cav551:
Thank you for that technical. I have just taken it (twice) for RBT which was the reason for the FWA query. 1st time it went with the trailer which was due at the same time. I have difficulty getting enough weight on anyway because all the units are down plated,( 4x2s 28t & 31t, 6x2 34t) so the trailers go only partially loaded to stay within their gross and with the load spread out so as not to overload the (4x2) units drive axle, although with enough room at the back for a couple of big blocks through the back doors so that the axle weights come up from about 4t to 9t.With the 6x2 it went with a short pin 30ft trailer, slider as far forward as the legs would allow and the heaviest achievable weight up front. Even raising the trailer suspension only put 3t on the midlift so its readings were a pathetic 45kg. It got 38% with axle 1 & 3 all locking, so passed service with more than half of the system locking, passed front secondary FWA and failed rear secondary insufficient weight on the drive … 7.75t with the trailer suspension still up.
Second time around using the ATF’s test trailer axle 2 was just under 1000kg each wheel with 5.5t on the midlift and 10t on the drive, so although OK I am not impressed.
It is most noticeable how much more rear overhang the 2 Ivecos have in comparison to the DAF which makes things a little awkward with clearance for the run up ramps when backing down the steep slope at one of the loading bays at a collection.
Edit: correction axke 1 &3 locking
Try sliding the fifth wheel all the way back, that’s what we do, it needs the drive to be overloaded on it’s airbags it should then adjust the airbag pressures to balance the weight across the axles. You just have to find an ATF with a heavy trailer or take your own with a real load of weight up to the headboard.
To be honest the real problem we’ve had is there seems to have been a spate of older tester retirements and they’ve been replaced with new younger guys who just aren’t experienced with what means what. Previously we’ve had them just cross through the midlift results and pass it because they know the reason why on a Stralis.
You will only ever scrape around 1000kgf on the midlift tops on a roller brake test and it won’t lock even with new chambers, calipers and everything. As has been mentioned it needs the downward force from dynamic braking really,
Edit to add: We did get on to DVSA technical last year when a unit would get consistent locked results on our RBT and we took the exact same unit and trailer to four different ATFs before it passed getting wildly varying results. They insisted we must have done work on the brakes and altered things but it all remained totally untouched throughout.