Front Wheel assembly will connect the wheel with the usual Dual A-arm (Double Wishbone) suspension system, and tie rod. As a designer, our duty is to design a robust wheel assembly, that can bear heavy road and impact loads, implements the suspension geometry provided by the suspension department ( king-pin inclination, scrub radius, relative positions of upper and lower ball joints with the axle).
Components of the front wheel assembly are ( starting from the wheel):
1. Stud Bolts
2. wheel Hub
3. Circlips/ Retaining Rings
4. Hub Bearings
5. Brake rotor and bolts fixing it
6. Brake Caliper
7. Steering knuckle
8. Upper and Lower Ball Joints & its spacers and bolts
9. Steering arm
1. Stud Bolts:
Stud Bolts (Wheel Studs) are the bolts on which a wheel is mounted and it facilitates easy assembling dissembling of the wheel. It takes the least space on the other side of the wheel hub with its stud head with minimum thickness. Thus, it provides better clearance from the fixed components and reduces the length of the axle thereby reducing the weight (our prime goal).
These are usually standard bolts, and we can select its dimensions from the hole size of the wheel rim. Though we can tweak it through some jugaad, but it's better to refrain from it, and go forward with general M10 bolts. We will get M10 lock nuts to fix our wheel on it and can tighten it using 16- number (ISO/ANSI) wrench.
2. Wheel Hub:
On the wheel hub, the brake rotor is fixed and The wheel Hub sits on the axle with the support of hub bearings. The detailed design of the Wheel Hub is content for a separate blog. Please refer to that! (coming soon!)
3. Circlips/ Retaining Rings are useful in positioning wheel assembly components (Bearings, Hub) Axially. These are OEM components with standard sizes available. Each circlip has specified standard groove sizes and dimensions, that we need to follow. Following is a nice source to access standard dimensions of the circlips:
Circlips are of two types: External and Internal and also according to its strength, there are types like standard, heavy-duty etc. The load-bearing capacity of each circlip is also available in the sources above, which can be utilized for the selection of appropriate circlips.
4. Hub Bearings: Hub bearings facilitate smooth rotation of the hub on the axle. All the road loads from the hub are transmitted via the hub bearings to the axle. We usually learn the calculations for selection of bearings in the typical machine design course. Yet if not, bearing catalogues are at our help. In any case, it is a must that we go through them before selecting the bearing. They provide detailed information on the types, selection, arrangements of the bearings.
Out of the many types available, the inclined roller bearings and deep groove ball bearings are mostly used as hub bearings. We need to counter both sides of axial thrust as well as radial loads of the vehicle weight as well as loads in the possible scenario of the impact. We need to calculate the loads and with a higher factor of safety, it is recommended to select bearings for infinite life >10E6 cycles. I will explain my load calculations later in this blog. We went with two deep groove ball bearings on each side of the vehicle in the front.
5. Brake Rotor/ Brake Disc as we might usually call it, is a part of the brake system that is bolted on the wheel hub and rotates with the hub and that's why in turn with the wheel. It will pass through the gap between the brake pads in the calliper. When the calliper applies the pressure on the discs through pads, the frictional braking force is transmitted from the brake rotor to the hub and in turn to the wheels through the mounting bolts of the brake rotor.
6. Brake Calliper: Brake calliper is the piston-cylinder assembly that applies pressure on the brake disc by pressing brake pads against it.
7. Steering Knuckle: Steering Knuckle provides the direction and motion to the wheel axle from the steering & suspension system.
8. Upper and Lower Ball Joints & its spacers and bolts: Upper and Lower Ball Joints connect the knuckle to the suspension A-arms.
9. Steering arm: Steering Arm joins the Steering Knuckle to the tie rods and thus to the steering system.
Before going to the designing of the components of the wheel assembly, it is necessary to determine the suspension geometry and steering geometry from the respective departments. These geometries are designed and optimized through ADAMS, LOTUS, MotionSolve etc softwares.
We require the coordinates of:
1. Wheel Center (it's usually the reference, so it might not be provided, and other dimensions are provided wrt it.)
2. Upper Ball Joint
3. Lower Ball Joint
4. Tie-rod mounting point
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