Aquaplaning formula

[Zummertor]

Having read a couple of posts about Aquaplaning which I suspect we've all had the experience of in one car or another I remembered that there is apparently a formula for speed and aquaplaning.

 

I guess the Mad Scientist will know this in detail.

 

This came from an EasyJet pilot I know and says this is as a good rule of thumb as any so as he talks in Knots, we'll multiply by 1.15'ish to get mph

 

Formula: aquaplaning may occur when min speed is greater than 1.15 * 9 * √ tyre pressure in psi

 

and then if started it won't stop until the speed drops below 1.15 * 7.5 * √ tyre pressure in psi

 

Once the depth of water exceeds the tyre tread the tyre behaves as if it were bald.

 

One strange point he made was that as aquaplaning occurs a pool of steam is generated under the tyre which causes the tyre rubber to start to melt. So not only a but it costs you £££££ even if you stay on road and don't hit anything.

 

I'm told that this in in aviation training and to get there it must be proven.

 

How much of this is true I don't know but it certainly makes you think, although doing that maths whilst driving in the wet and looking at speed, traffic etc. probably isn't going to happen.

[ir_fuel]

errr.

 

What is this gibberish?

 

It doesn't even take into account the tyre width nor the thread.

[Zummertor]

Apparently, tyre width has little difference as the tyre pressure makes the difference.

 

May be gibberish but having heard it I thought I'd share to see what you all thought.

 

Usually aviation industry does its research, don't know if this applies to cars though.

[ir_fuel]

So why do people tend to recommend smaller tyres in wet conditions then?

 

What you are saying is that driving a Porsche with 315 width rear tyres over a puddle of water will do the same as driving that car with a 185 width rear tyre?

 

And if width doesn't matter, why do motorcycles have nearly no risk of aquaplaning except for in very extreme circumstances?

[martinmac]

I have awful trouble with aquaplaning at work

[Zummertor]

We had this discussion, admittedly it had beers involved. What he was saying is that this is a good approximation because the difference becomes too small only a few mph, the point being that speed is by far the biggest factor regardless of tyre types / rubber mixtures, so don't expect a massive practical difference.

Obviously racing is pushing things to limits so tolerences are all needed to get the extra little bits.

 

Seemed logical at the time and given the refernces in the airline industry to this it may be transferable to cars but may need some constant adjustment.

 

The point is I don't know and thought maybe one of you much wiser people would know.

 

Out of curiousity has anyone tried to deliberately aquaplane in a car and then change to smaller tyres to see if it happens again ?

[ir_fuel]

maybe in aviation the speeds are a bit higher than when talking about cars?

[H5]

I thought the aquaplaning formula was

 

amount of water > amount of water displaced = aquaplaning.

[GC350z]

You want low tyre pressure if you are planning on driving on snow or water (if insane) to increase the surface area as there's virtually no grip.

 

My understanding of aquaplanning is that the tyre is no longer in contact with the ground and is effectively floating.

 

Aquaplanning in terms of airlines is a lot different from the road. A plane is not designed to turn at speed when on the ground.

[Tere]

Actually....

 

The formula is correct, presuming depth of water exceeding the tread depth. At that point all tires perform exactly the same because the tires can't displace enough water in time (which relates to speed) to make contact with the road. This guide is used primarily when landing jets under wheel braking on a wet runway; it presumes standing water AND no anti-skid equipment.

 

In a half-inch of standing water (a torrential downpour), the Z will begin hydroplaning at about 60 mph regardless of your tire make. The big danger occurs when you're tooling down the road at 80 and hit a huge patch of standing water AND come on the brakes (ABS off) as humans will instictively do -- you may as well try driving on ice at that speed. Given enough hydroplaning time in the water under braking (wheels will lock up instantly), the bottom tire surface will start to melt -- it's characterized by little beads of rubber on the tire surface. The next big problem is when you come out of that big puddle with the brakes locked -- instant blown tire(s).

 

Moral to the story is don't disable your ABS if you plan on hitting a huge patch of standing water over 60 mph AND slamming on your brakes. Second moral to the story is don't turn the wheels radically because when you come out of the water puddle you'll suddenly go wherever the wheels are pointing which may not be the direction you really want to go.

 

When water depth does not exceed tread depth, then tires and a thousand other variables come into play, and the hydroplaning formula goes out the window. Automotive technology has taken out a lot of the real hydroplaning dangers, BUT it can't protect a loose nut behind the wheel entirely.

 

By the way, a hydroplaning skid mark under braking is light colored instead of black -- the road gets a little steam cleaning. You'll see 'em occasionally on the road if you look.

[ir_fuel]

ok,

 

 

that makes sense. But that surface melting stuff sounds quite extreme, no? How many meters do you have to aquaplane to have that

[mbs]

I have awful trouble with aquaplaning at work

[martinmac]

I have awful trouble with aquaplaning at work

 

Glad somebody got it

 

Just working out aquaplaning formula for containerships on mud at the moment

[mbs]

I have awful trouble with aquaplaning at work

 

Glad somebody got it

 

Just working out aquaplaning formula for containerships on mud at the moment

 

Does that mean you are in for a big bonus?

[martinmac]

Contract work doesnt pay and the MCA tug on scene is on a none salvage contract. Its the same size as the one I got last month which didnt pay either. Still, record to date, 15 salvages and paid for 8 so still ahead of average

[stevie_350z]

Actually....

 

By the way, a hydroplaning skid mark under braking is light colored instead of black -- the road gets a little steam cleaning. You'll see 'em occasionally on the road if you look.

 

who the hell hits the brake when aquaplaning!!

 

Are you sure that those "light coloured" (notice the correct spelling - just teasing, Tere!) is not the skid marks from the drivers undercrackers?

[Tere]

ok,

 

 

that makes sense. But that surface melting stuff sounds quite extreme, no? How many meters do you have to aquaplane to have that

It would be a pure guess. In an aircraft at around 150 knots with the wheels locked, it takes about a couple hundred feet of planing before the rubber becomes significantly reverted. So in a car at 80 mph (about half the speed), the distance would be exponentially longer. I think in a car, it's more theoretical than practical. The bigger problem is exiting the water with brakes locked (they won't unlock unless you release the pedal) because it will grind through the tread in a few seconds, blowing the tire. In the Z with ABS/TCS/VDC on it's not much of a worry unless the driver gets extraordinarily dumb.

 

In the pilot training business, we used to keep a blown reverted rubber tire around for show-and-tell with the students. They tend not to forget it -- little beads of rubber surrounding a complete flat spot on an otherwise pristine tire where you can count all 12 plys and stick your pinky into the hole.

[AndySpak]

Is it just me or in the days before ABS did tyres just leave big black lines on the road when they locked up? Why if you come out of an aquaplane with locked wheels will the results be so dramatically different than when a non-ABS equipped car does an emergency stop?

[Ebized]

In the pilot training business, we used to keep a blown reverted rubber tire around for show-and-tell with the students. They tend not to forget it -- little beads of rubber surrounding a complete flat spot on an otherwise pristine tire where you can count all 12 plys and stick your pinky into the hole.

 

[Tere]

Is it just me or in the days before ABS did tyres just leave big black lines on the road when they locked up? Why if you come out of an aquaplane with locked wheels will the results be so dramatically different than when a non-ABS equipped car does an emergency stop?

In a panic stop on dry pavement it's virtually impossible to actually lock the wheel with standard braking systems (including non-ABS) -- the decelerative force simply exceeds the tire's grip capacity. You may think the wheel is completely locked but it's actually still rotating. You lose a lot of rubber, but it's lost somewhat evenly around the whole tire. When the tire is hydroplaning though it's quite easy to completely lock the wheel and it will remain locked (particularly if there's some level of rubber reversion going on -- melting or melted rubber) when the car exits the plane, thus almost instant pop.

[Digsy]

Is it just me or in the days before ABS did tyres just leave big black lines on the road when they locked up? Why if you come out of an aquaplane with locked wheels will the results be so dramatically different than when a non-ABS equipped car does an emergency stop?

In a panic stop on dry pavement it's virtually impossible to actually lock the wheel with standard braking systems (including non-ABS) -- the decelerative force simply exceeds the tire's grip capacity. You may think the wheel is completely locked but it's actually still rotating. You lose a lot of rubber, but it's lost somewhat evenly around the whole tire. When the tire is hydroplaning though it's quite easy to completely lock the wheel and it will remain locked (particularly if there's some level of rubber reversion going on -- melting or melted rubber) when the car exits the plane, thus almost instant pop.

 

When I was a student I locked the wheels on my Civic in an emergency stop and flat-spotted the tyres. It was a very long, smokey and scary stop though....

[AndySpak]

Is it just me or in the days before ABS did tyres just leave big black lines on the road when they locked up? Why if you come out of an aquaplane with locked wheels will the results be so dramatically different than when a non-ABS equipped car does an emergency stop?

In a panic stop on dry pavement it's virtually impossible to actually lock the wheel with standard braking systems (including non-ABS) -- the decelerative force simply exceeds the tire's grip capacity. You may think the wheel is completely locked but it's actually still rotating. You lose a lot of rubber, but it's lost somewhat evenly around the whole tire. When the tire is hydroplaning though it's quite easy to completely lock the wheel and it will remain locked (particularly if there's some level of rubber reversion going on -- melting or melted rubber) when the car exits the plane, thus almost instant pop.

I can see the logic in that. Thanks for the explanation.

[Tere]

When I was a student I locked the wheels on my Civic in an emergency stop and flat-spotted the tyres. It was a very long, smokey and scary stop though....

Good point, there's also the mass of the car variable that combines with the efficiency of the brakes and the inefficiency of the tires, along with the velocity when the brake pedal is jammed to the floor. So given the right combination of variables, you probably could completely lock the wheels up.