Tow Hook Design for Bus, Truck or Commercial Vehicle

Tow Hook: As the name describing itself, a tow hook is an accessory to use for towing any vehicle for transportation of vehicle from one place to other in case of breakdown.
While designing a Tow Hook first you have to know what a Tow Hook is and what is the application of Tow Hook. There are a lot of types of application of a Tow Hook, So first decide the proper application. (Here are few of them)

1. Tow Hook for Haulage application or for Tractors, Trolley, Thresher, Harrow etc.
2. Tow Hook for Small Vehicles like Cars, Jeeps, Van, SUVs and MUVs.
3. Tow Hook for Heavy Commercial Vehicles like Bus, Trucks and Tipper etc.
4. Tow Hook for Transportation carriers like Trailers, SPV (Self Propelled Vehicles), Ships etc.

As above mentioned a Tow Hook can be used anywhere as per requirement and so the design of Tow Hook is also very as per requirements. Here I am going to design a Tow Hook for a Bus which is a Heavy Commercial Vehicle. So the design should be robust because it has to pull or push the Bus.

Here I am going to design a Tow Hook to pull a 12.6 Ton Bus running with velocity 18 Km/Hour. On bus we can mount the towing arrangement either on the Chassis Frame side member or at front or rear side cross members. So I will design both Tow Hooks which can be mounted on side member of chassis frame assembly and cross members.

First of all I will do some calculation of Tow Hook designing; it will help me to validate my designed Tow Hook in simulation also. After designing I will do 3D modeling of Tow Hook in SolidWorks and will check it with FEA analysis with Cosmos Works embedded in SolidWorks itself for simulation. If required then I will also do the design optimization in Cosmos Works. As we have to Tow and pull the bus in case of breakdown, so first we need to know the pulling effort required to pull the bus or forces on tow hook. For this we will do some basic physics calculations. Basically the three forces which are resisting the motion of vehicle are Rolling Resistance, Gradient Force and Aerodynamic Drag. We can calculate the Force required for pulling the bus by adding the three resistances.

Rolling Resistance Calculation for Vehicle:

Fr = Cr *mg
Where:

Cr = coefficient of rolling resistance

m=mass of vehicle

Fr = Rolling Resistance

As mentioned above, the mass of bus (m) is 12600 Kg and the bus is rolling on the tyres. The tyres will generate rolling resistance which will oppose the movements of bus in pulling direction. 

Coefficients of Rolling Resistance
Road surface	Coefficient (Cr)
Good asphalt road	0.007
Wet asphalt road	0.015
Good concrete road	0.008
Rough concrete road	0.011
Block paving	0.017
Poor road	0.032
Dirt track	0.15-0.94
Loose sand	0.15-0.30

Suppose the bus is rolling on rough concrete road and the rolling resistance coefficient for tyre on concrete road is 0.011.
Fr = 12600*9.81*0.011 = 1360 N

Gradient Force Calculation for Vehicle:

Fg = mg* Sin θ
Where:

Fg = Gradient Force.

Assume the angle of Road toward up (θ) = 7°

Fg = 12600*9.81*0.122 = 15080 N

Aerodynamic Drag Calculation for Vehicle:

Fa = Cd*Af*ρ*v²/2
Where:

Fa = Aerodynamic Drag Force

Cd = Drag Coefficient

Af = Frontal Area (approx. 0.9bh m²) = 0.9*1.85*3 = 4.995 m²

ρ = Air Density (≈ 1.2 Kg/m³)

v = velocity (m/s) = 18000/3600 = 5 m/s

Typical values of Drag Coefficient
Sports car, sloping rear	0.2-0.3
Saloon, stepped rear	0.6-0.7
Convertible, open top	0.4-0.5
Bus	0.6-0.8
Truck	0.8-1.0
Motorcycle and rider	1.8
Sphere	0.47
Flat plate normal to flow	1.2
Long stream-lined body	0.1

Aerodynamic Drag Force:

Fa = 0.8*4.995*1.2*5*5/2 = 59.94 N or 60 N

So the total force required to pull the bus is Ft = Fr + Fg + Fa

Ft = 1360+15080+60 = 16500 N

So we need a force of around 16500 Newton’s to pull the bus on a concrete surface with a slop of 7° and 18 Km/Hr speed.

Now we can design a Tow Hook which can pull the bus and will be safe for the above load. Now we have to decide the mounting position or location of Tow Hook on vehicle. Here we have two options:

1. Tow Hook which is mounted on side member of Chassis frame Assembly.
2. Tow Hook which is mounted on front or cross member.

My first design of Tow Hook which can be mounted on side member of chassis is a simple U shape hook. This type of hook can be mounted on side member as well as cross member of chassis frame assembly.
So let’s start the design of U type Tow Hook as shown below. First we will do the Tow Hook calculation and then the 3D modeling and FEA simulation.

This type is very easy in manufacturing as well as can be mounted on side member of cross member of chassis frame assembly with two bolts. The diameter of the hook is denoted by Dh and the diameters of holes for mounting bolt are denoted by db.

This Tow Hook is designed to be mounted on chassis frame with two bolts, so we will need to design the bolt size First.
The two bolts are also will be in shear load after assembly with chassis frame assembly. The towing load will be shared by both bolts equally.

Shear Stress = Ft/Ab

Where:

Shearing Area of Bolts (Ab) =2*π*db²/4 OR 1.57 db² (Where db is diameter of Bolt)

Ft (Pulling Force) = 16500 N

The above table will be helpful while selecting the Shear Strength for bolt.
(Assumptions: For an 8.8 property class and bolt size < M16, Min. Shear Strength is 480 N/mm²)
Hence for repeated cycles or with a factor of safety of 5, we will take permissible Shear Stress = 96 N/mm²
So
96 = 16500 / (2*(π* db²/4)) = 16500 / (1.57* db²)
db² = 16500 / (1.57*96)
db = 10.46 mm
So two Bolts of M10 are sufficient to pull the Vehicle

Before proceeding we have to assume the material for Tow Hook. Material Selection for Tow hook is also very critical. So we have to select a material which will have a good Tensile and Yield Strength. I have selected St 42 material for Tow Hook, but it is not available in SolidWorks library so I have searched & selected ASTM A36 Steel in SolidWorks which have mechanical properties nearer to St 42.

Material	St 42 (IS: 1570, Part-1)	ASTM A36 Steel
Ultimate Tensile Strength	410 N/mm²	400 N/mm²
Yield Stress (Min.)	250 N/mm²	250 N/mm²

We will select ASTM A36 Steel which is also available in SolidWorks 2011 material library. This will be helpful in FEA analysis while simulation. While pulling bus the tow hook will be subjected with bending, shearing and tension loads. I will design the Tow Hook for shearing load at the center of bend and tension load at the holes for bolting.

Relation between Shear Stress and Ultimate Tensile Strength for steel:

Shear Stress = 0.577*Yield Stress

So Shear Stress for ASTM A36 Steel = 0.577*250 = 144.25 N/mm²

Hence for repeated cycles or with a factor of safety of 5, we will take Permissible Shear Stress = 28.85 N/mm²
The diameter of Hook (Dh) can be finding out by Shear force calculation equation.

Shear Stress = Ft/Ah

Permissible Shear Stress = 28.85 N/mm²

Ft (Pulling Force) = 16500 N

Ah (Resisting area) = 2*(π*Dh²/4)

So by putting the different values in shear stress calculation equation:

28.85 = 16500/ (2*(π*Dh²/4)) = 16500/ (1.57*Dh²)

Dh² = 16500/ (1.57*28.85)

Dh = 19.08 mm

So Rod of Ø20 is sufficient to pull the Vehicle.

The 3D modeling and FEA analysis of tow hook will be coming soon. So stay tuned..........