because they rev the engine to it's maximum revs repeatedly.

Unless you drive like a complete helmet, your engine shouldn't ever get to it's maximum revs during normal driving.

If the engine is revving far faster than normal then it seems fairly logical to me at least that the belt will also be rotating faster than normal, and therefore flexing from straight to curved round pulley and back to straight again and then curved round pulley and so on much faster than normal, with a higher drag co-efficient from the components being driven by the belt. Due to the higher stresses imposed on the belt by all of these factors, it would be more likely to snap at this time.

probably.

Optimus Prime said:

because they rev the engine to it's maximum revs repeatedly.

Click to expand...

im sorry, but if my local MOT centre did that, i would be knocking them out!!!!!!! mine just gives a quick blip on the throttle, probably doesnt even reach 3000rpm

However, the TD4 has no cambelt - chain driven. A plus point at last.

The engine has to be held at 2500 rpm (or half it's maximum if lower) for 30 sec in order to complete the test.
Any engine that can't withstand that is knackered anyway.

the new diesel smoke testers only need one light blip of the throttle now, if it fails that test then a further warmup to get the oil to 80 degress and five blips, no more than 2500rpm, thats how our new smoke meter works

With the engine at the rev limiter if the cam belt fails then there is so much inertia in the flywheel that there has got to be a lot of damage to the engine. A piston hits a valve and is brought to a rapid halt. The bottom end is going to take a hell of a shock force. Connecting rods coming through the side of the block and shattered bearing shells are possible results, let alone the piston heads, valves and valve guides. It'll be off to the big engine place in the sky.

ste220 said:

im sorry, but if my local MOT centre did that, i would be knocking them out!!!!!!! mine just gives a quick blip on the throttle, probably doesnt even reach 3000rpm

Click to expand...

andyb81 said:

the new diesel smoke testers only need one light blip of the throttle now, if it fails that test then a further warmup to get the oil to 80 degress and five blips, no more than 2500rpm, thats how our new smoke meter works

Click to expand...

Overhere the same, one gentle push of the pedal and that's it, not the close to max. revving up anymore as they used to do in the past.

maybe the information I have is out of date:

Metered Smoke Test (Diesel)
The metered smoke test has been conducted in
EU countries for many years. During this test, a
calibrated smoke meter is used to assess the
density of the smoke from compression ignition
(diesel) engined vehicles.
The engine will be accelerated up to governed
speed and the smoke density measured.
Engines that emit very little smoke and achieve
a meter reading of 1.5m-1 or less will pass the
test after the first acceleration. However, if the
test is not passed on the first acceleration a
further two accelerations will be carried out.
The average of the three acceleration readings
will be calculated and if the reading is at or
below 2.5m-1 for non-turbocharged engines
or 3.0m-1 for turbocharged engines, the
vehicle will pass this element of the test.
However, if the average is higher, a further
acceleration will be carried out, and the average
of the last three readings will be calculated. This
will continue up to a maximum of six
accelerations. If the average of the fourth, fifth
and sixth accelerations is higher than the
appropriate level, the vehicle will not pass the
test.
In addition, vehicles may be refused a
certificate if the exhaust emits excessive smoke
or vapour, to an extent likely to obscure vision.
A metered smoke test will not be carried out if:
The exhaust tail pipe is damaged or an
accessory is fitted which prevents
insertion of the smoke meter probe.
The exhaust system is extensively
fractured or holed.
There are obvious signs of an engine
defect such as an unusual noise or
emission of excessive smoke.
There is an insufficient or excessive
amount of oil in the engine or, low
engine oil pressure, which could cause
engine damage if the engine is
accelerated.
There are obvious signs that the
governors have been tampered with or
are not operating.
The vehicle owner / presenter cannot
confirm that the vehicle has been
properly maintained, the camshaft drive
belt has been replaced at the
recommended intervals and the engine
is in a suitable condition for testing.
In the above circumstances where a metered
smoke test is not carried out, the vehicle will fail
the test.
Note: It is important that vehicles are
properly maintained prior to testing
(including changing of the camshaft drive
belts in accordance with the manufacturer’s
recommendation) and presented for test at
normal working temperature. This may
require the engine to be kept running prior
to the start of the test.

its all changed optimus prime, had numerous notices about diesel smoke testing from vosa since last year, new legislation on the testing equipment and everything, but the smoke limits are the same! weve had to spend 4 grand on a new tester cuz it didnt meet the new specs, now u have to get the oil temp to 60 degrees and one light acceleration up to 2500rpm, if its in limits then its classed as a fast pass, if not then warmup to 80 degrees and five accelerations and an average is taken! theres so much bullsh*t that vosa keep proposing now, it make my job harder than ever

unless the vehicle was first used prior to 1st January 1979....

Treworgey 90 said:

unless the vehicle was first used prior to 1st January 1979....

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yep, then its just a visual test for dense blue or black smoke, dont get many pre 79 motors in at the mo though, all modern ****e wi too much electrics!

happy to be corrected andyb81.

Never wait with mine while it's mot'd so don't see it done. Usually drop mine off at the garage, then walk home to get the wife's car, which irritatingly is due at the same time. Then go back in her car to get the (usually bad news) on mine. Drive home in mine while they're doing hers, then walk back to get hers. Seems arduous, but works fine, nice pleasant stroll there and back by the river / creek, usually weighing up the pro's and cons of changing the damn thing !

never seem to be able to get rid though !

ive been threatening to burn/cut-up/sell/dump my series 3 recently but at the mo shes running fine (famous last words) bought a volvo for everyday use so the old gal can have a rest! ill have to post up some of the special notices vosa send, might help a few ppl out cuz something seems to change in the testers manual every month

OPTIMUS PRIME is right they use a decelerometer and road test it, for the foot brake and hand brake.

ianda12 said:

OPTIMUS PRIME is right they use a decelerometer and road test it, for the foot brake and hand brake.

Click to expand...

only if they've got a very old brake tester, which doesn't contra-rotate, or they're thick (more likely).

So if they put the back wheels on the rollers, how do they stop the drive trying to spin the front wheels ?

it has a differential.

Techno explination for those 'not in the know'.

Take the wheels on an axle of a 4x4, front or back, doesn't matter. The power to these pair of wheels comes from a differential gearbox that allows these two wheels to rotate at different speeds, say when going round a corner, but also allows the engine power to drive both of the wheels. Clever stuff!!.

With a standard differential gearbox, if one wheel is spun forward while the other is spun backwards at the same speed then the 'input' shaft to the differential does not turn. That is what a contra-rotation brake tester does. On a 4x4 this means that the 'other' axle can not receive drive from the axle under test.

On a Defender style Landrover, proving it is out of gear (and please don't fry me if this is wrong), the contra-rotation test is not necessary because this style of car has a different method of transfering drive to the two axles. The Freelander transfer system makes contra-rotation testing very necessary as the car would drive itself off the test rig if both wheels on a given axle were driven in the same direction, regardless of the set up of the controls.

Thus ends the lesson.

Having said all that. Take said 4x4 and put it on the contra-rotation tester. Hit the brake pedal. BUT one brake is duff. So one wheel slows but the other doesn't. We have drive to the other axle. Wayhey..........
:rolly:

Or doesn't the machine work that way?

The MoT testers I have spoken to all seem to have tales of 4x4s not behaving as they should, and road test them to play safe.