[-EndOfAnEra-]

Hello everyone,

It's been a great year with my car even if its been horrid everywhere else! So it was bound to not last forever. I have a problem that's totally shot my confidence in my car and I'm not sure if its me, the car, or just the freak weather.

The last three days in Cleveland we've seen some REALLY heavy rain. The kind that makes it almost impossible to see on the highway. I discovered on my morning commute two days ago that adaptive cruise would make my car hydroplane, lightly and then more severely. I noted an immediate improvement when I turned it off and reasoned that its attempt to constantly hit a specific speed (usually about 65 to 69 MPH) was overwhelming traction with so much water. Turning it off and relying on engine braking by letting the foot off the accelerator improved and I figured that was that.

Then today, same kind of very heavy rain on my commute home. I quickly turned off adaptive and had less problems, but the car still wanted to hydroplane at much lower speeds than I expected.

I checked my tire tread depth and it seems fine:

Left Front = 4.44 mm (between 5/32 and 6/32 of an inch)
Right Front = 4.41 mm
Left Rear = 5.4 mm (between 6/32 and 7/32 of an inch)
Right Rear = 5.08 mm

These are the tires I've had with the car when I picked it up in April 2020. I didn't drive the car much during the pandemic and swapped to winters in October of 2020 until April of 2021. My rule of thumb has always been to seek replacements for my summers when they hit 3 mm on an axle, or 4/32 of an inch. I'm closer than I'd expect but quite surprised at the life I've gotten from them.

Pressures are 42 PSI front, 44 PSI rear. Mind you, I had a devil of a time finding the "real" pressures for an American RWD M340i on staggered 792Ms so anyone has input please speak up.

My previous car, a RWD E90 335i 6MT never had any issues with hydroplaning, even on the same Michelin PS4S tires with less tread depth (less than 3/32" - never going that low again ). I'm hoping to debunk the variables here and get to the bottom of this issue. Anyone skilled in the science behind hydroplaning, please let me know your thoughts.

1. New car is heavier than the old, any factor?

2. New car tire specs are front 225/40/R19, rear 255/35/R19, old car's were front 225/40/R18 and 255/35/R18, so nearly identical but with a slightly taller sidewall from the diameter change, any factor?

3. New car is 8AT, old car is 6MT. Generally I would cruise the old car in 6th but in inclement weather might downshift to 4th for better engine braking. In the M340i, I tend to run in manual 8th on clean highway and use sports auto S6 when I feel I may need more engine braking or better reactivity.

Appreciate any and all thoughts for me!

[ST6IX]

I do not have the biggest insight into this but I do know that hydroplaning can be very random; it happens depending the circumstance. But I also do know that the lighter the car, the more likely you are to hydroplane, so weight is most likely not the reason for you to hydroplane more. Also I noticed you vehicle was RWD, I don't know much of a difference it makes, but RWD does have a harder time maintaining traction in rain. And finally, after what you said, I just think it was some bad luck, it can happen at any high speed at random, you could be doing 90 in rain and not hydroplane but another time you could do 55 and hydroplane.

[HighlandPete]

Probably a combination of speed, tire tread depth and amount of standing water.

I drive in the Highlands of Scotland and we get a lot of heavy rain, 4 - 5mm of tread is really not enough to cope with a lot of the conditions. Speed becomes your main control.

[6oclockshadow]

Heavy rain, enough to make it impossible to see travelling 70mph on cruise...
Not sure what you were expecting...

[hockeymen14]

I live in the Tampa Bay area of FL. We get torrential downpours often. Prior to the rainy season I was noticing slippage when really trying to put down the power even in dry conditions. I was at 18,000mi approaching two years of ownership and had 4-5/32 pretty evenly distributed on my PS4S. I run at 39PSI.

I wasn't surprised given the high performance tread of these tires and how I drive that I was getting low.

I didn't even want to risk it and had them replaced just before the rainy season started. I would suggest you do the same. I also wonder if your high tire pressure is leading to faster wear and maybe leading to some of your hydroplaning issues.

That was my experience. Better to spend the money then slide into a cement wall on the highway. Stay safe.

[Top Gon]

I wouldn't use cruise control in the rain

[Montaver]

At 44psi I suspect your tires are very over inflated, affecting the tires context patch. Check the drivers door sticker for the correct pressures, it's probably somewhere in the low/mid 30's.

[jlapicki]

Originally Posted by Montaver

At 44psi I suspect your tires are very over inflated, affecting the tires context patch. Check the drivers door sticker for the correct pressures, it's probably somewhere in the low/mid 30's.

Yup....tire pressure is way too high.......try 34 to 36 psi and don't ever use cruise control in rain storms....

[hockeymen14]

38 PSI is the recommended pressure for your car.

[jlapicki]

Originally Posted by hockeymen14

38 PSI is the recommended pressure for your car.

Those pressures are for 4 passengers...

[hockeymen14]

Originally Posted by jlapicki

Those pressures are for 4 passengers...

Interesting. I did not know that was for full capacity rating. Makes sense though. I just got these tires a month or so ago and thought the pressure was a little low but what you're saying makes me think this is what the dealership intended. They were at 35psi. I like the feel better and performance at 38psi but maybe I am accelerating tread wear. How are we actually supposed to know what ideal tire pressure is? I knew enough from the OPs 44psi that it was too high but low 30s seem too low.

Input or sources from others?

[TiMSport]

Like some have stated already, your tire pressures are way too high and also I can't imagine why anyone would use cruise control in torrential rains. That's just asking for trouble.

[-EndOfAnEra-]

Originally Posted by HighlandPete

Probably a combination of speed, tire tread depth and amount of standing water.

I drive in the Highlands of Scotland and we get a lot of heavy rain, 4 - 5mm of tread is really not enough to cope with a lot of the conditions. Speed becomes your main control.

Appreciate that insight!

Originally Posted by jlapicki

Yup....tire pressure is way too high.......try 34 to 36 psi and don't ever use cruise control in rain storms....

Thanks for that, so is the idea here that the higher the pressure the smaller the contact patch gets because the tire rides "high"?

Originally Posted by TiMSport

Like some have stated already, your tire pressures are way too high and also I can't imagine why anyone would use cruise control in torrential rains. That's just asking for trouble.

Appreciate the honest feedback. I had found adaptive to be a gamechanger in tight traffic situations as I could use it as a "safety net" to cover sudden stops or lane changes from drivers around me. I won't be using it in heavy downpours going forward.

Thanks to all for your responses! I realized another possible issue. My front tires are wearing faster than my rears, and especially the shoulders of the tire have much heavier wear than the inside.

All my records for my previous car have the tires wearing close to evenly, with rears always lower than fronts. It's a good possibility that my prior experiences in the old car didn't result in severe hydroplaning because the fronts could still grip and steer, while the rears benefited from the wider contact patch.

I've scheduled an alignment appointment with my mechanic, I will update this thread afterwards if indeed that was the cause of the uneven wear.

[Sportstick]

The pressures may be too high for most situations. However, the force exerted by the pressure also does go in the down direction through the contact patch, so a higher pressure will exert more downward force toward the road than a lesser pressure, and push aside more of the water the part of the tire surface between tread might otherwise "float" over. The general guideline for hydroplaning is to keep the speed below the square root of the tire pressure x 10. So, pressure of 42 psi has a square root of 6.48. The goal would be to stay under 64.8 mph. The lower the pressure, the lower the speed, so I would not agree with the suggestion to drop into the mid to low 30s.

And, yes, the door lable is an FMVSS regulatory requirement from NHTSA. The pressures represent GVWR...full passenger and cargo load. Typical daily driving recommendations will likely be somewhat lower.

[Montaver]

Originally Posted by Sportstick

The pressures may be too high for most situations. However, the force exerted by the pressure also does go in the down direction through the contact patch, so a higher pressure will exert more downward force toward the road than a lesser pressure, and push aside more of the water the part of the tire surface between tread might otherwise "float" over. The general guideline for hydroplaning is to keep the speed below the square root of the tire pressure x 10. So, pressure of 42 psi has a square root of 6.48. The goal would be to stay under 64.8 mph. The lower the pressure, the lower the speed, so I would not agree with the suggestion to drop into the mid to low 30s.

And, yes, the door lable is an FMVSS regulatory requirement from NHTSA. The pressures represent GVWR...full passenger and cargo load. Typical daily driving recommendations will likely be somewhat lower.

Drive your car on track at 35psi, then 44psi and see which has more grip. Overinflated tires deform the tires contact patch as the force is not distributed evenly across the center of the tire. It also means the tire is more likely to skip across road imperfections, increasing the chances of hydro planing.

[Blubaron79]

Slow down. If it's raining that hard you cannot see you should be going about 55-60 max

[HermanMunster]

-Have the right tires and in the right conditions for the job (all seasons, correct tire pressure, tread depth, etc)
-disable any driving automations such as cruise control,
+if you're already aqua-planning, when you regain traction, cruise control may cause jerkiness
+cruise control can't anticipate deep bodies of water in the road ahead and won't slow down to prevent aqua planning
- Most importantly , when it's pouring, SLOW DOWN!
+tire grooves are there to evacuate water, can't do it if you're going too fast
+ even with slick racing tires you can keep traction and control if you are driving slow
- remember you need traction not only to move forward but also to change direction and to brake

[GRB60]

The sticker on the door tells you the correct tire pressures as well as load information. It DOES NOT mean that the listed pressures are only for when you are fully loaded. Use 38/38.
If traction is a problem in the rain, simply slow down. There's too much water between the tire and the road.

[Sportstick]

Originally Posted by GRB60

The sticker on the door tells you the correct tire pressures as well as load information. It DOES NOT mean that the listed pressures are only for when you are fully loaded.

The OEM may also happen to use the same numbers as unladed pressure recommendations in the owner's manual. However, the label values are the maximum cold tire pressures front and rear, or if lower, at which they certify the vehicle at GVWR. These are not intended for ordinary and typical use for a lone driver.

https://www.govinfo.gov/content/pkg/...sec571-110.pdf

[GRB60]

[QUOTE=Sportstick;27922978]The OEM may also happen to use the same numbers as unladed pressure recommendations in the owner's manual. However, the label values are the maximum cold tire pressures front and rear, or if lower, at which they certify the vehicle at GVWR. These are not intended for ordinary and typical use for a lone driver.

Not trying to escalate this, but you attached a lot of information, none of which seemed to support your assertion.
Keeping in mind that most car owners don't spend great amounts of time reading government sites (or even their owner's manual), it seems much more likely that they'd put the RECOMMENDED cold pressures on the door, where it's easily referred to.

I don't own a BMW (yet) but the manual on my 2020 RAM 1500 says:
"The proper cold tire pressure is listed on the driver's side B-pillar or rear edge of the driver's side door."

[jlapicki]

[QUOTE=GRB60;27923379]

Originally Posted by Sportstick

The OEM may also happen to use the same numbers as unladed pressure recommendations in the owner's manual. However, the label values are the maximum cold tire pressures front and rear, or if lower, at which they certify the vehicle at GVWR. These are not intended for ordinary and typical use for a lone driver.

Not trying to escalate this, but you attached a lot of information, none of which seemed to support your assertion.
Keeping in mind that most car owners don't spend great amounts of time reading government sites (or even their owner's manual), it seems much more likely that they'd put the RECOMMENDED cold pressures on the door, where it's easily referred to.

I don't own a BMW (yet) but the manual on my 2020 RAM 1500 says:
"The proper cold tire pressure is listed on the driver's side B-pillar or rear edge of the driver's side door."

Wow!.....great discussion about tire pressures....by the way....the car will tell you what pressures are recommended for the conditions if you scroll to the screen to check your current pressures....

[hamera]

Any ECO drivers out there?