Tyres

[Speedy Builder5Participant @speedybuilder5]

Why do Tyre centres over inflate tyres. I recently bought 2 new front tyres. Recommended tyre pressure 32psi, measured pressure (3 different gauges) 45psi

Rear tyres which were 29psi inflated to 45psi.

No advice given like, check your tyre pressure after a week. Is it just so that the tyres wear down quicker ?

I have noticed that MOT stations also over inflate tyres before the brake tests – again no warning that it has been done.

Bob

[Speedy Builder5Participant @speedybuilder5]

[bernard towersParticipant @bernardtowers37738]

MOT centres are not allowed to pump up your tyres

[HOWARDTParticipant @howardt]

Probably overinflated to get the tyre to seat on the rim then some released but not enough to achieve correct riding pressure.

[John HinkleyParticipant @johnhinkley26699]

Bob,

As you reside in France, is it possible that the tyre fitter misread psi for bar? 2.9 bar = 45 psi. Also, I've noticed that some tend to whack in a goodly amount of air to get the beads to seal and then forget to re-check the pressures at the end of the fitting session.

I always check the pressures when I get home, prior to re-calibrating the under-inflation system.

John

[noel shelleyParticipant @noelshelley55608]

Hi Bob, Over inflate to seat on the bead, then re adjust to correct value ! MOT stations must test the vehicle as presented ! Alloy wheels when a few years old are notorious for slow leaks due to corrosion and leaking under the laquer. Good Luck. Noel

[Speedy Builder5Participant @speedybuilder5]

Bernard and Noel, I did omit that living here in France the vehicle test is the "Control Technique" and yes, they do over inflate the tyres.

John, Its a French Renault car and the plate on the door indicates pressures in both BAR and PSI. Its strange that the tyre bay inflated the rear tyres when it was only the front tyres being replaced.

[Jon LawesParticipant @jonlawes51698]

Bit of a sweeping generalisation there. Maybe if yours is a bit of a cowboy outfit they just fill it without checking the pressure.

[Martin KyteParticipant @martinkyte99762]

Couple of years ago our head of IT got a couple of new tyres fitted at a well known tyre and exhaust centre. The charged extra for filling with nitrogen. However Paul who is a qualified diver and mixes his own gasses decided to check the tyres with his gas analyser when he got home. Oh my just the same ratio of gasses as normal atmosphere. Big fuss with the company the next day. I don't think they offer that service any more.

;O)

Martin

[noel shelleyParticipant @noelshelley55608]

On the basis that the atmosphere is 70%(roughly) Nitrogen, did they say anything about it being pure ? Did they evacuate the tyre before filling with N ? Good money maker – must remember that one ! Noel.

[Don CoxParticipant @doncox80133]

After 15 years in my retirement job in the Motor Vehicle department of our local FE college where I had free and unlimited access to tyre fitting, wheel balancing and many other vehicle facilities, I finally retired and had to buy tyres fitted as part of the deal like everybody else. I bought them online from a well known national supplier and as part of the spiel on the day I was asked if I wanted a Nitrogen fill at extra cost. After explaining that I knew a bit about tyre fitting and expressing doubt that nitrogen would give any real benefit, I asked how it was possible to displace the existing air from the tyre when it was being pressurised and what percentage of oxygen it was expected would remain compared to the 20% that was normally present in air. All I got was a load of waffle and no satisfactory explanation. I doubt that they bother flogging that particular dead horse now either.

[Nigel Graham 2Participant @nigelgraham2]

I wonder how many customers were taken in by that little nitrogen racket, though?

[not done it yetParticipant @notdoneityet]

Posted by Nigel Graham 2 on 12/05/2022 22:47:24:

I wonder how many customers were taken in by that little nitrogen racket, though?

Any of those that don’t know that PV = nRT. Those that don’t know that if there is already any water in the case, it will not behave like an ideal gas. Those that don’t know they already have 78% Nitrogen in their tyres. Those that don’t know that Carbon Dioxide is a vapour, not a gas (ie the same as water vapour). Those that think HO improves engine efficiency. Those that are ‘taken in’ by any such fad.

In other words: lots of them.

Different for the likes of F1 racing cars where every little thing needs to be controlled, for the absolute maximum efficiency.

[Jon LawesParticipant @jonlawes51698]

Aircraft too, but that was less to do with efficiency and more to do with safety.

[John MCParticipant @johnmc39344]

Does anyone check the calibration of there tyre pressure gauges? Could be that the OP's gauges are wrong and the tyre fitter is correct? Statistically unlikely but who knows.

I've tried to check my gauges by making up a device that I hoped would work like a dead weight tester for pressure gauges. Not successful because of friction from the O-ring seal.

Any ideas as to how a tyre pressure gauge could be checked?

[not done it yetParticipant @notdoneityet]

Posted by John MC on 13/05/2022 07:21

….

Any ideas as to how a tyre pressure gauge could be checked?

Have more than one gauge? I check mine against any good gauges I come across. I don’t rely wholly on the pressure indicated – tyre wear is important – the tread should have the maximum contact with the road, for normal road vehicles.

Pressure too high means they will wear in the centre of the tread and if too low the edges will wear more quickly – but both mean less effective contact with the road surface.

Under-inflation is worse than slightly over – the tyre wall will flex more and heat up (and any loss of pressure may make things much worse very quickly.

[CirclipParticipant @circlip]

My 'cheap' Romanian Renault has a light on the dash telling me to 'Check tyre pressure' . Don't know how it can sense this, standard wheel nuts so there must be an inertial gravitational balance system built into the dashboard???? Yet another waste of electronic gizmo programming on modern cars.

Regards Ian.

[John HaineParticipant @johnhaine32865]

Posted by Circlip on 13/05/2022 10:36:46:

My 'cheap' Romanian Renault has a light on the dash telling me to 'Check tyre pressure' . Don't know how it can sense this, standard wheel nuts so there must be an inertial gravitational balance system built into the dashboard???? Yet another waste of electronic gizmo programming on modern cars.

Regards Ian.

They compare rotational speed between tyres on the same axle as measured by the antilock brake sensors. A/k/a wheel ticks. Since the sensors are there anyway it's just software.

[Speedy Builder5Participant @speedybuilder5]

John MC, start of thread "Recommended tyre pressure 32psi, measured pressure (3 different gauges) 45psi"

I suppose all 3 gauges could read high by the same amount.

Simple dead weight pressure test rig. Known weight on a piston of known area. I wonder how often the tyre gauges are checked at the tyre centre. Normal inflation kit is generally abused.

Bob

[Ex contributorParticipant @mgnbuk]

Don't know how it can sense this

As well as the ABS sensor wheel speed arrangement that JH described above, there are also battery operated pressure sensors built in to the fill valves. These look the same as normal unsensed fill valves from the outside. The batteries on these sensors are non-replaceable, so when they eventually die a complete replacement sensor must be fitted and "coded in" to the car system.

On my last car (which had a sensor based system for TP monitoring) I had a spare set of wheels with winter tyres fitted that didn't have the sensors – replacement sensors are not cheap & must be "coded in" when changed, so I didn't bother. When I swapped over to these wheels, it took about 4 miles for the pressure warning light to come on, which stayed on unitl about 4 miles after I refitted the sensored wheels. No idea about how long the batteries last – that car was over 6 years old when I changed it, though about a year of that was with the winter tyres fitted. Given the 4 mile lag between loosing (or regaining) the sensors, I guess that the sensors only have to respond to an occasional request from the car systems for their status & that the normal battery drain is very low..

My wife's current car displays the actual pressures for each wheel on the car display, so easy to see which one is low. This system appears to trigger at a 3 PSI difference from the setting on the door pillar & activated the first time when the ambient temperature dropped below freezing.

Nigel B.

(edit for spelling)

Edited By mgnbuk on 13/05/2022 11:53:54

Edited By mgnbuk on 13/05/2022 11:54:10

[SillyOldDufferModerator @sillyoldduffer]

Posted by Circlip on 13/05/2022 10:36:46:

My 'cheap' Romanian Renault has a light on the dash telling me to 'Check tyre pressure' . Don't know how it can sense this, standard wheel nuts so there must be an inertial gravitational balance system built into the dashboard???? Yet another waste of electronic gizmo programming on modern cars.

Regards Ian.

A useful warning rather than gizmo surely? Having tyres at different pressures on an axle pair is downright dangerous, worse if the condition is left to wear the treads faster on one side than the other for any time. Chap killed nearby a couple of years ago by steering into a bend a shade too fast on a wet road with a bald under-inflated tyre on one side and a new correctly inflated tyre on the other. Sliding into a bridge abutment did the rest.

I don't believe tyre-pressure gauges have to be particularly accurate. I doubt many of them are better than plus/minus 15%, and the acceptable range of tyre pressures allowed is rather large. Pressure can be reduced well below normal to improve grip in slippery conditions, or increased well above normal to reduce noise and improve mpg on good surfaces – motorway cruising in good weather. What's unwise is mixing tyres at different pressures on the same vehicle, a problem avoided by using the same gauge on all the wheels.

Nitrogen filled tyres? Plenty of chaps on this forum willing to spend money on perceptions of 'quality'. Like vanity number plates, if it makes folk happy, why not?

Dave

[not done it yetParticipant @notdoneityet]

What's unwise is mixing tyres at different pressures on the same vehicle, a problem avoided by using the same gauge on all the wheels.

Often different tyre pressures are stipulated for different axles. Further, the pressures can be different for different loads in the vehicle.

I would not dream of using a different pressure gauge for different tyres on the same vehicle!

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