Design of Hub and Knuckle
If you have ever participated in any student racing championship, such as mini BAJA, You must be aware of the tough competition given by the teams in all the aspects. Whether it is a static event or a dynamic event, we get to see a new height of engineering used by the students. Out of all the events, the most challenging event that affects both your static as well as dynamic performance is “Design Inspection”. If your design is good, you get to score in static as well as dynamic events.
Though there is a lot to do in the design of a vehicle, in this blog I will tell about the designing of one of the most important and expensive manufactured part in a buggy for BAJA.
Hub and Knuckle
Before going through the design procedure, let’s see what is actually a hub and a knuckle.
Hub, in an automobile, is the rotating part which holds rim and disc(if applicable). A hub can be connected to the driving shaft for the driving wheel or it can be free for the driven wheel. In both cases, it is supported by knuckle via bearings.
(Image for descriptive purpose only)
Knuckle is the stationary part which supports hub and thus connects the wheel to the vehicle via suspension system.
(Image for descriptive purpose only)
The images are for descriptive purpose only because you won’t like to design hub and knuckle with so much weight. A simple rule of designing is that your product should have strength, should be feasible in manufacturing and should be lightweight.
As stated earlier, Hub and Knuckle form a single assembly which connects wheels to suspension. So we need to understand how this assembly works.
Download sample Hub and knuckle from here to understand better
The hub knuckle assembly is such that it allows the hub to rotate freely on knuckle even when different forces act on it. For this bearings are used between hub and knuckle, but a major challenge that comes in this process is the selection of bearing. If the forces acting on wheels were the only radial, We would be using the most common type of bearing, i.e., “Single deep groove ball bearing”. But this is not the case here. The force that acts on wheels while running and especially turning is a vector sum of radial load due to vehicles weight and axial load due to steering. Ball bearings have a certain capacity to withstand axial loads above which their life and performance decreases. To withstand such kind of load, we use “tapered roller bearings” which looks something like this.
The tapered part allows the bearing to bear both radial as well as axial load. But these bearings cannot be used alone. They are always used in a pair. Each bearing to withstand axial load in two different directions.
The design for front and rear assembly is different. so a different design style is required for them.
To design front knuckle we need some data from other departments.
- Steering and Suspension department
- Caster angle
- Upper arm and lower arm length
- Camber angle
- Steering arm angle
- Track width
- Braking department
- Caliper position
- Caliper width
You have already crossed halfway if you have got the above information. Now the only part remaining is the mechanical design for durability. A line diagram which includes suspension and wheel geometry will be helpful in deciding the parameters for hub and knuckle
Here are some points to follow-up.
1. Load Calculation: The maximum load that will act on your knuckle will be when your vehicle collides and the first point of impact is your wheel. It is the similar impact what you assume while designing other parts such as rollcage. So the load is also similar. Take the same load what you took for your rollcage. But not less than 4G ( 4 times the weight of your vehicle in newtons).
2. Factor of Safety (fos): The idea factor of safety for knuckle is 2. There is no such standard for fos, this is just by experience. You can increase or decrease it based on your requirement.
3. Boundary Conditions: The load is applicable on the two area of spindle where inner and outer tapered bearings will fit. the arm which supports shock absorber will be fixed and the other arm will be frictionless support.
3. Material: Well, the most common materials for knuckle are cast iron, steel or aluminium of different grades. Each material has its advantage and disadvantage.
Cast Iron will make your manufacturing cost low but it will be very heavy. Steel will have slightly higher cost than iron but the strength will be very good and weight will also be lower than iron. Aluminum, on the other hand, will be very expensive but it will be very lightweight. In my experience steel of good grades like AISI 1018 or AISI 4130 is a good choice.
4. Spindle Design: You can accurately determine the Spindle length from the above line diagram. Now you just need to divide that into different parts of different lengths and diameters. Following activities needs to be done together, as one depends on other. An accurate spindle design is achieved by performing several iterations for a different combination of below parameters.
- Bearing selection: As stated earlier, tapered bearings are used in between hub and knuckle. Bearing selection is done by calculating the static and dynamic load on the bearing. A proper method can be found in Design of Machine Element by VB Bhandhari. Things to keep in mind while selecting bearings are that the bearings should be as small as possible both in width and Outer Dia. Also, make sure that the bearing you are selecting is available in your area.
For a list of available tapered bearings, you can refer this brochure of SKF bearings
- Spindle Diameter: A rough idea of spindle dia is made by using the bending moment formula for a cantilever beam.
Where, M = Force (4G) x (half of spindle length) x fos
σ = Tensile strength of your material
I = Moment of inertia of spindle, for cantilever circular beam it is given by πr4/4
Y = distance of neutral axis from the outer surface, here Y=d/2
From this formula, you can find a rough value of d.
But this will give you an average dia. In the above figure of the front knuckle, you must have observed that the spindle has two diameters, the outer part of the spindle is smaller. This is because then only our tapered bearing assembly will work. The step acts as a collar for the outer bearing.
Now to calculate the two diameters of the spindle, it is easier to go on a hit and trial method by using simulation in ANSYS or any other software.
To continue further, select two bearings from the brochure. One with Inner Dia (ID) ~5 mm less than our calculated Dia, D. and one with ID ~5 mm greater than D. Design the Spindle according to the bearing selected, with both diameters and width.
Give a taper between these two positions and an extension of 1in length at the end for the nut.
- Spindle Length: The complete spindle length is identified by the line diagram of suspension geometry. Divide that length into Collar, Bearings, taper, and thread according to the selected parameters.
5. Knuckle body: Rest of the body can be given a simple design as illustrated in the above image. For the thickness, you can use the same method as for spindle dia. Just put the correct value of all the variables. Or you can even go for hit and trial method. Take any thickness, maybe equal to the smaller dia of the spindle. And width 10 mm greater than the collar dia and adjust these parameters on the basis of simulation results.
Remember, the body of knuckle has a good scope of weight reduction. You can use various designs to get the optimum strength with minimum weight. All you have to do is Iterations.
6. Simulation: The simulation results will decide the ultimate design of your knuckle. For our study, we can use “Static Structural” from ANSYS. Import your solid model into ANSYS and apply the boundary conditions as stated earlier. Try to achieve the factor of safety near 2.5. Remove or reduce the areas with very high fos to reduce the weight. It all depends on your thinking capability to design a lightweight durable part.
For other parts, we will describe in the shortcut. The basic steps are same for all the parts.
For Hub, you need to know Pitch Circle Diameter (PCD), no. of holes and diameter of holes of Your Rim and brake disc. Then you have to make a hub to fit your wheel and disc and it should rest on the spindle properly.
This is an example of a front hub
The main thing to keep in mind is the setting of bearings. You need to provide a common collar for both the bearings to prevent them from slipping from their respective position. The final assembly will look something like this.
Simulation: Hub has to be tested for two conditions
- For impact
- For Torsion
- For impact: Force is same as that on knuckle. The bearing rest is fixed support and holes for rim bolt bears the load.
- For Torsion: Take torque which is maximum amongst the driving torque or braking torque. Find force by multiplying toque with PCD/2 and distribute it equally on either Rim holes or disc holes and fix the other one.
Try to achieve a fos of 2.5.
Rear Hub and Knuckle
The design procedure for Rear hub and knuckle is same as front one. Only the way of assembly is change.
The rear knuckle will vary according to the suspension geometry you use. The illustrated knuckle is only suitable for the double wishbone.
The simulation technique is also same for front and rear parts with same load and fos.
That’s all about the most critical part in a BAJA vehicle. I hope this post was helpful for you and now you are ready to compete with the best teams of BAJA by designing your own parts. If you liked this post, please hit the like button. If there is anything which I was not clear about, please comment. The team of FEADesign is always there to help you in designing.