CHOOS­ING MA­TE­RI­ALS

MORE COM­PLI­CATED THAN YOU THINK

Cycle World - - Ignition - BY KEVIN CAMERON

De­spite all the trick ti­ta­nium bolts or the oc­ca­sional con­nect­ing rod, car­bon-fiber-re­in­forced plas­tic (CFRP), and the odd bear­ing with sil­i­con ni­tride balls, mo­tor­cy­cles are still mostly made of steel and alu­minum. Do you re­call the prom­ise of plas­tic car en­gines that one per­son could eas­ily pick up? Or NASA’S fun and games with no-weight car­bon-car­bon pis­tons? Or the won­der­fully weird-look­ing car­bon fil­a­ment-wound con-rods that were so light that 25,000-rpm en­gines were just around the cor­ner?

All these won­der­ful pos­si­bil­i­ties do ac­tu­ally ex­ist, but mak­ing them prac­ti­cal takes se­ri­ous money of the kind that Boe­ing in­vested in its 787 “Dream­liner,” much of whose struc­ture is CFRP.

One mea­sure of ma­te­rial per­for­mance is spe­cific mod­u­lus, which is the stiff­ness of the ma­te­rial di­vided by its den­sity (weight per vol­ume). In ar­bi­trary num­bers, alu­minum, mag­ne­sium, ti­ta­nium, and steel all have closely equal spe­cific mod­uli. Then we look at beryl­lium—off the chart with a num­ber sev­eral times that of the other met­als! NASA has used the stuff for elec­tron­ics racks in satel­lite ap­pli­ca­tions, but the $10,000-per-pound cost of rock­et­ing loads into low earth or­bit makes it worth con­sid­er­ing—even though dust from ma­chin­ing it is poi­sonous, and the metal is trou­ble­some and ex­pen­sive to pro­duce.

Okay, but how about uni­di­rec­tional CFRP, tested along the grain? Its spe­cific mod­u­lus is 4.5 times greater than that of the com­mon met­als. Boe­ing is mak­ing a go of it; why can’t we? A mo­tor­cy­cle is pretty dif­fer­ent from an air­plane. Air­planes are ba­si­cally a large beam (the wing) sup­port­ing a big tube (the fuse­lage), and the im­por­tant stresses are mostly dis­trib­uted as lift over large wing and tail sur­faces.

On a mo­tor­cy­cle frame, stresses are con­the CFRP struc­ture, and while this has been done suc­cess­fully it has never been done eas­ily. It also adds weight to durably bond metal fit­tings (good at han­dling con­cen­trated loads) into CFRP, eat­ing away at its weight and stiff­ness ad­van­tages. In the present era, metal mo­tor­cy­cle chas­sis are mostly made of cast el­e­ments, welded to­gether by ro­bots, but high-class CFRP re­quires skilled hand­work of the kind that has only lately been ban­ished from chas­sis pro­duc­tion lines.

If you in­ves­ti­gate high-end bi­cy­cle tech­nol­ogy, you find that rid­ers think of alu­minum bi­cy­cle frames as be­ing dis­agree­ably stiff. How can this be, when the Young’s mod­u­lus of alu­minum is only one-third that of steel? Here’s how it comes down. The stiff­ness of tubes in­creases as the fourth power of di­am­e­ter, so to make a light bike that’s strong enough for the forces ap­plied to it, you in­crease tube di­am­e­ter and de­crease tube wall thick­ness. With steel—al­most three times denser than alu­minum—you quickly reach a wall “thin­ness” that is vul­ner­a­ble to dent­ing or out­right buck­ling. But when you switch to alu­minum tub­ing of the same weight per foot, you have three times the wall thick­ness, al­low­ing you to go far­ther with the big­ger tube/thin­ner wall con­cept be­fore dent­ing and buck­ling rear their ugly heads. As a re­sult, alu­minum bike and mo­tor­cy­cle frames have big­ger tubes than do steel frames. That big­ger tube size gives a very stiff struc­ture—even though alu­minum is one-third the stiff­ness of steel. And that is why many bi­cy­clists find alu­minum frames un­com­fort­ably stiff. The stiff­ness comes from the way the ma­te­rial is used (in large thin-walled tubes) and not from the

ma­te­rial’s own prop­er­ties.

We all ad­mire the beauty of forged mag­ne­sium wheels, but we know that mag­ne­sium yearns in­ex­press­ibly to be­come ore; it is very sus­cep­ti­ble to cor­ro­sion. This is why street wheels are alu­minum. Also, alu­minum wheels are cast close to net shape, though they need hub and rim ma­chin­ing they are cheap to make. But ev­ery sur­face of forged mag wheels has to be ma­chined, turn­ing more than half of the weight of a blank into chips (which be­cause of their ex­treme fire haz­ard need spe­cial han­dling).

Ever do any fiber­glass layup? It’s not too bad—the heavy glass cloth drapes well and mostly stays where you put it in the mold as you daub on polyester resin and in­hale the hearty fumes of MEK per­ox­ide. But car­bon? If you han­dle the raw fibers, be ready for them to float into any un­sealed elec­tric mo­tors or switch­boxes in your shop, where they cause shorts. The stuff is stiff, so it rises up out of the resin you’ve blor­ped onto it. That’s why the dom­i­nant tech­nique is to use uni­di­rec­tional lay­ers preim­preg­nated with un­cured resin (keep this pre-preg in the fridge un­til the mo­ment of use, to pre­vent cur­ing), held against the mold by vac­uum bag­ging, with the whole as­sem­bly go­ing into an au­to­clave for con­trolled-tem­per­a­ture cur­ing. Oh, and those uni­di­rec­tional fiber plies? They have to be laid down in se­quence, ori­ented to give the de­sired prop­er­ties.

In other words, more de­mand­ing than strolling into your 24foot boat mold with a chop­per gun and shoot­ing it with chopped fiber and resin.

Wow, ti­ta­nium. Yes, Ti has only 57 per­cent of the den­sity of steel but prop­erly alloyed can be heat-treated to equal the strength (which is dif­fer­ent from stiff­ness) of fancy steels. What are we wait­ing for? Let’s make ev­ery­thing out

YES, TI HAS ONLY 57 PER­CENT OF THE DEN­SITY OF STEEL BUT PROP­ERLY ALLOYED CAN BE HEATTREATED TO EQUAL THE STRENGTH (WHICH IS DIF­FER­ENT FROM STIFF­NESS) OF FANCY STEELS.

of it! Not so fast. First, be­cause Ti is less stiff than steel, re­plac­ing steel axles with ti­ta­nium makes your bike feel, well, floppy. And when con-rods are made of the stuff, you need two or three times the pis­tonto-head clear­ance re­quired with steel rods. By forc­ing you to make your squish ar­eas thicker, it makes them less ef­fec­tive in fend­ing off det­o­na­tion. So it’s back to steel rods for some ap­pli­ca­tions.

Ti­ta­nium valves? You bet—but only af­ter hard-fac­ing the seat­ing sur­face and coat­ing the stem to pre­vent galling or seiz­ing. And putting a hard lash cap atop the stem.

Weigh all the fas­ten­ers on your bike. Is sav­ing 43 per­cent of that worth the price of ti­ta­nium?

Bot­tom line? You can’t just scratch out where it says “heavy, bor­ing, ob­so­lete ma­te­rial” on your parts draw­ings; write in trick lighter ma­te­rial, and ex­pect to feel sig­nif­i­cant im­prove­ment on your next ride.

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