Motorway driving in heavy rain
Discussion
Inspired partly by a post in General Gassing concerning the misfortune of a boxster and also a trip along the A303 and M3 last Tuesday during very heavy rain, I was hoping to get some advice on aquaplaning.
Assuming we are on motorways/dual carriageways. Is the cause of aquaplaning dependant predominantly upon speed or can other factors such as depth of water, tyre wear, how the water sits, amount of water (i.e. 5m/10m/15m/20m) be more influential than the speed?
When approaching standing water should you maintain speed and only lift off if you start to aquaplane or should you lift off before you hit it? How can you judge what is an appropriate speed to pass through standing water?
Assuming we are on motorways/dual carriageways. Is the cause of aquaplaning dependant predominantly upon speed or can other factors such as depth of water, tyre wear, how the water sits, amount of water (i.e. 5m/10m/15m/20m) be more influential than the speed?
When approaching standing water should you maintain speed and only lift off if you start to aquaplane or should you lift off before you hit it? How can you judge what is an appropriate speed to pass through standing water?
I think you've identified most of the contributory factors involved in causing aquaplaning, but it's an interesting question.
Aquaplaning is often described as when a "wedge" of water builds up between the front tyres and the road surface, to the point when the tyres are effectively "floating" on the surface of the water. I find this a rather simplistic explanation of aquaplaning. To be more precise, aquaplaning occurs when the tyres can no longer disperse the water through which they're travelling, sufficiently to allow the face of the tyre to make contact with the road surface.
Contrary to what many people think, the tread pattern on a tyre does not create any grip with the road surface - the grip is created between the tyre compound itself in contact with the road. The tread pattern of a tyre is primarily there to disperse water - to allow the water to flow into the tread grooves, and therefore allow the tyre compound to make contact with the road surface.
There comes a point, however, when the tyre cannot cope with the amount of water. Yes, the depth of the water can be an element in the equation, but so can tyre wear - the shallower the grooves in your tyre tread, the less water they can cope with. Speed is also a factor - tyres can cope with standing water better at low speeds than at higher speeds. I personally find that (and this is just personal opinion - it's definitely not based on any scientific evidence) water which is flowing across a road surface seems to be more prone to causing aquaplaning than standing water.
So, how to deal with aquaplaning - or, more importantly, how to avoid it?
Firstly, observation skills are paramount. A good driver should be constantly taking note of changes in road surface conditions and scanning for standing or flowing water on a wet day. It's more difficult at night for obvious reasons, but if the weather is similar to what we've been enduring this summer so far, then it's safe to assume that you'll find standing water in the bottom of dips, adjacent to grids, in potholes, by the verges and in camber dips.
If you can spot the standing water, then you can plan for it and drive correctly through it. My advice for standing water has always been this...
NO INPUTS.
Sounds simple, really doesn't it? If the worst comes to the worst and you end up aquaplaning, it isn't actually a problem, providing you intend to continue in a straight line. The problem with water under your tyres is that it will prevent you from being able to steer, brake or accelerate, so as long as you don't do any of those three things, you shouldn't have a problem - like I said - no inputs.
If you're approaching a corner for which you'll have to brake and there is standing water on the approach, then bring your braking zone back away from the water, make sure your speed is correct for the bend before entering the corner, and as you drive through the water, simply make no inputs - no steering, brakes (including deceleration) or acceleration until you're out of the other side of the water.
If you're travelling at speed on a motorway or similar straight road, just maintain a steady speed through the water without braking, decelerating or accelerating, and you should roll through the water with a minimum of deflection.
Aquaplaning is often described as when a "wedge" of water builds up between the front tyres and the road surface, to the point when the tyres are effectively "floating" on the surface of the water. I find this a rather simplistic explanation of aquaplaning. To be more precise, aquaplaning occurs when the tyres can no longer disperse the water through which they're travelling, sufficiently to allow the face of the tyre to make contact with the road surface.
Contrary to what many people think, the tread pattern on a tyre does not create any grip with the road surface - the grip is created between the tyre compound itself in contact with the road. The tread pattern of a tyre is primarily there to disperse water - to allow the water to flow into the tread grooves, and therefore allow the tyre compound to make contact with the road surface.
There comes a point, however, when the tyre cannot cope with the amount of water. Yes, the depth of the water can be an element in the equation, but so can tyre wear - the shallower the grooves in your tyre tread, the less water they can cope with. Speed is also a factor - tyres can cope with standing water better at low speeds than at higher speeds. I personally find that (and this is just personal opinion - it's definitely not based on any scientific evidence) water which is flowing across a road surface seems to be more prone to causing aquaplaning than standing water.
So, how to deal with aquaplaning - or, more importantly, how to avoid it?
Firstly, observation skills are paramount. A good driver should be constantly taking note of changes in road surface conditions and scanning for standing or flowing water on a wet day. It's more difficult at night for obvious reasons, but if the weather is similar to what we've been enduring this summer so far, then it's safe to assume that you'll find standing water in the bottom of dips, adjacent to grids, in potholes, by the verges and in camber dips.
If you can spot the standing water, then you can plan for it and drive correctly through it. My advice for standing water has always been this...
NO INPUTS.
Sounds simple, really doesn't it? If the worst comes to the worst and you end up aquaplaning, it isn't actually a problem, providing you intend to continue in a straight line. The problem with water under your tyres is that it will prevent you from being able to steer, brake or accelerate, so as long as you don't do any of those three things, you shouldn't have a problem - like I said - no inputs.
If you're approaching a corner for which you'll have to brake and there is standing water on the approach, then bring your braking zone back away from the water, make sure your speed is correct for the bend before entering the corner, and as you drive through the water, simply make no inputs - no steering, brakes (including deceleration) or acceleration until you're out of the other side of the water.
If you're travelling at speed on a motorway or similar straight road, just maintain a steady speed through the water without braking, decelerating or accelerating, and you should roll through the water with a minimum of deflection.
Going back a year or two somebody (I think it was the IAM) came up with a formula that supposedly enabled one to calculate the speed at which aquaplaning would occur. IIRC this was based solely on tyre pressure, taking no account of any other factors, and it looked like complete nonsense to me. I tried using the formula taking a fairly average sort of tyre pressure, and it seemed to indicate that aquaplaning would take place at a speed as low as about 50 mph
One thing I would like to add to what Reg says though is that even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
The key to this is to read the road surface very carefully, make sure you spot the areas of standing water and get the speed down before you get there. Subject to that, and the usual caveat about stopping distances, it is possible to travel safely at very high speed in the wet, but it needs great care. HTH.
Best wishes all,
Dave.
One thing I would like to add to what Reg says though is that even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
The key to this is to read the road surface very carefully, make sure you spot the areas of standing water and get the speed down before you get there. Subject to that, and the usual caveat about stopping distances, it is possible to travel safely at very high speed in the wet, but it needs great care. HTH.
Best wishes all,
Dave.
TripleS said:
even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
That makes sense, as the tyres on the side with deep water will lose grip, the tyres on the other side will still grip, tending to push you away from the tyres with grip. It hasn't happened to me on wet roads, but I have noticed it on narrow country roads if you stray onto a wet verge or a wet verge is spilling over onto the road.saxmund said:
TripleS said:
even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
That makes sense, as the tyres on the side with deep water will lose grip, the tyres on the other side will still grip, tending to push you away from the tyres with grip. It hasn't happened to me on wet roads, but I have noticed it on narrow country roads if you stray onto a wet verge or a wet verge is spilling over onto the road.
This is a topic that has interested me ever since I once had the unnerving experience of driving an Austin Allegro (sorry, it was a long time ago!) at 70mph on a wet road with good condition tyres and suddenly realising that the revs were rising because the front wheels were spinning. One little rule I learned from that was to stay out of lorry ruts in the inside lane when standing water accumulates.
There are numerous factors that affect a tyre's resistance to aquaplaning, including the width of the tyre and the weight of the vehicle: simplistically, and up to a point, the narrower the tyre and heavier the car, the less likely it is to aquaplane. Tyres vary greatly in their characteristics, however, even when they are of similar size and type. Every now and then Autocar carries out a tyre test in which they compare different makes on the same car in a variety of conditions, including wet braking, resistance to aquaplaning in a straight line and resistance to aquaplaning on a curve. They also compare performance in damp and dry conditions. No single tyre is ever the best in all categories so it's a case of selecting the compromise that best meets your needs.
Some manufacturers specifically advertise particular tyres as having good wet weather performance: Goodyear's Ventura comes to mind. I fitted these to a Subaru I had and was amazed at how much more traction they gave me exiting wet roundabouts when compared to the different Goodyears that had been on the car before.
To me the key rule, however, is that whatever tyres you have, tread depth is the one critical factor. I can't remember the data but I recall that research shows that a tyre's ability to shift water when it's down to the legal minimum tread is atrocious, when compared to even a 3/4 worn tyre. Personally, I feel very uncomfortable if I'm on tyres on which the tread wear indicators are anywhere near the surface!
There are numerous factors that affect a tyre's resistance to aquaplaning, including the width of the tyre and the weight of the vehicle: simplistically, and up to a point, the narrower the tyre and heavier the car, the less likely it is to aquaplane. Tyres vary greatly in their characteristics, however, even when they are of similar size and type. Every now and then Autocar carries out a tyre test in which they compare different makes on the same car in a variety of conditions, including wet braking, resistance to aquaplaning in a straight line and resistance to aquaplaning on a curve. They also compare performance in damp and dry conditions. No single tyre is ever the best in all categories so it's a case of selecting the compromise that best meets your needs.
Some manufacturers specifically advertise particular tyres as having good wet weather performance: Goodyear's Ventura comes to mind. I fitted these to a Subaru I had and was amazed at how much more traction they gave me exiting wet roundabouts when compared to the different Goodyears that had been on the car before.
To me the key rule, however, is that whatever tyres you have, tread depth is the one critical factor. I can't remember the data but I recall that research shows that a tyre's ability to shift water when it's down to the legal minimum tread is atrocious, when compared to even a 3/4 worn tyre. Personally, I feel very uncomfortable if I'm on tyres on which the tread wear indicators are anywhere near the surface!
I suspect there are quite a lot of effects going on and it's interesting to think of what is happening. Essentially the there is the road, then water on top and then a tyre on top of that supporting a mass.
The essence is what is going on with the water between the tyre and the road.
If the road is just wet then you can visualise it as essentially two (lubricated) surfaces interfacing with lower levels of friction in comparison to dry (non-lubricated) surfaces. Once you get a 'layer' of water between the tyre and the road you have two interfaces with a fluid between - a very different kettle of fluid-dynamics monkeys.
Thus the tyre is sguashing the water out of the way as it rotates and the rubber hits the road. The activity can be resolved in 4 directions. Moving the water forward, back, left and right. The tyre has to accelerate the water in those directions. If it can do it, it's ok, if not then it builds up and you get aquaplaning. If there is good tread the water doesn't have to go far sideways until it finds a hole to be squirted out. If you have slicks you are never going to get it out sideways only forwards and backwards.
One obvious effect is that it takes a lot to accelerate the water forward out of the way as you have to accelerate it faster than the tyre IYSWIM. That creates a lot of drag.
I am sure that I received advice to de-clutch when approaching standing water. It would make sense to make the wheel rotation "free" to deal with varying and differing drag on each wheel.
I once spent a happy day on a very very wet airfield in a caterham. There was a big puddle. When entering it at speed (50mph plus), there was nothing at all I could do to stop some very violent spinning. Fun on an airfield, terrifying on a road.
Bert
The essence is what is going on with the water between the tyre and the road.
If the road is just wet then you can visualise it as essentially two (lubricated) surfaces interfacing with lower levels of friction in comparison to dry (non-lubricated) surfaces. Once you get a 'layer' of water between the tyre and the road you have two interfaces with a fluid between - a very different kettle of fluid-dynamics monkeys.
Thus the tyre is sguashing the water out of the way as it rotates and the rubber hits the road. The activity can be resolved in 4 directions. Moving the water forward, back, left and right. The tyre has to accelerate the water in those directions. If it can do it, it's ok, if not then it builds up and you get aquaplaning. If there is good tread the water doesn't have to go far sideways until it finds a hole to be squirted out. If you have slicks you are never going to get it out sideways only forwards and backwards.
One obvious effect is that it takes a lot to accelerate the water forward out of the way as you have to accelerate it faster than the tyre IYSWIM. That creates a lot of drag.
I am sure that I received advice to de-clutch when approaching standing water. It would make sense to make the wheel rotation "free" to deal with varying and differing drag on each wheel.
I once spent a happy day on a very very wet airfield in a caterham. There was a big puddle. When entering it at speed (50mph plus), there was nothing at all I could do to stop some very violent spinning. Fun on an airfield, terrifying on a road.
Bert
IainT said:
saxmund said:
TripleS said:
even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
That makes sense, as the tyres on the side with deep water will lose grip, the tyres on the other side will still grip, tending to push you away from the tyres with grip. It hasn't happened to me on wet roads, but I have noticed it on narrow country roads if you stray onto a wet verge or a wet verge is spilling over onto the road.BTW, I think it best that you're joking about whizzing through puddles and soaking pedestrians, as it has been known for drivers to get prosecuted for that.
Best wishes all,
Dave.
If I may add something else to this.
The most common experience of aquaplaning I have is on roads where ruts have been worn by passage of vehicles with studded tyres during the winter. It is difficult to avoid other than by driving out of position and on clear sections of the road surface.
Which brings me onto another subject, winter tyres.
I do believe that with the fashions for tyre sizes and tread designs we really do not do ourselves any favours. I have two sets of wheels and tyres, summer and winter tyres. The summer jobbies are the usual trendy wide low profile things which in part look a little like hand cut slicks. They are OEM fitment, with a few big grooves, but very few of the small sipes that used to be much more commonplace.
The winter tyres* have a deeper tread but only by the odd mm, the main difference being the very large number of sipes. Even in very wet conditions these tyres feel absolutely planted by comparison.
There are some conditions obviously where they have less grip and stability than summer tyres but that is dealt with easily by recognising it and driving appropriately. Having due regard to the conditions in which I travel it's worth it in my mind.
The most common experience of aquaplaning I have is on roads where ruts have been worn by passage of vehicles with studded tyres during the winter. It is difficult to avoid other than by driving out of position and on clear sections of the road surface.
Which brings me onto another subject, winter tyres.
I do believe that with the fashions for tyre sizes and tread designs we really do not do ourselves any favours. I have two sets of wheels and tyres, summer and winter tyres. The summer jobbies are the usual trendy wide low profile things which in part look a little like hand cut slicks. They are OEM fitment, with a few big grooves, but very few of the small sipes that used to be much more commonplace.
The winter tyres* have a deeper tread but only by the odd mm, the main difference being the very large number of sipes. Even in very wet conditions these tyres feel absolutely planted by comparison.
There are some conditions obviously where they have less grip and stability than summer tyres but that is dealt with easily by recognising it and driving appropriately. Having due regard to the conditions in which I travel it's worth it in my mind.
- * These are not studded but what are colloquially known as friction tyres, ie compound which does not harden at <7 deg C and the different tread pattern.
I lived for 6 months in Syracuse New York...we had 162" of snow that winter...snow tyres were mandatory.
If you got stuck in snow and held up traffic, or had a bump, $100 fine - no discussion!
The cars were crap, the US accident rate is horrendous, but driving to work in snow was quite peaceful.
Unlike Heathrow, the airport never closed...maybe a different kind of snow
BOF.
If you got stuck in snow and held up traffic, or had a bump, $100 fine - no discussion!
The cars were crap, the US accident rate is horrendous, but driving to work in snow was quite peaceful.
Unlike Heathrow, the airport never closed...maybe a different kind of snow
BOF.
TripleS said:
IainT said:
saxmund said:
TripleS said:
even if you're happy to continue in a straight line, your car may pull quite violently to one side if you hit deep water at one side only, and in my experience the car pulls to the side that hits the water.
That makes sense, as the tyres on the side with deep water will lose grip, the tyres on the other side will still grip, tending to push you away from the tyres with grip. It hasn't happened to me on wet roads, but I have noticed it on narrow country roads if you stray onto a wet verge or a wet verge is spilling over onto the road.BTW, I think it best that you're joking about whizzing through puddles and soaking pedestrians, as it has been known for drivers to get prosecuted for that.
Best wishes all,
Dave.
Flat in Fifth said:
If I may add something else to this.
The most common experience of aquaplaning I have is on roads where ruts have been worn by passage of vehicles with studded tyres during the winter. It is difficult to avoid other than by driving out of position and on clear sections of the road surface.
Which brings me onto another subject, winter tyres.
I do believe that with the fashions for tyre sizes and tread designs we really do not do ourselves any favours. I have two sets of wheels and tyres, summer and winter tyres. The summer jobbies are the usual trendy wide low profile things which in part look a little like hand cut slicks. They are OEM fitment, with a few big grooves, but very few of the small sipes that used to be much more commonplace.
The winter tyres* have a deeper tread but only by the odd mm, the main difference being the very large number of sipes. Even in very wet conditions these tyres feel absolutely planted by comparison.
There are some conditions obviously where they have less grip and stability than summer tyres but that is dealt with easily by recognising it and driving appropriately. Having due regard to the conditions in which I travel it's worth it in my mind.
Ho hum, UK Scooby Forums when the snow appears, new owners start threads about how it ain't as they thought it would be.The most common experience of aquaplaning I have is on roads where ruts have been worn by passage of vehicles with studded tyres during the winter. It is difficult to avoid other than by driving out of position and on clear sections of the road surface.
Which brings me onto another subject, winter tyres.
I do believe that with the fashions for tyre sizes and tread designs we really do not do ourselves any favours. I have two sets of wheels and tyres, summer and winter tyres. The summer jobbies are the usual trendy wide low profile things which in part look a little like hand cut slicks. They are OEM fitment, with a few big grooves, but very few of the small sipes that used to be much more commonplace.
The winter tyres* have a deeper tread but only by the odd mm, the main difference being the very large number of sipes. Even in very wet conditions these tyres feel absolutely planted by comparison.
There are some conditions obviously where they have less grip and stability than summer tyres but that is dealt with easily by recognising it and driving appropriately. Having due regard to the conditions in which I travel it's worth it in my mind.
- * These are not studded but what are colloquially known as friction tyres, ie compound which does not harden at <7 deg C and the different tread pattern.
Cue some, especially from more Northern Climates, mountainous areas, and the relevant parts of Japan, "Winter tyres", they exclaim.
As always, a great answer from Reg.
On Saturday I was on the M11 in lane three. I had overtaken some way back but wanted to maintain a big rear gap before pulling back in. On a slight incline so I lifted off the gas as reached the top of the incline and wanted to ensure I maintained a considerable gap to the car in front. In hindsight this was not the smartest overtaking move but can we put that to one side and focus on the nect part.
So, off the gas, decelerating and driving in a straight line in lane 3. I hit surface water and whoosh, I am half in lane one and half in lane 2. I felt the steering go completely limp but I did not steer - I just held tight and hoped.
Now this was pretty bloody scary, I got away in one piece with no damage to anyone else, but if the car is sliding violently then should you treat it like a normal skid or what??
On Saturday I was on the M11 in lane three. I had overtaken some way back but wanted to maintain a big rear gap before pulling back in. On a slight incline so I lifted off the gas as reached the top of the incline and wanted to ensure I maintained a considerable gap to the car in front. In hindsight this was not the smartest overtaking move but can we put that to one side and focus on the nect part.
So, off the gas, decelerating and driving in a straight line in lane 3. I hit surface water and whoosh, I am half in lane one and half in lane 2. I felt the steering go completely limp but I did not steer - I just held tight and hoped.
Now this was pretty bloody scary, I got away in one piece with no damage to anyone else, but if the car is sliding violently then should you treat it like a normal skid or what??
Chris French said:
How difficult is it to engineer a sufficient degree of camber into a motorway,when they build it, so that there is no standing water in the first place?
Guess like so much of our road infrastructure, everything is done on the cheap, for the short term or not at all.
I don't know, maybe something could be done along those lines, but if we're talking about really heavy rain I don't see what you can do about that. Even with some cambering there's still going to be a lot of water around, so ultimately it's drivers that need to be aware of the danger and try to spot the problem areas early enough.Guess like so much of our road infrastructure, everything is done on the cheap, for the short term or not at all.
To me it's just another item on the 'driver education' list.
Best wishes all,
Dave.
Chris French said:
How difficult is it to engineer a sufficient degree of camber into a motorway,when they build it, so that there is no standing water in the first place?
Guess like so much of our road infrastructure, everything is done on the cheap, for the short term or not at all.
It's interesting to note that different types of motorway surfacing in this country have a different effect on the amount of standing water. Most newer motorway road surfaces, and those of the "yellow" type, whilst I find to be noisier, I have found to be much better at "wicking away" standing water. Indeed, travelling up the new sections of the A1/A1M past the M62 interchange a couple of weeks ago, the lack of spray was phenomenol, allowing me to see much further and feel much more confident of the car's ability to respond.Guess like so much of our road infrastructure, everything is done on the cheap, for the short term or not at all.
I have also heard that some French Autoroute and German Autobahn surfaces are made of a specific type of tarmac, which acheives just this effect. I've not spent enough time on these roads to know, but I certainly noticed a lack of spray on the way to Le Mans this year!
Graebob said:
I've not spent enough time on these roads to know, but I certainly noticed a lack of spray on the way to Le Mans this year!
Didn't notice any lack myself - we were reduced to 40mph and less at times.To be fair though that was less spray and more direct from cloud to windscreen induced blindness.
graebob,
what you are probably experiencing is Porous Asphalt which errrr as its name suggests has an open texture thus allowiang drainage through the voids.
plus factors
better drainage hence less spray,
lower road noise.
minus factors
more expensive to lay
more susceptible to variations in laying technique
faster wear rates
needs anti-icing chemicals to be added at greater application rates.
Guess you can see why UK doesn't use it much these days and has largely gone over to SMA, which has its own problems.
what you are probably experiencing is Porous Asphalt which errrr as its name suggests has an open texture thus allowiang drainage through the voids.
plus factors
better drainage hence less spray,
lower road noise.
minus factors
more expensive to lay
more susceptible to variations in laying technique
faster wear rates
needs anti-icing chemicals to be added at greater application rates.
Guess you can see why UK doesn't use it much these days and has largely gone over to SMA, which has its own problems.
Edited by Flat in Fifth on Thursday 12th July 10:44
JohnG1 said:
Now this was pretty bloody scary, I got away in one piece with no damage to anyone else, but if the car is sliding violently then should you treat it like a normal skid or what??
The general principle of how to deal with the start of a skid is to take away the input that causes it. When aqua planing any input other than neutral can cause problems, so use neutral throttle (not total lift off) and steer straight. That is my understanding anyway
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