It`s Time

Why is it that a person, who would never dream of stepping into the path of a 44 tonne truck travelling at 90kph, drives one in fog at a similar speed? Sitting behind that wafer thin sheet of glass and metal waiting for the rear of a semi-trailer to appear out of nowhere – from normal day to death in just a few seconds.

A truck travelling at 90kph (56mph) is doing 25 metres (that`s 82 feet or 27 yards) every second.  Drivers, it is estimated, will have typical reaction times of between 0.6 seconds to 1.5 seconds.  The shorter time is considered pretty fast, an alert driver prepared for something to occur, maybe, so slower times within the range are often more common, although this will depend on a number of factors. Times greater than 1.5 seconds may also occur as a characteristic of some driving conditions, motorway driving, for example, which tends to be monotonous. (Interestingly, The Highway Code relies on a time of about 0.7 seconds for its `typical` stopping distances.)  At 56mph, times of 0.6 to 1.5 seconds represent distances travelled of between 15 metres and 38 metres (49 – 125 feet) as the driver reacts.  These are the distances the truck covers before any brakes are applied.  So, step out 125 feet in front of a driver with a 1.5 second reaction time and the front of his truck will strike you at 56 mph.  The same driver, travelling in fog where visibility is restricted to 125 feet would hit a stationary vehicle at a similar speed – and at exactly the same time his foot buried the brake pedal. 

Of course stopping completely takes longer.  If a truck under heavy braking achieved a deceleration of 0.7g, it would slow at a rate of about 7 metres per second (16mph) every second.  For our driver, stopping from a speed of 90kph, it would take 1.5 seconds of reaction plus the time for the vehicle to brake to standstill.  At a deceleration rate of 7 metres per second, per second, this would mean a total time of about 5 seconds.  In distance that`s 125 feet of reaction plus about 150 feet of braking.  A total of 275 feet (92 yards or 83 metres) to stop from 56mph (90kph) – or 5 artic truck lengths.

Why does a driver travelling 1 kph faster than the truck in front attempt to overtake it, when it takes several kilometres to do so, and such a manoeuvre often has little effect on the total journey time? The same driver wouldn`t think twice about spending 15 minutes idly chatting before going home at the end of the day.

In terms of time, a truck travelling at 1kph (about 0.3 metres per second, or just under 1mph) faster than the vehicle in front will take whatever `relative` distance it needs to pass it, divided by this `relative` speed difference.  The absolute minimum relative distance for the overtaking vehicle is from when its front passes the rear of the other vehicle, to when its rear passes the front of the other vehicle.  Assuming that both vehicles are artics, that`s a relative distance of about 33 metres, which, at 1kph, takes 1 minute and 50 seconds. At 90 kph that`s an absolute distance of just over 1 mile (just under 3 km).

On a journey of 124 miles (200km), a 1 kph difference represents a time difference of 4 minutes.  That`s 8 minutes if the difference is 2kph, or the journey is about 250 miles.

Why do drivers race towards red or amber traffic lights, braking sharply before coming to a halt, when, subsequently, every second they have to wait seems to last a lifetime.

If a traffic light phase, including amber and red and amber phases, lasts, say, 30 seconds, then the less time spent sitting waiting the less frustrating it seems.  Braking at a harsh rate of 0.7g, or 16mph every second, would mean coming to a halt from 56 mph (90kph) in 3.5 seconds, from 48 mph in 3 seconds and from 32 mph in 2 seconds.  But, by easing off and slowing at a much lower rate, an average of 0.2g, for example, which is about 4mph every second, would take 4 times as long as those times shown for 0.7g.  So, 8 seconds from 30 mph, that`s 6 seconds less of sitting, waiting time.

Of course it all depends on the distance available, how far the truck is from the lights when they change, but how often do you see vehicles, all types of vehicle, bowling towards lights that are changing to red.  They could ease off, relax, themselves and their vehicle, and take some time.

Why is it that when we are busy, time passes quickly, but when we are waiting, time passes slowly?

When a vehicle brakes hard, harsh enough to activate its ABS, the deceleration rate can be considered constant. At 0.7g, the vehicle will slow by 16mph every second.  Under these conditions speed and time have a linear relationship – one is directly proportionate to the other.  Work, the energy of a system, on the other hand, is not directly proportionate to either time or speed.  To do more work takes longer, so that speed loss at higher speeds needs greater distance.  To slow at 0.7g from 50 mph to 40 mph takes in the region of 12 metres. But from 20 mph to 10 mph it takes only something close to 4.5 metres.

Could it be the more we work, the more time passes? In our world, a world that seldom exceeds 90kph, time is constant and finite. Each second is the same as the next and time travels in one direction.  Time is not to be wasted by working unnecessarily. It`s time to slow down; It`s time to relax.  It`s time to appreciate time.

Over the Hill and Far Away

When I was young a sore back meant nothing.  Aching shoulders, neck, legs, wrists, you name it, were soon forgotten and tiredness didn`t even figure. With every new day I seemed to inherit a new body.  And I could sleep anywhere, on anything, and for any period of time, short or long, and always awaken completely recharged. I slept in digs with half a dozen drivers in every room, across the steering wheel in my cab, or in the trailer, it didn`t matter to me, not then.  Even days off were no different in terms of comfort: I spent my time wandering the hills, resting at night in an old bivi bag.

Then, as I weakened with age, so the truck manufacturers introduced sleeper cabs of increasing luxury. I now sleep on a wide, comfortable bed. I can walk from driver`s seat to passenger armchair.  The cab is so high that I am able to sit at a table and chair below the bunk. I feed myself from a fridge using a microwave and a coffee maker keeps me refreshed all day and into the night. And when the curtains are drawn, just like at home, there`s power to light the room, run my phone and tablet, and keep me warm. 

Even the EU recognized the restful qualities of modern sleeper cabs, their suitability and comfort. Or so I thought.  I`ve just been told by the European Commission (Julia Kremer, DG Mobility and Transport Unit.  D3 Land Transport. Julia.Kremer @ec.europa.eu) that a Regular Weekly Rest cannot be taken in a vehicle: this concession, it seems, only applies to Daily Rest and Reduced Weekly Rest. So, every fortnight, if I`m away, I must find somewhere else to spend 45 hours – maybe I`ll have to dig out that old bivi bag and sleep on the ground next to my home from home.

Building a Scania V8 R500 - Part 3. Suspension, front axle

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 3. Suspension (front axle)

A front spring.  The suspension spring assembly comprises two leafs (main and Secondary), saddle and U bolts, centre pin and bump stop, swinging (rear - right in picture) and fixed shackles. 



Leaf springs come in different shapes.  In the days of horse drawn carriages, full elliptical springs were sometimes used.  Today, we see parabolic, semi-elliptical and quarter-elliptical springs (see Pt 2  drive axle). 





Leaf springs.  The Scania`s front axle is a non-independent dead beam setup.With front axle weights typically 7.5t there is no real need for air springs. This system is the most efficient way to achieve steering axle geometry.  A main spring is wound into eyes at each end and attached to the chassis, a secondary spring is attached to the main spring.  The main spring is very important as it attaches the axle to the chassis and provides a chassis connection for the secondary spring to work on - if the main spring breaks, axle location and suspension will be lost. 






Although Scania use a parabolic spring, this term refers to a particular shape, the mathematical description of which is quite complex.  Sometimes it`s best to think visually, in this case how curves are formed in cones.

 



Some leaf spring shapes are derived from ellipsis





The Scania`s front leaf spring shape is parabolic.  If in doubt, they are simply leaf springs.

 

The rearmost shackle (the chassis seen here inverted) attaches the rear end of the spring assembly to the chassis.  It uses a swinging device that allows for spring (longitudinal) extension when the suspension deflects.  A fixed shackle will always be at the front end of the spring, as the vehicle drives and brakes through this rigid connection. 

A `C` anti roll (stabilizer) bar is used on the front axle.



Anti-roll bar in action.  Anti-roll bars reduce body roll but also increase spring rate in roll, transferring weight across the vehicle.  This will effect handling.  Body roll is important on trucks as increased roll can reduce stability (rollover).  The anti roll bar will attempt to level the body by lifting the inner wheel (on a bend) or the vehicle`s body itself.  It can only do this up to a certain point, after which the bar begins to twist more and there`s an increase in weight transfer.





The front axle is an I beam (because its cross section resembles the capital letter I).  The depth of the beam and the width of its flanges deal with bending loads and twisting (torque).



Next:  Part 4  Steering

Govern

Years ago, in the 1980s if I remember correctly, there were a number of haulage operators in London who employed day drivers on tipper work.  Most of these men lived locally and enjoyed a drink after work.  But one operator stands out in my mind because he opened a pub close to his yard and filled it with trucking memorabilia.  By all accounts his drivers loved the place and duly dispatched a proportion of their wages across the bar each week.  A cynic would point a finger at an unscrupulous employer – paying his drivers with one hand and taking his money straight back with the other.  Others, including many drivers, might say that he was providing a comfortable and familiar environment to relax in.  The fact is that if the beer was sold for a reduced profit they would be getting a bargain, and by not lining the pockets of some other local establishment, both driver and boss would benefit.  It`s with this type of rare, innovative thinking that The Government could do something to help the cash-strapped haulage operator. 

Don`t even think about a reduction in tax revenue; that`s simply not going to happen.  Look at light vehicle MOTs.  Common sense says that current technology and vehicle reliability only require an inspection every two years, but the government turned down such a recommendation and opted to stay with the current system.  The reason had nothing to do with road safety, it was purely down to revenue – they feared a reduction in garage employment, income and, therefore, treasury funds.  But The Government has always dabbled in income generation outside the tax system.  It sells bonds, it allows the Royal Family to parade themselves for fee paying tourists, and it allows donations.  So what about running its own fuel company?

As a bulk supplier to the haulage industry, The Government could trade in bunker delivery or volume purchases at the pumps.  The tax on diesel would not change, only the profit made.  Any decrease in tax revenue from the main fuel companies, due to a reduction in truck fuel sales, would be compensated for by the profit (all be it reduced) and tax the Government`s fuel company created. Governing the industry`s diesel supply could be the answer, and a rare situation where everyone benifits.