Bone question number 2; Tyre Pressures?

[Tosh_374320091]

I've an old 1994 Lunar Moonlight 5 (in good nick) with a motor-mover (which is great); no handbook; single axle, and I'm extremely carefull how I load the van and car; I understand the issues here.

But I haven't a clue what the tyre pressure should be on the van tyres.

I've taken it out twice and have always put 40 psi in each of them; why?

Just a guess really, I'm ashamed to say, but it seems to work. They're just about brand new tyres too.

But does anyone know the correct tyre pressure I should be putting in them?

 

[rob_jax]

Hi Tosh,

Several ways - hopefully they'll give reasonably similar answers - but first of all:

Look at the tyres fitted to your caravan - and write the tyre information down - possibly something like 175/80SR13 85T - and then also record the maximum tyre inflation pressure (usually in a small oval marking on the side wall) and the Load Index number (the 85 in the above example)

Find out the maximum weight of the caravan (MTPLM) - should be on the data placard near the caravan door.

And then:

1. Look on the Lunar owners web-site or forum for the inflation information - but check that you still have the same size and section as listed in their information

2. If you can't find the information - or if the tyres are different you can calculate the pressure from:

Pressure = (Caravan MTPLM weight on that wheel) divided by (The maximum tyre load from the load index) multiplied by (the maximum inflation pressure)

If your caravan has a MTPLM of 1000kg (500kg per wheel)

The tyre has a load index of 85 - that's a maximum of 515 kg - google "Tyre load index" for sites that tell you what the number codes mean

A maximum inflation pressure of 50 psig

That works out as: 500/515 * 50 or 48 psig... Your figures will be different...

Robert

 

[Tosh_374320091]

Hi Tosh,

Several ways - hopefully they'll give reasonably similar answers - but first of all:

Look at the tyres fitted to your caravan - and write the tyre information down - possibly something like 175/80SR13 85T - and then also record the maximum tyre inflation pressure (usually in a small oval marking on the side wall) and the Load Index number (the 85 in the above example)

Find out the maximum weight of the caravan (MTPLM) - should be on the data placard near the caravan door.

And then:

1. Look on the Lunar owners web-site or forum for the inflation information - but check that you still have the same size and section as listed in their information

2. If you can't find the information - or if the tyres are different you can calculate the pressure from:

Pressure = (Caravan MTPLM weight on that wheel) divided by (The maximum tyre load from the load index) multiplied by (the maximum inflation pressure)

If your caravan has a MTPLM of 1000kg (500kg per wheel)

The tyre has a load index of 85 - that's a maximum of 515 kg - google "Tyre load index" for sites that tell you what the number codes mean

A maximum inflation pressure of 50 psig

That works out as: 500/515 * 50 or 48 psig... Your figures will be different...

Robert

Robert,

Thank you for your helpful post; much appreciated.

Cheers,

Tosh.

 

[Iain_5062081]

Hi Tosh

I have an even older Lunar. A 1986 Jupiter 5 berth. The booklet recommends a pressure of 36psi.

The Jupiter is plated at 1050Kg.

Judging by what others are quoting on this forum 36psi seems low for a caravan's tyre pressure. But it's definately in the book.

Iain

 

[rob_jax]

Hi Iain,

"Judging by what others are quoting on this forum 36psi seems low for a caravan's tyre pressure. But it's definately in the book."

And the handbook can so easily be wrong...

Hymer (Eriba) have published the same 2.5 bar recommendation for tyre inflation for decades - the different models used different section tyres - but the same magic tyre pressure was recommended.

There's been some discussion about this on the UK, French and Dutch Eriba forums - particularly where the same tyre was fitted to one certain model where the load could vary by 500 kg on a 800 kg MiRO.

Long story short - the groups made contact with the engineering director at Hymer and were promptly rewarded with a very comprehensive and informative list of tyre pressures - showing how different pressures should be used for different loads - for every tyre combination...

The two main reasons for doing this were to satisfy any legal representations about using higher pressures, and also to satisfy those owners who implicitly believe the handbook value without question...

Robert

 

[Iain_5062081]

Hi Robert

Thanks. I seem to fall into the later category then.

I have just phoned Lunar but they are on their annual shut down until Monday. I'll try again then. In the meantime I'll try out the calculations above and see how they compare with what Lunar tells me.

Iain