Motorcycle Gearboxes

01
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Gearbox Development

A) Movable gear B) Fixed gear c) Dogs for engagement into another gear D) Selector fork groove. John H Glimmerveen Licensed to About.com

Over the years many different types of gearbox have been tried on motorcycles, but eventually, most manufacturers settled on what is now the norm or conventional gearbox: the multi ratio, sequential foot change type.

Motorcycle manufacturers began to fit gearboxes during the early 1900s to maximize the performance of their machines. The early machines were so low in power (typically 1.5 hp) that to achieve a better top speed than a regular bicycle, they had to have a gearbox.

During the evolution of motorcycles many of the components (and their designs) have become standardized; for example tires, spark plugs, and (eventually) gearbox operating principles.

The basic configuration of most motorcycle gearboxes (from the 60s onwards) consists of a fixed gear on one shaft which is mated with a movable gear on another shaft. The movement of the gear is controlled by a selector fork which in turn follows a rotating drum with grooves.

The operating principles of most gearboxes from the 1960s onwards are as follows:

1) The rider moves the gear change lever which is attached to a shaft

2) The shaft passes through the gearbox and pushes or pulls pegs located on a selector drum

3) The selector drum rotates for the distance required of one gear change

5) A gear (sitting on a selector fork) moves sideways until its dogs (large teeth, typically three or four in quantity, positioned radially around the gear) engage with another –fixed - gear

6) Output shaft rotates the final drive front sprocket or input gear of a shaft drive type

02
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Disassembly and Inspection

Image courtesy of: Harry Klemm groupk.com

Periodically (depending on the mileage) or during a restoration, a motorcycle gearbox should be checked for wear. In addition, if the gear change is not working properly or if the oil contains large amounts of swarf, the gearbox should be inspected.

Although access to the gearbox (and the design) may vary between makes and models, the basic mechanical skills needed for gearbox work are the same. Ideally, the mechanic should consult a workshop manual if one is available. If the mechanic does not have access to a manual, he should photograph each stage to ensure accuracy when the time comes to rebuild the box.

During the disassembly stage, the mechanic should attempt to loosen as many bolts, nuts or screws as possible while the engine/gearbox assembly is still in the frame. In particular, the drive gear bolt or nut on the end of the crankshaft (note: this may have a left hand thread), the clutch center retaining nut, and the final drive sprocket retaining nut (where fitted) should be loosened.

Horizontally Split Engine Casings

When the engine/gearbox casing half’s have been separated, the gearbox input and output shafts should stay in the bottom casings, along with the selector forks and drum. At this point, the mechanic should rotate the shafts to inspect each for run-out, and also each gear and its associated teeth. Any signs of wear or pitting will indicate the need for replacement parts.

Vertically Split Casings

As the mechanic separates vertically split-type cases, he should endeavor to keep all of the gearbox components in one half of the cases (typically in the right side case).

Inspection

After the gearbox components have been removed from the casings, the mechanic should remove the gears (where possible; some gears are fixed to the shafts-check the shop manual) for a more detailed inspection.

Besides damaged teeth on the various gears, they also suffer from damage or wear to the dogs; they typically get rounded corners sometimes resulting in a missed gear or jumping out of gear (improper engagement).

03
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Detailed Inspection

A professional stand will make inspection easier. John H Glimmerveen Licensed to About.com

To make disassembly of the gear from the shafts easier and to facilitate inspection, the mechanic should make a stand for the shafts. This can be as rudimentary as a large nail in a piece of wood to a machined stand such as the one shown in the photograph.

With the shafts placed on a stand, the mechanic can begin the process of disassembly. Normally, gears are retained on their respective shafts between circlips and thrust washer (in the order: circlip, thrust washer, gear, thrust washer, circlip). To ensure correct reassembly, the mechanic should inspect each item as it is removed from the shaft, and then place in order onto a suitably sized rod or pole (again, something as rudimentary as a large nail in a piece of wood will suffice).

Should the mechanic notice wear on a gear’s dogs, or the receiving hole on the engaging gear, both items should be replaced. It should also be noted that in some cases gears are sold as matched pairs.

When all of the gears have been removed from their respective shafts, the shafts should be placed between centers in a lathe and checked (with a dial gauge) for run-out. Each manufacturer will specify an acceptable amount of run-out; however, if no specifications are available, the mechanic should consider 0.002” (.0508-mm) acceptable, anything greater (up to 0.005”) should be considered suspect and anything above this in need of replacement.

Another typical high wear item is the selector forks where they interface with the spinning gear, where any sharp edges or thinning will indicate the fork must be replaced.

04
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Rebuilding the Gearbox

A schematic gearbox diagram will help with the reassembly sequence. John H Glimmerveen Licensed to About.com

When rebuilding the gearbox internals, the mechanic must replace all of the circlips and thrust washers. In addition, it is good practice to replace all of the bearings if their age/mileage is not known or if they have any play. (Bearings should also not make any noise when spun, after cleaning). All oil seals should replaced each time the gearbox is disassembled.

Reassembly is simply a matter of replacing the various gears, washers and circlips back in their respective locations. All of the components should be liberally coated with the same grade of oil that will be used in the finished gearbox.

During the reassembly, it is imperative that all of the components are perfectly clean.