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My A3 has a 'standard' inflation of 41 PSI all round. I've never had the need to increase to max load level.
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Well, I'm now on the 3rd mouthful of my hat... learn something every day!
As a supplementary question: why do cars have such a variation on tyre pressures from 'normal' load to max. load? Is it more to do with the % increase the extra load is compared to the overall vehicle weight, i.e. it's more on %-wise on a small car than a big car?
(and hence pro-rata the tyre deflection)
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>>i.e. it's more on %-wise on a small car than a big car?
I think there's quite a lot of truth in that, especially on a front wheel drive car that doesn't have any particularly heavy parts at the back, and also has a very small rear overhang.
Number_Cruncher
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The other thing to remember is that the higher pressure must be able to cope with a max legal rear axle load and being driven for hours down an autobahn, at V-max, with the road temperature in the high 140s.
Not conditions likely to be encountered in the UK over X-mess. [Unless that Global Warming can really do abrupt changes...]
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Its believable that the pressure does need to be increased to 46psi - but check the tyre rating plate on the car or in the handbook. Also check the tyre sidewall for 'max inflation pressure' data. Most EU-market tyres are 44psi or 51 psi max., although some extra-load (XL) are higher. The tyres fitted may not be the OE spec tyres. Most tyres reach maximum load capacity at about 40-42psi, although higher pressures may be used to limit sidewall deflection and hence heating of the carcass.
Tyre pressures do not measurably change when loaded (there's some straightforward physics behind that statement - think about it).
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without wanting to get into argument with the expert numbers people lest I embarrass myself badly...
My thinking is that Pressure . Area = Force
Assuming that the contact area is desired to be constant then F/P should be constant. So I guess weight distribution needs taking into account but assuming that a focus has little weight over the rear wheels, slapping three rear passengers + luggaeg in the back is likely to merit a substantial extra downwards force.
I know that the tyre has a full circumference/ total surface area etc, but the weight of the car is dealt with only by the contact patch, No?
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ac, I think you're generally right on all counts.
The increase in tyre pressure that you apply when running laden is to restore overall levels of tyre deflection and contact patch shape and area to close to their nominal levels.
The discussion above between cheddar and I is over a more subtle and esoteric point (IMO!).
Number_Cruncher
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The increase in tyre pressure that you apply when running laden is to restore overall levels of tyre deflection and contact patch shape and area to close to their nominal levels.
Agreed 100% for reasons previously stated.
However there is surely more to the pressure v load scenario, stiffness and elasticity of the tyre walls are critical, after all if the tyre were made of, say, steel then extra load would have no effect on the tyre at all and hence no effect on the contact area, the deflection or the pressure, on the otherhand it would be feasable to have a thin walled elastic tyre that would maintain a normal profile at a nominal load though would completely deform with an increased load.
So stiffness and elasticity surely has to be accounted for in the area, load, pressure equation?
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>>So stiffness and elasticity surely has to be accounted for in the area, load, pressure equation?
Not really - although I do know what you mean (and laboured under the same misapprehension myself for a long time!)
The stiffness of the sidewalls themselves is low compared with the stiffness of the inflated assembly. If you try to flex the sidewall of a car tyre when it's off the rim, you'll know there's no inherent stiffness there.
In this context, the only important thing a tyre does is not to expand like un-reinforced rubber material - it gains flexural stiffness by virtue of the air pressure within.
Number_Cruncher
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The stiffness of the sidewalls themselves is low compared with the stiffness of the inflated assembly. If you try to flex the sidewall of a car tyre when it's off the rim you'll know there's no inherent stiffness there.
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Agreed.
In this context the only important thing a tyre does is not to expand like un-reinforced rubber material - it gains flexural stiffness by virtue of the air pressure within.
As does a party balloon though that is very elastic, my point is that as tyre must have some elasticity, not as much as the un-reinforced rubber material you mention though much more that the steel example I gave before.
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>>my point is that as tyre must have some elasticity
Yes, but whether or not that elasticity is important or not depends upon how much load it takes compared with other load paths in the system. In this case, the sidewall flexure isn't dominant - the radial load is borne by the air, all the tyre is doing, is containing that air.
When you are cornering or braking, then the sidewall has a more dominant role to play.
Another way to consider it is even for the same material, you can have comparative flexibility even while the material is comparatively inextensible.
Number_Cruncher
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Yes but whether or not that elasticity is important or not depends upon how much load it takes compared with other load paths in the system. In this case the sidewall flexure isn't dominant - the radial load is borne by the air all the tyre is doing is containing that air.
Yes NC though surely its ability to contain the air is defined by its elasticity as opposed to stiffness? Hence both are important factors.
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>>is defined by its elasticity as opposed to stiffness?
Both words describe the same thing - there's no difference. Neither are hugely important in this case.
So far, I've presented some practical experience, an argument and a calc (which errs on the safe side), and Aprilia has stated that the pressure doesn't measurably depend on loading. What more do you want?
Number_Cruncher
Edited by Number_Cruncher on 20/11/2007 at 10:49
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My A3 has a 'standard' inflation of 41 PSI all round. I've never had the
need to increase to max load level.>>
Must be high speed (sometimes also labelled Extra Load) tyres.
- - - - - - - - - - - - - - - - - - - - -
What's for you won't pass you by
Edited by Stuartli on 19/11/2007 at 18:29
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what pressure would one need in the spare then? as its not under load when its being checked/inflated in the wheel well
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what pressure would one need in the spare then?
I tend to set the spares a few psi above the highest front/rear pressure for normal use.
As the pressure won't change (measurably!) when you fit it, there's no problem.
Number_Cruncher
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I keep the spare at the max pressure as moulded on the tyre wall - I always carry a tyre pressure gauge so I can just reduce it, when fitted, to the required pressure. As the laden/towing pressure is near the tyre's maximum I don't need to worry about increasing the pressure if I need to use the spare when laden/towing.
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"a full load of five people which will require the rear tyre pressures to be increased from 32 psi to 46 psi. "
Please bear with me in my total ignorance, but I have never owned a car that needs more than 2lb per Sq in increase between usual and fully loaded. Taking this in my logical way, but which may not be very logical, if you have to increase pressure by 14lb etc for 4 passengers (ie about 3.5 lbs per extra passenger) does this mean that if you have one passenger you should increase pressure by 3.5, if 2 passengers by 7lbs, etc., etc. Sounds a lot of trouble
Or does this extra pressure just come into effect when you have 4 passengers and not be needed when there are 3??
Just seems odd to me.
Or have I had too much red wine??
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Phil
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Please bear with me in my total ignorance but I have never owned a car that needs more than 2lb per Sq in increase between usual and fully loaded. Phil
Do you read your vehicle's handbook? Most ordinary cars have a significant variation between "normal" and fully laden/towing.
On the other hand, many 4x4s and trucks just have one set of pressures issued by the manufacturer.
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..... L'escargot. Maybe you could use search next time.
I've retracted my head into my shell in embarrassment!
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L\'escargot.
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