Apologies for another tyre inflation query.

[Dustydog]

Thanks Prof
Cutting the semantics Bailey got it right :kiss:
In regard to your earlier comments and some of Sir Sproket's my official recommended tyre pressure 29 psi is safe.
I have to say I was ready to challenge Bailey of the potential under engineering.
This doesn't seem to be the case.
So Sir Sproket.
You and Kev have very large heavy SAs. What is your axle rating?

 

[ProfJohnL]

I have to say that this discussion has brought the matter into the light, and hopefully given some readers a better insight into some of the issues with Twin axles.

 

[Sproket]

So Sir Sproket.
You and Kev have very large heavy SAs. What is your axle rating?

Mine is 1600kg running on 195 / 70R 15C 102/104 tyres i run them between 60 - 63psi (max is 65psi).
I have tried running them at 58psi what the calculated pressure is but the pressure increase is more evident on the tyrepal due to the heat generated by the flexing of the sidewalls, quite a few years ago i contacted Dunlop technical and was advised to increase the pressure to help the problem .....

Tyres that are run at low pressure due to flexing generates heat buildup and that can cause blowouts ....

 

[Gagakev]

So Sir Sproket.
You and Kev have very large heavy SAs. What is your axle rating?

Mine is 1600kg running on 195 / 70R 15C 102/104 tyres i run them between 60 - 63psi (max is 65psi).
I have tried running them at 58psi what the calculated pressure is but the pressure increase is more evident on the tyrepal due to the heat generated by the flexing of the sidewalls, quite a few years ago i contacted Dunlop technical and was advised to increase the pressure to help the problem .....

Tyres that are run at low pressure due to flexing generates heat buildup and that can cause blowouts ....

Mine is 1700kg......Mtlpm 1700kg running on 195 /70 R 15C 104/102R so the same tyre as Sprocket.
Running at 65psi
So max weight on recommend axle weight. Bit tight on margins 🙁

 

[Dustydog]

I have to say that this discussion has brought the matter into the light, and hopefully given some readers a better insight into some of the issues with Twin axles.

Thanks Prof

A very interesting thread.
Sir Gagakev's is right on the limit with no margin for error. Is that good engineering practice?
In my own case if one of the axles fails or a tyre has a puncture unknown to me then the other axle /tyres will be seriously overloaded.!
Time for a set Sir Sproket's tyrepals.

 

[Sproket]

Funny you should mention that. .....
The Bailey Unicorn now has them fitted as standard

 

[Dustydog]

Wyoming rear axle plate

 

[ProfJohnL]

Hello Dusty,

Bearing in mind what has been discussed earlier about rated loads which are specified and measured statically, and the effect of towing dynamics which will increase the actual loads applied by a considerable margin, responsible manufacturers will specify safe maximum loads, and in every mechanical respect there should be no concerns at operating up to those maximum loads because of the extent of the dynamic load margins they have to cater for.

That does not mean you can be cavalier about exceeding the specifications. The dynamic loads involved are almost all due to accelerative forces which don't mathematically add to the static loads but multiply them. So if you overload the static specification by say 10% then the dynamic loads will also be 10% bigger.

This same concept is applicable to nose loads, axle loads, tyre loads, Maximum permitted towed loads. All these features need to cater for very large dynamic loads compared to their static specifications.

For example, lets consider nose loads, if a tow hitch is rated at static 75kg, dynamically it will be designed to handle at least 3 times that. Consequently it's perfectly safe to leave the caravan coupled to the car when stationary and then enter the caravan to make and eat a meal.

In the case of tyres, we know from previous threads that tyre pressures are quite a contentious issue, but I hope there is general agreement that tyres pressures need to be set to keep the flexion of the tyre wall as the wheel rotates within a particular range. As the tyre rotates when towed the continual flexing of the tyre wall generates heat. This is normal and allowed for by the tyre manufacture.

If a tyre is underinflated this means the sidewall flexes more and will generate excess heat. Excessive heat is detrimental to the tyre compounds and will accelerate failure. But even a properly pressurised caravan tyre will frequently be subject to increased load due to the dynamics of towing, but the important difference is the excess will be momentary, and often followed by an almost equal but opposite underload which means the average heat generation will not be outside its safe range. So again a tyre can easily withstand more than its rated load when stationary or when negotiating a kerb/speed hump at slow speed.

I hope this puts peoples minds at ease.