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Sheldon Brown's Bicycle Glossary Dr-z

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Drag Brake
A brake that can be set so that it will stay on even when the rider lets go of the control lever. This is usually a drum brake operated by a friction-type shift lever. Tandems are often equipped with a drag brake when they are to be used for touring in mountainous terrain. Descending with a heavily-laden tandem using only rim brakes can cause the rims and tires to overheat, leading to blowouts.

See my article on Tandem Brakes.

Draft
To follow another cyclist (or motor vehicle) so closely that the leading vehicle takes the brunt of the wind resistance, and acts as a windbreak for the drafting cyclist.

Since air resistance is the major limitation on bicycle speed, many of the tactics used in bicycle racing are based on exploiting this effect.

Drainpipe
British term for a tandem's Keel tube.
Draisine
Before the invention of the bicycle, a German nobleman named von Drais invented the "draisine" which is like a bicycle without pedals. The rider would use his feet to push along the ground. The draisine walking enhancer, permitting the user to cover more ground with each stride, and to coast down hills.

When Pierre Lallement added pedals to a draisine in the 1850s, the bicycle was born.

Draisines are still made for small children, intended as a transitional vehicle between a tricycle and a bicycle, but I don't recommend buying these, because you can acheive the same effect by unscrewing the pedals from a real bicycle.

See my article on Teaching Children to Ride.

Driver
The part of an internal-geared hub or coaster brake that the sprocket attaches to, either by threads or splines.

See also threaded driver.

Drivetrain
The parts of a bicycle which have to do with generating forward motion. These include the pedals, cranks, chainwheels, bottom bracket, chain, derailers, rear sprocket(s) and rear hub.

The drivetrain area of a frame consists of those parts of the frame that are directly stressed by the drive train parts, specifically the chainstays, down tube, and seat tube.

Drop
  • Frame geometry: the difference between the height of the fork ends and the height of the bottom bracket.

    Smaller drop = a higher bottom bracket. This dimension is often preferred to the bottom-bracket height dimension, because it is the same whatever tires are installed on the bicycle.

    A bicycle with a shorter drop (higher bottom bracket) will be less at risk of striking a pedal on the ground during high-speed cornering or obstacle jumping.

    A bicycle with a longer drop (lower bottom bracket) will be slightly faster, and easier to mount/dismount.

  • Handlebars: the vertical dimension of a drop handlebar, from the horizontal part to the level of the drops.
Drops
The lower parts of a drop handlebar, the area below the brake levers.
Drop Bolt
An offset mounting bolt, to allow a short-reach brake caliper to be used where a long-reach one would otherwise be required. See my Home Made Drop Bolts article.
Drop Handlebar
A drop handlebar is one in which the middle of the bar is the highest point, or nearly. Most bicycles built for fast or long-distance riding have drop handlebars, which provide a range of different grip positions, allowing the rider to change positions for variety and to accommodate different road/wind conditions.

The most common style of drop handlebar is the "Mæs" bend. Variations include the "randonneur" and "anatomic" bends.

DESCRIPTION
Mæs
DESCRIPTION
Randonneur

The main advantage of drop handlebars is that they offer several different hand positions. For longer rides, the ability to change positions is very desirable. Riding for a long time in any one position tends to be uncomfortable.

People who think they don't like drop handlebars are often actually objecting to the position of the bars on the drop-bar bikes they have tried.

Bikes with drop bars often have the bars mounted rather low and far forward, so that the rider has to lean forward quite a lot to reach the bars, especially the lower "drop " position. If these people tried a bike where the drop bars were placed higher, and closer to the saddle, they might find they really liked them.

I have an article on this topic, called "Hands Up."

Most newer drop handlebars come in either single-groove or double-groove versions, with grooves along the upper section to acommodate brake (and shift) cables running under the handlebar tape.

These grooves are totally unnecessary. Indeed, the double-groove variety is often rather uncomfortable due to the rear groove's causing sharp ridges against the rider's hands.

Dropout
A type of forkend that allows the rear wheel to be removed without derailing the chain first.

Track and BMX bicycles do not have rear dropouts; they use fork ends that open to the rear.

A current fad has led to the revival of this inferior forkend style for single speed bikes.

Rear dropouts come in two styles:

  • Horizontal dropouts have a longish slot for the rear axle to fit into, which runs more-or-less horizontally along the dropout. They permit the wheel to be placed in various positions front to rear. Horizontal dropouts are necessary for bicycles which don't have derailers, because the axle must be moveable to adjust the chain tension.

    With horizontal dropouts, it is possible to mis-align the wheel in the frame if it is installed carelessly. The axle nuts or quick-release must be tightened quite securely, or the chain tension may pull the axle askew.

  • Vertical dropouts have a vertical notch for the axle to go up into, and the axle's position is not adjustable. With vertical dropouts, the axle cannot be pulled out of position, even if it is not properly secured.

    Vertical dropouts are generally intended for use with derailer gearing, and do not permit any adjustment of the position of the rear wheel, so there's no way to regulate chain tension if you don't have a derailer or pulley type chain tensioner.

    A few single-speed or internal-hub gear bikes use vertical dropouts with an eccentric bottom bracket to permit chain tension adjustment.

    White Industries makes a special hub with an eccentric axle, also to permit vertical dropouts to be used with a straight chain run.

On derailer-equipped bicycles, the rear derailer is attached to the right rear dropout, either directly to a hanger that is part of the drop out, or by way of an adaptor claw.

Dropout spacing varies among different styles of bicycles

Forged Dropouts
forged dropout forged dropout forged dropout forged dropout
Horizontal Dropout
Campagnolo 1010
Horizontal Dropout
Short
Semi-Vertical Dropout Vertical Dropout
Spoke Divider
Stamped Dropouts Investment Cast
dropf dropf dropf dropf
With Hanger Without Hanger Raleigh 3-speed Track Forkend
Not a dropout!
Drum Brake
A drum brake is a hand-operated brake which is built into, or attached to the hub of a wheel, with shoes that press against the inside of a cylindrical drum.

The drum may be the inside of an oversized hub shell, or may be a separate unit which screws on to the side of the hub, by threads like those to which freewheels attach.

Drum brakes are common on automobiles and motorcycles, but fairly rare on bicycles, mainly due to their weight. The greatest advantage of a drum brake is that it is unaffected by rain. Drum brakes are commonly used as drag brakes on tandems. See my article on Tandem Brakes.

Dry Rot
"Dry rot" is a fungus that infects cellulose-based materials: wood, paper, cotton and the like.

Sometimes people speak of bicycle tires as if they suffer from dry rot, but this is not generally correct. (The exception would be for cotton-cord tires, but those pretty much disappeared by the mid 1960s, at least as far as clinchers are concerned.)

What people commonly call "dry rot" is a deterioration of the rubber, usually on the sidewalls. This is particularly common with gumwall tires that have been exposed to ozone damage. (A common cause of this is storing a bicycle near a household furnace. The brush-type motors on such furnaces often create sparks, which in turn create ozone.)

This type of damage is ugly, but not structurally significant, as long as the cords (fabric) of the tire are intact.

Generally, if a tire isn't lumpy/misshapen when inflated, and has not had the tread area worn too thin, there is no reason to replace it, no matter how ugly the sidewalls get.

D.S., N.D.S.
For some obscure reason, some writers on bicycle matters seem to have a problem with the terms "right" and "left."

Instead of using these easily understood common English words, they have invented the terms "drive-side" for "right" and "non-drive-side" for left. (This relates to the fact that the chain drive is on the right side.)

As if this silly jargon were't confusing enough, they sometimes further muddy the waters by abbreviating these phrases as "D.S." and "N.D.S."

Dual-Pivot
A type of sidepull brake caliper. The dual-pivot design was originated by Altenburger in the 1960's, and popularized by Shimano in the 1990's.

The main feature of dual-pivot brakes is that they are easier to keep centered, due to the way the arms are linked together. They can have more mechanical advantage (and a resulting rest position closer to the rim) than conventional sidepulls. Newer designs permit fine tuning the centering of the arms by simply turning a screw.

The principal disadvantage of dual-pivot brakes is that they don't track imperfect rims as well as single-pivot sidepulls, possibly causing pulsating braking.

Thanks to Mark McMaster for his help with this entry.

DT ®
Swiss DT is the leading manufacturer of spokes. When DT spokes first became available in the U.S. market in the late 1970's, they revolutionized wheel building. Although stainless spokes had been available previously, the quality of the threading on DT spokes and nipples was quite a lot better than that of brands that had previously been available, allowing wheels to be built at considerably higher spoke tensions than had previously been possible.

"DT" stands for "Drahtwerke Tréfilerie." "Drahtwerke" means "wire works" in German; Tréfilerie means the same thing in French.

Dual Suspension
Refers to a bicycle with suspension for both wheels.
Dunlop
John Boyd Dunlop was born in Ayreshire, Scotland, in 1840. He studied veterinary science at the University of Edinburgh. In 1888,he invented the pneumatic tire, probably the most revolutionary and important invention to come out of the bicycle industry. J. K. Starley's chain-driven "safety" bicycle had been invented three years earlier, but was not fully practical due to the harsh ride of the smaller wheels with solid tires. With Dunlop's pneumatic tire, the modern bicycle was basically complete. No development in bicycle technology since then has been more than a minor refinement on the work of Starley and Dunlop.

Because an earlier patent for an inflatable tire had been taken out in France, his 1888 patent was invalid in Scotland, so he moved to Belfast, Northern Ireland, and started manufacturing tires in 1890.

The Woods valve is also sometimes referred to as a "Dunlop" valve.

The Dunlop tire company which he started still exists, but stopped making bicycle tires in the 1960's. This caused a crisis among cyclists for a couple of years, because Dunlop bicycle tires were incontestibly the finest available. The Dunlop "HPRR" (High Pressure Road Racing) tire was the high-performance "clincher"

Dura Ace
Shimano's top-of-the-line parts group for road racing bikes. See Shimano Models and Buzzwords.

There are interchangeability issues with older Dura-Ace parts, and I have an article on Dura-Ace interchangeability.

Duralumin, Dural
Duralumin is a trade mark name for an early popular structural aluminium (aluminum) alloy, and 'dural' is slang for it. Dural has a poor corrosion resistance, and (I hope) isn't used much for cycle parts nowadays. It needs a paint or lacquer finish if used out of doors in dirty conditions. (Thanks to Mark Irving)
Dustcap
  • A thin metal, plastic or rubber shield that covers the bearing balls of a conventional cup-and-cone hub.
  • A plastic or metal cap that screws or snaps into the extactor threads of a cotterless crank.
Dyna Drive ®
In the early '80's, Shimano introduced a special crank/pedal set, which used much larger diameter threading where the pedal screwed into the crank. This allowed the bearing to be built into the inside of the pedal thread, eliminating the need for a pedal axle. The purposwas to improve the biomechanics of the pedal by placing the bottom of the foot below the pedal axis.

The threading chosen was 1" x 24 tpi, same as a standard 1 inch headset, except that the left side was a left thread.

This was rather a good idea, but never caught on.

Dynamo
British term for generator. Technically, the term "generator" is preferred, because "dynamo" primarily refers to a generator of direct current, while bicycle generators all produce alternating current.

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