Ex. 22. Make up a plan to the text.

Ex.23. Render the text B using the plan of rendering on page 67.

VII. Oral Practice.

Ex. 24. Give information about a fixed-gear bicycle and its uses using the plan of ex.22.

Reading and comprehension

Ex.25. Read the text C without a dictionary for 5 minutes and point out advantages and disadvantages of fixed gear bicycles.

Advantages and disadvantages of Fixed Gear bicycles.

Fixed gear bicycles are ridden by cyclists for many reasons, such as their light weight, simplicity, low maintenance, or image.

Many people who ride fixed-gear bicycles simply find it more enjoyable than or as an alternative to riding bikes with freewheels. Although the bike has only one gear, the lighter weight of a fixed-gear bike over its multi-speed freewheel equivalent can provide increased performance. Although the rider cannot change to a lower gear, climbing hills on a fixed gear is easier than with a multi-speed freewheel because it is easier to maintain momentum as the cranks are pushed through the dead centers by the chain, though this could be because a fixed bicycle is lighter than its multi-speed freewheel equivalent. In slippery conditions some riders prefer to ride fixed because the transmission gives feedback on back tire grip.

Descending is more difficult as the rider must spin the cranks at a very high speed (sometimes at 150rpm or more), or use the brake(s) to slow down. Nevertheless, the enforced fast spin when descending is said to increase "souplesse" (a French word roughly meaning suppleness), which improves pedaling performance on any type of bicycle.

Riding fixed is generally considered to encourage a more effective pedaling style, which translates into greater efficiency and power when used on a bicycle fitted with a freewheel.

When first riding a fixed gear, a cyclist used to a freewheel has a tendency to try to coast now and again, particularly when approaching corners or obstacles. Since freewheeling is not possible, this can lead to anything from a 'kick' to the trailing leg, up to a loss of control of the bicycle.

Notes:

In slippery conditions – в умовах обвала

Descend – спускатися по Crank - кривошіп

 

Supplementary Reading

Texts for written translation with a dictionary

Read the texts and translate them in writing. Use a dictionary.

Mechanical advantage

Intermeshing gears in motion

The interlocking of the teeth in a pair of meshing gears means that their circumferences necessarily move at the same rate of linear motion (eg., metres per second, or feet per minute). Since rotational speed (eg. measured in revolutions per second, revolutions per minute, or radians per second) is proportional to a wheel's circumferential speed divided by its radius, we see that the larger the radius of a gear, the slower will be its rotational speed, when meshed with a gear of given size and speed. The same conclusion can also be reached by a different analytical process: counting teeth. Since the teeth of two meshing gears are locked in a one to one correspondence, when all of the teeth of the smaller gear have passed the point where the gears meet -- ie., when the smaller gear has made one revolution -- not all of the teeth of the larger gear will have passed that point -- the larger gear will have made less than one revolution. The smaller gear makes more revolutions in a given period of time; it turns faster. The speed ratio is simply the reciprocal ratio of the numbers of teeth on the two gears.

(Speed A * Number of teeth A) = (Speed B * Number of teeth B)

This ratio is known as the gear ratio.

The torque ratio can be determined by considering the force that a tooth of one gear exerts on a tooth of the other gear. Consider two teeth in contact at a point on the line joining the shaft axes of the two gears. In general, the force will have both a radial and a circumferential component. The radial component can be ignored: it merely causes a sideways push on the shaft and does not contribute to turning. The circumferential component causes turning. The torque is equal to the circumferential component of the force times radius. Thus we see that the larger gear experiences greater torque; the smaller gear less. The torque ratio is equal to the ratio of the radii. This is exactly the inverse of the case with the velocity ratio. Higher torque implies lower velocity and vice versa. The fact that the torque ratio is the inverse of the velocity ratio could also be inferred from the law of conservation of energy. Here we have been neglecting the effect of friction on the torque ratio. The velocity ratio is truly given by the tooth or size ratio, but friction will cause the torque ratio to be actually somewhat less than the inverse of the velocity ratio.

In the above discussion we have made mention of the gear "radius". Since a gear is not a proper circle but a roughened circle, it does not have a radius. However, in a pair of meshing gears, each may be considered to have an effective radius, called the pitch radius, the pitch radii being such that smooth wheels of those radii would produce the same velocity ratio that the gears actually produce. The pitch radius can be considered sort of an "average" radius of the gear, somewhere between the outside radius of the gear and the radius at the base of the teeth.

The issue of pitch radius brings up the fact that the point on a gear tooth where it makes contact with a tooth on the mating gear varies during the time the pair of teeth are engaged; also the direction of force may vary. As a result, the velocity ratio (and torque ratio) is not, actually, in general, constant, if one considers the situation in detail, over the course of the period of engagement of a single pair of teeth. The velocity and torque ratios given at the beginning of this section are valid only "in bulk" -- as long-term averages; the values at some particular position of the teeth may be different.

It is in fact possible to choose tooth shapes that will result in the velocity ratio also being absolutely constant -- in the short term as well as the long term. In good quality gears this is usually done, since velocity ratio fluctuations cause undue vibration, and put additional stress on the teeth, which can cause tooth breakage under heavy loads at high speed. Constant velocity ratio may also be desirable for precision in instrumentation gearing, clocks and watches. The involute tooth shape is one that results in a constant velocity ratio, and is the most commonly used of such shapes today.