Route X speeds still unexpectedly quick?

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acjy1985

acjy1985

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acjy1985
I know a while back there were some threads about unrealistic top speeds being achieved by cars on Route X. There was even talk of the drag model not being right on this one circuit. Do people think this is still happening (or never happened)? Reason I ask is as I planned to start my testing programme and Route X was supposed to be the first circuit I used to record the top speed of each car on the back straight. I didn't want to start testing though if this was going to give unreliable results. I remembered hitting about 160mph ~260km/h in a stock Clio R.S. which I thought was a bit high. I think the book speed for it is about 143mph but not sure if that is limited by a rev limiter or the natural drag overcoming power.
 
I've done some tests here with a stock C7 to compare the acceleration profiles at different tracks (1.04).
There were some differences at high speeds, but they can be explained with inclines at the other tracks. If they were due to drag, the effects would be seen already at lower speeds. I don't think the physics are different at SSRX, it's just the only place with sufficiently long and level straights to reach top speed.
17357411wk.jpg


To further look into this, I've used an Excel sheet to calculate acceleration profiles. Using a cd * A of 0.605 m² (I haven't found data for the C7 yet , so this is assuming a very good drag coefficient of 0.29 and the C6's frontal area of 2.08 m²), I can get a pretty good fit (orange line) to measured data from Road&Track (circles, data taken from here) up to 120 mph.
If I leave everything else the same, but only adjust cd * A to 0.4 m², the Excel model (red curve) now almost perfectly matches the in game results at SSRX (dashed green curve). So the exaggerated top speeds in GT6 can be fully explained by a drag coefficient that's far too small (or too low air density, which would give the same results).
Also note that the difference between the red and orange curves is quite a bit greater than those for the different tracks.
Just out of curiosity I've also tried increasing the power to reach the same top speed as in game with the more realistic (yet probably even still a little bit lowish) drag. This takes 494 kW (blue curve) instead of 338 kW.
17423714vc.jpg


TL;DR: As of 1.04, drag is still far too low in GT6, and not only at SSRX.
 
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What did he say?

:-)

Much work involved there. I hope someone in charge uses it to improve things to increase realism.

I believe the C5 Corvette had a cd of .29. I read that the C7 has improved aero and thus should be a lower cd than the C5. I remember this because my Audi Allroad has a cd of .29. (formerly owned)
 
Thanks Verbal. Some spectacular work there. At least it appears the drag is consistent (even if it may be not quite right). Your curves have allayed my fear that drag diminished at higher speeds. Your work gives me the confidence to just get on with the testing and not worry about the top speeds all being wrong. Cheers!
 
Verbal
I've done some tests here with a stock C7 to compare the acceleration profiles at different tracks (1.04).
There were some differences at high speeds, but they can be explained with inclines at the other tracks. If they were due to drag, the effects would be seen already at lower speeds. I don't think the physics are different at SSRX, it's just the only place with sufficiently long and level straights to reach top speed.
17357411wk.jpg


To further look into this, I've used an Excel sheet to calculate acceleration profiles. Using a cd * A of 0.605 m² (I haven't found data for the C7 yet , so this is assuming a very good drag coefficient of 0.29 and the C6's frontal area of 2.08 m²), I can get a pretty good fit (orange line) to measured data from Road&Track (circles, data taken from here) up to 120 mph.
If I leave everything else the same, but only adjust cd * A to 0.4 m², the Excel model (red curve) now almost perfectly matches the in game results at SSRX (dashed green curve). So the exaggerated top speeds in GT6 can be fully explained by a drag coefficient that's far too small (or too low air density, which would give the same results).
Also note that the difference between the red and orange curves is quite a bit greater than those for the different tracks.
Just out of curiosity I've also tried increasing the power to reach the same top speed as in game with the more realistic (yet probably even still a little bit lowish) drag. This takes 494 kW (blue curve) instead of 338 kW.
17423714vc.jpg


TL;DR: As of 1.04, drag is still far too low in GT6, and not only at SSRX.
Click to expand...

"Glad to know it's all wrong" - nice oxymoron!

Great job on the research 👍 What do you think about flat floors ? Once installed I found the cars (that allows it's install) achieve more realistic top speed and acceleration, often matching perfectly factory specs or missing them just by few km/h.
 
@Verbal : Looks solid. In order to test for drag coefficient vs air density, you could calculate what the air density would have to be in order to produce the results you got. Then you can try other cars and see if that air density value explains the differences in top speed for those as well. If it does, it must be the air density that's off. If it doesn't, it could still be air density, but it's also something else, possibly the drag coefficient.

Route X also has the advantage that it's right at sea level, so it's easy to calculate what the real air density would be. The screen before you start the run also shows air temperature. It's almost as if Route X was made for science :D
 
TurismoBad
What do you think about flat floors ? Once installed I found the cars (that allows it's install) achieve more realistic top speed and acceleration, often matching perfectly factory specs or missing them just by few km/h.
Click to expand...
I guess that's compensating one error with another one, trading off realistic downforce for realistic top speed...

eran0004
@Verbal : Looks solid. In order to test for drag coefficient vs air density, you could calculate what the air density would have to be in order to produce the results you got. Then you can try other cars and see if that air density value explains the differences in top speed for those as well. If it does, it must be the air density that's off. If it doesn't, it could still be air density, but it's also something else, possibly the drag coefficient.
Click to expand...
Air density would have to be off by the same factor, i.e. 50% 34% too low. Probably indeed easier to screw up that one rather than each car individually. Will test some more ;)
 
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I took the data for the Corvette C7 and reverse-calculated what the air pressure would be. The result is 80.2 kPa, which at a temperature of 27 degrees Celsius would be the atmospheric pressure at 2100 meter above sea level.

  1. Route X is obviously not at 2100 meters above sea level. More like 10-20 meters.
  2. At 2100 meters above sea level the engine would have lost about 95 BHP due to the thinner air, so that top speed would still not be possible to achieve.
  3. The train station in Rotenboden (Matterhorn track) is at about 2800 meters above sea level, for comparison.
  4. At sea level, the air pressure should be just above 100 kPa.
 
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acjy1985
I can't speak for everyone else, but I have been taking my reading from the back straight as this is flatter than the one you start on.
Click to expand...

Ah, well ok then. Just to inform people that the 1st straight an up-down hill section if you didn't already know.
 
Hoping for an aero fix in an update real soon, we need to get realistic aero back, GT5 had realistic top speeds and GT6 doesn't. By the way, expect a comment on this thread from some starry eyed GT fan about how 'top speed isn't important anyway' or 'GT isn't supposed to be a simulator' real soon.......... :rolleyes:
 
I made an Excel tool to test the hypothesis that the top speeds are caused by too low atmospheric pressure.

atmocalc1.jpg


I've only tested a few cars so far, the tricky part is to find accurate IRL top speed data to compare with the game. The results I have so far indicates an atomspheric pressure of somewhere between 70 kPa to 75 kPa, but with so few samples it's impossible to draw any conclusions just yet.

So if anyone knows a great web resource for finding top speeds of cars, please let me know!

Also, if anyone has got some of the more expensive cars that aren't available in arcade mode, such as the Bugatti Veyron, I'd aprreciate some help with testing their top speeds on Route X (on the flat straight, the downhill section does not count). Just to keep all tests as similar as possible, I've tested all cars at 12:00 track time with the track temperature of 26-27 degrees Celsius. All cars are stock of course, but if the stock gearbox makes the engine rev out before top speed is reached, it's allowed to fit a customizable gearbox.

The car's I've tested so far are:

Pagani Huayra '11
Corvette Stingray '14
Scion FR-S
Ford Shelby GT500 '13
Audi R8 '07

I'll have attached the Excel sheet below, if anyone want to play around with it.
It's based on the formulas found here: http://www.engr.colostate.edu/~allan/fluids/page8/page8f.html
 

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