TI­TA­NIUM

Beau­ti­ful, magic metal, find­ing glory in the light

Cycle World - - Elements - By KEVIN CAMERON / Pho­tog­ra­phy by DREW RUIZ

MMe­tals are a river that flows through our his­tory. Bronze Age horse­men from Asia’s steppe brought the mys­te­ri­ous stuff to the near-east­ern “Cra­dles of Civ­i­liza­tion.” Gold and sil­ver drove Span­ish New World ex­plo­ration and con­quest. And now ti­ta­nium’s use in aero­space and rac­ing has made that el­e­ment syn­ony­mous with the high­est per­for­mance.

Ti­ta­nium, sought for its com­bi­na­tion of high strength, low weight, and ex­tra­or­di­nary cor­ro­sion and fa­tigue re­sis­tance, be­came avail­able in com­mer­cial quan­ti­ties only af­ter 1953. Although com­mon in the earth’s crust, ti­ta­nium is dif­fi­cult (read: ex­pen­sive) to ex­tract from its ore. It was there­fore the ri­val­ries of the Cold War that bankrolled its large-scale pro­duc­tion. The Sovi­ets be­came mas­sive pro­duc­ers, mak­ing entire light­weight sub­marines of the stuff. Aero­space-grade ti­ta­nium cur­rently sells for $20 to $25 a pound, and 15 per­cent of the empty weight of Boe­ing’s 787 Dream­liner is ti­ta­nium.

The word “ti­ta­nium” has taken on near-mag­i­cal mean­ing; it has be­come a fash­ion color (shall we call it pale straw?), and like “turbo” has been used to im­ply spe­cial prop­er­ties in many com­pletely un­re­lated com­mer­cial prod­ucts. Is there any ti­ta­nium in a Ti­ta­nium Edi­tion Ford, for ex­am­ple?

Iron, the ba­sis of steel, has a den­sity of 7.8 times that of water, while that of ti­ta­nium, 4.5, is just 58 per­cent as much. This light weight is at­trac­tive in any ap­pli­ca­tion that re­quires rapid ac­cel­er­a­tion. Steels of ex­treme per­for­mance can be stronger than any ti­ta­nium al­loy but only at greater weight. Steels can reach ten­sile strengths more than 300,000 psi, but ti­ta­nium’s light­ness, even at its lower ul­ti­mate strength, gives it the edge in strength-to-weight.

This is the ba­sis for ti­ta­nium’s use in high-per­for­mance mo­tor­cy­cles, mainly in the forms of fas­ten­ers, ex­haust plumb­ing, valves, and con­nect­ing rods. Ti­ta­nium’s lower stiff­ness can make it un­de­sir­able as a ma­te­rial for axles and chas­sis, though Ital­ian ti­ta­nium spe­cial­ist NCR suc­cess­fully use spe­cial al­loys for both cur­rently. In 1966, BSA, seek­ing to ex­tend its suc­cess in 500cc Euro­pean mo­tocross, cre­ated a ti­ta­nium chas­sis, but as­so­ci­ated prob­lems made it un­suc­cess­ful.

A com­plete con­trast has been the use of ti­ta­nium in bi­cy­cle frames, where its lower stiff­ness re­duces ride harsh­ness, earn­ing the de­scrip­tion “magic ride.”

In 1956, the well-con­nected AJS en­gi­neer and for­mer TT rider Jack Wil­liams (fa­ther of Peter Wil­liams of Nor­ton fame) showed that con­nect­ing rods of high-strength

ti­ta­nium al­loy could re­duce bear­ing loads and there­fore fric­tion loss in in­ter­nal-com­bus­tion en­gines. Boe­ing pi­o­neered ti­ta­nium-ma­chin­ing tech­niques in pro­duc­ing the B-52 bomber in the early 1950s. In the 1960s, Bob Ni­chols, who worked 25 years at Douglas’ mile-long plant in Santa Mon­ica, Cal­i­for­nia (now empty), in­tro­duced ti­ta­nium con-rods to the open-wheel Champ car com­mu­nity.

It is mainly in dy­namic ap­pli­ca­tions such as valves and con­nect­ing rods that ti­ta­nium has made a place for it­self in high-per­for­mance en­gines. Even with the ne­ces­sity of pro­tect­ing re­ac­tive ti­ta­nium valve stems from seizure with anti-fric­tion coat­ings, and pro­vid­ing hard­ened seat­ing sur­faces and hard stem caps, its light­ness has en­abled en­gines to re­li­ably reach higher revs be­cause its lower weight can re­quire lower valve spring pres­sures than do steel valves. In the later 1970s, rac­ing mo­tor­cy­cles be­gan to use ti­ta­nium in­take valves, but ti­ta­nium ex­hausts soon fol­lowed. When Honda de­cided to re-en­ter US AMA road­rac­ing in 1980, early team man­ager Steve Mclaugh­lin (some call him “the fa­ther of Su­per­bike”) flam­boy­antly or­dered thou­sands of dol­lars’ worth of ti­ta­nium valves and rods from Jet En­gi­neer­ing.

Dur­ing the 500cc two-stroke era of Grand Prix rac­ing, engi­neers grad­u­ally sub­sti­tuted ti­ta­nium for the steel tra­di­tion­ally used to make ex­haust pipes, start­ing with the pipes far­thest from the bike’s roll axis. As spe­cial­ist shops ac­quired the tool­ing to work ti­ta­nium, four-stroke pipes were made of the light metal, re­sult­ing in com­plete four-cylin­der systems as light as 7 pounds.

Yamaha in­tro­duced ti­ta­nium springs to mo­tocross for the ob­vi­ous rea­son: weight; ti­ta­nium springs cut weight 30 per­cent but cost at least eight times more than steel. Sev­eral years ago, when I was given a tour of the Honda/ HRC Su­per­cross trans­porter, I was shown a draw­er­ful of ti­ta­nium foot­peg as­sem­blies—at $2,600 each. The ben­e­fits are that light weight im­proves nearly all as­pects of per­for­mance.

Ti­ta­nium’s high cost comes from the need to process or weld ti­ta­nium in ei­ther vac­uum or an in­ert at­mos­phere. This pro­tects the metal from ab­sorb­ing gases that can ren­der it brit­tle and prone to crack­ing.

Why is ti­ta­nium light and strong? Be­cause at the small­est level, this metal’s atomic ra­dius is fairly large, giv­ing the solid a lower den­sity, and its room-tem­per­a­ture hexag­o­nal close-packed crys­tal struc­ture puts ev­ery atom in con­tact with 12 oth­ers. Metals are duc­tile rather than brit­tle be­cause, with few outer-shell elec­trons, solid metal is ef­fec­tively an ar­ray of atomic nu­clei filled by a “gas” of loosely bonded elec­trons. Metals bend rather than snap be­cause this elec­tron gas lets in­ter­atomic bonds stretch, break, and then re-form with new part­ners. The high mo­bil­ity of the elec­tron gas is also re­spon­si­ble for the elec­tri­cal and ther­mal con­duc­tiv­ity of metals.

Ti­ta­nium’s ex­cep­tional fa­tigue strength owes much to the metal’s re­ac­tiv­ity, which in­stantly forms a pow­er­fully pro­tec­tive sur­face layer of ti­ta­nium ox­ide. Whereas ex­po­sure of steel and alu­minum to weather in­evitably pro­duces tiny cor­ro­sion pits that com­bine with cyclic stress to pro­duce even­tual fa­tigue fail­ure, ti­ta­nium’s pro­tec­tive layer greatly slows this process.

This self-pro­tec­tion gives pure ti­ta­nium its abil­ity to be ac­cepted by the hu­man body in the form of den­tal im­plants, ar­ti­fi­cial re­place­ment joints, and the plates and screws used to speed the heal­ing of bro­ken bones.

The rain­bow colors sur­round­ing the welds in ti­ta­nium ex­haust systems are cre­ated by im­per­fect weld con­di­tions. Ide­ally, ti­ta­nium weld­ing takes place in a “glove box” (vis­ually sim­i­lar to a bead blast cab­i­net) that is purged and filled with in­ert ar­gon gas, but suf­fi­cient oxy­gen re­mains to form sur­face ox­ide lay­ers on the work that ap­pear col­ored. These thin lay­ers act as in­ter­fer­ence fil­ters in the same way as do thin films of oil, float­ing on water.

A ti­ta­nium pro­duc­tion scheme us­ing less en­ergy than the present Kroll process might in time al­low price re­duc­tions, mak­ing the light metal more at­trac­tive for wider use in pro­duc­tion ve­hi­cles.

The ro­mance of ti­ta­nium in the mo­tor­cy­cle world be­gan when we could see the metal’s spe­cial gleam on fac­tory race­bikes. See­ing that, who could re­gard the stan­dard pro­duc­tion zinc-plated steel fas­ten­ers as any­thing other than ad­ver­tise­ments for ex­cess weight?

The beauty changes the closer you get. Ti­ta­nium is nearly unas­sail­able when fac­ing en­vi­ron­men­tal fac­tors that lead other metals to ox­i­dize, cor­rode, and even­tu­ally fail.

NCR ti­ta­nium cus­tom frame can be tuned for flex based on rid­ing style, vary­ing wall thick­ness, and di­am­e­ter of tub­ing. Welds are per­fect.

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