Roller brake tester fails

as dvsa says, look it up ont web, a reliable source? yeah right.

Why would you need the weight for a decel test when the results are calculated as a percentage of G (Gravitational Force) ?

duhh,cos its worked on gvw,that and load ratings are the primary differences from a 4.if you had lets say an empty 3500 and it passed at 16% decel its got about 3/5 of the required effort,service brake no probs but you cant work out the handbrake effort without knowing the presented weight now can you?if your gear weighs em you could work the maths backward so if it weighed lets say 1750 and you got 32% that would be about right,similar reason they warned us about camper conversions etc,ulw is no use cos it aint what it weighs,you could pass an unladen 7 and when he loads his tools back in it rolls off down the road with him chasing it

unknown weight is a decel,thats SERVICE brake,handbrake effort is based on gross weight for 7 and its a KNOWN weight,campers etc are 4,different rules and you cant test a 7 handbrake with a decel unless you know its presented weight to do the relevant maths.you could be passing a motor with 1/2 the required hand effort.dont do it son.....

you would need to know the presented weight and work back from there using 3500kg as a division,lets say 1750kg,32% would equal 3500 @16%,you get the idea,if your gear don't weigh then youre sorta stuffed.

on a 4 yes,not on a 7,thats worked out on fully loaded weight for the handbrake and you could only assess that accurately on a decal if it weighs 3500 dead when you test it,unknown test weight is for service brake on 7s,class 4 is presented weight so decal to your hearts content,its always been a bit of a gap in the manual and the ministry reply is errrrrr?(yes ive tried)think there used to be summat like assuming a nominal weight of 2600 kg but no info on how you were supposed to confirm it and I think its gone now anyway.lets take his above example cos that's pretty simple and maths aint my strong point,he has 24% on a 3500kg motor,thats 8% more than it needs yes?1/3 yes?i make a third of 3500 as 1166 recurring,lets say 1167.that makes 2/3 of 3500 2334 I think.so if the motor weighs 2334kg or more that equates to your 16% or higher yes?if on the other hand it weighs less then itll be below 16%,dont really matter cos it generally only comes into play when rollers die and even then only 48hrs.as an aside mystic robs prediction would be that its an unloaded panel van tranny lwb high roof,based on percentages tho this aint nailed on by any stretch of the imagination.yeah its all a bit rough and ready but itll get you close.

I think where the confusion is becoming clear here is in the fact that converted vehicles like camper vans have had the original test weights used when the van has been converted, DVSA gave a example of why using the wrong test weight would equate to the wrong pass/fail test result, they advised that the weight of the converted vehicle had increased so using the original BTW would give the incorrect test results. If your rollers have weighing facilities you will not see this problem, but, if you start converting these BTW figures for decel tests you will make a complete 'hash' of the results as I said 'Gravity' is always there as a constant on the road. With converted vehicles we were always advised to gain a weigh bridge ticket and use that weight when roller brake testing, if a decel test was not practical, this might be due to the size and configuration of the presented vehicle.

Thanks guys. I have relayed the info onto the relevant people. As for the company I work for not sorting it out as soon as the RBT failed, the RBT itself has been falling apart for years and no-one wants to spend money fixing it. Maybe now we can't do any tests, it might make the company dig deep or face having to turn tests away.

Sorry for quoting this Rob, but I need to read it as I ask questions about it!

1 / I don't understand what you mean when you say; that was the problem with decal test on 7s,an unloaded one will almost always give a higher hand reading cos its only stopping 2/3 of the mass/weight that your testing it to.

How can the weight change?

If the vehicle is presented unladen and you do a decel test, the weight is as presented, you don't add or subtract anything from the vehicle, it has a design gross weight of 3500 kg. If you test it fully laden then more energy will be required to decelerate the vehicle to gain the said percentage to pass it, but if the said vehicle is presented unladen the brakes will require less energy to decelerate the vehicle to the said percentage to pass it.

Correct me if I am wrong, but art you only supposed to use the GVW when the BTW is not available!

I have a class 7 transit, which is gross 3500 Kg, but I'm sure the BTW is somewhere around 1800 Kg, obviously there will be a significant difference with the brake test results depending upon which weight is chosen, but I was reasonably sure DVSA said at some distant point in past time that only the GVW should be used if and when the brake test weight is not available!

I'm not too sure what you mean when you say "only stopping 2/3 of the mass/weight that you are testing it to"

I think this is where the confusion is being generated, it seems you are deciding whether the vehicle is laden or not and then adjusting the brake test weight to suit?

I think the test should be carried out as presented, if the vehicle is fully laden and requires a decel test, then just decel it as is and take the measured results to indicate pass and fail, don't add or subtract 2/3 rds of any weight figures, its already accounted for, and if the said vehicle is a roller test result, if the said brake test weight is available, test using that, but if not use the DGW 3500 Kg. If the wheels prematurely lock in the rollers, the said brakes have passed anyway, and if you believe a defect may be present because of the wheel locking, I'd say that would be a P&A.