What's the expert's opinion on how to handle a hydroplaning Model S AWD at highway speeds?

[Galinette]

The thing is that snow tires are not practical for most of us who live in parts of the country with relatively moderate winter weather.

There is no such thing as snow tires (except maybe studded tires), but winter tires.
And you don’t need snow or ice to make winter tires useful, below 15C/59F they are more effective than summer tires. At those temperatures summer tire rubber is too hard. Winter tires are made of a softer rubber which is optimal in that case. In fact winter tires are even better for traction at high temperatures, but they become too soft when hot and you will wear and destroy them.

 

[shellderp]

I never had to lower my regen in winter, though of course since its cold the regen is often not usable anyway. In ludicrous all winter long, with some careful foot control

check out this whole vid, but at the timestamp he talks about why regen shouldn't be a problem in theory, as soon as the wheel loses grip regen can't slow it down anymore

 

[ZeeDoktor]

This may surprise some here, but the speed at which hydroplaning occurs is purely dependent on speed and tire pressure. In aviation, we approximate this in knots as 9 times the square root of the tire pressure in psi. There will be small variations to this due to the tire profiles used in automotive applications, but the general rule of thumb is surprisingly accurate.

So for my S performance at 45 psi, hydroplaning will set in above 9*sqrt(45) = 60 kts = 70mph = 111kph. In most other cars with 35psi tire pressure, you're looking at 62mph / 99kph.

It's really important to keep your tire pressure where it should be!

Since the only factor of the two you can influence while driving is speed, letting the car slow down will regain traction. I haven't tried changing tire pressure while driving.

 

[DCEV]

This may surprise some here, but the speed at which hydroplaning occurs is purely dependent on speed and tire pressure. In aviation, we approximate this in knots as 9 times the square root of the tire pressure in psi. There will be small variations to this due to the tire profiles used in automotive applications, but the general rule of thumb is surprisingly accurate.

So for my S performance at 45 psi, hydroplaning will set in above 9*sqrt(45) = 60 kts = 70mph = 111kph. In most other cars with 35psi tire pressure, you're looking at 62mph / 99kph.

It's really important to keep your tire pressure where it should be!

Since the only factor of the two you can influence while driving is speed, letting the car slow down will regain traction. I haven't tried changing tire pressure while driving.

Wow this is insightful.

Can someone explain the physics of the tire pressure though?

I always thought hydroplaning had more to do with the volume and rate of water flow. How are these two variables not a part of the hydroplaning equation? It seems logical that a fast flow of water sideways 6 inches deep could get your car to hydroplane at 50 mph even at 45psi.

 

[Wol747]

I was about to post the nine root P myself. It's important to note that once it's initiated hydroplaning can continue to quite a low speed, riding on the wedge of water.

>>Model S is just a car. There’s not really anything to do differently than any other car.<<

Oh, there is! If you have regeneration braking it's the same as using the brakes from the tyres' point of view. As others have pointed out you're probably best using the accelerator to bring the speed down slowly rather than let the regen throw out the anchor!

 

[UncleCreepy]

I always thought hydroplaning had more to do with the volume and rate of water flow. How are these two variables not a part of the hydroplaning equation?

And while we're at it, please also explain why tread depth and tread pattern are not a factor.

 

[murphyS90D]

If one or both wheels on an axle are hydroplaning there will be no regen braking. Because of the way a differential works regen braking needs both wheels in contact with a high friction point. If either wheel is free to slip no force will be transmitted to the motor via the differential input shaft. This does not apply to a limited slip differential but I do not think a Tesla has one of those.

 

[ZeeDoktor]

This thread has all the answers: How does tire pressure affect susceptility to hydroplane?

Check out the pictures here showing the ground contact tires make depending on inflation: http://www.tirerack.com/winter/tech/techpage.jsp?techid=3

Of course the tread will have an effect on how efficiently it can remove water from underneath the contact surface. As will the rubber hardness.

But by far the biggest effect is the size of the contact area, which is dominated by the inflation pressure.

 

[agloutney]

I had a scary experience a few years ago. It was raining and right about at the freezing point one early quiet morning. Roads appeared to be wet as they had been salted so driving was pretty normal. That is until I got into the EV lane which I guess no one else was using. It was wet ice and traction was instantly nil at highway speed. The mighty S started to slip sideways facing the left lane concrete divider. Instinctively I let off the accelerator which caused it to fishtail the other way facing the other cars in the right lanes all the while tracking straight on the wet ice as Newton intended. Once the accelerator feathered to 0 power/regen things started to slowly stabilize with the rumble of the anti skid (not ABS) system working overtime to own the ice claiming victory against long odds! It was savage!
Moral of the story is don’t accelerate or decelerate. The car will take care of you. I set regen to low since then in winter. It also helps keep the brakes clean.

 

[Wol747]

>>If one or both wheels on an axle are hydroplaning there will be no regen braking. Because of the way a differential works regen braking needs both wheels in contact with a high friction point. If either wheel is free to slip no force will be transmitted to the motor via the differential input shaft. This does not apply to a limited slip differential but I do not think a Tesla has one of those. <<

True. Also, thinking about it, cars have different treads than aircraft. Aircraft tyres have circumferential grooves with no cross-tyre ones. I guess that this is because runways are mostly grooved so water is dissipated via those rather than the tyres' sideways grooving. That may change the nine root P equation - it's above my pay grade!

 

[ZeeDoktor]

@Wol747 Your handle sounds like you might know a thing or two about airplane tyres? The grooves in airplane tyres (airliner size) are much larger and deeper than the tread patterns found in automotive tyres. Per surface area, they can accommodate a similar amount of water as car tyres. Hydroplaning is purely dependent on the tyre's ability to displace water from underneath the contact surface - which as shown in the links above is predominantly determined by the size of the contact area and thus, by maximising the amount of downward force per surface area, so more water can be displaced. The type of grooving really only plays a secondary role as long as the grooves are large and can accommodate a lot of water.

Runway grooves on the other hand don't form part of an airplane certification process. India for instance doesn't seem to have grooved runways, not even for most of their monsoon prone airports as the recent overrun illustrated. So for airplanes to be able to land and take off from a water contaminated runway they need to demonstrate being able to roll at landing speeds, on a contaminated runway, without hydroplaning. 130kts is a typical gate speed for most aircraft, which then explains the tyre pressures in a 747-400 for instance, which is around 210 psi (9*sqrt(210) = 130 kts incidentally). Max tyre speed (usually around 200 kts / 230 mph / 370 kph) is much higher though. A fully loaded 747-400 returning immediately after taking off and at maximum landing mass will touch down at about 185kts. A factor to consider when returning to a wet runway.

 

[Wol747]

I've been retired for 20 years or more and have forgotten the -400 numbers! Thanks for the reminder....