As we contemplate obtaining a new set of old-fashioned tyres for Heritage Commercials' 1937 Reliant van project, Jerry Thurston reminds us of a few salient facts about these everyday round black objects.
Tyres. Black, round and made from rubber - easy enough. Available in different sizes and widths - still fairly simple. Have different load capabilities and speed ratings - there's more to this than meets the eye. Many can be remoulded, some can be re-cut. Some are solid.
In the beginning there was the wheel. Made of wood, consiting of a hub, spokes and a rim. To keep these components in close contact with each other, a band of iron was heated to expand it, and dropped over the wooden rim. As the iron cooled, it contracted and pulled everything together. This band of iron was the first tyre.
Iron-shod wheels running on metalled roads are noisy, uncomfortable and offer precious little grip. This arrangement was fine on commercial vehicles such as carts, until the advent of the internal combustion engine, if you forget the experimental pneumatic tyre being developed in the late 1th century by John Boyd Dunlop. Private carriages and early bicycles were making do with a band of solid rubber stretched over a rim that that had a cup shape to prevent the rubber from rolling off in use. These solid tyres, although not ideal, did isolate the user from the worst of the road shocks and afforded plenty of grip - a good thing if you were riding a high bicycle.
All early motor vehicles adopted the solid tyre at first; cars quickly changing to pneumatics and all the attendant problems with punctures - think of how many tens of thousands of horse shoe nails were scattered on the roads! Commercial vehicles could not easily afford to be stopping to change tyres, so they continued using solids, the type being used being a thick layer of rubber vulcanised (using heat and pressure) onto a steel band, and these were then pressed onto the wheel.
Those using solid shod vehicles today (ie restores of vehicles from that particular era) are not in as much trouble with tyres as you might think, as there are a couple of companies specialising in re-rubbering (the mainstay of their business tends to be print rollers) that have the capacity to take commercial vehicle wheels. I'm told that the results, although expensive, are well worth while.
The pneumatic tyre really started to make sense once the horse started to disappear from the road, and surfaces improved. Pneumatics allowed the vehicle to travel at higher speeds, and driver comfort increased considerably. Light commercials such as vans could use what were termed 'beaded-edge' tyres, which were held onto the rim by air pressure. If tyre pressures were allowed to fall too much, the tyre could, and would, simply peel off. Heavy commercials had to wait a while, and still ran solids until the modern type of tyre had been developed.
By the end of the 1920's all vehicles were on tyres as we now understand them - black round rubber things that all look the same, and it's here where the confusion really starts.
Apart from size, what sets a tyre designed for commercial use apart from one for a saloon car? It's all in the construction; a saloon car tyre has to cope with comparitively little weight compared to its counterpart on a commercial vehicle, so is constructed in a lighter manner.
Let's go back to basics: a tyre is made up of a carcass and the rubber bonded to it. The carcass is overlapping layers of string for want of a better word. This 'string' was once natural fibre, but is now nylon, rayon, Kevlar, or even in some cases, steel wires. These 'strings' or plies, either run from side to side (crossply) or round and round (radial).
Cross-ply tyres have very strong sidewalls but are not resistant to wander (they are easily twisted). Radial tyres have much more flexible walls (hence you see that characteristic bulge at the bottom of a modern tyre) but are very resistant to wander and resist cornering forces very well. The radial had superseded to cross-ply in the passenger car market by the 1970's, but as seems to be the way, the cross-ply held on longer in the commercial market, mostly because of its superior weight-bearing capablities. Radial technology had to come on much further before it was superior to the old cross-ply.
The carcass of a commercial vehicle tyre has many, many miles in it, more than the tread, and hence the market in remoulded tyres. Once the tread has worn away, provided that the carcass is in good condition, the rest of the tread rubber can be buffed away, and a new tread area remoulded. We occasionally see a tread by the side of the motorway that has been thrown from a truck tyre. This is usually because the tyre has been run at too low a pressure. This causes heat to build up, and eventually the rubber loses its bond and separates from the carcass. Thankfully, this is now a rare occurrence as operators check the tyres on their fleets' vehicles on a more regular basis.
Certain makes of tyre can be recut. They have sufficient thickness of rubber for a second tread pattern to be cut once the first has worn out. Once the tread gets low, the tyre is taken off, and a device that looks like a wood plane with a loop of blade below is used to cut a new tread pattern. The magic part of this tool is the loop blade, which heats up and allows the rubber to cut easily. If there's enough rubber to cut a second tread, why didn't they give the tyre a deeper tread in the first place? It's all about rigidity, if the tread was cut deeply it would deflect unduly under cornering forces, scrub and wear out really fast.
The fact that tyres have differing speed ratings confuses many people. Heat build-up and centrifugal forces are the problems. Excessive heat build-up can cause a tyre to de-laminate and come apart. Tyres with low speed ratings should not be run at high speeds.
Do use the right tyres.
It is illegal to run a vehicle with insufficient speed-rated tyres, but it is legal to run a tyre with a better rating than is needed.
Do check what tyres you have fitted.
The old public information films urged us not to mix radial and cross-ply on the same axle. I'll take it one stage further; never, ever mix radials and cross-plies on the same vehicle, for their handling characteristics are so different the vehicle will never be right.
Do use your tyre pressure gauge
Correct tyre pressures are a must. Low pressures wear tyres out prematurely, can build heat up to unacceptable levels, causing carcass damage, and can badly affect the handling characteristics of the vehicle.
Do have sufficient tread depth.
Let's break a myth: the tread on a tyre has nothing to do with its grip when on a dry road. Grip is generated by the rubber moulding itself to the road surface. This is why racing cars use slick tyres on a dry track, for it gives maximum surface area and the tyre surface is nice and stable. Tyre tread comes into play the second it rains: it helps the tyre cut through the layer of water to the road surface below partly by increasing the ground pressure. Imagine cutting half the rubber area away. This will have the effect of doubling the weight on the rubber that remains. The tread pattern acts as water channeling, clearing the water away to let the rubber do its job. Within reason, the deeper the tread, the more water the tyre can clear. At a steady 50 mph think of the gallons of water each tyre is siphoning off the road surface every minute. 1.6mm is now the legal tread depth minimum.
Don't run with damaged tyres.
Our tyres have a hard enough job to do when in perfect conditions, but cuts to a sidewall will render a tyre scrap.
While checking the sidewalls, have a squint at the tread and remove any stones; we don't want them burrowing in and causing a puncture!
Look after your tyres, and they will look after you!
Heritage Commercials is published every month, and covers a wide range of commercial vehicles from car-derived vans to ballast tractors and tank transporters. Ages range from the days of the very earliest commercial vehicles to the 1970's.