Archive for November, 2006

Retreaded Tyres

The economics of the heavy trucking industry rely on the retreading industry to keep their tyre costs down. Truck tyres are commonly retreaded twice. However, trucking firms monitor their tyre performance closely. Tyres are rotated through steer, drive, and trailer positions, to maximise their life and the number of retreads they can attain.

The casual observer may think that this is not desirable, because of the strips of rubber that can be seen alongside the main trucking highways. But it is not the retread that has separated; it’s the casing components that have separated, which can be seen from the strands of steel wire attached to the rubber tread strips.

Retreading bonds a new tread compound to a freshly prepared surface of the old tread. The process, called ‘buffing’, presents a scored creped clean surface to the new rubber. New retread rubber can then be applied using unvulcanised rubber spirally wound around the tread, precured with a bonding layer sandwiched between the old and new, or unvulcanised strip rubber with a bonding layer on its underside. All systems have their advocates. The bond of new rubber to the old is quite strong, and service reliability of the retread is not usually a problem. Trying to get the last bit of life out of a tired casing is.

Aircraft tyres are retreaded using the spiral wrap hot application process, and six or seven times is quite normal. The casing inspection standards are quite rigid though, as you can imagine.

Nowadays, passenger retreads do not enjoy the same acceptance. Several factors have influenced this:

  • Casing mileage has been doubled with the introduction of steel belted radials compared to bias ply.
  • Motorists’ expectations have risen regarding performance. Motorists realise the difference that a set of good tyres can make to their car.
  • Casing reliability standards deteriorated, considering that the retreaded radial was expected to do four times the life of a new bias ply tyre.
  • The relative costs and efficiency of manufacture of new tyres lowered the cost difference between the two processes. Similar things happened to the engine reconditioning industry.

However, when times get tough and the pocketbook can’t meet the cost of new tyres, retreads are always there, and are infinitely better than a bald tyre once it starts to rain.

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November 30, 2006 at 6:06 am Leave a comment

10 Tips for Tyre Safety

Safety on the road is each driver’s responsibility. Tyre safety is part of the process of maintaining a safely functioning vehicle. While there may be more to the complex design and structure of modern tyres, some basic tips will help you in this quest for safe driving.

The trouble is that tyres are so reliable these days. ‘Fit and forget’, ‘Always in a hurry to refuel’ could describe the motorist’s attitude to their tyres.

Yet they still operate in the same service environment as they always did, so consider some basic tips which might help you get the best out of them.

  1. The design of the valve that holds the air inside the tyre hasn’t changed in 100 years. Their performance is boosted if you do one simple thing- fit a valve cap to keep dirt and dust from under the valve seat, which will cause it to leak air.
  2. Check your tyre pressures around once a month using the same gauge. 50 years ago, motorists had to do this weekly, because tubes leaked 4 p.s.i  (30 kPa ) a week. Nowadays, once every three months with tubeless tyres. But an eyeball check in between does no harm, but it is not reliable. A tyre can be well down in pressure before it becomes readily apparent that it is going flat.
  3. Horse and buggy drivers carried a pocket knife to remove stones from their horse’s hoofs. Even in these security conscious days, it’s not a bad idea, particularly for country motorists, to check your tyres for stones and puncturing objects periodically. Stones can get caught in the groove, and be punched right through the casing over time, ruining it.
  4. Vehicles with I.R.S. wear their inside shoulders on the rear, right down to the steel, without being seen from the outside. The wheel well is full of tyre and rim, and the part you can see, may still have lots of tread pattern. So check from under the rear bumper sometimes if your tyre’s life is getting on a bit. Better still, put the car on a hoist, or have your serviceman check it.
  5. If a tyre keeps going down and there is no puncture and the valve is O.K., suspect the rim! Rims can crack due to fatigue in service, generally on caravans and trailers, because they’re run at maximum pressures.
  6. Wet road performance declines gradually and progressively as the tyre wears down. It doesn’t all happen at once, when the tread pattern wear indicator bars appear. Have a good look at your tyres before the wet season starts- it could be good insurance.
  7. You get more punctures in the wet, in the last 10% of the tread life, than at any other time. This is because wet rubber cuts easier than when dry. So to get that last 10% out of a punctured tyre by repairing it and refitting , might be false economy. You possibly will get another one!
  8. Unidirectional tyres are just that. Unidirectional! You might just have to put up with the noise.
  9. If you’re going for a long, hot trip, the air pressure hose at your service station is your friend. An extra 4 p.s.i. (30 kPa), and it’s free, will give you more margin of safety.
  10. If you do get a puncture, make sure that your repairman strips the tyre from the rim, and has a look inside for signs of casing breakdown (run flat) or secondary damage from the point of the puncturing object. Those plugs put in from the outside are quick, but your safety is more important than his time! And NEVER allow more than one plug in the hole- it’s damaged, and dangerous, if the hole is that big.

November 30, 2006 at 5:52 am Leave a comment

The Changing Shape of Tyres

Sure, they are still black and round. But have you noticed that the tyre and wheel assembly seems much larger than it used to be? Particularly the wheel part! It fills the wheel well in the mudguard. ‘Why is it so?’ asked the professor.

Partly it’s fashion. Vehicle designers don’t like gaping holes under the bodywork, it looks more functional to fill the wheel well, but there really is a good reason.

The ‘footprint’ of a tyre dictates the contact surface it has with the road surface.

Fat, wide wheels have a short contact patch, where the tyre meets the road. In the wet, this short footprint doesn’t allow much time for any water trapped under the tread to be displaced, either to the side, or straight through to the back of the tyre. If to the side it has further to go, also. So the risk of the tyre riding up on a wedge of water, and losing grip, is increased. In the dry, it really doesn’t matter- grip is good anyway. But if the tyre has to be run at a higher pressure, to stiffen the walls and allow high cornering speeds, the footprint is even shorter still. Then if it rains, you can be in big trouble grip-wise.

A long footprint allows more time for the pattern to do its job, displace the water, and allow the tread rubber to grip the road surface, not a film of water. Exactly the same principle applies to the farmer’s tractor. A long skinny tractor tyre footprint will out-pull a big widey, though you must agree that it doesn’t look as macho. There’s just more grip available. The ultimate extension of this idea is the tracks on an Army tank, and that’s what you would call a very long footprint, with so much surface area that the weight of the tank is distributed over a large area, enabling it to travel even where there is no road at all. This is called high flotation, but the idea of a car tyre is to have enough contact pressure to squeeze the water out from under the tyre so it can get down to the ‘nitty gritty’, the road surface.

But what if the road itself is smooth. Unfortunately, this is commonplace, on heavily trafficked roundabouts, at bus stops, and roads that are just plain worn out. Yes, roads wear out. The basalt, crushed rock, or screenings of a local rock, set in a bed of asphalt become worn and polished, and like water smoothed river gravel, (which is the last thing that a roads engineer would use), offer only reduced amounts of grip compared to when the road was re-surfaced. Incidentally, the rate of tread wear slows also. You see the signs ‘Road slippery when wet’, or a yellow and black signed graphic of a skidding vehicle, and you should realise that there is less grip available from the road surface at this particular spot, so take care.

 

This highly desirable “long footprint” is obtained by increasing the rim diameter. This allows the designer to use a wider, low profile tyre, which displaces more water, but without impairing the ability to shift water from under the tyre footprint, because of the longer time available for water displacement.

November 30, 2006 at 5:52 am 1 comment

Installing a Tyre

If you have ever tried to fit a tyre yourself, you will know that it is not easy.

This is because it is not the air pressure that holds a tyre on its rim – nor is it the safety humps on the rim ledge. It’s an interference fit between the beads and the rim that makes it stay there.

The mounted tyre rides over the safety humps, generally with a loud ‘pop’, slides up a tapered ledge, and stops when it hits the rim flange. During this fitting process, the rubber and fabric composite caught between the steel hoop built into the tyre, and the steel or alloy ledge built into the rim, is compressed. It requires localised force to break the seal caused by this compression.

Archimedes said ‘Give me a lever long enough and I will move the world’. Your tyre dealer uses a powered lever instead, called a tyre fitting machine. Air pressure is used to dislodge the beads into the wheel well, and then the steel hooped part of the tyre is levered over the rim flange. Lubricant is used to help the process, and also to stop the rubber bead tearing. If it does tear, the tyre may leak or bubble when refitted.

The choice of lubricant, as simple as soft soap, is basic. Once it has done its job, it should disappear. It’s biodegradable. This is because the part of the bead (the ledge) that sits on the rim is transmitting the power, steering, and braking forces to the road.

The ‘pop’ that you hear is caused by the tyre beads sliding over the humps once there is enough sideways force generated by the inflation pressure, to drive them over. It should happen between 12 and 25 p.s.i. of pressure. Then the beads slam up against the flange, are checked that they are centrally located using guide lines engraved in the sidewall, and pressure is adjusted to road service requirements.

You will also have been charged for a new tubeless valve. This is because the rubber ‘snap in’ valves flex during service. High-speed photos of long valves, or those with extensions to make them accessible when deep hub-caps are used, show them to bend right over and touch the rim at speed. Ultimately they will crack, or de-laminate from the brass core. Some alloy rims may require clamp in tubeless valves, due to service requirements, or the rim thickness at the valve hole. Since these don’t flex, they don’t have to be replaced every time new tyres are fitted, though they cost more.

But wait, there’s more. After fitting, the whole assembly is balanced statically (vertical plane), and dynamically (lateral plane) on a sophisticated machine, which process adds kilometres to the tyres performance, smooths the ride, lessens mechanical wear on your car’s suspension, and reduces the risk of flat spotting.

Finally your friendly tyre dealer will refit the wheel to the car, and replace the rub cap evenly, having kept it clean and unscored by dirt on the workshop floor. Cheap at half the price!

November 29, 2006 at 3:58 am Leave a comment