods later (over 320 whp) only got to 146. It has however been clocked at 157 over a 3mile stretch. Comparing that to a Mini, I would think you would need somewhere in the neighborhood of 325 engine hp to keep up.

 

I'll bring my gps with velocity measurement function; **** on those 'corrected" speedos

 

dgszweda- unfortunately, in this scenario, only acceleration is affected by weight (mass).
yes the heavier car (with greater mass momentum BTW)will take longer to get there, but the top speeds will be affected by the CD of the car way before the mass becomes a factor.
i think that you are right regarding loading the car with 1000 lbs reducing top speed but not for reasons you've stated. (compressing tires, reducing aero gap between the car and the road, etc.)
the point i believe flyboy was making was that drag was the major factor. in a vacuum this would not be the case, and dgszweda would be correct. f=ma

Brother illustrates a good point about the search for speed, and the monster increases in HP required to gain a few MPH.
you have to face the fact that the MINI has a terminal velocity of probably around 150MPH at 300BHP+/- if you want to go faster, save yourself the hassle and buy a Porsche.

 

I always wondered about the effectiveness of the rear spoiler. Maybe it causes additional drag, but maybe it lowers the drag coefficient by adding sort of a 'tear-off edge', allowing the vortex to disconnect from the car easier.
Has anybody an idea if the spoiler generates any down force? Under hard breaking at high speeds the car gets very very light in the rear! More than once me and the electronics needed the full width of the lane to slow the car down from above 180 km/h.

 

My guess is that the spoiler mostly creates a lot of drag. Hatchbacks tend to have bad aerodynamics because of form drag. Also I'm just guessing here but I wouldn't be suprised if you ended up with laminar bubbles at the front of the roof which means that there is a good chance that the flow does not reattach by the rear spoiler.

Let's put it this way... in the field of aerodynamics the MINI is a brick.

Patrick

 

Well actually, turning the A/C off will act like you shed over 200 lbs. The spoiler on the MCS is what probably increases it's Cd to .36 from .35 of the cooper. At high speeds, it's needed to keep you on the road. The MINI is a brick in terms of Cd and you can't expect it to have a high top speed. It never has and never will.

 

I think dgszweda1 and flyboy2160 are both right. Weight will affect top speed because it takes X hp to keep X lbs accelerating and to keep it there(constant velocity)....more weight, more hp needed, including at top speed. All which is something the MINI lacks alot of...HP. The more weight(lower car as well), the more rolling resistance is in the equation, drag will be reduced. You would think that the lower drag would mean higher top speed, but it still takes X hp to move X lbs, and that means less hp to overcome the drag...even though it's lower.

 

No disrespect but You guys are way out of your element. These are the guys that go for it. http://www.scta-bni.org/

 

Weight has nothing to do with top speed other than rolling resisitance issues. That's a fact. I'll stake my PhD in physics on it. The statement about time and distance to speed are also correct, but unrealted to the statements about top speed and wieght.

For the post that mentioned the friction being dependant on wieght, that's what rolling resistance is! It the friction on the systems that use a bearing surface (like wheels).

The dynamic issue of lower mass increasing acceleration is very true and obvious. Look at 0-60 times for motorcycles.

Some other forces that come into play: Lift. Early "aero" body cars could actually lift the car off the road because the bodies acted like wings. So you add a wing to create downforce, at the price of drag. But this drag isn't a wieght issue, it's one of aerodynamics. The weight of the car helps with traction in steering on non-banked turns. The reason really fast cars have wings that push down isn't for speed, it's for handling in turns. And they pay a price in speed, via the aerodynamic drag of the wings.


o4yellows & dgszweda1: Not quite there. Given enough distance, wieght has no effect on top speed. Like stated earlier on in the thread, the evo did it's thing sooner, but there are cars with less power that will go faster. Heck, my old 450 cc honda 2 cylender MC did a 0-60 in a bit over 4 seconds! Here you're mixing a dynamic (acceleration) issue with an equilibrium (flying-mile type top speed). This is apples and oranges.

olyeller pretty much nailed it.

If you really want to know about this stuff in detail, we can start with some good undergrad mechanics texts, and then get hammered by math by fluid dynamics for the airflow!

Hay, it's even possible for adding weight to lower the car and reduce drag! Maybe faster! There are lot's of forces at work here.....

Matt

 

I have a couple of physics degrees, too, and I'll second Dr Obnxs' comments. Weight has nothing to do with top speed. Power is the measure of work done per unit time. It doesn't take a given amount of HP to move a given mass at a constant speed in the abscence of external forces. It does take a given amount of power to counteract the resistive forces, which in this case are rolling resistance, air resistance, and internal friction. Aero forces are by far the most significant because they go as the square of the velocity and are essentially all that matters. Rolling resistance, though a function of weight, is essentially constant with speed. Not just linear, but constant.

In fact, land speed record cars are typically quite heavy. They're not worried about weight because they have, effectively, all the time they need to get up to speed.

Mark

 

I should add that we're talking about level ground here. Going up a hill is a very different thing since then you're doing work against gravity.

Mark

 

How much does the ratio of 6th gear play in all of this? I would imagine that makes a difference...

 

Oh, I was doing about 6,000 RPM in 6th gear and I was doing 140, if I remember correctly.

 

And one more thing! The aerodynamic resistance goes as the square, like we said, but the power required to overcome it is equal to the force times the velocity, so it goes as the cube. A small increase in speed can take hugely more power.

Mark

 

Dr. Obnoxious,

Weight has an affect on rolling resistance, and it can be quite great. I agree taking rolling resistance out of the equation, weight has no affect. And the car is primarily limited by drag forces anyway. And most high speed competition cars have very narrow tires to reduce rolling resistance. But next to the huge loss associated with drag, rolling resistance is still a great factor. For a street car with large tires, it is even much greater than the thin wheeled high speed cars. That was all I was trying to say, not trying to rewrite physics.

 

It's interesting to try plugging in some actual values. http://http://www.europhysicsnews.co.../article8.html has an interesting graph showing rolling and aerodynamic resistance for "typical" values. The author takes 0.01 as the coeffecient of friction, so rolling resistance equals that multiplied by weight, or 1000 kg x 9.81 m/s^2 * 0.01 = 100 N (approximately). That's linear with the weight of the car so, for example, double that for a car that's twice as heavy. It's also (approximately) not a function of speed at all so it's a flat line on his graphs of force vs velocity.

Aerodynamic drag is equal to the coefficiant of drag times the frontal area times one-half the density of air times the square of the velocity. It's that squared term that makes aerodynamic drag by far the most important contributor at high speeds. It's true that rolling resistance does matter quite a bit at low speeds. In his example, aero drag and rolling resistance are equal at 50 km/h. That's why ultra high-mileage cars have special low rolling resistance tires, but it's clear from the graph that rolling resistance begins to matter much less at higher speeds.

Again, plugging in numbers, you get about 450 N of aero force at a speed of 30 m/s. Double that and you quadruple the force, so 1800 N at 60 m/s while rolling resistance is still only 100 N. In other words, at 135 MPH (or 60 m/s), aero drag is 18 times greater than rolling resistance.

Mark

 

The reason gearing affects top speed has to do with torque multiplication and the power curve of the motor. At you go from first to 6th, the torque multiplication of the driveline goes down, and the force with which the motor can "push" the car forward (as measured at the drive wheels) is reduced when the motor is at a given rpm.

another way to think of it is that you have to do a lot of work at very high speeds. A motor turning higher RPMS is burning more gas and releasing more energy per unit time. There are limits to this picture, but it captures the idea.

If you didn't lug your motor, you could accelerate from a standing start in 6th, but it might take days to do the quarter mile!