Harley-Davidson Roller-Bearing Crankshaft Retro Charm Explained | Cycle World

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Harley-Davidson Roller-Bearing Crankshaft Retro Charm Explained

Why does The Motor Company operate differently than other motorcycle manufacturers?

Harley-Davidson’s Big Twin of today, the Milwaukee-Eight

Harley-Davidson’s Big Twin of today, the Milwaukee-Eight, cut away in tantalizingly few places. The four chromed pushrod tubes converge on the centerline of the circular cap behind which rotates the silent-chain-driven single camshaft. Below that circular cap we can see the oil pump assembly. Behind all this whirls the crankshaft, its mass necessary to push large pistons through compression at idle. At lower left the oil reservoir—a part of the bolted-on six-speed gearbox—can be seen. So many motorcyclists love cooling fins, but as cylinders grow bigger, physics loves them less. That has meant supplemental cooling must be provided by circulating cooled liquid through passages in the cylinder heads, routed close to the very hot exhaust-valve seats.

Courtesy of Harley-Davidson

Sometime in the early 1990s I spoke with Harley-Davidson’s retired racing manager, Dick O’Brien. “If you had it to do again,” I asked, “would you still give the XR750 dirt-track engine a roller crank?” Normally given to salty speech and sharp one-word answers, “Obee” squinted at the sky and replied, “I’d like…to think I would.”

The reason for O’Brien’s introspective moment was because that aspect of the XR’s design occurred in 1970, just as the rest of the world was turning away from rolling-element-bearing cranks. True, Kawasaki’s iconic four-cylinder roller-crank Z1 would hit the market three years later, followed shortly after by Suzuki’s likewise-roller-equipped GS series. But Porsche, Formula 1’s last roller holdout, had switched to plain journal bearings five years earlier.

Rollers and balls had long borne the name “anti-friction bearings,” and it had been reflexive among engineers to assume such bearings were the last word in friction reduction. Honda had equipped every one of its world championship-winning four-stroke Grand Prix bikes of the 1960s with rolling bearings.

Yet today, Harley-Davidson’s Big Twins and Sportsters are among the last holdouts for rolling bearings. Crankshafts and connecting rods in cars, trucks, and bikes worldwide turn nearly 100 percent on plain journals. Ditto Formula 1, MotoGP, and World Superbike. A few four-stroke motocross engines continue with rollers, mainly because they survive well on little oil.

In the beginning, the form of motorcycle crankshafts was determined by two things: 1) the need to function without automotive-style pumped recirculating oil systems; and 2) the ability to be manufactured on the most basic tooling. The classic five-piece roller crank that resulted would carry Big Twins through 1999! The five pieces are the two large circular flywheels, two main shafts, and one crankpin. The flywheels were cast in iron, a technology widely understood because of demand from the railroad and machine-tool industries. The three shafts were simple lathe and milling-machine work.

The five pieces were assembled by means of tapers and keyways held together by thin nuts. Final alignment was achieved (as it is with such cranks to this day) by “bumping” (thumping the “proud” flywheel on the bench top or striking it with a brass or copper hammer), followed by inspection for “truth” with a dial gauge.

M-8 crankshaft

Here is a M-8 crankshaft installed in just the right-hand (timing side) crankcase half. The crankshaft axis is vertical here, and the pistons and cylinders have been removed. You can see the small ends of the two connecting rods projecting from between the two very substantial flywheels. Half surrounding each small end are the openings for the two cylinders. In early days, the fork-and-blade connecting rods turned on four rows of quarter-inch rollers, but as racing experience with the XR750 accumulated it became clear that many more smaller, lighter rollers lasted longer because they could better follow the changes of bearing speed as the rods swung this way and that. The same concept was eventually adopted on the Big Twin.

Courtesy of Harley-Davidson

The many millions of American V-8s that have been built are really just close-packaged sets of four V-twins, and every one of them has its plain bearing con-rods side by side on each of its four crankpins, not both in the same plane, as Harley’s fork-and-blade con-rods are arranged. Why is Harley so different? First of all, there were Harleys for decades before the Chevrolet small-block was conceived, and second, there was a special reason to choose fork-and-blade rods.

The special reason is that the first motorcycle engines were singles. When the need for more power made builders consider twins, they naturally chose a design that required the fewest changes to implement. That design was to put a second cylinder on the original crankcase in vee form and find a way to connect both pistons to the original single crankpin. If they had put the con-rods side by side as they are in a V-8, they would have needed to offset the cylinders and possibly have made the crankcase wider. Best to avoid all that. Just find a way to put both rods, pistons, and cylinders in the same plane by making the rods in fork-and-blade form. Big Twin con-rods remain in fork-and-blade, roller-bearing form to this day.

texas half mile

End of a very long and successful road for Harley-Davidson’s XR750? Racing in no uncertain terms tells us what can and what cannot survive. In 1972, the aluminum XR dirt-track engine was the first H-D to have crank main shafts forged in one piece with the flywheels. It was also first to have cylindrical crankpins pressed into those flywheels with many tons of installation force. And it showed that big-end connecting-rod bearings of small, lightweight needles and cages could survive operation above 9,000 rpm.

Courtesy of Harley-Davidson

Vincent Motorcycles—made eternally famous by the iconic photo of Rollie Free lying horizontal on his bike at 150 mph on the Bonneville Salt Flats while wearing only bathing trunks—did decide to put the rods of its postwar twin side by side. But that was a brand-new design, owing little to previous tradition.

Do I hear objections that dividing the stress on the fork rod (to make room for the blade rod between) weakens it? When engineers at Rolls-Royce in Britain and at Allison in the US laid out the two great V-12 aircraft engines of World War II, they chose fork-and-blade construction, albeit with plain journal bearings. Think of them as “the 2,000-hp test.”

When five-piece roller cranks were raced by a number of manufacturers, they found that shafts worked loose in cast-iron flywheels. Therefore stronger flywheels of either rolled or forged steel were the next step.

When Harley-Davidson went racing with the first OHV iron XR750 in 1970, that engine still had a classic five-piece crankshaft. But when the stroke had to be shortened the next year, holes for main shafts and crankpin became too close together. To overcome this problem, the flywheels and main shafts were forged together, resulting in a stronger assembly. Instead of the usual tapers, keyways, and nutted crankpin, a straight pressed-in crankpin was adopted, while retaining the fork-and-blade con-rods with their rows of caged rollers.

The aluminum XR dirt-track V-twin of 1972 began life at 7,600 rpm but, over its four-decade reign, that number increased to 10,000 rpm. Crankshafts vibrate at high revs, with the two massive flywheels opening and closing like elephant ears flapping. The “spring” on which they vibrated was the crankpin, and all its fussy shape features—keyways, threads, and a radius or two—became stress raisers from which cracks could propagate. Therefore a simpler shape was put in their place: the straight crankpin.

This experience was the model on which later changes to the Big Twin’s crank would be made. After 1999, Big Twin flywheels and main shafts were forged in unit, just as XR wheels and main shafts had been. A straight pressed crankpin took the place of tapers and nuts.

Why still fork-and-blade roller rods and roller main bearings? Common experience (and mental health advisors) tell us not to change too many things in our lives simultaneously—all in good time. Through the many years when Indian and Harley-Davidson were market competitors, Indian was more often the innovator and Harley the conservative. Yes, Harley patents from 1919 described an OHV V-twin of advanced design, but the OHV E and EL production models—also known as the Knucklehead—didn’t hit the market until 1936–’37. Conservatism won the day. The original Indian company frequently teetered on the edge of business failure, finally ceasing production in 1953. Harley continued, remaining conservative to this day.

Perhaps one day the Big Twin will make the leap to fully modern construction, with side-by-side, split-and-bolted plain journal con-rods and all the rest of it. But management can’t afford to upset the many who look back with satisfaction on more than 100 years of air-cooled roller engines.