îHE HISTORY OF COmPOilEltfSThis section of the journal deals with one of the metal components used in the automotive industry, recording thechanges in its design. At the same time, the many ways in which the techniques and materials employed in itsmanufacture have developed will be describedThis approach helps to emphasize the vital role played by metal component manufacturers in developing the basicmaterials and working out production processes.The article in this issue deals with the wheel.

WheelsGiovanni Riccio

Abstractln the eaily days of this century, automobile pioneers were still arguing about whether wooden or metal wheels gave the better performance.But soon, in 1924, the first wheel in light alloy, mounted on a Bugatti, made its appearance on the occasion of the French Grand Prix.The article, following the evolution of wheels for cars and heavy vehicles, focuses on their characteristics and the innovations brought in,affecting both the materials used and the manufacturing process.

RiassuntoLe ruoteN,ei primi anni del secolo. i pionieri dell'automobilismo disputavano ancora sulla maggior convenienza delle ruote in legno o in metallo. Ma già nel1924,in occasione del Gran Prìx di Francìa, la prima ruota ìn lega leggera, montata su una Bugattì, faceva la sua comparsa.L'articolo ripercorre l'evoluzione dei vari tipi di ruote per automobili e veicoli industriali, evideÀziandone le caratterlstiche e le innovazìoniapportate nella scelta dei materiali e nei processi di fabbricazione.

Fig. 1 -The prehistoric wheel (4000 B.C.).

According to Tchakravartin, aTibetan sage, seven things wereprecious to the world: the wheel(Fig. 1), the horse, the elephant,woman, the pearl, the house-steward and the counsellor.To-day, some things have changedin this list, but the wheel, and inparticular the automobile wheel,retain its importance. Thisautomobile component consists of ametalbody, disc shaped or spoked,and a rÌm shod with a pneumatictyre. Such a wheel can easily bemounted on a fixed hub, whichgenerally carries the brakingsystem.The characteristics of the wheel arenow stated on the basis of the

experience and long history of theautomobile. First of all we talk aboutlightness, inasmuch as the wheelrepresents the greatest unsprungweìght and contributes to the qualityof the suspension, the roadholding,etc. The wheel must be compatìblewith effective ventilation for coolingthe brakes and the tyre itself, andmust be particularly endowed withresistance to fatigue, especiallywhen the vehicle is entering a curveor accidentally strikes an obstacle.Fìnally, the wheel has an importantfunction in the general aesthetics ofthe automobile.From the first years of thedevelopment of the automobile,solidity ìn the wheel was sought, tobe able effectively to transmit theimpulse of the engine, to resist theforces of sudden braking and towithstand impacts of various kinds.The most convenient diameter ofthe wheel was considered, relativeto the weight of the vehicle and toits position, f ront or rear. Thetractive effort, the weight of thevehicle, the radius of the wheel andthe height of the obstacle to besurmounted are connected in arelation of the typeT: P!* where T: tractive effort,P: weight,R: radius,

h : height of the obstacle.

On the width of the rims, theorywasn't much exhaustive andaccount was chiefly taken of thestate of the road bed. For example,given roads with a soft bed, the rimswere made wider; but if the sameroads were strewn with stones thewide rim increased the number ofobstacles that had to besurmounted.At the end of a competition forheavy vehicles, held in Liverpool inMay i 898, it could be aff irmed:"The irregularities of the ordinaryroads are the main causes of veryhigh costs of maintenance anddepreciation of vehicles and theprimary cause of risks in any publicservice with mechanically drivenvehicles".The argument about theconvenience of wooden or metalwheels filled the first years of thelife of the automobile (Fig. 2)Metal is elastic while wood is not.Wood shows variations in volumewith humidity and temperature, butin the opposite direction to that ofmetal. Therefore wooden wheelswith metal hubs and tyres causeddangerous separations betweenthese parts.On the other hand, wood has avaluable characteristic that metalhas only in a much reducedmeasure; that is, the capacity toabsorb the shocks provided by the

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Fì9. 2 - Autom0bile wheels n w00d with metal reinforcement ('1900)

road (ìrregularties, potholes,stones). The metalwheel in thiscase is subject to vibrations lhat aretransmitted to other components ofthe vehicle.The damping action of wood thusaugments that of the outer elastic

tyre on the rìm.A great advantage of the woodenrim appeared during the Peking-Paris "raìd" (1907), durìng which a

wheel was rebuilt by the work of a

Central Asian artisan, enabling thejourney to continue (Fig. 3).

At the start of the Century, wheelsfor motor vehicles were divided intothree main categories: 1)woodenspoked wheels, 2) metal spokedwheels, 3) solid metalwheels.Wooden spoked wheels were madewith the hub of elm wood or moref requently of bronze, case-hardenedand tempered iron or steel, thespokes were of acacia wood or, forvery heavy vehicles, of oak. Therims were of elm or ash andoccasionally of metal.At that time, wooden wheels gavesatìsfactory results and were widelyused.Wheels with metal (steel) spokeswere derìved from those used onmotorcycles and light cars. Metalspokes worked only in tension, incontrast to wooden ones, whichworked in compression. Theirsectìon used to increase f rom thehub towards the rim, wrth a direct(radial) or tangential arrangementaccording to the circumference.Solid metal wheels are an innovation

Fig. 3 - Wooden wheel rebullt in 7 hours by a Siberian art san (Pek ng Paris"Raid",

1 907)

M eta I I u rgica I Sci e n ce and Tech nol ogy

of the last years of last century. Totake an example, the wheels of NewYork taxis from the Electric VehicleCo. had the spokes replaced by solidsteel segments 4 mm thick, andarranged according to truncatedcone surfaces like ordinary spokes.The rims were of wood withconcave sections to receive thepneumatic tyres.At the same time, for the comfort ofpassengers, and for adequateprotection of the variousmechanisms, the wheels of vehiclesthat must maintarn high (relative)speeds were provided with elastrctyres. Heavy vehicles thatmaintained lower speeds wereequipped with metal tyres. But thetendency to use rubber for the outertyres of wheels was increasingrapidly (Fig. a).The solid rubber tyre had to be fixedsolidy to the rim to avoid tearingaway. One precaution was to makethese tyres wide enough so as notto slot into tram tracks. Fitting thetyre was done by forcing, welding orwith circular elements with orwithout bolts.Hollow rubber covers had nosuccess because the air in the cavityat atmospheric pressure was notenough to ensure an empty spacethat would support the weight of thevehicles. Their performance inpractice was equivalent to that of asolid rubber tyre.At the Paris Exhibition of '1899, 80%of the cars on show had adopted thepneumatic tyre with air at highpressure. Dunlop was starting toproduce pneumatics for cars builtwith a weldless inner tube with thejoint vulcanised, as also was thepatch carrying the valve. A tapeprotected the inner tube from theheads of the metal spokes. The tyrewas composed of 7-B robust canvaswebs with other supplementarywebs for the tread, held togetherwith a special rubber.The whole was vulcanised onceonly, without the subsequentapplication of a tread. Four or fivesecurity bolts fixed the cover to therim, which could be of metalor

Fig. 4 - Solid rubber type {end of XIX Century}.

wood. The air pressure was 4-5 kg/cm'.Arguments soon arose about thebest pressure of inflation forpneumatic tyres in relation to rollingresistance presented by the newversion of the wheel, comparedwith the others. A fundamentalfactor in this regard was presented

Fig. 5 - Michelin pneumatìc tyre {end of XIX Century).

by the variability of the road bed.Despite the f requent punctures andtheir very costly maintenance,wheels equipped with pneumatrctyres spread rapidly (Fig. 5).However, their weakness led to therealìsation of some curiousalternative solutions. A typicalexample was the so-called "elasticwheel". This consisted of a metalouter rim coated with rubber,connected to the hub by spings inplace of the traditional spokes. Thusa certain elastic displacement of theouter rim was possible whichsimulated the behaviour of apneumatic tyred wheel. A solutionof the same type will then beadopted for the first lunar vehiclebuilt in the USA in 1971 .

Some interesting attempts weremade to simplify production withintegrally metallic wheels, like theSankey wheel (Fig. 6), comprisingtwo parts laminated from pressedsteel and welded together so as toform a spoked wheel.ln 1914 appeared the disc wheelproduced by Michelin (Fig. 7).Thanks to its lower cost, this typequickly replaced the spoked wheelwhich, however, was still retainedon sports cars (Fig. B).The first wheel in light alloy made itsappearance on the occasion of theFrench Grand Prix of 1924, mountedon a Bugatti. The innovation

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Fig. 6-Sankey wheel, meta (1920)

FÌg. B - Spoked wheel for sports car, '30s

Fig 7 Sheet melal d sc whee (1925).

awakened lively interest ìn theadvantages offered by lightness.However, this solution did not haveimmediate success for reasons ofCOSt.

The first definitely to adopt the lightalloy wheel were the Americans inrhe late '50s. ln 1963-1964 Ferrariadopted wheels in light alloy both onhls "monoposto" and his seriesproduction cars,ln the years between 1920 and 1930a basic development of vehrcles andpneumatic tyres occurred. Theshape of the coachwork graduallymoved away f rom the typical opencar of pre-war days. Greaterconsideratlon was given to comfortand saf ety, and the pneumatic tyrebecame safer and more reliable. Thefirst refined designs of tread in relìefappeared and rims changed so thatthe "clincher" became"straightsided".The "clrncher" gripped the bead ofthe tyre like a pincer, whrle the" straightside" was characterised bya bead rendered undeformabie by a'#++t

Metallurgical Sctence and Technology

metal hoop supported and held inplace by the pressure of inflation.The spread of trucks introducedgìant pneumatic tyres that replacedthe earlier solid rubber ones.ln 1923 appeared the low-pressurepneumatic, which was generallyadopted ìn the years immediately tofollow. With a low rnflation pressurethe carcass no longer needed amassive construction ; f rom thisresulted an increase in flexibilitywith greater passenger comfort.The larger section of these tyresearned the popular name "Balloon".Coming to the more recent years ofthe second and, we hope, the lastpost-war period, we can trace apanorama distinguishing the wheelsfor cars f rom those for heavyvehicles. With regard to cars, wecan say that the most widespreadwheels are those of disc type.These are formed of a channel rim,welded or riveted to a pressed disc,shaped so as to enhance rigidity andresistance to transverse stresses.The disc is of extra-soft steel platefor deep drawing, in a thickness of3-5 mm. Generally the central part ofthe disc has a greater thickness andis ribbed in the zone where rt

mounts to the hub with the aim ofincreasing strength and avoidingcomplete contact between disc andflange. Often tlre disc has holes forlightness and to provide for coolingthe brakes. This type of wheel owesits wìde adoption to itscharacteristics of strength, long lìfeand low cost.The light alloy wheel (aluminìumand magnesium), after a phase ofuse on competrtion and special cars,has seen the development of twomarkets. The first market is that ofthe light alloy wheel as an optionalreplacement for the steel wheel onseries productron cars, especially forthe up-market sector. The secondmarket is that of the wheel ìn lightalloy covered by the firmsmanufactu ri ng high-performance(sports or competition) cars wherethe advantages of the same are holdin hìgh regard.The styling of the lìght alloy wheel

has always had great importance inthese two markets; but it isnecessary to consider the effectiveadvantages as regards performanceand safety, which have not alwaysbeen in evidence.These advantages are: 1) highdimensional accuracy, whìchensures a better balanced wheel; 2)a machined sealing for the tYrebead, wìth consequent bettercontact between the tyre and therim; 3) higher thermal capacity,which allows better dissipatron ofthe heat f rom the tyre and brakes;4)a reduction in the unsprungweight, with better handling of thevehicle.When the processes ofmanufacture of wheels in light alloyare referred to, casting must rankfirst. This is subvided into casting insand moulds, gravity casting inmetal die and casting in metal die at

Fig. 9 - Cast steel wheel for heavy vehicles.

high and low pressure. The choiceof the most expedient method istaken bearing in mind that highquality and uniformity in the productare essential for a stressedcomponent like a wheel, and thefact that we must operate under themost economrc conditions.To achieve the required mechanicalcharacteristics of strength andelongation, we resort to heattreatment. To combat porosity in thecastings, which would be disastrousfor the heat treatment, pore-freeprocesses have been adopted.A recent system using aluminiumwheels is that presented byAluminia S.p.A. at the XIXCong resso Nazionale ATA(Modena, 1983). The rim is drawnout from a segment of tubeextruded in aluminium alloy, and isthen welded to the disc.The latest innovatìon in aluminium

Vot. 5 [2](1987)

wheels is the "sectron wheel"carried out by Spiith GmbH in WestGermany. "Section wheels" havenot yet gone into mass productionbut their design and manufacturingtechnology can be consideredcomplete for the most part.ln the field of automobile wheels,attention must be drawn to productsin resin reinforced with specialfibres while other novelties can beforeseen in connection withdifferent conditions of service.ln every case, the wheel has a roleof primary rmportance in theoperational safety and theroadholding of the vehicle.Wheels for heavy vehicles, trucks,buses and trailers, because of theexceptional loads they carry, verysoon began to adopt spoking in caststeel (Fig. 9) and subsequently inmalleable cast iron and spheroidal

Fig. 10 - Triplex rim for heavy vehicles (1934).

graphite iron. The part in contactwith the ground was orrginally madeof solid rubber or semi-pneumaticsfixed to the outer flat surface of thewheel. Later, f rom 1925 to 1930,even on lorrìes, pneumatic tyrescame to be adopted, for whichdetachable rims were developed,Irke those still in use (Fig. 10).To-day wheels for commercìalvehlcles present two typicalsolutions, that is: 1 ) disc wheelssimilar to those on cars but, ofcourse, with adequate thickness forthe greater stresses which theyundergo; 2) spoked wheels, with ahub equipped with spokes to whichthe rubber tyred rims are drrectlyattached. The choice between thesesolutions is made by firms buildingcommercial vehicles in appreciationof the different factors, such asspeed, capacity and the weight of

the vehicle. ln general, disc wheelsare preferable for the light andmedium classes, and those withspokes for the heavier classes.The rims of these wheels are of twotypes: the so-called continuous withdetachable rims which are the mostwidespread throughout the worldand those radially divided into threesegments, which prevail in Europe.Various attempts to use light alloysto manufacture wheels for heavyvehicles have been made from theearly'20s. The results obtainedwere not always satisfactory.On the basis of past history andcurrent trends, we can say that thefuture development of the wheel forautomotive use wìll be evolutionaryrather than revolutionary (Fig. 1 1).We must not lose sight of the factthat motor vehicles arepredominantly work tools, and must

M etal I u rg ical Science a nd Tech nology

REFERENCES

(1) Lavergne, G. Manuel de l'automobilesur route, Librairte Polytechntque, Paris,1 900.

(2) OMNIA. Revue pratique delocomotion. Paris. 2 fevrier 1907.

(3) Fifty years of growth, Dunlop BubberCo, England, 1938.

(4) Uccellt, A. La ruota e la strada,Editoilale Domus, Mtlano, 1946.

(5) Encyclopédie Universel leA,utomobile, Edition du Livre, Montecarlo,1951.

(6) Lief , A. Firestone story, WhittleseyHouse, NewYork,1951.

(7) Buss, H. Aus der Entwicklung derRader fur Lastungen und Omnibusse,Georg Fisher AG, Schaffhausen, 1 952

(B) Architettura della ruota, GianettiS.p.A., Saronno, 1963.

(9) Bencini, M. La trazione anteriore, Ed.Automobile, ACl, Roma, 1967.

(10) Matteuccr, M. Storía dell'automobile,Ediprint, Toilno, 1 967.

(11) lchimura, Y. Automotive tires,J apa n's Auto m otive I nd u st ry I nte rnat t on a I,

Suppl. of Ashai Evenrng News, Tokyo, 1969.

(12) Ruote leggere, ruote veloct. TavolaRotonda Auto 70, Tonno, aprtle 1 972.

(13) Milleruote, ed. Domus, Mtlano, 1974.

(14) Storia di un pneumatico. lndustrrePirelli S.p.A., Mtlano, 1980.

(15) Sternberg E.R. A hrstory of motor truckdevelopment SAE Paper493 (198l)

(16) Spath W. Felgenfertrgung ausAl u m i n i u m Strangpre ss p rof i I e n. ATZAuto mobi ltech n isch e Zeitsch rift 88( 1 0 )(1 986)

Fig. 11 - Rim for tubeless pneumatic tyre (Formula '1).

function in an efficìent andeconomical way, with the minimumof downtime. Therefore everystrongly innovative proposal mustprove its validity on the wholenormal lrfe of the vehìcles before itcan be accepted in any degree.

Vot. 5 [2] (1987) 65