Overview - General Automotive Engineering Notes

Table of Contents

Frame / Chassis

Types of Solar Car Frames

There are three main options when it comes to selecting how to give a solar car structure: space frame, unibody or monocoque. To date, all of Midnight Sun's solar cars have used a space frame chassis except for MSXI which used a monocoque. Related information about frame design can be found on MSUN 101: Overview of Solar Car Design. 

MSXII will use a steel space frame. 

Metal Space Frame

A space frame uses lengths of round or square tubing, or other structural metal shapes to form the chassis structure. Although many solar car teams are trying to use more composites in their cars, many teams such as Principia, Appalachian, Iowa State and ETS Quebec still build space frame type vehicles. The aforementioned teams all competed successfully at FSGP and ASC 2016. Space frames are also commonly used by other design teams such as FSAE and Baja. 

Unibody 

Increasingly, solar car teams are choosing to use a unibody frame where joined composite panels form the chassis structure. In general, an equivalent space frame requires more material and thus more weight relative to a composite unibody. However, composite frames are generally more difficult to produce and service. It is also more difficult to recognize damages/defects on them. 

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. 

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

SimCafe

SimCafe is a wiki created by Cornell University that has some great resources for learning about FEA. 

ANSYS

Scale Models 

While designing a space frame, it may be useful to make a scale model of the truss structure (perhaps either 3D printed or laser cut from balsa wood). This will allow team members to better understand the shape of the car and how it will be constructed, especially those not on the mech team. A scale model can also allow you to "feel" the strength and stiffness of the vehicle. Although a model's strength can depend heavily on it's material (balsa wood for example is highly orthotropic), the ability to feel the structure in your had can still be useful. 

Manufacturing

Tube Cutting and Notching

Welding

Suspension

Components

	Picture	Description
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.

Geometry and Terminology

Offset

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. 

Toe

Toe-In or Toe-out is a slight angle of a wheel/tire measured from a line running longitudinally (along the length of the vehicle).

Caster

Caster is the angle, measured from vertical (90° perpendicular to a flat road surface), of a line that runs through the mounting points of the upright/knuckle, when viewing the wheel/tire from the side. Positive caster is usually used on vehicles to help the wheels move back to steering straight when the steering wheel is released. 

Steering Axis Inclination

Steering axis inclination is the measurement in degrees of the steering pivot line when viewed from the front of the vehicle. Steering axis inclination is used to change the scrub radius and can help the wheels move back to steering straight when the steering wheel is released.  

Scrub Radius

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

1. Four wheel independence (disturbance on one wheel doe not necessarily induce a disturbance on the others)
2. Zero change in toe in / toe out while suspensions is moving
3. Minimal change in camber as the wheel translates due to longitudinal load transfer (accelerating, decelerating, aerodynamic effects) or road irregularities 
4. Minimal change in camber (with respect to the road) as the car rolls when cornering