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Is a tire's propensity to hydroplane proportional or inversely proportional to tire pressure? That is, if a tire is over inflated or under inflated, is it more or less likely to hydroplane?

Edit: This subject came up in this thread and the moderators agreed that it deserved its own question. That's why it's posted here.

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In this article by the NHTSA a formula is given for calculating the speed at which a tire might be expected to hydroplane. The formula assumes "...the water depth exceeds the capability of the tread design to remove water." The formula is:

Hydroplaning speed = 10.35 x √ inflation pressure
As some examples they give this table:
  • At 30 psi, hydroplaning could occur at 56.7 mph
  • at 25 psi, hydroplaning could occur at 51.8 mph
  • At 20 psi, hydroplaning could occur at 46.3 mph.
Clearly, the lower the tire pressure the more likely a tire is to hydroplane.

There are two main reasons for this

  • A tire with lower inflation pressure has more contact area, and it, therefore, takes less pressure to lift the tire off the pavement.
  • A tire with lower inflation pressure presents a slightly concave profile to the pavement. This causes water trapped under the tire to be directed towards the tire's center line rather than away from it.
Source

Conversely, a tire with higher pressure presents less contact area, which therefore requires more pressure to lift the tire off of the pavement, and it presents a slightly convex profile to the pavement, which directs water towards the outside of the tire rather than towards its center.

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This discounts tread type and tread depth. Tire rubber hardness is also in play. A rain tire and a slick will perform very differently in similar wet conditions. Best advice would be to always slow down in wet conditions.

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