The physics of aquaplaning?

[jagman21]

Yesterday in the rain, had a bit of a lets say hairy moment, going round a bend (left, gentle one at that) lost the car and it volently over steered (PS its FWD) accross three lanes i then reversed locked it and turned the car around still having no grip until my car came to a halt accross the road. Thankfully, no-one was behind me, since i went accross three lanes twice i was very lucky i was not rear ended or sided.

My tires are not premium quality but the thread is well within legal requirements, neither was my speed massively accessive, in the dry i'd hammer the bend in question.

After i had corrected the car, ie started travelling in parralel with the road rather the across it. I started wondering about the physics of the situation, and have a number of questions.

1) why was it so easy to loose grip, and then not able to regain it until the car came to a halt? surely there must be some mid point where the car should be controllable.
2) did the gentle rain cause a floating layer of oil which exuberated the situation? rather then wash it way.
3) how was i able to steer the car into the other direction while still have virtually no grip to otherwise control the car?
4) On the point of the curve that i lost grip, should the car not have travelled in perfectly straight tangent to that curve, until it came to a halt or regained grip? why was this not the case.

I have a few theories but would anyone like to add their 2p worth?

jag

[sniff petrol]

Tyres can disperse X amount of water in a given time, so above a certain speed there's simply too much water for them to move so they float on the surface.

[biglepton]

From what you say, that doesn't sound like aquaplaning to me, more like a patch of diesel. What speed were you doing and how much water was on the road?

[Bushmaster]

exacerbated

[jagman21]

the road was throughly wet, as it had been spitting down for a while, and my speed was probable 20-25mph, note that in the dry the bend can be had at 35-40mph, with only a slight hint of tyre screeching.

[lunchbox]

Sounds more like a very greasy road than aquaplaning, correct me if I'm wrong, but aquaplaning IME is less of a snap oversteer moment and more gradual.

[Frik]

I can't see how you ended up oversteering unless it was your rear tyres that were the ones aquaplaning. I can't see how that would happen either.

[kambites]

Frik said:

I can't see how you ended up oversteering unless it was your rear tyres that were the ones aquaplaning. I can't see how that would happen either.

Depends on the car. Elises aquaplane like this because the rear tyres are (a lot) wider than the fronts - hence in the wrong conditions, the fronts cut through the water and the rears plane over it leading to snap oversteer if you're cornering. I think that's where their reputation for being unmanageable in the wet seems to come from.

In a FWD car, the nose-heavy weight distribution will tend to mean that the rears plane first, assuming it has the same width tyres front-to-back.

Edited by kambites on Monday 11th August 18:43

[jagman21]

i thought aqua planing was a prolonged "floating" effect on water not a snap movement

[Fidgits]

lunchbox said:

Sounds more like a very greasy road than aquaplaning, correct me if I'm wrong, but aquaplaning IME is less of a snap oversteer moment and more gradual.

not really...

aquaplaning comes of a layer of water being between the rubber and the road (the tread not dispursing it) so generally it happens suddenly...

however a reduction in speed should easily correct the situation (except in the case of an increasing water mass)

Unless the OP braked (and continued to do so) when he felt the front end aquaplane, thus causing the rear wheels to lock during the aquaplane, i cannot see how the incident occured based on simple aquaplaning.

More likely is the solution offered by others that there was diesel on the road you could not see due to the water coverage and this caused the rear end to break away.

[kambites]

jagman21 said:

i thought aqua planing was a prolonged "floating" effect on water not a snap movement

When the wheels leave the ground you lose all grip. So if you hit a puddle and the rears lose grip, you will snap oversteer if you are putting much lateral stress on the tyres.

[MrKipling43]

Sounds like diesel to me.

[mattley]

Aquaplaning only happens over standing water. Just losing it like that sounds like diesel on the road to me.

[Crash Test Dummy]

jagman21 said:

i thought aqua planing was a prolonged "floating" effect on water not a snap movement

Yes but imagine this situation. All 4 wheels are clearing water and the tyres have some adhesion to the surface. The for one reason or another weight transfers over the front wheels.

There is therefore less force acting downwards on the rear wheels, making the back end become lighter. At this time you hit a patch of water and the car begins to act like a pendulum, pivoting on the front wheels.

Quite a common problem [snap over-steer] , even in the dry with FWD hot hatches where the rear suspension is too stiff or the car is over-strutted

Edited by Crash Test Dummy on Monday 11th August 18:47

[kambites]

MrKipling43 said:

Sounds like diesel to me.

If it was diesel, you'd expect all the tyres to have hit it, the car would have initially understeered (as the front wheels hit the diesel) and then slipped into a four wheel drift (while all the wheels were on it) then snap oversteered (as the fronts passed the spill).

If the OP was driving fast, he might not have consciously registered the changes but at 25mph it takes a significant amount of time to travel a car's length.

Edited by kambites on Monday 11th August 18:48

[Frik]

kambites said:

In a FWD car, the nose-heavy weight distribution will tend to mean that the rears plane first, assuming it has the same width tyres front-to-back.

That's a good theory but I'm not sure that it'd be the case. It's certainly never happened like that to me.

Sounds more like the OP lifted off at some point during the manoeuvre.

[kambites]

Frik said:

kambites said:

In a FWD car, the nose-heavy weight distribution will tend to mean that the rears plane first, assuming it has the same width tyres front-to-back.

That's a good theory but I'm not sure that it'd be the case. It's certainly never happened like that to me.

Sounds more like the OP lifted off at some point during the manoeuvre.

Yeah the difference in weight is probably insignificant. Lifting off mid corner, especially if it was as the front encountered grip again would probably be far more dramatic.

[jagman21]

Frik said:

kambites said:

In a FWD car, the nose-heavy weight distribution will tend to mean that the rears plane first, assuming it has the same width tyres front-to-back.

That's a good theory but I'm not sure that it'd be the case. It's certainly never happened like that to me.

Sounds more like the OP lifted off at some point during the manoeuvre.

yes i did, i didn't have the balls to use the trottle when the cars in a state like that! Even though i knew i should have, its hard to repell instinct and use the trottle when you want to slow down and control the car!!

[jagman21]

looks like the general concensus is that it was diesel, it makes sense given the facts, so would the same thing have happened in the dry then?

[john_r]

I don't think it was diesel. I think you were driving too fast for the conditions - you claim to have been doing 20-25 mph at the most, yet you were able to spin back and forth across 3 lanes? Also if the bend causes tyres to squeel at 35mph in the dry then it's a fairly sharp bend...

Chalk it down to experience...