Tyre balancing

"Chris Whelan" wrote

If you watch a washing machine spin up you will notice it has a critical speed where the oscillation gets very large before reducing. The idea for the washing machine is to spin up quickly to get past that point. The principles are the same but unfortunately a car doesn't always have the luxury of avoiding that critical point.

Imbalance means the CofG does not coincide with the axle. Below the critical speed the CofG and axle increase their orbits away from the centre of rotation (CR) so the shake gets worse as speed rises. Above the critical speed, things flip and the centre of rotation then moves towards the CofG so things get calmer.

CR----------Axle----CofG (below critical) Axle--CR--CofG (above critical)

The above is just static imbalance that causes up/down movement. Dynamic imbalance causes shake around the steering axis and is corrected by distributing the weights round both rims.

Good analysis.

But it doesn't really matter when it's just a piece-of-shit Focus, does it?

He wasn't very good sport, was he?

Nope; he's off-form ATM.

Chris

Anyone that checks tyre pressure so infrequently that permeation of oxygen gives a measurable pressure drop won't have the ability to do the physics to prove the pressure change is due to oxygen permeation and not simple ambient temp change.

Dry air and Nitrogen or any other gas behaves as a ideal gas and obeys the ideal gas law, tyre temp is about 60-80°C (F1 may hit 100°C). Starting with 2.2bar at 15°C the pressure at 60°C will be 2.78bar and at

80°C will be 2.89bar. That's for dry air, or nitrogen, or argon, or carbon dioxide or any other dry gas they care to pump in.

Even soaking wet doesn't make a whole lot of difference to pressure. More likely to cause issues with rim corrosion. At 2.2 bar the boiling point of water is around 130°C and increases with higher pressure as tyre warms the air. Water doesn't boil, there is no violently expanding steam. It's a saturated vapour mixture. At most extreme case 100% rel humidity and liquid water still present at running temperature the increase in running pressure over dry air is about 0.2bar at 60°C and

0.5bar at 80°C.

Clearly Motorsport "engineers" don't do the air conditioning unit of thermodynamics (if they do thermo at all?). If they did they would know how to use this.

It would cause imbalance if there was enough water vapour to condense out into a puddle and it froze. That would either mean fitted wet (you have an air line blow it out) or filled from a compressor air tank that had no drier, hadn't been vented of water, had a water trap full of water and had water in the lines. I'm sure that some people that run tyre shops really are muppets to the extent they will let the air supply get to that state. They only find out when the drain c*ck rusts off and spurts a lot of rusty water across the workshop floor.

What happened was F1 teams started taking air bottles on the grid and setting tyre pressures hot. Someone got a nicely painted one. Everyone else had to have the same.

The only reason aircraft use nitrogen is that a locked wheel (100 tons doing 150mph) will see tyre temp hit the point that the inner wall of the tyre catches fire and the weakened tyre can fail explosively. In one case a tyre exploded while stowed when the aircraft reached altitude, resulting in loss of the aircraft. By not having oxygen in the tyre it can't burn and become weakened.

Peter Hill wrote: [snip]

Also the difference in molecular diameter between N2 and O2 is so small (~10%) that there's no appreciable difference between nitrogen and air as far as loss of pressure by diffusion is concerned. It's all sales bollocks backed up mechanics bollocks.

They all measure gauge pressure. That's pressure relative to the ambient atmospheric pressure.

10 millibar is 0.145psi

As the pressure goes from low to high (yippee) across the UK the pressure can easily change by 30 millibars (0.435psi) in just a few days. That means the gauge pressure of the tyre will reduce by 0.435psi. Far greater and faster change than any diffusion can cause.

Can we declare it as being Mega bollocks?

When they filled my tyres wth nitrogen the reason given was better fuel consumption and less tyre wear and as oxygen wasnt present they wouldnt burn in an accident !!!!!!!!!!!!!

"steve robinson" wrote: [snip]

Mega bollocks it is then. There's plenty of oxygen outside the tyre to permit them to burn.

As for fuel consumption and tyre wear it cannot be better than that of a correctly inflated tyre that uses air.

Aircraft tyres have used nitrogen since the 60's and that's because of a reduced fire risk. Of course the worry there is the tyre exploding after the undercarriage has retracted and the tyre is in an small sealed enclosure.

reduced fire risk.

But as Peter has explained for a different reason, with higher temperatures. Even so aircraft tyres do catch fire.

Quite.

They explode. The inner liner gives off vapour that auto-ignites resulting in very rapid pressure rise. An aircraft tyre or large truck tyre explosion is a major hazard, the pressure rise exceeds the tyre's capability. Car tyres, have small size - less mass being combusted, don't run high pressure - less O2 to burn and have a much stiffer carcase that will usually contain the pressure rise.

The requirement is less than 5% O2. The easy cheap way of doing that is and was Nitrogen fill. It's been a mandatory Airworthiness Directive sine 1987 due to the loss of an aircraft and 167 people in 1986.

list was (I think) every large western civil aircraft in operation at that time.

They don't actually fully fill the tyre with nitrogen. The initial volume is air with oxygen and water vapour, then the added volume to bring it up to pressure is Nitrogen. At 200psi inflation pressure for an aircraft tyre from an initial 14.7psi the result is about 1.4% O2.

Air has about 21% O2. A car tyre is filled to about 2.2bar gauge (3.2bar absolute). The result is 6.56% O2 which is NON COMPLIANT with the AD requirement of less than 5%.

Yet more proof that the car service industry not only can't do physics they can't even do simple maths. The tyre would have to be purged by being let down and re-inflated to reduce the O2 level below 5%. It would then be filled from an initial 6.56% O2 resulting in 2% O2.

Truck tyres do catch fire / explode all to often - you see the remains on the roadside. They are higher loaded than car tyres, do more miles and a dragging brake can go unnoticed. They are also inflated to much higher pressure than car tyres.

document linked to above has a recommended 5.5% 02 and suggests that a purge should be done. Filling an air filled tyre with nitrogen to 2.8bar (41psi) is enough to to achieve that 5.5% O2. Min pressure for a light truck tyre on chart below is 4.5bar (3.8% O2) and heavy trucks and buses are 6bar (3% O2), a purge isn't required for truck tyres.http://www.conti-online.com/generator/www/uk/en/continental/transport/misc/technical_information/download/air_pressure_table_pdf_2010_en.pdf Nitrogen fill of aircraft tyres is maiming and KILLING aircraft tech staff. (I'm an Engineer, I can't spell or do joined up writing that I can read.)

Huge spake thus:

I heard it was a by-product of custard powder manufacture.

In message , Peter Hill writes

Its not the nitrogen that's maiming and killing people its the high pressure and rate of inflation. It would be just the same or worse with compressed air. Even the heading of the link you gives states:- "In each case, an unregulated supply of nitrogen or AIR was responsible for the explosion".