Saturday, March 25, 2017

Tire Monitor Systems (TPMS) and high performance cars

Tire monitoring systems first appeared on passenger cars about 20 years ago.  They’ve been mandated on passenger cars in the USA for a little more than a decade.  The idea was that underinflated tires contribute to over 250,000 accidents every year, and they waste fuel.  A vehicle whose tires are underinflated by 10PSI will burn 3% more as a result (according the studies quotes by our National Highway Traffic Safety Administration.)

The need for a low tire warning system was reinforced by government-sponsored studies that found 50% of cars examined by researchers driving on underinflated tires.  Modern tire monitor systems should alert you when your tires are at less than 75% of the vehicle manufacturer’s recommended cold tire pressure. 

That’s good enough to protect a typical passenger car driver against most (but not all) under-inflation dangers.  The average passenger car tire has a recommended inflation pressure in the 28-38PSI range.   Studies have found that under-inflation damage (and accidents) become a lot more likely when tire pressures drop below 20PSI.  The current standard provides adequate warning to protect against that.

However, not all cars are average. And some drivers subject their tires to stresses far beyond the common range. For those people, tire monitor systems CANNOT provide assurance that they will be safe under all conditions.  Tire monitors are far better than nothing, but they do not take the place of careful inspection and checking of tires with a good manual gauge, before driving at or near the limits – particularly for supercars.

Let’s look at tire monitoring on a new Bentley.  The Flying Spur is one of the world’s most popular supercars, if we define supercars as those vehicles whose top speed is above 175mph and whose overall performance in above the 90th percentile of all cars on the road.  The 2017 Flying Spur has a flexible and sophisticated monitor system, as shown below.

The system in these cars allows drivers to select comfort, normal, or aggressive driving modes, with the difference being progressively greater air pressures.  The higher air pressures give increased stability and reduced tire heating at high speed, at the cost of noticeably harsher ride as we go from one extreme to the other.

Bentley sedans may be fitted with more than one tire size, and they may be fitted with summer or snow tires.  The monitor system allows tire size and type to be selected too.

Once the system is set it shows a screen with the target tire pressures and the actual pressures.  When any tire drops below the 75% warning limit the driver is alerted by a light in the center of the dash, and the tire can be identified on the center console screen.

This model of car is capable of reaching speeds of roughly 200mph, and could attain that velocity on a minute’s notice given a long enough stretch of clear road. The tire monitor system cannot assure a driver that the vehicle will be safe if suddenly accelerated to that speed.  I’ve been surprised to encounter supercar owners who trust these systems in those situations, and they tell me the systems or the cars must be faulty if they don’t work.  The fact is, no built in electronic monitor can assure you of probable safe tire performance at three times the American highway speed limit.  Nothing short of a physical inspection can do that.  Here are some of the issues and what you can and should do about them, before driving triple-digit speeds.

First, look at your tires and make sure they carry the proper rating.  The fact that you have a near-new car is not an assurance that the tires are suitable for all roads and all speeds.  In the case of supercars, such tires do not exist.

On this tire, the ZR and (103Y) designations specify a tire that is designed to be safe "above 189 mph" which is the highest speed rating in use today.  Such tires are designed in conjunction with carmakers and are presumably matched to the potential of the vehicles for which they are intended.  This particular tire is for Bentley. 

Here is the standard tire on the Bentley as an example:

The first concern is potential damage to the tire or the rim.  Supercars use very low profile tires to get the required sidewall stiffness for responsive handling at triple-digit speeds.  The thin sidewalls make the tires and wheel rims much more vulnerable to damage from curbs and potholes.  Tires develop egg-shaped bruises that can blow out. Rims may bend and crack, and potentially come apart at speed.

This is what sidewall cracking looks like.  Tire monitors can't detect this condition, and it is a common cause of sudden blowouts.

The Pirelli P Zero is one of the finest extreme performance tires made today.  Models were specially developed for fitment on the Bentleys, plus the Aston Martin DB9, Audi R8, Ferrari 599 GTB Fiorano, Lamborghini Murcielago, Maserati Quattroporte Sport GT and Mercedes-Benz AMG vehicles.

The tire is called a “maximum performance summer tire.”  When Pirelli wrote those words, they meant that, and only that.  Few drivers realize that.  Here’s what Pirelli says: warranty does not cover tires that develop compound cracking due to use in ambient temperatures below 45° Fahrenheit (7° Celsius), so the P Zero, like all summer tires, is not intended to be driven in near-freezing temperatures, through snow or on ice.

Drivers who only move their supercars in warm weather don’t need to worry about this.  Drivers in most of California and the south where they seldom see freezing weather don’t have to worry either.  All the rest of us – in New York, Boston, Chicago and Montreal – should take Pirelli’s words to heart.  And bear in mind I chose Pirelli as an example but similar restrictions apply to supercar tires from other brands.

We can phrase this another way for clarity:  There is no such thing as an all-season tire rated for use at or near the 200mph potential of today’s supercars.  Furthermore, in the case of supercar tires, the “summer” tires that are standard fit on all supercars will be damaged by use on freezing roads.  If you want to drive your supercar year round, you need winter tires for cold season.  And when you fit those tires, you must (for safety’s sake) limit your speed to the lower of the safe limit of a winter road or the rating of the tires you have chosen (which will be far below the ultimate speed potential of the car.)

Winter tires lack the sidewall stiffness of supercar summer tires.  This imposes a much lower practical limit on spirited driving.  If you have one of the super-SUV vehicles (like a Bentley Bentayga, a Mercedes G63, or a Supercharged Range Rover) currently available winter tires will begin to feel squirrely at speeds over 90moh and they will become dangerous on many road surfaces far below their rated top speed.

The next thing to be aware of is the possibility of cracking and incipient failure in your tires. Cracks cause sudden blowout failures; the worst thing that can happen at high speed.  Supercar tires – due to their formulation – are more prone than ordinary tires to cracking for various reasons.  Most supercars spend most of their time parked – few are driven more than a few thousand miles per year. That means it’s common to see supercars with tires that superficially look good, but which are 5, 10, or 15 years old.

Don’t be lulled into complacency if you drive fast! Tires like that can be deadly.  Standing beside the car you can only see about 1/3 of the tire’s surface.  If the tire is cracking on the inside, or of the tread is starting to come apart, you may not see it.  In Europe – where some countries allow very high speeds – tires must be replaced every 5 or 10 years.  We don’t have such a rule in America but it’s still good and prudent practice.  Here’s an example of a date code on a Pirelli tire.  This tire is fitted to a 2017 car and was made in week 38 of 2016.  

Newer tire monitor systems are only intended to warn against low pressure under normal, legal, American driving conditions.  If you have your tire monitor set for “comfort” and you go to a track event, you could get an overheat failure of a tire.   When driving fast, ALWAYS first raise tire pressures to the higher level required for safe high speed. 

Use common sense, which is actually not at all common.  If your car recommends pressures for light or full loads, and comfort or fast driving, bear in mind that YOU may create an unsafe situation if you load the vehicle to its max weight, then set the tires to comfort pressures, then drive at the outer limits of what's possible. You may go over the edge, and the car's tire monitor can't protect you because it cannot know the weight load or speed in advance.  Only you know that and you are responsible for the safe operation of your machine.  Its warning systems are only there to assist you.

The load that any tire can safely carry is a function of the tire design rating, inflation pressure, and speed.  In America the standards we use are promulgated by the Tire and Rim trade association.   You can buy their book here.  Car manufacturers can only specify a safe range of recommendations.  They have no way to know if you follow them.

The final limit on tire monitors is response speed.  Today’s sensors do not respond rapidly enough to warn drivers of sudden tire damage and leakage. There is no way around the fact that some tire failures take place in less than ten seconds and nothing but very fast reflexes and luck can save a driver in that situation.

In summary, tire monitors are a good addition to all passenger vehicles. If used properly they will warn us of the worst tire conditions, but they are not a substitute for physical inspection.  When driving a supercar anywhere near its potential the driver should ALWAYS check the tires and vehicle carefully before setting out, and at every fuel stop.  When auto performance moves upward drivers must apply a standard of care that is more akin to that of a jet pilot than that of a Sunday driver.

(c) 2017 John Elder Robison

John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Bentley, BMW/MINI, Mercedes, Land Rover, and Rolls-Royce restoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the car clubs, and he’s owned and restored many fine British and German motorcars.  Find him online at or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.  

Monday, March 6, 2017

Things to look for when buying a Bentley Continental GT

The Continental GT is the first Bentley to be developed under VW ownership.  The car was introduced for the 2003 model year and remains in production as of this writing (2017)  The Continental GT has been the most successful Bentley model of all time, significantly outselling all other models.

Bentley GT coupe (c) J E Robison

Today new GT prices start at about $200,000, and late model examples remain pricey.  Cars less than 5 years old are available through the Bentley dealer network with factory-backed certified warranty, and they are not the subject of this article.  Rather, we will focus on older examples.  Now that the car has been on the market 14 years we are seeing good early cars selling under $50,000.

The Continental GT has retained a consistent appearance since its introduction.  Trim, wheels, and colors have changed but the basic bodywork has had a long production life.  This is consistent with Bentley’s traditional postwar practice, where the previous Continental R was built without major bodywork revision from 1991 through the arrival of the GT in 2003.

Continental GT is the first mass-produced Bentley motorcar.  The prior Continental R was entirely hand built.  There is a lot of hand work in each GT but it was designed to be a mass produced product like other offerings from VW Group.  That allowed Bentley to offer the new GT at little more than half the MSRP of the previous R model, and sales immediately took off.

Bentley GT interior, MY 2005  (c) J E Robison

GT cars contain more sophisticated technology than any prior Crewe built automobile.  In developing this car VW and Bentley engineers worked together to retain much of the “look and feel” of traditional Bentley, while using modern manufacturing techniques.  That meant – for example - that dashboards were molded from foam rather than being hand crafted from wood and metal.

All of the driveline components on the new GT were previously proven on other cars in the VW/Audi line.  While the W12 (and later V8) engines were totally new to Crewe and Bentley they were well developed in the other car lines.  Modifications to personalize those components for Bentley did not change their essential nature.  By sharing driveline components across multiple brands and platforms VW group was able to build larger numbers of fewer parts. That allowed them to optimize quality - an issue that had plagued Crewe for years due to their low volumes.

Right from the outset the Continental GT series cars had less trouble with things like gasket failure, water pump leaks, and pesky check engine lamps.  The reasons for that were twofold - improvement through higher volume production and greater depth of experience in the now combined VW-Bentley engineering department.  We can mourn the passing of the Conti R - the last 100% handmade Bentley - but we have to appreciate the improved day to day quality modern mass production delivers.

Bentley GTC in front of Robison Service  (c) J E Robison

Mechanically the new Bentley was built on the same platform as the VW Phaeton and Audi A8.  Technicians who are trained in high end VW/Audi are able to adapt quickly to servicing these cars, and the VW electronic test systems support the Bentley product too.  Any prospective purchase should be scanned for faults in all computers, using dedicated VW software.  Owners can access most of the functionality of the factory test system with the VAG tester, available online for less than $1,000.

Technological changes to Bentley Continental through the years have tended to follow the rest of the VW lines.  The first update was for the 2005 model year, where they added voice activation for in-car telephone in six languages.  Bluetooth phone integration came shortly after and Bluetooth music streaming later still.   With every year the range of color choices in paint and leather shifted and in many cases expanded. 

When looking at an older Continental you can’t help being struck by the outdated appearance of the electronics screens.  The navigation and radio displays are particularly striking, as compared to even a basic VW of today.  That electronic obsolescence is a new issue with collector cars and the GT is really the first manifestation of that in the Bentley line.

Early GT cars were sold with built-in phones, the earliest of which use analog technology that is no longer supported in the USA.  Even the newer phones only offer limited compatibility with the 4G systems of today.  Some owners are stripping out the phones while others keep them for a period look.

At ten years of age many of these cars are showing wear in the interior.  The leather and carpet used in the GT appears to be a step down in durability from that found in earlier offerings from Crewe.   That means the leather is more easily perforated or torn, and carpets are more vulnerable to damage.  Traditionally these cars had wood trim that consisted of veneers overlaid on thick hardwood or plywood substrate.  In the interest of accident safety the newer Bentley offerings use a laminate of soft aluminum, wood veneer, and plastic coating.  Wood is often splitting on the older cars and repair of this new composite trim requires a different process than the older materials.

In the Robison Service shop we have seen several instances of fallen headliner fabrics on 2003-2006 cars.  Other shops have reported similar experiences to us.  A falling headliner is easy to spot and can be surprisingly costly to fix, as the proper cure is a complete new assembly from Crewe at a cost of several thousand dollars.

GT cars built after 2006 came with tire pressure monitors.  Those monitors depend on sensors built into each wheel rim, and those parts had a design life of 8-10 years.  When they fail (as most all older ones have) the fix requires new sensors and a new control unit, at a total cost approaching $3,000. 

Brake hoses should be updated on all cars at 8-10 year intervals.  That’s an important service that is often deferred on these motorcars.  If you look at a car pay attention to whether the hoses were done, or if they are original.  The hoses on the first GT cars have been superseded so replacement will require fitment of some additional pipes and brackets at a total cost over $2,000.

Brakes and tires can be very expensive on these cars, particularly on variants with the high performance brakes.  A complete brake job can range from $3-10k.  Several sizes of tire have been used, and prices vary widely from $300 to over $1,000 per tire.

Engines are generally rugged, but some were damaged by too-infrequent oil services.  An engine that has been clogged with sludge cannot be cleaned short of total teardown, and proper repair of that issue can cost $50k.  I suggest you be sure any car you buy had received annual service, and if in doubt look inside the engine with a fiber optic camera.  Anything less than clean and shiny is unacceptable; there are enough of these cars on the market that I suggest passing on marginal examples.

Design engineers made extensive use of plastic for coolant and vacuum lines and even parts of the engine.  Once a car gets 10 years old you can expect those plastic parts to become brittle.  That may mean that seemingly simple repairs cascade as one part after another brakes on removal or manipulation.  It’s hard to predict which cars will have brittle hoses – one example can be fine where everything you touch on another seems to break.

Paint and bodywork is usually very good on the GT cars.  These vehicles were painted by an automated process and are the first Bentley cars where you could expect a consistent 4 mil paint thickness on a new vehicle.  Paint thickness on a Continental R could vary from 3 to 10 mils and it was hard to judge whether you were looking at a car that had rework in the Crewe factory, or later repair in the field. In a GT any deviation from the standard paint thickness indicates field repair.

From a driver perspective the GT is absolutely the finest product Bentley has created.  These cars are fast, agile, and smooth.   All of the GT cars have all wheel drive, and it is wise to drive any prospective purchase in tight left and right circles with windows down to listen for any tire scrubbing that would be evidence of driveline problems.  Cars should be free of rattles and noises on rough roads.

The first GT cars were coupes.  A convertible version followed shortly after (the GTC.)   Convertible tops had a distressing tendency to suffer bow and fabric damage which can only be corrected by fitment of a complete top assembly at a cost near $20k.  Read my 2013 article on that here.  If you are looking at a GTC pay close attention to the cables on either side of the top as it folds.  Look for tears on the underside and small perforations in front of the rear window, particularly on the left side.  If you see any of the aforementioned plan for a new top assembly soon.

In model year 2006 Bentley released a four-door version of the GT – the Continental Flying Spur.  The mechanical issues of these cars are similar to those of the coupes.  Overall they have been good cars.  The Flying Spur name was previously used briefly on a turbocharged Rolls-Royce in 1995 and before that on a famous derivative of the Bentley S series of the 1950s.

For model year 2010 the GT series received a significant facelift.  Expect prices to jump sharply between 2010 and 2011 examples as a result.  The revised GT series also included a V8 car at a lower price point.  Experience with the V8 has been good.

Expect general maintenance to be pricey.  These cars hold three gallons of synthetic oil and they have an exacting specification for all lubricants. Ignore that at your peril!  Most versions of the GT use Mobil 1 0-40.  Any work on the belt drives or front of the engine requires removal of the nose of the car (similar to Audi timing belt work) and some engine repairs require powertrain removal.  Make sure any service provider you choose is trained and tooled for the job.  Remember the technology in this car is totally different from that in any prior Bentley automobile.

Users can access the factory service system online at Bentley Erwin.  Subscription is required but you get access to the same info as the dealer techs including a lot of technical training.

One tip for keeping these cars trouble free:  If you plan to park your GT for more than 3-4 days between drives you should get a trickle charger installed.  Bentley includes chargers in the new cars and sells a kit for retrofit.  You can also buy and adapt generic chargers.  If the batteries run down you will get spurious fault codes set and systems may malfunction.  When a service person reads those wrong codes they may be sent down false trails, wasting your money.  Best to avoid the whole thing by keeping batteries charged.

Overall the GT is a very fine car.  When buying a used example be very careful of cosmetic damage.  Look for evidence of both deferred maintenance and neglect, and substandard accident repair.  An original well cared for example should give years of good service.  These cars are far more common than any prior model Bentley and with that in mind there are no "one of a kind" GT cars.  It's worth taking the time to find a good example, and have it carefully inspected by a professional who is trained in the VW-Bentley series cars before spending money on it.

More on this topic:
Buying a good used Crewe era Bentley or Rolls-Royce
Bentley and Rolls Royce - yesterday and tomorrow
Rebuilding the Crewe V8 used in Mulsanne, Turbo R, Azure
John Elder Robison

(c) 2016 John Elder Robison
John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Bentley, BMW/MINI, Mercedes, Land Rover, and Rolls-Royce restoration and repair in Springfield, Massachusetts.  John is a longtime technical consultant to the car clubs, and he’s owned and restored many fine British and German motorcars.  Find him online at or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff.  So it's good for you.  But don't take that too far - printing and eating it will probably make you sick.