Motorcycle Gear Friction

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Motorcycle engines contain gear drives. Primary drives (engine-to-gearbox) have the highest meshing speed among them. When the camshafts are gear-driven (racing or near-racing applications) there can be torques 10 times greater than a back-of-an-envelope calculation would suggest. Transmission gears—providing typically five or six speeds—have the highest tooth-to-tooth pressures because the torque reaching them is engine torque multiplied by the primary ratio. There may also be gear-driven accessories such as oil and water pumps or, less often, a gear-driven alternator.

There are three basic sources of friction loss in gear drives:

1. Tooth-to-tooth sliding contact: Tooth profiles are designed to minimize sliding, but some sliding between teeth does occur. Because tooth-to-tooth pressure can be very high, even a small amount of sliding consumes power, turning it into heat.
2. Oil churning: Any but the slowest-moving and lightly loaded gears require lubrication, and in fast-spinning gears the power consumed in rapidly pumping-out oil trapped in the mesh and in flinging oil about in the gear case can be significant. This loss raises lube oil temperature. Dyno operators from the old days of Harley racing have spoken of gearboxes too hot to touch after a test session.
3. Bearings: Friction torques occurring in the bearings supporting the gears.

The simplest way to lubricate gears is by “splash,” which means filling up the gear case at least until the largest gears dip into its surface. This was the “system” employed on popular two-stroke production bike engines of the 1970s.

Related: Journal Bearing in Motorcycle Engines

Gears in a motorcycle engine are a source of friction, but some surprising things happen when those teeth come together. (Jeff Allen /)

Allowing gears to churn masses of oil eats some power. Racers of the 1970s discovered their engines could reach a few hundred rpm higher on top-end if they reduced gearbox oil level after first practice. Back in 1961 Yamaha engineer Noriyuke Hata had decided to do something about such losses in that company’s earliest GP roadracers. He added a tiny oil pump, supplying gear oil to a gallery running across the gearbox, just above the gear meshes, from which a tiny oil jet was directed down into each mesh. To lubricate the free-spinning gears on their shafts, the shaft centers were also supplied with oil and the shafts were radially drilled to lead that oil where needed. The gearbox oil level was far below.

For mesh speeds above 12,000 feet/minute gearing texts suggest use of air-oil mist, applied on the opening side of the mesh.

As to loss from tooth-to-tooth sliding, in early days gear engineers were able to calculate that a complete hydrodynamic oil film could not possibly exist under the pressures they knew to be present between the meshing teeth of heavy-duty truck gears. Why didn’t they immediately fail by scuffing and scoring (surface-to-surface welding and tearing)? Much later a fantastical explanation was discovered: That under extreme tooth-to-tooth pressure, some unknown mechanism was multiplying the viscosity of oil by hundreds of times until the “fluid” becomes a nearly solid lubricant. This is given the name “elastohydrodynamic” lubrication because part of the picture is that curved tooth surfaces flatten elastically against each other under load, increasing contact area. Computer simulation has suggested that under such pressure, the carbon chains of oil molecules align parallel with each other, making escape from the loaded zone extremely difficult. The result in well-designed gears is protection of tooth surfaces from scoring failure.

Related: TECH ESSAY: Separate Gearbox Oil?

The work of overcoming friction is force times distance. Here, distances are short (the sliding in tooth contacts) but the forces are large.

An example of modern power gearing is the compact star-type reduction set driving Pratt & Whitney’s Geared Turbofan (GTF) engine. At takeoff, the gearing sends 13,000 hp to the fan with an efficiency of 99.3 percent. They are able to get sufficient oil into contact with the gear teeth to carry away excess heat, while pulling the oil out of there to prevent much larger loss from oil churning. Star gearing looks like planetary gearing, but with the planet carrier held stationary.

The rotating bearings associated with gearboxes present the usual sources of friction torque. Plain or journal bearings use oil’s viscosity to allow the rotating part to drag oil into the loaded zone. This is effectively pumping of oil into a high-pressure zone and so consumes a small amount of power. Rolling bearings can be tricky; when long-ago Norton race chief Joe Craig was studying the big-end roller bearing of the company’s Manx racing engine, he found that increasing oil delivery to the bearing drove its temperature up faster than a decrease by a similar amount. Too much oil forces the rolling elements to rapidly squish raceway oil ahead of them, generating heat in familiar oil-churning fashion. It’s like the sharp deceleration you feel when you drive your car into standing water on the roadway: Some of the momentum of the car is consumed by rapidly accelerating the liquid aside.

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