Discussion

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???

williamp said:

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.

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.

williamp said:

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 said:

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.

williamp said:

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.

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.

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.

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

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