Goneoff Posted August 2, 2019 Share Posted August 2, 2019 We recently purchased a Laika Kreos 4009 on a Fiat Maxi chassis with air-ride, GVW 4500kg and yes we like it but can’t understand why we have to travel with only 20% of fresh water in the tank. Historically we have owned 2 Rapido’s and 1 Hymer and could travel with any amount of water in the fresh water tanks without restrictions. I’m assuming it’s something to do with stability but why just Laika or are there other motorhome brands applying restrictions on the amount of water you can travel with onboard. Does anyone know the science behind this restriction and does the fact we have air-ride and an additional 1000kg change anything? Link to comment Share on other sites More sharing options...
weldted Posted August 2, 2019 Share Posted August 2, 2019 These tanks are made for more than one model, as they may have a lower payload. Hence the 20% for traveling. Link to comment Share on other sites More sharing options...
Clive_Adams Posted August 2, 2019 Share Posted August 2, 2019 I have a similar option on my tank, however, don’t think it’s compulsory that I have to use it while I am travelling, it’s there for the convenience of being able to control your payload better and of course added consumption. Link to comment Share on other sites More sharing options...
Keithl Posted August 2, 2019 Share Posted August 2, 2019 Phil, It is normally done to keep a MH's weight under the plated limit. As you have a 4,500 kg max this hopefully will not apply to you. The way to find out is fully load the MH ready for travel then either fill the tank to 20% or 100% and get the MH weighed. From the weight obtained you should easily be able see or to calculate whether you are within your limits. For info one litre of water weighs one kilogram so simple to calculate the difference from 20% to 100% assuming you know the tanks volume. Keith. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted August 2, 2019 Share Posted August 2, 2019 Phil When I began motorcaravanning in 1998 it was the norm for UK motorhome manufacturers to state a MIRO (Mass in Running Order) figure that included no allowance for carrying fresh water. Subsequently a standard formula was used where the MIRO needed to include an allowance of 90% of the motorhome’s fresh water tank’s capacity. More recently (though some years ago) this was changed so that the fresh water allowance element of the MIRO can be zero or any other figure the motorhome manufacturer chooses, and the norm now is for a motorhome’s fresh water tank’s capacity to be stated as two figures - one for ‘when staiionary’ and another for ‘when travelling’. For example, Rapido defines my 2015 640F model’s tank capacity as 120/20, indicating that the tank is able to hold 120 litres of water, but the MIRO is based on a 20 litres allowance. The Rapido brochure carries a footnote “Fresh water tank can be filled to 120 litres, which results in a lower payload”. It’s commonplace now for a motorhome’s fresh water tank to have two drain valves (though my Rapido has just one) with one valve draining the tank to the ‘while travelling’ figure and the other valve draining the tank completely. This arrangement is mentioned in this 2017 MHFun discussion. https://www.motorhomefun.co.uk/forum/threads/fresh-water-tank-capacity-limited-to-20-ltr.148324/ A litre of water weighs 1kg, so there’s a 100kg difference between 20 litres of water and 120 litres and (in my Rapido’s case) that translates to a 100kg reduction in potential payload if I chose to completely fill the fresh water tank before driving the motorhome, rather than travel with just 20 litres in the tank. But, as my usage of the Rapido is such that I have plenty of user payload left when the fresh water tank is full, I always fill the tank to its maximum capacity. Obviously, if doing this meant that the Rapido’s weight maxima (overall or axle loadings) were exceeded, to drive the motorhome legally I’d accordingly need to reduce the amount of water I put into the tank. There may be persuasive technical reasons for not driving a motorhome with a lot of water onboard. For instance, if the vehicle has a long rear overhang and the fresh water tank is large-capacity and located right at the rear, the more weight of water in the tank the more effect this will have on the motorhome’s handling. And every ‘unnecessary’ kilogramme a motorhome carries will impact on its acceleration, braking and fuel consumption. But (as Clive opines) there is no legal compulsion demanding that a motorcaravanner drive their motorhome with a restricted quantity of water onboard - the legal imperative is that the effect of putting water in the tank does not result in the motorhome becoming overloaded. Link to comment Share on other sites More sharing options...
Goneoff Posted August 2, 2019 Author Share Posted August 2, 2019 Derek Uzzell - 2019-08-02 7:30 AM Phil When I began motorcaravanning in 1998 it was the norm for UK motorhome manufacturers to state a MIRO (Mass in Running Order) figure that included no allowance for carrying fresh water. Subsequently a standard formula was used where the MIRO needed to include an allowance of 90% of the motorhome’s fresh water tank’s capacity. More recently (though some years ago) this was changed so that the fresh water allowance element of the MIRO can be zero or any other figure the motorhome manufacturer chooses, and the norm now is for a motorhome’s fresh water tank’s capacity to be stated as two figures - one for ‘when staiionary’ and another for ‘when travelling’. For example, Rapido defines my 2015 640F model’s tank capacity as 120/20, indicating that the tank is able to hold 120 litres of water, but the MIRO is based on a 20 litres allowance. The Rapido brochure carries a footnote “Fresh water tank can be filled to 120 litres, which results in a lower payload”. It’s commonplace now for a motorhome’s fresh water tank to have two drain valves (though my Rapido has just one) with one valve draining the tank to the ‘while travelling’ figure and the other valve draining the tank completely. This arrangement is mentioned in this 2017 MHFun discussion. https://www.motorhomefun.co.uk/forum/threads/fresh-water-tank-capacity-limited-to-20-ltr.148324/ A litre of water weighs 1kg, so there’s a 100kg difference between 20 litres of water and 120 litres and (in my Rapido’s case) that translates to a 100kg reduction in potential payload if I chose to completely fill the fresh water tank before driving the motorhome, rather than travel with just 20 litres in the tank. But, as my usage of the Rapido is such that I have plenty of user payload left when the fresh water tank is full, I always fill the tank to its maximum capacity. Obviously, if doing this meant that the Rapido’s weight maxima (overall or axle loadings) were exceeded, to drive the motorhome legally I’d accordingly need to reduce the amount of water I put into the tank. There may be persuasive technical reasons for not driving a motorhome with a lot of water onboard. For instance, if the vehicle has a long rear overhang and the fresh water tank is large-capacity and located right at the rear, the more weight of water in the tank the more effect this will have on the motorhome’s handling. And every ‘unnecessary’ kilogramme a motorhome carries will impact on its acceleration, braking and fuel consumption. But (as Clive opines) there is no legal compulsion demanding that a motorcaravanner drive their motorhome with a restricted quantity of water onboard - the legal imperative is that the effect of putting water in the tank does not result in the motorhome becoming overloaded. Derek I have to say thank you for your comprehensive reply it is appreciated; you are a top man and a true stalwart of this site. I remember you taking the time helping me out of a difficult situation we had with our Truma boiler when we were in the South of France. Many thanks Link to comment Share on other sites More sharing options...
Goneoff Posted August 2, 2019 Author Share Posted August 2, 2019 Thank you everyone for all your replies Link to comment Share on other sites More sharing options...
EJB Posted August 2, 2019 Share Posted August 2, 2019 It has been customary for manufacturers of MH/Caravan toilets to dictate that toilet holding tanks should be empty when travelling. I'm sure that most travelled with them full...we certainly did. Whether that was for weight, safety or technical reasons I don't know! As suggested the current fresh water tank dictate is to allow for payload when travelling, again I am sure that most ignore it. Link to comment Share on other sites More sharing options...
Don636 Posted August 2, 2019 Share Posted August 2, 2019 I’m sure the instructions for our Thetford toilet cassette states that you should not travel with the holding tank more than half full and I had assumed that this was to prevent damage to the fairly delicate internal mechanisms. Seems unreasonable to state that it should be empty as this would mean you could not use the toilet whilst travelling. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted August 2, 2019 Share Posted August 2, 2019 Thetford’s advice has been to travel with the toilet’s FLUSH-WATER reservoir empty (to avoid spillage/damage) and does not relate to the contents of the cassette. It’s referred to here https://www.practicalmotorhome.com/forum/tech/860-thetford-c-402c-toilet (If the toilet’s flush-water is supplied from the motorhome’s main fresh-water tank, that advice will not be relevant.) The cassette has a feature that shuts off the cassette’s interior if it’s so full that the waste might be be forced out through the air-pressure equalisation valve during driving. Link to comment Share on other sites More sharing options...
Don636 Posted August 2, 2019 Share Posted August 2, 2019 Just checked the user instructions for my cassette and it states: “Please avoid to travel with a waste-holding tank that is more than 3/4 full. This may cause leakage through the venting system.” I was wrong about half full and Derek’s explanation ties in with the above so now I know. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted August 2, 2019 Share Posted August 2, 2019 Thetford’s warnings about the flush-water tank and the cassette can be read on this link https://www.jacksonsleisure.com/content/pdf/thetford/c502_user_manual.pdf The potential problem with the cassette is that, if the cassette is full, this will close the pressure-relief valve. This will seal the cassette and a pressure increase occurring inside the cassette may harm the valve. Even if the valve is not damaged, anyone attempting to use the toilet with the pressure-relief valve stuck shut and high pressure within the cassette will have an exciting experience when the ‘slide’ in the toilet bowl is opened and the waste erupts upwards. Link to comment Share on other sites More sharing options...
EJB Posted August 2, 2019 Share Posted August 2, 2019 Thanks Derek I didn't explain the fresh water flushing tank supplying the toilet cassette quite clearly enough, Thanks ;-) Link to comment Share on other sites More sharing options...
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