Weights, tyres and pressures.

[Guest Brian Kirby]

This may be of interest as the new season approaches. It has been stated many times that before setting off, motorhomes should be weighed to check that their overall, and individual axle, loads are within legal (i.e. plated) limits. Now, many owners find that their motorhomes have a hard ride. This is partially due to the commercial vehicle origins of the suspension systems, but also to the high tyre pressures (around 5BAR or 72psi) commonly set by base vehicle manufacturers and converters alike. This has frequently been debated in this forum, usually with the recommendation that, provided the fully laden vehicle has been weighed and the individual axle loads noted, and the tyre manufacturer consulted, lower pressures may be permissible. Having duly visited my local weighbridge and proved the ‘van legal, but finding its ride uncomfortably hard, I followed this route. Result: tyre pressures could be reduced from 5BAR all round, to 3.9BAR (56psi) front and 3.4BAR (50psi) rear, and a much improved ride. Bliss! However, that weighbridge visit gave no more that a snapshot of the load on the day, and you shift weight (mainly water) around as you travel along. OK, the ‘van gets lighter as you use the water, but quite a lot finds its way into the waste tank, and you can’t always empty that before you resume your road. Transferring water from fresh tank to waste tank, unless the two are close together, will changes axle loads and, possibly, the desirable tyre pressures. Problem is; you can’t keep re-visiting the weighbridge to see what you’ve done! Enter Mel Eastburn’s article of August 2004, p205, “It ain’t heavy, it’s my motorhome”, wherein there is a neat little formula, and the bones of a spreadsheet, from which one can calculate the affects on axle loading of various distributions of weight within the vehicle. In effect, it provides a mathematical model of the van load. It took a bit of work - and another trip to the weighbridge to weigh the ‘van empty - to get it all together, but the results were quite instructive. Our ‘van is a 6 metre, Fiat long wheelbase, fixed rear bed, front lounge, low profile, with well distributed storage, an inboard 130 litre water tank mounted amidships and a rear mounted 90 litre waste tank. It takes two 13Kg gas bottles within the wheel base, and has a 15 litre Thetford toilet tank just aft of the rear axle. Oh, and we go too! Normally laden for travel, with empty waste and toilet tanks and the fresh water tank full, the weighbridge ticket said front axle 1680Kg and the rear axle 1570Kg. However, if the waste tank were full (with a corresponding volume deducted from the fresh tank), the toilet full and three dozen bottles of wine were stashed in the under bed boot (possible on the way home!), the axle loads shift to 1630Kg front and 1720Kg rear. The correct tyre pressures for our ‘van should then be 3.7BAR (54psi) front and 3.8BAR (55psi) rear. Of course, it’s hardly practical to go around fiddling with the tyre pressures all day, but it does seem wise to cater for the worst case at each end, and adopt the higher rear pressure. I know 5psi isn’t proportionately that great, but without it, we’d be under inflated relative to our load: not all of the time, but definitely some of the time. It’s only through Mel’s spreadsheet that I’ve been able to check this: running off to the weighbridge to check all the possible variations would have been totally out of the question. And, yes, the calculations do tally with the weighbridge. Weighing all our stuff, and distributing it around the spreadsheet model of the ‘van, gives calculated laden axle loads of 1684Kg front and 1569Kg rear. Close enough?! So, belatedly, thank you Mel, a valuable tool; and to those who use reduced tyre pressures, do use it to check how your axle loads might vary as you travel. Be safe. Regards Brian

[Guest Mike Chapman]

Hello Brian, Great posting and let us hope that it is taken notice of. See posting "Tyres Again" for some more discussion. Regards, Mike Chapman.

[Guest PeteC]

As a VERY experienced van driver (My first ever vehcle was a van in 1959)and I have driven various vans ever since (appart form 3 Land Rovers when towing) Most problems stem from CAR drivers who want a soft ride If yuo drive a van with reduced pressure in the tyres to get a softer ride yooucan feel them roll on corners - you will also find that they do not wear correctly - OK reduce the pressure - IF YOU MUST - when unloaded BUT blow them up again when they are loaded

[Guest Brian Kirby]

Pete Perhaps I wasn't clear. The pressures I have used are those recommended by Michelin, not some I invented. According to Michelin, they are the correct pressures for the vehicle weights I quoted above. In that sense thay are not reduced pressures, they are the correct pressures. What the base vehicle manufacturer, and the converter, recommend are, according to Michelin, excessive pressures designed to ensure an overloaded motorhome doesn't suffer tyre failure. This stems from the knowledge that many motorhome owners simply don't check their wiights, they just assume if they fill the cupboards the weight must be OK. No one who reads MM would do that, of course, but we get the tyre pressures aimed at those who do. It does make sense to reduce the pressures from this excess, but the extent of reduction will depend on the weight of your 'van. Ours is plated at 3400Kg, so not heavy, and 5 BAR is way too high. If your tyres are too hard the roadholding and braking drop off as well, so geting it right is important. I think the tyre maker is much more likely to get it right: the others are covering their nether ends in case of someone's stupidity. Regards Brian