MOT Workshop 23 – Tyres, Steering and Suspension


Turning and burning!

Tyre technology and the MOT

Since the MOT began in the 60’s tyre technology has changed dramatically – they’re now predominately radial ply with better grip than 40 years ago when cars were slower and 0-60 acceleration took minutes, not seconds and some sports cars had only 65bhp – imagine that! Today’s tyres cope with greater stresses, are more expensive and don’t last as long – as important to road safety as steering and suspension, let’s have a quick look at how tyres and developing tyre technology could affect a future MOT.

tyre-wear2This tyre is shown as it was after removal following an MOT failure – imagine how long it had run before being discovered during an MOT!

The MOT check of tyre type, use and condition has changed little in over a decade. Simply summed up, the Tester has to decide if it’s the right type, the right size, properly fitted on the correct axle and in acceptable condition. Well, not quite, there are a few other ‘wrinkles’, including speed rating for class 7 vehicles, whether or not the tyre fouls the vehicle, some other checks for twin wheeled vehicles, and of course, the condition of the valve. But is that enough?

Speed and performance

In the BBC ‘Top Gear’ programme Jeremy Clarkson demonstrated that a modern standard saloon car will outperform (especially on acceleration) both an E-type Jaguar, and an Aston Martin of the 1960s era – both ‘supercars’ of their day. Modern tyres have to work much harder now than they have ever done in the past. With lighter vehicles, and better acceleration coupled with traction control systems enabling vehicles to go right to the edge of loss of grip, tyre technologies are forced to keep up.

So what determines a ‘good’ or a ‘bad’ tyre? Well, that depends on whether you are a vehicle manufacturer, or an aftermarket tyre supplier.

Quiet… but with confidence

Car makers are interested in vehicle handling, comfort and reducing tyre noise – so when you test drive that new Ford, Mazda or whatever, you expect it to be comfortable, quiet and handle well. For noise reduction the block size of the tread pattern will be smaller, resulting in better dry grip, but clever tread design is needed to ensure that as much water is pumped away in wet driving conditions – more easily achieved with larger (but noisier) block sizes. These days car makers are also interested in reducing rolling resistance to improve fuel consumption – so the tyre material has more silica in its composition. Yet this isn’t necessarily the case for aftermarket products where a lower selling price or higher mileage performance may be more desirable to increase sales.

Some of this can affect the MOT. With those clever tread patterns it isn’t always easy to make pass/fail decisions on wear. Yet the one factor that can really affect tyre performance – tyre pressure – we don’t even measure – and we should!

No pressure…

Pile-of-tyresJust a sample of the tyres changed over a few days in a small but busy MOT garage.

The MOT is an EU road safety Directive (96/96), but tyre pressure measurement is not a requirement – so it isn’t in the MOT. Yet there’s nothing to stop the Department for Transport from including it if  they wanted to – in France tyre pressures are measured prior to every ‘MOT’. And in terms of road safety, environmental considerations and to better carry out an MOT it makes a lot of sense.

Under inflated and over inflated tyres are dangerous. Overheating could cause catastrophic failure. There is variable grip across axles so steering and braking will be adversely affected and tyre wear accelerated – all affecting road safety. But there’s more. Under inflated tyres result in increased rolling resistance hence greater fuel consumption, wasted energy and increased emissions.

Then there’s the MOT. We are required to keep the rollers on our RBT equipment in good condition, and ensure the vehicle standing area for the headlamp check is level to fine tolerances. Yet conducting an MOT on vehicle’s with incorrect tyre pressures makes a nonsense of it all. Brake readings and headlamp measurement are both inevitably inaccurate and distorted.

It seems very odd that whilst VOSA nit-pick about how Testers carry out an MOT, which must be precise and to the letter, Testing a vehicle with incorrectly inflated tyres is no problem at all!

Running flat

We’ve talked about ‘run-flat’ tyres before – and specifically about disagreements in the industry as to whether or not they can be repaired following failure – an issue still unresolved. It is idiocy. If, on road safety grounds a ‘run flat’ tyre should not be repaired then that should be included in the MOT – or not – if the opposite is the case.

Then there are those remote tyre pressure sensing systems – a requirement if ‘run-flat’ tyres are fitted. With electronically controlled steering systems a flat tyre will not necessarily be detected by the driver’s ‘feel’ of the car. In which case an alternative way of knowing that a tyre is deflating could be vital to avoid a unexpected ‘blow out’.

Such systems operate in two different ways. The simplest, and least accurate uses the wheel speed detector in the ECU to pick up wheel speed differences – because a tyre is flat. Whereas the more sophisticated system uses a pressure transducer in the wheel which transmits a remote signal to the vehicle’s ECU. Maybe, just as we check that ABS electronics are functioning, the same should be true of the tyre pressure measurement systems – especially if run flat tyres, or electronically controlled steering systems are fitted.

Avoiding death and injury

According to statistics from VOSA’s computer, tyre MOT failures are running at 8.02%, that’s almost two and a half million every year. This must save a lot of lives, and prevent injuries yet even more could be saved if we checked inflation levels, and, according to Richard Edy, the Chief Executive of the National Tyre Distributor’s Association, if the minimum tread depth were increased – which would vastly reduce braking distances in wet conditions.

Tyres are also a vital element in the argument against going to two yearly Testing – with a good chance that the majority of those two million plus defective tyres currently failed at MOT time would still be rolling on the road without that vital annual MOT to pick up their dangerous road safety defects.

MOT Testing of tyres isn’t exactly complicated, once you’ve got your head around those ‘Tyre size, ply rating and Load Index’ tables, and the various axle to axle requirements. But what about the regulations themselves, should they be ‘tighter’? We talk to Richard Edy, Chief Executive of the National Tyre Distributors Association who has some very strong views on tyres and road safet

Gripping stuff!

tyres1This Pagani ‘Zonda’ has a 7.2 litre Mercedes engine and can do 220 mph. Unlikely perhaps, but it could be legally fitted with lower speed rated tyres!

Road safety is the underlying foundation of the MOT Test. It is the very reason for the Test in the first place – and if we get it wrong it is the “threat to road safety” that is used as the basis for any disciplinary action.

It is against this background that Richard Edy, the man in charge at the National Tyre Distributors Association, believes that the current MOT regulations do motorists a disservice when it comes to road safety. For a number of reasons he feels that the authorities should be paying more attention to the road safety implications inherent in some of the regulations.

Tread depth

On tread depth, for example, Edy has strong views. He believes that the 1.6mm minimum should be across the whole width of the  tread, and not just “…the central three quarters of the breadth of the tread…” quoting from the Manual.

Edy thinks this is deplorable, noting “…tyres are about safety and on a wet road any baldness on the tyre could be fatal… yet we allow tyres to be used which are 25% bald, that cannot be acceptable in road safety terms…”.

Edy also expressed concern about tread depth and speed rating. “Why not”, he said “insist on a greater minimum tread depth on higher speed rated tyres?” He suggested that for tyres with ‘S’ and ‘T’ speed rating, a minimum tread depth of 2mm should be enforced and for really high speed ‘H’ rated tyres the minimum should be as much as 3mm.

He also suggested that the normal minimum tread depth should also be increased, noting “At the NTDA we are aware of tests which clearly indicate that even an extra 0.5 mm of tread depth offers a significant reduction in stopping distance on a wet road…”.

Finally he accused the Government of a hypocritical attitude to the subject of tyre tread depth regulations. He pointed out that Government Ministry vehicle fleet managers operate a policy of changing the tyres on their own vehicles when the tread depth reaches 3mm – almost twice as much as the 1.6mm minimum. “This”, commented Edy, “…directly contradicts the Government’s own legislative policy on tyres. If it is necessary for Government Ministerial transport, then why aren’t safer regulations enforced for the rest of the motoring public?

Speed rating

Apart from providing higher speed rated tyres with more tread, Edy suggested that putting lower speed rated tyres on an essentially high speed vehicle was also a potential threat to road safety. In Germany, for example, the tyre fitted as original equipment when the vehicle leaves the production line has to be noted in the vehicle’s registration document. Motorists are obliged to only use replacement tyres which have an equivalent speed rating – not so in Britain…

Edy again: “the car makers know what they are doing. If they believe that for safe performance a vehicle should have tyres of a specific speed rating, then it is not unreasonable to assume that anything less is counter to road safety…”.

Mix and match

Another aspect of tyre safety which is not part of the MOT is mixing different makes of tyres and tread patterns. Richard Edy made the point that whilst it is acceptable to mix different brands it would be inadvisable to mix tread patterns across an axle. Although there are no specific regulations about this, he feels that different tread patterns will have different grip characteristics and fitting tyres of the same pattern across an axle is, as he says “good practice”.

We also asked him about the difference between ‘budget’ and ‘branded’ tyres. He noted that in safety terms there is little difference and that often the budget tyre is made in the same factory with the same materials but using a different mould.

The reason being that the brand leaders are keen to avoid being excluded from the budget tyre market and so a proportion of their output is at the more economic end of the scale.

Colours and wear bars

During our discussions Edy emphasised the importance of wear bars and how motorists often don’t realise that any wear below the wear bar in the average central three quarters of the tyre means it is illegal.

He told us that the car makers are looking into using coloured rubber as a more noticeable ‘tell-tale’ that the tyre needs replacing. This would certainly aid MOT Testers when reaching pass/fail decisions on tyre wear.

A Gripping interface

When you think about it, it is surprising that the tyre regulations aren’t more strict. After all they are the key ‘gripping interface’ between the vehicle and the road. Car makers can put as much effort as they like into ABS, traction control and the like but it all means nothing if the tyres aren’t up to their job.

Testers and Authorised Examiners take pride in their work. Our efforts do make a contribution to road safety. Yet increasingly we have to question legislative decisions about the standards we apply in the MOT Test itself. And tyres are very near the top of the list in road safety terms.

If you want to find out more about tyres, have a look at the NTDA’s website

On some modern tyres there are two separate depths of tread. A recent addition to the manual says that if part of the tyre has a shallower tread and no wear bars, then that tread does not need to comply with the 1.6mm minimum depth rule!

During a recent Test in the Editor’s MOT station his Tester came across a tyre where only 40% of the centre tread pattern was ‘Testable’. The remaining 60% (30% either side to the shoulders) was of a shallower depth and had no tread depth bars and was therefore not Testable.

The tyre in question was almost bald for 60% of its width – but still had to be considered acceptable for the Test. If you find this you must pass the tyre but we recommend you issue an advisory note.

We contacted VOSA who said they have been informed about this already, and say this apparent anomaly is an aspect of modern tyre design.

So, as we only have to check the middle three quarters of the ‘Testable’ width of the tyre in question (40% of the total width), that means the tyre could pass the MOT with only 30% of its total width having the minimum tread depth of 1.6mm. Road safety?

Keeping it on the road!

Steering (and suspension) is a large and complex section of the Manual, representing about 12% of all MOT failures. So this inspection makes a significant contribution to road safety. Here we’ll have a look at some issues surrounding these checks, difficulties in the Manual, pitfalls for Testers to avoid, and recent developments with automated Test lanes, one person Testing and electric steering.

The very first part of the manual deals with the steering wheel and the steering column – including the ‘free play’ check, straightforward stuff. Not quite – in the information column on page 1 of section 2.2, the Manual unequivocally states “The inspection must be carried out with the vehicle over a pit or a raised lift”.

Electric powered steering (EPS)

Electric powered steering is becoming more and more popular with vehicle manufacturers, and there are two key reasons for this – energy efficiency and system control. The implications for the future regarding the MOT Test could be far reaching.



Here the power steering’s pump is driven by an electric motor. However this is now becoming less popular with vehicle manufacturers, and isn’t really electric steering as there is no facility for electronic control of the steering operation. And as the pump is driven all the time, not just ‘on demand’, it is energy inefficient.

Column electric power steering (CEPS)

In this version of full EPS the power assistance is applied by an electric motor which assists turning the steering shaft when the driver applies a steering input. Desirable in terms of the ability to install electronic control systems, car makers are less enthusiastic because they cost about 25% more to manufacture and install than good old fashioned hydraulic systems.

There is, however, a fuel saving benefit which they can use as a selling point, and the use of this system is predicted to grow as future costs reduce. A future MOT implication will be how to check the electronics, especially with sophisticated ‘control’ systems expected in the near future.

Rack electric power steering (REPS) 

As it ‘says on the tin’, this is where an electric motor operates the rack on receiving an input signal as the driver turns the wheel. The advantage here, it seems, is that it can be used with heavier vehicles in the luxury car market as it can take heavier steering loads.


Active steering (AS) 

This can apply to both CEPS and REPS systems and includes such refinements as variable steering ratio (as developed by Bosch), auto park, and ultimately ‘steer-by-wire’ systems, although in safety terms we are a long way off from that in the current situation.

Checking the software

In road safety terms, the active steering systems will be controlled by software in the vehicle’s ECU, and as we have seen in the CITA ‘Autofore’ report (see Testing Times), these systems could be part of an MOT check after 2010 – so, as ever the MOT is struggling to keep up with the car makers’ technological developments here, regarding steering control systems.

When ‘play’ is not much fun…

So how does that work on a raised lift? Picture it, the Tester standing on the runners, perched precariously over the edge, gripping the steering wheel for dear life whilst attempting to rock it from side to side and craning his head out over the void to look at the wheels and check when they are just moving. No chance!

We contacted VOSA HQ at Bristol. They agreed it was wrong but couldn’t offer an explanation. How strange that nobody at VOSA had picked this up before – and they’re always telling us we should read the Manual!

Good ‘play’, bad ‘play’…

In the same information section entitled “Types of movement”, it says “Relative movement due to excessive wear MUST be distinguished from relative movement due to built-in clearance or spring loading of a joint.”

So how does a Tester know? In one case a Tester was disciplined by VOSA for failing a joint because it had a small amount of ‘play’. The VE insisted the Tester should have ‘passed and advised’.

Infuriated, the AE confirmed with the car makers that any wear in the joint is unacceptable and must be replaced. The situation was never really satisfactorily resolved, but highlights the difficulty of expecting NTs to make such decisions. That was prior to computerisation, so hopefully now, as these issues arise and become resolved on an engineering basis, appropriate vehicle specific information will be available, so always check.

Vague and unhelpful

When checking steering and suspension Testers must make judgement calls. The Manual is liberally sprinkled with vague statements. For example, on ‘rear wheel steering’, a ‘reason for rejection’ only applies when “…the front wheel steering is adversely affected”, and later “a component… deformed so that it is unserviceable”. How is a Tester to decide? He or she may never have been specifically trained on rear wheel steering systems, and never worked on a vehicle on which it has been fitted. Of course VOSA would argue that this is where a Tester’s experience and qualifications must help – but some Testers working in dealerships may never have come across the complexities of the steering and suspension of other vehicle makes.

Of course the answer is better training, something about which we rant and rail every now and again – but that’s a different story. In the meantime on these value judgements, follow VOSA’s golden rule, if you’re in doubt – pass and advise!

Jacking or joking!

As long as I can remember, where to jack the front suspension has been a bit of a problem. Exasperated VEs have told me how shocked they have been at how many Testers just don’t seem able to work it out.

This is less of problem now with better and more specific diagrams in the Tester’s Manual, and the Vehicle Specific Information (VSI) document off the computer tells you exactly where to place the jack. It is still as well to be careful – you may think you know and inadvertently get it wrong – and if a VOSA man is watching, you’re in trouble. So it must be worthwhile from time to time just referring to the VSI on the computer and check you’ve got it right, and really do understand the principle involved!

Shake rattle and roll!

The examination of suspension and steering joints is time consuming. Both Tester and Assistant must “…grasp the top of each front wheel and rock it vigorously in and out to check for play”. The Tester must “Grasp each wheel at 3 o’clock and 9 o’clock, and shake vigorously…”, and “Rock each wheel by hand or, where appropriate, with a bar in the wheel” – the Assistant must also lift the wheel with a bar beneath it whilst the Tester observes the suspension joints from beneath the vehicle. All this shaking, pushing and pulling takes time.

Yet with the shaker plates on ATLs and one person Test lanes, this can all be done by the Tester alone. To load the suspension the plates move in and out across the track of the vehicle, loading the appropriate joints and to check the steering joints the plates shake the wheels from side to side – a significant time saving.

The invisible man…

When VOSA carry out ‘mystery shopper’ MOTs, you may not spot the seemingly innocent punter asking for an MOT on a ‘doctored’ VOSA vehicle. Invisible as a VOSA man, he’ll watch everything – especially how the Test is done – and with the steering and suspension checks amongst the most complex in the Manual, that is where Testers are most likely to get caught out. And with the new ‘traffic light’ disciplinary system (see VOSA Matters), you do not want to suddenly drop into that red zone.

So remember – ensure all the checks are done and done properly – or alternatively just buy an ATL, or a one person Test lane which greatly simplifies the whole currently complex process of checking steering and suspension.

motw MOT Workshop Magazine



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