Sheldon Brown's Bicycle Glossary G (original) (raw)

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-- Sheldon (d. 2008), Harriet, John.
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Gage

Alternate spelling of gauge.

Gain ratio

One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the only system that takes crank length into account, giving a true value for the relative leverage of different gears on bicycles with different size wheels and cranks. Gain ratio has the further advantage of not needing any units. It is a pure ratio, and is the same whether you use metric or inch-based units to calculate it. See also gear inch and development.

For more detail on gain ratios, see my article on the subject.

You can calculate gain ratios., gear inches or meters development. with my Online Gear Calculator or with your slide rule

Gas Pipe

A derisive term for the "high tensile" tubing used to build cheap bicycles. Since low-end bicycles are made of low-quality steel, the builders compensate by using heavy, thick-walled tubes.

Gauge

• A measuring instrument, most commonly in bicycle contexts, a device for measuring the air pressure in a tire.
• A measurement of thickness, particularly of wire. The major use of gauges in bicycle technology is for spokes. There are several different national systems of gauge sizes, and this has been a great cause of confusion. A particular problem is that French gauge numbers are smaller for thinner wires, while the U.S./British gauge numbers are larger for thinner wires. The crossover point is right in the popular range of sizes used for bicycle spokes:

  U.S./British 14 gauge is the same as French 13 gauge
  U.S./British 13 gauge is the same as French 15 gauge
  Newer I.S.O. practice is to ignore gauge numbers, and refer to spokes by their diameter in millimeters, usually rounded to the nearest tenth of a millimeter.:
  U.S./British 13 gauge is 2.3 mm
  U.S./British 14 gauge is 2.0 mm
  U.S./British 15 gauge is 1.8 mm
  U.S./British 16 gauge is 1.6 mm
  U.S./British 17 gauge is 1.4 mm
  The gauge system is basically obsolete as explained in this excerpt from "Machinery's Handbook" 21st edition, 1980 p463:
  WIRE AND SHEET METAL GAGES

  The thickness of sheet metals and the diameters of wires conform to various gaging systems. These gage sizes are indicated by numbers and the following tables give the decimal equivalents of the different gage numbers.Much confusion has resulted from the use of gage numbers and in ordering materials it is preferable to give the exact dimensions in decimal fractions of an inch... [millimeters preferred nowadays-SB ]

  ...the decimal method of indicating wire diameters ...has the advantage of being self-explanatory, whereas arbitrary gage numbers are not. The decimal system of indicating gage sizes is now being used quite generally, and gage numbers are gradually being discarded. Unfortunately there is is considerable variation in the use for different gages. For example a gage commonly used for copper, brass and other non-ferrous metals may at times be used for steel, and vice versa...

  The wire gage system used by practically all the steel producers in the United States is known by the name Steel Wire Gage or to distinguish if from the Standard Wire Gage (S.W.G.) used in Great Britain it is called he United States Steel Wire Gage. It is the same as the Washburn and Moen, American Steel and Wire Company, and Roebling Wire Gages. The name has the official sanction of the Bureau of Standards at Washington, but is not legally effective. The only gage which has been recognized in Acts of Congress is the Birmingham Gage...the Birmingham Gage is, however, nearly obsolete both in the United States and in Great Britain, where it originated...

  In Great Britain one wire gage has been legalized. This is called the Standard Wire Gage (S.W.G.), formerly called Imperial Wire Gage.
  The U.S. system, S.W.G. and Birmingham gages are all the same for 15 gage, 0.72" (1.8288 mm.)
  Below is a small excerpt from the table referred to:

  Gage Number	U.S. Steel Wire Gage	British S.W.G.	Birmingham Gage
  12	.1055" 2.6797 mm	.1040" 2.6416 mm	.1090" 2.7686 mm
  13	.0915" 2.3241 mm	.0920" 2.3368 mm	.0950" 2.413 mm
  14	.0800" 2.032 mm	.0800" 2.032 mm	.0830" 2.10 mm
  15	.072" 1.8288 mm	.072" 1.8288 mm	.072" 1.8288 mm
  16	.0625" 1.5875 mm	.0640" 1.6256 mm	.0650" 1.651 mm
  17	.0540" 1.3716 mm	.0560" 1.4224 mm	.0580" 1.4732 mm

Gear

The "gear" of a bicycle relates to the mechanical advantage of the whole drive system. In a low gear, the pedals are easy to turn, but you have to spin very fast to get any speed up. In a high gear, the pedals are hard to turn, but you don't have to make them turn very fast to make the bicycle go fast.

The gear of a bicycle depends on the ratio between the sizes of the front and rear sprockets, and the size of the drive wheel. If the bicycle is equipped with planetary gears, they also affect the gear. There are several ways of designating gears numerically. See gain ratios, gear inches, and development.

For information on using your bicycle's gears, see my article: Everything You Wanted to Know About Shifting Your Bicycle's Gears, But Were Afraid to Ask

Gear Inches

One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. This gives a convenient two- or three-digit number. The lowest gear on most mountain bikes is around 22-26 inches. The highest gear on road racing bikes is usually around 108-110 inches. Unfortunately, the handwriting is on the wall for all inch-based measurement systems.

See also gain ratios and development.

You can calculate gear inches, gain ratios, or meters development. with my Online Gear Calculator or with your slide rule

Gear Case

A chainguard that totally encloses the chain, usually found only on roadsters. Gear cases (also known as "chain cases") are extremely practical for utility bicycles. They keep the chain clean in all weather, and permit cycling in ordinary street clothes without risk of soiling. Unfortunately, a satisfactory chain case to work with derailer gears has not been developed, so they are only found on one-speed bikes or those with internal-gear hubs. Sometimes referred to as an "oil bath", though most of them are not tight enough to act as an actual "oil bath." An exception is older Sunbeam models, where there was actually a pool of oil contained in the bottom of the gear case.

Chain replacement with a gear case:

• You will need to remove the disc that covers the chainwheel. Pry it with a flat-blade screwdriver, as with the lid of a paint can.
• Remove the pieces at the back by undoing the obvious screws.
  That's as far apart as it needs to come for chain replacement.
  It's very helpful if you have a proper workstand that lets you tilt the bike up and down, but if you don't, a strong friend can handle the bike for you.
• Set the bike vertical with the front end downward, and drop one end of the chain into the upper section of the chain case until you can thread the end of the chain onto the chainring.
• Once the end of the chain has engaged the chainring, pivot the bike vertically so that it's facing straight up. Turn the cranks forward to feed the end of the chain through the lower run of the chain case until the end appears at the rear opening.
• Join the chain, either with the master link or with a chain tool, and put it all back together. (To actually remove the chain case, you need to remove the right crank. There's a bolt hidden behind the chainwheel, screwed into a special fitting on the chainstay.)

Gel

A type of closed-cell foam, in which the bubbles are under higher-than-normal pressure. Originally developed by Spenco for wheelchair cushions, gel is also popular for saddles and saddle covers, and for padding the palms of cycling gloves.

Gel has been rather over hyped as a cure-all for saddle related discomfort.

Generator

A generator (or "dynamo ") is a device for turning mechanical energy into electricity. Generator-powered bicycle lights have been available for many decades. Generators have the advantage of being ready to use at any time and costing nothing to operate.

Unfortunately, the output of a generator depends on the bicycle's speed. Generator lights go dark when the bicycle is stopped, and are dim at low speeds. Fast riding can cause the voltage to rise to the point that bulbs burn out prematurely.

The efficiency of LEDs has made generator lights more practical. Some generator systems now charge a small \battery or capacitor and keep the light shining when the bicycle is stopped.

The widespread availability of rechargeable batteries has considerably reduced the desirability of generator powered systems. The vast majority of bicycle generator systems only put out 3 watts; a very few put out 6 watts (when running at optimal speed.) Since rechargeable systems commonly put out 15 watts or more, at all speeds, they are generally preferable for most applications.

• "Bottle" generators, the most common type, are shaped like a bottle, with a small wheel at the "cap" which rubs against the side-wall of the tire.
• "Bottom-bracket" generators mount below the chainstays, and have a spring-loaded roller which presses against the tread of the tire. Bottom bracket and bottle generators frequently slip and fail when ridden in wet conditions.
• Hub generators are built into one of the wheel hubs. They usually consist of a stationary armature (coil) attached to the axle, with a revolving multi-pole ring-shaped magnet surrounding the armature. The best-known of these is the no longer available Sturmey-Archer Dynohub.
  Hub generators were unavailable for several years, but they are undergoing something of a renaissance in this century, with models from Busch & Müller, Shimano (Nexus ) and Sturmey-Archer. Hub generators are more expensive than other types, but they are more efficient, more reliable and silent.

Ghost Shifting

Refers to when a bike shifts gears spontaneously. See my separate article on Autoshifting

Gizzmo ®

See: Erickson Gizzmo ®

Gloves

Gloves are a good idea for cycling, even in warm weather. They considerably reduce hand discomfort on longer rides, partly due to their padding, and partly because they aren't as slippery on the handlebars, reducing the need to grip the handlebars so tightly and reducing hand fatigue. They also allow use of a hand to break a fall without injury.

Traditional cycling gloves are fingerless, with leather palms, often double layered with a foam sandwich under the heel of the hand. The backs were traditionally a crocheted mesh, though Lycra ® is more common of late.

Goat head

The seed of a low-growing flowering plant common in warm climates. Each seed has thorns which can puncture bicycle tires, like a thumbtack.

More detail at Wikipedia

Goofy Footed

When you coast with the cranks horizontal, ideally you should have the left foot forward, right foot rear. This way, the stresses on the crank/bottom bracket interface are in the usual direction.

If you coast with your right foot forward, that's sometimes called "goofy footed" and can contribute to loosening up or damaging the crank, since it applies stress in the opposite direction than the stresses of normal pedaling.

This term comes from the world of surfing, where left-foot forward is the norm.

Goose Neck

Handlebar stem, particularly a stem with a long forward extension.

Granny Gear

Slang term for the smallest chainwheel on a triple crank set.

Note that this term is not a general term for the lowest gear of a bicycle, but specifically refers to just the smallest chainwheel on a crankset with more than two chainwheels.

Grease

A lubricant in the form of a viscous paste, rather than a liquid oil. Grease is generally a more difficult to apply, because most bicycle parts cannot be greased effectively without disassembly. Grease lubrication is commonly used on all ball bearings. Good mechanics also use grease (or oil) on the threads of most threaded fittings and fasteners, and also inside the steerer (to keep the stem from becoming stuck) and the seat tube (to keep the seatpost from becoming stuck.)

There are a great many different greases on the market with different special features, mainly for automotive applications. For bicycle use, almost any grease is adequate, since the loads and temperatures are generally low. In wet conditions, a water-resistant grease is preferable. Coaster brakes need a heat-resistant grease.

Greaseguard ®

A Sun Tour system with fittings to allow grease to be injected into bearings without the need to disassemble them; especially intended for mountain-bike use, in which a bicycle may be partly submerged in water.

GripShift ®

SRAM trademark for twist-grip shifters.

Group

A group is a set of parts for one bicycle from (usually) a single manufacturer. The practice of selling parts as a group (or "gruppo" in Italian) probably originated with Campagnolo in the 1960's.

A group would normally include (as a minimum) the following:

• Hubs
• Bottom bracket
• Derailers
• Shifters
• Brakes

If a particular manufacturer doesn't make one or more of these items, it may have some made under its name by another manufacturer to fill in a group. For instance, when Mavic was offering groups, it had everything listed above except for brakes, so it had brakes made under its name by Modolo and Dia Compe, and pedals by Look.

If the manufacture offers them, a group could also include:

• Pedals
• Headset
• Seat post

Note the distinction between a "group" and a "kit": In addition to the group, a kit includes everything needed to turn a frame into a bicycle: built-up wheels (rims, spokes, tires, rim tape, tubes), handlebars, saddle, etc.

Grouppo

Common misspelling of "Gruppo."

Groupset, Group Set

Redundant term for "group." More common in British usage than American.

Gruppo

Italian for "group." The use of the term "gruppo" is sometimes considered an elitist affectation, especially when referring to parts which are not of Italian manufacture.

Gum

Tan, semi-translucent rubber, commonly natural rubber. This was once commonly use on the sidewalls of tires and on brake hoods. Natural gum rubber lacks the additives which would help defend it against ozone and other chemicals. Gum rubber tends to crack and deteriorate into a sticky mass over time. Some tan rubber is not natural gum rubber, and is more immune to this deterioration. See skinwall, blackwall, gum hoods.

Gusset

A reinforcement welded or brazed to one or more tubes of a frame to reinforce an area of high stress.

Gyro ®

A particular type of rotor, from Odyssey ®. The Gyro features a split cable, so that the rotor is pulled evenly from both sides. This avoids any mechanical friction from the rotor, since it does not slide along the stem shaft as with other rotors. In fact, the rotor part of the system touches nothing but the cables.

In addition, the Gyro has a ball-bearing built into it, so it has very little friction while turning.

Gyro adjustment:

Every part in a brake system has a certain amount of "travel" available. For instance, the lever can go from its rest position (which is often adjustable for hand size by a small screw) as you squeeze it to the point where the lever bumps into the handlebar grip.

The caliper has a limited amount of travel too, limited by the brake shoes hitting the rim on the inward side, and by whatever stops the arms from springing out when the brake is released.

On any braking system, the outward (rest position) travel should be limited by the lever's bumping against its stop, but the inward travel should be limited only by the brake shoes' hitting the rim. You should be able to squeeze the lever as hard as you can without having the lever bump into the grip, because once the lever bottoms out, no more braking power is transmitted to the brake shoes, no matter how much harder you squeeze.

The Gyro has a limited amount of travel available to it also, and it is important to ensure that the Gyro itself is not the limiting factor in either inward or outward travel.

Start by hooking up the upper cable between the lever and the Gyro. Operate the lever with one hand while pulling downward on the Gyro assembly. The adjusting barrels on the Gyro (and on the lever, if there is one) should be set so that the lever just barely stops the Gyro from bottoming out against the headset when you release the lever.

Make sure that the adjusting barrels on both sides are set the same way, so that the Gyro is horizontal. Otherwise, the braking action may vary as you turn the handlebars.

Once the upper cable is set, connect the lower cable to the brake in the usual manner.

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