........it's easy 
Twin axle caravans are fitted with lower load rated tyres by the manufacturer since the do not carry as much weight as most single axle tyres. (the weight is spread over 4 tyres)
Because they are carrying lower weights they require relatively lower pressures......usually around 30 psi.
Most single axle caravans require higher load rated tyres that need higher pressures.....maybe up to 65 psi.
....it is a mistake to think that the two types of tyre are the same.
Twin axle caravans are fitted with lower load rated tyres by the manufacturer since the do not carry as much weight as most single axle tyres. (the weight is spread over 4 tyres)
Because they are carrying lower weights they require relatively lower pressures......usually around 30 psi.
Most single axle caravans require higher load rated tyres that need higher pressures.....maybe up to 65 psi.
....it is a mistake to think that the two types of tyre are the same.
EH52ARH said:
Hello Hutch,
Yes I'm afraid you have missed two important points. The first is that old chestnut nose load plays a significant part in the relative loading on TA wheels and the effect if the spring rates and compression characteristics of caravan suspensions.
I'm not going to on long about TA nose loads, but suffice to say your nose load on your TA that will translate to a differential on the axles (and therefore tyre loading) in the roughly the same ratio of the distance from the hitch to the CoG divide by the distance between the axles. The longer the caravan the bigger the difference will be but it will be at least 3:1 and could be as high as 5:1 This is one of the reasons why getting nose loads set up properly is difficult but vitally important for TA caravans.
Conventional springs exhibit a linear relationship between load and extension/compression, but caravan suspensions are far from linear. They tend to stiffen up dis-proportionally as the load increases. This is why most caravans do not seem to vary their height very much even when heavily loaded. But it also means if a suspension unit is unloaded (i.e. extended) the load it actually supports diminishes more rapidly than for a conventional spring.
As the side of a TA caravan drops when a tyre fails, the difference in height must apply a greater load to suspension on the remaining wheel. But at the same time as the weight of the caravan has not been changed that means the failed wheel must be carrying less load. This is also demonstrated by the fact the failed tyre allows its axle to drop further which unloads the suspension.
Hello Dusty,
just for clarity:
Boyles law states that: the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature
I cannot deny that temperature changes will occur in working tyres, but to try and keep the explanation as simple as possible concerning the effects of volume on pressure I deliberately chose to remove them from the discussion hence my comment in my posting "ignoring temperature changes" meaning to infer constant temperature. I considered it reasonable because they will be relatively small compared to the effect we are discussing.
Your reference to the footprint is very pertinent, and over or under inflation is bad news, but within the tyre manufactures remit must be a remit to ensure that for any given pressure the tyre will continue to perform satisfactorily with a range of applied loads. were not talking about F1 edge of working limits here. But I did use the concept of the "contact area" which is the same as the foot print.
As an area is defined as its width times its length, if one dimension is constant (i.e width) then the change in length is directly proportional to the change in area. Again, I deliberately chose not to vary the width of the tread because it would over complicate the point I was making. In practice the tread width does not change very much once the pressure has been set so it was I believe reasonable to consider it a constant in this principle.
I do concede I could have described the tyre slightly differently:
Looking at a side view of tyre on any vehicle it has two distinctive parts to its circumference. There is flat portion where it is in contact with the road, and the remaining part which is a segment of a circle. Bearing in mind my simplification that the width of the tyre does not change, the the area of the side of the tyre is proportional to its volume.
Considering the same tyre but with different applied loads, if you increase the load the contact area will increase which will be represnted by a lengthening of the contact area and to accommodate this the axle has to drop accordingly, thus the side wall area of the tyre has reduced, which in turn means the volume has decreased. Due to Boyles Law, that means the pressure will also have risen but not in proportion to the load but in proportion to the change in tyre volume.
Importantly if you double the load, you don't lose half the volume of the tyre, only a relatively small proportion, which is why the pressure changes are also relatively small. Boyles law applies.
just for clarity:
Boyles law states that: the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature
I cannot deny that temperature changes will occur in working tyres, but to try and keep the explanation as simple as possible concerning the effects of volume on pressure I deliberately chose to remove them from the discussion hence my comment in my posting "ignoring temperature changes" meaning to infer constant temperature. I considered it reasonable because they will be relatively small compared to the effect we are discussing.
Your reference to the footprint is very pertinent, and over or under inflation is bad news, but within the tyre manufactures remit must be a remit to ensure that for any given pressure the tyre will continue to perform satisfactorily with a range of applied loads. were not talking about F1 edge of working limits here. But I did use the concept of the "contact area" which is the same as the foot print.
As an area is defined as its width times its length, if one dimension is constant (i.e width) then the change in length is directly proportional to the change in area. Again, I deliberately chose not to vary the width of the tread because it would over complicate the point I was making. In practice the tread width does not change very much once the pressure has been set so it was I believe reasonable to consider it a constant in this principle.
I do concede I could have described the tyre slightly differently:
Looking at a side view of tyre on any vehicle it has two distinctive parts to its circumference. There is flat portion where it is in contact with the road, and the remaining part which is a segment of a circle. Bearing in mind my simplification that the width of the tyre does not change, the the area of the side of the tyre is proportional to its volume.
Considering the same tyre but with different applied loads, if you increase the load the contact area will increase which will be represnted by a lengthening of the contact area and to accommodate this the axle has to drop accordingly, thus the side wall area of the tyre has reduced, which in turn means the volume has decreased. Due to Boyles Law, that means the pressure will also have risen but not in proportion to the load but in proportion to the change in tyre volume.
Importantly if you double the load, you don't lose half the volume of the tyre, only a relatively small proportion, which is why the pressure changes are also relatively small. Boyles law applies.
ProfJohnL said:
Tyres are made out of semi-rigid material that does not stretch or shrink and therefore the tyre side wall cannot reduce. The side wall at the lower portion deforms outwards and that outward bulge will contain part of the volume of the tyre, so the overall volume has not decreased.
WoodlandsCamper said:
Hello Woodlands
If you read my post I specifically excluded that aspect to simplify the principle. Yes; the deformation of the tyre does occur as you describe, and but even so there is still a small reduction in volume, which results in a slight rise in pressure.
The important point I am trying to express is that Chrisn7 reported there was no significant change in tyre pressure between his TA's axles even though the caravan was not towed level. He assumed incorrectly that the loads on each tyre must therefore be the same.
We have I think and hope we've been able to explain why he saw no difference. And how when a TA's tyre fails the bulk of load is transferred to the remaining tyre.
Sorry Prof, you are incorrect, there is no reduction of volume inside of the tyre, if the side wall deforms. the volume is constant, for the volume to decrease inside tyre, the ambient air pressure would have to be considerably higher than the internal pressure of the tyre. This is why the temperature increases due to a friction from the road and tyre wall flexing.
Hello Dusty
I forgot to answer this:
The AL-KO type "axle" in a caravan is not a true axle. In fact its a securing tube into which a swinging arm suspension units is fitted at each end, and the true axle(s) are the rotational centres of the wheels on the ends of the swinging arms. These are actually fully independent of each other.
Its convenient to describe the cross tube as an axle, as that's how AL-KO supply them, So when they describe an axle assembly as having a load capacity of 1000kg in reality it means 2 x 500kg units
Again as with so many pieces load bearing kit, the manufacture will specify a static load limit, with the full knowledge that in practical use the dynamic loads will exceed the static limit. The system will be design with sufficient margin of strength to with stand more than the normal dynamic loads. Consequently if a unit has a static load limit of 500kg, its probably capable of withstanding 100% or more overload under dynamic conditions. Protracted overloading should be avoided as it will be detrimental to the long term life of the product.
In the context of your question a situation like climbing a kerb where both wheels on one "axle" are lifted off the ground will occur at low speeds and will not be sustained for long times, these should be Ok. but if a caravan was pitched for a longtime with a pair of wheels left hanging then the suspension units in the remaining pair may be damaged.
In the context of the thread and the loss of a tyre, then you still have three wheels with a nominal 1500kg capacity, and again provided this is not sustained for hours on end it should not damage the axle units.
I forgot to answer this:
Dustydog said:
The AL-KO type "axle" in a caravan is not a true axle. In fact its a securing tube into which a swinging arm suspension units is fitted at each end, and the true axle(s) are the rotational centres of the wheels on the ends of the swinging arms. These are actually fully independent of each other.
Its convenient to describe the cross tube as an axle, as that's how AL-KO supply them, So when they describe an axle assembly as having a load capacity of 1000kg in reality it means 2 x 500kg units
Again as with so many pieces load bearing kit, the manufacture will specify a static load limit, with the full knowledge that in practical use the dynamic loads will exceed the static limit. The system will be design with sufficient margin of strength to with stand more than the normal dynamic loads. Consequently if a unit has a static load limit of 500kg, its probably capable of withstanding 100% or more overload under dynamic conditions. Protracted overloading should be avoided as it will be detrimental to the long term life of the product.
In the context of your question a situation like climbing a kerb where both wheels on one "axle" are lifted off the ground will occur at low speeds and will not be sustained for long times, these should be Ok. but if a caravan was pitched for a longtime with a pair of wheels left hanging then the suspension units in the remaining pair may be damaged.
In the context of the thread and the loss of a tyre, then you still have three wheels with a nominal 1500kg capacity, and again provided this is not sustained for hours on end it should not damage the axle units.
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