Page 2 of 2

Re: MOT & brakes

PostPosted: Thu Feb 02, 2017 4:27 pm
by Chancer
Absolutely not! Although I understand your cynicism.

Its computerised, it gives a graph plot of the frequency versus oscillation and the the reaction force, plus the percentage disparity between individual dampers, you can even see secondary resonant frequencies.

That said the French CT is very thorough and correct but most things are just let go with the equivalent of an advisory, even if it is a death trap the fail certificate enables you to drive "legally" for the next 2 months at which time you could just repeat the process :D

My pal has an MX5 which we do track days with (the circuit is a state secret!) which we bought in the UK together with most of the upgrade parts, it has a set of adjustable springs and dampers, unbeknown to me he had just set them all to the same number thinking they would be identical and didnt do a bounce test, I drove it on the circuit and didnt feel anything untoward, it was later picked up on the CT damper test and the disparity was outside the acceptable limit, of course they let it go! I later set up all 4 corners myself then took it to the CT centre where we tweaked each pair to be equal, its far more sensitive and repeatable than I can do myself as its a relatively heavy vehicle with hard springs, my Caterham 7 I can get to bounce enough from my own force to do a fairly good job, not the Mazda.

Seems crazy that a shot damper that has just been jet washed clean and no longer has any o?l to leak will now pass a UK MOT in the name of EU harmonisation yet another EU country has a decent test.

Re: MOT & brakes

PostPosted: Thu Feb 02, 2017 8:16 pm
by Galwaylotus
In the NCT in Ireland - based on EU Directives - the braking effort of the vehicle must be not less than 55% of the test weight of the vehicle (Service Brakes). Brake effort on any wheel must be not less than 25kgf. There must not be more than 30% difference in braking effort side-to-side on the same axle. Parking brake effort must be not less than 20% of the test weight of the vehicle (vehicles registered before 01 July 1964 with a single braking system). This figure rises to 27.5% on single line braking vehicles registered on or after 01 July 2964. For all vehicles with dual line braking systems the braking effort must be not less than 16% of the test weight of the vehicle. Parking brake imbalance must be not less than 50%.
The relevant EU Directive on which Irish legislation is based and, I believe, on which much of UK testing regulations are based (until Brexit!) is http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0045&qid=1415792757598&from=EN. If you page down to the tables you'll find the relevant section on Brakes and Brake Performance.
By the way, passenger cars are classed M1. This includes our Elans.

Re: MOT & brakes

PostPosted: Thu Feb 02, 2017 11:11 pm
by englishmaninwales
Chancer wrote:Absolutely not! Although I understand your cynicism.

Its computerised, it gives a graph plot of the frequency versus oscillation and the the reaction force, plus the percentage disparity between individual dampers, you can even see secondary resonant frequencies.

That said the French CT is very thorough and correct but most things are just let go with the equivalent of an advisory, even if it is a death trap the fail certificate enables you to drive "legally" for the next 2 months at which time you could just repeat the process :D


OK, so I'm right.....but for the wrong reason :D

Given the definitive test is impractible ie remove the damper and put it on a test rig, this test described can only be a screening test. Do the administrators of the French CT publish levels of sensitivity and specificity this test achieves for damper performance?

Re: MOT & brakes

PostPosted: Thu Feb 02, 2017 11:47 pm
by Chancer
If you want to split hairs (and you started it :lol: ) its a test of the whole spring/mass system, the unsprung weight that is, its testing how well the wheel remains in contact with a bumpy road. If you drive over vibrating rollers, perhaps a very undulating road in an earthquake area it is very relevant :D

As for the question in the second sentence i dont really understand it but the answer will be NON! with a gallic shrug of the shoulders :D

Re: MOT & brakes

PostPosted: Fri Feb 03, 2017 12:06 am
by englishmaninwales
In other words, as you say, it may be a sensitive screening test for the performance of whole system (mass, spring, damper, ARB torsion etc) but it might not necessarily be a specific test of damper performance per se (and I have no reason to doubt the value of such a test other than my natural scepticism :D )

Re: MOT & brakes

PostPosted: Fri Feb 03, 2017 9:22 am
by Chancer
Well apart from the hyst?r?sis of the rubber suspension bushes, friction if solid bushes are used, the hyst?r?sis of a Mini rubber suspension cone there is nothing any significance that creates damping in a motor vehicle suspension spring mass system, even the aforementioned are de minimis, theoretical but non measurable outside of a laboratory.

I'm struggling to recall if hydrolastic systems used seperate dampers also the Citro?n sphere system.

Re: MOT & brakes

PostPosted: Fri Feb 10, 2017 12:43 pm
by rcraven
By chance I've now found this which explains why a specific test of the dampers was removed from the MOT: https://mattersoftesting.blog.gov.uk/your-questions-answered-why-were-shock-absorbers-removed-from-the-mot-test/

Re: MOT & brakes

PostPosted: Fri Feb 10, 2017 5:24 pm
by Galwaylotus
That legislation has been superseded at least until 2014/45/Eu which includes the following:
5.3.2.1. efficiency testing of damping (X)2 Use special equipment and compare left/right differences (a) Significant difference between left and right. X (b) Given minimum values not reached. X
Any failure on this item is considered "Major".