zoom44

btw im on darrin's side that was just a dig for the hell of it

dmp

lol :D

Poop-head.

- Darin.

:D

zoom44

dude darrin has 2 r's

dmp

Not if you account for frictional losses.

zoom44

oh right-im with you there

1.3L

First off, I appologize for being sarcastic earlier.

Secondly, I want to point out that you keep stating that top speed of a car is the SAME regardless of whether it has a load of bricks (or whatever) in it or not. Now, I do agree that the heavier car will take longer to reach whatever top speed it can obtain and I also agree that aerodynamic drag is the bigger factor (on fast cars), however, I want to point out that the very link you supplied does state that there will be a loss of top speed as weight increases. The author guessed maybe a loss of 10 MPH with their example car filled with 2 tons of lead. That very statement is at odds with your claim that there would be NO loss of top speed. You even made that claim again in the above quote.

And BTW, isn't suggesting that someone fill their car with bricks a bit sarcastic

1.3L

dmp

The top speed wouldnt be reduce because of the added weight, but because of the strain on tires/ etc. The 4000 pounds of lead statement was to address how the tires would freak the hell out, and cause massive amounts of friction (probably because they'd go flat). Assuming the tires had NO increase in friction due to the weight, the car's top speed would be the same.


And suggesting somebody fill their car with bricks isn't sarcastic in the least. (shrug).

toca

ive hadf mine up to 154 on the freeway coming back from az to ca on the 10 fwy

BoosTED

iTrader: (2)

So on the dyno the 8 does over 180mph if it had the power it should be able to get there correct?

Especially if you turned your mirrors in

MazdaManiac

iTrader: (3)

A feather and a rock are dropped in a vacuum (no drag) - which hits the ground first?

BZZZZZZZ. Wrong. They both hit the ground at the same time. Mass made no difference, all that mattered was the "horsepower" of gravity was the same on both objects.
Get it?

BoosTED

iTrader: (2)

BUT since we don't live in a vacuum environment, the car has to fight against the drag. With more horsepower the car should be able to overcome the resistance from drag.

1.3L

Oh, I see; speed would be reduced because of "strain on tires/etc." (I think we're making some progress here). So, if it's not caused by weight, what is it exactly that is causing this mysterious strain on the tires?


1.3L

dmp

In the morning when I get on the scale, I say to my wife "Wow! i've gained a LOT of friction!"

No? It's weght. Assuming the tires could support the car with 4000lbs of lead in the passenger seat, without the tires squishing and popping like balloons, the top speed of the car would be the same as a run w/o the 4K lbs.

Adding passengers...several hundred pounds easily, to an rx8 weighing 2800 to begin with will NOT affect the Rx8's top speed.

Not sure how much more clear I can make that. Weight 'indirectly' causes increases in drag, at times. Not every time. You can probably get a NerdCalculator out and figure out adding 3 250lbs passengers w/o increasing the air pressure in the tires by 4 psi 'could' cause a .000343234mph reduction in top speed. Sure...go for it..try it.

but for those of us who live in the real world, (adding) weight to your car won't change it's top speed.

But..but...what if I dumped 40,000lbs of concrete on teh car! CERTAINLY that would affect the car's top end. Of course...it would, the car would be FLATTENED.

BUT...if the car could withstand it...tires, suspension, body, etc, could stand it...AND the load didn't increase wind resistance, the top speed would be the same.

dmp

Thanks!

Melchior

Yes I agree with dmp. Thinking back to physics now, it makes sense.

But, having more weight is not a good thing. It decreases acceleration and decreases the time spent at top speed. Which almost effectively makes the average velocity the car can achieve lower. (Because it only stays at top speed for a certain amount of time before reaction destination or having to slow down.)

Either way, lighter is better I think we can conclude

But thanks all for the responses. I think 150 is plenty for a car, and plenty illegal to drive at too!

1.3L

What a bunch of double-talk. You completely deflected and circumvented my question. Again, what EXACTLY, causes the increased strain on the tires (remember, you made this statement about increased strain on the tires)? Follow up! Support your claims with something other than personal opinion and BS about concrete and what, "friction," on your personal weight scales? What on gods green earth has that got to do with it? BS! And then you make the totally absurd statement that 40,000 lbs. would crush the car - well no duh. Get real.

I totally reject your idea that you can get something for nothing. Your assertion defies logic. To be totally clear, your claim that your car will reach EXACTLY the same top speed whether it weighs in at 2800 lbs. or 3800 lbs. is pure baloney. If that were really true, I think some super-secret government spooks would like to talk with you (sarcasim intended). There ain't no free lunch.

Again, answer my question.

1.3L

1.3L

So, please explain where you can get something for nothing, Mr. physics.
1.3L

Hornet

I'm no expert but I think the extra strain on the tires would come from the airflow over the car. Basically the airflow should have the affect of pushing down on the car and of course that would include pressing down on the tires causing the sidewalls to basically bow! It would be like adding weight as you go faster!

dmp

Here's what you are doing:

You are asking a stupid question, it's being answered in a way which makes sense...since the answer is now what you want to hear you throw out the whole 'that defies logic' comment. You're hopeless. Go read a few physics books...search google...do whatever you want. I'm sorry you can't come to terms with the reality that a 238hp RX8 weighing 2800lbs or 3800lbs will have the same top speed, assuming the tires didn't pop or flatten under the increased weight. Your insistance that increasing the WEIGHT of an object will change it's TOP SPEED sure SOUNDS right...I used to believe it too...until I opened my mind to reality.

Good luck in life.

Here it is, Last time:

Increasing the DRAG a car has can lower it's top speed. Increasing the cars WEIGHT will not. Two different animals, not neccessarily related. If you can't wrap your mind around that concept, you're hopeless.

dv/dt

Back of the envelope calculations:

RX8: Cd = .31
L × W × H (in): 174.3 × 69.7 × 52.8
fA = 69.7 X 52.8 = ~3680 in^2 = ~25.6 ft^2 However, the car is not a brick, so we'll estimate 23 ft^2
Weight (lbs) = ~3000
Gross Vehicle Weight (lbs) = ~4000

Rolling resistance (lbs) = vehicle weight (lbs) x 0.012 to 0.015 (we'll just use .013 as an average):

So: Rolling resistance = 3000 X .013 = 39 lbs
So: Rolling resistance = 4000 X .013 = 52 lbs
So: Rolling resistance = 5000 X .013 = 65 lbs

Air resistance (lbs) = fA x Cd x 0.00256 x speed^2 (speed in mph):

So: 23 X .31 X 0.00256 X 150^2 = 410.7 lbs air resistance @ 150mph

Power to overcome rolling resistance = weight x 0.013 x mph / 375

So: 3000 X .013 X 150 / 375 = 15.6 rwhp to overcome rolling resistance @ 3000 lbs of weight.
or: 4000 X .013 X 150 / 375 = 20.8 rwhp to overcome rolling resistance @ 4000 lbs of weight.
or: 5000 X .013 X 150 / 375 = 26.0 rwhp to overcome rolling resistance @ 5000 lbs of weight.

Power to overcome air drag = fA x Cd x 0.00256 x mph^3/ 375

So: 23 X .31 x 0.00256 X 150^3 / 375 = 164.3 rwhp to overcome aerodynamic drag at 150mph. (note that the RX8 would have to weigh 32000 lbs in order for the rolling resistance to equal the aerodynamic drag. Ouch!)

And: 15.6 + 164.3 = 179.9 total rwhp to travel at 150mph @ 3000 lbs, so it looks like my numbers are close enough to real world.

or: 20.8 + 164.3 = 185.1 total rwhp to travel at 150mph @ 4000 lbs, which is Mazda's fully loaded weight of the car.
so: 185.1 - 179.9 = 5.2 extra rwhp needed to carry an additional 1000 lbs.

or: 26.0 + 164.3 = 190.3 total rwhp to travel at 150mph @ 5000 lbs, which would undoubtedly wreck the car in some way.
so: 190.3 - 179.9 = 10.4 extra rwhp needed to carry an additional 2000 lbs.

So it only takes an additional 5rwhp to carry each additional 1000 lbs to 150 mph.

How does this effect the top speed? Let's approach this another way:

hp = Total drag x mph / 375

so: hp/ total drag X 375 = mph

total drag @ 3000 lbs: 39 + 410.7 = 449.7 lbs of drag
total drag @ 4000 lbs: 52 + 410.7 = 462.7 lbs of drag
total drag @ 5000 lbs: 65 + 410.7 = 475.7 lbs of drag

So let's assume that the RX8 has 185 rwhp:

so: 185/449.7 X 375 = 154.3 mph @ 3000 lbs (remember, we've added 5.1 rwhp from the 179.9 estimated previously)
or: 185/462.7 X 375 = 149.9 mph @ 4000 lbs
or: 185/475.7 X 375 = 145.8 mph @ 5000 lbs


So our theoretical top speed drops by about 4mph with each increase of 1000 lbs. Realistically, that's not enough to even worry about. Change the Cd or fA of the car and the numbers change much more dramatically.

In short, aerodynamic drag is a MUCH higher contributor to a vehicles maximum top speed.

(The formulas used were taken from here .)

BTW, yeah, I know I should be using newtons for force and meters/sec for velocity. However, the calculations above seem to coincide nicely with reality.

knight7

iTrader: (1)

wow is nice to see that in this thread almost everybody voided their warranty...didnt someone here got the whole car outta warranty cause he said he did 100mph on the 8?

dmp

I don't know what any of that math means, but, the top speed would be 'reduced' assuming drag increases with added weight. Drag wouldn't neccessarily increase. In fact, drag could be reduced to some extent perhaps, by lowering the car?




oh...and "NERD ALERT!" :D

dv/dt

The drag incurred from rolling resistance increases with weight, yes. We don't need to assume this, it's a fact. However, the effect of this increased drag is insignificant when compared to the aero drag. This is mostly due to the fact that aero drag is a function of the mph^3 while the rolling resistance is a function of mph^1.

Does this added weight also contribute to lowering the vehicle enough to significantly lower aerodynamic drag? How much lower will the car sit? On one hand, you decrease the fA (frontal area) of the car when you lower it, which decreases aero drag. On the other hand, if the decrease in ride height increases downforce you end up loosing more to rolling resistance. So, I would say that it depends. You'd have to actually run to top speed with increasing weight to know for sure.

Either way, any impact that weight has on top speed is insignificant at best.

1.3L

dv/dt just answered the question perfectly. Increased weight does, in fact reduce top speed. If you go back and read everything I wrote in this thread, you'll notice that I never made a claim that the top speed would be reduced by a great amount; just that it would be reduced (no matter how little). And I was right. You have repeatedly claimed that there would be no reduction, which is wrong. As to aerodynamic drag, I continue to agree that it is the biggest factor by far in limiting any vehicles top speed.

Above all, this argument should be totally academic, meaning few, if any RX-8 owners will ever take the car to such speeds, with or without passengers (or bricks).

So, peace and have a wonderful holiday.

1.3L

dmp

No - he wrote 'weight can increase drag - which affects top speed'. Would drag be increased if we used solid steel wheels and ran the car on a track like a train?

The weight would make no difference in the top speed because the drag would not be increased. Drag would only increase if the tire contact patch increased or changed due to weight. Maybe I add more air to the tires to keep them from deforming by increased weight? Or maybe I just engineer a tire which maintains the same drag regardless of weight?