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John_b_45

Confused - Tyre Pressures

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Caravan MTPLM 1425kg. Tyres 185 R14 C with 900kg @ 65PSI as loading moulded on wall. I've read the helpful pdf file posted here by a member that gives really sound advice, along with tables showing tecnical details, for Tyres fitted to caravans. I am happy that my tyres conform in every way with the recommendations in this leaflet, including the 90% recommendation for leeway between actual and max. permissible loads etc. The tyres are original equipment but my can, newly purchased, is nearly 2 years old.

 

I have checked tyre pressures with an accurate guage and both are 54 PSI. The handbook for my caravan tells me the pressures should be 73 PSI and the tyre manufactures refers to 65 PSI as a basis for loading values.

 

Question. I would like to assume the caravan manufactures knows what the correct pressure should be for the combination of tyre and caravan - 73PSI. But, I have had the van only a few weeks (not used it yet) and tyres are at 54 PSI (and this after a full service from a reputable "main" dealer). Would I be advised to inflate to 73 PSI and assume the extra 8 PSI over the rating moulded on the tyre wall is not an issue? Or should I leave at 54 PSI?

 

I know "stiffer" tyres on both car and caravan help control - as does "stiffer" car suspension in my experience. I would like to go with the 73 PSI but would welcome comment from members.

 

My apologies if this topic has been raised before - I'm sure it has - but tyres are crucial when safety is considered so a further airing might be useful for others as well as myself.

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Using a formula I've seen on here the pressure should be

 

Max tyre press = 65psi

Max tyre load = 900Kg

MTPLM = 1425Kg

 

(1425 X 65) / (900 X 2) = 51 psi

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73psi seems incorrect. If the markings on the tyre wall show 65psi at 900kg, i. e 1800kg caravan load, this is the max pressure for the tyre. 54psi seems more realistic for the 1425kg load you have. if you prorate the load 65 x 1425/1800, it comes to 52psi so add on 2-3psi, 54/55psi would seem to be about right.

What make and model of caravan do you have?

Brian

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73 seems high,mine on a senator oklahoma are supposed to be around 60 but i have them around 55-57 and have a pretty stable tow.

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You have not made mention of the tyres load index which should be 100/102 for that size tyre.

 

If you look in the tables on this web page ( http://www. tyresafe. org/data/files/caravan%2008. pdf ) and extrapolate between 1390kg and 1469kg - you will see that a pair of 185R14C tyres with a load index of 102 will easily carry 1425kg at an inflation pressure of 53psi.

 

I would sugggest that the handbook has a misprint and should read 53 not 73

 

The pressure and load indicated on the tyre side wall are maximum figures and should not under any circumstanses be exceeded. :huh:

 

Over inflated tyres will wear prematurely in the centre, are liable to overheat and fail, not to mention being illegal. :angry:

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Basically agree with Reluctant, however, there is no such 185/14 tyre listed with what would be a load index of 104 for 900kg?

 

102 for 850kg is listed and 52-53psi would be correct for this tyre, while perhaps 53-54psi would be correct for a tyre with a 900kg max.

 

Seems like I'm splitting hairs but no, I'm simply pointing out one more anomaly to add to the rest.

 

So is the load index 102 or 104? because in this case you should ignore the 900kg and go with the load index number which I believe will be 102 and will then remove the uncertainty

 

74psi is quite obviously wrong and would indeed be illegal, don't matter if it is a typo, such an error is potentially dangerous and should have been found and corrected long before the manual went to print.

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Most definitely the 73 psi printed in the owners manual is a dangerous error. The OP doesn't make any mention as to who the manufacturer of his van is, it would be very interesting to know.

 

As a side, some years ago when compiling all of the detail data and information for the Dunlop Tyres tyre pressures and fitment chart (I did this exacting project every year for 10 happy years!) I came across some widely varying tyres pressures for a Toyota product, my enquiries revealed that Toyota had one set of pressures in the trade workshop manual, another set of pressures in the owners handbook and another set of pressures reproduced on the cars 'B' pillar sticker and all 3 of them were totally wrong!!

In fact every single year we discovered widely varying and totally inaccurate information given by the motor manufacturers, or as Mercedes used to say when approached "it is our job to sell cars and your job to sell tyres" !!

 

Returning to the OP, you should always ignore the MAX pressure moulded into the tyres sidewall, that is the tyres maximum pressure and should never be approached when inflating, so going by what has been offered here, I would agree that a pressure somewhere between 52 and 55 should be an ideal compromise

Edited by bigjimmy

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Basically agree with Reluctant, however, there is no such 185/14 tyre listed with what would be a load index of 104 for 900kg?

 

102 for 850kg is listed and 52-53psi would be correct for this tyre, while perhaps 53-54psi would be correct for a tyre with a 900kg max.

 

Seems like I'm splitting hairs but no, I'm simply pointing out one more anomaly to add to the rest.

 

So is the load index 102 or 104? because in this case you should ignore the 900kg and go with the load index number which I believe will be 102 and will then remove the uncertainty

 

74psi is quite obviously wrong and would indeed be illegal, don't matter if it is a typo, such an error is potentially dangerous and should have been found and corrected long before the manual went to print.

 

The OP quotes a tyre size of 185R14C which is listed in the table I posted

 

 

 

185R14C 102 1062 1146 1245 1326 1390 1469 1532 1593 1624

 

First seven digits are the size. ..next three are load index and the rest are axle load in Kg

 

Tyres of this size are readily available, I have just fitted Bridgestones to my van in this exact size/load index.

 

No need to split hairs. ..it is all there in black and white :)

Edited by reluctant

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Oh dear, it would appear from the tone you think I'm somehow getting at your reply, which I am not!

 

I'm simply pointing out one more anomaly so as to answer another possible doubt about John trusting the tables to find the correct pressure.

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wow - Thank everyone for your advice. Sincerely appreciated.

 

Update: This morning I had to take my car to have a puncture repaired and took the opportunity to talk with the mechanics. The first guy looked at my figures (I'd jotted the details of the tyre on a bit of paper) and told me he thought near 73 psi would be correct. Whilst paying for the repair I asked the same question to the manager who thought 54 psi (as they currently are at) more appropriate. He went on to say that vans they deal with fitted with similar tyres run as low as 36 psi in some cases.

 

I left the garage and drove straight to the main dealer who serviced the van, and from whom I had purchased it, and asked for clarification. I was taken into the workshop and shown a generic chart pinned to the wall which from memory was not dissimilar to that in the pdf leaflet I refer to in my original post. Here the tyre pessures for my van (actually weight ranges slightly below and around 40kg above mine) is shown as 54 psi. The dealer will ring Lunar (yes it's a Lunar Ultima EBW which is a Lexon EB with a few fancy bits added) and ask for clarification. I've looked again at the handbook and there are quite a number of vans shown with 73 psi and 185 R14 C tyres. (Quaser 524, 525, 534, 546, Lexon EB, SI, SB, Clubman EB, ES, SI. Other vans with the same tyres but lighter MTPLA Zenith 5, 6 and 7 have 60 psi as does the Stellar and Quaser 462.) I can only hope there are misprints but are we now looking at two misprints? The 73 psi and the 60 psi both of which are well over the 54 psi which your replies support.

 

These figures are in a 2009 Service Handbook - I wonder if they are the same in 2010/11 versions? I'll report back on the information I receive from Lunar via my dealer early next week but it worries me to find this situation.

 

PS. The load index shows on the tyre as 104/102N. I am fairly certain these figures refer to the tyre fitted as a pair and singly (ie 2 wheels per axle or 4) Again I am fairly certain 102 is the figure to use and cleary gives a greater margin of safely. And the tyre is a Kargomax C ST-6000.

 

PPS. I was advised by a mechanic that balancing the wheels was well worth the cost - around £5 per wheel. His view was that whilst the inbalance would never be felt in the car the constant "wabbling effect" on the wheel could contribute to premature damage to the bearings! Sooooooooo Next week I hope to have balanced wheels with tyres at the correct pressure!

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Basically agree with Reluctant, however, there is no such 185/14 tyre listed with what would be a load index of 104 for 900kg?

 

 

The 185 x 14 OE tyres on my caravan are marked 102/100P plus another marking in an oval ring 104N

 

To run at load index 104 (900kg) the speed index P (93 mph) is reduced to N (87 mph)

 

There are tyres with 104 load index to be found on websites

Edited by beejay

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My caravan handbook states a tyre pressure of 54 psi (MTPLM is 1430kg).

For the car, I follow the handbook and increase the tyre pressures to the 'fully loaded' pressures specified.

 

One word of caution concerning motorhomes. When we had motorhomes, the handbook pressures were the same as for those specified for the vehicle by the vehicle maker.

I felt that the ride was too hard at (as specified) 50 psi(front) and 70psi(rear). When I had new Michelin tyres fitted, I contacted Michelin for advice. They told me to have the vehicle weighed as used, and get three readings: front axle, rear axle, and total weight, and then contact them again. I did this and they then calculated the proper pressures. The front tyres remained at 50psi and the rear tyres dropped to 50psi. The vehicle then rode much more comfortably.

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An update as promised. I contacted Lunar who tell me a change of supplier/manufacturer of wheels/tyres part way through 2009 should have been reflected in details being added to the handbook advising pressures of 73psi or 65psi depending upon tyre brand fitted. I seem to have a handbook which was issued when only one tyre type was being fitted - not the KargoMax that I have. They insist that the correct inflation pressure for my KargoMax tyres is 65psi!

 

John

 

 

 

 

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I've just purchased a 2009 Lunar Stellar. I checked the tyre pressures @ 58psi. The Service Manual states 44. I contacted Lunar who say it should be 60! This inconsistency is worrying. According to my own calculations based on formulae I have seen on forums about 46/47 seems correct. The data is as follows: Maxmiler CX 185 R14 102/100 maximum pressure 65psi. Max load 850 single, 800 dual. Caravan's MTPLM is 1150kg. Therefore, 1150 x 65 divided by 800 x 2 = 46. 7psi.

 

I just wish I could be certain. As I say, worrying.

 

mbp

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I've had a similar problem. My 94 Abbey with mtplm is on 175R13 8 ply tyres, and pressures in the handbook suggest 42psi. When I had it serviced they were put upto 54psi, and next time we went out in it it was really bouncy, after looking on here and finding the forumula I worked out it should be 49ish, so tried 50 and it was a whole lot different to tow, no bouncing when hitting slight dips/potholes.

 

I'd be using the formula and taking it for a drive to see how it fairs, and then maybe add or remove a couple of psi.

 

My old car was on low profile tires, with very stiff suspension (think track prepped) and if I ran at the figures it said it was a nightmare to drive on the road, just felt like it bounced over every bump. Lowered pressure slightly and it was much better.

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The two above posts are easy to answer, the 42psi and 44psi will refer to original fit 'reinforced' tyres which all have a maximum working pressure of 42psi for their maximum load.

 

Current tyres are 'commercial' tyres which depending on which type, have 65psi or 54psi as a maximum, (apart from the 185/75R14C which is 69psi)

 

So for 1150kg on 185R14C 102 the correct pressure is 40psi,

As for the Abbey on 175R13C 97, the 'formula' as I've said many times, is inaccurate and gets worse the further from the tyres maximum load/pressure you get, if then it says '49ish' and from the chart, 48psi is for 1139kg and 51 for 1194psi. 1194kg

 

:wub:

Edited by Arc Systems

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. .. 1194psi.

Gee that's a lot of pressure ;)

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