Archive for December, 2006

Understeer and Oversteer

Most cars have understeer built into them, because it is inherently safer. The car when driven into a corner too fast, will tend to become heavy on the steering, run wide at the front, generate excessive tyre scrub, and thus warn the driver to “back off”, with all these signals being transmitted through the steering.

An oversteering car will tend to have the rear run wide, and the driving technique required to correct it is quite unique. Extreme examples of understeer, are high powered front wheel drive vehicles aka SAAB Viggen “goes like a bullet, handles like a lead tipped arrow”, the reviews said. The old Vee –Dub, and the infamous Chevrolet Corvair. as good examples of oversteerers. Both had a pronounced weight bias to the rear. The Chevvy Corvair resulted in a book called “Unsafe at any Speed” by Ralph Nader, which lambasted the American auto industry for manufacturing such a vehicle.

Vee-Dub pressures were set at up to 30 p.s.i. on the rears, and as low as 17 p.s.i. on the fronts, to help curb the excesses of oversteer. In other words, adjusting tyre pressures can affect the steering characteristics of your car.

So if it’s so desirable, why would you want to change an understeering car. Some drivers prefer that the car be “balanced”, and might even go to the extreme of buying a mid engined car such as a (later model) Toyota MR2, or choose a Alfa Romeo with 50/50 weight distribution, front to back. Using reverse logic to the Vee-Dub, you can make an adjustment to your tyre pressures to reduce the understeer on your family sedan, or small F.W.D. An extra 4 p.s.i. (30 kPa) in the front tyres, and lower 4 p.s.i. in the rear tyres, will change the steering characteristics of your car. Of course, the usual disclaimers apply. You must always have enough air in the tyres to carry the load, so don’t do it when the car is fully laden for your vacation.

Other performance vehicle makers will tailor the handling by having a wider tyre on the rear, maybe even a lower profile, so that the rear will always out corner the front tyres at the same slip angle, and won’t overtake the front of the car in a spin, which is very embarrassing, even dangerous for the driver. Yet they only provide one spare wheel!


December 12, 2006 at 12:02 am Leave a comment

Mag Wheels

Many cars today come equipped with “mag wheels’ of aluminium alloy. Makes the car look sporty, has a hint of prestige, and the variety of designs sets them apart from the pack. But there’s another reason why they are favoured by car designers and engineers. It’s called the MacPherson strut front suspension.

The late Colin Chapman , designer of the Formula 1 winning cars of the sixties, chased down the sources of unsprung weight on his racing cars to extremes. ”If it didn’t break, it was too heavy”, was a statement attributed to him. Unsprung weight is all the components that go to make up the suspension, brake, and drive components. The remainder of the car sits on springs. Springs oscillate at particular frequencies, so that if a vibration is fed into a sprung system, the oscillations can compound and become quite noticeable, even destructive. For example, the low speed vibration caused by an army of marching men on a long bridge, can get quite a rock up in the bridge surface, so they are told to break stride. The amplitude of the oscillation generated by something out of balance, or out of round, feeding up into the sprung suspension of a motor car can cause a vibration felt by the occupants-the vibration resonates.

The late unlamented Falcon XB series had a vibration at 117 km/h, which became quite noticeable. Like many modern cars, it too had a MacPherson strut front suspension. Keep in mind that the wheel assembly had already been balanced off the car, yet there was still a vibration. Much development work went into developing a fix. It was realised that the steel wheel, centred on the hub by the wheel nuts, was one of the causes of imbalance. So the bore hole in the wheel was used to locate the wheel centrally on a machined hub, the wheel nuts being relegated to their real role of holding the wheel on. Today, this hub fit can be quite tight, and if the wheel has been on a while, hard to dislodge.

Investigation then turned to the wheel itself. A steel wheel is heavier, stronger, particularly laterally, than a modern alloy wheel, though alloys have stringent safety standards to meet for lateral impact resistance. But being a punching made in a die, with the hub assembled into the rim in a secondary process, there exists a greater potential for it to be out of round than a mag wheel. If the press die is out of round, the wheels are out of round. Mag wheels are cast, then machined on a computer controlled machine, which drills the bore hole and machines the bead ledges in the one operation. The machining tolerances can be set much tighter than for a steel pressing. Also, being lighter, the spinning mass may not generate a noticeable vibration until the car reaches a higher speed- you may never notice it.

So that got rid of two sources of vibration. That only leaves the tyre, which is a composite lay-up of many components, with plenty of scope to be out of round, and out of balance. This will be subject of another article. However, once the car engineers tracked these sources down, tyres came under even greater scrutiny. Car design engineers specialise in tracking down these sources of noise, vibration, and harshness. It’s a very specialised world of engineering.

As to Colin Chapman’s mag wheel, the first time you pick up a real magnesium alloy wheel from a true race car, you will be very surprised indeed to find out how light the whole assembly is! However, it will never be subjected to the everyday abuse that a road wheel will be subjected to, so it is not surprising that it is much lighter.

A final word on cleaning mag wheels. Some are coated with a clear protective finish, some are not. Using a specialised cleaner to spray on, hose off brake dust is recommended for periodic maintenance and detailing but make sure that you buy the correct type of cleaner for the wheels on your car.

December 12, 2006 at 12:01 am Leave a comment

Alignment – Tell me a story

Your worn tyres will tell an experienced tyre man a great deal about the mechanical condition of your car, and the way you drive it.

Tell-tales on a rear wheel drive are rear tyres worn in the crown (hard driving), rear tyres worn on the inside only (I.R.S. camber /load relationship), and tyres worn scored or smooth-surfaced (jack rabbit starts or not).

Front wheel tyres worn excessively on the shoulders show up misalignment, fast cornering, and underinflation. That’s just some of them.

Front wheel drive cars are different. The front tyres can wear nearly twice as fast as the rears, and can show wear in the crown, and excess wear on the shoulders, at the same time, for all the reasons listed above.

Yet the salesman will spend much more time trying to sell you the flavour of the month, and then recommend that you have a wheel alignment after new tyres are fitted.

What are the benefits? After all, alignment can cost quite a bit, and it might seem hard to justify. For the dealer, the installation is a big outlay in both equipment and training for the operator.

However, tyres are an expensive item, so here are some of the benefits:

  • The car is easier to drive and steer, so long trips are less tiring.
  • Tyre life is increased, particularly if the alignment on the previous set was out of whack
  • Restoration of the steering angles designed into the car will make it handle the way the designer intended.
  • You’ll get your money back in extended tread life, provided of course that you don’t indulge in the improved handling and cornering that result.
  • Adjustment to your suspension camber can be made to suit the way you use your car. For example, suspension can be adjusted if the car has a high negative camber for fast cornering, but is used mainly on freeways. Aftermarket kits are readily available to permit these changes.
  • If you’ve had a bingle, make sure that your car is completely aligned before you sign the release form from the panel works.
  • If you have kerbed your front wheels badly, it will pay to have them checked.
  • After any front end work on steering or drive components, alignment is highly desirable.
  • Ask the tyre salesman before he launches into his spiel, to assess the wear pattern on your tyres, and advise him how you actually use the car, before he makes a recommendation on the best tyre for YOU.

See also the following article on misalignment to the thrust line of the vehicle, particularly on front wheel drives.

December 11, 2006 at 11:56 pm Leave a comment

Tyre Pressures

Every new car sold in Australia has a “Tyre Placard’ fixed to the vehicle, generally on the front driver’s side door pillar or door. Information is given there on what tyres are permitted to be fitted to the car, and pressure requirements for varied operating conditions, such as basic, high load, and high speed. Dependent on which model of the car, these may vary. Maybe the luxury version differs from the turbo-charges sporty version- at least you would hope so!

The Australian Design Rules compel manufacturers to fit tyres which match or exceed the performance of the car under various load and speeds. The tyres sizes fitted today have more than adequate load carrying capacity. Fashion has taken care of that, as car buyers like to see the wheel well full of wheel equipment, not the mechanical underpinnings.

From the mass of the car (see handbook), split it into four, consult the Tyre and Rim Standards, and it will reveal that the placarded tyre pressure comfortably exceeds those specified by the tyre makers, if load was the only consideration. Vary the speed, add in the lack of maintenance of tyres generally, and the need for a buffer over the minimum pressures specified is apparent.

The car engineers also want to get the ride and handling package right for the car buyers. The tyre is contributing to the total suspension package, and adjustment to pressures contributes. Higher pressures give better fuel consumption too! There is also the question of reliability and durability to be considered.

The placarded pressures are set for the fully fuelled vehicle with two in a two seater, three occupants each 68 kg in a five seater. Higher load means for five occupants and their luggage, which weighs 13.6 kg for each seat. The weight of passenger and luggage are defined in the Standards. These increased loads are compensated for by higher pressure.

The speed factor cannot be ignored. A higher speed rating on a tyre wall implies that you can go faster, with appropriate pressure increases. But the law won’t allow it is the cry from all but the Northern Territory. What it really means to the motorist is that the premium tyre has design features which resist degradation due to heat, and deformation due to speed or overload. It holds together better under duress.

The Tyre Standards represent the accumulated wisdom of car and tyre engineers over the years, and should not be ignored. When in doubt, or if the car is going to be used under more severe conditions than the basic levels outlined, tend towards the higher end of the pressure ranges specified on the placard.

December 11, 2006 at 11:48 pm Leave a comment

With Rims, Inches rules!

After flirting with different systems, tyre size codes have gone almost completely metric. There are still a few relics around in R.V. and aircraft tyres, based on inches, but passenger tyres are coded in metric. All except the rim size, so really it becomes a hybrid of both. Why didn’t they get rid of the inches then.

The answer, is that there are just too many inch rims to change. For example, U.S.A. makes over 60 million rims a year, and they last forever. Rims are used all over the world, from Afghanistan to Zagreb, ending up on all sorts of vehicles. They all rely on the fit between tyre and rim, which is designed to tight tolerances to hold it on. It just got too hard to effect the change. Conservatively, it would take 50 years or more of confusion and danger to change the world over to all metric. It was easier to call it a “nominal rim size” in inches.

There are in fact, metric rim sizes, which are designed to be used with only two types of tyres- Michelin TRX and its derivatives, and Dunlop Denloc run flat tyres. These are specially matched tyres and rims, of different diameters to the inch norm. They must NEVER be fitted to inch coded rims.

The rim diameter is known as the “nominal rim diameter”. A fourteen inch rim doesn’t measure fourteen inches anyway, because of the designed interference fit between the tyre bead and the rim ledge. It is measured only with a specially designed ball tape, because the rim ledge is a taper, and dependent on just where the measurement is made on the taper, you yet a different result, so a tape measurement is useless. For example, a 14 inch rim (nominally 14) has a specified diameter of 13.968 inches, with a plus and minus tolerance from 13.9838 to 13.9523 inches. The rim taper leads the tyre bead up to a radiussed flange which stops the bead going any further.

There is a further complication. So that the bead will not unseat readily if deflated accidentally or purposely, a “safety hump” is placed inboard of the flange, of diameter just below the final specified ledge diameter. During fitting, the tyre is inflated until the pressure is sufficient to slide the lubricated beads over the hump. Placed far enough down the taper to allow the beads of the tyre to seat on the ledge between flange and hump, it holds the deflated tyre in position hopefully long enough for the motorist to slow to a halt.
The logic is that you can still steer the car until the beads become dislodged, after which it becomes uncontrollable.

While it is possible to design a tyre/rim combination to pass this bead unseating test (see Dunlop Denloc above), the easiest way is to incorporate these safety humps. The bead unseating test is a requirement of the Australian Design Rules, and the rims used in the test procedure do not have a safety hump- the tyre is a tight fit anyway.

As with all things engineering, an unfavourable combination of tolerances can result in a very tight, or very loose fit between tyre and rim. Some tyre makes have the reputation amongst fitters of being difficult to break the seal. The design manual for the engineers from that particular tyre maker may specify a tight squeeze of the rubber and fabric between bead hoop built into the tyre, and the rim. Others are a little more relaxed about it. Different styles, or applications even within the one make, may be tight or loose, dependent on how the tyre is to be used.

December 11, 2006 at 11:40 pm 2 comments

Original Equipment

Tyre manufacturers chase the business of having their tyres fitted to new cars very assiduously. Of course it helps if you own a large slice of the car company, as does one such company, but most compete strenuously for the business.

This because they know that further down the track, the car buyer will have to replace his tyres, and they can have a profound influence on his or her choice, particularly if the customer has ‘had a good run’. Besides, there is the compatability of the (probably) un-used spare with any other tyre, the calibration of the A.B.S. braking system, compatability with the inbuilt traction control sensors, and many other factors to be considered. To the customer, it might just get too hard, so they take the easy choice first.

Important to the tyre manufacturer is that the car company requirement helps build the volume of that sized tyre, which spreads the development costs over a greater number of units.

So when you come to replace your tyres, what do you do?

If you’ve been reading our panels of information, you know that there is a wide choice of tyres available on the Australian market. What do you do about the spare? Leave it there? If you do, the next time you have to replace tyres, it will be six years old, not three. You will be missing out on six years development and improvements in tyre design. It will be a little crazed in the sidewall rubber from the heat in the boot, and the rubber will be harder and less flexible. So it seems like a good idea to use it, by matching it with an equivalent tyre, if you can still buy one the same. Tyre fashion changes too, you know.

Is your car front wheel drive or rear wheel drive? If FWD, the front tyres wear out twice as fast as the rears, normally, because they are doing all the work- steering, braking, driving. The rears hold up the corner, and come along for the ride. So probably you are up for two new tyres on the front first.

A matched pair of tyres on the one axle won’t upset the calibration of your traction/braking systems. If one tyre is worn out, the other part worn, buy two, put the part worn tyre in the boot or the garage. It’s false economy to only put one new tyre on an axle when the other is almost worn out. The new tyre rides better, the drive system is balanced, and the alignment is as the designer intended. There is also less risk of punctures with new tyres.

If your car is rear wheel drive, dependent on the type of rear wheel suspension, and whether you have diligently rotated your tyres in either a 4 wheel or five wheel rotation (including the spare), you may find that you are up for five new tyres at once. Shock! Horror! That blows the budget! Most of us leave the spare in the boot though. So wear is much more evenly balanced around the car, and it is likely that you have the choice of two or four tyres to be replaced immediately.

It would be a rarer circumstance where you replace only one tyre. That would likely be due to damage. If the remaining tyres ‘still have plenty of meat on them’, you could assume that one is the spare, so less worn, so match the new tyre with that on the same axle. The one that comes off goes in the boot, but pump it up first to close to the maximum so that you can forget it till you need it next. Everybody else does! Just be aware that it will probably be on a dark and stormy night in the rain, because that’s when you are most likely to get a puncture.

December 11, 2006 at 11:38 pm 1 comment

Tyres hang around

There are only so many kids’ swings or white garden swans that you can build from discarded tyres. Worn out tyre disposal is a world-wide problem, so you are not alone with those secondhand tyres you’ve stored in the garage for a rainy day, which you will probably never use. What to do with them? Where do they go when you load them on the trailer for a trip to the tip, and find out that the tip won’t take them. That’s why they figure so prominently in the news after every “Clean Up Australia Day”. Don’t add to the problem!

Australia discards around 12 million, plus or minus a million, every year. U.S.A. probably 70-100 million. Outside L.A., there is a mountain of tyres one mile square, and last seen over 200 metres high. An absolute haven for snakes, rats, vermin, and mosquitoes. The owner of the site charges tyre services which flock to his site, for the privilege of dumping them there. He is convinced that one day, some entrepreneur will find a use for them. Meanwhile, it’s a huge fire risk. The bureaucrats of the European Common Market have just awakened to the fact that they have an even bigger problem. One enterprising truckie in N.S.W. established his own dump in the bush on the Central Coast, and was sprung when that caught fire, which took months to extinguish at his expense.

Recently, dumping of a poisonous European petrochemical refinery by-product in Niger in Africa, highlighted the trade in by-products that no-one wants. Cardboard and paper, and all the recyclable plastics, find a ready trade, mostly to China, where they are sorted and recycled. Unfortunately, there is no major source of disposal for old tyres, and the problem grows apace.

Another promising disposal tried was as a marine reef, offshore, to break up wave action, and provide a harbour, or lessen erosion. Great efforts were made to lock the tyres together with concrete blocks, chains, anchors, and anything heavy. However, the power of the sea cannot be denied. After a few years, the reefs broke up, and tyres ad infinitum were dumped on the beach, to be picked up again. Geelong in Victoria, Australia was one such trial, another you might like to read is about a reef established with the best intentions in Florida.
It’s the shape of tyres that causes one of the problems. They contain a high volume for their weight. The other reason is that the chemicals used in rubber to prevent it perishing in sunlight and weather, also are very persistent, and prevent marine organisms from attaching themselves to the rubber.
I well remember being asked to identify a tyre dredged up from Darwin harbour in 1976, as a curiosity. It was still inflated, and attached to an artillery piece. The romance of the story fell in a heap when the serial number disclosed that the tyre had been made two months after the war ended. The tyres and the gun had made it to Darwin just in time to be dumped in the harbour, having no further use. The point of the story is that the serial number was still quite readable 30 years later, no decomposition having occurred.

Let’s look at the possibilities.
Because of their shape, tyres are difficult to dispose in land fill. They don’t rot, and trap air inside the casing, so that it is difficult to consolidate the ground. After a few years, they tend to work their way to the top again. When split into two halves circumferentially, and the two halves laid on top of each other, this objection is removed. However, it requires a special machine which has to be capable of cutting through two layers of steel under the tread. You can fit a lot more tyres on the truck for the trip to the tip, though.

A better approach is to cut the tyre up into chips of rubber if they are to be used for landfill. This is slow, and takes a lot of power in a heavy, robust machine. Once chipped however, other processes can be applied to recover the steel, to chop it up even finer for use as playground surfaces, bowling green surfaces, in road asphalt, and the like. It makes asphalt more flexible, less likely to crack in snow country. But the outtake volume is just not there.

It’s a terrific fuel. Rubber tyres burn beautifully, hot, and smokily, which can be corrected by furnace design. But rubber is cured using sulphur. There is a huge push to clean up power station and diesel engine emissions of sulphur, by cleaning up their fuel. Oxides of sulphur emitted from their exhausts combine with elements in the atmosphere, to cause “acid rain”. So that’s out. Experiments have been done on distilling the tyres in retorts to recover the oil. The economics aren’t there either, particularly if you can’t heat the retort with old tyres- a Catch 22 situation.

Tyres can be retreaded if they pass stringent inspections to ensure their reliability. It is entirely dictated by economics. To retread a tyre for a second tread life is not unusual. Consider however, that the modern steel belted radial has already done twice the tread life of a bias ply tyre, when retreading of bias was commonplace. Retread it, and you expect four times the casing life of the old bias ply tyre, with 100% reliability. You are ‘stretching the envelope”. Realistically all you are doing is delaying the day when eventually the casing has to be disposed of, though you have reduced the demand for a new tyre.

So your tyre dealer will ask you for an “Environment Fee” to take your old tyres off your hands. A truck calls round to his tyre service, sorts the retreadable casings” from the scrap, and drives away, bound for a municipal tip that still takes tyres. Their number is reducing too. You could ask which tip he uses, if you want to dispose of them yourself.

So arise all you entrepreneurs, and put your thinking caps on. It’s not only Houston that has a problem of monumental proportions.

December 11, 2006 at 11:35 pm 1 comment

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