Table of Contents
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| A production car's unibody. | A look at Stanford's unibody. |
Monocoque
A monocoque is a structural system where loads are supported through an object's external skin, similar to an egg shell. MSXI used a monocoque as its frame which presented numerous challenges with regards to structure, manufacturing and serviceability. Due to these challenges, most solar car teams do not elect to use a monocoque.
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| A Lamborghini Aventador has a carbon fibre central monocoque, with front and rear steel subframes. | MSXI's monocoque prior to vinyl wrapping. |
System Design Considerations
Impact on Suspension
Centre of Gravity and Handling
Material Considerations for Space Frames
4130 Chromoly
4130 is often seen as superior to mild steels like 1018 and 1020 for the use of space frames (ex. FSAE).
- Both chromoly and mild steels have the same elastic modulus
- Ultimate tensile strength of 4130 is higher
- Density is approximately the same
- Since UTS is higher, you can use thinner walled members in the space frame and save weight
- Chromoly could be around 50% more expensive (estimate)
- A chromoly frame could b around 80% of the weight of a mild steel frame (estimate)
- If stiffness if the limiting factor, chromoly will not help since elastic modulus is the same
Strength
Torsional Rigidity
Torsional rigidity is a measure of a vehicles resistance to twisting from a moment applied along it's longitudinal axis. It is often expressed in Nm/degree (torque required to create one degree of twist). Torsional rigidity is very important to vehicle handling, especially with regards to cornering at speed.
Modelling and Analysis
FEA
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| Picture | Description | |
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Tires |
| Tires form the only point of contact between the vehicle and the road. The grip provided by the tires help propel the car forward, maneuver it a round a corner or bring it to a stop. Excessive wear on a particular area of a tire can be an indiction of misalignment in the suspension. |
| Rims |
| Tires are mounted to rims which then join to the hubs. position of the point of contact between the hub and the rim is called the wheel "offset". |
| Hub / Knuckle / Uprights |
| A hub is the central part of a wheel that connects the control arms and axle or spindle to rim. A hubs contains bearings which allow the wheel to spin. |
| Spindle |
| On a non-drivewheel, the spindle is the axel that connects the hub to the upright or knuckle. |
| Control Arm |
| S hinged suspension link between the chassis and the suspension upright or hub that carries the wheel. |
| Tire-rod |
| The linkage between the steering system and hubs. |
| Shock |  | The spring damper system that helps to eliminate vibrations and give the passages a smoother ride. |
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Offset is the distance between the central axis of the wheel and the mounting plane between the hub and the rim.
Camber
Camber is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or rear.
| Widget Connector | ||
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| Widget Connector | ||
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Scrub Radius
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The scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road.
Sprung and Unsprung Mass
The mass of the body and other components supported by the suspension is the sprung mass. Unsprung mass includes the mass of components such as the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the weight of driveshafts, springs, shock absorbers, and suspension links.
Roll Center
The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll.
Double wishbone suspension
Although there are several different types of suspension systems used in the automotive industry, we shall focus on one in particular: the double wishbone suspension. As Carroll Smith explains in Tune to Win, "this is where eight year of motor racing has held us... virtually every serious racing car has implied on for or another of the four bar link independent suspension [(double wishbone)]" - pg 42. Although Tune to Win was published in 1978, the double wishbone suspension is still the most commonly used system on solar cars today.
Objectives of a Suspension System
- Four wheel independence (disturbance on one wheel doe not necessarily induce a disturbance on the others)
- Zero change in toe in / toe out while suspensions is moving
- Minimal change in camber as the wheel translates due to longitudinal load transfer (accelerating, decelerating, aerodynamic effects) or road irregularities
- Minimal change in camber (with respect to the road) as the car rolls when cornering