The Motor Forum

Big TC · Posted: Tue Nov 03, 2009 11:29 am Post subject: Soggy tyres

I spend several days at a time where I don't use my car (I generally work from home). I can often go all week without driving.

When it's been idle for several days, my cars tyres are all a bit soggy - the rears particularly so. It needs to be not used for at least 4 or 5 days for me to require the local tyre pump (8 miles away....). In that time, the rears can have dropped to well below 20psi, the fronts to the low 20s.

If I use the car regularly, the pressures stay pretty much at what they're supposed to be and don't need to be checked quite so regularly.

I've never experienced this before on previous cars. I've had two new tyres (and valves) on the front this year, and even they loose pressure.

Anyone else experience this phenonomen?
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It's a '57 Pontiac Star Chief, dontchaknow.

SpecB · Posted: Tue Nov 03, 2009 11:31 am Post subject:

Could be corroded rims or dodgy valves.

The Cherokee I had a couple of years ago had badly corroded alloys that meant the tyres kept losing pressure. The local tyre place ground off the corrosion on the rim using a bench grinder and remounted the tyres and all was well.
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Mark

TimR · Posted: Tue Nov 03, 2009 1:22 pm Post subject:

I should probably be ashamed to admit this but the BM has done the grand total of 10 miles - all in 1 trip - since October 9th yet it's tyres all appear to be still decently inflated.

Must be porous rims or something?

woof woof · Posted: Tue Nov 03, 2009 1:59 pm Post subject:

If the tyres deflate less when the car is used more could the difference be the air temperature in the tyre? If the rims are a bit corroded could heated air and slightly higher pressure within the tyre create a better seal?
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Big TC · Posted: Tue Nov 03, 2009 2:02 pm Post subject:

Its got alloys - would corrosion be a problem with them? They're pretty damned dirty - I'll check to see if they're corroded.
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It's a '57 Pontiac Star Chief, dontchaknow.

TimR · Posted: Tue Nov 03, 2009 2:05 pm Post subject:

From the distant mists of time my memory has dredged up that a car mag had a lateish model original Impreza on test that had porous alloys yet the car was less than 1 year old.

I think that just lost pressure anyway.

Could it be a poor seal between tyre and rim.

Dr. Hfuhruhurr · Posted: Tue Nov 03, 2009 2:27 pm Post subject:

The Allegro Equipe limited edition was also famous for porous wheels. Gratuitous excuse to post this picture ...

DradusContact · Posted: Tue Nov 03, 2009 2:34 pm Post subject:

I would of.
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Bob Sacramento · Posted: Tue Nov 03, 2009 2:36 pm Post subject:

Never had the problem myself but then again the air is thicker in Newcastle.
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Keep Calm & Carry On.

Jasper · Posted: Tue Nov 03, 2009 2:59 pm Post subject:

DaveGibson · Posted: Tue Nov 03, 2009 3:41 pm Post subject:

Back in the eighties, my sister had an MG Metro which lost tyre pressure because of corroded rims.

The IS loses pressure in one of the rear wheels but its only a pound or two a week so I keep my electric pump in the boot, ready to re-inflate it should it get worse.

scamper · Posted: Tue Nov 03, 2009 3:54 pm Post subject:

Had problems last winter with the MX5 loosing tyre pressure, sitting around not used. I think it may be corroded rimms as pointed out to me when i got a new front boots from ATS but had no problems over summer. Not sure if you can do anything about the rims? A pity as they are in cosmetically good condition on the outside.

Dr. Hfuhruhurr · Posted: Tue Nov 03, 2009 4:00 pm Post subject:

Tyres will lose pressure anyway at this time of year, due to sudden drops in ambient temperature. Always used to get false alarms from the M3's TPM in the autumn. Going by the GT3's actual pressure display, the difference from summer to winter can be as much as 0.4-0.5 bar.

DaveGibson · Posted: Tue Nov 03, 2009 4:16 pm Post subject:

Ignoring the fact that the tyre volume will change slightly with a change in pressure, the pressure will vary in proportion to the absolute temperature. So a change in temperature from 20C to 10C (i.e. from 293 to 283 degrees K), will result in a pressure drop of about 3.5% - or, in terms of a typical 30psi setting, about 1psi.

Big TC · Posted: Tue Nov 03, 2009 4:46 pm Post subject:

"him" · Posted: Tue Nov 03, 2009 4:48 pm Post subject:

What you really need is red Bora, now if he really wanted to help...
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Turbonutter · Posted: Tue Nov 03, 2009 5:45 pm Post subject: