Design Analysis of a Portable Manual Tyre Changer

DOI: http://dx.doi.org/10.24018/ejers.2020.5.11.2224 Vol 5 | Issue 11 | November 2020 1 Abstract — In this paper, an efficient, affordable, and portable manual automobile tyre changing tool was designed. The tool operates on the principle of second-class levers, where the load to be overcome is situated between the fulcrum and the effort point. The mechanical advantage (MA) of the bead breaker arm and pry bar assembly of the tool was determined to be 11.5. The standard tube size of the bead breaker arm that can withstand stress due to bending moment was determined to be a 2” (50mm) diameter Schedule 80 steel tube. Also, the dimensions of the standard hexagonal bolts to be used at the fulcrum and load point of the bead breaker arm are M12 × 1.5 × 75 mm, while the standard size of hexagonal nuts to be used with the bolts is M12 × 1.5 mm with a height of 10mm. Moreover, the mechanical advantage (MA) of the mount/demount arm and pry bar assembly of the tool was determined to be 5.22. Also, by comparing the angles of twist of two sizes of the mount/demount arm when the tool is used in mounting or demounting a tyre on a 16” × 7” wheel rim, the standard size of the mount/demount arm that can withstand stresses due to bending moment and torsional moment was determined to be a 60mm × 60mm × 5mm square tube. The cost of materials needed to fabricate the tyre changer summed up to ₦21,000. Ergo, the design provides an alternative portable and relatively affordable tyre changing tool that can be afforded by tyre technicians across Nigeria, and other developing or underdeveloped countries.


I. INTRODUCTION
A tyre is a torus-shaped rubber covering that surrounds the wheel rim and transfers the load of a vehicle from the axle, through the wheel rim to the ground, thereby providing traction on the surface over which the tyre-rim combination travels. Tyres consist of a tread which provides traction, and a body which contains a certain quantity of compressed air. Therefore, majority of tyres are air inflated structures, which provide an elastic cushion that reduces the effect of shock as the tyre rolls over rough features on the ground. Studies show that it is important to maintain the correct inflation pressures in tyres in order to keep vehicle dynamics, handling performance and braking at their best [1]- [4]. The basic functions of the tyres on a vehicle are supporting the weight of the vehicle, guiding the vehicle along trajectories decided by the driver, transmitting braking or acceleration forces and absorbing irregularities on the road. It is for these reasons that the tyre-rim combination has been described as one of the most important and critical components of an automobile in several studies [5]- [10]. Basically, modern pneumatic tyres are made from natural rubber, synthetic rubber, carbon black, fabric and wire, as well as other chemical compounds. The structure and layers of a typical tyre are shown in Fig. 1. Fig. 1. Tyre structure and engineered layers [11].
As tyres carry out their functions over time, they fail due to wear from friction or damage in the form of blowouts, and the subsequent replacement of these tyres becomes necessary. After the wheel rim and tyre assembly are removed from the vehicle, tyre changing tools and machines help tyre technicians demount and mount tyres on the wheel rim. It is pertinent to state that the tyre changing process can be extremely laborious, thereby making the technician prone to injury and the wheel rim susceptible to damage, if proper tyre changing tools, machines or techniques are not used during the tyre changing process. Over the years, studies have shown that most of the injuries that occur while servicing tyres happen during the tyre inflation process, with the head and face of the tyre servicing personnel being the most commonly injured regions [12]- [15].
The tyre bead is the innermost diameter of the tyre that interfaces with the rim of the wheel. The surface of the bead creates a seal between the tyre and the wheel rim to prevent leakage of air. Unfortunately, the bead can become stuck to the rim after corrosion occurs. Therefore, tyre changing requires a process known as bead-breaking. Bead-breaking is the process of separating the tyre beads from the rim, by applying force on the sidewall of the tyre, in close proximity to the wheel rim flange.
Even though various designs of tyre changing machines are available for loosening beads and changing tyres automatically, these machines are usually ponderous and not affordable by the majority of tyre technicians across Nigeria and most parts of other developing or underdeveloped countries. Moreover, the locally fabricated bead-breaking tool which is the sole tyre changing tool @ Design Analysis of a Portable Manual Tyre Changer Ovundah King Wofuru-Nyenke commonly available at the tyre service centres of tyre technicians across Nigeria is not portable, barely useful for the bead-breaking operation, and completely inadequate for the tyre mounting and demounting operation. This makes the tyre changing process at these tyre service centres characterised by drudgery and waste of time. The commonly available, inefficient locally fabricated bead breaker is shown in Fig. 2. From the foregoing, there is the need for more portable, complete and affordable tools or machines that can carry out the tyre changing process efficiently. The portable manual tyre changing tool designed in this paper works based on the second-class lever principle, in which the load to be overcome is situated between the fulcrum and the effort point. The design is based on standard tube sizes, making it relatively affordable and easy to fabricate.

II. LITERATURE REVIEW
There are various tools, machines, and methods for changing the tyre on a wheel rim. When the appropriate tools, machines and methods are not utilized, the tyre changing process can be both hectic and hazardous. Apparently, the first step of replacing a bad tyre on an automobile is to remove the wheel from the wheel hub of the automobile. The conventional method of carrying out this operation is to use a lug wrench to loosen the lug nuts attaching the wheel to the wheel hub of the vehicle, one lug nut at a time. However, machines that can loosen and tighten all lug nuts on the wheel simultaneously have been developed in several studies [16]- [23], thereby saving the tyre servicing personnel valuable time and energy. The next step of the tyre changing process is to deflate the tyre and separate it from the rim. Adetan et al. [6] described various tools, machines and techniques used in separating tyres from wheel rims. They described the least expensive technique as a process of using a rubber mallet to repeatedly hit the sidewall of a tyre until it separates from the rim. Clearly, this method takes time and is labourintensive. Similarly, another crude method described by Adetan et al. is the drive-over technique. In this technique, a vehicle with a good fully inflated tyre is driven over the sidewall of a bad deflated tyre to be changed, in order to separate the tyre from the rim. This method is dangerous because if it is not done properly, the deflated tyre can stand upright and hit the side of the vehicle, or worse still, injure bystanders. Another dangerous method of bead-breaking described by Adetan et al. is the high-lift jack technique, where the deflated tyre is placed under the bumper of a vehicle, and the high-lift jack is positioned between the tyre and the bumper. The jack is then activated. This method is dangerous because the tyre can lift up on the side opposite the jack, and roll in unanticipated directions. The design proposed by Adetan et al. in their paper, is a bead breaker that utilizes compound levers to carry out the bead-breaking task. Though the tool designed by Adetan et al. is portable and affordable, the tool lacks the ability to mount or demount a tyre on a wheel rim.
After the bead-breaking step, the tyre is demounted from the wheel rim. Conventionally, the demounting operation is carried out with the aid of tyre bars, and can be a very tedious task. Finally, a new tyre is mounted on the wheel rim, inflated and replaced on the wheel hub of the vehicle.
Scrap tyres which have lost their useful life need to be recycled. Recycling and recovery of waste tyres is a major environmental problem because vulcanized rubbers usually take several years to decompose naturally and thus, remain for long periods of time in the environment. About 78 percent of scrap tyres end up in overcrowded landfills, and thousands more are strewn across empty car lots, highways and illegal tyre dumps [24]. Therefore, options such as discarding end-of-life tyres as waste in landfills or incineration for energy recovery are no longer acceptable for the environment as they pose numerous environmental and health risks.
Some of the present usages of scrap tyres are tyre-derived fuels, artificial reefs, erosion control and crumb rubber, an asphalt additive. However, all of the recycling, re-use and recovery practices combined only consume about 22 percent of discarded tyres [24]. Various studies [24]- [29] have shown that old tyres can be broken down into smaller aggregates known as ground tyre rubber (GTR) that can be used as fillers in civil engineering applications, as well as blended with polymeric matrices such as thermoplastics, thermosets or virgin rubber. Though rubber from scrap tyres can be added to concrete as coarse aggregates to achieve durability, less ductility and greater crack resistance in the concrete, it was discovered that rubber reinforced concrete has a low compressive strength when compared with ordinary concrete [29].
Apart from the ecological advantages of utilizing tyrederived fuels, there has been a growing interest in using these fuels as a result of the fact that tyre-derived fuels have a higher heating value than both coal and wood [30], [31]. Moreover, waste tyre pyrolysis which is the chemical decomposition of waste tyres at elevated temperatures in the absence of oxygen, produces important chemical substances such as synthesis gas, benzene, toluene and xylenes. Synthesis gas can be used for fuel, electricity and chemicals, while benzene, toluene and xylenes are used as primary feedstocks to produce plastics, resins, fibres, surfactants and pharmaceuticals.

III. MATERIALS AND METHODS
The material chosen for the tyre changing tool is plain carbon steel 30C8. Plain carbon steel 30C8 was selected because it possesses good strength and stiffness to resist applied forces and stresses.

A. Calculation of Permissible Stresses
The allowable bending stress, σb, and allowable shear stress, τ, were obtained from [32] as: Allowable Bending Stress, where Syt = Yield strength in tension (N/mm 2 ). fs = Factor of safety.

B. Design Calculations for the Bead Breaker Section
The bead breaker section of the tyre changing tool has a bead breaker arm which is basically a circular tube into which a telescoping pry bar can be fitted. The assembly was modelled as a second-class lever as shown in Fig. 3. From Fig. 3, point A is the fulcrum, while points B and C are the load and effort points respectively. The load arm, l1B, is 100mm long while the effort arm, l2B, is 1150mm long. Since, l2B is greater than l1B, the mechanical advantage of the lever is greater than 1.

Determining the Mechanical Advantage of the Bead Breaker Arm and Pry Bar Assembly
The mechanical advantage of the bead breaker arm was obtained from [33] as: where l2 = length of effort arm (mm). l1 = length of load arm (mm).

Determining the Load, RB
The force applied at the effort arm of the lever by the operator, RC = 733N, which is the maximum power grip strength of a man working with two hands [34]. This value was obtained by assuming that the maximum power grip on the effort arm of the lever was performed using two hands, with an optimal grip span (50 ± 20 mm), neutral wrist posture and while wearing heavy heat-treated gloves. It was also assumed that the fingers of the operator can curl around the pry bar being grasped and that the surface of the bar is not slippery.
Taking moments about the fulcrum, A 3. Determining the Reaction at the Fulcrum, RA Considering equilibrium of forces and summing vertical forces, taking upward forces as positive and downward forces as negative:

Determining the Tube Diameter of the Bead Breaker Arm
The bead breaker arm of the tyre changing tool was designed to be made from a plain carbon steel 30C8 circular tube. Therefore, it was necessary to determine the appropriate size of the circular tube considering the forces acting at various points on the tube. The bending stress, σb, was obtained from [35] as: where Mb = Applied bending moment (N·mm). y = Distance from the neutral axis to the most extreme fibre of the shaft (mm). I = Area moment of inertia (mm 4 ).
The area moment of inertia, I, of a circular section was obtained from [36] as: 4 4 where r = Radius of the circular section (mm).
Therefore, the area moment of inertia, I, of a circular hollow section is given by where ro = Outer radius of the circular hollow section (mm). ri = Inner radius of the circular hollow section (mm). Also, where do = Outer diameter of the circular hollow section (mm). By substituting (7) and (8) into (6) Assuming By substituting (10) into (9) Tube wall thickness,

Determining the Sizes of Hexagonal Bolts and Nuts
The shanks of the bolts at A and B are subjected to double shear stress. The shear stress, τ, was obtained from [37] as where P = Load (N). A = Area of the shank of the bolt (mm 2 ).
Since the bolts are subjected to double shear stress, The diameter of the shank of the bolt at A, dA, and that of the shank of the bolt at B, dB, were calculated from (13).
The height or thickness, h, of the standard nut to be used with the bolts at A and B was obtained from [32] as where d = Nominal diameter of the bolt (mm).

C. Design Calculations for the Mount/demount section
The mount/demount section of the tyre changing tool has a duckhead bar on which a plastic duckhead is attached. The duckhead bar was designed to be adjustable on a mount/demount arm between 40 mm and 330 mm from the centre of the centring post sleeve, which serves as the fulcrum. It was assumed that the tyre changing tool was being used to demount or mount a tyre on a 16-inch wheel rim of 7 inches width. So, the duckhead bar was adjusted on the mount/demount arm to about 200mm from the fulcrum, which is the centre of the centring post sleeve. The duckhead bar was also adjusted to a length of about 300mm from the centre of the mount/demount arm to the surface of the wheel rim. Therefore, the assembly of the mount/demount arm and the pry bar was modelled as a second-class lever as shown in Fig. 4. While mounting or demounting a tyre, the assembly of the mount/demount arm and the pry bar is rotated clockwise or anticlockwise about the centre of the centring post sleeve which serves as the fulcrum. The rotation of the mount/demount arm causes a rotation of the duckhead bar, which carries the duckhead, in the same direction. The clockwise or anticlockwise rotation of the duckhead between the tyre and the wheel rim subjects the mount/demount arm to stresses due to bending moment, as well as stresses due to torsional moment. Therefore, it was necessary to use a square tube for the mount/demount arm, in order to prevent rotation of the duckhead arm about the longitudinal axis of the mount/demount arm.

Determining the Mechanical Advantage of the Mount/demount Arm and Pry Bar Assembly
From Fig. 4, point D is the fulcrum, while points E and F are the load and effort points respectively. The load arm, l1M, is 200mm long while the effort arm, l2M, is 1045mm long. Since, l2M is greater than l1M, the mechanical advantage of the lever is greater than 1. The mechanical advantage was calculated from (3).

Determining the Load, RE
Again, the force applied at the effort arm of the lever by the operator, RF = 733N, which is the maximum power grip strength of a man working with two hands [34]. This value was obtained by assuming that the maximum power grip on the effort arm of the lever was performed using two hands, with an optimal grip span (50 ± 20 mm), neutral wrist posture and while wearing heavy heat-treated gloves. It was also assumed that the fingers of the operator can curl around the pry bar being grasped and that the surface of the bar is not slippery.
Taking moments about the fulcrum, D,

Determining the Reaction at the Fulcrum, RD
Considering equilibrium of forces and summing vertical forces, taking upward forces as positive and downward forces as negative,

Determining the Tube Size of the Mount/demount Arm
The mount/demount arm of the tyre changing tool was designed to be made from a plain carbon steel 30C8 square tube. Therefore, it was necessary to determine the appropriate size of the square tube considering the forces acting at various points on the tube.

a. Determining the Tube Size of the Mount/demount Arm based on Stresses due to Bending Moment
The bending stress, σb, is given by (6). The area moment of inertia of a rectangular section about a centroidal axis, I, in (6) where I ̅ x ′ = Area moment of inertia about the centroidal xaxis (mm 4 ). I ̅ y ′ = Area moment of inertia about the centroidal y-axis (mm 4 ). b = Length of rectangular section (mm). h = Width of rectangular section (mm).
For a square section, b = h, therefore the area moments of inertia, I, in (17) become where b = Length of a side of the square section (mm). While for a square hollow section, the area moments of inertia, I, in (17) become where bo = Length of a side of the outer square section (mm). bi = Length of a side of the inner square section (mm). Also By substituting (19) and (20) into (6) Assuming bo = 1.2bi By substituting (22) into (21) Tube wall thickness,

b. Determining the Tube Size of the Mount/demount Arm based on Stresses due to Torsional Moment
The torque, T, setup in the mount/demount arm by the clockwise or anticlockwise rotation of the duckhead between the tyre and the wheel rim was calculated from where RE= The force acting on the duckhead arm (N). L = The perpendicular distance from the longitudinal axis of the mount/demount arm to the force RL (mm). The shearing stress at any given point of the mount/demount arm wall, τ, was obtained from [38] as τ = T 2tα (26) where T = Torque applied to the hollow member (N·mm). t = Tube wall thickness (mm). α = Area bounded by the centre line (mm 2 ). Assuming where lo = Length of a side of the outer square section (mm). li = Length of a side of the inner square section (mm). Then, tube wall thickness, Also, length of a side of the square section bounded by the centre line,

IV. RESULTS AND DISCUSSION
The design equations were applied, and with the required inputs, results were obtained. The design inputs, results and discussions are presented in this section.

A. Design Input Parameters
Some design inputs were required for computing the results of the design. These input parameters and their corresponding values are shown in Table I.

B. Design Calculations and Results
The design input parameters were substituted into the design equations and results for component parts of the manual tyre changer were obtained. The calculations are shown below.

Calculation of Permissible Stresses
Equations (1) and (2) were used to calculate the allowable bending stress, σb, and allowable shear stress, τ, as follows:

Determining the Load, RB
By taking moments about the fulcrum, A, the load, RB was determined from (4) as follows:

Determining the Reaction at the Fulcrum, RA
The reaction at fulcrum A, RA was also determined from (5) as follows:

Developing the Shear Force Diagram of the Bead Breaker Arm and Pry Bar Assembly
The shear force diagram of the bead breaker arm and pry bar assembly is shown in Fig. 5, and the values were calculated as follows:

Developing the Bending Moment Diagram of the Bead Breaker Arm and Pry Bar Assembly
The bending moment diagram of the bead breaker arm and pry bar assembly is shown in Fig. 6, and the values were calculated as follows: The load, shear force and bending moment values of the bead breaker arm and pry bar assembly are listed in Table  II.  Therefore, the maximum bending moment, Mb is 769700N·mm, and it occurs at point B on the bead breaker arm.

Determining the Tube Diameter of the Bead Breaker Arm
Equation (11) was used to calculate the inner radius of the circular hollow section of the bead breaker arm, ri, as follows: Equation (10) was used to calculate the outer radius of the bead breaker arm, ro as follows: Equation (12) was used to calculate the bead breaker arm tube thickness, tB, as follows: However, from the ASME B36.10M standard for tube sizes [39], the standard tube size for the bead breaker arm was selected to be a 2-inch (50mm) diameter Schedule 80 steel tube.

Determining the Sizes of Hexagonal Bolts and Nuts
On the bead breaker arm, the bolt at A was assumed to be subjected to a force of 7697N while the bolt at B was assumed to be subjected to a force of 8430N. Therefore, it was necessary to determine the sizes of bolts that can withstand the forces at those points. Equation (13) was used to calculate the diameter of the shank of the bolt at A, dA, as follows: π × 40N/mm 2 = 11.07mm However, from the ASME B18.2.6-2006 standard for hexagonal bolt sizes [40], the standard size of the bolt at A was selected to be M12 × 1.5 × 75mm.
Again, equation (13) was used to calculate the diameter of the shank of the bolt at B, dB, as follows: However, from the ASME B18.2.6-2006 standard for hexagonal bolt sizes [40], the standard size of the bolt at B was selected to be M12 × 1.5 × 75mm. Therefore, the size of the bolt at A is the same as that of the bolt at B, this reduces variety and facilitates interchangeability of parts. The dimensions of the hexagonal bolt are shown in Fig. 7. Equation (14) was used to calculate the height or thickness, h, of the standard nut to be used with the bolts at A and B as follows: However, from the ASME B18.2.6-2006 standard for hexagonal nut sizes [40], the standard size of nuts to be used with the bolts at A and B is M12 × 1.5 with a height or thickness of 10mm. The hexagonal nut is shown in Fig. 8.

Determining the Load, RE
By taking moments about the fulcrum, D, the load RE on the mount/demount arm and pry bar assembly was calculated from (15) as follows:

Determining the Reaction at the Fulcrum, RD
The reaction at the fulcrum, RD, was also determined from (16) as follows:

Developing the Shear Force Diagram of the Mount/demount Arm and Pry Bar Assembly
The shear force diagram of the mount/demount arm and pry bar assembly is shown in Fig. 9, and the values were calculated as follows:

Developing the Bending Moment Diagram of the Mount/demount Arm and Pry Bar Assembly
The bending moment diagram of the mount/demount arm and pry bar assembly is shown in Fig. 10, and the values were calculated as follows: The load, shear force and bending moment values of the mount/demount arm and pry bar assembly are listed in Table III. It was necessary to determine the appropriate size of the mount/demount arm considering the forces acting at various points on the tube, based on stresses due to bending moment, as well as stresses due to torsional moment.
14. Determining the Tube Size of the Mount/demount Arm based on Stresses due to Bending Moment From Fig.10, the maximum bending moment occurs at point E. Equation (23) was used to calculate the length of a side of the inner square section of the mount/demount arm, bi, as follows: The length of a side of the outer square section, bo, was calculated from (22) as follows: Next, the tube thickness of the mount/demount arm based on bending stress, tMB, was calculated from (24) as follows: Tube wall thickness, t MB = 44.77mm−37.31mm 2 = 3.73mm From ASTM A500 tables for standard square tube sizes [41], the standard square tube to be used for the mount/demount arm of the tyre changing tool in order for the arm to withstand stresses due to bending moment is a 50mm × 50mm × 4mm square tube.

Determining the Tube Size of the Mount/demount Arm based on Stresses due to Torsional Moment
The tube size of the mount/demount arm was also determined based on stresses due to torsional moment. Equation (25) was used to determine the torque setup in the mount/demount arm as follows: Equation (30) was used in determining the length of a side of the inner square section of the mount/demount arm based on torsional stress, li, as follows: Also, the wall thickness of the mount/demount arm based on torsional stress, tMT, was calculated from (28) as follows: Finally, the length of a side of the square section bounded by the centre line, lc, was calculated from (29) as follows: l c = 1.1 × 49.15mm = 54.07mm From ASTM A500 tables for standard square tube sizes [41], the standard square tube to be used for the mount/demount arm of the tyre changing tool in order for the arm to withstand stresses due to torsional moment is a 60mm × 60mm × 5mm square tube.

Bead-breaking Operation
To replace a tyre on a wheel rim with the tyre changing tool, the tyre is first deflated and placed flat on the base of the bead breaker section of the tool. The bead breaker shoe is then placed on the sidewall of the tyre near the edge of the rim, and a downward force is applied at the free end of the bead breaker arm and pry bar assembly. The tyre-rim assembly is then rotated through 180 o about the vertical axis and the bead breaker shoe is again placed on the sidewall, close to the edge of the rim, and a downward force is applied at the free end of the bead breaker arm and pry bar assembly. The tyre is then flipped over and the beadbreaking process is repeated for the underside of the tyrerim assembly.

Demounting a Tyre from a Wheel Rim
After carrying out the bead-breaking operation, the tyrerim assembly is placed on the mount/demount section of the tool, centred with the aid of the locking pin and the centring post and fastened with the aid of the centring cone and the centring post sleeve. The area where the edge of the wheel rim meets the tyre is properly lubricated to allow for easy demounting of the tyre. Next, the duckhead is moved into position, between the rim flange and the tyre bead. The mount/demount arm and pry bar assembly are then rotated clockwise or anticlockwise until the tyre bead completely lifts off from the wheel rim. The process is repeated for the underside of the tyre-rim assembly, the tyre can then be easily separated from the wheel rim.

Mounting a Tyre on a Wheel Rim
The wheel rim is placed on the mount/demount section of the tool, centred with the aid of the locking pin and the centring post and fastened with the aid of the centring cone and the centring post sleeve. The tyre bead edge is properly lubricated to allow easy mounting of the tyre on the wheel rim. Next, the duckhead is moved into position, between the rim flange and the tyre bead. The mount/demount arm and pry bar assembly are then rotated clockwise or anticlockwise until the flange of the wheel rim is completely above the tyre bead. The process is repeated for the underside of the tyre-rim assembly, until the tyre bead is completely above the flange of the wheel rim.

V. CONCLUSION AND RECOMMENDATIONS
The design of a portable manual automobile tyre changer has been developed in this paper. Loads, stresses, shear forces, bending moments and torsional moments were considered in selecting the component parts of the tool. The bill of materials in Fig. 13 shows that it costs only ₦21,000 to purchase the materials needed to fabricate the tool. Therefore, the design provides an alternative portable and relatively affordable tyre changing tool that can be afforded by tyre technicians across Nigeria, and other developing or underdeveloped countries.
Obtaining a higher mechanical advantage of the mount/demount arm and the bead breaker arm of the tyre changing tool will improve the overall effectiveness of the tool. Thus, if the tool is installed in an area with ample space, the overall mechanical advantage of the manual tyre changer can be further increased by increasing the length of the effort arm or pry bar. However, if there are space constraints, compound levers should be utilized instead.
Therefore, it is recommended that more designs of tyre changers that utilize compound levers could be developed, in order to produce a cost-effective tyre changing tool that possesses an improved overall mechanical advantage.