The wheel is one of the essential parts of all forms of land transports. The wheel is generally thought of as the assembly of hub, discs or spokes, rim and over it the tyre. The wheel supports the weights of the vehicle and helps to protect it from road shocks. In addition the rear wheels must transmit the power, the front wheels must steer the vehicle, and all must resist braking stresses and withstand side thrusts. All wheels must be perfectly balanced. For discussion the assembly is divided into wheels, rims and tyres. Earlier types of wheels were fitted with solid rubber tyres and as a wheel was not desired to be readily detachable, the wheel and hub were usually made integral. The fitting of pneumatic tyres with their shorter life and liability to puncture necessitated easily detachable wheel. The wheels of most Motor Transport Vehicles are interchangeable and readily detachable; and additional wheel is usually in the vehicle as a spare.
A few type of vehicles use wire spoked wheels but the most widely used wheels are of the disc pattern. With this type, the center part, termed the nave, is a steel pressing to which the rim, also a steel pressing, is either riveted or welded. The rim may, or may not incorporate one or both the flanges, which is necessary for retaining the tyre in position.
The load carrying capacity determines the diameter and width of the rim The wider rim permits the beads of the inflated tyre to be further apart, thus, making the vehicle more stable, and providing flatter tread contour which increases the potential mileage of the tyre. The rim base on this type of wheel has a slight taper of approximately 5° from each flange Tractors and similar vehicles are not fitted with types of wheels already mentioned. The wheels usually fitted to these vehicles are of the cast type fitted directly to the driving shaft.
CLASSIFICATION OF WHEELS
The wheels are usually classified by the type and overall diameter of the rim. Thus a measurement of a wheel is as:
(a) Diameter of the wheel in inches is taken across the tyre bead seats, i.e, the diameter of that portion of wheel the tyre fits.
(b) The width between the rims in inches. The measurement is taken across the rim from one bead seat to the other, i.e the inner distance between the bases of each flange of the wheel.
TYPE OF WHEELS
(a) Disc type wheel. This type of wheels are most commonly used on MT vehicles, because of their constructional features, it carries maximum load and balancing of the vehicle is maintained during the running of the vehicle.
(b) Wire Spoked Wheel. This type of wheel possesses considerable strength in proportion to its weight and is therefore suitable for light vehicles, cycles and motorcycles. However, this type is much more easily damaged than the pressed steel, and for the reason has been replaced by the latter on many vehicles. The wire spokes run from the hub to the outer rim around which they are evenly spaced so as to ensure that the stresses are spread everywhere over whole wheel.
TYPES OF DISC WHEELS
(a) Well Base Wheel. This type is virtually a one-piece wheel because the rim pressing incorporates both type retaining flanges is either welded or riveted to the nave pressing. A well or recess around the centre of the rim facilities the fitting and removing of the tyre. This type of wheel is mainly used on passenger cars and similar vehicles.
(b) Two-piece Wheel. The wheel consists of steel pressing, one flange of which is detachable. The detachable flange may be in the form of split ring, or a plain continuous ring. A groove is provided in the rim edge opposite the fixed flange for both types of flanges. A special lever known as “rim tool” is used when removing or fitting the flange.
(c) Three-piece Wheel. The three-piece wheel, is similar to the two-piece, the difference being that the detachable flange is a continuous ring fitting comparatively loose on the rim. The flange is recessed to accommodate to a locking ring, either of the continuous or of the split type.
(d) Four-piece Wheel. In this type, both flanges are detachable, and the rim base contour is flat. One edge of the rim is beaded to retain one flange and the other is groove to accept a locking ring for the second flange. The four pieces of the wheel are, therefore the nave (with rim), the two flanges and a locking ring.
Note: Before removing the locking ring with the aid of a ‘rim tool’, ensure that the air pressure is fully released.
(e) Divided Wheel. This type of wheel forms a very convenient method of mounting for large type tyres. It consists of similar steel pressings each of which incorporates a nave and a half rim with a flange. The pressings are bolted together at the nave and thus form a wheel with two fixed flanges. Although this wheel is made of two pieces, it must not be referred to a two-piece wheel.
Warning. The nuts clamping the halves of a divided wheel are painted “RED” and these must never be slackened until the tyre has been completely deflated.
(f) Tractor Wheel. These wheels are generally attached to the axle shafts and consequently they are not usually considered as detachable types. The wheel (Refer Figure No.6.7) consists of two pieces, the nave is a heavy casting incorporating the hub, and is bolted to a pressed steel rim having a well base. Some tractor wheels are cast in one piece also.
TYPES OF ATTACHMENTS
Generally there are two method of mounting a detachable wheel:-
(a) Stud Mounting. The most common method of mounting detachable wheels is to secure them by nuts to studs, which are situated around the hub face.
(i) The stud holes in the wheel nave are slightly larger than the studs and are spherically counter sunk on the outside to take the suitable shaped base of the securing nuts. This allows for positive centralizing of the wheel, and helps in the prevention of damage to the studs when the wheel are being removed or fitted.
(ii) The number of studs varies with different vehicles and is mainly dependent upon the load carrying capacity of the vehicle. On heavy vehicles the wheel studs on the left hand side hubs may have left hand threads, the right hand side having right hand thread. Thus the forward rotation of the road wheels tends to tighten the nuts.
(b) Splined hub Mounting. This method of mounting wheels is now mainly confined to sports cars and racing cars where time taken when changing wheels is vital. The hub of the wheel is internally splined to make with external splines cut on the hub of the axle. The locking nut which is in the form of a large ring has a circular tapered groove cut in its inner face and these mates with the levelled outer face of the wheel hub, thus providing a wedge action to prevent the nuts slackening off when in service. The locking nut is provided with two lugs, which can be struck with a lead hammer or a mallet for loosening or tightening the nut. The locking nuts are tightened in the opposite direction to the forward rotation of the road wheel.
It is important to ensure that the wheel complete with its tyre is properly balanced especially on passenger car and light vans. While the effects of unbalanced wheels are not important, at low road speeds of 30 miles per hour or less, they become of increasing importance at higher speeds, being serious above 50 mph. Any out of balance of the front wheels will affect the steering, and unbalanced rear wheels will give rise to vibration.
It is usual for tyres and tubes to be checked for balance by the manufacturers, the lightest part of the tyre being marked with a white spot or spots and the heaviest part of the tube with black spots. The tyre and tube should be assembled to the wheel with these spots in line. Whenever any repair is being carried out on wheel, tyre or tube then the balancing of wheel is to be carried out on wheel balancing machine.