Erik

Well that is off course correct, at least if its done in a vacum, NO air.

At increasing speed the mass is getting less and less important since the major recisting factor is the air resistance, but since you obviously are an expert in the area you already know that.

Another thing is, the Cw value is only a factor telling us how slippery the body/shape is, to get the air resistance you have to multply with the Area to puch thrue the air.

So in low speed like the 0-60 sprint the air resistance in of no practical importance, only weight to power ratio ( and gearing, traction...)
but from high speed like 155 mph it is off huge importance and weight are getting a realivly lesser and lessser important factor.

And why dont you stop the stupide Gustav bashing, it does not give you much credit.

And to put WW2 into this is pointless, and I actually do not belive the freedom of speech was the main motivation for the US to finaly involve themselves in the War.

Lets stick to the overwhelmingly important discussion about how much or little the E55 will be beaten by the M5. Cant think of anything more important than that.


Personally I have had enough of this nonens.

W210

Liar, you enjoy this which is why you keep coming back for more!

Erik

:p

reggid

Improviz is correct, take any car double its mass and the instantaneous acceleration halves regardless of drag or anything.

but at speed power to weight comparsions are pointless (unless same cars are used) because of drag.

if drag and resistance are absent acceleration = Force/mass

this is analgous to power to weight ratio.

if you include drag then

acceleration = (Fe-Fd)/me where

Fe =engine force/thrust at rear wheels
Fd = drag force
me = equivalent mass (translational and rotational mass)

which is analgous to (power - drag) / mass and no longer power/mass

the reason people believe that mass is less important at high speeds is as follows:

Take two cars of the same physical size and shape. Car A weighs 10000lbs and has 500hp and car B weighs 5000lbs and has 250hp. So both cars have the same power to weight. The drag is the same for both cars because of them being of the same shape and size. The heavier more powerful car has twice as much thrust so when you subtract the drag it has less of an effect to the net thrust meaning it will be faster at high speed. This leads people to believe that the weight has litte effect which is wrong according to Newton.

Its the relative magnitude of drag to engine thrust (rwtq/wheel radius or rwhp/vehicle speed) that governs high speed performance.

reggid

i've looked at some test data and believe the test by Autocar claiming a 119mph trapspeed is wrong my numbers show 117mph.

the test data is

0-110 11.5
0-119 12.8
0-120 13.3

just by looking at the numbers they seem unrealistic becasue 110-119 (difference of 9mph) in 1.3sec but 119-120 (difference of 1mph) in 0.5sec and this is with a gear change at 125mph so that isn't the cause.

Other data

Sportauto

0-100km/h 4.5sec
0-200km/h 13.8sec

my interpolated values
60ft 2.2sec
0-60mph 4.35s
1/4mi 12.55@116.7mph

Autozeitang

0-100km/h 4.4sec
0-200km/h 13.9sec

my interpolated values
60ft 2.15sec
0-60mph 4.15s
1/4mi 12.45@116.8mph

Autocar

0-60mph 4.6sec
0-120mph 13.3sec

my interpolated values
60ft 2.45sec
0-100kph 4.9s
1/4mi 12.9@116.9mph

i fit an equation to the time to speed data and integrated the equation to get time to distance. Based on how well my eqation matches the original data points from which the equation was derived i believe my trapspeeds are +/- 0.5mph and time to distance accurcay about +/-0.1s.

I predict about 117mph trapspeeds at 12.4-12.5 ET.

i compared the test of the preproduction m5 and cls55 from automotor and sport

m5 cls55
0-100 4.7 4.7
0-200 14.8 15.6

my calcs show both did approx 12.9s the m5@115mph and cls55@112mph its worth noting that this m5 was 1 sec slower from 100-200kph than any other test i have since found.

enzom

We have many scientific engineers on this board.

Actually, the last few posts have been informative, illustrative and helpful. Even if somewhat speculative. Nice work, guys. Seriously. It is nice to read for a change.

Reggid - I don't know how your formula works, but I will try to remember this prediction when the car is timed in the 1/4 eventually.

There used to be a guy in the car and driver forums that had created a formula to measure your horsepower based on e.t., speed, frontal area, cd, gearing, etc. He had built up a database. I don't know how accurate it was, but it was still kind of neat. I have tried some of the rather crude hp calculators that use your trap speed and weight. I can tell you that they are waaaaaaayyyyy off.

Good day, fellas.

enzom

[QUOTE=Improviz]Wrong and wrong. Force=mass*acceleration. This holds true whether the body is in motion or not. Less mass with same force = more acceleration. Try taking a physics class.

/QUOTE]


You know, I never understood something about this equation. Yes, it is sort of but maybe one of you guys can help me with this.

If Force = Mass x Accel., what if a car is travelling at a steady 60 mph? The acceleration component is equal to 0. If that car hits you, we all agree that a force is involved. But anything times zero is zero. So how do you caclulate force with this equation of there is no acceleration?

regor60

Right, there is no net force on the car if it is not accelerating. The torque supplied by the engine, a force, offsets wind and other resistance forces.

But when the car hits another object, their is a deceleration, and the force necessary to produce this can be estimated by using the F=MA equation. For example, estimate how long it takes for car to come to rest, divided into speed, gives the deceleration.

tommaey

[QUOTE=enzom]If the magical car crashes into you at 60mph and doesn't slow down, then 0 force is experienced. But thats not going to happen

What is confusing you is that you don't realise slowing down is a form of acceleration - negative acceleration! (yay)

So the car is travelling at around 27m/s (physics units), and it hits a car. It accelerates at -25m/s/s (deceleration of 25 metres a second every second).

So: Force = mass*-25m/s/s

hurray

reggid

In simplistic terms if you are standing stationary in the middle of the road and get hit by a car travelling 60mph and you stick to the front bumper then your body has basically started from zero and accelerated to 60mph in a fraction of a second hence you have accelerated very quickly, this produces the large forces.

The magnitude of force generated depends on the time it takes to accelerate you upto the speed of the car, the longer it takes the better in terms of producing lower forces. Deformation of the car (crumple zones etc) and your body squashing help increase the time. If both objects were perfectly rigid there would be infinite force generated.

One of Newtons laws says that ehere is an equal but opposite force that you apply to the car slowing it down(the driver would have to give it some gas to maintain his 60mph speed). Remember that since the vehicle probably weights many times more than you it doesn't slow down as much as you speed up after the impact.

the speed of the two oblects after impact is

Velocity after Impact= (Mass A*velocity A + Mass B*Velocity B)/ (Mass A + Mass B).

So if a 1500kg car travelling at 100kph hits you who weigh 80kg while you are stationary then the velocity after will be 95kmh. A force must be provided by the engine to keep the car travelling at 100kmh

Improviz

Quite a bit, actually, per the data I posted which you're ignoring. Cars which were several seconds apart at 120-130 mph were shown to be tied or a few tenths apart at the 1/4 mile. Ergo, it follows that two cars a shorter distance apart at 120-130 will be closer still at the end of the 1/4. Ergo, your 0-3-0.4 claim is prima facie unsupported by the 0-125 acceleration figures for the E55 and M5.

First, prove that the M5 traps at 119, which would require 550 horsepower in a 4000 pound car.

Second, you are very deliberately and intentionally dodging my point. Cite one instance of any RWD car tested 0.8 faster to 125 mph was 0.3 or 0.4 faster than a second car to the 1/4 mile.

One. Stop dodging and start producing.

You can't, of course, hence the dodge. The cars' 1/4 mile times will be virtually identical, not 0.3-0.4 apart.

Improviz

...which is what I was trying to point out all along. Thank you.

1/4 times and trap speeds are several tenths (and mph) slower than what US mags have gotten, though...for apples to apples, we'll have to wait until the same mag does a direct comparo.

Still, data above shows that my inclusion of SL55 data was valid in the context of our discussion, so thanks again.

Improviz

It is correct in air as well, Erik. A given force will cause a given mass to accelerate at a given rate. Of course, the coefficient of drag must be factored in, but mass is still a huge factor, and Gustav's statement that it has no effect at higher speeds was, and still is, prima facie false.

Can you support this claim with an actual equation? Because I maintain that regardless of speed, in these comparisons, mass will be a huge factor, and in fact will be more predominant than air resistance which is roughly the same for the vehicles discussed here.

Consider bullets, which travel far faster than any object being discussed here. A .357 magnum round has the same Cd as a .38 special round and weighs the same, but has a much higher charge of gunpowder (force).

Which round has the higher velocity, Erik? Here's a hint: it's the .357.

Or compare a .22 rifle round with a .460 Weatherby round or a .50 cal machine gun round. If your assertion that aerodynamics predominate at the velocities being discussed here were true, then the .22 should be faster by virtue its hugely smaller surface area. It isn't.

And here we are discussing automobiles with roughly the same Cd. Power and weight are thus the dominant factor, becuase Cd is roughly equal for these vehicles.

The drag coefficient **is** the measure of how slippery the body shape is, Erik.

Further, prove to me that the M5's aerodynamics are sufficiently superior to those of the E55 for it to become the predominant factor, over force and mass that is, between 125 and 155 mph. I maintain that since Cd is very close for these cars, mass and horsepower will predominate.

And again, you are moving the topic off course. Point is whether or not Gustav's assertion that mass isn't a factor at high speeds is true. It is not. Period. If you can deterministically prove otherwise, I'd like to see it. Mass is *always* a factor, and a large factor, particularly at the speeds being discussed here.

No, they are not getting less; aerodymics are getting more. Mass is still a huge factor, end of story, particularly with vehicles like this with similar Cds.

That's your opinion, which here you have the freedom to express without fear of being banned.

On Gustav's board, by his own admission, you do not.

I did not say that it was the main motivation, but the end result is that as a result of WWII (and the Cold War), millions of people who did not have it now do. And to insult or demean the principle itself in the manner I've seen done here is offensive, and so I will exercise my freedom to criticize Gustav's actions, here, where he cannot ban me for speaking my mind.

I can. But this is fun, and all work and no play...

So untrack the thread and stop posting.

enzom

This is going beyond the scope of my scientific knowledge. From what I know, the higher the speed, the more important resistance becomes, and the less important mass becomes. That is not to say that mass is irrelevant at any point. I don't know that Erik is saying anything different. If you pick a range of speeds for acceleration, I think mass becomes less of a factor. For example - in relation to aerodynamics, mass is more important going from 60 to 90 mph than it is going from 120 mph to 150 mph. I think, anyway. Maybe somebody can demonstrate that as a matter of physics, I am right or wrong.

People are free to post, read threads or refrain from posting or reading threads. I think Erik's posts (like Improviz's) add some interesting color to the to and from. Keep them coming.

It is taking a while, but this may actually turn into a civilized discussion. Who woulda thunk it?

enzom

I suspect the 1/4 mile times will be very, very close. So I agree with your conclusion. But I don't know that 550 hp is required to get a 4,000 lb car through the traps at 119. If you change the gearing on two identical cars, won't the trap speeds change? Lots of guys on the Z06 board swap out their gears and run quicker and faster. The weight stays the same, as does the flywheel horsepower. I guess you do need a lot of hp to move 4,000 lbs to 119 mph, but it may not be 550 with the right gearing. I am willing to believe the M5 can trap in the 119 range. WIth LC and the seven speed box. I haven't seen it done yet, but I am open to it.

As far as producing the test examples that you are asking about - I don't know how anybody can dig up that data. Just trying to be fair.

Improviz - you and I (and others) will go to our local strips and see how the M5 runs. If the M5 runs one or two tenths quicker or slower, it is still a fast car. If it runs 11.8 @ 120, then it is a very, very fast car. Kudos.

Gustav

Have you? Were your tests instrumented? Didn't think so. Do you have any instrumented tests from 200 up? Haven't seen any yet, but if you do, please post them.

OK, Imptoviz, I credit you with soem fair points and I was wrong regarding aerodynamics. You ahve to agree that the SL55 AMG looks more aerodynamic than the E55 AMG

However the speedo indicated 250 km/h on the meter, maybe that is under 200 km/h real speed on a E55

PS: There is a 8 MB and 32 MB surprise somewhere you shoudl see

wolverine

Interesting discussion indeed. I've got a BS in Physics, MS in Mechanical Engineering, and have been in design for 25 years. I've done some consulting work for some major car companies, so I'll throw my two cents in here. I'm going to neglect a number of things to keep it simple.

A car moving at a certain speed V will be using part of it's horsepower to overcome the drag force of the air, and let’s say the rest is available to accelerate the car. If a 400 HP car uses 40 HP at 50 mph to overcome the drag force, then it has 360 hp available to accelerate the car. The car will accelerate pretty quickly from 50 mph with 360 HP available.

As your speed V increases, the drag force, (given as f = -1/2cpAV**2) increases as the square of the velocity. So at double the speed, say 100 mph, the drag force increases by a factor of 4. At 100 mph, the 400 HP car is using 160 HP to overcome the drag force, and has only 240 HP available to accelerate the car. Acceleration from 100 mph would be good, but less than what it was at 50 mph.

By 150 mph, the drag force would be 9 times as much, or 360 HP would be required just to keep the car moving. That leaves only 40 hp to accelerate the car, which would gain speed very slowly.

Now, let’s say another car has a slightly higher drag, say 10% more. That would mean that at 150 mph, the car would be using 360 + 36 = 396 hp just to maintain the speed. That leaves only 4 hp available to accelerate the car, basically nothing.

Now you can look at your F=ma calculations and see that even if the horsepower numbers are equal, a heavier car with a better drag number can accelerate faster than a lighter car, if the speed is high enough, because the lighter car is using up more of it’s HP to overcome the drag force. A 4000 lb car with 40 HP will out accelerate a 2000 lb car with 4 HP every time!

I’ve left out friction, time vs distance, and a bunch of other things just to keep it simple. Hopefully this gives people a good idea of how drag influences acceleration.

Jakpro1

Yeah...uhhhhh....what he said. So there.

Erik

Very good responce

That was what I was trying to put across, I knew it was like that but I just dont have the theoretical skills to describe it that good.

Opss, I forgott I did not want to contribut anymore, but since this all of a sudden made a turn to the better cold not resist, sorry

Improviz

Interesting. So, let's say that the Cd of one car in question, namely the E55, is 0.27 , while for the second car in question, namely the M5, it is 0.31 .

Which car would be more affected by drag at high speeds? Would it not be the car with the lower Cd? I believe it would...given that the Cd in the equation you provided is the drag coefficient.

Improviz

Unfortunately for your argument, the E55's drag coefficient is better than the M5's by 13% (0.27/0.31 = 0.87). Hence, at high speeds the E55, not the M5, has the aerodymanic advantage. My previous post to Wolverine has links to both BMW and Mercedes sites giving the Cd for both cars. Feel free to click them.

So you still lost on the point, even after shifting the argument away from Gustav's central contention, which was that weight has NO effect at higher speeds. Clearly, it does, and clearly, you were in error in defending his incorrect assertion.

The example Wolverine provided was an extreme case, and would buttress your argument if and only if the M5's drag coefficient were vastly superior to the E55's. Unfortunately, the reverse is true. In fact, his 10% better example is more applicable here, given that the E55's Cd is 13% lower than the M5's--not a favorable result for the M.

Further, the cp in the drag equation provided by Wolverine is given as "cd" in other listings of the equation, as in Coefficient of Drag, which is what it represents. Yes, that's right: the drag is directly proportional to the car's drag coefficient, Erik.

I.e., the E55's drag should be lower than the M5's, as its drag coefficient is 13% lower than the M's.

schwarzwagen

after peter b and belmondo stopped posting, the thread turned decidedly intellectual...

fwiw, the e55 is 1 fender faster than the sl55 from 50-140mph. 1 driver each car, no passengers. i was driving the sl and the owner of both cars was in the e55.

i am a little surprised that so much time has been invested over a few tenths of a second in *straight line* acceleration. don't you guys think that driving dynamics as a whole are a little more important? or at least more useable, therefore more pertinent to a discussion of whether or not the m5 is 'better.' however pointless arguing over something as subjective as that is.

reggid

what about frontal areas? Anyone got numbers on those. The e55 may have a lower Cd than the SL55 and M5 but what about the frontal areas

f=0.5*rho*Cd*A*V^2

Wolverine if

Power = f*V = 0.5*rho*Cd*A*V^3

the power increases as V^3 so to double the speed needs 8 times the hp, but the actual force is 4 times higher!

Also as i said in an earlier post mass is always important but at high speeds heavier powerful cars usually have an advantage, provided they aren't very large in physical size which would strongly affect their drag properties.

Improviz

I thought of this, but since the M5's Cd is 13% smaller than the E55's, the E55's frontal area would have to be 13% larger than the M5's for the two cars' drag force to be equal...extremely unlikely given their measurements. And that would be just to get a tie in drag force....for the M to have *less* drag force, the E55's frontal area would have to be larger still.

Belmondo

Wow, car talk "intellectual"??
Do you want to confirm that? What else do yo put in that category, Bob Vila's DIY Do it Yourself Home Improvement and Home Repair?? Are you serious?

May be civil, respectful, crazy, educational, entertaining, helpful, biased, but INTELLECTUAL??

"Car talk" radio show "intellectual"??
Parhaps you can redirect me to "intellectual" discussion on www of home appliances---I'm looking for new washer & dryer.........