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