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Old 03-14-2009, 02:11 PM   #1 (permalink)
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tire width vs. drag (Cd)

The question of the effect of tire width on drag seems to come up once in a while. So here's more data to add to the pile, from Hucho, 1998.

---

Influence of the tire width on drag, lift, and yawing moment, after H Kerschbaum Fig 5.72



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

Keep in mind the increase in Cd comes with an increase in A (frontal/projected area) too, so you're getting a double whammy.

Add to this from Phil's notes:
  • 1986, wind tunnel development work for Subaru XT show a drag increase of 5.1 % when tire size is increased from 155 to 185 series radials.
  • 1984-1987 HONDA CRX shows jump from 165 to 185 series radials increase drag 9.3 %.
Phil's notes also point out that the increased drag from wider tires can be partially offset by managing the airflow ahead of them (dams/spats).

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