WHAT’S SHAKIN’?

The many forms of en­gine vi­bra­tion and how we stop it—or don’t

Cycle World - - Tdc - By KEVIN CAMERON

TThe ter­ri­ble truth is that no mat­ter how finely you add or sub­tract bal­ance weights from your crankshaft us­ing a high-tech bal­anc­ing ma­chine, a ro­tat­ing coun­ter­weight can never can­cel the shak­ing force of a pis­ton mov­ing back and forth in a straight line. The only way a sin­gle can be bal­anced is by gen­er­at­ing an equal and op­po­site in-line shak­ing force.

Here’s why. Let’s start with a sin­gle-cylin­der en­gine whose ro­tat­ing parts—the crankpin, its bear­ing, and the big end of the con­nect­ing rod—have all been 100 per­cent bal­anced by ro­tat­ing coun­ter­weights at­tached to the crankshaft at 180 de­grees to the crankpin. All that’s left now is the straight-line shak­ing force gen­er­ated by the pis­ton, its rings, wrist­pin, and con­nect­ing rod small end. As we add coun­ter­weight to the crank, the up-and­down shak­ing force de­creases, but we are also cre­at­ing a new shak­ing force that acts for­ward and back. Adding coun­ter­weight to bal­ance 100 per­cent of the “shak­ing parts” just moves the shak­ing from ver­ti­cal to hor­i­zon­tal.

One way out of this is to fight fire with fire: Build a sec­ond en­gine of the same bore and stroke, hav­ing the same re­cip­ro­cat­ing mass but ori­ented in the op­po­site di­rec­tion, with its crankpin phased so both pis­tons reach top and bot­tom dead cen­ters to­gether. Now the two shak­ing forces can­cel each other. This is BMW’S flat twin.

Shucks. The re­sult isn’t per­fect be­cause there is no sim­ple way to place both cylin­ders on the same cen­ter line. The dif­fi­cult way would be to have three crankpins, the cen­ter one at 180 de­grees to the other two, one on ei­ther side of it. The con-rod on the cen­ter pin would con­trol one pis­ton, and the outer pair would drive the other pis­ton. This could be made to can­cel all shak­ing forces if the two outer con-rods weighed ex­actly the same as the sin­gle in­ner one. Also, the crankshaft would be se­ri­ously weak­ened by hav­ing three crankpins joined in this way.

So de­sign­ers com­pro­mise. They have two crankpins at 180 de­grees, lo­cated as close to each other as crankshaft strength will per­mit. The re­sult, when BMW first did this in 1923, was an en­gine much smoother than the vig­or­ously shak­ing sin­gles of most of its con­tem­po­raries. Be­cause the two cylin­ders are not on the same cen­ter­line but are sep­a­rated by the width of one con-rod plus that of what­ever crankshaft ma­te­rial joins the two crankpins to­gether at 180 de­grees, there is a force gen­er­ated tend­ing to twist the en­gine back and forth around a ver­ti­cal axis. As long as the en­gine’s two pis­tons are small and light, this is unim­por­tant. But when BMW be­gan to make flat twins over 1,100cc dis­place­ment, pis­ton mass be­came much greater, and this os­cil­la­tion made it­self felt. That ul­ti­mately

caused the com­pany to add a bal­ance shaft that would can­cel it.

An­other way to can­cel vi­bra­tion is to com­bine two sin­gles side by side as a par­al­lel twin, phas­ing their two crankpins at 180 de­grees to each other. Now when one pis­ton is ar­riv­ing at TDC, the other is ar­riv­ing at BDC. Honda and Yamaha in the 1960s and ’70s built hun­dreds of thou­sands of par­al­lel twins this way. Just as with the flat twin, this looks like most of the shak­ing forces of the two pis­tons should can­cel each other. But no, they can’t, be­cause the cen­ter­lines of the two cylin­ders have to be at least as far apart as two halves of one cylin­der bore, plus two cylin­der thick­nesses. If the cylin­ders are air­cooled, we might also de­cide there needs to be some cool­ing fins be­tween the two cylin­ders, spac­ing them even far­ther apart.

So now the two shak­ing forces are be­com­ing widely sep­a­rated. What re­sults is called a “rock­ing cou­ple.” As the right-hand pis­ton de­cel­er­ates to­ward BDC, it pushes the right-hand cylin­der down­ward. But at the same time, the left pis­ton is de­cel­er­at­ing to­ward its TDC, yank­ing the left-hand cylin­der up­ward. The lever­age for all this yank­ing is one-half of the sep­a­ra­tion be­tween the two cylin­der cen­ter­lines. The re­sult is a whack­ing great sideto-side rock­ing mo­tion from the en­gine. Be­cause shak­ing force is pro­por­tional to the square of speed, vi­bra­tion in­creases rapidly with rpm. Yamaha’s par­al­lel twin street­bikes, mak­ing peak power at maybe 7,500 rpm, pro­duced only half as much rock­ing cou­ple as did its 10,000 rpm road­race ver­sion. That was too much for the racer’s chas­sis, whose en­gine mounts were, one by one, bro­ken by the fa­tigue stress (at an av­er­age 8,500 rpm, a 10-minute race ap­plied 85,000 stress cy­cles, so it didn’t take long).

We think this over and de­cide to join two of these rockin’ par­al­lel twin en­gines to­gether, end to end, to make a flat-crank (all crankpins in the same plane) in-line four. Ex­cel­lent! The two rock­ing cou­ples can­cel, leav­ing this en­gine with no pri­mary pis­ton shak­ing force. The re­sult, with Honda’s CB750 Four of 1969 and Kawasaki’s 903cc Z1 of 1973, was an en­gine that was very smooth for its time.

But alas, pri­mary shak­ing force (oc­cur­ring in step with the crankshaft) is not the only shak­ing force.

There are rid­ers who en­joy some vi­bra­tion to re­mind them that there’s dynamic ma­chin­ery down­stairs.

Also vary­ing the heights of the pis­tons is rod an­gu­lar­ity, which changes twice per rev­o­lu­tion, gen­er­at­ing a sec­ondary shak­ing force (twice per rev­o­lu­tion) roughly one-quar­ter as great as the pri­mary force. Again, as long as the pis­tons are light in re­la­tion to to­tal en­gine mass, the re­sult­ing vibes aren’t too bad. But when Honda built its CBR1100XX Black­bird, sec­ondary forces had out­grown hu­man tol­er­ance, and bal­ance shafts had to be added to shush them. It was an ex­cep­tion­ally smooth en­gine.

Like a 180-de­gree par­al­lel twin, a 120-de­gree in-line triple has a pro­nounced rock­ing cou­ple, even though its cen­ter of mass sits still. That cou­ple is can­celed in modern triples by a bal­ance shaft, but as with par­al­lel twins, there’s an­other way: Join two triples end to end in mir­ror-im­age fash­ion. Now ev­ery­thing cancels, leav­ing us with an en­gine so smooth that BMW won’t leave it alone. If you equate smooth­ness with ex­cel­lence, noth­ing beats an in-line six or a V-12.

Yet even to­day, there are rid­ers who en­joy some vi­bra­tion to re­mind them that there’s dynamic ma­chin­ery down­stairs. The fo­cus groups get to de­cide just how much: Har­ley’s new Big Twins are can­cel­ing 75 per­cent of pri­mary shak­ing force. If by the time of the next re­design the con­sumers have changed their minds, that per­cent­age can be ad­justed ei­ther up or down.

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