It’s All About Weight!
Weights, payloads and legal criteria.
Chrissy is a Chausson 738 xlb (xlb meaning extra large bed!). There is a Fiat chassis and a remapped 130 bhp engine with front wheel drive. As a result, this gives enough power to cope with hills and hairpin bends. Suspension is a single rear axle, leaf springs and a legal weight limit of 3500kg with a motorhome payload of 350kg. (See later for a motorhome payload calculator.) This means you can drive with an ordinary car licence.
As the driver, in order to keep to weights, payloads and legal criteria, you must to ensure all payloads and axle weights adhere to legal limits. Attention must be paid to balancing the weight over the axles, we will explain this fully at the handover.
This article was taken from CARAVANGUARD , read it carefully, it is an excellent explanation of weights and loads.
Motorhomes are limited to the weight which they can carry and how the weight should be distributed throughout the vehicle.
It is extremely important to stick to these limits as overloading can have a detrimental effect on handling. The motorhome weight affects performance, stopping distances, and overall safety and stability. These are all factors which could lead to an accident.
This motorhome payload calculator explains the jargon surrounding motorhome weight limits, so you can calculate the legal weights for your motorhome. This includes calculations for adding things such as a rear bike rack.
First of all, let’s tackle the different weight limits which need to be considered and what each of them mean.
Mass in Running Order (MIRO)
The MIRO is the weight of your motorhome as it left the factory. This is with a full tank of fuel and an average driver weight of 75kg, but before any other contents are added.
Maximum Technically Permissable Laden Mass (MTPLM)
The MTPLM is the maximum amount which your fully laden motorhome can weigh and still be legal to drive.
(The MTPLM is the MIRO plus the Payload).
Payload
The payload is the weight of passengers, equipment and belongings (gas bottles, clothing, food, solar panels, batteries, bike racks) carried on your motorhome.
Your maximum payload is usually given by your motorhome manufacturer.
Calculate your actual MTPLM by going to a weigh bridge (there is normally a small charge). Going over the MTPLM figure means you need to reduce your payload by removing items from the motorhome.
Calculating the effect of adding weight behind the rear axle
Many motorhome owners like to add a bike rack. Before you adding, you must carefully calculate the effect this will have on your motorhome’s front and rear axle loading. These both have their own specific limits (which can be found on your motorhome’s weight plate.) The diagram below shows an Alko chassis weight plate for an Auto-Trail motorhome.
The Pivot Point and Centre of Gravity
When adding weight behind the rear axle the back tyre becomes a pivot point. This means any added weight will increase the load on the rear axle but actually decrease it on the front one.
The diagram below shows the different measurements you need to be able to accurately calculate the effect of adding a new rear load.
Your motorhome’s size measurements should be in your handbook, and your front and rear axle load can of course be calculated at a weigh bridge by simply placing only the front or rear tyres on the sensor area (some weigh bridges may have equipment which can calculate front and rear axle loading independently automatically).
Formulas
Once you know the weight of the rear load you are adding (e.g. the weight of the bike rack plus a scooter) you can then use the below calculations to work out the effect of this on your front and rear axle loading:
New front axle load (new F) = F – [L x (O ÷ W)]
New rear axle load (new R) = R + L + (F – new F)
The diagram below gives some example measurements.
In the above example we are adding a new load (L) of 200kg to the rear overhang of the motorhome.
To work out the effect this will have on the motorhome’s front and rear axle loads we can use this formula, simply replacing the letters with the values in the example diagram above.
The new front axle load (which we’ll call “new F”) is:
1500 – [200 x (1000 ÷ 4400)] which equals 1454.54kg.
The new front axle load (once the new overhang weight is added) works out at 1454.54kg and can then use this figure to work out the new rear axle load (which we’ll call “new R”).
The new rear axle load (“new R”) is:
2000 + 200 + (F – new F) which equals 2245.46kg.
Thus, for this motorhome, adding a new overhang weight of 200kg (perhaps a motorbike rack) would add 245.46kg of load to the rear axle, whilst taking 45.46kg off the front axle load.
So if the maximum permissible rear axle load was 2400kg and the MTPLM was 4250kg, (see chassis plate photo) then adding 200kg to the rear overhang would still be well within the permissible limits.
Rear loading dangers
You need to be very careful not to overload the rear axle and overly reduce the front axle load. It’s not only unlawful but will alter the pivot point. This will cause your motorhome to tip at the rear, leading to potentially dangerous handling, steering and traction issues. We’d always advise calculating the load on both axles when your motorhome is fully loaded to make sure you’re within safe limits.
Always refer to your motorhome payload calculator when you change the contents or configuration of your motorhome.