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X6 E71 / E72 (2008 - current)
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  #1  
Old 11-06-2011, 11:32 PM
Steve J Steve J is offline
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Disparities In Acceleration Times

Why is it when professional drivers drive the same car they emerge with vastly different results?
Let's take the BMW X6M 0 - 60mph times
Motor Trend 4.0 seconds
Car and Driver 4.3s
Edmunds 4.3s
Autoblog 4.5s
Left Lane News 4.7s

There is a gulf of difference between 4.0s and 4.7s. It is as if they were driving different cars. Why is there such disparity?

As an anecdote it is interesting that UK editorials are the opposite!
Car 4.7s
CarBuyer 4.7s
Top Gear 4.7s
The above three times are measured for a velocity change of 0 - 62mph

UK confers; USA differs....why so?

Last but not least, is it not time for the USA to start using metric measurements? Afterall this is 2011.
Is it not time for the UK to switch to left hand drive cars? Why operate the gears with the weaker hand?
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  #2  
Old 11-15-2011, 07:24 AM
Steve J Steve J is offline
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Mein Auto:
I wonder whether they make up the figures! Look, even the 2011 Porsche Cayenne Turbo has alot of variation in tested acceleration times:
Insideline 0 - 60mph 4.7s
Autoweek 0 - 60mph 4.3s
The use of sophisticated timing equipment should make time differences down to hundreths of a second.
How can some drivers test a BMW X6M and have a difference of 0.7 seconds? Could it be quality control ie lack of it on BMW's side? Two cars may look identical but there may be measurable differences when driven to extremes.
Could it be varying skills of the drivers?
Could it be non-standard parameters eg some use launch control, some don't.
There has to be an agreed set standard when testing acceleration times. The car magazines should be clear about parameters eg the type of surface, the relative humidity, ambient temperature, windspeed, type of tyres used, weather conditions. It should be a proper scientific report and not indisciplined waffle.
Since launch control is available, conduct two tests: with and without it.
Motor Trend 0 - 60mph 4.0s
Left Lane News 0 - 60mph 4.7s
Manufacturer 0 - 60mph 4.5s

Using simple maths 60mph = 29.33yards/s
According to Left Lane the X6M will reach 60mph in a distance of 137.85 yards (about 126.05 metres)
According to Motor Trend 117.32 yards (107.28m)
It is like testing two completely different cars.
Obviously the Left Lane car will be treated to the tail light LED Display of Motor Trend!
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  #3  
Old 11-19-2011, 12:05 PM
Steve J Steve J is offline
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I don't have much time now but I must say that although rate of change of velocity describes acceleration, passing rates of change of acceleration figures are seldom given. If one would scrutinize the latter, it is clear that this figure is decreasing with time. In other words it takes a shorter time to get from 30-60mph than it does from 60-90mph. A great engine would be one that accelerates from 60-90 as quickly as it would do 30-60.
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  #4  
Old 12-23-2011, 07:23 PM
ithatel ithatel is offline
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Rolling starts

I read a while ago that most car mags are using a rolling start. In an attempt to minimize wear and tear on press cars.
So, if you launch at 5 mph vrs 2 mph you might get those differences.

I'm thinking it is a lack of talent among the press.

I remember that motorcycles 0 - 60 and 1/4 mile runs we're done by the same hired gun who could launch the crouch rockets and live to tell about it.
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  #5  
Old 01-05-2012, 09:31 AM
HIRISC HIRISC is offline
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Though technically correct (rolling start), I think what you read is that some magazines use 'rollout' in their calculations and some don't.

Rollout should be used only for 1/4 mile type runs (head to head). It is inappropriate and sensationalistic to use it for 0-60.

Using a rollout for 0-60 shaves about .3 seconds. Most U.S. magazines use them, most European ones don't.

There are other factors that determine time of course - car in question, DA, driver, etc., but most 'magazine' variances can be explained by the use or non-use of rollout for testing.

Rollout explained (from a different Forum).

Quote:
Originally Posted by Racer_X View Post
I've always been perplexed by the great variance in 0-60 times published by different sources. More specifically, I've been perplexed by the 0-60 times cited in some U.S. magazines, which seem to be significantly better than those cited by the manufacturers and other sources. Some have speculated that the manufacturers sandbag the numbers, but this never made any sense to me. Clearly, manufacturers would post the best 0-60 numbers they could in an effort to sell more cars.

Anyway, I was reading an article that discussed how Car & Driver tested the 2012 Audi TTRS and found it to go from 0-60 in 3.6 seconds, which was 0.3 seconds faster than other sources posted. The article noted in parentheses, however, that C&D uses "rollout" in their 0-60 testing. Many of you may already know about this practice, but it was new to me so I did a search. Below is what I found in thread from another forum (forums.anandtech.com). Seems to explain a lot:

This topic comes up from time to time. Working in the Auto industry, I see some of this first hand and am in a position to elaborate on the subject.

We sometimes see rather large discrepancies between acceleration test times when comparing different magazines or magazines to manufacturer given figures.

Why is that?

Most of the time, it's a difference in the measurement practices. The biggest factor comes down to a phenomena called "rollout".

Rollout defined: The concept relates directly to NHRA dragstrips and the measurement methods used in quarter mile racing. For the unfamiliar, this is how it works. When you pull into the staging area or starting line, there are two light beams which are aimed perpendicular to the track. The first light beam is the "pre-staging" light. It has no real function except for telling you when you're getting close to the starting line. The 2nd light beam, called the "stage" beam, is the actual starting line. Ignoring staging strategies (shallow stage, deep stage, etc), we'll say the car pulls up to the staging line until the beam is just barely broken by the front tires...as this red Civic shows below (the black wheel).

When the light turns green, the driver hits the gas. However, the clock doesn't know that the car is moving until the front tire moves far away from the light beam to allow it to fully shine across the track. This is demonstrated by the grayed out tire in the photo. In reality, the car moves a distance of about 12" and 0.3 seconds for free...the clock hasn't started yet. Once the clock starts, the car is already moving 3 mph. Here Car and Driver discusses the importance of rollout.

That brings us back to modern day test measurements. Car magazines and car manufactures don't test on dragstrips very often. They use sophisticated computerized GPS or "5th wheel" type measurement systems. A commonly used system comes from a system called a Racelogic VBOX.

How does this all tie together? Well, this fancy measurement system eliminated the need to have an optical start/stop line like a dragstrip does. However, magazines want to publish times that relate to what the average Joe can accomplish if he takes his car to the local NHRA dragstrip...so all the major US car magazines still test with a 12" simulated rollout. This also makes acceleration times look faster on paper, which of course sells too.

Car & Driver, Road and Track, and Motortrend all use this simulated 12" rollout. That means when you read any acceleration time in those magazines, it's not a true 0-60, 0-100, or 0-150 mph time. It's actually measuring 3 mph to 60, or 3 mph to 100, or 3 mph to 150 mph. The car starts from a stand still, but the clock doesn't begin to run until the car has moved 12", gained 3 mph, and traveled for 0.3 seconds.

GM and Ford also use rollout when claiming their factory times.

Road & Track admits to using rollout.

Here Car and Driver states how they use rollout.

Quote:
"Before you take out your car to try to equal our times, remember that our results are adjusted for weather conditions [see "Correcting for Weather," page 152]. We also average the best runs in two directions to cancel out the effects of wind, and we use a 3-mph rollout. And of course there is car-to-car variability."

Motortrend calls rollout "US Traditional" as they use it too.

Automobile mag does NOT use rollout.

Edmunds does NOT use rollout, but GM does...

Because NHRA and dragstrips are basically non-existant outside the US...ONLY US BASED MAGAZINES AND MANUFACTURERS TEST WITH ROLLOUT.

This means that British, German, or Japanese magazines will clock times that are 0.3 seconds slower than US magazines for the same car as they time true 0-60 and true quarter mile times. They use no rollout.

That partially explains why German cars often perform much better in US magazine testing than the manufacturer specifications have you believe. People often wonder why German companies publish so conservative acceleration times, especially in 0-60 mph. The lack of rollout is one major reason.

Cliffs Notes:
-Most US car magazines don't really test 0-60 mph times...they only give you 3-60 mph. It's sort of cheating.
-You cannot directly compare magazine times with different measurement practices. Edmunds and Automobile don't have slower drivers than C&D; they use other measurement practices.
-Only the US uses rollout

Also, some magazine "test cars" aren't exactly as bone stock as you'd like. I've personally been in catless, semi tuned magazine test cars by major car companies. They all do it, don't fool yourself.
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Last edited by HIRISC; 01-05-2012 at 09:34 AM.
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  #6  
Old 01-12-2012, 04:22 AM
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also tire temp, oil temp, air temp, track condition/material, weight of car (full tank vs near empty + driver), etc. all play a part. Rarely are all of these things specified in the final figures so a true comparison is not likely
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Old 03-20-2012, 12:56 AM
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TGray5 TGray5 is offline
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