Sophie2 Posted March 6, 2016 Share Posted March 6, 2016 The perceived wisdom if trying to establish correct pressures for your motorhome is to check with the tyre manufacturer, giving details of tyre and relevant axle weights. This I did and for several months drove around with 58psi all-round-though I must admit it felt a bit low to me. When browsing through the Fiat handbook the other day I came across the page headed "Cold Tyre Inflation Pressure (bar)" Under my tyre "225/75 R16 CP.. Maxi range Camping tyres" the recommended pressure is 5.5 bar...around 79 psi. The same pressure is shown on the inside door frame "A plein charge" Now even for allowing that I'm not "fully laden" there's a helluvah difference between 58 and 79 psi!! So what do you suggest folks? I'm of a mind to up the tyres to around 70psi and see how that is -but confusion still reigns! Link to comment Share on other sites More sharing options...
Brian Kirby Posted March 6, 2016 Share Posted March 6, 2016 What make are your 225/75 R16 CP tyres, and what were the fully laden axle loads you quoted? You can download (at least I did this in 2013!) the Continental Technical Databook from their website, which will give you the correct pressure for load figures for your size tyres (in bar), according to axle location. However, this is only relevant to Continental tyres. In recent years Michelin have refused to recommend any pressure for rear axle tyres below that suited to the maximum permissible axle load, irrespective of actual load, but will recommended a lower pressure for the front axle. 58psi is approximately 4 bar which, from the Conti Databook, should suit a front axle load between 2,170kg and 2,180kg, and a rear axle load between 1,840kg and 1,940kg - for their 225/75 R16 CP 116R tyre (but not for anyone else's). The reference to "plein charge" means at maximum load. I assume your van is French made. My Fiat handbook also quotes, for the same size CP tyre on an X250 based van, 5.5 bar all round. This is presumed suitable for the maximum permissible axle loads for the "maxi" versions of the van at 2,100kg front and 2,400kg rear. OTOH, the Conti databook, for your tyres, recommends 4.0 bar front and 5.25 bar rear at these axle loads. (The maximum permissible loads and pressures for these tyres is 2,500kg at 5.5bar rear and 2,500 kg at 4.75 bar front.) Vehicle manufacturers and converters are very nervous that their vehicles will be overloaded (risking tyres overheating and failing catastrophically), so invariably recommend pressures that a) cater for the maximum permissible load for each axle (irrespective of the fact that any vehicle so loaded would exceed its permissible maximum weight) and/or b) the maximum permissible pressure for the tyre (irrespective of the fact that this caters for loads exceeding the permissible axle loads). This is a fail-safe ploy, intended to protect the uninformed from the consequences of overloading their vehicle. The usual consequence of these higher pressures is somewhat "twitchy" handling, some loss of adhesion, and a harsh ride. If your van feels uncomfortable, or unstable, at the pressures recommended by the tyre manufacturer (who should be judged the ultimate expert on what is appropriate), there is no reason not to increase the pressures to anything between that recommended by the tyre manufacturer, and that taken from the Fiat Owner's Handbook. But, adopting pressures below those recommended by the tyre manufacturer for your axle loads would be very unwise. Link to comment Share on other sites More sharing options...
Pete-B Posted March 6, 2016 Share Posted March 6, 2016 Mine are Michelin and I contacted them with my weighbridge reading, which, I guess, remain much the same as we tend to carry the same stuff, or roundabout the same. They replied saying the pressure should be 80psi rear and a suggested 51psi front. When I tried that it seemed a very hard ride and the backend seemed to bounce a bit (probably my imagination) but I reduced the rear to 75 and upped the fronts to 54 which seems to give a fairly good ride. All I need to do now is stop some of the rattles in the back! When I cure one, perhaps plates or whatever,something else starts up, I guess I'll never win that one! Link to comment Share on other sites More sharing options...
Sophie2 Posted March 6, 2016 Author Share Posted March 6, 2016 THanks. Max wts on the vehicle are 2100 and 2400 (front/Rear) 4250 overall max. It's on a Fiat and the "Plein Charge" is in Italian as well as French. It's on a Hobby. I certainly wouldn't go lower than the figures quoted by Continental so reckon I'll try at around 70psi as I'm not fully laden with a view to increasing when we go away for longer periods with bikes etc etc. Link to comment Share on other sites More sharing options...
Brian Kirby Posted March 7, 2016 Share Posted March 7, 2016 Sophie2 - 2016-03-06 4:19 PM THanks. Max wts on the vehicle are 2100 and 2400 (front/Rear) 4250 overall max. It's on a Fiat and the "Plein Charge" is in Italian as well as French. It's on a Hobby. I certainly wouldn't go lower than the figures quoted by Continental so reckon I'll try at around 70psi as I'm not fully laden with a view to increasing when we go away for longer periods with bikes etc etc. So, the tyres are Continental Vanco Camper 225/75 R16 CP 116R, and the base vehicle is a Fiat maxi, plated at 4,250kg, with plated axle limits of 2,100kg front and 2,400kg rear (I'm being a bit "forensic" with this as accuracy is important if you want to move away from the tyre pressures recommended in the vehicle handbook). On the basis of your post above, I would say that you should not presently vary your tyre pressures from the Fiat handbook recommendation of 5.5bar all round, but should first take your van to a weighbridge and get its actual laden, and axle, weights. Then, having got these (the actual, fully laden gross vehicle weight, plus individual axle loads - making sure you get, and keep, the printed weighbridge certificate), I would suggest you should then do one of two things. Either: 1) e-mail Continental's technical department with the exact details of the tyres fitted to the van, and the actual axle loads as gained from the weighbridge, and ask them to confirm back what tyre pressures they recommend (printing and keeping their reply with the weighbridge certificate), or: 2) go to Continental's website and download the latest available edition of their Technical Databook, and look up the correct pressures for the weighbridge axle loads (printing and keeping the relevant page with the weighbridge certificate). You will then have a permanent reminder of the weights and tyre pressures for your van but, more importantly, you will have the evidence that a) your van is not overloaded, and b) that the tyre pressures you have adopted are those recommended by the tyre manufacturer as appropriate for those actual loads. I think obtaining and keeping that evidence is important because, in its absence, you leave yourself exposed to allegations of negligent vehicle maintenance, which could have implications for your insurance, or legal liability, were you to be involved in an accident. Unlikely I know, but rather difficult to prove after the event, should the worst happen. Link to comment Share on other sites More sharing options...
Robinhood Posted March 7, 2016 Share Posted March 7, 2016 ...not always all that easy to find the Conti Technical Databook, they keep moving it ;-) The latest is available for download here: http://blobs.continental-tires.com/www8/servlet/blob/681286/30cec2cae7fa004e1eae3b0f560e449f/download-technical-databook-data.pdf ...and the tyres/pressures in question are on pages 82/83 (make sure the CP version is referenced). I'm on those tyres myself, and run at well below the door pillar figures, but based on a fully loaded weighbridge session (on the day of departure for a 4 week continental holiday) with a 10% extra allowance added for each actual axle weight. I can assure you the ride and handling are somewhat improved from the 5.5 bar all round figure. Link to comment Share on other sites More sharing options...
Sophie2 Posted March 7, 2016 Author Share Posted March 7, 2016 Many thanks to you both for that. I did find a Continental Data Book but certainly not the one quoted in the last Post so that was very helpful. Looking at pp 82.83 then, and taking account of my last weighbridge visit (1600 front and 1960 rear including driver/.passenger and about 15litres water, half tank fuel)....... I read my required pressures as being a shade over 3.0 front(45) and 4.0 rear (58) However I haven't a clue about "Load Index" and "Service Description" so am not even sure if I have read along the correct column! Can you please enlighten me if I have got that right? (BTW I have emailed Continental as well so it will be interesting to see if they come up with the same result this time! Link to comment Share on other sites More sharing options...
Robinhood Posted March 7, 2016 Share Posted March 7, 2016 ...well, on the figures you quote, close, but no cigar ;-) The front axle value should be taken from the "FA S" (Front axle, single) line, and the rear axle value from the "RA S" (Rear Axle, single) line (assuming you haven't got twin rear wheels). Your value for the rear appears to have been read correctly, but not the front. BUT (a few big caveats): The weighbridge values you're using don't appear to me to be fully-loaded values, and this would lead to lower than safe tyre pressures. The front value is below the table entries in the document, and could be potentially extrapolated to a lower value. However, when Michelin used to quote differently, they always added about 10% to the actual front load to allow for weight transfer under braking, and this generally brings the value into the table range. (I suspect that, in a similar way, Conti will probably quote somewhat higher front pressures than you might otherwise expect for a given weight). To restate, get the 'van weighed at it's absolute heaviest (full tanks, all people, all kit) before adjusting the pressures accurately. Trying to determine the required pressures without these values is futile. I would (my own preference) then add 10% to front and rear actual axle loads to allow for some flexibility, and determine the pressures from those values (either via the databook if you are confident in interpreting - or post here for confirmation of that interpretation, or via Conti-supplied values). I can assure you that (unless you are overloaded, which seems unlikely at the figures you've quoted) this will lead to tyre pressures that will give a less filling-threatening ride. The load-index is a numerical value that defines the maximum weight a tyre can support. Yours have a load index of 116, which is 1250kg per tyre, and is sufficient for your (non-overloaded) maximum axle-load values (2 x 1250 being greater than the max 2400 rear axle load). The values are in a table on page 8 of the databook. Link to comment Share on other sites More sharing options...
Sophie2 Posted March 7, 2016 Author Share Posted March 7, 2016 Robinhood - 2016-03-07 2:25 PM To restate, get the 'van weighed at it's absolute heaviest (full tanks, all people, all kit) before adjusting the pressures accurately. Thanks again...but "absolute heaviest" I have a problem with. About twice a year we have 2 small grandchildren with us. On a few other occasions we have 2 elec bikes in garage. We never have a full tank of water. One a year perhaps we have loads of food, no kids and 2 bikes!! In other words, very rarely will the vehicle be at its "absolute heaviest" (and if you mean the max permitted weight, then I would say "never" ) I'm trying not to be obtuse but I'm beginning to think that every time we go way and change the "variables" I'll have to go to a weighbridge! Wouldn't it be acceptable (and I thought the norm) to take some sort of "average" or even "median" and then adjust the pressure up or down accordingly?-()eg weight of water, fuel, bikes are fairly easy to determine. ) in the same way that a "tugger" may have one set of pressures when towing, and another set when solo?- On the basis of the weights I gave Continental they have told me that the front would be 42psi and the rear 60psi. With two bikes plus accessories at 40kg in the back then I'd increase the rear to say 64psi. AS regards weight transference to the on braking, Continental said "this only tends to be for short periods whereas overloading at the rear axle can be present for long periods when the tyres are rotating at high speed. This si the main reason why with CP Camper tyres the pressures at the rear are deliberately set higher." (Seems to differ from the Michelin stance?>) Certainly the next time we go away for a few days I'll go back to the weighbridge and then re-assess. Thanks again. Link to comment Share on other sites More sharing options...
Brian Kirby Posted March 7, 2016 Share Posted March 7, 2016 Robinhood - 2016-03-07 2:25 PM.......................The weighbridge values you're using don't appear to me to be fully-loaded values, and this would lead to lower than safe tyre pressures. The front value is below the table entries in the document, and could be potentially extrapolated to a lower value. However, when Michelin used to quote differently, they always added about 10% to the actual front load to allow for weight transfer under braking, and this generally brings the value into the table range. (I suspect that, in a similar way, Conti will probably quote somewhat higher front pressures than you might otherwise expect for a given weight). To restate, get the 'van weighed at it's absolute heaviest (full tanks, all people, all kit) before adjusting the pressures accurately. Trying to determine the required pressures without these values is futile. I would (my own preference) then add 10% to front and rear actual axle loads to allow for some flexibility, and determine the pressures from those values (either via the databook if you are confident in interpreting - or post here for confirmation of that interpretation, or via Conti-supplied values)......................... All agreed, and this cannot be overemphasised. The weight at which the van must be weighed is in the heaviest state in which you will use it, including driver and all passengers, all pets, books, clothing, food, full fuel, full water, full gas, etc. etc: in short, the lot! Hence, fully laden. Anything less, and you may as well just guess. Then, apply those loads to the Databook, using the pressure above the actual load for each axle. So, if your front axle load were 1,875kg, you'd use 3.5bar. If the actual load is at, or very close to, one of the weights cited in the Databook, select the next pressure up so, if the axle load were 1,940kg, I'd use 3.75 bar. Or, just do as Robin suggests and add 10% to the actual axle loads and select pressures around that - though this may over-egg the rear pressures somewhat. As you travel in your van you will consume some of its contents, and that can result in the load at one axle, usually the rear, increasing even if the overall van weight falls. You do need a margin to allow for this. The danger is under-inflated tyres, having them a bit too hard (subject to the maximum permissible for the tyre) only affects ride comfort, it is not dangerous. Regarding using pressures below 3.0 bar, I did query this with Conti a few years back, and they weren't then keen - on the basis that being a commercial vehicle tyre, they hadn't been assessed for such low loads. Link to comment Share on other sites More sharing options...
Robinhood Posted March 7, 2016 Share Posted March 7, 2016 ...I can't add much more to that. The calculation at maximum loaded state (not the MAM, the maximum weight that you are likely to carry) ensures that the pressures remain safe, (never being under-inflated) if not entirely optimal for use with lower loads. The addition of a margin gives a little more peace of mind re changes in load distriblution and/or the temptation to bring back a little more wine ;-) (and I'd agree that the 10% is probably high, but it dates back to when we travelled 4-up, and the loading and weight distribution was a little more unpredictable than with just the two of us). Whatever the merits, this method has always given me "recommended" pressures considerably lower than the manufacturer's default, and that reduction has invariably resulted in better ride and handling. Link to comment Share on other sites More sharing options...
Sophie2 Posted March 8, 2016 Author Share Posted March 8, 2016 Thank you both and I take the points you've raised. The "fully laden" weighbridge trip will have to wait until a (planned) long term trip in a few weeks but until then I'll make sure we're not under-inflated. Link to comment Share on other sites More sharing options...
George Collings Posted March 9, 2016 Share Posted March 9, 2016 Its the compensation culture that has driven so many manufacturers to decline giving perfectly reasonable advice over subjects such as LGV tyre pressures. On high performance cars where the tyres are closely matched to suit the vehicle and in some cases the vehicle is matched to a specific tyre, pressures are highly critical. On this class of vehicle deviation from specified pressures for the operating conditions may be unwise in the extreme. On commercial vehicle operating weights vary a great deal between laden and unladen but the hard unladen ride is tolerated for practical reasons. It seems to me that legal advice to tyre companies has led to blanket rules over tyre pressure advice so every case is treated as a worst case scenario. For tyres worst case is full or overload,, high ambient temperature and prolonged high speed. If its hot drive 10 mph slower to improve the margin of safety. Link to comment Share on other sites More sharing options...
silverback Posted March 9, 2016 Share Posted March 9, 2016 I just bat on with whats on the door pillar 5.5 bar or 80 psi and i have just changed all 4 tyres and they were still under inflated! cos the edges of the tyres were just legal and in the middle was well within legal limits, 20000 miles on conti's and yes the weight of the van is 100kg below the limit, so just bat on, but that is just my opinion, as above it says on my agillis 80 psi with i think 118Q rating so thats what i do, i don't think the ride is harsh at all, life is too short to faff about taking air out then putting it back in, if i could inflate higher i would! again just an opinion. Jonathan Link to comment Share on other sites More sharing options...
Brian Kirby Posted March 10, 2016 Share Posted March 10, 2016 Jonathan, we don't take air out and put it back, we just set the tyre pressures to those recommended by the tyre manufacturer for the maximum fully laden axle load, and then keep to those. Its no more demanding than keeping to tyres to 80psi. How's your rear axle load, BTW, in full travelling trim? I'm a bit surprised the edges of your rears were worn more than the centres. With fronts it's normal, but the rears should be about even side to side. Link to comment Share on other sites More sharing options...
silverback Posted March 10, 2016 Share Posted March 10, 2016 Brian Kirby - 2016-03-10 7:09 PM Jonathan, we don't take air out and put it back, we just set the tyre pressures to those recommended by the tyre manufacturer for the maximum fully laden axle load, and then keep to those. Its no more demanding than keeping to tyres to 80psi. How's your rear axle load, BTW, in full travelling trim? I'm a bit surprised the edges of your rears were worn more than the centres. With fronts it's normal, but the rears should be about even side to side. hi Brian, haven't got the figures with me but got weighed before coming down to Spain, overall and back axle and i was coming in at 50kg light on the back axle so ok, yes i got all 4 changed down here and on inspection all four were worn on the edges, i would say though not by much maybe 2mm more tread in the middle, the reason for the pressure change was because Sophie said that when they would put the bikes on the back they would increase the pressure so i am assuming that when they got back and not carrying the bikes they would decrease the pressure hope that clears it up, oh and yes the fronts were more worn than the back, but i did rotate them at 12000 mile cos the fronts were well wearing more than the back Jonathan Link to comment Share on other sites More sharing options...
George Collings Posted March 10, 2016 Share Posted March 10, 2016 Suggest a steering alignment check if you have not had a recent one. Springs settle and suspension bushes age apart from the odd contact with kerbs etc. Link to comment Share on other sites More sharing options...
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