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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. This is because descending with a heavily-laden tandem using rim brakes only 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 wind-break for the drafting cyclist.

Since air resistance is the major limitation on bicycle speed, most 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 basically a bicyle without pedals. The rider would use his feet to push along the ground. This was basically a walking enhancer, permitting the user to cover more ground with each stride, and to coast down hills.

When Pierre Lallement adde 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 tricyle and a bicyle, but I don't recommend buying these, because you can aceive 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.

Drive Train

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

The drive train area of a frame would consist of those parts of the frame that are directly stressed by the drive train parts, specifically the chain stays, down tube, and seat tube.

Drop

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
Mæes
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 un-necessary. Indeed, the double groove variety is often rather uncomfortable due to the rear groove causing sharp ridges against the rider's hands.

Drop Out

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

Track and BMX bicycles do not have rear drop outs, they use fork ends that open to the rear.

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

Rear drop outs come in two styles:

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

Drop-out spacing varies among different styles of bicycles

Forged Dropouts:
Horizontal Dropout
Campagnolo 1010
Horizontal Dropout
Short
Semi-Vertical Dropout Vertical Dropout
Spoke Divider

Stamped Dropouts:Investment Cast
With Hanger Without Hanger Raleigh 3-speed Track Fork End
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 gum wall 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 side-pull 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. This makes it possible to design them with more mechanical advantage (and the 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, which can cause pulsating braking action.

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. This allowed wheels to be built at considerably higher spoke tensions than had previously been possible.

"DT" stands for "Drahtwerke Treflerie." "Drahtwerke" means "wire works" in German; Treflerie 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, Dr. J.B. Dunlop, a veternarian of Belfast, Ireland, invented the pneumatic tire, probably the most revolutionary and important invention to come out of the bicycle industry, the pneumatic tire. 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 the 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 popular an early 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. (Mark Irving)

Dustcap

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 them to build the bearing into the inside of the pedal thread, eliminating the need for a pedal axle. The purpose of this was 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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