Building a Scania R500 - Part 6. Tyres

I`m building a Scania R500 - from a 1:24 scale kit produced by ITALERI. 

I`ll be looking at aspects of truck construction as I go along, hopefully highlighting some basic truck technology.  It`s easy to think that modern vehicles bear no resemblance to those of the past, but that`s not true.  Suspension and steering; engine, transmission and final drive; and tyres are all there to maximise the laws of physics, and have retained the same configuration since diesel replaced steam.  Cost and natural performance limits have meant that the chassis abandoned long ago in car design is still used in truck manufacture.   Yes, your truck is computer controlled and a modern marvel;  but so is the modern cruise liner - which is still a Titanic underneath.

Building a Scania V8 R500 Part 6 - Wheels and Tyres

Tyres are probably the most important component of any vehicle.  As strange as it may seem, like all that electronic wizardry, tyres are complicated bits of engineering. They also comprise a number of different materials.  All the digital technology and hardware that`s now a part of any truck, works on small patches of rubber in contact with the ground. 

Tyres will have a number of different rubber (elastomer) compounds and, maybe, rayon or nylon used for the ply, as well as steel cords. With tyres, the critical points are:

• Most modern trucks use tubeless, radial tyres fitted to one piece wheels
• Two or three piece wheels are usually used with stiff, cross-ply tyres for ease of fitting. (These wheels are found on military trucks used in the field or those operated in remote areas where tyre fitting has to be done by the vehicle's crew.) Wheels split in this way need inner tubes
• correct pressure is very important - heat destroys tyres, and it has to be created.  In short, low pressure shortens a tyre`s life and uses fuel
• correct fitting and balance ensures correct service life. 
• cross-ply construction is not shown on a side wall.  Radials are marked R or radial (bias-belted, B or bias) 
• Inflate a newly fitted tyre in a cage or a van - tyre fragmentation at 140psi can kill
• bits of tyre on the hard shoulder of a motorway are not necessarily from remoulds, they will probably be from a poorly maintained tyre that`s got too hot 
• Aspect ratio or profile is used to alter a tyres performance.  There are disadvantages to low profile tyres but the advantages are many.  These include: load bearing, handling and weight.

Wheels fitted to most modern trucks are one piece construction and dished between the rims (wells).  The dish shape ensures that tyres can be fitted, i.e. one bead is dropped into the well, allowing the tyre`s other bead to be brought across the wheel rim.  In the past, with very stiff cross-ply tyres, two or three piece wheels had to be used.  Some special applications still use split rims or two piece wheels for ease of fitting.  However, these are usually on vehicles used out of the range of a professional tyre fitter.

Radial ply tyres are found on all road vehicles these days - even motorcycles and trucks, where cross-ply tyres or bis-belted tyres were once used.  Radial ply is at 90 degrees to the bead (straight across inside the tyre`s rubber) and runs cooler than cross-ply construction.  Heat uses more energy, so creates higher rolling resistance, and destroys tyres.  Radials last longer and use less fuel.  They are not as tough as cross-ply tyres, so are not always used on heavy duty/rough terrain applications.  Radials handle most dynamic forces better. 

Side wall markings include size information in metric and imperial.  This radial (R) tyre has 22.5 inch wheel. Lower aspect ratios have many advantages including better handling but they also reduce the suspension effect of the tyre.  With trucks they are sometimes used simply to lower the overall height of the vehicle. Other side wall markings may include European load index numbers and speed index letters.  US weights are also shown on some tyres.  A DOT stream of numbers relates construction and ends with a number that indicates the tyre`s age. E.g.  2012 means the tyre was made in the 20th week of 2012.

Aspect ratio (profile) is a very important part of a tyre`s characteristics.  Care should be taken to match tyre sizes properly as increased stress can be induced in incorrectly paired units.  When radials replaced cross ply tyres, the advantages were similar to those now offered by lowering profile.

Aspect ratio is a simple calculation

Next:  Part 7 -  Brakes

Stopping

I read a magazine article this week where a journalist attacked the stopping distances quoted in the Highway Code.  Modern braking systems and tyre compounds, he claimed, have shortened the distances in which we can bring our vehicles to a halt.  The whole thing, he seemed to be suggesting, was so out of date it was next to useless.

Yes, I thought, braking distances for some vehicles may have shortened, but the overall picture is not quite that simple.  The Highway Code may be old in detail but the general message still holds true.  Here`s why.

The Highway Code uses a short reaction time (0.7 seconds). 

The distance a vehicle will travel while the driver reacts lengthens as reaction time increases.  Most drivers take a little longer, in fact on a motorway reaction times can reach 1.5 to 2 seconds. 1 second is considered a reasonable representation, but by no means average.  Coefficients of friction between the tyres and the road have increased with modern compounds and construction.  The Highway Code uses 0.65, which is no longer typical for most cars - they can now exceed 0.8. Many trucks and buses, however, still have tyres that only achieve about 0.65. 

A speed of 30mph is used for the calculations above. The deceleration rate and reaction time calculated is the same for all speeds quoted in the Highway Code.  The figures for s, reaction distance of 9 metres and braking distance of 14 metres, are taken from the code.



The Highway Code stopping distances compared with those representing greater tyre friction coefficients and increased reaction times.  

The end result is similar stopping distances.  It should be noted that impacts that occur at the same point along these distances will vary in speed between the two sets of graphs (e.g. at 50mph, the Highway Code figures show the vehicle braking before 20m has been covered. With a longer reaction time, the driver doesn`t start braking until after 20m).  The dotted line on the Highway Code graph illustrates the squared relationship between energy (work) and speed. The braking distance (red) at 60mph is about 4 times that of 30mph, despite the speed being only doubled. 



Building a Scania V8 R500 - Part 5. Drive

I`m building a Scania R500 - from a 1:24 scale kit produced by ITALERI.

I`ll be looking at aspects of truck construction as I go along, hopefully highlighting some basic truck technology.  It`s easy to think that modern vehicles bear no resemblance to those of the past, but that`s not true.  Suspension and steering; engine, transmission and final drive; and tyres are all there to maximise the laws of physics, and have retained the same configuration since diesel replaced steam.  Cost and natural performance limits have meant that the chassis abandoned long ago in car design is still used in truck manufacture.   Yes, your truck is computer controlled and a modern marvel;  but so is the modern cruise liner - which is still a Titanic underneath.

Building a Scania V8 R500 Part 5 - Driveline

All vehicles use diesel (DERV - diesel engine road vehicle) power.  Modern engines utilize extremely high, common rail fuel systems, where injectors are opened electronically - as opposed to the older, lower pressure systems that used fuel pressure created by the injection pump to open injectors.  Exhaust gas driven turbos, used to increase are mass in the combustion chambers, are common.

All gearboxes are conventional, synchronized, manual types but many have automated clutches.  Manual boxes are often 4-speed with additional input and out put ratios (range-change and splitters).

After the gearbox, the driveline comprises: universal (Hooke`s) joints; splined joints; differential; half-shafts:

Prop-shaft has a universal joint (UJ) at each end.  And a splined joint somewhere along its length

Looking at the UJ at the back of the gearbox.

The centre of the UJ, the centre cross.  The joint is based on this small component.  UJs are needed to allow for change in direction of the prop-shaft.  A UJ is not, however, a constant velocity joint. 

The prop-shaft angle will change as suspension deflects, so its effective length will change in addition to its angle.  A splined joint allows for this extension.

The `diff`is the housing that contains the final drive crown wheel and pinion gear as well as a differential.  Differentials are necessary because the rear, driven wheels will not always run at the same speed (when the vehicle is cornering). Bevel gears rotate in the spider attached to the crown wheel.  Half shafts are splined into the spider.  If one half-shaft slows compared to the other, torque is transferred to the other shaft, via the bevel gears.

Next  Part 6 - wheels and tyres

Wave Goodbye

If there`s one thing that should make even the most ardent atheist reconsider their position, it`s quantum physics.  How can something so remarkable and inconsistent with our every day experience not be the work of a higher being? Quantum physics is not some abstract theory  despite being concerned with particles so minute we can only observe them through special microscopes  - quantum physics is about everything.  What it says is that the very building blocks of our world, the atoms that all things are made of, are themselves constructed from particles that we are not allowed to fully understand.  And there is not a scientist living or dead that really knows or knew why this is the case.

Electrons are just one particle in the structure of an atom and a comparatively small part at that.  Apparently, they whizz around outside the atom`s nucleus in what is mainly empty space.  I read once that in a scaled up version, an electron would be a speck of dust in the dome St Pauls while the nucleus would be a pea suspended in the middle (well, it was something like that; I`m sure you get the picture). The important thing is that with electrons, as with other sub-atomic particles, you cannot know where they are at any instant in time if you also want to know their direction and speed. You can have either one or the other, but not both.  There are rules it seems about just how much we are allowed to know of our existence.

What we do know is that electrons can behave like both waves and particles but have the absolute properties of neither.  Experiments have shown that they can exhibit wave behaviour but then change to that of a particle when an attempt is made to observe them.  Waves create distribution patterns that show constructive and destructive lines.  Imagine two small openings in a harbour breakwater.  Waves crashing through them fan out in the harbour and the peaks and troughs from one set meet the peaks and troughs from the other.  The peaks that come together create very high peaks and the troughs that meet peaks cancel each other out, resulting in flat water. When this `interference` pattern reaches the harbour`s inner wall the super high peaks splash violently up against it, but at intervals and spacing along the wall dictated by the wavelength and amplitude that has been formed.  It would be the same with light waves passing through two slits cut in a piece of card and shining on a screen behind.       

Remarkably, sub atomic particles will behave in the same way but with a surprising result when any attempt is made to look at their movement in detail.  Scientists have fired electrons at the slits and found a wave pattern, but when they tried to discover through which slit each electron had passed, the behaviour reverted to what you would expect from particles – that similar to firing bullets through slits and where no interference pattern is formed.  Even single electrons, fired at the slits undetected, create interference wave patterns. The totally unintuitive conclusion is that, when we are not trying to peek at them, each particle must be considered to be passing through both slits at the same time, and only predictions based on statistical information can be used to estimate their position, direction and speed.  The material of the universe, it turns out, is not constructed in quite the way we once thought.  There are many theories emerging from this discovery, not least the uncertainty principle and its implications for probability.

Up until the discovery of the quantum nature of matter, physical systems were considered deterministic – everything could be predicted or retrodicted.  Probability was associated with error margin and precision; a normal consideration when accuracy was discussed. As we know it, any variation in precision is caused by the quality of the available data – error is often considered a manmade limiting factor.  Quantum theory, though, says that probability is a natural phenomenon and that there is no alternative to its implications.  With quantum physics a particle can be in more than one position until some effort is made to see it; then it becomes real, well in our interpretation of the word.  A famous thought experiment put forward the notion that a cat in a box with a phial of poison could be both alive and dead until the box was opened to reveal whether the phial had broken or not. 

The proposition of the multiverse has since emerged: multiple universes all existing together, each resulting from the many alternative paths suggested by quantum theory. As philosophy seeks to explain our experiences, so physics describes the world that shapes them.  Looking at the past and into the future there are many alternative possibilities of reality; personal consequences following on from physical effect. In the end, only this instant, `the now`, may be true and everything else merely a range of potential states of being. Isn`t it true that our memory is never as black and white as we imagine and history is merely a collection of recollections that together produce a likely account? The image of the past may seem clear but that doesn`t mean it`s accurate.  What is looming on the horizon can often be predicted with some level of certainty but it can never be guaranteed; the expected, sometimes obvious, is not always the result.  In what may seem like a wave of happenings, life flowing from one second to the next, there may be a number of alternatives to our individual perceptions.  Don`t be fooled by the apparent clarity of your assumptions or apply unwarranted colour to your expectations; in the mechanics of all things, certainty does not exist.

Beauty is in the 'I' of the beholder

The Daily Mail journalist Samantha Brick`s claim that her beauty makes life difficult, not least because other women become so jealous they end up disliking her, created an enormous amount of vitriol - and interest.  Some questioned how a woman who doesn`t appear to be anything special could have the arrogance to imagine herself beautiful.  Others thought such delusion must be the result of subconscious low self esteem. When the initial furore had subsided, the debate continued with many people agreeing that exceptional physical attractiveness was not always easy to live with, and that some women felt threatened by those who were beautiful.  Brick`s comment certainly made us look closely at the subject of appearance.

The whole affair has exposed our fascination with all things we judge to be good looking, and highlighted something about jealousy that means this is not about Samantha Brick: it`s about us, and when we look at another person we are simply making comparisons with ourselves. If someone does everything within their power to be physically more attractive, all that results is that others look in the mirror more closely. Money on clothing, cosmetics and hair styling is spent so others can ponder their own worth and significance.

We are surrounded by items of beauty that provoke similar emotion.  I`ve often thought that by buying an expensive new car I am simply providing a moving display for others to fantasize over.  They are not going to look at me, sitting there behind the wheel, and think, I am pleased for him, but instead imagine themselves behind the wheel, and what it would be like to own such a car.  All I get in return for my money is a view of the dashboard – and one dash looks very much like any other, believe me.  The real beauty will be appreciated by those outside, looking on.

Truck designers place great emphasis on the image their vehicles present to the world.  And although they use basic symmetry to achieve the looks we find most attractive in people, there`s a lot more going on. Just like fashion can dictate hair styling, the use of facial studs and other adornments, truck appearances change to reflect contemporary attitudes.  In the modern world of full control, where every second of the day has to be accounted for and everything is expected to work to maximum effect, every moment presents a challenge.   

With so many demands and so much complexity, the reaction is obvious: everyone`s on the attack.  We mirror confrontation and become confrontational ourselves. All around this attitude is interpreted as normal, the natural consequence of our new systems.  You can see it in Scania`s R800 and its twin, the new Actros. The word in truck design at the moment seems to be `aggressive`, a physical representation of the new order.  These are the machines of the digital age; a compilation of discrete shapes coming together to complete a task, like the computers that run them. It would be a brave creation that went against such a trend; anything outside the current vogue for confrontation would surely be seen as unattractive.  We want to see something in the trucks appearance that we imagine we need to see in ourselves.

Samantha Brick need not worry about the reactions of others to her looks. It`s not her they are worried about, it`s themselves.  But if she is still concerned, she can always create an image for herself that no one else would find desirable.  Reflect something no one would wish to see in the mirror. Even if natural beauty is used as a base to build on, cosmetic changes are easily achieved.  If the current thing is long blonde hair and business suits, she could cut her hair, dye it black and wear an afghan coat.  But even then it`ll probably all be a waste of time and money, and she would be better off staying as she is.   She`ll soon be replaced in the “I” of others.

Friction with the Law

Knowledge, as they say, is power.  And I can`t think of a time it is more important than when you`re driving a vehicle that can weigh up to 44 tonnes, be over 18 metres in length, has by far the largest proportion of its weight temporarily attached and, when subject to excessive side force, has a tendency to roll over rather than slide sideways.  Just a little extra understanding of what`s controlling all that weight will help keep the load secure, the truck upright and you out of the mire.  With the right tools you`ll be able to meet any challenge with confidence, argue your case if necessary and act appropriately when something is about to go wrong.  So, when it comes to vehicle and load stability, all you need are: a copy of the Department for Transport document, Code of Practice, Safety of Loads on Vehicles, which can be downloaded from the VOSA website; a pair of furry dice; and one of those photocopies of your backside you get by dropping your pants and sitting on the copying machine. 

Load restraint and vehicle stability are intrinsically linked - just think of what goes through your mind every time you enter a bend a bit quicker than you intended – and there are laws that govern them both: the laws of physics and the Road Traffic Act.  The laws of physics tell you that the force needed to get the load sliding on a trailer or truck bed is proportional to the weight of the load itself.  The size of this proportion is determined by the friction properties of the surfaces in contact with one another - wooden pallets, concrete slabs, sheet metal, carried on a wooden bed, all have different friction values. Similarly, the force needed to topple a load is in proportion to its weight; although now dimensions and how they correlate to the height of its centre of mass become important.

The way to make the load less likely to move is to increase the force pushing down on it without increasing the mass of the load itself; in other words, push the load onto the bed with something other than its own weight. And this is where the dfT`s Code of Practice comes into its own: not only does it tell you how to do this (with straps etc) it also represents the official line on how it should be done.  In one document the laws of physics and the law of the land are covered: the load is secured properly, so nature can`t shift it, and it`s been done according to government guidelines, so an enforcement officer can`t criticize it. 

The Code also has a very telling diagram that shows a truck and the amount of load restraint required, forward, backward and to the sides.  The load, it says, has to be secured for a minimum of its full weight forward but only half of its weight to the sides.  What the diagram is doing, although not intentionally, is illustrating the relative accelerations an entire truck can endure before it becomes unstable. It`s an interesting fact of the relationship between the forces acting on a body (in this case, a truck) that in some aspects of stability only accelerations need to be considered: the acceleration due to the force of gravity (g) and the accelerations due to forces created as a vehicle brakes or travels through a curved path.  The diagram is saying that braking force can be quite high and vehicle deceleration may approach the value of g, so loads have to be secured adequately.  However, side accelerations of 0.5g are often enough to destabilize a truck, so a load secured for half its weight is unlikely to move before the truck rolls over. 

And it`s the problem of rollover that bring the furry dice into play.  The vehicle will be subject to the same accelerations as the furry dice hanging in its windscreen, despite their weights being so different.  By measuring the angle that the string the dice hang on makes to the vertical, an estimate of side acceleration compared to the acceleration due to gravity can be made.  It`s how, after all, a tilt test is conducted for static stability.  A vehicle is placed on a tilting bed and the angle is found at which the upper wheels lift.  The angle varies for different types of vehicle, of course, and the test doesn`t allow for the `in use` characteristics of load or other important dynamic considerations.  Increasing the height of the centre of gravity, for example, would reduce both the angle and the vehicle`s stability.  But from the angle obtained, the basic stability is interpreted as a ratio between the acceleration to the side (trying to pull the vehicle over) and the acceleration due to gravity (trying to keep it upright).  

So, if you are unlucky enough to find yourself heading into a bend a little too quickly (because of its relationship with side acceleration, small variations in speed can make a significant difference to stability) and your truck is not equipped with ESP, look at the dice.  If they have swung out to more than say, about 25 degrees to the vertical, kiss the photocopy of your backside and say goodbye.   

Building a Scania V8 R500 - Part 4. Steering

I`m building a Scania R500 - from a 1:24 scale kit produced by ITALERI.

I`ll be looking at aspects of truck construction as I go along, hopefully highlighting some basic truck technology.  It`s easy to think that modern vehicles bear no resemblance to those of the past, but that`s not true.  Suspension and steering; engine, transmission and final drive; and tyres are all there to maximise the laws of physics, and have retained the same configuration since diesel replaced steam.  Cost and natural performance limits have meant that the chassis abandoned long ago in car design is still used in truck manufacture.   Yes, your truck is computer controlled and a modern marvel;  but so is the modern cruise liner - which is still a Titanic underneath.

Building a Scania  R500    Part 4. Steering



The steering for the rigid beam, non-independent suspension system is a simple, time honoured layout. Brake backplates are mounted on stub-axles that swivel on king pins at each end of the axle. Wheels rotate on bearings on each stub axle. 




The steering column from the driver`s wheel works the integral PAS (power assisted steering) box, mounted on the chassis.  The box converts this rotating motion through 90 degrees, working a drop arm.  The drop arm works the drag link, via a ball joint.  This then operates onto a steering arm (upper), which forms part of the stub axle fitted to the end of the axle behind the position of the steering wheel.  The stub axles (left side and right side) are connected to one another by lower steering arms and a track rod

 



Steered wheel angles are important.  KPI (king pin inclination) reduces scrub by reducing drag on the steering and bending and sheer forces on the king pin.  True centre point steering (not always used) is created by ensuring that an imaginary line passing through the centre of the king pin and the centre of the wheel meet at ground level.  Wheel scrub radius is further reduced by using dished wheels and wheel camber.  Caster creates steering self centreing.





The Ackerman set-up  is needed to reduce scrub as the vehicle turns, and is created by angling the steering arms at each end of the track rod.  As the steering is turned, each wheel describes a different angle of turn, ensuring that the tyres travel in different directions according to the distances they are covering (the radius of the inner tyre is different than that of the outer tyre). 



The steering box is mounted on the left side chassis rail (LHD) and a drag link connects it to the left side steering arm, which forms part of the stub axle.

Inside the steering box a piston acts as the power ram for PAS.  A control valve directs fluid under pressure to either side, depending on which way the steering wheel is turned.  The piston rides on a worm gear, driven directly (through the control valve) from the steering column. Recirculating ball bearings reduce friction in the box.  The piston operates onto a sector shaft that is connected to the drop arm.  The control valve uses a cam system, or a torsion bar, to sense the driver`s requirements and regulate the amount of effort given by the PAS.

Next.  Part 5  Drive Train

 

Count Vernacular

For those not familiar with the plot of Bram Stoker`s horror story, Dracula, it goes something like this. The blood sucking count has a captive, naive and incredible population to feed on; from Exeter to Whitby, no one is safe.  But he`s thwarted by a small group led by a crafty professor who uses shock tactics to expose the count for what he is.  By showing the result of the count`s plans, before they can be fully unleashed on an unsuspecting public, the professor rallies support.

Tactically the plan is faultless: exposing, creating alarm and then riding the wave of hostility you`ve created to defeat an enemy.  And morally, despite his slyness, the professor is surely right – blood is life, after all.  Once the initial criticism dies away and everyone realizes the necessity of the professor`s purpose, his plan comes to fruition, the count is defeated and the country settles back into normality.

I recently re-read the book while sitting at home, not wishing to venture out into the maelstrom of panic fuel buying which had taken over the country. It struck me that the professor is not quite what he seems: he`s too nice, too appreciative; he tries to be `all things to all men` and ends up coming across a bit smarmy, as they say around here.  In his efforts to beat the count, he exposes his own ruthlessness and despite all the claims of reason, he`s a bit of a count himself. But at least he stopped the vampires sinking their teeth into us, sucking the life`s blood from our veins and leaving us for dead – well, for now, anyway.