23.2.
Pneumatic Tyre
The pneumatic tyre
(a) supports the vehicle load,
(6) cushions the wheel against small road shocks,
(c) transmits driving and braking forces between the wheel and the road surface, and (of) converts steering effort into directional movement.
Tyres may be broadly divided into tubed and tubeless constructions, and cross-ply and radial-ply constructions.
23.2.1.
Inner Tubes
The inner tube of a tyre is a flexible tube container, which when inflated with compressed air expands until it bursts unless the expansion is restrained by the tyre, acting as a casing.
Fig. 23.6. Commercial-vehicle wheel and tyre construction. A. Tubed tyre and wheel. B. Tubeless tyre and wheel.
The inner tube (Fig. 23.6A) then takes the shape of the tyre cover, which protect it and contains this trapped pressurized air over long periods.
The tube is manufactured from extruded rubber sheets, which are shaped on a former and cured (vulcanized) in a steam-heated mould to the cover shape. The tube is constructed to about 10% undersize. To provide an outlet or inlet to the tube interior, a valve stem is sealed to the inside circumference during vulcanization.
23.2.2.
Tubeless Tyres
The tubeless tyre (Fig. 23.6B) constitutes an outer cover and instead of the inner tube an unstretched rubbejiHning is bonded to the inside of the cover wall. The rubber lining continues under the beads dredges of the cover to join up with the outer side-wall rubber. The bead portion of the tyre m^fes an effective air-tight seal between the tyre and the rim. The taper rim seats on each side of the weft base and adjacent to the rim flanges, and assists initial sealing between the bead and the wheel rim during inflation of the tyre. Car and commercial vehicle rims have taper angles of 5 degree^and 15 degrees respectively.
In this layout, the tyre valve is generally positioned in a hole formed in the well base of the rim. Car valves are sealed by rubber grommets, whereas commercial vehicle valves have -^all-metal threaded system and is secured to the rim by nuts.
23.2.3.
Merits of Tubeless over Tubed Tyres
Following are the benefits of using tubeless tyres over tubed tyres.
(a) Air Retention. In the tubeless tyre, the inner lining does not stretch unlike ordinary inner tubes, and the cover wall itself resists air passage, therefore air loss is very much slower. The tyre bead and the rim also provide seal between them so that no leakage of air around the tyre rim is allowed.
(b) Safety. The tubed tyres can suddenly puncture without warning. With a tubeless tyre, if a tyre is ruptured a slow loss of air takes place giving enough time to take care of the problem.
(c) Ride Comfort. The tubeless tyre is lighter than the inner tube and tyre, giving less unsprung-mass reaction on the vehicle’s suspension system, which reduces wheel bounce.
(d) Tyre Cooling. When there is no inner tube, the heat generated in the compressed air is dissipated to atmosphere directly through the metal rim. The inner tube being relatively poor conductor of heat, a greater temperature builds up in tubed tyres.
(e) Wheel Assembly Balance. A commercial-vehicle tubeless tyre does not incorporate a tube flap, loose flange, and lock-ring. In this case the tyre bead fits concentric to the taper wheel rim. This layout provides a better balanced assembly.
if) Assembly of Tyre to Wheel. There is no chance of the inner tube being nipped or punctured during assembly as only the cover has to be fitted over the wheel rim. Also there is no danger of loose components flying off during inflation of tubeless tyre due to distortion or
carelessness.
23.2.4.
Run-flat Car-tyre
The conventional tubeless tyre in the inflated condition contains air and does rolling. When deflation occurs, there is a possibility of the bead of the tyre to move across the rim into the well base, due to which the tyre may slide over the rim flange causing its buckling and folding. The rim may then plough into the road, causing loss of control.
The run-flat tyre developed by Dunlop operates effectively with or without air, and provides acceptable handling qualities when deflacted and excellent handling qualities when inflated. In the run flat concept the tyre bead is held against the rim flanges when the tyre deflates. It absorbs all possible stresses caused by deflation. It remains undamaged by internal friction and heat generated when running flat. It self-seals any punctures, so that the remaining air is trapped and heat is generated, which reinflate the tyre within limits.
The run-flat tyre bead and rim are designed to enable the bead to engage a small circumferential groove in the bead-seat area of the rim. The bead remains locked in position under inflated and deflacted conditions. The tyre does not dislodge from the wheel (Fig. 23.7A) even in the event of a high-speed blow-out. The tyre uses a low-profile radial-ply construction and the tyre side walls are made thicker to sustain the increased stresses created when running deflated. The tyre is fitted on to a rim which is narrower than the tyre tread. The rim flange supports and does not cause any damage to the tyre during deflated conditions. The friction and consequent heat generation are minimized by the application of a one-stage lubricant, known as Polygel, to the underside of the tread at high temperature during the final stages of tyre manufacture. Polygel also provides self sealing properties against a puncture (Fig. 23.7A to D).
Fig. 23.7. Dunlop “Denovo 11″ run-flat car tyre. A. Nail punctures type. B. Polygel seals puncture hole.
C. Polygel seals any other small puncture at low pressure to trap the remaining cold air.
D. Movement of the wheel and tyre deflection generate heat which raises the trapped-air temperature and
pressure to re-inflate the tyre.
The principle of construction of tyre having self sealing properties against a puncture has been extended to develop bullet-proof tyres. In the event of a bullet hitting or passing through the tyre, the self sealing characteristic seals the effected area without loss of appreciable pressure. Bullet-proof tyres are normally used in military vehicles and special category vehicles.
23.2.5.
Captive-air Tyres
This category of tyres is basically tubeless tyres having two distinct air chambers (Fig. 23.8). The captive-air inner chamber is generally of a 2-ply nylon construction and is inflated by the air entering into it through a standard valve. The outer chamber is the space formed between the outer tyre and the inner chamber wall. This chamber is inflated through a needle valve.
Fig. 23.8. Captive-air tyre.
When a puncture occurs, the air stored in the outer chamber escapes out to atmosphere through the punctured tyre wall. The inner chamber then supports the weight of the vehicle. The vehicle in this way can be driven without loss of any stability to a nearest service station. Captive-air tyres are generally used on racing cars.
