Fhwa Class and Vehicle Layouts

Chapter 1 — Traffic Estimation and Section 19 — FHWA Vehicle Classification Forecasting Figures

Section 19

FHWA Vehicle Classification Figures

FHWA Vehicle Classifications

Figure 1-14: FHWA Class 1 — Motorcycles

Figure 1-15: FHWA Class 2 — Passenger Cars (With 1- or 2-Axle Trailers)

Figure 1-16: FHWA Class 3 — 2 Axles, 4-Tire Single Units, Pickup or Van (With 1- or 2-Axle Trailers)

Figure 1-17: FHWA Class 4 — Buses

Figure 1-18: FHWA Class 5 — 2D - 2 Axles, 6-Tire Single Units (Includes Handicappe- Equipped Bus and Mini School Bus)

Figure 1-19: FHWA Class 6 — 3 Axles, Single Unit


Figure 1-20: FHWA Class 7 — 4 or More Axles, Single Unit

Figure 1-21: FHWA Class 8 — 3 to 4 Axles, Single Trailer

Figure 1-22: FHWA Class 9 — 5 Axles, Single Trailer

Figure 1-23: FHWA Class 10 — 6 or More Axles, Single Trailer

Figure 1-24: FHWA Class 11 — 5 or Less Axles, Multi-Trailers


Figure 1-25: FHWA Class 12 — 6 Axles, Multi-Trailers

Figure 1-26: FHWA Class 13 — 7 or More Axles, Multi-Trailers

1Vehicle layouts and some simple vehiclestructures

Topics covered in thischapter

The types of forces to which a vehicle structure issubjectedUnitary constructionChassis-built vehiclesFront- and rear-wheel drive and the layout of thesesystemsPassenger protection e crumple zones and side impactbarsVehicle shape e air resistancePros and cons of front- and rear-wheel-drive systemsEnd-of-life vehicles and methods of disposal

Light vehicles

The term light vehicle is generally taken to meanvehicles weighing less than 3 tonnes. These are vehiclessuch as cars, vans, and light commercial vehicles.Various types of light vehicles are shown in Fig. 1.1.Most of these vehicles are propelled by an internalcombustion engine but increasing concern aboutatmospheric pollution is causing greater use of electricpropulsion. Vehicles that incorporate an internalcombustion engine and an electric motor are calledhybrid vehicles.

Vehicle structure

Figure 1.2 shows some of the forces that act on a vehiclestructure. The passengers and other effects cause a down-ward force that is resisted by the upward forces at theaxles. The vehicle structure acts like a simple beamwhere the upper surface is in compression and the lowerone in tension. When the loading from side to side of thevehicle is unequal the vehicle body is subject to a twistingeffect and the vehicle structure is designed to havetorsional stiffness that resists distortion through twisting.

There are basically two types of vehicle construction:one uses a frame on to which the vehicle is built; theother is called unitary construction where the bodyand frame are built as a unit to which subframes areadded to support the suspension and other components.

The frame is normally made from low-carbon steelthat is formed into shapes to provide maximum strength;box and channel sections are frequently used for thispurpose. The frame shown in Fig. 1.3 has a deepersection in the centre area of the side members becausethis is where the bending stress is greatest. In areaswhere additional strength is required, such as wheresuspension members are attached, special strengtheningsupports are fitted.

Unitary construction

Most of the vehicle structure is made from steel sectionsthat are welded together to provide a rigid structurewhich is able to cope with the stresses and strains thatoccur when the vehicle is in use. In most cases the mate-rial used is a deep-drawing mild steel that can be readilypressed into the required shapes. An example of unitaryconstruction is shown in Fig. 1.4. In some cases, theouter panels are made from plastics such as Kevlar(Fig. 1.5), which has excellent strength and resistanceto corrosion.

Body panels are normally lined with sound-deadeningmaterial which is either sprayed, or glued, on to theinside. In order to protect body components againstrust and corrosion, they may be galvanized, or treatedwith some other form of protection.

Passenger protection

In addition to providing the strength required for normalmotoring conditions, vehicles are designed to protect theoccupants in the event of a collision. Two areas of vehicleconstruction are particularly related to this problem:

1. Crumple zones2. Side impact protection.

� 2011 Allan Bonnick and Derek Newbold. Published by Elsevier Ltd. All rights reserved

Crumple zones

A vehicle in motion possesses kinetic energy. Becauseenergy cannot be destroyed, some means has to be foundto change its form. Under braking conditions, thekinetic energy is converted into heat by the friction inthe brakes e this heat then passes into the atmosphere.When involved in a collision the kinetic energy is

absorbed in distorting the vehicle structure e if thisdistortion can take place outside the passenger compart-ment, a degree of protection for the vehicle occupantscan be provided. The front and rear ends of motor carsare designed to collapse in the event of a collision; theareas of the bodywork that are designed for this purposeare known as ‘crumple zones’ (Fig. 1.6). At the design

Van

People carrier

SUV

SaloonHatchback

Pickup truck

Fig. 1.1 Types of light vehicles

Vehicle structure

Bending effect

Twisting effect

Twisting effect

Fig. 1.2 Forces acting on vehicle structure

2 A Practical Approach to Motor Vehicle Engineering and Maintenance

Cross members

Suspension mounts

Box section side membersFront

Rear

Fig. 1.3 A typical vehicle frame

Radiator grill panel

Wing valance

Bulk head

A Pillar

Sill

Floor panel

Cant rail

B Pillar

C Pillar

Fig. 1.4 Features of a unitary construction body

Boot lidmade fromKevlar

Fig. 1.5 Kevlar body panel

Fig. 1.6 Crumple zone e impact test at the British Transport

Research Laboratory (TRL)

Image Courtesy of StaraBlazkova at the Czech language Wikipedia

Vehicle layouts and some simple vehicle structures 3

and development stages, and prior to introduction intogeneral use, samples of vehicles are subjected torigorous tests to ensure that they comply with the stan-dards that are set by governmental bodies.

Side impact protection

In the event of side impact, a degree of protection tooccupants is provided by the bars that are fitted insidethe doors (Fig. 1.7).

Vehicle shape

A considerable amount of engine power is consumed indriving a vehicle against the air resistance that is causedby vehicle motion. The air resistance is affected bya factor known as the ‘drag coefficient’ and it is depen-dent on the shape of the vehicle.

Air resistance 5 Cd 3 A 3 V2, where Cd is the dragcoefficient, A is the frontal area of the vehicle, andV is the velocity of the vehicle relative to the wind speed.The way in which engine power is absorbed in over-coming air resistance is shown in the graph in Fig. 1.8.

Streamlining

The efficiency of the streamlining of a vehicle body isa major factor in reducing the drag coefficient; otherfactors, such as recessing door handles and shaping ofexterior mirrors, also contribute to a lowering of drag.Another factor that contributes to the lowering of dragis the air dam that is fitted to the front of a vehicle(Fig. 1.9); this reduces under-body turbulence.

End-of-Life Vehicles Directive

The End-of-Life Vehicles (ELV) Directive aims toreduce the amount of waste produced from vehicleswhen they are scrapped. Around two million vehiclesreach the end of their life in the UK each year. Thesevehicles are classed as hazardous waste until theyhave been fully treated.

What does the directive mean?

The directive requires ELV treatment sites to meetstricter environmental standards.

The last owner of a vehicle must be issued witha Certificate of Destruction for their vehicle andthey must be able to dispose of their vehicle free ofcharge. Vehicle manufacturers and importers mustcover all or most of the cost of the free take-backsystem.

It also sets higher reuse, recycling and recoverytargets, and limits the use of hazardous substances inboth new vehicles and replacement vehicle parts.

Strengthening member

Tubular protection bar

Fig. 1.7 Side impact protection (Toyota)

Rolling assistance

45

Speed [km/h]

90

Pow

er lo

ss [k

W]

Transmission

AerodynamicDHAS

Fig. 1.8 Power used against air resistance


Who implements the directive?

In the UK, the directive is implemented throughELV Regulations issued in 2003 and 2005, andthrough the Environmental Permitting (EP)Regulations 2007.

The 2003 regulations deal with information require-ments, certificate of destruction requirements, andrestricting the use of hazardous substances in new vehi-cles. The 2005 regulations cover recycling targets andfree take-back for ELVs.

The 2007 regulations extended the treatmentrequirements in the UK to all waste motor vehicles(including coaches, buses, motor cycles, goodsvehicles, etc).

Authorized Treatment Facilities

Authorized Treatment Facilities (ATFs) are permittedfacilities accepting waste motor vehicles, which areable to comply with the requirements of the End-of-LifeVehicle (ELV) and Environmental Permitting (EP)regulations.

In the UK, most local authorities have vehicle recy-cling companies that are authorized to deal with ELVs:

� Vehicle recyclers must dismantle vehicles in anenvironmentally responsible manner and achievebetween 75% and 85% recycling targets.

� The vehicle owner must dispose of their unwantedvehicle in a legal and responsible manner by using anAuthorized Treatment Facility.

Layout of engine and driveline

Front-wheel drive

The majority of light vehicles have the engine at thefront of the vehicle with the driving power being trans-mitted to the front wheels.

In the arrangement shown in Fig. 1.10 the engine andtransmission units are placed transversely at the front ofthe vehicle, which means that they are at right angles tothe main axis of the vehicle.

Some of the advantages claimed for front-wheel driveare:

� Because the engine and transmission system areplaced over the front wheels the road holding isimproved, especially in wet and slipperyconditions.

� Good steering stability is achieved because thedriving force at the wheels is in the direction thatthe vehicle is being steered. There is alsoa tendency for front-wheel drive vehicles toundersteer, which can improve driveability whencornering.

� Passenger and cargo space are good because there isno need for a transmission shaft to the rear axle.

Possible disadvantages are:

� Complicated drive shafts are needed for constant-velocity joints.

� Acceleration is affected because load transfer to therear of the vehicle lightens the load on the drive axleat the front.

� The turning circle radius is limited by the anglethrough which a constant-velocity joint canfunction.

Front-engine rear-wheel drive

Until reliable mass-produced constant-velocity jointsbecame available, the front engine and rear drive axlearrangement shown in Fig. 1.11 was used in most lightvehicles.

In the layout shown in Fig. 1.11 the engine ismounted in-line with the main axis of the vehicle.The gearbox is at the rear of the engine and power istransmitted through the propeller shaft to the driveaxle at the rear. The gearbox, propeller shaft, and rearaxle make up what is known as the driveline of thevehicle.

The advantages of a front-engine rear-wheel drivearrangement are:

Fig. 1.9 An air dam (DuPont)

Fig. 1.10 Typical front-wheel-drive arrangement


� The front axle is relatively simple.� Acceleration and hill climbing are aided because

load transfer to the rear of the vehicle retains tractionat the driving wheels.


� Reduced space for driver and front passengerbecause of the bulge in the floor panel that is required toaccommodate the gearbox and clutch housing.

� The raised section known as the propeller shaftaffects available space throughout the length of thepassenger compartment.

� Long propeller shafts can cause vibration problems.

Rear-engine rear-wheel drive

Figure 1.12 shows an arrangement where the engine ismounted transversely at the rear with the drive beingtransmitted to the rear axle.

The advantages claimed for the rear-engine layout are:

� Short driveline because the engine, gearbox, and finaldrive can be built into a single unit.

� A preponderance of weight at the rear of the vehiclegives improved traction during hill climbing andacceleration.


� A tendency to oversteer.� Difficulty accommodating liquid cooling of the

engine.� Difficulty accommodating the fuel tank in a safe zone

of the vehicle.

� Space for luggage is reduced.� Difficulty steering in slippery conditions.

Four-wheel drive

In this system the engine power is transmitted to all fourwheels of a light vehicle. In the arrangement shown inFig. 1.13(a), the engine is placed at the front of thevehicle. Power to the front wheels is provided throughthe gearbox to the front axle and from the gearbox tothe rear axle via the propeller shaft.

Permanent drive to all four wheels (Fig. 1.13(a) and(b)) poses certain difficulties with braking and steeringthat require the use of sophisticated electronicallycontrolled devices. A common approach to four-wheeldrive makes use of an additional gearbox that is knownas the transfer gearbox. This additional unit allows thedriver to select four-wheel drivewhen driving conditionsmake it beneficial and for cross-countrywork the transfergearbox provides an additional range of lower gear ratios.

The advantages claimed for four-wheel drive are:

� Better traction in all conditions.� Wear of tyres and other driveline components is more

evenly shared.


� Increased weight and initial vehicle cost.� Increased maintenance due to the complexity of

transmission systems.� Increased fuel consumption.� Possibly difficult to accommodate anti-lock braking

systems.

Fig. 1.11 Front-engine rear-wheel drive


Self-assessment questions1. What happens to a motor vehicle that has reached

the end of its useful life?2. Find out the names of the Authorized Treatment

Facilities in your area.3. What percentage of a vehicle is recycled? What

happens to the steel that is reclaimed whena vehicle is scrapped?

4. Examine a manual for a vehicle that you work onand describe the features that the design

incorporates to protect the occupants in the eventof a collision.

5. Write a few notes to describe why you think thatfour-wheel drive vehicles are now popular for useas family cars.

6. What is meant by the term ‘oversteer’. Why do youthink that a rear-engine vehicle may be more proneto oversteer than a front-engine vehicle?

7. What is the purpose of an air dam?8. What measures should be taken to protect

paintwork when a vehicle is being worked on?9. In which position on vehicle panels is soundproofing

applied?10. If the speed of a vehicle is doubled, by what factor is

the air resistance increased?11. What features of vehicle design affect the drag

coefficient?12. Which part of a four-wheel drive transmission

system permits the four-wheel drive to beengaged?

13. In which positions on a vehicle are the side impactprotection bars fitted?

14. Make a list of the external parts of a motor car thatare made from plastic.

15. What methods of joining body panels are used inmodern vehicle construction?

16. What materials and methods are used to preventwater entering the interior of a car?

17. Describe, with the aid of a diagram, the type ofengine mounting that is used to attach an engine toa vehicle frame.

Gearbox

Transfer box

Visco clutch and 60/40 diff

Front engine with 60/40 differentialand visco clutch

Transverse engine withtransaxle

(b)

(a)

Fig. 1.13 (a) Optional four-wheel drive. (b) Permanent four-wheel

drive

Fig. 1.12 Rear-engine rear-wheel-drive layout


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Fhwa Class and Vehicle Layouts