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- Dave Baker
August 30, 2012, 7:07 am
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tails tucked between their legs for safety I used to find that front tyre
balancing was fairly critical on my old rwd cars like Marinas and Mk1/2
Escorts. Front wheel bearings tended to be "nip up hand tight and then back
off a tad for clearance" type things and any play in them got rapidly
exacerbated by out of balance tyres leading to the steering wheel shakes at
about 60 mph usually depending on resonant frequencies.
My mate has just stuck some good s/h tyres on the front rims of my Focus and
also on a Mondeo I'm selling to someone else with his tyre changer and as
his balancer is kaputt we didn't bother. We just left the existing weights
on the rims. In fact he's been running unbalanced tyres for years on his own
modern vehicles and said it didn't seem to make any odds. Well to my
surprise I don't notice any issue on either of the above cars after testing
them at various steady speeds.
In fact one of the tyres we took off the front of my Focus had a bulge in
the tread which was causing the steering wheel to oscillate by an inch at
the rim if you took your hands off it at 20 mph as the tyre rotated over the
high spot. This must also have been causing quite an imbalance at higher
speeds but it never manifested as shake or vibration.
So either modern fwd cars have front wheel bearings and suspensions that are
not that critical to wheel balance or modern tyres are better balanced as
made than older ones or there's summat else I'm missing. Not having to
bother paying for this to be done is a good saving if you don't have your
own tyre balancer.
A mate who works at the local council tells me that they also don't bother
having tyres balanced anymore on council vehicles after looking into the
cost / benefits.
What are the thoughts of the collective?
--
Dave Baker
Re: Tyre balancing
Balancing is primarily a revenue stream for the tyre places. Time was, only
if a wheel wobbled after tyre fitting would any balancing be done. When I
was selling and fitting part worns years ago, I never balanced any, and only
once did I hear of a wobble problem.
You may recall a while ago there was a trend towards the boy racers wanting
special boy racer gas in their tyres. I haven't heard anything about that
for a while. That too was a revenue stream matter.
Steve
Re: Tyre balancing
"Filling your tyres with nitrogen may seem odd but that’s exactly what
motor sport and aviation professionals have been doing for years. Nitrogen
is completely safe. And by using it in a mixture with oxygen to inflate
your tyres the theory is that it’s possible to negate the issue of
slow deflation, which is caused by oxygen slowly infusing through the
tyre wall from the atmosphere. "
Unmitigated bollocks.
--
Today is Boomtime, the 23rd day of Bureaucracy in the YOLD 3178
"If there is anyone here who I have not insulted, I beg their
pardon." Johannes Brahms (1833-97).
Re: Tyre balancing
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.
http://www.linric.com/webpsysi.htm
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 cock 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
replace nospam with domain host name to reply
Re: Tyre balancing
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?
--
Peter Hill
replace nospam with domain host name to reply
Re: Tyre balancing
<1301959829368298244.081514%steve%-malloc.co.uk@news.eternal-september.or
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.
--
Paul Giverin
My Photos:- www.giverin.co.uk
Re: Tyre balancing
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.
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/55850E6389EFBA3C8625695B006723A3?OpenDocument&Highlight=87-08-09
The 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.
http://www.dmp.wa.gov.au/documents/Guidelines/MSH_G_TyreSafetyFiresAndExplosions.pdf
The 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.
http://www.boeing.com/commercial/aeromagazine/aero_05/m/m03/index.html
(I'm an Engineer, I can't spell or do joined up writing that I can read.)
--
Peter Hill
replace nospam with domain host name to reply
Re: Tyre balancing
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".
--
Paul Giverin
My Photos:- www.giverin.co.uk
Re: Tyre balancing
[...]
My experiences are very different; I find that the much heavier wheels
and tyres fitted to modern cars make any out of balance *very*
noticeable, to the degree that I usually need to get the fronts
rebalanced half way through their life in order to eliminate vibration.
I find that the critical speed for the Focus is around 75mph. 5mph either
side of that, it's OK. Rebalancing always removes it.
Son's Beemer threw a self-adhesive weight, and it was almost undriveable
above 50 until rebalanced.
Chris
--
Remove prejudice to reply.
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