Tyre pressures

[ItalianTuneUp]

Since I discovered the great handling with correctly inflated tyres, I try to keep them spot on.

I use one of the digital auto cut-off Michelin tyre inflators.

However, even a few days after inflating them, I've noticed that they lose air. This was the case with the last set of tyres and the new ones too. They are Continental Contact Sport 5 tyres.

It's a heavy car (S320CDI), but I would have expected the tyres to hold the air better.

Does anyone have an idea if this air loss is normal, or whether it indicates a problem with the tyres, or perhaps the way they were fitted?

 

[paul_f]

If it happened with the last tyres as well, it would suggest to me that is what the wheels or the valves at fault.

 

[markjay]

The air could be leaking from the area where the tyre's rubber meets the rim's alloy. More so if the alloy is cracked, chipped, or had become porous.

Otherwise.... leaking valves, though I would have thought that when changing the tyres they would have fitted new valves anyway.

 

[jgy6000]

As said, it's either the bead around the wheel not seating correctly due to corrosion or the valve. Normal practice is to change the valve when you change the tyre. If it's happening on all the wheels then I'm guessing it's not the valves.

If it's the bead around the wheel then usually a good clean up with a wire brush will sort it.



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[Deleted member 65149]

Valves are nearly always replaced when new tyres are fitted so it's unlikely to be them. But it's a little known fact that what many people call "dust caps" are actually there to help maintain the pressure. The Schraeder valve inserts are just a means of getting air into the tyre, not to maintain the pressure on their own. So always replace the caps.

But I'll agree that it could well be a problem with the wheel. Does the pressure drop on all wheels?

 

[ItalianTuneUp]

I think the valves seem new after new tyres are fitted, the rubber valve stem and caps look brand new, and the valve brass threads too, so I assume the internals are too.

So perhaps the wheel rim/bead needs cleaning up?

I've never checked the pressures methodically on all wheels...I just attach the inflator, observe the pressure shown on the digital display and set the required pressure, and switch it on. But I would say that all tyre pressures seem to have reduced a little...the magnitude dependent upon the interval since the last inflation.

However, apparently atmospheric air pressure changes all the time too, so I assume this affects readings too? I press the yellow pressure calibration button before an inflation session.

 

[John Jones Jr]

So always replace the caps.

With truck ones.

 

[ItalianTuneUp]

With truck ones.

Why?

 

[Druk]

Soapy water and a brush round the rim and valve. See if it bubbles.

 

[paul_f]

I've never checked the pressures methodically on all wheels...I just attach the inflator, observe the pressure shown on the digital display and set the required pressure, and switch it on. But I would say that all tyre pressures seem to have reduced a little...the magnitude dependent upon the interval since the last inflation.

Are you checking the pressures with the tyres cold? Atmospheric pressure won't make much difference, but hot tyres makes a difference.

Also next time you do it, maybe check the pressures with another gauge - I am always suspicious of automatic tyre pressure inflators

 

[ItalianTuneUp]

^ Good point re warm/cold tyres. Usually they are a bit warm, but maybe best to inflate when cold.

I'll check them first with a Draper pen-style tyre pressure gauge first.

 

[John Jones Jr]

Why?

They are metal with a built in seal. If the valve fails the tyre won't lose pressure like the standard plastic ones. Also too they are far easier to find if you drop them

 

[paul_f]

They are metal with a built in seal. If the valve fails the tyre won't lose pressure like the standard plastic ones. Also too they are far easier to find if you drop them

Kids also like them on their bikes, don't last long around here

 

[Deleted member 65149]

^ Good point re warm/cold tyres. Usually they are a bit warm, but maybe best to inflate when cold. I'll check them first with a Draper pen-style tyre pressure gauge first.

Not just a good point, an important point. Pressures must be checked when the tyres are cold. And never rely on the gauge on the compressor, they're notoriously inaccurate.

 

[ItalianTuneUp]

Not just a good point, an important point. Pressures must be checked when the tyres are cold. And never rely on the gauge on the compressor, they're notoriously inaccurate.

If the compressor is inaccurate regarding exact pressure, won't the tyres all be inaccurate by the same amount, thus equal?

 

[paul_f]

If the compressor is inaccurate regarding exact pressure, won't the tyres all be inaccurate by the same amount, thus equal?

It might do, but it might be inaccurate in that it doesn't give a consistent reading

 

[ItalianTuneUp]

It might do, but it might be inaccurate in that it doesn't give a consistent reading

Before I got the digital compressor I used a manual tyre gauge but it's hard to see how accurate to the absolute nearest one unit of PSI the tyre pressure is with it.

Maybe there is a manual digital one that is dead accurate? I like my tyres spot on ad the ride in the S-class is sublime when they're right.

 

[ItalianTuneUp]

The tyre inflator I use is this one:

http://www.amazon.co.uk/dp/B001NI9XWO/?tag=amazon0e9db-21

It's pretty good, has good reviews, and is one of the better ones from what I can see.

The proof is in the pudding, and after using it, the car is extremely stable when driving, which tells me that it's quite accurate...from all I can tell anyway.

 

[paul_f]

That is probably a lot better than most, but it is good to compare against something else.

 

[ItalianTuneUp]

^ I'll have a word with the tyre fitters and see what they have to say about the air loss.

From those Amazon reviews, this is quite reassuring:

"The pressure gauge is very accurate and always agrees with my expensive digital pressure gauge."