williamp
Original Poster
12,518 posts
159 months

I went to an IAM training sessiontoday, and they mentioned the Aquaplaining formula a rough guide as tho when a tyre will start to aquaplane. It is the square root of the tyre pressure (in psi) multiplied by nine. This is the maximum speed obtainable befoere aquaplaining. hence my tyre pressure of 36 psi will be 6x9 = 56 mph. I'm not convinced. What about you???


yiw1393
9,682 posts
147 months

Surely tread depth and pattern have got to play a part as well?


unlicensed
7,585 posts
136 months

Forget the forumla, just go really bloody fast.


mojocvh
15,092 posts
148 months

Sorry but to many variables not taken into account, like corner or total weight and width for starters. cheers MoJo.


nonegreen
7,803 posts
156 months

williamp said:I went to an IAM training sessiontoday, and they mentioned the Aquaplaining formula a rough guide as tho when a tyre will start to aquaplane. It is the square root of the tyre pressure (in psi) multiplied by nine. This is the maximum speed obtainable befoere aquaplaining. hence my tyre pressure of 36 psi will be 6x9 = 56 mph. I'm not convinced. What about you??? 
This is bollox but I will check it out with the IAM.

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TheLemming
4,099 posts
151 months

Surely this is also affected by the size of the tyre, the weight of the car, as well as a host of other factors. The pressure of the tyre and the speed travelled are relevant factors, but there are a lot of others that affect it as well to quite a significant degree. A rule like that, I would have thought, would be about as usefull as the stopping distances on the highway code  ie grossly innacurate.


srider
709 posts
168 months

williamp said:I went to an IAM training sessiontoday, and they mentioned the Aquaplaining formula a rough guide as tho when a tyre will start to aquaplane. It is the square root of the tyre pressure (in psi) multiplied by nine. This is the maximum speed obtainable befoere aquaplaining. hence my tyre pressure of 36 psi will be 6x9 = 56 mph. I'm not convinced. What about you??? 
I'm not convinced either, my tyres run 40psi, and I've aquaplaned at 45mph (scary stuff in a 2.5 tonne vehicle with joystick steering ).


seafarer
1,278 posts
139 months

I wouldn't think so either. The coefficient of friction (road surface, tire surface) and normal force of the car (weight), along with the velocity would determine the degree of wheel turning in combination with sliding. Tire pressure and the mass of the car would determine the surface area of contact between the car and the road. If there's more contact surface, then presumably a higher velocity would be needed to skid. If there's hydroplaning, you have an effect sort of like bearing stress on a fluid between two surfaces where the ability of the liquid to shear is greater than the tires and weight (or lack of weight) of the car can break the surface tension and drive through the water to displace it (why wheel tread has channels)rather than sliding over top of it. But what the hell do I know. I'm only guessing.


seafarer
1,278 posts
139 months

I hydroplaned and spun my MINIVAN 180 degrees while stopping at a light just after it had rained. Must have hit an oil slick. Anyway, I was only going about 25 mph, lower speed than the tire pressure, with plenty of room to stop, so I hadn't slammed on the breaks. My van isn't exactly light weight either.


agent006
11,211 posts
150 months

Tread pattern can make a huge difference to this. I remember a test a few years back that showed nearly 15mph difference in aquaplaning speeds.


streaky
19,311 posts
135 months

A bit worrying that ... given the takeoff and landing speeds of aircraft. According to the formula, they'd all aquaplane (no pun intended!) in the wet. Mind you, runways tend to be fairly well drained. But perhaps it explains why the safety briefing includes ditching  Streaky


Superflid
2,254 posts
151 months

Would have expected depth of water to make a difference as well. Fairly easy to aquaplane when finding a foot and a half of standing water across a Scottish single track road in the dark.


chrisjl
781 posts
168 months

seafarer said:I hydroplaned and spun my MINIVAN 180 degrees while stopping at a light just after it had rained. Must have hit an oil slick. Anyway, I was only going about 25 mph, lower speed than the tire pressure, with plenty of room to stop, so I hadn't slammed on the breaks. My van isn't exactly light weight either. 
That just sounds like a very slippy surface to me, which definately isn't the same thing. Aquaplaning is when there is sufficient quantity of water that the pressure it exerted on your tyres by the water as it tries to squeeze out of the way is enough to lift the tyres off the road, instantly reducing your coefficient of friction almost to zero. As has already been said, the formula suggested ignores far too many factors to be even vaguely useful.


GavinPearson
5,469 posts
137 months

williamp said:I went to an IAM training sessiontoday, and they mentioned the Aquaplaining formula a rough guide as tho when a tyre will start to aquaplane. It is the square root of the tyre pressure (in psi) multiplied by nine. This is the maximum speed obtainable befoere aquaplaining. hence my tyre pressure of 36 psi will be 6x9 = 56 mph. I'm not convinced. What about you??? 
It's total rubbish. The Bosch handbook has a table of mu values against speed for new and worn tyres. Fact is that if there is deep enough water you will aquaplane. But that can be from driving thorough a small puddle or on a flat road with no run off. If the water is draining from the surface then you have considerably more margin of safety.


TripleS
4,294 posts
128 months

With all due respect to the IAM, their aquaplaning formula does not sound reliable to me. It ignores far too many important factors. At the risk of provoking serious criticism of my driving, I report the following experience: The road layout  a high class dual carriageway, slightly uphill, a very large radius RH curve over a gentle summit and into a long straight, slightly downhill. The road surface  wet, moderately rough, decent drainage, no standing water. The weather conditions  daylight, an overcast afternoon but good visibility, not raining at the time. Traffic conditions  passed one HGV while accelerating, no other vehicles in sight. The car  a Jaguar Series 3 Sovereign V12 with very good tyres on. The driver  me. Guilty as charged  again! The speed  115 mph. The result  no problem, given suitable care in the execution of the experiment. For anyone minded to point out the great stopping distance required in such conditions, please rest assured that a very large stopping distance was available, but not required. The key to success in any such extreme 'antics' is to keep it all smooth and gentle. Hope this helps! Best wishes all. Dave.


Riki
223 posts
128 months

just out of interest.but 6x9=54 not 56...just thought i'd mention that so you can recalculate


Mr E
15,447 posts
145 months

I once worked out the speed you'd need to do to get a racing pushbike slick tyre to aquaplane. 100psi or so and a very narrow contact patch. Something like 300mph required..... Can't remember the formula. It was probably wrong.


XM5ER
3,697 posts
134 months

Pehaps they mean the square root of the pressure on the surface of the tyre i.e a product of vehicle weight divided by the total contact patch area in inches (giving psi).


TripleS
4,294 posts
128 months

Mr E said:I once worked out the speed you'd need to do to get a racing pushbike slick tyre to aquaplane. 100psi or so and a very narrow contact patch. Something like 300mph required..... Can't remember the formula. It was probably wrong. 
Don't worry, it was probably rather more credible than the IAM formula. Dave.


captdm
1 posts
98 months

Hi, Just to clarify the aquaplaning formula that is very accurate. Firstly the formula applies to aviation and the answer is expressed in Knots (nautical miles per hour) so get the answer and multiply by 1.15 to give you the answer in statute miles per hour (vehicle MPH) There are many criteria to be addressed in using the formula. For a start, the tread of the tire is ineffectual once the depth of standing water equals the depth of tread on the tire. The tire behaves the same way as if it was bald once this happens. The tire pressure is the underpinning criteria due to it's direct effect on the tire footprint loading, that is, the smaller the footprint the higher the loading on the surface. formula (weight supported divided by tire contact surface area) So, using that formula the higher the tire pressure, the higher the aquaplaning initiation speed will be. Also, the interpretation of the formula maintains that the value expressed is the speed that aquaplaning MAY OCCUR, NOT WILL OCCUR. The second part of the formula states that once the aquaplaning is established the speed that it will cease is 7.5 times the square root of the tire pressure. These formulae weren't just made up, they were derived by testing and noting the results. In aviation the word "demonstrated" takes a new and powerful meaning. Virtually everything in aviation has to be "demonstrated" before it is written in the aviation training syllabus and accepted as fact. It is interesting to note that when hydroplaning or aquaplaning occurs, a steam pool is created beneath the tire, between the tire and the road. This causes the tire tread to boil and melt (rubber reversion) and ruins good tires. Also, the steam pocket is located forward of the center of gravity in the direction of travel, located directly under the center of the axle and perpendicular to the road. Because of this the wheel will rotate BACKWARDS! Thought this might help Regards to all

