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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This comes up in Airline Transport Pilot Licence exams. The correct answer is that the Hydroplaning speed increases with higher tyre pressure.– Chris HarryApr 15, 2020 at 19:13
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2 Answers
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 pressureAs some examples they give this table:
Clearly, the lower the tire pressure the more likely a tire is to hydroplane.
- 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.
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.
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.
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.
