Quote:
Originally Posted by serialk11r
I still think tires is the biggest difference. Porsche uses the exact same bodies across a lot of models. Between the Cayman S and non-S for example, the drag is the same, but the S is over an inch lower. Lowering 1 inch takes away as much drag as adding 1 inch of tire (mainly in the back). I think the number cited for each of these changes is about 0.01 Cd, which is quite a lot (3% of the whole car's drag).
Same story with the 911, you can see the 4S models with their fat tires having a couple hundredths of Cd added, but it's only an inch or so of tire width making that difference.
For the MB CLA, we're talking a huge change in tire width from the BlueEfficiency up to the AMG, I could see that alone explaining half of the increase in drag.
|
I couldn't find a general chart for tire width vs drag... But here is an example of increased drag from width.
CD --- tire & rim size (for a 1991 BMW 318i)
0.293 --- 155 R 15; 5 1/2 Jx15 St. with wheel covers
0.294 --- 165 R 15; 61 2 Jx15 St. with wheel covers
0.297 --- 175/70 R 15; 6 Jx15 St. with wheel covers
0.305 --- 185/65 R 15; 61 Jx15 St. with wheel covers
0.311 --- 205/60 R 15; 61 2 Jx15 St. with wheel covers
0.314 --- 205/60 R 15; 7 Jx15 LM
0.319 --- 225/55 R 15; 7 Jx15 LM
(
http://ecomodder.com/forum/showthrea...g-cd-7475.html
Going from 6.1" (155) wide to 7.2" (185) increased by .012 CD, to 8.07" (205) is an additional .006 increase in CD... So if we use this as a benchmark just for conversation, a 1" increase will get an average of +/- .009 increase in CD. That's right in line with the .01 you cited above.
From Mercedes website, and the MB forums:
180 Blue Efficiency, stock 195/16 205/16 depending on country (7.6/8.07")
AMG model, stock 235/18 (9.25")
That's only 1.65" difference in tire size? So by your examples and the above chart, tires should/could account for less than .02 or .03 increase in CD
I understand that percentage wise, that accounts for a lot of the cars drag, but it's still less than a third of the increase in CD from .22 to .32.
~C