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 www.robisonservice.com 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.  

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