For the 2019 BAJA SAEINDIA season, I served as Design and Analysis Head of IIT Indore's team- Team Engines and Demons. I was responsible for making important design decisions and performing design, analysis and selection of components. With an aim to design an optimized vehicle with minimum weight, we started from scratch and surfed through various research papers, books and resources to find out optimum parameters for our vehicle. I designed the wheel assembly with utmost attention to details and nearly spent 3 months until I received satisfaction with my designs. It involved numerous iterations, simulations, learnings and finally, I was able to bring the weight of the knuckle and hubs down to 350 gms each. Here, through a series of articles I wish to share my knowledge obtained with my juniors as well as other BAJA enthusiasts. I am planning to explain the whole design process using my own design and analysis addressing issues like the selection of the bearings, FBDs and selection of boundary conditions.
While there may be some flaws, I'd be glad to address those if some of the readers point it out. Feel free to reach out to me in that regard.
In this article, I want to discuss the constituents of a wheel assembly and possible configurations.
Wheel Assembly connects the wheel with the suspension system. It also supports the brake system, attached to the wheel. The following figure shows a typical wheel assembly.
The wheel assembly is termed a front or rear wheel assembly according to the wheel it attached to. Depending upon whether its FWD, RWD or 4WD, the configuration & components of the wheel assembly may change.
Generally all automotive use front-wheel steering system. So, front-wheel assembly of a car usually consists of steering knuckle with steering arm, brake rotor, wheel hub. Rear-wheel assembly consists of: rear knuckle, brake rotor and wheel hub.
Wheel Hub connects to the wheel rim and is usually free to rotate about the axle. The stud bolts provided on the wheel hub. (No. 6 in the figure).
[ Why stud bolts? - we can plug the wheel and tighten the bolt easily. Imagine lifting the heavy wheel, matching it with the holes on the hub and tightening the nut bolts. even with threaded holes, plugging nuts into those holes would be too tedious and time-consuming- not a smart idea! ]
[ Why not normal bolt and stud bolt instead? - very less clearances with the brake rotor/disc in case of normal bolts.]
Brake Rotor / Brake Disc is attached to the wheel hub- and is responsible for stopping the rotational motion of the wheel with friction induced by brake pads of the calliper (in disc brakes) or brake shoes (in drum brakes)
Steering Knuckle supports the stationary component of the brake system (calliper in case of disc brake), supports the wheel axle/ wheel hub, connects the wheel to the suspension system (and steering system for front-wheel assembly).
Usually, all BAJA Buggies are RWD because of simplicity in the design of the cockpit area and reduced weight. In rare case, some teams have attempted mid-engine buggy or recently 4WD in BAJA. We had employed RWD in our car. (I will soon write a separate blog post for the design selections of the transmission system, but here I will focus on the wheel assembly design.) In the RWD, the front-wheel assembly has passive axle, and the rear wheel assembly has an active axle.
The configuration of the wheel assembly is broadly of two types. (Names are given by me- not official):
1. Passive axle integrated with knuckle (with/without stub axle)
2. Passive axle integrated with the wheel hub
1. Axle integrated with the knuckle
In this configuration, as the name suggests, the axle is fixed (does not rotate) with the knuckle and the wheel hub supported by the bearings rotate on the axle. Passive axles (Not powered) are generally (but not necessarily) of this type. I have seen this configuration generally used in the front wheel assembly of most of the RWD BAJA buggies.
2. Axle integrated with the wheel hub
Active (powered) axles usually use this configuration. Rear-wheel assembly in BAJA is generally of this type. In this case, the axle is supported by bearings in the cylindrical bore of the knuckle and rotates with the wheel. This axle is joined to the transmission shaft usually a CV axle with tripod/Rzeppa joint or universal joints. OEM axle can be fixed with the rear hub through the splines and or castle nut provided.
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