The functions of wheel in the Automotive vehicle are,
1.To observe the impact forces of the road surface and to bear the vehicle load.
2.Transfers the rotational movement of the axles to Tyres.
3.Absorbs and transfers the braking and Accelerating forces as well as cornering forces.
4.Dissipates the braking heat to the atmosphere and
5.Acts as a seal in case of tubeless tyres.
Types of Wheels:
1) Disc wheels: It consists of wheel disc also called as wheel nave and the Rim. The wheel disc connects the Rim with the Hub and the Rim holds the tyre.
2) Cast or forged wheels: Cast disc wheel consists of a ready cast part or a cast blank that is then forged. The wheels are then cut to shape. The cast wheels are made from light-alloy, malleable cast iron and cast steel wheels. The usages of cast steel wheels are limited due to its heavy weight. Cast and forged wheels are more expensive than the welded steel disc wheels but they are preferred due to its high stability and precisely machinable quality.
In the case of multi-piece system the Rim parts are bolted to the wheel rim. If the wheel rim remains on the hub it is easier to change the wheel but the disadvantages are its heavy weight and high manufacturing cost.
Wheel weight and load bearing capacity:
The dead weight of the wheel should be as low as possible and at the same time it should have high load bearing capacity in order achieve low unsprung masses. The advantage of light weight wheels are
-Improved vibration Characteristics
-Sensitive suspension response
-Low fuel consumption and
-Higher pay load.
By changing over from steel to alloy rims approximately 200 kg weight can be saved on a three axle semitrailer tractor.
Rims are having the functions of holding the tyre as well as sealing function in case of tubeless tyres. They are classified as follow based on their construction.
-Drop centre rims
-Flat base rims
-Slanted taper bead seat rims
-Taper bead seat rim
Drop centre rims and Taper bead seat rims are made up of one piece, slanted taper bead seat rims are multi piece rims. A raised part all around the bead seat in case of Taper bead and Drop centre rims, improves the fixation of the tyres.
Drop centre rims:
These are one piece rims used in light commercial vehicles and cars. The raise in the bead seat is 5 degree. They are not suitable for high load bearing capacities due to their design and being used in commercial vans and mini buses.
Taper bead seat rims:
In order to exploit the advantages of tubeless tyres also on heavy trucks, the taper bead seat rims with drop centres were developed. The bead seat rises on these rims by approximately 15 degrees. Taper bead seat rims are one piece and having low weight with good load bearing capacities. In this design wide rims are used to fit wider tyres and hence it is very suitable for heavy loads.
Illustration example: 9.00 x 22.5
Rim width in inches = 9.00
One piece = x
Rim diameter in inches = 22.5
Slanted taper bead seat rims:
Slanted taper bead seat rims are multi piece rims for commercial vehicles with inner tubes. The rise in the bead seat is around 5 degree. The tyre is held on one side by removable combination or side and fastening ring. In such type of rim system, it is very easy to change the tyres but it does not have good running characteristics as compared to one piece system.
Illustration example: 8.50 – 20
Rim width in inches: 8.50
Multi piece = “–“
Rim diameter in inches = 20.
Trilex, Tublex and Unilex Rims:
The three piece wheel system as shown in the sketch below allow easy tyre mounting, but they do not seal the rim, hence they need an inner tube.
It normally comes with higher dead weight due to separate rim parts and wheel rim. These rims are used very rarely now days where there are no service stations are available. Tublex and Unilex rims are also suitable for tubeless tyres but here the rims are not split axially (like in case of Trilex) but they have removable rim flange and an additional ring as shown in the sketch below.
Wheel Weight and Load Bearing Capacity:
All wheels are assessed mainly by the two criteria 1) The wheel weight and 2) Load bearing capacity. Light wheels with high load bearing values increases the pay load of the any kind of vehicles.
Specific Load Bearing Capacity:
The usage extent of the wheel or Rim construction is referred by Specific load bearing capacity. It is the ratio of load bearing capacity or ultimate load (in KG) to the Dead weight of the wheel in KG and hence it is a direct measure of efficiency.
Specific Load-bearing capacity “t spec” =Load capacity of wheel / Dead weight of wheel in Kg.
The higher the value of the specific load bearing capacity , the more load the wheel can bear In relation to its dead weight and higher pay loads can be used. At the same time the rolling properties are improved.
The specific load-Bearing capacity can be increased by choosing the light alloy and aluminium wheels by saving more weights.(Fig below)
For Super wide tyres with weight of rims and tyre put together 246 and 304 kg, the specific load bearing capacity and mean value of load bearing capacity is shown graphically as below.
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