MOT weights

[plwsm2000]

Just had my van MOT'ed and the paperwork includes a sheet with the 4 separate wheel weights and brake efficiencies (it passed BTW). The MOT center specializes in HGVs etc. and I am not sure if this is now standard for all MOTs. The sheet states "Class IV M1 Sept 2010 onwards Dual Line System"

I have a couple of observations and would be interested in other people's opinions.

 

1) Although the front & rear axles were well within the limits on my VIN plate, there was nearly 100kg difference on the left and right hand front wheels (the right was heavier). The van was not fully loaded for a long trip but if it had been, I suspect I would be near to the tyre limit on the right front although still under the front axle limit. The right rear was only 25kg heavier than the left. The weights are measured with the driver inside the van.

 

100kg is quite a big difference and could affect the recommended tyre pressures. I am not suggesting different pressures in each tyre, but don't just assume that each wheel sees exactly half the axle weight.

 

2) The sheet also has a "presented weight" figure (just the sum of all 4 wheels) and measurements of the braking forces on each wheel for the foot brake and the hand brake (rear only). There is then a calculation of braking efficiencies measured against the "presented weight" and there is a pass/fail limit applied to this calculation. I assume that if your brakes are a bit on the low side it might fail this test if you present your van on the heavy side but pass if lighter. The weight of the MOT tester might also be a factor if you are close to the limits. Seems a bit odd to me.

[colin]

For the MOT I only saw a total vehicle weight, and this was done at a separate weight bridge on site, your MOT station must have fairly new equipment.

When it comes to tyre pressures, I have taken the loaded weight, then added a good margin to allow for offset loads or changes in load.

[Robinhood]

I have had my 'vans MOT'd at a local coach and bus firm for some years, and have always received detailed information (including weights) from the brake test during that time.

 

I've not received the same level of information for the cars, so I'm assuming it is the nature of the test station, and the willingness to share all the info that is at play.

 

If your 'van is RHD, could it not be the weight of the driver that is causing the imbalance? Given the geometry, almost all the weight of a driver will be taken by the nearest front wheel. If it is LHD, then it is possibly a bigger problem (and I'm assuming no-one in the passenger seat).

 

It's been obvious to me since getting the above braking figures at MOT that there may be some significant advantage in presenting the 'van for MOT in as light a state as is possible. Since the target braking performance is a %ge of the vehicle weight as presented, it should assist with meeting (exceeding) the requirements. Given the common issue of Ducato handbrake performance, it might be doubly important to some owners.

[Deneb]

I'm not sure what happens now as I'm no longer involved, but brake efficiency for cars and light commercial vehicles was always calculated using the brake force achieved and the approved operating weight for the make and model which had to be obtained from MOT approved reference data. With the advent of computer testing I assumed that the data previously published on reference charts was now incorporated into the MOT test software and automatically selected according to the make and model of the vehicle entered for the test.

 

Since larger goods vehicles have to be unladen when tested, the testing stations used equipment that applied a known reference weight to the vehicle during the brake test.

 

The issue with motorhomes is of course that although they are subject to class 4 testing, they often have to be tested at goods vehicle equipped testing stations due to the inability of testing stations aimed at cars and motorcycles to accommodate them, and if tested at an HGV facility it is not possible to load them with a reference weight by applying a force to the load bed.

 

Ordinarily, I wouldn't expect braking efficiency to be calculated on the unladen or partially laden weight of a vehicle, because as stated it could be possible for a vehicle with poor brakes to achieve the minimum required efficiency in those conditions, yet be unable to meet the required efficiency when at normal operating weight.

[Robinhood]

For a Class IV test (to which motorhomes are subject) the current (automated) practice is to use the actual weight as presented:

 

Calculating brake efficiency

 

For most vehicles the MOT testing service will calculate brake efficiencies automatically.

 

If MTS is not working, add the brake efforts from each wheel for the system that is being tested and carry out the following calculation:

 

% efficiency = total brake effort / vehicle test weight x 100

 

The vehicle test weight will depend on the vehicle test class.

 

For Classes 3 and 4 use the weight shown by the brake test equipment. Otherwise, take the weight from a weight data chart or some other reliable source.

 

Clases 5 and 7 use reference weights or MAM or similar.

[Deneb]

Robinhood - 2020-08-12 5:30 PM

 

For a Class IV test (to which motorhomes are subject) the current (automated) practice is to use the actual weight as presented:

 

 

Thanks, although I note there is a fallback to the original system, and that GVTS still use reference weights or MAM.

 

Otherwise, take the weight from a weight data chart or some other reliable source.

 

Clases 5 and 7 use reference weights or MAM or similar.

[plwsm2000]

Thanks for the comments.

It would make more sense if the brake calculation used the VIN plate values rather than "as presented". I personally hadn't given any thought to the weight of the van when I took it to the MOT centre and could easily have unloaded it if I know the brakes were suspect just to get it to pass. I am more concerned about the potential safety of it rather than just a tick in the right box.

That said, it is fairly easy to recalculate the efficiency yourself using the VIN plate details (or the typical weight of the van for travelling) and compare this to the pass/fail criteria for your own peace of mind.

 

[Tracker]

For me it always seemed right to get the van MOT'd empty of everything including water tanks and loo.

If nothing else at least it gave it all a clear out once a year and it can even make you smile when you find things that thought you had lost!

[DAZID1]

Motorhomes are a funny thing as if the same base vehicle was tested as a class 7 then, as you say it would be calculated against the plated weight 3,500 in a lot of cases. If unladen, the tester should then be putting that info into the system so it can then do a slightly different calculation, this is to allow for the load sensing valve having reduced the braking efficency to the rear, if both fronts lock, the rears have to achieve 100kg per wheel. Motorhomes like you said are tested as class 4, where historically the system gave the vehicle a test weight somewhere inbetween kerb and laden. That doesn't always work on coach built mpotorhomes and I have seen testers enter all sorts of weights into the system. The issue with the load sensing valve isn't normally an issue as we all know they are heavy sometimes quite close to the gross weight.

The ATL test is as also stated presented weight, ATL is basically one man testing where the brake tester weighs the vehicle.