With effect from Monday there are some changes being made to the class 5 MOT test in relation to brakes. The online manual will not get updated until the date of change.
Everything is on the most recent special notice http://www.dft.gov.uk/vosa/repository/SN%2003%202013.pdf

It basically bring the class 5 brake test more aligned to class 6 (PSV)

Wheel imbalance is now calculated on maximum effort rather than part/half pressure and now applies to both axles with a max 30% varience irrespective of locks.
A check for brake fluctuation which in other words is the ovality test as on a class 6.
Still no dispensation to test against ULW rather than MAM/GDW if at least one lock obtained on each section as on class 6
Introduction of secondary brake test using handbrake on single line/circuit vehicles

HTH

Mark

My bus number (if any): RM1414 sort of

Always a sting in the tail. The imbalance 'concession' for class V now stipulates "irrespective of locks". An important alteration that is different to Class 6 requirement which are detailed below, as is a description of "ovality". It can be seen that 'irrespective of locks' is an important concession.

From VOSA instructions for the use of roller brake testers:


7.4.4 Imbalance
For HGVs, PSVs and Trailers all wheels are tested individually, thus it is not possible to assess imbalance throughout the whole range of brake force. Imbalance is assessed only at the maximum brake effort achieved for each wheel on an axle. The following formula is used:

Imbalance (%) =Higher Brake Effort - Lower Brake Effort x 100 divided by
Higher Brake Effort

For the service brake only a failure shall be recorded if the imbalance is:
Greater than 30%

If both wheels on an axle lock, the imbalance criterion is not applicable.

If one wheel on an axle locks, the following rules shall be applied:

a) If the brake force from the locked wheel is less than the non-locked wheel, the imbalance criteria defined above is not applicable as the locked wheel is deemed to be capable of a greater brake force.

b) If the brake force from the locked wheel is greater than the non-locked wheel, the imbalance criteria defined above shall be applied, as the non-locked wheel may be defective.

7.4.5 Ovality
Ovality is measured only on front steered axles.

In the CCS for HGVs and PSVs, the trigger for measuring ovality comes in at either 65% of the FWA or after 4 seconds of brake application, the latter is to deal with brakes that do not get to 65% of the FWA. The following formula is used:

Ovality (%) =

Max Brake Force - Min Brake Force x 100 divided by
Max Brake Force

A failure shall be recorded if, when a wheel is rotated with a steady brake pressure applied, the braking effort varies by greater than 70%.

Be interesting to see how much the failure rate goes up by

It should reduce failures on front brake imbalance IMHO

Don'T see why they just couldn't fully align it at all, after all all preserved buses will have been operating buses in their life so why do they have to have a more complicated and demanding brake test in preservation??

Mark

My bus number (if any): RM1414 sort of

For a "standard" RM undergoing a Class 5 MOT brake test, what "I/P Weight" are MOT testers basing their brake efficiency calculations on? I would guess most would use the "wheel weights" measured by the RBT plus the usual calculation of 63.5kg x number of passengers seats...so an RM should be required to achieve 45% of something like 12,000 tonnes? Or a minimum of three locks for a guaranteed pass. Or do they use the ULW marked on the side of the vehicle which, in my experience, always seems to be significantly less than the vehicle weight as measured by the RBT? Or to they use the ULW plus the additional calculated four tonnes or so for passenger load?

For those who Class 6 their RM's, what are the correct DTP brake codes that the testers are inputting into the RBT's prior to the start of the test? At a guess 122202 is the correct code for a standard RM?

Did any refurbished RM's/RML's ever end up with "plated" Gross Vehicle Weights marked on them anywhere?

I'm conducting some research at the moment into inconsistencies in the way the published MOT "rules" are being interpreted and, as the number of roadworthy RM's clearly far exceeds the number of any other type of preserved bus and coach, it seems sensible to use routemasters as examples.

Is there anyone out there who has failed to get an RM through a Class 5 (could not achieve the imbalance standard)and so resorted to a Class 6 test solely because of the significantly easier brake test?

Any thoughts much appreciated folks!

My bus number (if any): Nothing, as yet, with RM as a prefix!

The RBT is not designed to be a weighbridge, but it is however supposed to be accurate to within 3%... Having recently had some plainly ridiculous weights: 9626kg for an RML and 9313kg for an RM, I have resorted to a diversion via a public weighbridge on the way to test. RM 7710kg 7t 11cwt 3 qr 2lb which is in line with what I would expect an RM to be with one passenger and some tools. I had previously accepted the RBT weight and based all calculations on those figures, but these most recent were obviously incorrect.

The most frequent error on the print out is usually that of classifying the vehicle as Post 68, which only applies to about 10 RMLs.

Failure to meet the pre 23/6/13 imbalance requirement can often be down to the driver's input. There is a knack to pressing the pedal if you want it to pass. However the vehicles do struggle, sometimes needing a couple of extra goes before the RBT passes it. We shall see on Tuesday how the new procedure goes.

As for efficiency I like to see 45% minimum at a more realistic 82.5 kg passenger weight. I also expect to meet the secondary brake requirement at this weight regardless of class V or 6.

I spoke to Bristol technical some years ago who advised that the Design "GVW" is assumed by them to be 13 tons 5 cwt.

I've found that providing the brakes are correctly adjusted and the accumulators are properly charged, I've not had problems getting RMs through the test, but then most of mine have been class 6. Remember the last 1/2" of travel on the brake pedal gives full emergency pressure and "should" lock out all round.

I had one the other week that locked o/s/r but n/s/f would not lock, but gave a much higher reading! I usually visit the local RBT facility before test to make sure they're all adjusted properly. I done that with RML2532 and found the results consistent with the class 6 test data, give or take a few %. 68% service, 38% handbrake! All locked. Imbalance on handbrake was 6%.

Most interesting Roy...with locks on a Class 6 you will have got 68% of the unladen weight, as measured by the RBT (I think!). Putting the issue of "locks" to one side for a minute I think this barely equates to a pass if you were trying to achieve 45% of the GLW as you do on a Class 5 for a pre 68 vehicle.

What does the sum of your brake readings equate to as a percentage of the notional Class 5 test weight of the vehicle (ULW plus 63.5kg x number of seats?).

The point I am trying to prove (to myself initially) is that it is perfectly possible to have a vehicle with fully functional brakes produce rolling road figures (commonly with only one or two locks out of four, three being an automatic pass) that only just meets (or just fails to meet) the Class 5 requirements if the GLW figure is correctly calculated, or, on more modern vehicles, the plated weight is used.

Thank you very much indeed for your assistance!

My bus number (if any): Nothing, as yet, with RM as a prefix!

I'm not all that well up on class 5 brake testing as I rarely do them, they're a pain in the backside. The last couple of class 5's I've done have locked all round and passed.

Other roy.

In theory RM unladen to LT specification sheet 7512 kg. 64x 63.5= 4064 so presented weight = 11576kg which requires 1310 kg (=5240kg total)per wheel to reach 45%

RML theoretical weight 7874kg 72x 63.5= 4572 so presented weight = 12446kg which requires 1410kg per wheel(=5640kg total) to reach 45%

1580kg per wheel for the last few RMLs to reach 50%

But this just isn't going to work in practice because the RBT isn't recording those unladen weights. Nor of course is 63.5 kg realistic for those who actually carry a full complement of passengers.

The last RM I took on Thursday locked out all round with a total of 7542kg minimum reading 1715kg.

Great stuff both Roy's...really useful! Thank you very much!

Just one last question...if you tested an RM and, say, the NSF locked at 1500...and the day after on the same rolling road you tested another RM where the NSF locked at 1700, which bus would you say had the more efficient NSF brake?

My bus number (if any): Nothing, as yet, with RM as a prefix!

It would not be possible to say. Why worry, as long as it stops!

Remember too the EU has recently amended the allowable passenger weight to something like 65kg to allow for people's eating habits changing over recent years. they're also talking of increasing the permissible axle weights for that reason and to allow for the increase in baggage that people now carry.

Which makes me wonder if this would affect the tyre ratings! Years ago, I COIF'd an RT for LT Museum. The inspecting officer at Mitcham was adamant it would not leave the place with 900x20 tyres as it was over his calculated weight on the front axle for those tyres, despite the fact there were over 4,700 of them like it! A hasty phone call to Roger Wright to get the number of his COIF'd RT meant the inspecting officer had to issue the COIF, on the bus with 900 x 20 tyres!

I don't place too much reliance (no pun intended) upon two separate RBT readings either of weight or effort, for anything other than a guide.Having had a trailer fail last year which had neither been unloaded nor been used in the week between a pre-test RBT which it passed easily and its MOT, I spoke to a few colleagues. This is a not uncommon experience, one reported taking a vehicle for RBT in the morning before lunchbreak, parking it 20 yards away in the DP parking area and testing it in the afternoon; when it failed. It is also noticeable that if the vehicle is removed from the rollers for another attempt, then the recorded weight will be different and occasionally quite significantly. As the VOSA leaflet says never rely on locks.

Yesterday's test was a Cummins RML weighbridge recorded 8250 kg RBT 9802kg.
Tester very interested since this was the 1st class V since the change in procedure. The RBT software update has not yet been received from VOSA so the test was identical, but the figures are then entered into the system which works out the imbalance from the maximum force figures. This proved to be exactly the same 15% as had been recorded by the RBT for a progressive/held 1/2 pressure application.

n/s/f 1699 L
o/s/f 1982 L
n/s/r 1799 L
o/s/r 2077

Handbrake
n/s 1545
o/s 1785

front imbalance 15%
rear imbalance 14%

so allowing an extra 100kg for a 2 man crew we have:
ULW 8350
82.5 x 72 5940
Total 14290


Calculated results including front/rear split not needed for class V:

max effort 7567 =53%
Front secondary 3691 =26%
Rear secondary 3876 =27%
Handbrake 3130 =22%

Since all the vehicles I work on end up carrying a potential full load I do not base anything on 10 stone men.