Rising Duty

TDC

Rising duty

Kevin Cameron

ON THE WAY TO HIGHER PERFORMANCE, parts get broken. Just when engineers think they’ve got things fixed, riders and tuners push the revs and the temperature up, and trouble starts all over again. Engineering never ends-problems aren’t so much solved as they are just pushed to a higher level of performance.

Yamaha designed its first two-stroke Twins back in the 1950s. Naturally, they used needle-bearing con-rods that could survive on very little oil. That required cranks pressed together around strong, non-split, one-piece con-rods. Any pressed-together Twin crank needs something to join its two halves. Yamaha did this by making the inner mainshaft of one crank as a tube, and that of the other as a solid shaft. The tube carried the center main bearings, and the solid shaft was pressed into it to join the halves. Yamaha engineers learned the value of providing generous fillets where shafts grew out of flywheels; if the transition is too sharp, the stress concentrated there may generate a crack that, in time, will break the shaft. These couplings were reliable either at 7500 street rpm, or at the 9500 rpm of the production roadrace engines.

When Yamaha updated its twostroke Twins for 1972, this successful tube-and-shaft crank-coupling system was continued as proven technology. However, certain other changes began to push the system harder. First, stroke had been increased from 50mm to 54mm, requiring bigger flywheels. This put larger masses at either end of the coupler, each trying to go its own way. The design was still reliable on the street-uncounted thousands of R5/RD Yamaha Twins ran millions of trouble-free miles. But on the roadrace version, heavier flywheels $nd higher revs excited torsional vibration that caused failures. Unusual measures were taken to make these cranks raceworthy: The center coupler was increased in diameter by 2mm, requiring inner main bearings with an expensive, non-standard, oversized bore.

Other problems cropped up. As rpm rose with development, the press-fits of the hollow crankpins in the flywheels became insufficient to prevent flywheel spreading-a steady increase in crank width as vibrating flywheels “walked” slowly off their crankpins at a few thousandths of an inch per hour of operation.

These engines were designed to be rebuildable. Crankpins were separate pieces, not forged as part of the flywheels as in today’s more rigid designs. Separate crankpins meant that one of the inner flywheels had two press-fits in it: one for the crankpin, one for the mainshaft of the opposite crank half. Because the racing mainshafts had been increased by that 2mm, those two press-fits were now just a bit closer together, putting extra stress on the flywheel material between them. To make balancing easier, crankpins had been made as hollow tubes, but now pins were made solid to increase crank stiffness. Press-fit force went up from around three tons to five. This raised stressespecially on the flywheel with the two press-fits in it. At the originally intended 10,500-rpm racing redline, all was well, but who looks at the tach when victory is at hand?

The result-especially on the TZ750 with its heavier pistons-was occasional outright breakage of the twopress-fit flywheel, sometimes even in new cranks. Stressed by the tighter press-fit and the vibration of 12,000rpm-plus operation, a crack would form in the fillet that joined the flywheel and its coupler tube. Later, the flywheel would suddenly open up like a pair of brake shoes, demolishing the crankcases. For privateers, the effective answer was to maganaflux the critical fillet area to find cracks, then shot-peen the good parts before crank assembly. Fillet radii that had been solidly reliable on the earlier Twin now needed help to survive at higher duty.

Main bearings acted up. At 12,000 rpm, the balls in the 1972-73 main bearings beat their steel-ribbon ball separators to pieces. When the balls ran over the pieces, everything stopped. Rugged bronze-caged rollers were substituted, while a switch to a snap-in plastic cage made the inner mains reliable. Race crank life jumped from 350 miles to just over 900. The new bearings, while reliable, were expensive (retail for an outer is now $144).

Knowing the history, Yamaha engineers improved the design when they created the evolutionary RZ350. To avoid having two press-fits in any one flywheel, they sensibly forged the crankpins integral with the inner flywheels and pressed them into the outer flywheels. The crank halves were joined by a single shaft, pressed into each of the two inner wheels.

When a big-end needle bearing fails, the friction and heat of its seizure wrecks the crankpin surface. The early Twins, designed in an era when many owners kept their bikes long enough to wear them out, needed separate crankpins that could be replaced. But when the RZ was designed, considerations of strength and durability at higher duty took precedence over rebuildability. The new RZ design banished center-coupling failures, as it had been designed to do, but if you needed a new crankpin, you had to buy the flywheel, too.

It was no surprise when Yamaha released the first of its productionbased V-Twin TZ250 roadracers in 1991 that they had big, RZ350-style integral crankpins (therefore no center-coupler problems) and good old industrial-quality ball main bearings. As a bearing engineer had told me years before, it’s possible by playing with internal clearance, and with clearance of the balls to their separator, to make quite ordinary-looking standard-construction bearings perform reliably at higher rpm. Yamaha’s TZ250 engines now regularly go more than 1500 miles on a crank, spinning to beyond 13,000 rpm. □