How it Works: The Tyre

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How It Works: Tyres

John Boyd Dunlop

The pneumatic tyre has come a long way since surgeon John Boyd Dunlop hit upon the idea of fastening inflated rubber tubes to bicycle wheels. This was a truly an epoch-marking invention, because it transformed the bicycle from a boneshaker - ridden only by fanatics - into a respectable means of conveyance, and it opened the way to faster modes of transport as we know it today. And, for such a relatively old invention (at least in automotive terms) there has been no great leap forward in the fundamental idea of the tyre.

Beaded Edge Tyre

But while the basic principle has remained pretty much the same for decades, the tyre itself has evolved into an entirely new beast. The earliest examples used on cars were of relatively tiny sections fitted to rims of large diameter, and were run at high pressures otherwise the woven rubberised canvas composing the casing developed so much heat through internal friction that the treads would come away, and for a long time tyres were the Achilles heel of racing cars.

Straight-Sided Cord Tyre

Apart from this, the beaded-edge construction made tyre changing difficult and the walls frequently suffered from concussion-bursts on bad roads. A big step forward came with the invention of the straight-sided cord tyre, with wired beads fitted to well-based rims, similar to the type then already in use on bicycles. In this form, the centre of the rim was depressed, so that the tyre bead could be forced down into this well at one side and could then be levered over the other side with much less effort, than was required for the old "clincher" type tyre.

The cords were at first continuous - wound on in layers or plies crossing each other at an angle, but previous to winding the cords were coated with latex rubber, so that each was insuited from its neighbors and less frictional heat was generated by the flexure of the cover. This not only reduced the tyre temperature, but absorbed less of the available engine power. Subsequently, the costly continuous-ccrd system was superseded by a method cf building up the casing from strips cf cord material cut on the cross and wound on a former or mandrel, after wich the cords were folded back over the bead wires, and this construction with detailed variations became and regains the standard method of making crcss-ply covers.

Balloon Tyres

The improved flexibility and low heat generation of cord tyres led to the invention of extra low-pressure "balloon" tyres which, by a not uncommon coincidence, were invented simultaneously in two widely separated countries, France and the U.S.A. These balloons gave a very soft ride, but some were so exaggerated in size that they rolled badly cn corners and when used with the then-conventional beam front axle were likely to generate axle-tramp and wheel-hop to a horrifying extent, especially when fitted to a rim of 24 or 28 in. diameter. Consequently, there was a move towards medium pressures and a reduction in rim size, while keeping the larger section for comfort.

Nobody really realised for a long time that a pneumatic tyre had properties which profoundly affected the behavior of a car. If you wanted to go fast, you blew the tyres up a bit harder and put up with the rough ride, until it was discovered that both the straight-line running and the handling on corners was greatly influenced by two properties possessed by the tyre. One of these is an inherent tendency to run straight, due to a self-aligning force developed by the tread in contact with the road, and which imparts "feel" to the steering; the other is that when subjected to a lateral force, as when running in a cross-wind or when cornering the tyre will proceed along a path which is at a small angle to the line along which the wheel is actually pointing.

The Slip Angle

This was called the "slip" angle, a silly and misleading term because as long as the wheel is rolling and not skidding, no actual slip takes place at all. What happens is that the tread rubber and the casing flex or distort sideways as each element of the tyre makes contact with the road and comes under the action of the lateral force, and remains flexed until contact is lost at the tail-end of the contact patch. This local distortion can be seen quite clearly if a car with slack tyres is pushed from side to side. On the road, if the slip-angle of the front wheels is greater than those at the rear, the car will "under-steer" and will need to be held in to a corner, whereas if the rear slip-angle is greater the car will "oversteer", maybe to such an extent that opposite-lock correction may be necessary on a tight corner, which calls for a lot of wheel-twiddling and can be very fatiguing.

However, this subject of steering is covered seperately in another "how it works" page here at Unique Cars and Parts. It is sufficient to say that low pressures, deeply-patterned treads and wide tyres on narrow rims are conducive to understeer. Another less obvious factor is the angle at which the cords are laid, the squarer they are to the tread the higher the cornering power. Conversely, almost everything which increases the cornering power is detrimental to bump-absorption and sometimes leads to harsh suspension and the noise known as tyre thump.

The B.F. Goodrich Tubeless Tyre

B.F. Goodrich takes credit for developing the tubeless tyre and seems to have marketed such an apparatus as early as 1947. The tubeless tyre, as we know it today, was invented by a B.F. Goodrich engineer Frank Herzegh, of Shaker Heights, Ohio. He applied for a patent on this very basic invention on December 14, 1946, then assigned it to his employer. B.F. Goodrich started manufacturing tyres of this type in late 1947 and had them on general sale in the US by February, 1948.

The Herzegh patent was infringed and led to important legal action. During the course of these proceedings no less than 66 prior patents for tubeless tyres were considered in the evidence. One of these patents - and a French one at that - dated from 1904. Mr Herzegh, from the text of his US Patent No 2,587,470, stated "One difficulty heretofore is that devices proposed for sealing against leakage of air at the bead portions of the tyre have been overly cumbersome and difficult to install, have required special rim constructions or have not provided satisfactory sealing. Another difficulty with constructions proposed heretofore is that without the benefit of an inner tube the tyre wall has had to take the whole of the gradient of air pressure from inside to outside and as a result of diffusion of air into the wall has been vulnerable to the formation of blisters in the wall and separation of the embedded fabric plies from the rubber, which has led to early failure of the tyre.

"This difficulty has been present especially in tyres for high speed service where the heat developed from the rapid flexure of the tyre walls has caused expansion of air diffused into and pocketed in the wall, which in turn has caused or aggravated separation of the tyre material. Rubber compositions heretofore used in tyre wall construction have not provided a sufficiently high resistance to diffusion of the air into the tyre wall to prevent this difficulty in a satisfactory manner.

Michelin's Radial Tyre

As successes in racing when adroitly advertised had a great effect on sales, a few makers such as Dunlop developed tyres specially for the job, but it was the non-racing firm of Michelin who introduced a startling innovation in the form of a tyre with the cords disposed radially from the beads and therefore square across the tread, with an in-extensible band woven from fine steel wire interposed between the cords and the tread. This band, while flexible radially, was relatively stiff laterally and so almost eliminated the local flexure of the tyre at the contact patch which creates high slip-angles. This design therefore provided great cornering power, without sacrificing ride quality too much, because the side walls were very thin and flexible, enabling low inflation pressures to be used without overheating.

This was the first of the "radial-ply" tyres and as far as the patent situation allowed, other companies developed similar tyres with braced treads though with non-metallic material such as rayon or nylon in the circumferential cords. In any case, synthetic fibres have largely replaced cotton as the basic casing material, even in cross-ply covers. Due to their superior handling characteristics, radials soon became the standard wear for fast cars and they were intensively developed by Dunlop, Goodyear and Firestone when these three began to vie with one another in dead earnest in international racing.

Up to this point of time the tyre section was approximately circular, this being the shape with the greatest area in relation to its perimeter, but as a round tread is undesirable except on a single track vehicle, a flat area of contact was obtained by building up the shoulders with rubber. This is feasible only up to a point, and consequently the tread width was limited to about two-thirds of the section diameter. However, by suitably disposing and tensioning the circumferential cords, the centre of the tread could be prevented from spreading outwards and the section then resembled a circle with a flattened top. A wide tread could then be moulded on without the necessity for thick heat-generating shoulders and the tyre gained in cornering power and also in adhesion on dry surfaces.

Elementary physics taught the engineers that the friction between two surfaces is dependent only on the weight involved and is not dependent on the area, but this statement does not hold good where a stiff rubber-like material is in contact with a rough surface such as concrete. The rubber, in effect, conforms with and becomes geared to the road and the force required to cause sliding depends more on the hardness and composition of the tread material, rather than on the area. Likewise, on dry surfaces the pattern is not of great importance, as witness the fact that the tyres used in drag racing, where acceleration is the sole object of the exercise, the only type of thread used is flat and devoid of any pattern at all, adhesion being gained solely by the grade of rubber employed.

Drag Racing - Adhesion by Rubber, Not Tread

On a hard wet surface, things are different, and a pattern is necessary to enable water to be squeezed out and ejected through the drain-slots provided, so that the trailing portion of the contact patch is running on a relatively dry surface. Sharp edges on the tread studs also assist grip, by dipping through on to the surface, but become rounded-off in time. Very narrow saw-cuts do not suffer as much in this way and remain sharp-edged for long periods, hence their use in many tread designs. Saw-cutting of standard tyres was introduced as the "Pneu-grippa" process in England in the late 1920s, but was not taken up by tyre makers until a method of moulding the saw cuts into new tyres was devised.

Still pursuing the search for greater cornering ability, the stability of the tyre on the rim was increased by widening the latter, and the section began to resemble a rectangle rather than the original circle. Racing tyres got wider and wider, and the rims followed suit, until the treads on a G.P. car were 15 or 16 inches across, and the rims were occasionally a little wlcer. Obviously, tyres for ordinary car work cannot be as specialised as those used in F1, and instead must be able to cope with ail conditions and yet give good mileage. The cult of the wide tyre spread to touring and sports cars in the 1960s, a time when the now mandatory "low-profile" sections were developed, their special feature being that the width of the section was considerably more than the depth. In conjunction with wide rims, these tyres gave exceptionally good dry-weather performance (and would also be good on sand) and many owners anxious to keep up with the times were going in for the widest tyres and rims they can obtain.

Retro-fitting Wide Tyres To Older Cars

The problem with retro-fitting low profile tyres on an older car is that wheels jump around a bit more than you might think inside the guards and there often isn't enough working clearance to use very wide tyres without danger of ruining them on rough roads. Another snag is that the increased width entails more overhang from the axle bearings and the suspension joints, especially if thick packers are also used to obtain clearance - and this, plus the additional cornering power places much greater strains on the running gear. Another snag which must be remembered is that wide tyres are no better than conventional tyres on smooth wet surfaes or on grease, and a driver who habitually takes advantage of their tyres to get around corners quickly on dry and corners may find themselves in trouble if they encounter a slippery patch half-way round. If you are modifying your older or classic car, we suggest a sensible plan is to consult the tyre manufacturer or distributor before pulling the trigger and spending large on a new set of wide wheels and tyres, which has the potential to end up being a big dissapointment.
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Aspect Ratio:

Aspect Ratio
Refers to the height of the tyre expressed as a percentage of the cross-section profile. Hence a 70% aspect ratio refers to a tyre that is 70% as high as it is wide.
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Bias Belted:

Bias Belted
A tyre construction technique developed in the US that consists of a mixture of cross-ply and radial-ply construction. It is aimed at producing a superior ride comfort level than that found with a radial ply tyre without sacrificing any road-holding capabilities. The bias-belted tyre fell from favor during the late 1970’s.
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