Every motorhome comes with certain limits as to the weights which it can carry and how the weight should be distributed throughout the vehicle. Sticking to these limits is extremely important as overloading can have a detrimental effect on handling, performance, stopping distances, and overall safety and stability which could lead to an unecessary motorhome insurance claim.
This guide explains the jargon surrounding motorhome weight limits, and gives advice on how to calculate the weights which your motorhome can handle when adding things such as a rear scooter 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, 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).
The payload is the weight of passengers, equipment and belongings (gas bottles, clothing, food stuffs, solar panels, leisure batteries, bike racks etc) which you carry around with your motorhome.
Your maximum payload is usually given by your motorhome manufacturer.
With your motorhome fully loaded ready for your trip you can easily calculate your MTPLM by going to a weigh bridge (there is normally a small charge). If you’re over the MTPLM figure, you obviously 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 rear bike rack for pedal cycles, small motorbikes or scooters. But it’s important that if you are adding a rack then you carefully calculate the effect this will have on your motorhome’s front and rear axle loading, as these both have their own specific limits (which can be found on your motorhome’s weight plate usually found under the bonnet or inside the cab door). The diagram below shows an Alko chassis weight plate for an Auto-Trail motorhome.
When adding weight behind the rear axle the back tyre becomes a pivot point meaning 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).
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 the formula, and simply replace 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.
So we now know that 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.
In other words, 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, as per the chassis plate in the photo further up, 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 cause your motorhome to tip at the rear causing potentially dangerous handling, steering and traction issues. We’d always advise calculating the load on your front and rear axles when your motorhome is fully loaded to make sure you’re within safe limits.