Trailer tire pressure

[jgr2015]

Thanks again. As @MetalMonkey and others said, my reference to car tyres is because that's my experience to date for tyres and pressures. A sheltered life.

The advice regarding pressure helping avoiding damage from kerbing, potholing and overheating are all good to know.

 

[mart]

He means car tyres are typically in the region around 30 - 35 psi, so based on this experience 90psi would seem rather high.
However trailer, caravan and motorhome tyres are typically ran at significantly higher pressures. HGVs are another step up again.

I agree.
It is good that @jgr2015 is making the effort to check and be safe.

 

[Bornfree]

I always thought the max pressure on the tyre sidewall was just for seating the bead not what pressure to actually run them at?

Ie if the bead hasn't seated by 30 psi etc you need to demount and relube or theres a risk of damaging the tyre bead

Tyres normally have max loading figure at a max pressure. The bead seating pressure is normally marked as such.

 

[mart]

Tyres normally have max loading figure at a max pressure. The bead seating pressure is normally marked as such.

I've never noticed the bead seating pressure on tyres. Is it usually marked in smaller lettering?

 

[Bornfree]

I've never noticed the bead seating pressure on tyres. Is it usually marked in smaller lettering?

It's normally on agricultural tyres but I have seen it on 4x4 tyres. Usually says " never exceed ***psi when seating beads"

 

[Justme]

Trailer & Caravan Tyre pressure calculator from Western Towing

Make sure you caravan or trailer tyres are at the best pressure for your tyres and gross weight using this tyre pressure calaculatoir from Wesytern Towing.

www.westerntowing.co.uk

 

[Justme]

The tyre pressure for these tyres seems to be 90 psi which is a lot coming from car tyres.

Car tyres tend to be bigger and potentially carry less weight per tyre.

Remember its PSI (pounds per square inch) so tyre size affects carrying capacity.

The more surface area for the air to touch the less the pressure has to be to carry X pounds of weight.


If you over inflate or under the trailer tyre you will wear the tyre out quicker.

 

[Erie Fred]

It's normally on agricultural tyres but I have seen it on 4x4 tyres. Usually says " never exceed ***psi when seating beads"

See it on car & truck tires now as well.
I can't figure why a tire rated at 40 psi max. has the warning "Do not exceed 70 psi during mounting"

 

[not done it yet]

See it on car & truck tires now as well.
I can't figure why a tire rated at 40 psi max. has the warning "Do not exceed 70 psi during mounting"

I can. The 40psi is a simple static pressure, whereas the 70psi is the maximum dynamic load of the tyre if dropped from a height with a considerable load on the axle.

Like the comment earlier about 500kg on each tyre. Just not true - might be, on average - as all the weight could be on one tyre of the axle at times. Simple safety limits, much like most things, there are SWLs and tested loads. Think here maybe a simple bungee cord? In equilibrium the load extends the bungee until the forces are in equilibrium. Jumping off a bridge parapet will stretch that same bungee by quite a lot…..

 

[bricol]

"Equilibrium"



A word most of us haven't heard since their last school Physics lesson

 

[Bornfree]

See it on car & truck tires now as well.
I can't figure why a tire rated at 40 psi max. has the warning "Do not exceed 70 psi during mounting"

I love the anticipation as you get over 50psi and the bead still hasn't popped on. It starts to get life or death.

 

[Erie Fred]

I can. The 40psi is a simple static pressure, whereas the 70psi is the maximum dynamic load of the tyre if dropped from a height with a considerable load on the axle.

Like the comment earlier about 500kg on each tyre. Just not true - might be, on average - as all the weight could be on one tyre of the axle at times. Simple safety limits, much like most things, there are SWLs and tested loads. Think here maybe a simple bungee cord? In equilibrium the load extends the bungee until the forces are in equilibrium. Jumping off a bridge parapet will stretch that same bungee by quite a lot…..

I think you have that backwards.
The 40 psi rating is the max for driving, yet somehow we are allowed to exceed it by almost 2x when mounting.
FWIW I never allow it to go above 20 psi when mounting.

 

[knighty]

I think you have that backwards.
The 40 psi rating is the max for driving, yet somehow we are allowed to exceed it by almost 2x when mounting.
FWIW I never allow it to go above 20 psi when mounting.

why?


nissan blows tyres up to 90psi before sending cars away on transporter ships - I was told it's to stop them getting flat spots when they sit for a long time, but that seams a little off to me... but whatever the reason, they do do it

 

[Erie Fred]

why?


nissan blows tyres up to 90psi before sending cars away on transporter ships - I was told it's to stop them getting flat spots when they sit for a long time, but that seams a little off to me... but whatever the reason, they do do it

I Learn something new each day.

I always take the maximum as just that, "Do not exceed" same as with any pressure vessel.
Tire explosions (especially when mounting) are horrific, and many have been killed.

 

[mart]

See it on car & truck tires now as well.
I can't figure why a tire rated at 40 psi max. has the warning "Do not exceed 70 psi during mounting"

Basically I would say the 40psi is the safe limit of the tyre when driving.

The 70 psi is a very temporary pressure for sealing on to the rim. After sealing, air would then be released down to 40psi for driving.

 

[Erie Fred]

Basically I would say the 40psi is the safe limit of the tyre when driving.

The 70 psi is a very temporary pressure for sealing on to the rim. After sealing, air would then be released down to 40psi for driving.

Yes, that's what I figured as well, but I change allot of tires, many of them are used.

So I don't want to tempt fate.

 

[mart]

Yes, that's what I figured as well, but I change allot of tires, many of them are used.

So I don't want to tempt fate.

You mean because you never know the exact condition of the used tyres.?

I would imagine that generally the sealing of the bead on to the rim would be accomplished before the maximum pressure is reached/required.

 

[Erie Fred]

You mean because you never know the exact condition of the used tyres.?

I would imagine that generally the sealing of the bead on to the rim would be accomplished before the maximum pressure is reached/required.

Eggzachary....
While many keep inflating a tire until it seats, I stop at around 20 psi, if it hasn't at point I simply
give it a few seconds, and maybe wack it with a rubber mallet.

If it still won't seat. I demount and try again.

Local garage friend was at a dodge dealership one day, said the kid on the tire machine was above 70 psi
before it seated,.....knocked all the dust off the roof trusses when it finally went, and I think the office staff
came running out as well.

I've dealt with split rims before, tractor tires, large truck tires, and understand the forces involved, don't need that kind of excitement.

 

[not done it yet]

I think you have that backwards.
The 40 psi rating is the max for driving, yet somehow we are allowed to exceed it by almost 2x when mounting.
FWIW I never allow it to go above 20 psi when mounting.

No, I definitely do not have anything backwards. If the force is applied to an object iit will either accelerate in the direction of that force unless/until there is an equal and opposite force to counteract it. Look up Newton’s Laws of Linear Motion!

If the force is increased on an inflated tyre, something must change. If it is restrained by, say the road surface, the tyre must deform. At some point the tyre will become completely squashed if it has not burst before that point. Squashing a tyre which does not stretch like ballon will increase the internal pressure according to Boyles Law (under isothermal conditions) If the internal volume is decreased adiabatically, the air inside the tyre will heat up as the pressure increases.

Hotter gas at constant volume, let alone constant anything else, will exhibit a higher pressure. Think here of a compression-ignition engine which reaches over 350 psi at cranking speed, making the gas hot enough to ignite and burn the injected fuel spray.

These processes all follow basic physical laws, which most on here would not use, or perhaps even understand, The simple piston compressor is yet another example - reduce the volume of air above the piston and the pressure increases. Nothing different than the occasional instantaneous load on a pneumatic tyre of any description. Please, get used to the Laws of Physics. They are Laws for a very good reason. They are theory proven beyond any doubt whatsoever under normal conditions.

Newton’s Laws hold true for all normal velocities until the velocity approaches the speed of light - when the mass of an object increases so much that it can no longer be accelerated by any increased force - hense nothing can travel faster than the speed of light (which, incidentally, is energy without mass - hence Einstein’s E=mc^2.

Typically Newton’s Laws hold true (enough for all practical purposes) until items travel in excess of about 0.6c (that’s 0.6 of the speed of light).

If anyone feels they need to discuss any of the Laws of Physics, please feel free to PM me.

 

[MetalMonkey]

Max pressure is the value the manufacturer has specified as their safe maximum pressure.
Max mounting pressure, is the safe momentary maximum pressure specified by the manufacturer to seat the bead (not something I've heard of on anything other than ag stuff?).

The vehicle or equipment manufacturer will have probably also specified a range of operating pressures based on the application in question, and no doubt after consultation with the tyre manufacturer.

That's all there should be to it - nothing more complicated. No physics lecture required.
The exact details of how those figures are arrived at will be down to design and testing done by the tyre manufacturer.

I'm not a tyre designer, so I don't pretend to know the process - but I feel fairly confident in saying that it will no doubt include lots of theory, empirical evidence and simulation.
There are specific formulas surrounding tyre modelling (magic tire formula IIRC), many years of data collected on different compounds, construction and loadings.

While I suspect that many of us on here do actually have a reasonable understanding of the basic laws of physics (thank you very much ).
I would be the first to admit to not having any experience in the design and manufacture of tyres, and I suspect I'm not alone.

Therefore I would suggest the best course of action is to assume the manufacturer probably knows what they are doing and just do "wot it says on the side".

A diligent tyre fitter should do just that, follow the instructions, inflate in a cage and observe safety precautions and maximum ratings.
An average tyre fitter will more than likely get the apprentice to sit on it, give it what for with full shop air and upload the result to youtube.

Car tyre explosions can cause you significant damage, HGV and large plant tyres can easily kill you and not leave much left to identify.
There will be safety margins built in, best not to discover what they are IMO.